Method for producing fibre cement products with fibre cement waste

Abstract

The present invention relates to a method to produce fibre cement products comprising heat-treated fibre cement waste material. The method saves energy and reduces CO2 emission compared to traditional production of fibre cement products.

Description

[0001] METHOD FOR PRODUCING FIBRE CEMENT PRODUCTS COMPRISING FIBRE CEMENT WASTE

[0002] Field of the invention

[0003] The present invention relates to methods to produce fibre cement products comprising fibre cement waste material. More particularly, the present invention provides methods for the production of fibre cement products, at least comprising the steps of providing cured fibre cement material and optionally comminuting said material to a cured fibre cement waste powder, heating the cured fibre cement waste material and comminuting it to a powder; or heating the comminuted fibre cement powder; and subsequently forming a fresh green fibre cement product from an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and the heat-treated cured fibre cement powder, which heat-treated powder substitutes a part of the ordinary hydraulic cement in the fresh green fibre cement products / sheets. The present invention further relates to fibre cement products obtained by curing the green fibre cement product / sheet as well as to uses of the fibre cement products in the building industry.

[0004] Background of the invention

[0005] Fibre cement material is a composite material typically comprising cement, cellulose fibers, and at least one of silica sand, synthetic fibers and fillers. It is widely used in building construction and can take the form of a plurality of products, such as for example corrugated sheets for roofs, small sheets for tiles (slates), sheets for sidings, cladding, boards, etc.

[0006] Cured fibre cement waste material has a chemical composition similar to the corresponding fibre cement products from which the waste is derived, and therefore could, and ideally should, be recycled and reused. However, the mineralogical composition will be changing over time due to carbonation and hydration of the cement phases during curing, i.e. air-curing or autoclave-curing, which makes a direct reuse of the cured fibre cement waste undesirable.

[0007] Prior art patent documents WO2018065518, US20080168927, US20080072796, JP200306385, JP2004217482 and WO20120844677 relate to methods for reuse of fibre cement material waste.

[0008] WO2018065518 discloses methods for the production of cured fibre cement products comprising cured fibre cement waste material, comprising the steps of providing a cured fibre cement waste powder, providing an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and a cured fibre cement waste powder, forming a green fibre cement sheet from said aqueous fibre cement slurry and pressing the green fibre cement sheet at 100 to 300 kg / cm2 before autoclave-curing said pressured green fibre cement sheet. The amount of reused fibre cement waste powder is stated to be between 10 and 30 mass% in the product.

[0009] Recycling waste material from cured fibre cement products, e.g. finished products being out of spec, demolition and / or construction waste and alike, remains a major challenge.

[0010] It is for instance very difficult to produce a good, comminuted fibre cement material from air-cured fibre cement waste and especially from autoclave-cured fibre cement waste (because of the higher degree of conversion of the starting material during autoclave heating and optional pressure), which is suitable to be used as an ingredient to produce new fibre cement products.

[0011] Also, the process features that are necessary for producing qualitatively good fibre cement products from comminuted fibre cement waste material are presently not known. Any attempts in the past to use ground waste material to produce new fibre cement products resulted in products not fulfilling the quality requirements as prescribed by national regulations, i.e. strength requirements, density and waterresistance. In fact, the use of a certain amount of fibre cement waste in new products resulted in an increased water absorption and / or a low mechanical strength of the end product, which made these products unusable for different reasons (increased risk for molds, extensive hygric movement (including e.g. swelling) and cracking).

[0012] Reusing fibre cement waste for various new purposes remains, however, the main, if not the only, approach to avoid disposal of large fibre cement waste streams.

[0013] Summary of the invention

[0014] It is an object of the present invention to provide a novel and improved method to produce new fibre cement products by using comminuted and powdered cured fibre cement waste material as one of the raw materials.

[0015] The inventors have found that heat treatment of fibre cement waste material before use solves the longstanding problem with recycling old fibre cement, which typically cannot be reused in new fibre cement in larger quantities without further processing.

[0016] Fibre cement typically consists of a mixture of natural (cellulose) and artificial (PVA plastic) fibers in a blend of cement and other additives, typically limestone. Cellulose helps hold the sheet together during production, while PVA contributes strength to the finished sheet. When cement reacts with water during the curing of fibre cement, the so-called clinker minerals, primarily alite and belite, transform into C-S-H (Calcium-Silicate-Hydrate), which is the phase that imparts strength to cured cement products.

[0017] For fibre cement products, this reaction is typically accelerated in a curing tunnel with elevated temperature and possibly humidity (autoclave-curing). However, not all clinker minerals react fully, and a limited amount of reactivity can theoretically be achieved by recycling crushed fibre cement. Nevertheless, this process is not without problems, and previous studies in other contexts have shown that typically not more than about 4% old fibre cement can be reused in new products before quality and durability issues arise. In WO2018 / 065518, the problem was tried solved by applying a pressure of between 100 and 300 kg / cm2to the green sheet comprising cured fiber cement powder before auto- clave-curing.

[0018] The present inventors are attacking the problem from another angle. They have found that parts of the reactive minerals present in ordinary hydraulic cement (e.g. Portland cement) can be substituted by cured fiber cement waste when the waste has been exposed to a heat-treatment, which restores parts of the reactive minerals (calcium carbonates, calcium silicates, etc.). By heating old cured fibre cement (e.g. air-cured or autoclaved-cured fiber cement waste or a mixture thereof) to over 500°C and preferably higher, for example from 750°C up to about 950°C, a variety of reactive minerals are formed from the waste fibre cement, including, but not limited to, typical clinker minerals found in ordinary cement, especially belite, as well as ferrite and minerals known from intermediate stages in cement production, such as ternesite, free lime and spurrite, depending on the temperature of treatment (see Figure 1). Additionally, both plastic and cellulose fibers are burned off in the process as a bonus.

[0019] The minerals obtained at temperatures from about 500°C, such as from about 600°C to 750°C, are typically less reactive than fresh cement but is well-suited for the specific curing conditions of fibre cement production in a curing tunnel. At higher temperatures, i.e. from above about 750°C to about 950°C, it is possible to create a mineral composition more similar to what is seen in ordinary Portland cement, but this requires, however, significantly more energy to achieve and thus result in a significantly larger environmental impact.

[0020] It is usually estimated that ordinary Portland cement has a CO2 footprint of 750-800 kg CO2 per ton of cement. Heat-treated fibre cement can be produced almost carbon- neutral, depending on the method, with an estimated footprint of 100-150 kg CO2 per ton. Heat-treatment of fibre cement below 750°C will result in estimated CO2 footprint of 100-150 kg CO2 / ton, were as heat-treatment above 800°C will result in estimated CO2 footprint of 300-400 kg CO2 / ton, given the assumption that end-of-life fibre cement products show carbonation contents of app. 50 %wt. However, this figure is associated with some uncertainty. It is certain, though, that it never comes close to the use of ordinary cement due to much more favorable chemistry.

[0021] From Figure 1, it can be seen that the proportion of reactive phases is optimal at temperatures of 850°C and above. Below this temperature, there is a large proportion of calcite, which is non-reactive and irrelevant. However, a temperature below 850°C, such as about 800°C, about 750°C or as low as 650°C may be sufficient and useful in certain situations where the higher temperatures are not practically achievable, necessary or feasible.

[0022] An uncertainty related to the data in Figure 1 is that the exact chemistry is poorly known, making it difficult to determine the accurate proportion of reactive phases in the sample. Thus, the mineralogy as determined by X-ray Powder Diffraction (XRD) shows that there is still around 40% amorphous / unidentified material in the sample after 15 minutes of heat treatment at 950°C. C-S-H is typically amorphous on XRD, but at 950°C, it is far outside the stability range of C-S-H, and everything should have recrystallized into other phases. At lower temperatures, the amorphous content is higher (55-56% at both 650°C and 800°C for 15 minutes).

[0023] When waste fibre cement is heat treated for use as a substitute for ordinary cement, a relatively large amount of free lime is formed. When in contact with water, it reacts to form Portlandite (CafOH ), which is an essential ingredient in almost all cases where cement is otherwise replaced with, for example, calcined clay or other pozzolanic materials. This enhances the option for such substitution as well.

[0024] For example, a typically low clinker cement like a CEM ll / C-M could consist of 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum. With the present invention, a large portion of the 50% clinker can be replaced with heat-treated cured fibre cement waste, achieving further CO2 savings without affecting the chemical reactions with calcined clay, as that reaction can be maintained thanks to Portlandite from the heat- treated fibre cement waste. This will enable cement blends with as low as around 20% high-quality clinker. Products with an even lower content of high-quality clinker may be achieved with further optimization.

[0025] In this respect, the present inventors have developed a method to produce qualitatively excellent new fibre cement products making use of cured fibre cement waste material which reduces the CO2 emission considerable compared to the prior art.

[0026] These methods at least comprise the steps of providing heat-treated comminuted cured fibre cement waste powder and forming a fresh green fibre cement product / sheet containing the heat-treated cured fibre cement waste powder as a raw material and subsequently curing the green fibre cement product / sheet by autoclave curing or air curing. The cured fibre cement waste materials are air-cured or autoclave-cured waste products or a mixture of cured waste material, which are comminuted to produce a powder, ready for heat-treatment or pre-treated to produce pieces of waste in sizes that can be heat-treated in an oven and subsequent comminuted into a powder.

[0027] It was observed that the new fibre cement products produced by the method of the present invention show a lower density when compared to fibre cement products not containing any waste material, while the density corrected bending strength is similar compared to fibre cement products not containing any waste material. The bending strength is well above the classification limit according to the standard for fibre cement, thus showing that the present invention results in a beneficial new product resulting a profitable use of fibre cement waste for the benefit of the environment.

[0028] Thus, with the methods of the present invention, the inventors have found a new way to process fibre cement waste for reuse in fresh fibre cement products resulting in a huge saving of CO2 emission without affecting the mechanical strength significantly to a level below the limit for fibre cement and at the same time decreasing the density of fibre cement products to an acceptable degree for beneficial use.

[0029] The property of decreased density remains an important aspect regarding enhanced workability of the fibre cement products in general.

[0030] In a first aspect, the present invention provides a method to produce new fibre cement products comprising cured fibre cement waste material, comprising the steps of:

[0031] (a) Providing cured fibre cement waste material and optionally comminuting said cured fibre cement waste material into a cured fibre cement waste powder;

[0032] (b) 1) Heating the cured fibre cement waste material to a temperature of at least 500°C for at least 10 minutes and comminuting said heat-treated cured fibre cement waste material to prepare a heat-treated cured fibre cement waste powder, or

[0033] 2) Heating the cured fibre cement waste powder to prepare a heat-treated cured fibre cement waste powder;

[0034] (c) Providing an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and said heat-treated cured fibre cement waste powder;

[0035] (d) Forming a green fibre cement sheet from said aqueous fibre cement slurry; and

[0036] (e) Curing said green fibre cement sheet thereby providing a fibre cement product comprising cured fibre cement waste material.

[0037] Curing may be by way of air-curing or autoclave-curing, which are commonly known in the art. The cured fibre cement waste material to be used in the method includes discarded or demolished used cured fibre cement products ("end of life fibre cement"), cured fibre cement waste from production and other cementitious waste, e.g. concrete, etc. If necessary, larger chunks of the cured fibre cement waste material are pre-treated to provide pieces small enough to enter a suitable oven for the heat-treatment or small enough to be comminuted into a powder before the heat-treatment. The pre-treatment should provide pieces of the cured fibre cement waste material not larger than 10 x 10 cm, preferably not larger than 5 x 5 cm or even as small as about 2 x 2 cm or smaller.

[0038] In one embodiment of the present invention, the cured fibre cement waste material (in the right sizes) is heated for a suitable time before it is comminuted to prepare a heat- treated powder for further use in the method. Cured fibre cement waste material, such as discarded and / or "end of life fibre cement" products may be heat treated prior to being comminuted to a powder.

[0039] In another embodiment of the present invention, the cured fibre cement waste material is comminuted to prepare a powder which is heated for a suitable time to prepare a heat-treated powder for further use in the method.

[0040] In a particular embodiment of the method according to the invention, the cured fibre cement waste material or cured fibre cement waste powder is heated to a temperature above 500°C, preferably above 600°C or 650°C, more preferably above 700°C, above 750°C, and most preferably above 800°C, above 850°C or above 900°C, such as about 950°C. A temperature in the range 500°C to 750°C, such as 600°C to 750°C is suitable in some instances where only a partial conversion of the chemical components in the fiber cement waste is sufficient for the further use. If more conversion is desirable, the heating temperature should be between about 750°C and about 850°C. A temperature between about 850°C and about 950°C is most suited for the heat treatment of the cured fibre cement waste powder if conversion of the chemical components in the fiber cement waste should be taken as far as possible. A temperature around 950°C may be desirable. The heat-treatment should last from 1 minute to 60 minutes, preferably from 5 to 40 minutes, preferably between 10 and 30 minutes, such as around 10 to 20 minutes. The heating time can be selected independently of the selected heating temperature.

[0041] In a further embodiment of the method according to the invention, the step of providing an aqueous fibre cement slurry comprises mixing at least water, cementitious binder, natural and / or synthetic fibers and the heat treated cured fibre cement powder, such that the heat treated cured fibre cement waste powder is present in the aqueous fibre cement slurry in an amount of at least 5 mass% of the slurry. "mass%" refers to the weight% percentage of the mass of the component over the total dry mass of the composition, i.e. all components except water. Preferably, the heat treated cured fibre cement waste powder is present in the fibre cement slurry in an amount between about 5 mass% and about 90 mass%, more preferably between about 10 mass% and about 90 mass%, between about 20 mass% and about 90 mass%, between about 30 mass% and about 90 mass%, between about 40 mass% and about 90 mass%, even more preferably between about 50 mass% and about 90 mass% (such as 50 mass% or 55 mass%), between about 60 mass% and about 90 mass% (such as 60 mass%, 65 mass%, 70 mass%, 75 mass%, 80 mass% or 90 mass%).

[0042] In an embodiment, the ratio of heat-treated cured fibre cement waste powder to fresh cement in the fibre cement slurry is between 10% and 100%, such as 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90% or 95%. Preferably the ratio is between 20% and 80%, and more preferably between 30% and 60%, such as between 30% and 50%.

[0043] In a further embodiment, the method according to the invention may additionally comprise a step of pressing the green fibre cement sheet prior to the curing step. The step of pressing the green fibre cement sheet may comprise compressing the green fibre cement sheet with a pressure of at between about 10 kg / cm2 and 300 kg / cm2 such as between about 20 kg / cm2 and 250 kg / cm2 such as at about 100 kg / cm2

[0044] The step of pressing the green fibre cement sheet comprises compressing the green fibre cement sheet either in a stack press or in a single sheet press. Methods for pressing green fibre cement sheets are commonly known in the art. In these embodiments, the methods of the present invention have the additional technical effect that the specific strength of the produced fresh air or autoclave-cured fibre cement products increase in comparison with non-pressed products also produced with the methods of the present invention.

[0045] In an embodiment of the methods according to the present invention, the steps of providing cured fibre cement waste powder comprises pre-treating and / or comminuting air-cured fibre cement materials or an autoclave-cured fibre cement materials or pretreating and / or comminuting a mixture of air-cured and autoclave-cured fibre cement materials. The pre-treatment and comminuting may take place by grinding, crushing, or milling, for example by use of a Herzog mill, a pendulum mill or a similar mill suitable for the present purpose.

[0046] In a second aspect, the present invention provides an air or autoclave-cured fibre cement product obtained using the method according to the present invention.

[0047] In a third aspect, the present invention provides the use of the air or autoclave-cured fibre cement products as obtained by the methods of the invention, as a building material. Brief description of the drawings

[0048] Figure 1 shows a mineral composition normalized to 100% at different Temperatures (bottom row) and Holding Times (top row, in minutes).

[0049] Figure 2 shows the transition temperatures for a corrugated fibre cement sheet measured by thermogravimetry evolved gas analysis (TG / EGA) and differential scanning calorimetry (DSC).

[0050] Figure 3 shows an example of a reference recipe without addition of heat-treated material. The outer ring illustrates the amounts of the individual components, whereas the inner ring illustrates the contribution to Scope 3 emissions of the product from the individual raw materials. For example, 80 % cement in the recipe contributes with 75% of the total scope 3 emission. Scope 3 encompasses emissions that are not produced by the company itself and are not the result of activities from assets owned or controlled by them, but by those that are indirectly responsible for up and down its value chain. An example of this is when buying, use and dispose of products from suppliers.

[0051] (https: / / www.nationalgrid.com / stories / energy-explained / what-are-scope-l-2-3-carbon- emissions).

[0052] Figure 4 shows an example of a recipe with the addition of heat-treated material. The outer ring illustrates the amounts of the individual components, whereas the inner ring illustrates the contribution to Scope 3 emissions of the product from the individual raw materials. Note that the emissions have been normalized to be comparable with the reference on figure 2. For example, the reduction in Scope 3 emission is 25%.

[0053] Figure 5 shows the bending strength of the produced products after 21 days of aircuring.

[0054] Figure 6 shows the density corrected and normalized bending strengths. For comparison, the normalized cement content has also been shown. It can be seen that in example 3 the bending strength is more than 90 % compared to the reference even with a 40% reduction of the cement content.

[0055] Detailed description of the invention

[0056] The present invention will be described with respect to particular embodiments.

[0057] The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

[0058] The terms "(fibre) cementitious slurry" and "(fibre) cement slurry" as referred to herein generally refer to slurries at least comprising water, fibers and cement and / or heat- treated fibre cement waste. The fibre cement slurry as used in the context of the present invention may also further comprise other components, such as but not limited to, limestone, chalk, quick lime, slaked or hydrated lime, ground sand, silica sand flour, quartz flour, amorphous silica, condensed silica fume, micro silica, metakaolin, wollastonite, mica, perlite, vermiculite, aluminum hydroxide, pigments, anti-foaming agents, flocculants, and other additives.

[0059] "Fibre(s)" present in the fibre cement slurry as described herein may be for example process fibers and / or reinforcing fibers which both may be organic fibers (typically cellulose fibers) or synthetic fibers (polyvinylalcohol, polyacrilonitrile, polypropylene, polyamide, polyester, polycarbonate, etc.).

[0060] "Cement" present in the fibre cement slurry as described herein may be for example but is not limited to ordinary Portland cement, cement with high alumina content, ordinary Portland cement of iron, trass-cement, slag cement, plaster, calcium silicates formed by autoclave treatment and combinations of particular binders. In more particular embodiments, cement in the products of the invention is ordinary Portland cement.

[0061] "Cement clinker" is a solid material produced in the manufacture of ordinary Portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually between 3 and 50 millimetres in diameter. It is produced by sintering (fusing together without melting to the point of liquefaction) limestone and aluminosilicate materials such as clay during the cement kiln stage.

[0062] "Ordinary Portland (clinker) cement" essentially consists of four minerals: two calcium silicates, alite (Ca3SiO5) and belite (Ca2SiO4), along with tricalcium aluminate (Ca3AI2O6) and calcium aluminoferrite (Ca2(AI,Fe)2O5). These main mineral phases are produced by heating at high temperatures. Ordinary Portland cement clinker is made by heating a homogeneous mixture of clays and limestone in a rotary kiln at high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about 1450°C.

[0063] Upon addition of water, clinker minerals react to form different types of hydrates and "set" (harden) as the hydrated cement paste becomes concrete. The calcium silicate hydrates (C-S-H) (hydrates of alite and belite minerals) represent the main "glue" components of concrete. Cement is a hydraulic binder whose hydration requires water. Water is essential to its hardening and water losses must be avoided at a young age to avoid the development of cracks. The concrete does not dry, but it sets and hardens. After initial setting the concrete continues to harden and to develop its mechanical strength. The first four weeks are the most critical for the hardening.

[0064] "Heat-treatment" means that fibre cement waste in suitable sizes is heated to a temperature between 500°C and 950°C for a suitable time in a suitable oven to decompose limestones and carbonates to form new phases of minerals present in cement clinker (see Figure 1). During the heat-treatment, any water in the fibre cement waste is evaporated and natural and / or synthetic fibers are fully decomposed (see Figure 2). Natural and / or synthetic fibers are normally fully decomposed at 300- 500°C.The heat-treated fibre cement waste is cooled and subsequently milled and optionally sieved to produce a comminuted heat-treated fibre cement waste powder, which powder is ready for use in preparing new fresh fibre cement products.

[0065] In an alternative heat-treatment procedure, the fibre cement waste is milled and optionally sieved to produce a fibre cement waste powder, which powder is subsequently heated to a temperature between 500°C and 950°C for a suitable time in an oven suitable to remove water, decompose fibers, and to decompose limestones and carbonates to form new phases of minerals present in cement clinker. The heat-treated powder is cooled and ready for use in preparing new fresh fibre cement products.

[0066] The fibre cement waste product or material is preferably pre-treated (pre-crushed) to prepare suitable small pieces for the milling / comminuting process. The pre-treatment may take place in a crushing step and / or in a cutting step. Pieces in the size of less than 10x10 cm, such as less that 5x5 cm, preferably less than about 3x3 cm or 2x2 cm are particularly suited, but both larger and smaller pieces may also be used depending on the capacity of mill and / or the oven.

[0067] To investigate the effect of thermal treatment of cured fibre cement waste, cured fibre cement samples were heated to different temperatures to follow the decomposition of the fibre cement matrix as explained in detail in Example 2. The initial weight loss is primarily attributed to water release. At lower temperatures, free and physically bound water is released from both the cement matrix and the cellulose fibers. As the temperature increases to 240°C, major dehydration of gypsum occurs, along with the dehydration of ettringite-like phases and the release of some chemically bonded water in the C-S-H phases. In the temperature interval from 240°C to 485°C the breakdown of cellulose into smaller molecules occurs. Cellulose pyrolysis is the first exothermic process in this region, followed by the dehydroxylation of Ca(OH)zinto calcium oxide (CaO) and water vapor (HZO) around 470°C. In the temperature range of 500-700°C, significant structural changes occur in the C-S-H gel, leading to the formation of calcium silicates with varying C / S ratios. This stage is marked by substantial weight loss and structural transformations. Temperatures above 700°C result in the further formation of calcium silicates, such as dicalcium silicate (CZS) and tricalcium silicate (C3S). At this temperature and higher, a significant endothermic effect is observed due to the thermal decomposition of CaCO3, which releases a large amount of COZand forms CaO (Figure 2).

[0068] At the beginning of the heating step, at temperatures above 500°C, fibers and water disappear quickly. The limestone starts to decompose at 750-800°C or higher depending on crystal modification and crystal size, and at higher temperatures, such as 800-900°C the carbonates will decompose to form new phases as shown in Figure 1. The new phases are similar to the phases seen in Portland cement clinker produced by the conventional process of heating a mixture of limestone and clay at about 1450°C. Because the mixture of minerals present in cement clinker formed by the heating process can replace all or a part of the conventionally produced Portland cement clinker large savings in heating energy and a huge reduction in CO2 emission are obtained by applying the present invention.

[0069] The term "bulk density" as referred to herein, is to be understood as the property of a powder or granules, or another particulate solid, especially in reference to mineral components (cement particles, filler particles, or silica particles). The bulk density is defined as the weight of a certain number of particles of a specific material divided by the total volume this number of particles occupies. The total volume includes particle volume, inter-particle void volume, and internal pore volume. The bulk density of powders as referred to herein is also called the "freely settled" or "poured" density, i.e. the bulk density measured after pouring the powder, without applying any further compaction process. The bulk density is expressed in kilogram per cubic meter (1 g / ml = 1000 kg / m3) or in grams per milliliter (g / ml) because the measurements are made using cylinders. The bulk density of a powder can be determined by any standard method for measuring bulk density as known to the skilled person.

[0070] For example, the bulk density of a powder can be determined by measuring the volume of a known mass of powder sample that may have been passed through a sieve, into a graduated cylinder, or by measuring the mass of a known volume of powder that has been passed into a measuring vessel. A person skilled in the art would readily know how to perform the measurements as described in the prior art.

[0071] A "(fibre cement) sheet" or "fibre cement sheet" or "sheet" as interchangeably used herein, and referred to as a panel or a plate, is to be understood as a flat but might also be corrugated. It is usually a rectangular element; a fibre cement panel or fibre cement sheet being provided out of fibre cement material. The panel or sheet has two main faces or surfaces, being the surfaces with the largest surface area. The sheet can be used to provide an outer surface to walls, both internal as well as external a building or construction, e.g. as facade plate, siding, roofing etc.

[0072] The present invention concerns a novel and improved method to produce cured fibre cement products by using heat treated cured fibre cement waste material as one of the raw materials.

[0073] In particular, it was found by the inventors that by combining at least the step of heating cured fibre cement waste material followed by a milling to prepare a heat-treated powder or comminuting the cured fibre cement waste material to prepare a powder before heat-treatment, followed by forming a fresh green fibre cement product containing the heat-treated cured fibre cement waste powder as a "raw material" for substituting parts of ordinary cement and subsequently curing the green fibre cement product by either air-curing or autoclave-curing, a fibre cement product with excellent quality can be produced, starting from cured fibre cement waste material. More specifically, it was observed that the mechanical strength and the density was minimally reduced, when compared to fibre cement products not containing any waste material. The strength was well above the standard for fibre cement and the density corrected bending strength was similar to fibre cement products not containing any waste material.

[0074] An advantage of producing fibre cement sheets or boards with lower densities compared to conventional (i.e. non-waste containing or non-recycled) fibre cement products is that the products obtained by the methods according to the present invention are lighter than non-waste-based products of equal dimension and as a consequence have an improved workability. Workability encompasses the ease with which the board is handled and installed.

[0075] The main advantage of producing fibre cement sheets or boards with heat treated fibre cement waste compared to conventional (i.e. non-waste containing or non-recycled) fibre cement products and fibre cement waste-containing products of the prior art is that the production of the products according to the present invention facilitates the reuse and incorporation of fare more fibre cement waste as a substitute for cement for the benefit of the environment in form of a reduced amount of fibre cement waste to be disposed and a considerable reduced emission of CO2 from production of raw materials, due to the substantial lower temperature, giving a lower energy requirement, in the heat-treating step of cured fibre cement material compared production of high quality clinker in new cement, e.g. ordinary Portland cement. Furthermore, the production of clinkers also gives rise to considerable CO2 emission from calcining the raw materials used. Therefore, the present invention embodies a reduced environmental impact, as less of the energy intensive high-quality clinker needs to be produced for use in fresh fibre cement products. Heat-treatment of fibre cement waste offers a way to re-gain from the cured waste some of the active material in ordinary cement, including calcium carbonate and calcium silicates, that can replace some of the ordinary cement in new fiber cement products and still being capable of forming calcium oxide (CaO) in the presence of water during the subsequent curing step in formation of solid fibre cement end products.

[0076] In a first aspect, the present invention provides methods to produce a fibre cement product comprising cured fibre cement waste material, comprising the steps of:

[0077] (a) Providing cured fibre cement waste material and optionally comminuting said cured fibre cement waste material into a cured fibre cement waste powder; (b) 1) Heating the cured fibre cement waste material to a temperature of at least 500°C for at least 10 minutes and comminuting said heat-treated cured fibre cement waste material to prepare a heat-treated cured fibre cement waste powder, or

[0078] 2) Heating said cured fibre cement waste powder to prepare a heat-treated cured fibre cement waste powder;

[0079] (c) Providing an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and said heat-treated cured fibre cement waste powder;

[0080] (d) Forming a green fibre cement sheet from said aqueous fibre cement slurry; and

[0081] (e) Curing said green fibre cement sheet thereby providing a fibre cement product comprising cured fibre cement waste material.

[0082] The first step in the methods according to the present invention comprises providing cured fibre cement waste material for a pre-treating (pre-crushing) process and optionally comminuting said pre-treated waste material into a powder to obtain a cured fibre cement powder.

[0083] The cured fibre cement material to be pre-treated / crushed and optionally comminuted to a powder before heat-treatment, is typically waste material, e.g. demolition waste, production waste from the fibre cement production plant, waste from the construction sites or rejected fibre cement products. The pre-treated and comminuted cured fibre cement material in form of heat-treated cured fibre cement powder, can be used as "raw" material for new cured fibre cement products, in which the cured fibre cement waste is recycled. Cured fibre cement powder for use in the new cured fibre cement products according to the invention can be pre-treated and comminuted air-cured fibre cement waste powder or pre-treated and / or comminuted autoclave-cured fibre cement waste powder or a mixture of both air-cured and autoclave-cured fibre cement waste powder, which powder is heat-treated.

[0084] In an embodiment, pre-treating / crushing and comminuting is effectuated by grinding, crushing or milling, for example by making using a Herzog mill, pendula mill or similar crushing and comminuting means known to a person skilled in the art.

[0085] Alternative to comminuting the waste material to a powder before heat-treatment, the fibre cement material may be heat-treated before comminution to a powder. However, in order for the heat-treatment to take place with the waste material, the size of the material must be suitable small for being introduced into the heating equipment, e.g. an oven, such as a rotary kiln or other heating processes known in the art. Mechanical pretreatment of large-sized waste material, such as whole size "end of life" cured fibre cement material products or debris therefrom, may therefore be used to down-size the waste material to a suitable size before the heat-treatment. Subsequently the heat- treated cured fibre cement waste material is comminuted to a powder for further use in the present method.

[0086] In one embodiment, the cured fibre cement material for use as a starting material to produce pre-treated and / or comminuted cured fibre cement material has a water content of less than or equal to about 10%w. In a further embodiment, the cured fibre cement material for use as a starting material for the production of pre-treated and / or comminuted cured fibre cement material has a water content of less than or equal to about 10 %w, such as less than or equal to about 8%w, for example less than or equal to about 6%w, such as less than or equal to about 5%w.

[0087] In certain particular embodiments, the cured fibre cement waste material for use as a starting material for the production of heat-treated cured fibre cement waste powder is air-cured fibre cement material.

[0088] Alternatively, autoclave cured fibre cement product may be used to provide the heat- treated cured fibre cement waste powder. A combination of air-cured and autoclave- cured fibre cement waste materials may be pre-treated and / or comminuted, or aircured fibre cement waste powder and autoclave-cured fibre cement waste powder maybe combined before heat-treatment to provide the heat-treated cured fibre cement waste powder.

[0089] One of the most important advantages of the comminuted fibre cement powder as produced according to the methods of the invention is that the particles have a granulated, sand-like texture and have a flowing behavior that is similar to the flowing behavior and the bulk density (as defined herein) of cementitious powder, silica flour or limestone flour. In one embodiment, the comminuted powder as produced by the methods according to the present invention is specifically characterized by a bulk density between 900 kg / m3 and 1900 kg / m3, and preferably between 1000 kg / m3 and 1600 kg / m3, such as between 1000 and 1300 kg / m3.

[0090] In this way, the powder is suitable to be recycled into the fresh autoclave-cured fibre cement products, without the necessity to make major changes in the production process (e.g. the Hatschek process) for producing fresh cured fibre cement products. Moreover, the produced cured fibre cement powder particles of the invention have a particle size distribution, which is similar to the particle size distribution of a cementitious binder material (e.g. cement) or a siliceous source (e.g. sand or quartz) or a filler material (e.g. CaCO3). In particular embodiments, the produced cured fibre cement powder particles are characterized by a particle size distribution, which is similar to the particle size distribution of cement. In particular embodiments, the produced heat- treated cured fibre cement powder particles are characterized by a particle size distribution, which is similar to the particle size distribution of a siliceous source. With "the matching of particle size distribution" of different materials, such as cement, fillers, siliceous material (e.g. sand) and the comminuted fibre cement product, as used herein is meant that these materials can be used together in a process to provide a fibre cement slurry for making fresh autoclave-cured fibre cement products, in particular using a Hatschek process, without the necessity to fundamentally change the process settings. Thus, the cured fibre cement powder may replace part of the filler, such as limestone, and / or cement used to provide the fresh autoclave-cured fibre cement sheets. The heat-treated cured fibre cement powder replace part of or all the cement and may replace part of the siliceous source (e.g. part of the ground sand) and / or may replace part of the fillers (e.g. ground limestone) typically used in autoclave-cured fibre cement products.

[0091] Preferably the cured fibre cement waste material, which is provided as the starting material for the production of comminuted cured fibre cement waste material, is provided in parts having a maximum size of not more than 10x10 cm, for example not more than 5x5 cm, typically as rectangular-like pieces with sides of not more than 3 cm or even not more than 2cm, before it is comminuted, for example by using a Herzog mill or a pendulum mill.

[0092] The pre-crushed material can be fed at a flow rate of about 350 kg / hr to about 800 kg / hr to a pendular mill type roller mill of the Poittemill Group (FR), in which the material may be comminuted at a rotation speed of between about 100 and about 400 tr / min. Alternatively, a Herzog mill, a roller mill or a hammer mill may be used. A combination of any two of these mills may also be used.

[0093] In one embodiment humidity in the materials should be avoided. Energy from the crushing may be enough to dry the crushed product. Alternatively, to compensate the humidity of the pre-crushed material, hot air (at a temperature between about 20°C and about 100° C was fed together with the pre-crushed material, to instantaneously suppress the humidity of the pre-crushed material and the ground fibre cement powder obtained. This may be important if the powder is stored before the subsequent heattreatment

[0094] Comminuted cured fibre cement waste material powder can be obtained as for example disclosed in WO2018065518.

[0095] Fibre cement powder as obtained by pendular milling has a good consistency (not woolly or fluffy), a suitable bulk density (between about 1000 kg / m3 and about 1300kg / m3) and a good particle distribution, to be used to produce new fibre cement products. The pendular milling may provide a particular suitable comminuted fibre cement waste powder because with this technique the fibre cement waste is squashed or flattened as opposed to other milling techniques, which typically mill by crushing or grinding. However, other milling techniques may be customized to produces a suitable powder for the heat-treating or after heat-treating to be efficient in forming the reactive minerals, including, but not limited to, typical clinker minerals found in ordinary cement.

[0096] In an embodiment, the desired particle distribution of the comminuted cured fibre cement material can be obtained by milling the cured fibre cement material in the absence of sand or another silica source. This further facilitates the heat-treated comminuted cured fibre cement material to be used as a good raw material for fresh aircured or autoclave cured fibre cement products.

[0097] In further particular embodiments, comminuting of the cured fibre cement material is done in so-called dry state, i.e. the cured fibre cement material is not to be brought in suspension of a liquid (typically water) to enable the milling, as is the case for some other milling techniques. As a result, a relatively dry comminuted cured fibre cement material in the form of powder is provided. This facilitates storage of an intermediate product before using it for e.g. making fresh cured fibre cement material. The heattreatment may be done before storage or after storage just before use in the production of new fibre cement products.

[0098] The second step of the methods according to the present invention comprises heating the comminuted cured fibre cement waste powder to a temperature of above 500°C for at least 10 minutes. Preferably the temperature is above 700°C, more preferably above 800°C, such as above 850°C. Preferably the heating time is between 10 and 60 minutes.

[0099] Heating of the cured fibre cement waste powder or pre-treated / crushed cured fibre cement waste material takes place in a proper heating process such as in an oven, rotary kiln, flash-calciner or similar heating equipment known in the art.

[0100] The heat-treatment of cured fibre cement waste powder or pre-treated / crushed cured fibre cement waste material according to the present invention results in formation of a variety of reactive minerals in the product, including, but not limited to, typical clinker minerals found in ordinary cement. Thanks to the formation of the new lime phase and the increased amount of belite, the thermally treated waste serves as a cementitious binder and may replace the original components in higher amounts.

[0101] The third step of the methods according to the present invention comprises providing an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and the heat-treated cured fibre cement waste powder.

[0102] In particular embodiments, the fibre cement slurry comprises at least 10 mass% of heat- treated cured fibre cement waste powder, advantageously at least 20 mass% of heat- treated cured fibre cement powder. In further particular embodiments, the fibre cement slurry comprises preferably more than about 40 mass% of heat-treated cured fibre cement powder, advantageously between about 20 mass% and 80 mass% of heat- treated cured fibre cement powder, more advantageously between about 40 mass% and 80 mass% of heat-treated cured fibre cement powder, such as about 40 mass%, about 50 mass%, about 60 mass%, about 70 mass% or about 80 mass% of heat-treated cured fibre cement waste powder. The slurry may in certain embodiments comprise 90 mass% or 95 mass% heat-treated cured fibre cement waste powder.

[0103] In respect of the above, the unit "mass%" refers to the mass percentage of the component over the total dry mass of the composition for preparing a fresh fibre cement material, i.e. all components except water.

[0104] In the next step of the methods of the present invention, new fibre cement materials or products are made from the fibre cement slurry, which is formed in a so-called green fibre cement product.

[0105] The fibre cement slurry typically comprises, in addition to the heat-treated cured fiber cement waste powder, water, process and reinforcing fibers which both may be natural organic fibers (typically cellulose fibers) and synthetic organic fibers (polyvinylalcohol, polyacrilonitrile, polypropylene, polyamide, polyester, polycarbonate, polyethylene, etc.), which fibers may be surface treated (chemically or mechanically) or not, synthetic inorganic fibers, such as glass fibers, cement e.g. ordinary Portland cement, limestone, chalk, quick lime, slaked or hydrated lime, ground sand, silica sand flour, quartz flour, amorphous silica, condensed silica fume, microsilica, metakaolin, wollastonite, mica, perlite, vermiculite, aluminum hydroxide, anti-foaming agents, flocculants, and other additives such as hydrophobation agents or water repellants. Optionally, a color additive (e.g. pigments) can be added to obtain a fibre cement product which is so-called colored in the mass.

[0106] Fibre cement products, also referred to as fibre cement sheets or fibre cement panels, are usually made by using the well-known Hatschek-process, flow-on process or Magnani- process, or suitable combinations thereof.

[0107] In particular embodiments, the green fibre cement products are optionally pressed before curing.

[0108] In particular embodiments, the optional step of pressing the green fibre cement product is performed by making use of one or more mechanical presses, including but not limited to single sheet press and one or more stack presses.

[0109] In an embodiment, the optional step of pressing the green fibre cement product is performed at a pressure of between 10 kg / cm2 and 300 kg / cm2 such as between about 20 kg / cm2 and 250 kg / cm2 such as at about 200 kg / cm2.

[0110] In particular embodiments of the methods according to the invention, the optional step of pressing the green fibre cement product comprises compressing the green fibre cement sheet during a time period of between about 5 minutes and about 15 minutes. The pressure applied to the green, i.e. uncured, fibre cement sheet causes the density of the green fibre cement sheet to increase.

[0111] The density of the fibre cement end products as obtained using the methods according to the present invention, may vary from about 1.0 kg / dm3 to about 2.5 kg / dm3, such as from about 1.3 kg / dm3 to about 2.0 kg / dm3, preferably about 1.5 kg / dm3.

[0112] In particular embodiments, the unpressed green fibre cement sheet may have a thickness in the range of about 3mm and about 25mm, such as between about 4mm and about 20mm, such as between about 4mm and about 12mm, preferably about 5mm.

[0113] In particular embodiments, the pressed green fibre cement sheet has a thickness in the range of between about 2mm and about 20mm, such as between about 3mm and about 15mm, such as between about 3mm and about 10mm, preferably about 4mm.

[0114] Finally, the methods of the present invention comprise the step of curing the optionally compressed green fibre cement sheet thereby providing a new cured fibre cement product. The curing may be air-curing or autoclave-curing.

[0115] The "green" fibre cement product, after being made by a sheet providing process, such as the Hatschek-process and optionally being pressed, may be pre-cured in air at ambit temperature, after which the pre-cured product may be further autoclave-cured if desired until it has its final strength.

[0116] The fresh green sheets, after being provided, may be stacked with metal sheets placed between the stacked green fibre cement sheets, and pressed in stacked form. Alternatively, the fresh green sheets may be pressed individually and thereafter stacked with metal sheets placed between the stacked and pressed green fibre cement sheets. The fresh green sheets may be formed, like corrugated, before being stacked with intermediate, formed metal sheets placed between the fibre cement sheets. For formed, e.g. corrugated sheets, the sheets are typically pressed individually.

[0117] The pre-curing step may take several hours, e.g. between about 1 hour and 10 hours, such as between 2 hours and 8 hours, such as between 3 hours and 5 hours, preferably about 4 hours, during which the temperature of the sheets rise due to the exothermic curing reaction of the cement. The pre-curing may take place in controlled conditions controlling the humidity, temperature or both.

[0118] After a first "pre-curing" step, curing the green sheets in air in stacked form with intermediate metal sheets, the sheets may be restacked while removing the metal sheets from between the green fibre cement pre-cured sheets. After removal of the metal plates, the pre-cured green fibre cement sheets are further cured in air during an air- curing step, which may take several days, typically 2 to 4 weeks. Alternatively, after the first "pre-curing" step, autoclave-curing in a high-pressure steam curing process can be used to accelerate the hydration reactions of the cementitious materials. The high-pressure steam accelerates the hydration of cement, forming stable calcium silicate hydrates (CSH) that contribute to higher strength, improved durability and dimensional stability, as well as reduced shrinkage and cracking. The pre-cured elements are placed inside an autoclave, which is a pressure vessel designed to withstand high temperatures and pressures, whereafter the autoclave is sealed, and steam is introduced gradually to raise the temperature and pressure. Typical autoclave curing conditions are a temperature between 170 and 200°C; pressure between 8 and 15 kg / cm2; duration from 6-12 hours (depending on the material and product specifications). After the curing cycle, the pressure is released gradually to avoid thermal shock, and the autoclaved elements are allowed to cool before being removed. Additional air curing may be done before the products are used or transported.

[0119] In a further aspect, the present invention provides air-cured or autoclave-cured fibre cements products as obtained by the methods of the invention.

[0120] The thickness of the fibre cement end products as obtained using the methods according to the present invention, may vary from about 4mm to about 20mm, such as from about 6mm to about 13 mm.

[0121] The length and width of the fibre cement end products as obtained using the methods according to the present invention may vary from about 1 meter to about 1.7 meter in width and from about 1 meter to about 3.6 meter in length.

[0122] In yet a further aspect, the present invention provides uses of the air-cured or autoclave-cured fibre cements products as obtained by the methods of the invention, as a building material. It is an advantage of certain embodiments of the present invention that the resulting air-cured or autoclave-cured fibre cement sheets are suitable and may be used as building boards, sheets, planks, etc. which require the presence of a high amount of cement.

[0123] The invention will now be further illustrated in detail with reference to the following Examples.

[0124] EXAMPLES

[0125] It will be appreciated that the following examples, given for purposes of illustration, are not to be construed as limiting the scope of this invention. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention.

[0126] Example 1

[0127] Production of air-cured comminuted fibre cement powder as produced with a pendular mill may be produced as disclosed in WO2018065518.

[0128] The desired particle distribution of air-cured comminuted fibre cement material can be obtained by milling air-cured fibre cement material in the absence of sand or another silica source. This further facilitates the comminuted cured fibre cement material to be used as a good raw material for both fresh air-cured and autoclave-cured fibre cement waste material after being heat treated.

[0129] Comminuting of the cured fibre cement material before heat-treatment can be done in so-called dry state at a humidity of between about 5% to about 10%, preferably between about 5% to about 6%. The provided cured fibre cement material is thus not to be brought in suspension of a liquid (typically water) to enable the milling, as is the case for some other milling techniques. As a result, a relatively dry comminuted cured fibre cement material in the form of powder is provided. This facilitates storage of an intermediate product before treating it with heat. The powder can also be stored after the heat treatment.

[0130] The cured fibre cement powder obtained by comminuting cured fibre cement material has a particle size distribution similar to that of the standard fresh starting materials to produce fibre cement, e.g. cementitious particles, ground silica particles and / or ground lime particles.

[0131] In one embodiment, the cured fibre cement material to be fed to a Herzog mill or alternatively to a pendular mill (i.e. for comminuting) are pre-cut or pre-crushed into pieces having maximum size of not more than 5 cm, typically as rectangular-like pieces with sides of not more than 3 cm or even not more than 2cm for an efficient comminuting or milling to obtain powder of the desired quality, such as particle size and uniformity.

[0132] The total humidity of the pre-crushed cured fibre cement waste material, for example air cured fibre cement material may be about 5%-6%w based upon dry weight. However, because of the following heat-treatment, a very low humidity is not crucial.

[0133] The %w based upon dry weight is the weight difference between the material as samples and the material dried in a ventilated furnace at 105°C until constant weight is obtained. The pre-crushed material can be fed at a flow rate of about 350 kg / hr to about 800 kg / hr to a pendular mill type roller mill of the Poittemill Group (FR), in which the material is comminuted at a rotation speed of between about 100 and about 400 tr / min.

[0134] As such the comminuted cured fibre cement material, i.e. the powder, is obtained of which the particle size distribution curve, measured using laser beam diffraction on dry dispersed material at 3 bar by means of the apparatus Malvern Mastersizer 2000, can be obtained.

[0135] This fibre cement powder as obtained by pendular milling has a good consistency, i.e. not woolly or fluffy, a suitable bulk density (between about 1000 kg / m3 and about 1300kg / m3) and a good particle distribution, to be used to produce fresh fibre cement products after the heat treatment of the powder. Alternatively, a Herzog mill may be used to crush the pre-treated fibre cement pieces.

[0136] The comminuted cured fibre cement powder obtained after treatment in a suitable mill is then fed into a suitable furnace or oven, such as a rotary kiln for the heat-treatment. The temperature is selected in accordance with the content of the comminuted waste fibre cement, as explained in connection with for example Example 2 and Figure 1.

[0137] Example 2

[0138] Analysis of heat treated, cured fibre cement products before comminuting.

[0139] Cured fibre cement boards were cut into pieces roughly 3x3 cm and heated at different temperatures up to about 950°C in an experimental setting (muffle furnace Controls, model 10-D1418, 3 900 W, Serial No. 16600645. The resulting materials were analysed by the Central Laboratories (Laboratory of Thermal-Gravimetric Analysis) at UCT in Prague. The measurement is Thermogravimmetric and differential thermal analysis combined with mass spectrometry (TG-DTA-MS). For evolved gas analysis (EGA), a combination of a TG-DTA thermal analyser (Setaram Setsys Evolution, France; operating range of 25-1600 °C) with an OmniStarTM quadrupoule-type mass spectrometer (Pffeifer Vacuum, Germany) is used.

[0140] Samples were heated to various temperatures to observe phase transformations during the thermal decomposition of the fiber cement matrix. The initial weight loss, starting at 50°C (Fig. 2, A), peaking at 110°C, and concluding at 240°C, is primarily attributed to water release (Fig. 2, C). This process is significantly endothermic (Fig. 2, B). At lower temperatures, free and physically bound water is released from both the cement matrix and the cellulose fibers. As the temperature increases to 240°C, major dehydration of gypsum occurs, along with the dehydration of ettringite-like phases and the release of some chemically bonded water in the C-S-H phases. The temperature interval from 240°C to 485°C is characterized by a significantly exothermic peak. During this range, the breakdown of cellulose into smaller molecules occurs. Cellulose undergoes pyrolysis, resulting in the formation of volatile compounds, char, and gases, primarily CO2. This leads to a pronounced CO2peak at 340°C (Fig. 2, D). Additionally, PVA fibers undergo thermal degradation in this region, producing acetic acid and other degradation products. Both CO2and water are released during this process. Cellulose pyrolysis is the first exothermic process in this region, followed by the dehydroxylation of Ca(OH)2into calcium oxide (CaO) and water vapor (H2O) around 470°C, accompanied by a significant endothermic effect.

[0141] In the temperature range of 500-700°C, significant structural changes occur in the C-S-H gel, leading to the formation of calcium silicates with varying C / S ratios. This stage is marked by substantial weight loss and structural transformations. Temperatures above 700°C result in the further formation of calcium silicates, such as dicalcium silicate (C2S) and tricalcium silicate (C3S). At this temperature and higher, a significant endothermic effect is observed due to the thermal decomposition of CaCO3, which releases a large amount of CO2and forms CaO. The purity, particle size, and crystallinity of the calcium carbonate influence the decomposition behavior, resulting in two peaks or shoulders on the CO2release curve.

[0142] At higher temperatures the carbonates are decomposed to new phases of minerals resembling the phases in Portland cement clinker (see Figure 1).

[0143] After cooling the heat-treated materials were comminuted in a Herzog mill for 30 seconds and sieved through a 90-micron sieve to prepare the powder.

[0144] Samples that were heated at high temperatures (15 minutes @ 900°C and cooled at air temperature) were tested on the Mini-Hatschek line at R&D Switzerland. Thanks to the formation of the new lime phase and the increased amount of belite, the thermally treated waste serves as a cementitious binder and may replace the original components in higher amounts. This contrasts with the use of fibre cement waste without heattreatment.

[0145] Example 3

[0146] The fibre cement slurry formulations as presented in Table 1 were used to provide green sheets of fibre cement on a state of the art Mini-Hatschek machine. The green sheets were pressed at 190 kg / cm2 and air-cured. Sample 1 is a reference sample without any heat-treated fibre cement waste material. In Test Sample 2, 20% of the cement (CEM ll / A-LL (EN197-1) is replaced by heat-treated fibre cement waste powder produced according to Example 2. In Test Sample 3, 40% of the cement is replaced by heat-treated fibre cement waste powder.

[0147] Table 1 * CEM ll / A-LL: used as alternative to CEM I + 15% limestone.

[0148] The density of the samples was measured by first saturating the samples during 72 hours in tap water. The weight of the samples was subsequently determined both under saturated and immersed conditions. Afterwards, the samples were placed to dry for 48 hours at about 105°C. For each of the dried samples, the weight was determined again.

[0149] The density (X) for each sample was calculated by dividing the dry weight (C) by the difference between the immersed weight (B) and the saturated weight (A), according to the following formula: X = C / (A-B).

[0150] It was determined that the density of the Test Samples (1.64 (20%) and 1.58 (40%) kg / m3) was lower as compared to the reference Sample (1.72) not containing any waste powder. Bending strength was also lower in the Test Samples (25.0 and 22.5 MPa) compared to the reference Sample (28.2 MPa), but still significantly higher than the classifications limit for class 4 fibre cement sheets according to EN12467, which is 18 MPa (see Figure 4). The density corrected bending strength is only minimally lower when substituting at least some of the cement by heat-treated fibre cement waste in Test Samples 2 and 3 compared to Reference Sample 1. The density corrected, normalized bending strength is almost similar for the 3 samples.

[0151] Thus, from the above, it can be concluded that air-cured fibre cement products comprising from about 20 mass% to about 40 mass% of heat-treated cured comminuted fibre cement waste powder as produced by the method of the present invention perform almost as good as the reference fibre cement product not comprising any comminuted waste powder with respect to density corrected, normalized bending strength (see Figure 5). Further, the products have sufficient bending strength and a beneficial lower density than the reference fibre cement product.

[0152] Example 4

[0153] CO2 account. Based on the laboratory / pilot experiments presented above and an existing fibre cement product recipe (B7 corrugated sheets) it was possible to reduce CO2 emissions from cement by 30%, for a total scope 3 reduction of around 24%, if 40% of the cement is replaced with heat-treated fibre cement. See Figures 3 and 4. The outer circle represents the composition in weight % while the inner circle represents CO2 impact. As can be seen, cement and limestone currently constitute the primary mass in the product, while cement and PVA fibers account for most of the CO2 emission. In an example recipe with 40% substitution, a saving of just under 25% CO2 is achieved, as the heat-treated fibre cement emits relatively little compared to its weight percentage.

Claims

CLAIMS1. A method to produce fibre cement products comprising cured fibre cement waste material, comprising the steps of:(a) Providing cured fibre cement waste material and optionally comminuting said cured fibre cement waste material into a cured fibre cement waste powder;(b) 1) Heating the cured fibre cement waste material from step a) to a temperature of at least 500°C for at least 10 minutes and comminuting said heat- treated cured fibre cement waste material to prepare a heat-treated cured fibre cement waste powder, or2) Heating the cured fibre cement waste powder from step a) to prepare a heat-treated cured fibre cement waste powder;(c) Providing an aqueous fibre cement slurry comprising water, cementitious binder, natural or synthetic fibers and said heat-treated cured fibre cement waste powder;(d) Forming a green fibre cement sheet from said aqueous fibre cement slurry; and(e) Curing said green fibre cement sheet thereby providing a fibre cement product comprising cured fibre cement waste material.

2. The method according to claim 1, wherein step (a) comprises a pre- treatment / crushing of larger lumps of cured fibre cement waste material to sizes that fit into an oven for subsequent heat-treatment or for comminuting the pre-treated material to prepare a powder before heat-treatment.

3. The method according to claim 1 or claim 2, wherein said step (c) of providing an aqueous fibre cement slurry comprises mixing at least water, cementitious binder, natural or synthetic fibers and the heat-treated cured fibre cement powder, such that the heat-treated cured fibre cement powder is present in the aqueous fibre cement slurry in an amount of at least 10 mass% of the dry basis of said slurry.

4. The method according to any one of claims 1-3, wherein said step (c) of providing an aqueous fibre cement slurry comprises mixing at least water, cementitious binder, natural or synthetic fibers and the heat-treated cured fibre cement powder, such thatthe heat-treated cured fibre cement powder is present in the aqueous fibre cement slurry in an amount of at least 20 mass% of the dry basis of said slurry.

5. The method according to any one of claims 1-4, wherein said step (c) of providing an aqueous fibre cement slurry comprises mixing at least water, cementitious binder, natural or synthetic fibers and the heat-treated cured fibre cement powder, such that the heat-treated cured fibre cement powder is present in the aqueous fibre cement slurry in an amount of between about 20 mass% and about 80 mass% of the dry basis of said slurry, preferably between about 40 mass% and about 80 mass% of the dry basis of said slurry, such as between about 50 mass% and about 70 mass% of the dry basis of said slurry.

6. The method according to any one of claims 1-5, wherein prior to said step (e) of autoclave-curing said green fibre cement sheet, said green fibre cement sheet is pressed by means of at least one or more mechanical presses.

7. The method according to any one of claim 1-6, wherein said heating in step (b) is at a temperature of at least 700°C for at least 10 minutes, preferably at a temperature of at least about 800°C for at least 10 minutes.

8. The method according to claim 7, wherein said heating is at a temperature of at least about 900°C, such as about 950°C for at least 10 minutes.

9. The method according to any one of claims 1-8, wherein heating is for at least 15 minutes, such as for between about 15 minutes and 60 minutes.

10. A cured fibre cement product obtained using the methods according to any one of claims 1-9.

11. The cured fibre cement product according to claim 10, which is a fibre cement sheet.

12. The cured fibre cement product according to claim 10 or claim 11, which is a autoclave-cured fibre cement product or an air-cured fibre cement product.

13. Use of the cured fibre cements product according to any one of claims 10-12 as a building material.

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