Method of manufacturing a fiber cement product

Abstract

Method of manufacturing a fiber cement product, comprising the steps of: - Providing a fiber cement paste comprising cement, fibers and water; - Extruding the fiber cement paste to form a green article; - Curing the green article to obtain the fiber cement product, Wherein the fiber cement paste comprises a cementitious material obtainable by carbonating a cured cementitious material.

Description

[0001] Method of manufacturing a fiber cement product

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of manufacturing a fiber cement product, comprising the steps of:

[0004] (i) Providing a fiber cement paste comprising cement, fibers and water;

[0005] (ii) Extruding the fiber cement paste to form a green article, and

[0006] (iii) Curing the green article to obtain the fiber cement product,

[0007] The invention further relates to the resulting fiber cement product and a fiber cement paste for use in the method.

[0008] BACKGROUND OF THE INVENTION Extrusion has been known since long as a production method for fiber cement products. Traditionally, it has been used for manufacturing of tubes and the like. More recently, extrusion has also been mentioned as a manufacturing method for fiber cement sheets or beams. However, extrusion of fiber cement materials is not easy. For a material to be extrudable, it must be soft enough to flow through the extrusion die. Furthermore, the pressure required for extrusion must be reasonably low, so as to control manufacturing costs. Phase migration must be avoided and the material should be shape stable. Additionally, the resulting fiber cement sheet should meet performance requirements for exterior building materials, such as freeze-thaw resistance, dimensional stability.

[0009] Generally, fiber cement paste comprise cement, fibers and water. Typically, a cellulose ether is used as a rheology modifier, and silica fume may also be added. Kuder and Shah, Construction Materials 24 (2010), 181-186, used a composition of 33% class F fly ash, 12% silica fume, 14% cement, 39% water and 1% water-reducing admixture (i.e. rheology modifier). They do not mention the use of fibers.

[0010] WO2013 / 082524 mentions use of a paste comprising cement, fibers in the form of pulp and of reinforcement fibers and one or more air entrainment agents in addition to water. The paste preferably comprises 35-70% cement, 0-15% reinforcing fibers, 10-70% silicious aggregates, 0-0.1% air entrainment agent and 0.6-1% viscosity enhancement agent. High-melting polymer-based synthetic fibers such as polypropylene fibers are preferred due to their higher viscoelastic performance than wood-based fiber. The resulting green product is hardened by autoclave-curing. The specified application is for sidings, which are plank-shaped panels for use in fagade coverings.

[0011] US5,891,374 mentions extrusion of fiber cement paste with a variety of fibers, including polyvinyl alcohol (PVA), carbon, microsteel, cellulose and polypropylene (PP). The length of the PVA and carbon fibers was 6 mm, that of PP 19 mm, and that of cellulose 2.55 mm. The diameter was from 15 pm for PVA fibers to 30 pm for PP fibers and 30-120 pm for cellulose fibers. It is shown in comparative tests relative to casting, that the extrusion process increases the strength of the resulting product. The fibers therein show a high degree of preferential alignment in the extrusion direction. Such fiber alignment is further mentioned by de Koker & Van Zijl, 'Extrusion of Engineered Cement-based Composite Material', Proc. 6thRILEM Symposium on FRC, January 2004, 1301-1310. However, strength development in one preferential direction is a disadvantage for building products such as panels and even sidings.

[0012] Overall, there is therefore a need for improvement of extrusion processes for the manufacture of fiber cement products. It would be desired to provide extrudable compositions that lead to shape-stable products which have sufficient strength both in the extrusion direction and also in a direction perpendicular to the extrusion direction.

[0013] SUMMARY

[0014] It is therefore a first object of the invention to provide an improved manufacturing method for fiber cement products, that may overcome one or more of the mentioned limitations.

[0015] It is another object of the invention to provide an extrudable composition for use in such a manufacturing method.

[0016] It is a further object of the invention to provide a resulting product.

[0017] According to the invention, the first object is achieved in a method of manufacturing a fiber cement product, comprising the steps of: (1) Providing a fiber cement paste comprising cement, fibers and water; (2) extruding the fiber cement paste to form a green article, and (3) curing the green article to obtain the fiber cement product, wherein the fiber cement paste comprises a cementitious material obtainable by carbonating a cured cementitious material.

[0018] The second object is achieved in an extrudable fiber cement composition comprising fibers, cement and water, further comprising a cementitious material obtainable by carbonating a cured cementitious material.

[0019] The further object is achieved in a fiber cement product obtainable with the method of the invention.

[0020] In the invention carbonated cementitious material is added into the fiber cement paste. The terms 'extrudable fiber cement composition' and 'fiber cement paste' are herein used as having identical meaning .The term 'paste' indicates a distinction to aqueous slurries used in Hatschek processing, as the amount of added water in the extrudable fiber cement composition is much less, such as at most 50% by weight based on the total dry weight of the composition, and preferably at most 30% by weight, based on the total dry weight of the composition. Such carbonated cementitious material is cured, as it typically originates from recycling of cement products, such as recycled fiber cement products or concrete, such as recycled concrete paste (RCP). The material is typically in the form of powder or fines, as this improves both uniformity of distribution in the extrudable fiber cement composition, and moreover increases the surface area and therewith enhances the carbonation process. For this reason, the added material obtainable by carbonating of cured cementitious material is also referred to as carbonated cement fines, or more generally carbonated cement powder. For sake of clarity, it is observed that this carbonated cement powder may be an autoclave- cured powder and an air-cured powder. The carbonated cement powder itself may be part of a fiber cement composition for air-curing or for autoclave-curing. The carbonated cement powder may originate from fiber cement waste or from other sources, such as from the recycled concrete powder (RCP) in concrete waste.

[0021] The invention is based on the insight that the carbonated cement powder is by itself a powder, such obtained by sieving through a sieve in the range of 75-200 pm, for instance a sieve of 100 pm or 150 pm. A mean average particle size (d50 as defined by laser diffraction) in the range of 10-40 pm is preferred, such as between 15 and 25 pm. The cured powder is thus equally fine or less fine than fresh cement. It can be considered as a fine aggregate, and hence may provide a counterforce against too much fiber alignment as a consequence of the extrusion process. Moreover, carbonated cement powder is a pozzolanic material that is capable of reacting quickly with calcium hydroxide. Such calcium hydroxide is liberated from cement during hydration in water, and then deemed available in aqueous form. This process happens also in the course of the extrusion processing of the cement paste and directly thereafter. Therefore, the carbonated cement powder can react with the liberated calcium hydroxide early in the manufacturing process and contribute to shape stability; the hydration products resulting from the reaction between calcium hydroxide and carbonated material will again be a solid and thus may help to replace the dissolved and thus removed calcium hydroxide.

[0022] Moreover, the use of carbonated material is shown to provide increased flexural and compressive strength, particularly in direct comparison to use of a corresponding amount of noncarbonated recycled cement material. In comparison to the use of non-carbonated cured cement powder, the use of carbonated cement powder may further reduce the water demand of the fiber cement paste. Water demand is to be understood as an amount of water necessary to obtain a predefined rheology of the fiber cement paste, as has been identified appropriate for processing the paste on the used extruder apparatus and in accordance with specified process settings.

[0023] Preferably, the amount of carbonated cement powder in the extrudable fiber cement composition is in the range of 5 to 40% by weight based on the dry weight of the extrudable fiber cement composition, more preferably 10-30%. In this manner, approximately 10-40% by weight of cement and / or silica-based materials such as quartz and fly ash could be substituted by carbonated cement powder. It was found in preliminary experiments that the use of carbonated cement powder provides an increase in both compressive strength and flexural strength when comparing the use of carbonated cement powder and the use of non-carbonated cement powder that was otherwise corresponding.

[0024] Preferably, the carbonated cement powder has a particle size distribution such that the d90 is less than 150 pm, preferably less than 100 pm, and wherein the d50 is within the range of 10-60 pm, preferably 10-40 pm, more preferably 15-30 pm. Such a particle size distribution may be obtained by recycling cement waste and processing it through comminuting and sieving. A preferred way of comminution uses a roller mill, and especially a vertical roller mill with an integrated classifier.

[0025] The carbonated cement powder may be obtained from material that has been cured in any feasible way, such as air-curing, autoclave-curing or even carbonation (CO2)-curing. The type of curing has an impact on the mineralogy of the cement material, and hence is distinct for air-cured material as opposed to autoclave-cured materials.

[0026] In one embodiment, use is made of carbonated material obtained by carbonation of autoclave-cured cement powder. Such material turns out to have stronger pozzolanic properties and an improved composition than cementitious powders obtained from carbonation of either recycled concrete paste or other air-cured cement material, such as air-cured fiber cement material. The improvement particularly resides in a comparatively high content of the alumina-silica phase (also referred to as alumina-silica gel) in the carbonated powder and therein a comparatively high concentration of silica. For sake of clarity, autoclave-curing is a method of steam curing of a material comprising both a source of calcium oxide and a source of silicon oxide, wherein the product is put to steam at a temperature above 100°C and a pressure well above atmospheric pressure, such as at least 5 bar, for a predetermined period of time. Under such conditions, calcium oxide and silica provided as raw materials will react to form calcium silicates, that might be present in amorphous form, typically referred to as a calcium silicate hydrate (CSH) gel, or in the form of crystalline calcium silicate. Autoclave-curing particularly results in crystalline material, whereas air-curing rather results in the amorphous gel material. An important crystalline phase is tobermorite, which is understood to contribute to the strength of the resulting material. Other crystalline phases of calcium silicate include xonotlite and wollastonite.

[0027] In an alternative embodiment, use is made of carbonated material obtained by carbonation of air-cured cement powder, further comprising fibers. The fibers in air-cured fiber cement include synthetic fibers in addition to cellulose fibers. Moreover, the total amount of fibers is less than the amount in autoclave-cured fiber cement. Such lower concentration of fibers and especially cellulose fibers is deemed beneficial to keep the water demand of the composition low. The water demand is deemed determined, at least primarily, but possibly entirely, by the rheology of the composition, i.e. the amount necessary to ensure appropriate flow of the composition through an extruder. Increasing the water demand implies that more water will be removed from the resulting fiber cement product, and thus typically giving rise to higher porosity, and concomitantly less optimal mechanical properties.

[0028] Rather than using merely autoclave-cured cement powder or air-cured cement powder for carbonation, a mixture of both may be provided. A benefit of such mixing is that it allows a broader supply of waste material. Furthermore, it becomes feasible to tune the composition of the alumina- silica phase in the carbonated powder, and more particularly the silica-content therein. This may be useful to ensure that the composition of the alumina-silica phase will remain stable, notwithstanding variations in the supply of waste material. The mixing may be performed before or after carbonation. It appears preferable to perform mixing prior to carbonation. The mixing ratio A / B between autoclave-cured material (A) and air-cured material (B) is preferably chosen in the range of 0.25 to 100 on a weight-basis; a mixing ratio of at least 1 is deemed preferable, and a mixing ratio of at least 3 (25% air-cured material), at least 6 or at least 9 (10% air-cured material) is even more preferable. In one further embodiment, the mixing ratio may be in the ratio of 6 to 19 (5 to 17% aircured material) and is intended to arrive at a predefined composition notwithstanding variations in supply of waste material.

[0029] In again a further embodiment, the cement powder obtained from a waste source may be mixed with a further material prior to carbonation or after carbonation, and preferably prior to carbonation. According to a first implementation thereof, such further materials include a recycled cement paste (i.e. without fibres), and / or a slag material, such as GGBFS as known per se to the skilled person. Hence, the further material is preferably a carbonatable material. According to a second implementation, thereof, the autoclave-cured waste material is mixed with another material, so as to arrive at a predefined density. A crystalline calcium silicate material typically has an initial density below 1.0 kg / dm3, whereas a waste material for high-density fiber cement products may have a density of at least 1.5 kg / dm3, or even more. Mixing sources may thus be useful. In one specific implementation, autoclave-cured calcium silicate waste material is mixed with autoclave- cured fiber cement waste material, preferably fiber cement waste material from so-called mediumdensity or high-density products. Such calcium silicate material is known by itself and is for instance sold by Promat® under the tradename Promaxon® and in the form of fire-resistant plates.

[0030] According to an important embodiment, the carbonated cement powder is obtainable from carbonating recycled fiber cement waste. As a consequence, the carbonated cement powder may further comprises fibers, such as synthetic fibers, cellulose fibers and optionally inorganic fibers. Fiber cement is a very pure material, as it does neither include aggregate nor a significant level of organic additives. This is beneficial for its reuse. The amount and type of fibers depends on the type of fiber cement, with autoclave-cured fiber cement material comprising more fibers than air-cured fiber cement material. However, the amount of fibers may be reduced by treatment of fiber cement waste material. A preferred way of manufacturing fiber cement products is by means of the Hatschek or the flow-on process, with the Hatschek process being most preferred.

[0031] In a further implementation, the carbonated cement powder obtainable from carbonating recycled fiber cement waste has the same particle size distribution and moreover has a fiber content of at most 6% by weight based on the carbonated cement powder. By further preference, the fiber content is at most 5% by weight. A cement powder with such a distribution is obtainable by using in sequence a vertical roller mill with an integrated air classifier and a sieve, especially a sieve with a sieve size of 150 pm or 100pm. More preferably, the vertical roller mill comprises a chamber that is heated by means of flow of heating gas, such as air. The heating up of the chamber and the cement powder therein is deemed beneficial to obtain a full separation between cement and fibers. Moreover, drying is relevant to ensure efficient and effective sieving. Passing the air classifier, which operates as a centrifuge, may aid in obtaining said separation of the fibers from cementitious particles. Investigations of the resulting cementitious powder by means of optical analysis have shown that most fibers, if not all fibers, are liberated from the cementitious particles. Most may herein be understood as at least 90% by weight based on the total weight of the fibers. Preferably, this is even at least 95% by weight. In one specific implementation, the milling chamber was heated to a temperature in the range of 70-90 °C. This principle is further elucidated in the nonprepublished application PCT / EP2023 / 067870 in the name of the applicant of the present application, which is included herein by reference. Based on this liberation a major portion of the fibers may be removed from the waste material, for instance at least 30% by weight, or even at least 40% by weight.

[0032] In again a further implementation, the carbonated cement powder comprises fibers with an arithmetic average length in the range of 0.25 to 0.50 mm, preferably in the range of 0.30 mm to 0.45 mm. Such short fibers are especially organic fibers, such as cellulose fibers and / or synthetic fibers. The short fibers are shorter than conventional organic fibers used in fiber cement compositions. Compared to conventional organic fibers, such short fibers will have positive impact on flowability, but the contribution to strength may be reduced. The arithmetic average fibre length can be distinguished from the length weighted average fiber length, as often used in the wood pulp industry. It can be is herein obtained by optical analysis. The ratio of the two different fiber length is the polydispersity, which is a measure of the broadness of the length distribution.

[0033] In a further embodiment, the recycled cured cement powder is wetted prior and / or during the carbonation treatment. Wetting preferably occurs to a mass ratio of water over binder (w / b) of at least 0.20, and preferably at most 0.4, and more preferably in the range of 0.25-0.35, when measured at the start of the carbonation step. The presence of sufficient water is understood to contribute to effective carbonation, i.e. the formation of COs2-from CO2 and H2O via H2CO3 and / or HCOs", as known per se.

[0034] The carbonation step may itself be carried out in various ways. In one implementation, the carbonation is performed in a plurality of cycles, wherein prior to each cycle the cementitious powder is de-agglomerized and wetted. Alternatively, a specific reactor may be provided, wherein the cementitious powder is wetted and / or de-agglomerized during the carbonation, for instance by means of a mixing device. One implementation is disclosed in Zajac et al, Cement and Concrete Research, 130 (2020)), 105990.

[0035] For sake of clarity, it is deemed preferable that such mixing occurs after comminution of the waste material and - if applied - after removal of fibers from the waste material. As the different waste sources may have a different density and / or different mechanical properties including hardness, it is deemed preferable to perform mixing powders rather than mixing blocks. The removal of fibers is herein helpful so as to ensure flowability of the powder and thus adequate mixing into a uniform powder mixture.

[0036] The extrudable fiber cement composition comprises cement and fibers, in addition to the carbonated cement powder. In case that the extrudable fiber cement composition is configured for air-curing, the total amount of cement-type ingredients is preferably at least 40% by weight, based on dry weight of the total composition and more preferably at least 50% by weight, or even at least 60% by weight. In an example, the cement-type ingredients in an extrudable air-curable composition are present in an amount in the range of 40-800% by weight, or even 50-70% by weight. The percentage of carbonated cement powder compared to said total amount of cement and carbonated cement powder is preferably in the range of 15-45%, such as between 20 and 40% by weight.

[0037] In the context of this application, cement-type ingredients include any cement-containing material in cured or uncured form, and therewith comprise recycled cement material, typically present as fines, fresh cement material, such as Portland cement, and recovered materials, especially materials recovered during industrial manufacture of other materials than cement, such as slag. An extrudable, air-curable fiber cement composition comprising slag as a cement-type ingredient is preferred.

[0038] Good results have been obtained in preliminary experiments, wherein the total amount of fresh cement was reduced to less than 30% by weight, more particularly to less than 25% by weight, and even to less than 20% by weight, such as 8-18% or 10-15% by weight, by adding a slag to the air- curable composition. The amounts of fresh Portland cement, slag and carbonated cement powder are suitably each between 10% and 30% by weight, wherein the amount of slag may be up to 35% by weight, or up to 40% by weight. For instance, the amount of fresh cement is in the range of 10-23% by weight, the amount of carbonated cement powder is in the range of 10-25% and the amount of slag is in the range of 10-35%. Preferably, the said amounts of fresh cement, carbonated cement powder and slag are in the ranges of 10-20%, 15-25% and 15-30% by weight. As stated hereinabove, it is deemed beneficial that the ratio of carbonated cement powder to the total of cement-type ingredients is in the range of 15-45%, preferably 20-40% by weight.

[0039] Preferably, the slag has a mean particle size that is smaller than the mean particle size of the carbonated cement powder or the fresh (Portland) cement. The mean particle size is herein defined as the d50-value that may be obtained by measurements using laser diffraction, Suitable equipment thereto is supplied by Malvern Pananalytical and known as MastersizerTM 3000. More preferably, the slag has a d50-value below 10 pm, such as from 4-8 pm, whereas the Portland cement has a d50 value in the range of 10-20 pm and the carbonated cement powder may have a d50 value in the range of 10-30 pm, preferably 15-25 pm.

[0040] In a further preferred embodiment, the air-curable composition further comprises calcium carbonate or a source thereof. The amount of calcium carbonate is for instance 10-40%, such as 15- 35% by weight, preferably 20-30% by weight in a composition without slag. However, in case of an air-curable composition comprising slag, the amount of calcium carbonate may be higher, such as from 20-50% by weight or preferably 30-50% by weight, especially when at least 10% or at least 15% slag is present in the air-curable composition. Therewith, the lower concentration of calcium in slag than in Portland cement gets compensated. It is observed that carbonated cement powder includes calcium carbonate as well, in addition to the amorphous alumina-silica gel. Therefore, the desired amount of calcium carbonate may decrease when increasing the amount of carbonated cement powder.

[0041] According to one preferred implementation, the air-curable extrudable fiber cement composition comprises Portland cement in an amount between 8 and 30 % by weight (wt%), carbonated cement powder in an amount between 10 and 30 wt%, slag in an amount between 10 to 40 wt% and calcium carbonate in an amount between 20 to 50 wt%, wherein said amounts of cement, carbonated cement powder, slag and calcium carbonate are based on dry weight of the fiber cement paste. One even more preferred composition comprising Portland cement in an amount of 10-30%, carbonated cement powder in an amount between 10 and 30wt%, slag in an amount of 10-40wt% and calcium carbonate in an amount of 30-50wt%. Another more preferred composition comprises Portland cement in an amount of 8-20wt%, carbonated cement powder in an amount of 20-40wt%, such as 22-36wt%, slag in an amount of 15-40%, such as 20-35%, and calcium carbonate in an amount of 20-50%. This lowers the content of Portland cement further. The air- curable extrudable fiber cement composition further comprises fibers, for instance in an amount of up to 5% by weight, preferably up to 3% by weight, which amount do not include any fibers present in the carbonated cement powder. Especially preferred amounts for each of these ranges are specified in the foregoing, and may be optimized to reduce the amount of -fresh - Portland cement to less than 20% by weight, or even in the range of 10-15% by weight.

[0042] According to another preferred implementation the air-curable extrudable fiber cement composition comprises Portland cement in an amount between 30 and 60 % by weight (wt%), carbonated cement powder in an amount between 10 and 40 wt%, calcium carbonate in an amount between 10 to 40 wt%, wherein said amounts of cement, carbonated cement powder and calcium carbonate are based on dry weight of the fiber cement paste. Preferably, said contents of carbonated cement powder and calcium carbonate are each 15-35 wt%. The air-curable extrudable fiber cement composition further comprises fibers, for instance in an amount of up to 5% by weight, preferably up to 3% by weight, which amount do not include any fibers present in the carbonated cement powder. Especially preferred amounts for each of these ranges are specified in the foregoing.

[0043] In case that the extrudable fiber cement composition is configured for autoclave-cured, a source of silica will be present in the fiber cement composition. Suitable sources of silica are for instance quartz and fly ash. The total amount of silica source in the fiber cement composition is preferably in the range of 30-60% by weight based on dry weight of the fiber cement composition. Preferably, the silica source is provided in a molar ratio between calcium oxide (CaO) and silica (SiO2) of 0.2-1.3, more preferably 0.25-0.8 and even more preferably 0.40-0.70. The added carbonated cement powder may serve as a source of calcium oxide and of silica. The effectiveness as a source may depend on its origin. A relatively high amount of SiO2, and hence a low molar ratio (CaO / SiO2) seems preferable for an extrudable composition. Such a low molar ratio is for instance a ratio of less than 0.70 or even less than 0.60, and preferably still higher than 0.40 or even 0.45 or more. For sake of clarity, it is observed that the carbonated or uncarbonated cement powder is not taken into account for calculation of this molar ratio between calcium oxide and silica.

[0044] Preferred autoclave-curable extrudable fiber cement compositions comprise 30-50% by weight of Portland cement, 30-50% by weight of a source of silica, i.e. SiO2, 10-30% by weight of carbonated cement powder. The Portland cement and silica are preferably present in amounts of 32-45% by weight, and the carbonated cement powder is preferably present in amount up to 25% by weight, for instance 15-25% by weight. The amount of added fibers is for instance up to 5% by weight, such as 3-5% by weight.

[0045] In a preferred embodiment, the extrudable fiber cement composition further comprises fibers. A variety of fibers are known for fiber cement and include synthetic fibers, cellulose fibers and inorganic fibers. Cellulose fibers and synthetic fibers such as polyvinyl alcohol and polypropylene are among the fibers commonly used in fiber cement production, but other fibers are not excluded. Use of inorganic fibers in combination with cellulose fibers and optionally synthetic fibers may for instance be beneficial for fire resistance properties. The amount of fibers in the composition is preferably in the range of 3 to 15% by weight, based on the dry weight of the fiber cement composition and excluding any - comparatively short - fibers introduced with the carbonated cement powder. Preferred amounts of fibers may be in the range of 5 to 10% by weight.

[0046] In again a further embodiment, the extrudable fiber cement composition further comprises a filler. Examples of fillers to fiber cement include for instance low-density additives such as perlite and microspheres, other sources of silica, calciumsilicate and / or alumina, such as wollastonite, amorphous silica, kaolin, metakaolin, mica, aluminium trihydroxide (ATH). The amount of fillers is suitably in the range of 0 to 30% by weight based on the total dry weight of the fiber cement composition, for instance 5-25% by weight, or 10-20% by weight.

[0047] In again a further embodiment, the extrudable fiber cement composition comprises one or more pigments. Therewith a product can be obtained that is mass-coloured and does not need to be coated. Pigments added to fiber cement compositions may need to withstand the alkaline environment of cements and are therefore typically chosen from inorganic pigments. Most preferably but not exclusively such inorganic pigments are chosen from the group of iron oxides and titanium oxides. The amount of pigment in the fiber cement composition is preferably in the range of 1-10% by weight based on the total dry weight of the fiber cement composition. An amount of up to 6wt%, or even up to 5wt% is deemed sufficient and beneficial for cost reasons.

[0048] The extrudable fiber cement composition may further include a variety of known additives, such as a rheology modifier or viscosity enhancement agent, a biocide, a hydrophobization agent, an air entrainment agent if any, dispersant, flocculant and so on. Such agents are typically used in an amount of at most 1% by weight per agent (based on total dry weight of the fiber cement composition), as known per se. A typical rheology modifier for cement applications, such as a fiber cement paste, is a cellulose ether, and is preferably present.

[0049] For sake of clarity it is observed that any of the implementation, embodiments and options as specified hereinabove, or hereinafter in the description of examples or in the dependent claims are applicable to all the aspects of the invention. Furthermore, any percentage referred to in the context of a composition is to be understood as a weight percentage based on the dry weight of the composition, unless otherwise indicated. Furthermore, the terms 'carbonated cement powder' and 'carbonated fines' have identical meaning. Similarly, the terms 'uncarbonated cement powder' and 'uncarbonated fines' have identical meaning. Additionally, in cases wherein a plurality of ranges with weight percentages are used, it is understood that the total amount will add up to 100%, including any further ingredients which remain equal, such as fibers and rheology additives. Moreover, the term 'Portland cement', in the context of weight percentages, refers to CEM1 type Portland cement, although alternative types of Portland cement are not excluded.

[0050] INTRODUCTION OF FIGURES The method and the material of the invention will be further elucidated with reference to figures, wherein:

[0051] Fig. 1(a) and (b) show DTG graphs for the carbonated and uncarbonated material; Fig 1(a) relates to autoclave-cured material, fig 1(b) relates to air-cured material.

[0052] Fig. 2(a) and (b) show FTIR graphs for the carbonated and uncarbonated material; Fig. 2(a) relates to autoclave-cured material and Fig. 2(b) to air-cured material.

[0053] EXAMPLES

[0054] Example 1

[0055] Test samples 1 and 2 were prepared with compositions as shown in Table 1. A fiber cement sheet was prepared therewith using the Hatschek process. Composition 1 was cured by subjecting the fiber cement sheet to precuring at atmospheric conditions, followed by autoclave curing at 7 bar for 12 hours. Composition 2 was pre-cured during 1 night in a climatized room at around 50°C and thereafter cured in ambient conditions during 3 weeks. The samples had a density of around 1.4 g / cm3. They represented recycled waste.

[0056] Table 1 - composition of samples, all values as weight% relative to dry weight of composition

[0057] The cured sheets were cut in smaller pieces, after which the pieces were milled with a cutting mill (Retsch SM300 with a 500 pm sieve) to obtain a powdered material. This powdered material was dried in a laboratory oven at 105°C for 24 hours, and subsequently milled with a disk mill of type Retsch RS200 during 2 minutes. Finally, the powdered material was sieved over a 100 pm sieve. The residual waste from this process was 25% by weight.

[0058] X-ray fluorescence (XRF) was applied to identify the elemental composition of the samples. More generally, the amount of CaO in autoclave-cured samples is in the range of 20-30 wt%, and the amount of SiO2 is in the range of 45-65 wt%, such as 50-60wt%.

[0059] Table 2 - Elemental composition prior to carbonation measured at 1000°C by X-Ray diffraction (XRF)

[0060] Example 2

[0061] Part of the waste fibre cement powder (at least 500 gram per test sample) produced in Example 1 was put aside to be tested as non-carbonated material. Another part of the waste fibre cement powder (at least 500 gram per test sample) from Example 1 was carbonated. Thereto, the dried and milled powders were spread out in aluminium dishes (about 250 grams per dish). The weight of the dishes and the dry powders were measured, after which the dry powders were moistened with water under a mass ratio of water over dry powder of 0.35 was achieved. The moisture was applied by spraying. Thereafter, the weight of the powders was measured again. The powders were then subjected to pre-hydration by placing them in a chamber at a temperature of 60°C and a relative humidity RH of 90% during 72 hours. The powders agglomerated during this pre-hydration process. Hence, they were disturbed and de-agglomerated with a spatula, after which the powders were wetted again to a mass ratio of water over dry powder of 0.35. Weight was measured before and after wetting. The powders thereafter were subjected to carbonation in five consecutive cycles. Each carbonation cycle had a duration of 24 hours and was performed in a carbonation chamber at a temperature of 60°C, a relative humidity RH of 90% and a constant CO2 inflow of 1.5 l / min to achieve a CO2 concentration of 18% by volume or higher. Between each of the cycles, the agglomerated powders were disturbed and de-agglomerated with a spatula, and subsequently wetted until a mass ratio of water over dry powder of approximately 0.35. The samples were weighted before and after wetting. The powders were dried at 105°C for 24 hours after the last carbonation cycle, and the weight of the powders was measured before and after drying.

[0062] Example 3 - determination of CC -content

[0063] Generally, the CC -content in a sample can be derived from the total inorganic content (TIC), by multiplying the TIC with the ratio of molar mass of CO2 to C (i.e. 3.667). The TIC can be found by measuring the total carbon content (TC) and the total organic carbon content (TOC) in a sample. The TIC is the difference of TC and TOC. As specified for instance in EN13639:2017, TOC and TC can be determined by oxidizing a sample at a temperature of 1300°C. In order to measure the TOC, the matter contributing to TIC needs to be removed first. This is done by treatment of a dried sample with strong acid, such as phosphoric acid (for instance in a 1:1 ratio). This determination of CO2- content was performed for the cementitious powders 1 and 2 obtained in accordance with Example 2. Use in made of a carbon / sulfur analyzer EMIA-320V from Horiba to quantify TC and TOC, enabling calculation of TIC. The carbonation degree is determined as the CO2 degree relative to the theoretically maximum CO2 degree as determined from XRF. The results are shown in Table 3

[0064] Table 3 - C02 content and carbonation degree (measurement in mass% compared to total dry weight)

[0065] Table 3 shows that the TIC and CO2-degree in composition 2 was much higher before carbonation than in composition 1. This is due to the presence of limestone in the product composition, see Table 1. The increase in CO2 content is higher for composition 2 than for composition 1. The degree of carbonation is about 80% indicating that the laboratory method of carbonation in a plurality of cycles preceded by wetting is sufficiently good, though not perfect.

[0066] Example 4 - characterization of the carbonated material by DTG

[0067] DTG (Difference Thermogravimetry) measures a ratio of Dm (weight loss or weight increase) at heating / cooling / isotherm, interpretation by Dm over T or time (-dm / dt). The DTG curve is the first derivative of the Thermogravimetry. Thereto, samples were heated from room temperature to 1000°C under Nj atmosphere with a heating rate of 10 °C / minute.

[0068] Results are shown in Fig. la and Fig lb for compositions 1 and 2 respectively. Each figures includes a graph for the uncarbonated sample (solid line) and carbonated sample (dotted line). These graphs allow several observations.

[0069] First, a major peak is seen in the carbonated samples around 800°C, which is absent in the uncarbonated sample of composition 1 and smaller in the uncarbonated sample of composition 2. This peak originates from the CaCOs, particularly in the form of well crystallized calcite.

[0070] Secondly, in Fig. la, it is apparent that a second peak is somewhat diminished and furthermore sharper. This peak originates from the cellulose in composition 1 and can be attributed to pyrolysis of fibers. In Fig. lb, a peak attributed to a complex of polyvinyl alcohol (PVA) fiber and Ca(OH)2 disappeared during carbonation and was replaced by a minor peak for PVA only. All this indicates that the sharpening of the cellulose peak in Fig. la (as a consequence of carbonation) may be due thereto, that the fibers in the carbonated sample are less or no longer adhesively bound to the inorganic matrix.

[0071] Thirdly, the graph of the carbonated sample in Fig. la includes a broad peak in the range of up to 200°C, which is absent in the uncarbonated sample and which is less pronounced in the carbonated sample of Fig. lb. This broad peak is attributed to the presence of silica gel.

[0072] Example 5 - characterization of carbonated samples by FTIR

[0073] Further characterization was performed using Fourier Transform Infrared (FTIR) measurements, in order to understand what phases of silica were present in the carbonated samples. According to literature, a peak at 960 cm'1is associated with the vibration of Si-0 bonds in the calcium-silicate hydrate gel (CSH). Fig. 2(a) show results for uncarbonated and carbonated samples of composition 1. Fig. 2(b) shows results for uncarbonated and carbonated samples of composition 2.

[0074] The graphs for the uncarbonated samples are quite different, but both include a peak around 960 cm'1that can be attributed to the CSH gel. This peak has shifted and is increased in size in the carbonated samples. It furthermore is broader. This indicates a change in silicon-oxide bonding type. The peak at 960 cm'1is representative for Ql-configuration, wherein merely one oxygen atom bonded to a silicon atom is also bonded to a further silicon atom. The peak around 1045 cm'1is representative of a Q2-configuration, wherein two Si-O-Si bonds are present for a single Si-O-unit rather than one. The peak has a shoulder around 1160 cm'1, that seems representative for Q3, i.e. even higher degree of polymerization of the Si-0 unit. There is some variation between the graphs of the carbonated samples in Fig. 2(a) and Fig. 2(b). This may be due to the amounts of aluminum replacement of silicon in the Q2 and Q3 configurations.

[0075] It is furthermore observed that Fig 2(a) shows a peak representative for SiO2 in the carbonated and non-carbonated sample. That implies that non all quartz has been converted into the aluminate-silicate gel. In addition, both graphs include peaks at 718 cm'1and 1414 cm'1that can be attributed to calcium carbonate in crystalline form.

[0076] According to Zajac et al, Cement and Concrete Research 134 (2020), 106090, the carbonated material would contain calcite (47%), alumina-silica phase (50%) and a minor quantity of other (3%). In the present example, the carbonated autoclave-cured material contains 8% of cellulose fibers, and some residual quartz that is not converted. The overall amounts thereto are roughly 7% fiber, 13% residual quartz, 42% amorphous alumina-silica phase and 38% calcite. The mutual ratio of the alumina-silica phase and the calcite phase is about 1.10 in the present case and 1.06 according to Zajac et al.

[0077] It is noted therein that the material originating from autoclave-cured material comprises relatively more SiO2 than the Portland cement used in Zajac et al. This is also apparent from the composition of the alumina-silica phase. According to Zajac et al, the phase comprises 67% SiO2, 19% AI2O3 and 14% other components, including 6% alkali metal oxides (Na2O, K2O), 3% earth alkali metals (MgO, CaO), 2% Fe2O3 and 3% SO3. In the present situation, the amount of SiO2 is estimated 85%, the AI2O3 is about 7%, and the others are about 8%, including 3% Fe. So, more generally speaking the amount of SiO2 is at least 75% by weight, or even at least 80% by weight, and the amount of alumina is less, up to 12%. Furthermore, the amount of alkali metal oxides is less. This leads to an alumina-silica phase, that is richer in silica, and hence would be characterized by a stronger pozzolanic behaviour.

[0078] Example 6 - BET surface area

[0079] BET specific surface areas were determined by nitrogen gas adsorption (porosimetry) based on the BET isotherm. Use was made of a Quantachrome Apparatus (Autosorb iQ. Station 1) for automated gas sorption.

[0080] Table 4 - specific surface area of samples according to BET isotherm

[0081] It can be seen in this table that the specific surface areas strongly increased as an effect of carbonation. The resulting value is very high, since typical BET specific surface areas for limestone and silica fume (amorphous silica) are 1.3 m2 / g and 20 m2 / g. The BET value before carbonation is attributed to the presence of amorphous calcium-silicate hydrate (CSH) gel. The inventors believe that the further increase of the BET value induced by carbonation is due to the decalcification of the CSH-gel, the inclusion of alumina in this gel to obtain an alumina-silica gel. The high BET value is deemed representative for a high pozzolanic character of the material, that may contribute to a quick reaction with cement.

[0082] Example 7 - mortar sample preparation

[0083] Several binders for mortars were prepared by mixing the uncarbonated and carbonated samples with Portland cement CEM I, 42.5 R, in accordance with Table 5. For the preparation of three mortar samples per formulation, 450 gram binder, 1350 gram standard sand and 225 gram tap water are used. The binder includes herein both the Portland cement CEM I and the carbonated material, in the ratio set out in Table 5. Hence, for sample 13, the effective amount 360 gram CEM-I, 90 gram carbonated material, 1350 gram sand and 225 gram water.

[0084] Table 5 - prepared mortar formulations; * comparative examples

[0085] At the age of 3 days, 7 days, 14 days and 28 days, three mortars (4 cm x 4 cm x 16 cm) per formulation are tested for compressive strength and for flexural strength. The strength data (expressed in MPa = kg / m2.s) of each of the samples was measured by making use of a UTS / INSTRON apparatus (type 3345, cel = 5000N).

[0086] Example 8 - compressive strength development of mortar formulations comprising composition 1 (waste fibre cement powder from autoclave cured material)

[0087] Table 6, 7 and 8 show the resulting data for the reference and the mortar formulations 11-16 for the compressive strength. All formulations include waste fibre cement powder from autoclave-cured material. Table 6 provides the absolute strength values. Table 7 shows the increase or decrease in strength relative to the reference. Table 8 shows the difference in strength between the carbonated and uncarbonated samples

[0088] Table 6 - compressive strength (MPa)

[0089] Table 7 - compressive strength increase or decrease relative to reference (in %)

[0090] Table 8 - compressive strength difference between carbonated and uncarbonated samples.

[0091] The percentages are calculated as the difference in compressive strength (in MPa) between the corresponding carbonated and uncarbonated sample, divided by the compressive strength value for the uncarbonated sample.

[0092] These tables demonstrate that the compressive strength of the mortars is improved significantly due to the carbonation. There is furthermore an improvement of the compressive strength for 5% and 10% addition of carbonated samples. The increase is most significant for the 5% addition after 7 days. After 28 days, the compressive strength of the formulations with 5 and 10% addition of carbonated material is nearly the same as that of the reference. This seems to imply that the carbonated material accelerates strength increase, without leading to an overall added strength.

[0093] The data for the 20% addition of carbonated material are highly interesting. In absolute terms, the compressive strength is lower than those of the reference. However, as is most easily visible in Table 7, it appears that the strength development is rather different than for the samples of 5% and 10%. Whereas the strength development of those samples is quite similar to that of the reference, but slightly quicker, the strength development of the sample with 20% addition is slower. After 28 days, the compressive strength is still lower than that of the reference, but the difference is decreased. It may be expected that the difference would be even smaller after 49 or 90 days. Furthermore, the difference in strength in comparison to the uncarbonated sample with 20% addition, only increases with the age of the mortar. This behaviour is different from the 5% and 10% additions. An explanation might be that at low addition percentages, the carbonated material does not have a major impact on the hardening of the cement. At higher addition percentages, however, the microstructure of the resulting mortar might change. A further explanation may be that the water-demand of the 20% addition is higher, and hence required more time and / or more water. It is observed in this respect, that the binder with 5% carbonated material effectively contains about 0.36% of cellulose fibers. This fiber content is about 0.72% for the 10% addition and 1.44% for the 20% addition. This fiber content is quite high for the 20% addition, even when taking account that the fiber content in the mortar is further diluted due to the addition of sand. Still, the fiber content may have had a negative impact on the strength levels for the 20% addition. However, fiber levels of fiber cement waste may be further reduced, for instance by means of the technology disclosed in W02024 / 003277A1.

[0094] The addition of uncarbonated material to the mortar leads to a decrease in strength. As it appears from Table 7, the relative decrease does not increase or decrease over time, and is further roughly linearly dependent on the added amount. Thus, the addition of uncarbonated material seems to be equivalent to the addition of filler material.

[0095] Example 9 - compressive strength development for samples comprising waste fibre cement powder from air-cured fiber cement

[0096] Table 9, 10 and 11 show the resulting data for the reference and the mortar formulations 11-16 for the compressive strength. All tables include data for samples comprising waste fibre cement powder from air-cured material. Table 9 provides the absolute strength values. Table 10 shows the increase or decrease in strength relative to the reference. Table 11 shows the difference in strength between the carbonated and uncarbonated samples.

[0097] Table 9 - compressive strength (MPa)

[0098] Table 10 - compressive strength increase or decrease relative to reference (in %) Table 11 - compressive strength difference between carbonated and uncarbonated samples. The percentages are calculated as the difference in compressive strength (in MPa) between the corresponding carbonated and uncarbonated sample, divided by the compressive strength value for the uncarbonated sample.

[0099] The compressive strength levels of the samples including waste fibre cement powder from air-cured material are consistently lower than that of the reference. As can be most easily seen from Table 10, the decrease in strength increases with the amount of added material, both for the carbonated and the uncarbonated material. As can be most easily seen from Table 11, the addition of carbonated waste fibre cement powder from air-cured material provides gives better result than the addition of uncarbonated material. The difference seems to biggest after 7 days for the 10% and 20% addition, after which it decreases again. For the 5% addition, the same pattern is visible, except that the biggest improvement in strength is seen after 3 days. When comparing the values of Table 10 and Table 7, it appears that the decrease in strength is even bigger for the addition of uncarbonated waste fibre cement powder from air-cured material than for uncarbonated waste fibre cement powder from autoclave-cured material. The abnormal pattern found for the addition of 20% carbonated waste fibre cement powder from autoclave-cured material is not recognized for the addition of 20% carbonated waste fibre cement powder from air-cured material. Apparently, in the samples comprising waste fibre cement powder from air-cured material, the large content of CaCO3 gives rise to a filler-like behaviour. In fact, as can be seen from Table 10, the loss in compressive strength after 28 days is quite linearly dependent on the amount of addition of carbonated material.

[0100] Example 10 - flexural strength development

[0101] MPa

[0102] MPa Table 10 and 11 show resulting flexural strength data for the reference in MPa and for the carbonated and uncarbonated samples. Table 10 relates to the addition of (waste fibre cement powder from) autoclaved material, Table 11 relates to the addition of (waste fibre cement powder from) air-cured material. It is apparent from the data that the addition of carbonated autoclaved material provides the best results. The flexural strength for the addition of 5% and 10% is very close to the reference value. As it was seen for the compressive strength, the flexural strength values for the 20% addition improve with age, but at 28 days the strength is still less than that of the reference. The data on flexural strength have larger spread than those for compressive strength. The behaviour found for flexural strength seems generally in line with the behaviour found for compressive strength.

[0103] Example 11

[0104] A further test sample was prepared from calcium silicate boards, which are in use as fire-resistant building boards. Sample 3 was obtained from a calcium silicate board manufacturing in a Hatschek process followed by autoclave-curing and comprising 4% by weight of cellulose fibers, wherein the weight% is based on total dry weight of the composition. The density of the calcium silicate board was 870 kg / m3when measured after drying at 105°C. The cured sheets were cut in smaller pieces, after which the pieces were milled with a cutting mill (Retsch SM300 with a 500 pm sieve, and thereafter milling with a Retsch disk mill RS200 for 2 minutes) to obtain a powdered material. The material was sieved through a sieve of 100 pm. Carbonation was performed batch wise as specified in Example 2 at a temperature of 60°C, relative humidity of 90%, CO2-concentration of 18 % by volume and a water / solid mass ratio of 0.35 at the start of carbonation. After carbonation, the samples were dried at 105°C for 24 hours and milled with a Retch disk mill RS200 for 30 seconds.

[0105] The mineral composition before and after carbonation was determined by XRF. Loss on Ignition (LOI) was determined by heating to 1000°C. Results are shown in Table 11.

[0106] Table 11 - XRF and LOI results for compositions 1, 2 and 3.

[0107] Table 11 shows that the composition of the calcium silicate board is comparable to the autoclave- cured fiber cement board, however with more CaO and less SiO2. Further characterization tests were done to determine the total organic content (TOCO, total inorganic content (TIC), CO2 content, density, specific surface area (BET) and tobermorite content. Methods were used in accordance with examples 3-6. The density was determined by pycnometry. Tobermorite content was determined by XRD. Results are shown in Table 12. Results for the powder from air-cured fiber cement is put to the right hand of the table.

[0108]

[0109] Table 12 - characterization of samples

[0110] The total inorganic content of the calcium silicate boards lies between the values for the autoclave- cured and the air-cured fiber cement materials. The increase in TIC after carbonation is however less than in the air-cured fiber cement material. This seems consistent with the lower content of CaO in the material, as shown in Table 11.

[0111] The density of the material decreases upon carbonation in the same manner as for the autoclave-cured fiber cement material. This is opposed to the comparatively large increase in density of the air-cured material. The latter increase in density for the air-cured material is believed to be due to conversion of amorphous calcium silicate hydrate gel into calcium carbonate. In the calcium silicate material, the tobermorite content has decreased from more than 20% to 0%. It is believed that the tobermorite is converted into a calcium silicate gel and into calcium carbonate.

[0112] The specific surface area as measured in accordance with the BET isotherm using nitrogen adsorption increases. The increase for sample 3 is somewhat less than for the sample 1, both autoclave-cured materials (17 m2 / g versus 19 m2 / g). The increase in specific surface area for sample 3 is significantly less than for the air-cured fiber cement material (29 m2 / g). The resulting surface area of sample 3 also is lower, although it is nevertheless much bigger than the specific surface area of limestone and silica fume.

[0113] It can be concluded that the behaviour of sample 3 in carbonation corresponds to that of sample 1, especially with respect to changes in density, surface area and tobermorite content. Therefore, it is expected that the partial substitution of cement by sample 3 after carbonation results in a stronger pozzolanic effect and advantageous flexural and compressive strength, as seen for sample 1 in examples 8 and 10.

[0114] Example 12 - fiber content of the powders

[0115] Production waste from autoclave-cured fiber cement material with a density of 1.6 kg / dm3was recycled. Thereto, the material was first crushed into crushed material by means of a shredder followed by a hammer mill. The resulting material has a characteristic size from about 3 mm to 5 cm in diameter. The crushed material is transported to a micronizing apparatus. This apparatus comprises a vertical roller mill and an air classifier. The vertical roller mill is present in a milling chamber through which the ground material may flow to the air classifier. The milling chamber is heated by means of heated air flow to a temperature of approximately 70-90 °C. The roller mill is operated at comparatively low pressure so as to form agglomerates of inorganic powder and fibers. The heated air flow in the milling chamber leads to drying of the agglomerates. The thus dried agglomerates may fall apart into inorganic powder and fibers. The operation of the air classifier may further contribute thereto. The air classifier has a characteristic setting in the range of 100-150 pm, for instance 125pm. Ground material passing the air classifier is sieved over a sieve of for instance 150 pm. The sieve is effective to remove a significant portion of the fibers. Herewith, the organic content of the material was reduced from 3.03% to 2.01% as defined by organic carbon content. 1% organic carbon content corresponds to approximately 2.5% by weight of the composition. A particle size distribution before and after sieving is shown in Table 13.

[0116] Table 13 - particle size distribution of cementitious powder before and after sieving over a 150pm sieve.

[0117] Example 13

[0118] Test were performed to determine the average fiber length in samples after sieving through a 150pm sieve. This material comprising both fibers and inorganic powder was treated in a Resch Vibratory Sieve Shaker during 5 minutes. In the apparatus, sieves of 150 pm, 100 pm and 75 pm were used. On each sieve, a fibre-rich layer was generated on top of the inorganic powder. Thereafter, the fibre-rich layers were removed from the inorganic powder and were combined into one fiber sample. The removed fiber sample still comprises inorganic powder. This contamination is however sufficiently small so that it does not hamper or disturb operation of the measurement apparatus. It was observed that the fibre-rich layer was thickest on the 150 pm sieve and thinnest on the 75 pm sieve.

[0119] The average fiber length was thereafter determined for the separated fibres by means of a Valmet FS5 apparatus, which is a fiber image analyzer for automated fiber measurements. This apparatus comprises a camera using UHD (ultra-high density) image resolution, which is also marketed as 4K resolution having a resolution of 4096 x 2160 pixels. The fiber length is measured by the apparatus in accordance with the ISO 16065-2 standard, hence using unpolarized light.

[0120] The measured fiber length can be specified in different ways. A distinction is to be made between the arithmetic average length of fibre L(n), as can be obtained by optical microscopy, and the length weighted average fiber length L(i), as often used in the wood pulp industry. Herein: L(n) = I(ni *li) / (ni) (1)

[0121] L(i) = ([li*ni]*li) / [li* ni] (2).

[0122] The ratio of the two different average fiber length (L(i) / L(n) is the polydispersity, and a measure of the broadness of the length distribution.

[0123] The average fiber length was determined for the cementitious powders with compositions 1, 2 and 3 obtained after sieving through a 150pm sieve in accordance with Example 1 and Example 2 (Samples 1 and 2 respectively). It was furthermore determined for another cementitious powder obtained from autoclave-cured production waste (Sample 3) and for a reference sample. This reference sample was a fresh cellulose mixture for use in an autoclave-curable fiber cement composition. Results are shown in Table 14

[0124] Table 14

[0125] The sieved portions of sample 2 were analysed separately. Results are shown in Table 15

[0126] Table 15

[0127] From the results in Table 14 it can be derived that the fibers in the cementitious powder have an arithmetic average length L(n) in the range of 0.30 mm to 0.45 mm, a length weighted average length L(i) in the range of 0.35 mm to 0.50 mm, and a polydispersity L(i) / L(n) in the range of 1.10- 1.20. It is apparent that the average fiber length is well above the 150 pm sieving.

[0128] In comparison to the fresh fibers, the average length is clearly much shorter. In fact, taking account thereof that approximately one third of the fiber weight is removed and that the fiber length is reduced to 30-50%, such as 35-40% (0.33 / 0.88 = 0.375), the overall number of fibers in the cementitious powder per unit of mass will be larger than the number of fibers in the waste material (or the fresh material).

[0129] Furthermore, the polydispersity is reduced in comparison to a sample of fresh cellulose fibers. The low polydispersity is an indicator that the fibers have not been pulverized. Moreover, this low polydispersity is an indicator that the cementitious powder is useful for reuse into a variety of cementitious products such as fiber cement products, concrete, mortars and as in cement material. Due to the comparatively short length of the fibres, the cementitious powder is well dispersible.

[0130] Example 14

[0131] Fiber cement compositions were prepared that are configured for extrusion and subsequent aircuring (thus extrudable and air-curable). Carbonated fines and uncarbonated fines were obtained from recycling of fiber cement waste as produced in accordance with Example 12. In accordance with Example 12, the fiber cement waste was autoclave-cured waste with 2.0% organic carbon content, with a d50 of 17 pm. However, in a further trial use was made of air-cured waste, with merely 1.3% organic carbon content and a d50 of 22 pm. The Calcium carbonate was obtained from Carmeuse, Belgium (Calcitec 2001). The synthetic fibers were polyvinyl alcohol fibers with a diameter of 2 dtex, a length of 6 mm and a tenacity of 11.5 cN / dtex such as specified in EP2172434A1, which is herein included by reference. Cellulose ether was obtained from Dow under the trade name Walocel as suitable for cement based applications. Formulations are indicated in Table 16.

[0132] Water is added in an amount of 20-30%. The amount of water increases when including fines. More water is needed when using uncarbonated fines than when using carbonated fines. Process settings such as temperature, seem to have an effect on the amount of water needed, in line with well known observations that viscosity decreases with increasing emperature.

[0133] Extrusion occurs on a pilot line, using extrusion equipment comprising a mixer and an extruder. The extruder is a single screw extruder as supplied from Handle GmbH, Muhlacker, Germany. The extruder is provided with a die such that the extrudates are plate-shaped (85 x 22 mm crosssection). The resulting extrudates are put on a curing rack for curing or brought to an autoclave for curing. In the operation of the pilot line, pellets from the mixer are fed into the extruder manually. percentage based on dry weight of the composition. Water addition is 20% for the reference.

[0134] Example 15

[0135] Further extrudable and air-curable compositions were prepared that further include a source of slag, more specifically granulated ground blast furnace slag, as commercially available from Ecocem as a superfine grade. Same fibers and cellulose ether additive were added in the same amounts as in Example 14. The combination with slag material reduces the amount of Portland cement to a low amount of 25% or less, up to as little as 10%. This is highly beneficial to achieve a sustainable product, which can be produced cost-effectively.

[0136] Table 17 - cement formulations for extrusion followed by air-curing. All numbers as weight percentage based on dry weight of the composition. Water addition is 20% for the reference. Example 16

[0137] Further test compositions were prepared to be extruded and subsequently autoclave-cured. The formulations thereof are shown in Table 17. The use of water is increased, in view of the higher amount of fibers. Ingredients originate from the same sources as indicated in Example 14.

[0138] Table 18 - - cement formulations for extrusion followed by autoclave-curing. All numbers as weight percentage based on dry weight of the composition. Water addition is 27.5% for the reference. Thus, by means of adding carbonated cement powder to an extrudable fiber cement composition, it is feasible to obtain a product that has improved strength. It furthermore does not have significantly higher water demand for forming the paste, which is beneficial for the resulting porosity of the product and therewith the dimensional stability. This is opposed to extrudable fiber cement compositions comprising uncarbonated.

Claims

CLAIMS1. Method of manufacturing a fiber cement product, comprising the steps of:Providing a fiber cement paste comprising cement, fibers and water; Extruding the fiber cement paste to form a green article;Curing the green article to obtain the fiber cement product,Wherein the fiber cement paste further comprises a cementitious material obtainable by carbonating a cured cementitious material.

2. The method as claimed in claim 1, wherein the cementitious material is obtainable by carbonating a cured fiber cement material.

3. The method as claimed in any of the claims 1-2, wherein the cementitious material is a carbonated cement powder originating from recycling of cement products.

4. The method as claimed in claim 3, wherein the powder has a particle size distribution with a d50 in the range of 10-30pm, preferably in the range of 15-25pm, wherein said particle size distribution is measured by laser diffraction.

5. The method as claimed in claim 3 or 4, wherein the powder comprises fibers in an amount of at most 8% by weight based on dry weight of the powder, preferably at most 6% by weight, more preferably at most 5% by weight based on dry weight of the fiber cement paste.

6. The method as claimed in any of the claims 1-5, wherein the cementitious material is present in the fiber cement paste in an amount between 10% and 30%, based on dry weight of the fiber cement paste.

7. The method as claimed in any of the claims 1-6, especially claim 6, wherein the fiber cement paste further comprises a slag and the green article is cured by air-curing.

8. The method as claimed in claim 7, wherein the slag is present in an amount of 15-45% by weight, preferably 20-40% by weight based on dry weight of the fiber cement paste.

9. The method as claimed in claim 7 or 8, wherein the fiber cement paste further comprises calcium carbonate in addition to the cement, the cementitious material obtainable by carbonating a cured cementitious material and the slag.

10. The method as claimed in claim 9, wherein the amount of calcium carbonate is 30-50% by weight based on the dry weight of the fiber cement paste.

11. The method as claimed in any of the claims 7-10, wherein the cement is Portland cement, which is present in an amount of at most 30% by weight based on dry weight of the fiber cement paste, preferably at most 25% by weight, more preferably at most 20% by weight.

12. Fiber cement product obtainable by the method of any of the preceding claims.

13. Fiber cement paste comprising fibers, cement and water, further comprising a cementitious material obtainable by carbonating a cured cementitious material.

14. Fiber cement paste as claimed in claim 13, having a composition comprising:- Portland cement in an amount between 10 and 30 % by weight (wt%);- said cementitious material in an amount between 10 and 30 wt%;- slag in an amount between 10 to 40 wt%;- calcium carbonate in an amount between 30 to 50 wt%, Wherein said amounts of cement, cementitious material, slag and calcium carbonate are based on dry weight of the fiber cement paste.

15. Fiber cement paste as claimed in claim 13, having a composition comprising:- Portland cement in an amount between 30 and 50 % by weight (wt%) - said cementitious material in an amount between 10 and 30 wt%;- silicium dioxide in an amount between 30 to 50 wt%,Wherein said amounts of cement, cementitious material and silicium dioxide are based on dry weight of the fiber cement paste.

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