Method of manufacturing a fiber cement sheet

Abstract

The method of manufacturing of a fiber cement product comprising the steps of: (1) providing an aqueous slurry of a fiber cement composition at least comprising fibers and cement, (2) manufacturing a fiber cement green sheet by means of a Hatschek fiber cement production process from the aqueous slurry, and (3) Carbonation curing the fiber cement green sheet Herein, the cement comprises first cement phases chosen from the group of CS (CaSiO3), C2S (Ca2SiO4), C3S2 (Ca3Si2O7) and combinations thereof, and second cement phases chosen from the groups of C3S (Ca3SiO5), C3A (Ca3Al2O6) and C3AF (Ca4AlFe2O10). The fraction of the second cement phase in the cement is 2-50% by weight, preferably 3-30% by weight, more preferably 5-20% by weight based on the dry weight of the cement.

Description

[0001] Method of manufacturing a fiber cement sheet

[0002] FIELD OF THE INVENTION

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

[0004] Providing an aqueous slurry of a fiber cement composition at least comprising fibers and cement,

[0005] Manufacturing a fiber cement green sheet by means of a Hatschek fiber cement production process from the aqueous slurry, and curing the fiber cement green sheet.

[0006] The invention further relates to a fiber cement composition and to a fiber cement sheet obtainable by the method.

[0007] BACKGROUND OF THE INVENTION

[0008] Shaped fibre-cement products are manufactured starting from an aqueous slurry having a fibre cement composition comprising one or more hydraulic binders, reinforcing and processing fibres, and possibly fillers and additives. The hydraulic binder is herein typically Portland cement, more precisely Portland CEM I cement. Since the discovery of the cancerous effects of asbestos fibres, the processing fibre is typically cellulose and the reinforcing fibre is either cellulose or a synthetic fibre such as polyvinyl alcohol or polypropylene.

[0009] This aqueous suspension is mixed in order to obtain a uniform distribution of the components. This suspension is then dewatered. The so obtained green fresh product can be shaped into a flat sheet, a corrugated sheet or a tube. The green shaped product is then hardened under atmospheric conditions or under specific pressure, temperature and humidity conditions. The common hardening procedures are either air-curing (curing under atmospheric conditions) or autoclave-curing (steam curing under pressure at temperatures above 100°C). The fibre cement composition is different for air-curing than for autoclave-curing.

[0010] The Hatschek process is most widely known fiber cement production process for the manufacturing of fibre-cement products. This process, initially applied to the asbestos-cement, is exhaustively described in the book " Asbestzement" by Harald Klos (Springer Verlag, 1967 ). Other manufacturing processes known by the man skilled in the art which can be cited are Magnani, Mazza, Flow-on, extrusion and injection. The Hatschek process, particularly suited for the manufacture of flat sheets, corrugated sheets and tubes, is based on the use of a dewatering cylindrical sieve. In this way, a layer originating from a diluted suspension of fibres, cement, fillers and additives contained in a vat is transferred to a felt, through a cylindrical sieve; this layer is then enrolled on a forming drum until the required thickness of the sheet is obtained.

[0011] The fibre-cement sheet shaped on the forming drum is cut and removed from the drum, once the desired thickness is obtained. This sheet is then subjected to a forming step, which in the case of corrugated sheets consists of putting it between corrugated templates, for instance metal corrugated templates which have been oiled. Afterwards, it is submitted to a hardening stage at room temperature (air curing). For certain applications the green product is compressed between the forming and the curing stage (post- compression). We call these pressed fibre-cement products opposed to non-pressed fibre-cement products. The pressed fibre-cement products are compressed at pressures equal to or higher than 4.9 MPa. In order to obtain high-density products with a density of at least 1.5 kg / dm3, the applied pressure during compression is typically at least 20 MPa.

[0012] In recent years, efforts are made towards more sustainable cement products. Initiatives have been launched to produce cement by means of carbonation curing. Herein, the conventional curing of cement by means of hydration is replaced through hardening in a CO2-enriched environment. The CO2reacts with water (in the theoretical form of H2CO3or HCO3_to carbonate the calcium in cement. The remaining silicate or calcium silicate is transformed into a gel. As with the conventional aircuring, such (calcium) silicate hydrate (CSH) gel contributes to the strength of the material. It is herein observed that carbonation curing is distinct from a carbonation treatment that may be applied on fibre cement after conventional curing, such as for instance taught in W02020 / 099597A1. The carbonation treatment is performed in order to accelerate ageing, which otherwise would occur during the lifetime of the cured cement material. Typically, such ageing results in strength loss of the fiber cement material due to decomposition of the CSH-material.

[0013] Evidently, in order to ensure that the correct reactions occur, it is beneficial to optimize the cement composition thereto. Cement is neither a chemical element nor a pure chemical material or salt, such as quartz (SiO2) or calcium carbonate (CaCO3). Rather cement is a mixture of several cement phases with mutually different compositions. Portland cement is very rich in a phase with composition Ca3SiO5orC3S in cement chemist notation. This C3S-phase is known as alite, and reacts quickly in hydratation curing.

[0014] Cement material configured for carbonation curing have been proposed by Solidia Techologies, and are disclosed in W02016154021A1, WO2016154024A1, and the publication of S. Sada and R. Meiniger, 'Performance of CO2-reducing cement based on calcium silicates', 15th International congress on Chemistry of Cement (ICCC), Sept 16-20, 2019 (available at the website of Solidia Technologies). The disclosed cement materials comprise CS-phases, C2S-phases and C3S2 phases, and more specifically wollastonite, pseudo-wollastonite, belite and rankinite. Further phases such as mellites, brownmillerite and amorphous material is also disclosed to be included in such cement materials. The CS-phase is present in amounts ranging from 15 to 51% by weight, the C2S- phase is present in amounts in the range of 1 to 14% by weight, the C3S2-phase in an amount of 0 to 19% and the amorphous phase from 23 up to 30% and in one example even 68% by weight. These phases are stated to be reactive in said patent publications, with exception of the amorphous phase that would be partially reactive. The C3S-phase is absent. It has been explained in literature, that the C3S-phase would generate CaCO3-crystals growing on the surface of the particles in a manner so as to hinder or block further carbonation of the particle. This quick growth of CaCO3 could for instance be due to early hydration.

[0015] It would clearly be useful to generate fibre cement products based on such carbonatable material and cured by carbonation curing. Still, fiber cement products need to meet requirements of their use, particularly as decorative building materials for the exterior of buildings, such as roofing, fagade, decking (terrace), and as building boards in the interior, for instance in wet areas. Particularly, it was observed in preliminary experiments with fiber cement compositions comprising carbonatable cement, that a compression treatment, such as at 25 MPa, leads to deformation of the sheets. Moreover, a difference in porosity of up to 10% was observed between measurements performed in air-dry conditions and water-saturated conditions. Such a difference indicates that the material may expand significantly when exposed to humidity such as rain, and thus be susceptible to impact of water, typically resulting in low resistance against free-thawing and / or bending problems of the material after exposure to sun and humidity.

[0016] SUMMARY OF THE INVENTION

[0017] It is therefore a first object of the invention to provide a method of manufacturing a fiber cement sheet that comprises the step of carbonation curing and which has improved performance.

[0018] It is a second object of the invention to provide a fiber cement composition for use in such method. It is a further object to provide fiber cement sheets obtainable in such a process.

[0019] According to a first aspect, the invention provides a method of manufacturing of a fiber cement product comprising the steps of: (1) providing an aqueous slurry of a fiber cement composition at least comprising fibers and cement, (2) manufacturing a fiber cement green sheet by means of a Hatschek fiber cement production process from the aqueous slurry, and (3) carbonation curing the fiber cement green sheet. Herein, the cement comprises first cement phases chosen from the group of CS (CaSiO3), C2S (Ca2SiO4), C3S2 (Ca3Si2O7) and combinations thereof, and second cement phases chosen from the groups of C3S (Ca3SiO5), C3A (Ca3AI2O6) and C3AF (Ca4AIFe2O10), and wherein the fraction of the second cement phase in the cement is 3-30% by weight, preferably 5- 20% by weight based on dry weight of the cement.

[0020] According to a second aspect, the invention provides a fiber cement composition comprising fibers and cement, wherein the cement comprises first cement phases chosen from the group of CS (CaSiO3), C2S (Ca2SiO4), C3S2 (Ca3Si2O7) and combinations thereof, and second cement phases chosen from the groups of C3S (Ca3SiO5), C3A (Ca3AI2O6) and C3AF (Ca4AIFe2O10), and wherein the fraction of the second cement phase in the cement is 3-30% by weight, preferably 5-20% by weight based on dry weight of the cement. This fiber cement composition for use in the method is thus a carbonation-curable fiber cement composition.

[0021] According to further aspects, the invention provides a fiber cement sheet obtainable or obtained from the said manufacturing, as well as a fiber cement product comprising such a sheet.

[0022] It has been found by the inventor in investigations, that a minor addition of such second cement phases, such as predominantly present in Portland cement, may improve a fiber cement product made with the Hatschek process. The minor addition is an amount of up to 30 weight% based on dry weight of the cement. It is believed that the second cement phases contribute to formation of a better fiber cement product, as a consequence of the layer-wise manufacture in the Hatschek process, as hereinabove specified, with the ongoing densification due to continuous water removal during the process, i.e. on the sieve, on the felt and further during transfer to the accumulator roll. It is believed that the processing of the aqueous slurry in the Hatschek process contributes to a microstructure that is better suited for carbonation curing. In fact, experiments have been done that show that the total CO2 uptake of fiber cement compositions according to the invention is higher than with prior art carbonatable cement only. This CO2 uptake is considered as a representative measure of the degree of carbonation curing.

[0023] In an advantageous embodiment, the fraction of the first cement phase in the cement is 50- 95% by weight, preferably 60-80% by weight based on dry weight of the cement. This has the benefit of thorough carbonation curing. In a further embodiment, the total amount of the first and the second cement phase in the cement is at least 90% by weight, preferably at least 95% by weight, more preferably at least 98% by weight, or even 100% by weight. This would exclude any further phases and unknown effect on the resulting product. In a further embodiment, wherein the second cement phase comprises for at least 75% by weight alite (C3S) based on dry weight of the second cement phase. Alite is known as a cement phase in Portland cement, which cures relatively quickly by means of hydratation. The benefit of using Alite is not fully understood, but it may be that the alite may involve in hydratation reactions already during the preparation of the green fiber cement sheet and directly thereafter. In fact, the Hatschek fiber cement manufacturing process is characterized by a very fine distribution of cement in an aqueous layer on a felt, which is gradually dewatered. As a consequence, hydratation and other processes are not excluded to occur already during this process.

[0024] The cement comprising the first and the second cement phases may be prepared by mixing appropriate quantities of raw materials. Alternatively, the cement may be obtained by mixing or blending two or more separately prepared cement materials. Preferably, it is prepared by mixing or blending a carbonatable calcium silicate cement and a Portland cement. Such mixing or blending may occur in dry form (blending), or also in wet form (mixing). Such wet form mixing typically occurs when preparing the fiber cement composition. At that stage, the said carbonatable calcium silicate cement and Portland cement may be combined jointly or one after the other with the water or aqueous solvent of the aqueous slurry. The ratio of blending or mixing of carbonatable calcium silicate cement and Portland cement is for instance in the range of 19 (95:5) to 3 (75:25). A preferred range is from 8 to 4. A lower ratio, such as from 6 to 4 may be even more beneficial.

[0025] In principle, any type of Portland cement can be used as an ingredient to arrive at the cement comprising the first and the second cement phases, such as type I, type II, type III or even type IV or type V. Since, the Portland cement constitutes an ingredient, the amounts can be calculated to arrive at an appropriate composition. Preferably, Portland cement type I is used, also known as CEM-I. Exemplary implementations are for instance CEM-1 42.5 N, CEM-I 52.5 N, as known to the skilled person in the art.

[0026] Preferably, the fiber cement composition comprises at least 50% by weight of cement, for instance between 60% and 90% by weight of cement, based on the total dry weight of the fiber cement composition. Good results have been achieved with fiber cement compositions comprising between 70% and 85% by weight of cement. The preferred amounts of cement as specified herein relate to the sum of the first cement phase and the second cement phase, and further cement phases if any. As opposed to a concrete composition, the fiber cement composition is preferably free of aggregate. As opposed to fiber cement compositions for curing in an autoclave-reactor, under high pressure in an atmosphere comprising steam, no reaction between silica and cement is required to take place in carbonation curing. Hence, there is no need for the addition of quartz or other silica source in significant quantities of typically at least 25% by weight based on the fiber cement composition.

[0027] In a further embodiment, the fiber cement composition further comprises calcium carbonate, preferably in an amount of 5-30% based on dry weight of the fiber cement composition, such as 10-20% by weight based on dry weight of the fiber cement composition. It is believed that calcium carbonate is useful for acceleration of the carbonation curing. The calcium carbonate particles may act as nuclei for subsequent formation of calcium carbonate through carbonation curing.

[0028] In a further embodiment, the cement and the preferably used calcium carbonate are chosen so as to arrive at particle size distribution with a mean particle size (d50 value) of at least 10 pm, preferably at least 12 pm or even at least 15 pm. Carbonatable calcium silicate cement may have a mean particle size (d50) below 10 pm. By addition of Portland cement and calcium carbonate, this mean particle size may be increased. Preferably, a Portland cement is added with a mean particle size in the range of 15-25 pm, such as 17-20 pm. The calcium carbonate is preferably chosen to be comparatively coarse, having a mean particle size in the range of 15-25 pm, more preferably 20-25 pm. For sake of clarity, the mean particle size is defined as the d50 value as measured by means of particle size distribution methods, especially by means of laser diffraction measurement systems.

[0029] In again a further embodiment, the fiber cement composition further comprises amorphous silica, preferably in an amount of 1-10% based on dry weight of the fibre cement composition. Amorphous silica has been found to improve mechanical properties of the material, either due to its small particle size or due to its pozzolanic character.

[0030] The fiber cement composition preferably comprises 2 to 15% by weight of fibers, more preferably 4 to 10% by weight, based on the dry weight of the fiber cement composition. As known to the skilled person, fibers in fiber cement serve as processing fibers, so as to enable processing of the aqueous slurry in the Hatschek process, and / or serve as reinforcement fibers, providing strength to the resulting product. As carbonation curing does not affect stability of the fibers, the selection of fibers is not limited by stability considerations, such as with fibers used for compositions to be cured in autoclave-curing.

[0031] In an implementation, the fibers comprise organic fibers, such as cellulose fibers and / or synthetic fibers. The cellulose fibers are preferably based on wood pulp, and may be any softwood or hardwood pulp or a combination thereof, and preferably is a chemical wood pulp such as kraft pulp or sulfite wood pulp. The chemical wood pulp may be bleached or unbleached or a mixture of both. Preferably, use is made of Kraft pulp. The synthetic fibers are preferably chosen from the group of polyvinyl alcohol fibers, polyacrylonitrile fibers, polypropylene fibers and / or combinations thereof. In one implementation, polyvinyl alcohol fibers are used, preferably with a titre between 1.5 and 9.0 dtex, such as 2.0-7.0 dtex and with a length from 4 to 10 mm, such as 6-8 mm.

[0032] Good results have been achieved with compositions in which cellulose fibers had a SR fineness in the range of 10 to 40, preferably 20 to 30, such as 25. The SR fineness is the Schopper- Riegler fineness as measured in accordance with ISO5267-1: 1999. This value indicates a relatively low degree of milling of the fibers. More preferably, this type of cellulose fibers was present in the composition in an amount between 5 and 12% by weight, such as between 7 and 10% by weight, as based on the total dry weight of the fiber cement composition. The resulting sheets had highest compressive strength, particularly when measured under air-dry conditions. In one implementation, the cellulose fibers are the only fibers in the fiber cement composition. In other implementations, the cellulose fibers may be combined with another fibre, such as a synthetic fiber and / or an inorganic fibers.

[0033] In another implementation, synthetic fibers were used. When using synthetic fibers, it is deemed preferable to combine these with cellulose fibers as processing fibers. For instance, cellulose fibers with an SR fineness in the range of 40-70, preferably 50-60 can be used as processing fibers.

[0034] In another or further implementation, the fibers comprise inorganic fibers, such as basalt fibers, glass fibers or the like. Particularly preferred as alkaline-resistant glass fibers. Such fibers are preferably used as reinforcing fibers in combination with cellulose fibers as process fibers. The advantage of such fibres is that they do not contain organic material, and hence the resulting sheet has a low organic content, resulting in a beneficial fire resistance performance.

[0035] In another embodiment, the method further comprises the step of compressing the green sheets at a pressure in the range of 10 to 30 MPa prior to carbonation curing. The compression leads to lower porosity, lower water content, higher strength and better dimensional stability. In view of the comparatively high water absorption of fiber cement products comprising carbonatable cement, as compared to corresponding conventional products (with merely Portland cement, a compression step is deemed very advantageous for the resulting properties. The preferred pressure is at least 20 MPa. Best results were obtained with pressures above 22 MPa, such as above 24 MPa. It is believed that the use of the fiber cement compositions of the inventions is beneficial to perform such a compression step before carbonation curing. The early immanent curing of the second cement phase will facilitate handling, which is of importance for the compression, that is conventionally performed by means of a stack press.

[0036] In again a further embodiment, the method further comprises the step of curing by hydratation after carbonation curing. Such curing by hydratation preferably is performed in humid conditions at elevated temperatures, for instance from 40 to 200°C and preferably from 50 to 100°C. A suitable duration is 4 to 24 hours and a preferred duration is 6 to 12 hours. Such additional hydratation curing may further increase strength and properties of the products.

[0037] The resulting fiber cement sheet as obtainable with the method of the invention may have a porosity of less than 15%, preferably less than 14% or even less than 13%, when measured under water-saturated conditions. As porosity has an impact on water absorption, a low porosity is highly preferred. In addition to lower overall porosity, finer pore sizes seem generated. The inventor believes that the lower (and finer) porosity is due to early formation of an amorphous calciumsilicate hydrate (CSH)-gel due to hydratation curing of the second phase. As such, the lower porosity has a beneficial impact on water permeability. Even though the CSH-gel may be further converted during carbonation curing, the initial microstructure is believed to persist at least partially in the resulting product. For instance, the CSH-gel may lead to an other gel. It is against this background that the presence of the second phase is limited to a maximum of 30% by weight of the cement. The inventor has understood that if too much CSH-gel is formed, this may have delaying effects on distribution of humidity, and thus also on removal of humidity from the green sheet, implying loss of stability and reduced handling of the green sheet.

[0038] By further preference, the sheet has a density of in the range of 1.7-2.2 kg / dm3, preferably 1.8-2.0 kg / dm3. Sheets with such a high density can be used as covers in building applications, such as for facades (in the form of panels and / or planks known as sidings) and / or for roofing (in the form of slates). Use in the form of corrugated sheets is not excluded.

[0039] The sheets may be coated with one or more coatings, as known to the person skilled in the art. Alternatively, the fiber cement compositions may comprise one or more pigment so as to achieve mass-coloring. The application of a coating, such as a transparent or translucent coating on top of a mass-coloured fiber cement sheet is not excluded. Further additives may be applied for sake of protection. Such further additives include hydrophobation agents, typically silanes, water repelling agents or waxes, biocides, processing aids, surfactants and the like. The fiber cement composition may be further modified according to known practices, for instance to add a low- density modifier such as perlite or microspheres.

[0040] For sake of clarity, the term 'green sheet' is used herein to refer to a cementitious sheet which has not been subjected to a curing treatment, and wherein no or no significant strength development has occurred. Curing of cement may generally be hydratation curing under atmospheric conditions, autoclave-curing and carbonation curing. Hydratation curing under atmospheric conditions is also known as air-curing. Time is required for strength development, which time may be one week up to several weeks. The strength development originates from the formation of amorphous calcium silicate hydrates due to a hydratation reaction of Portland cement. Autoclave-curing is a steam curing treatment under higher temperature (of above 100°C) and higher pressure. A reaction occurs between SiO2 and a source of calcium, typically a cement, to obtain crystalline calcium silicate hydrates, such as tobermorite. Carbonation curing involves a curing reaction of cement with COZ. Portland cement is not well suited for carbonation curing. Therefore, specific CO2-curable cement compositions have been developed. Carbonation curing should be distinguished from a carbonation treatment after full hydratation curing. Such a carbonation treatment may be applied to mimic or accelerate ageing, which otherwise occurs during use of cement panels. As mentioned hereinabove, it cannot be excluded that a hydratation reaction would occur when processing the cement composition of the present technology during the Hatschek process and / or directly thereafter. This occurrence of a hydratation reaction cannot be equated with hydratation curing. Its duration and the amount of hydratable cement phases within the cement composition of the present technology do not allow strength development typical for curing.

[0041] The term 'fiber cement product' herein refers to the ultimate product, which may be a coated sheet and may further comprise neans for assembly of the sheet to a support

[0042] INTRODUCTION OF FIGURES AND EXAMPLES

[0043] These and other aspects of the method, the fiber cement composition and the fiber cement sheet of the invention will be further elucidated with reference to the Figures and the Examples, wherein:

[0044] Fig. 1 is a graph showing the CO2-uptake for fiber cement sheets made with different fiber cement compositions comprising cellulose fibers as the only fibers. A series of individual values is shown, and

[0045] Fig 2 is a graph showing the CO2-uptake for fiber cement sheets made with different fiber cement compositions comprising cellulose fibers and polyvinyl alcohol fibers. A series of individual values is shown.

[0046] Example 1

[0047] Tests were performed based on compositions shown in Table 1. The Polyvinyl alcohol fiber had a thickness of 2 dtex and a strength of 11-13 CN / dtex, and a length of 6 mm. The cellulose fiber was a mixture of chemical wood pulp, more particularly Kraft pulp with a length of at least 1.5 mm, when measured using Valmet FS5 fiber analysis apparatus. The cellulose fiber was refined to a fineness of about 10 to 30 for samples 1 and 2, and to a fineness of 40 to 70, such as 55SR for samples 3 and 4. The CEM I 52 5 was European Portland cement. It had a conventional particle size distribution with its peak (in volume%) at 30 pm. The limestone had a particle size distribution with 5 wt% smaller than 2 pm, 65wt% between 2 pm and 60 pm and 29% larger than 60 pm. The d50 (median) was 22.3 pm. The carbonatable calcium silicate cement was supplied by Solidia Technologies, Piscataway,, USA under the tradename Solidia Cement™. It had a dlO of 1.52 pm, a d50 of 14.56 pm and a d90 of 44.30 pm, according to S. Sada and R. Meiniger, 'Performance of CO2-reducing cement based on calcium silicates', 15thInternational congress on Chemistry of Cement (ICCC), Sept 16-20, 2019.

[0048] Table 1 - compositions tested The cement clinker phase composition of Solidia Cement™, as measured by XRD, is given in Table 6 in Sada and Meiniger (see above). The cement composition of CEM I 52 5 as given in Table 1 of B. Larbi et al, EPJ Web of Conferences 56, 01006 (2013), and comprises 65.3% alite (C3S), 18.2% belite (C2S), 4.4% aluminate (C3A) and 7.1% cellite (C3AF). Based thereon, the tested samples had the phase compositions as specified in Table 2. It is apparent from Table 2, that the main difference between sample 1 without Portland cement and the samples with 10% Portland cement resides in the presence of the C3S (alite) phase, with an additional minor change in the C2S phase.

[0049] Table 2 - cement phase compositions for cement in the mixtures according to Table 1 (additives such as limestone and clay are not included)

[0050] Example 2

[0051] Fiber cement products were manufactured in according with the Hatschek process on a pilot line. Use was made of the compositions shown in Table 1. Products were produced with a total of 20 / 30 ? monolayers, with a specified monolayer thickness of 0.34 mm, and a density of 1,25 - 1,35 g / cc for the samples 1 and 2, and a density of 1.30-1.40 g / cc for samples 3 and 4. The green sheets obtained from the pilot line were compressed with a pressure of 26 MPa to obtain high-density products. Thereafter, the pressed green sheets were installed in the carbonation curing device for 18 hours. In this device the humidity content of the FC samples was first brought down to an appropriate level for carbonation, after which the curing air was monitored for temperature and CO2 concentration, at level of ca. 60°C resp. 90-100 % CO2. Mechanical tests were performed on the fiber cement products made. The modulus of rupture (SMOR, typically expressed in Pa = kg / m2.s) he E-modulus (also referred to as Young's modulus or elastic modulus) of each of the samples was measured by making use of a UTS / INSTRON apparatus (type 3345, cel = 5000N). The limit of elastic deformation and the stress at the limit of elastic deformation (dMOR, iMOR) were measured by making use of a UTS / INSTRON apparatus (type 3345, cel = 5000N) with software Bluehill by Instron (Norm EN12467 and related norms).

[0052] The mechanical tests were performed under saturated conditions. Herein, the samples received a pretreatment of submerging in a water bath overnight. All measurements were performed on sample material of 30 x 30 cm in size. Each reported value in Table 3 is an average of measurements made on three individual sample materials, and then performed fourfold (thus 12 measurements in total). For E-modulus, SMOR, iMOR, dMOR measurements were done in the parallel and the perpendicular direction. The averages for both directions (i.e of 12 measurements) were then combined in one average.

[0053] Table 3 mechanical test results as measured under saturated conditions

[0054] It is apparent from the test results that the addition of some Portland cement to the carbonatable calcium silicate cement leads to a reduction in the porosity, and an increase in both the elastic modulus and the flexural strength of the samples. The porosity is lowest for the samples comprising synthetic fibers, which however also demonstrate lower values for the limit of elastic deformation and the stress at that limit (dMOR, iMOR). In view of the difference in phase composition between sample 1 (reference) and the other samples 2-4, the presence of the C3S-phase (alite) is apparently beneficial.

[0055] This is a surprising result, as the C3S-phase is known and understood to be quickly hydratable, but inert with respect to carbonation curing or rather even negative. According to Groves et al, Adv in Cement Research 3 (1990), 117-125, C3S paste will decompose into a CH-phase and an CSH (Calcium Silicate Hydrate gel)-phase. The result would be that the crystalline calcium carbonate (calcite) layers would cover the CSH-structure and therewith prevent the CSH from being fully carbonated. It is furthermore observed that some early and quick hydration of the C3S phase leads to the same result. Herein, early hydratation results in formation of CSH-material on the surface of C3S-particles limiting ingress of CO2 and thus hindering carbonation of the material.

[0056] In contrast, the addition of clay to the carbonatable cement did not provide in any major improvement or change as compared to the reference. This holds true both for the sample with cellulose fibers (sample 5) and with synthetic fibers (sample 6). The sample with synthetic fibers turns out to have a higher elastic modulus, and also lower values for the limit of elastic deformation and the stress at that limit.

[0057] For sake of clarity, it is observed that the measurement under saturated conditions does not have a major impact on the porosity or density. Generally, E-modulus and SMOR are higher when measured in air-dry conditions, and IMOR and DMOR are lower. Table 4 provides a comparison between the values as measured under air-dry condition (after overnight drying at 105°C) and water-saturated conditions. The samples have been ordered in the Table, for best comparison. Herein, samples 1, 5 and 2 contain cellulose fibers as the only fibers, in an amount of 8% dry weight. Samples 6, 3 and 4 contain cellulose fibers and polyvinyl alcohol fibers in a total amount of 5.5% by weight.

[0058] Table 4 - comparison of measurement results as measured under air-dry and water-saturated conditions. For sample 1, no measurements under air-dry conditions are available.

[0059] The measurement condition does not have a significant impact on the density, for any of the samples. The porosity is generally lower when measured in air-dry conditions. The effect on the porosity is the biggest for the comparative samples that include 5% kaolin. A major effect of the measurement method on porosity was also observed in other preliminary experiments in which the reference composition was used.

[0060] The thickness of the samples is much more dependent on the measurement method for the samples containing polyvinyl alcohol fibers than those containing merely cellulose fibers.

[0061] The elastic modulus and the flexural strength (SMOR) are both higher when measured under air-dry conditions than under water-saturated conditions. The pattern observed and discussed for the measurement under water-saturated conditions apply to air-dry conditions as well. Again, the fibers in the samples have an impact on the results, particularly with respect to the flexural strength (SMOR). Example 3

[0062] Further tests were performed to understand the hydric movement and the water content. Samples are prepared by cutting two samples from a plate. Each of the samples has dimensions of 300 mm x 300 mm. Per sample, four measurement points are created located in a virtual square and at a mutual distance of 250 mm. This occurs by drilling a cavity in the sample plates to a depth of 4 mm and with a diameter of 3.5 mm. Copper sensing elements are attached by means of an epoxy glue. The thus created samples are conditioned in a conditioning room at room temperature (23°C) with relative humidity of 50% ± 5% during at least 7 days. The distances between the four measurement points per sample are measured. This results in distance Lc.

[0063] Subsequently, three steps are performed on the first sample (or a plurality of first samples):

[0064] 1 drying the sample, following by measurement using a dilation meter. For the drying, the samples are put into an oven at 105°C, until weight does no longer vary. For the measurement, the first sample is taken out of the oven and the distances between the four measurement points is measured using a dilation meter. This results in distance Lw

[0065] 2 Cooling down the sample to room temperature. Thereto, the samples are put under vacuum or in a closed plastic bag and cooled down during two hours to room temperature, after which the first sample is again measured with the dilation meter. This results in distance LD

[0066] 3 Placing the first sample into a water bath at room temperature, until no further weight changes are seen. At that stage, the sample is taken out of the water bath, dried with a towel and measured again. This results to distance LN

[0067] The second sample is treated correspondingly, except that step 3 precedes step 1 and step 2. The thermal movement dLTis calculated as dLT= (LW-LD) / LDin 10-6mm / m°C. The hydric expansion (in mm / m) is calculated for the first sample as dLDN = (LN-LD) / LDand for the second sample as dLcN = (LN- Lc) / Lc. The hydric shrinkage (in mm / m) is calculated for the first sample as dLco = (LD-Lc) / Lc and for the second sample as dLNo = (LD-LN) / LN.

[0068] The hydric shrinkage (in mm / m) is reported in Table 5. This is the value obtained during a second cycle of drying and saturating. Table 5 further reports the water content at saturation. This content is expressed as a percentage relative to the weight after drying in an oven at 105°C.

[0069] Table 5 - hydric movement and water content for samples 1-5 It is observed that these values are very good, when compared with conventional products, and especially in comparison to air-cured products. It is believed that this can be explained in that the cement in products obtained by carbonation curing is present - primarily or predominantly - in crystalline phases rather than in an amorphous phase, such as the amorphous calcium silicate hydrate (CSH) gel, as in air-cured products. This amorphous CSH-phase will considerably shrink and expand in view of hydric movement. This behaviour is understood to be due to the presence of large quantities of fine capillaries and the water transport therein. It is deemed surprising that the hydric movement of the samples according to the invention is not larger than that of the reference samples. One would expect that addition of Portland cement leads to formation of an amorphous CSH-phase, and hence to larger hydric movement.

[0070] Example 4

[0071] The CO2-content Y in a sample can be derived from the total inorganic content (TIC), by multiplying the TIC with the ratio of molar mass of CO2to C (i.e. 3.667), i.e.:

[0072] Y = TIC x 3.667 = (TC- TOC) x 3.667 (1)

[0073] 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 samples 1-6. 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 CO2degree as determined from XRF.

[0074] The CO2uptake Z as a weight percentage of the product is derived from the CO2-content in a sample, by subtracting the carbonate content initially present due to the CaCO3in the mixture (15mass%) (Yo), and by dividing over the remainder of the composition excluding the CO2:

[0075] Z = (Y - Yo) x (100 / 100-Y) (2)

[0076] In order to compare the data, one may take into account the mass fraction a of carbonatable calcium silicate cement in the formulation. This is performed by:

[0077] Z* = Z / a (3)

[0078] This analysis was performed for several samples of a composition subjected to carbonation curing, and additionally to two samples before pressing and before carbonation. Table 5 provides averages of the samples after carbonation curing. Herein, apparent outliers were not included in the average calculation. A graph for the individual samples with only cellulose fibers is included as Fig. 1. A graph for the individual samples with cellulose fibers and synthetic fibers in included as Fig. 2. The individual samples are sorted based on the Z-value in the Figures.

[0079] Table 6 - average values for carbonation related data.

[0080] Table 6 shows values for the total carbon content (TC) in the second column. It can be observed that the values for samples 1, 2 and 5 are higher than those for samples 3,4 and 6. This is due to the higher content of organic carbon material, i.e. more fibers. The third column should the total inorganic carbon content (TIC). The values of samples 5 and 6 are herein lower than the values of the other samples. Indeed, samples 5 and 6 contain less carbonatable calcium silicate cement and no Portland cement. The reduction of TIC content compared to sample 1 is therefore not surprising. However, samples 2-4 contain less carbonatable calcium silicate cement too and still, the TIC is higher. This again indicates that the addition of the Portland cement has a beneficial impact on the carbonation curing. The same pattern is visible in the subsequent columns for Y and Z.

[0081] Z represents the CO2uptake. The Z* values in the last column however provide the most appropriate comparison. Herein, the CO2uptake has been correct for the amount of carbonatable calcium silicate cement present. From these Z*-values, it becomes apparent that the carbonation of samples 5 and 6 including kaolin (clay) is not worse than the reference. In fact, the clay addition may be slightly beneficial, although the effect seems marginal. A bigger impact has the exchange of the cellulose fibers for synthetic fibers, leading to a reduction of the CO2uptake. The likely background is that the fibers have an impact on porosity, pore structure and humidity spreading.

[0082] The Z*-value for the carbonated samples including Portland cement is higher than the Z*— value for the comparative examples 1, 5 and 6. This may be explained in different ways; first of all, the Y1

[0083] Portland cement contains carbonatable cement phases too. Traditionally, it is believed, such as expressed in WO2016 / 154024A1, that the C2S phase may be carbonated; The C3S phase is not known to be effective for carbonation. The extra amount of C2S phase in the samples 2-4 was however not very big. A further explanation is that the Portland cement starts hydrating from the first contact with the water during mix preparation. So, at the time the fresh FC sheets are subjected to the carbonation curing, quite some Ca(OH)2 (i.e. so-called Portlandite) has been generated in the FC product, which carbonates easily. But also during the carbonation curing, some further hydration, and by that Ca(OH)2 generation may take place, which most probably will be instantaneously carbonated. The Hatschek production system is especially suited for the generation of Ca(OH)2 because of the abundance of water since the FC mixture is prepared, temporarily stored and finally fed to the Hatschek machine as a very watery slurry. So, compared with the concrete and mortar application, the Hatschek-based FC products will be characterized by a significantly higher degree of early hydratation of the Portland cement. The Hatschek manufacture is herein specific in that the cement is dispersed in an aqueous slurry, which is transformed into monolayers (or thin films) that are stacked upon each other to form a sheet. Some spread in the Z-value (CO2-uptake) can be seen in Fig. 1 and 2. The spread is generally not bigger or smaller in the reference and comparative samples than in the samples according to the invention. Still, it appears that the samples 3 and 4 in particularly contain some outliers. The presence of some spread seems inherent in the Hatschek manufacturing process. Also, there is an unavoidable spread due to the sample taking and measurement process.

Claims

CLAIMS1. A method of manufacturing of a fiber cement product comprising the steps of:Providing an aqueous slurry of a fiber cement composition at least comprising fibers and cement,Manufacturing a fiber cement green sheet by means of a Hatschek fiber cement production process from the aqueous slurry, andCarbonation curing the fiber cement green sheet, wherein the cement comprises first cement phases chosen from the group of CS (CaSiOj), C2S (Ca2SiO4), C3S2 (Ca3Si2O7) and combinations thereof, and second cement phases chosen from the groups of C3S (Ca3SiO5), C3A (Ca3AI2O6) and C3AF (Ca4AIFe2Oi0), and wherein the fraction of the second cement phase in the cement is 2-50% by weight, preferably 3-30% by weight, more preferably 5-20% by weight based on the dry weight of the cement. wherein the fraction of the first cement phase in the cement is 50-95% by weight, preferably 60- 80% by weight, based on dry weight of the cement.

2. The method as claimed in claim 1, wherein the second cement phase comprises for at least 75% by weight alite (C3S).

3. The method as claimed in any of the preceding claims 1-2, wherein the cement comprises a mixture of a carbonatable calcium silicate cement and Portland cement.

4. The method as claimed in any of the claims 1-3, wherein the fiber cement composition further comprises calcium carbonate, preferably in an amount of 5-30% based on dry weight, such as 10-20% by weight based on dry weight of the fiber cement composition.

5. The method as claimed in any of the claims 1-4, wherein the fiber cement composition further comprises amorphous silica, preferably in an amount of 1-10% based on dry weight, based on dry weight of the fiber cement composition.

6. The method as claimed in any of the claims 1-5, wherein cement is present in the fiber cement composition in an amount in the range of 60 to 90% by weight.

7. The method as claimed in any of the claims 1-6, wherein the fibers comprise organic fibers, such as cellulose fibers and / or synthetic fibers.

8. The method as claimed in any of the claims 1-7, further comprising the step of compressing the green sheets at a pressure in the range of 10 to 30 MPa, preferably 20 to 30 MPa prior to carbonation curing.

9. A fiber cement composition comprising fibers and cement, wherein the cement comprises first cement phases chosen from the group of CS (CaSiO3), C2S (Ca2SiO4), C3S2 (Ca3Si2O7) and combinations thereof, and second cement phases chosen from the groups of C3S (Ca3SiO5), C3A (Ca3AI2O6) and C3AF (Ca4AIFe2O10), and wherein the fraction of the second cement phase in the cement is 2-50% by weight, preferably 3-30% by weight, more preferably 5-20% by weight, based on dry weight of the cement, and wherein the fraction of the first cement phase in the cement is 50-95% by weight, preferably 60-80% by weight based on dry weight of the cement.

10. A fiber cement sheet obtainable by the method of the preceding claims 1-8.

11. The fiber cement sheet as claimed in claim 10, wherein the sheet has a density of in the range of 1.7-2.2 kg / dm3, preferably 1.8-2.0 kg / dm3.

12. The fiber cement sheet as claimed in claim 10 or 11, wherein the sheet has a porosity of at most 15%, preferably less than 14% and more than 12%, when measured under water-saturated conditions.

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