Aerated fiber cement building products and methods of making the same

Abstract

Disclosed herein are low density fiber cement articles, such as fiber cement building panels and sheets, comprised of multiple overlaying fiber cement substrate layers having small and uniform entrained air pockets distributed throughout. Also disclosed herein are air entrainment systems and methods for manufacturing aerated low density fiber cement panels or sheets with consistent air void content and uniform air void distribution. Also disclosed herein are air entrainment technologies adapted to work in conjunction with the Hatschek process to produce aerated fiber cement articles having controlled air void content and distribution.

Claims

1. A building article comprising: a plurality of thin overlaying fiber cement substrate layers, each of said overlaying fiber cement substrate layer is bonded to an adjacent overlaying substrate layer thereby forming a fiber cement matrix wherein the fiber cement matrix comprises an air entrainment agent; and a plurality of closed cell air voids formed by the air entrainment agent that do not form continuous air channels with adjacent voids, the air voids defined by air pockets entrained in the fiber cement matrix such that the air voids are formed directly in the fiber cement matrix with no material separating the air voids from the fiber cement matrix, said air voids being dispersed uniformly throughout the fiber cement matrix to reduce the density of the fiber cement matrix such that the cumulative volume of the air voids is greater than 3% of the volume of the fiber cement matrix, said air voids having an average diameter greater than 20 microns.

2. The building article of claim 1, wherein the average diameter of the air voids is between 20 microns to 100 microns.

3. The building article of claim 1, wherein the volume of air voids is between 5% to 20% by volume of the fiber cement matrix.

4. The building article of claim 1, wherein the density of the fiber cement matrix is between 1.0 to 1.3 grams per cubic centimeter (g/cc).

5. The building article of claim 1, wherein at least 90% of the air voids are closed-cell.

6. The building article of claim 1, wherein each overlaying fiber cement substrate layer has a thickness of between 20 to 450 microns.

7. The building article of claim 1, wherein fiber cement substrate layers comprise cellulose fibers.

8. The building article of claim 1, wherein the cumulative volume of the air voids is between 5% and 20%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a low-density fiber cement article according to one embodiment of the present disclosure.

(2) FIG. 2 is a SEM photo of voids dispersed throughout the fiber cement article of FIG. 1 shown at 100× magnification.

(3) FIG. 3 is a SEM photo of voids dispersed throughout the fiber cement article of FIG. 1 shown at 400× magnification.

(4) FIG. 4 is a SEM photo of the structure of a single air void of the fiber cement article of FIG. 1.

(5) FIG. 5 shows a comparison of pore diameter distribution of low density fiber cement product of a preferred embodiment and equivalent low density fiber cement product having ceramic microspheres as additive;

(6) FIG. 6 is a schematic illustration of a process flow of one embodiment for manufacturing a low-density aerated fiber cement sheet in conjunction with the Hatschek process;

(7) FIG. 7 is a schematic illustration of a process flow of another embodiment for manufacturing a low-density aerated fiber cement sheet in conjunction with the Hatschek process; and

(8) FIG. 8 are SEM photos comparing an aerated low-density fiber cement sheet of one preferred embodiment with a control low-density fiber cement sheet made with micro sphere additives.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(9) Disclosed herein are low density fiber cement articles comprised of multiple overlaying fiber cement substrate layers having small and uniform entrained air pockets distributed throughout. Also disclosed herein are air entrainment systems and methods for manufacturing aerated low density fiber cement panels with consistent air void content and uniform air void distribution. Also disclosed herein are air entrainment technologies adapted to work in conjunction with the Hatschek process to produce aerated fiber cement articles having controlled air void content and distribution.

(10) FIG. 1 illustrates one embodiment of a low density aerated fiber cement article 100 of the present disclosure. The fiber cement article 100 includes a plurality of overlaying fiber cement substrate layers 102, which together form a fiber cement matrix 106. As shown in FIG. 1, a plurality of air voids 104 are formed in the fiber cement matrix 106 by air entrained in the matrix 106. The air voids 104 are preferably sized within a pre-selected range and are uniformly distributed throughout the fiber cement article 100. In one implementation, the diameter of the air voids 104 is greater than 20 microns (μm), preferably between about 20 μm to 100 μm. In another implementation, the average diameter of the air voids 104 is between about 20 μm to 60 μm. In another implementation, the air voids 104 are more than 5% by volume of the fiber cement article 100, preferably between about 5%-20% by volume, preferably between about 5%-10% by volume, preferably between about 9.2% by volume.

(11) FIG. 2 is a SEM photo of air voids 104 dispersed throughout the fiber cement matrix 106 shown at 100× magnification. FIG. 3 is a SEM photo of air voids 104 shown at 400× magnification. FIG. 4 is a SEM photo illustrating the structure of a single air void 104 shown at 3000× magnification. As shown in the SEM photos, the air voids 104 formed in the fiber cement matrix 106 are generally mixtures of single and clusters of air bubbles. Unlike low-density additives such as microspheres, the air voids 104 are air pockets formed directly in the fiber cement matrix by air entrainment such that the walls defining each void is part of the fiber cement matrix. The air voids are preferably closed cell voids that do not form continuous air channels with adjacent voids. In some embodiments, at least 50%, 60%, 70%, 80%, 90%, or over 90% of the air voids in the article are closed cell voids. The shape and volume of the voids are preferably determined by the amount of air entrained in the fiber cement article. While air entraining techniques have been used to aerate concrete, it is very difficult to form uniformly distributed predominantly closed cell air voids in fiber cement panels or sheets using conventional concrete aeration techniques.

(12) One of the challenges in forming uniformly distributed, closed cell air voids with the desired morphology in fiber cement panels or sheets is the difficulty in entraining air bubbles in the fiber cement that can survive the Hatschek process intact. The Hatschek process is widely used as a commercially viable method for making fiber cement panels or sheets with multiple overlaying substrate layers. To the best of the inventors' knowledge, aeration of fiber cement building sheets produced by the Hatschek process has never been considered as a viable process for forming uniform air voids because of the difficulty in producing air bubbles or pockets in the low solid content slurry and the bursting of air bubbles due to the numerous vigorous processing steps of the Hatschek process. Some of the processing steps that present obstacles to uniform air bubble formation are the slurry agitation, pumping, and filtering steps; the fiber cement film generation and fiber cement layer buildup steps; the high vacuum treatment of the thicker fiber cement layer and multilayer fiber cement green sheet; the high nip pressure and density increasing roll pressures on the green sheet; and the high pressure long autoclaving curing step.

(13) Furthermore teachings from the aeration of concrete, which typically has water to cement ratios of less than 1, provide no guidance in determining whether or not the aeration of fiber cement building sheets produced by the Hatschek process would be feasible. For example, studies have shown difficulties in predicting the air content of aerated uncured concrete when subject to forces or pressure. Some have recommended against pumping air-entrained concrete at pressures in excess of 300 psi. Additionally, the effect of air entrainment additives on concrete cement is difficult to predict because of the numerous factors affecting performance. For example, excessive vibration can result in as much as 50% of entrained air being lost after three minutes of vibration; very fine (<150 μm) and coarse (>1200 μm) aggregates decrease air content, but aggregates between 150-1200 μm increase air content; and an increase in concrete temperature will significantly decrease the air content.

(14) The size of the air bubbles present in the uncured concrete cement can also be affected by the forces imposed on it. Suction and dissolution that occur in pumping concrete could result in very few, if any, air bubbles with diameters below 50 μm being present afterwards. Suction occurs in pumping when the wet concrete is subject to a negative pressure brought on by vacuum when the pump piston chamber fills with the wet concrete paste. Dissolution occurs when the wet concrete paste is subject to pressure with the smaller air bubbles dissolving in water but not re-forming when the wet concrete paste is depressurized. Furthermore the current knowledge in the field of entrained air in fresh uncured concrete paste appears to be that the solubility of the entrained air is the same as that for free air and water. As such, based on Henry's Law, a fresh concrete paste that is subject to a pressure of 400 psi will likely dissolve a total amount of air comprising 52% of water volume present.

(15) Accordingly, as explained above, to the best of the inventors' knowledge, there is no information from the aeration of concrete and the properties of fresh concrete paste that teaches that the aeration of fiber cement building sheet or panel with multiple overlaying substrate layers made using a Hatschek process would be technically feasible. Given the pressures, forces and temperatures encountered by the material processed through the Hatschek process, all previous low cost method of reducing the density of fiber cement building sheets and panels have been based on the addition of a low density additive, such as hollow ceramic or glass microspheres.

(16) The present disclosure provides various embodiments of fiber cement articles having multiple overlaying substrate layers with air pockets distributed throughout. Preferably, the fiber cement article is made using a Hatschek process. In some embodiments, air pockets are introduced into the matrix by air entrainment during the manufacturing process of the fiber cement article. In other preferred embodiments, air pockets are introduced by the use of sacrificial materials in the fiber cement formulation wherein the materials disintegrate during the manufacturing process, leaving air pockets within the matrix of the end products. The fiber cement article may be a panel or a sheet and may be used as a building product.

(17) As discussed in greater detail below, air entrainment according to preferred embodiments of the disclosure may be achieved by vigorous mixing of the formulation slurry in the Hatsheck process to produce air bubbles, coupling with the use of suitable aerating agents for bubble stability. In some preferred embodiments, the mixing condition is adjusted so that air bubbles are substantially within the 20 to 100 μm in diameter. It has been found that higher mixing rates produce smaller bubble size. In some embodiments, air bubbles are pre-produced as a foam, using a high sheer foam generated pump, before being introduced into the slurry.

(18) Aerating agents as used herein can be chemicals or compounds that have the ability to help bubble generation and/or bubble stabilization within the Hatschek process. In some embodiments, these chemicals can be long chain polymers, acting as surfactants to strengthen the bubble shells. While there are many types of long chain polymers, the selection of suitable aerated agents for cellulose fiber cement is not obvious. The process water in Hatschek process for fiber cement product typically contains dissolved form of cements such as Calcium (Ca.sup.2+), Sodium (Na.sup.+), Potassium (K.sup.+), and Sulfate (SO.sub.4.sup.−). Thus, the process water is high in pH and selected aerating agents must be able to survive in a high pH environment. In some embodiments, preferred aerating agents are cationic surfactants made with ammonium substitute, anionic surfactant such as wood resin, fatty acid salt or synthetic detergent, with carboxylate group (R—COO—), sulfate group (R—SO.sub.4—) and sulfonate group (R—SO.sub.3—)). More preferred aerated agents are sodium vinsol resin and/or sodium benzene sulfonate. In some embodiments, aerating agents are used in the amount of between 0.1% to 5% by weight or 0.3% to 2% by weight of the total fiber cement formulation. In more preferred embodiments, the amount of aerating agents used is 1% to 3% by weight. In some implementations, aerating agents are used in conjunction with other low density additives, such as microspheres, to reduce the density of fiber cement article. In one implementation, the aerating agents comprise about 0.13% by weight of the total fiber cement formulation and microspheres comprise about 1% by weight of the total fiber cement formulation.

(19) Other preferred embodiments utilize suitable sacrificial materials or fillers to produce the air voids in the fiber cement matrix. The sacrificial materials or fillers are selected for their ability to disintegrate during the Hatschek process, leaving air pockets in the matrix of the end product. In some preferred implementation, the sacrificial fillers can be starch, ammonium carbonate and/or sodium bicarbonate by consideration of cost efficiency. Sacrificial material is preferably introduced into the formulation in a finely ground form so that it can be mixed and distributed throughout the formulation during the manufacturing process, thus leaving well distributed air pockets within the matrix of the end product. Preferred particle sizes of the sacrificial materials are less than 250 μm (60 mesh).

(20) Concrete Air Entrainment Additives (AEA)

(21) In some implementations, certain concrete air entrainment additives (AEA) are selected and modified for fiber cement sheets or panels formed by the Hatschek process. There are four commercially available types of AEAs used in the concrete cement industry: wood resins, synthetic detergents, petroleum acid salts, and fatty acids salts. Laboratory tests on the foam production from commercially available AEAs mixed with highly alkaline fiber cement processing water were performed to determine whether any of the AEAs could be used as a potential air entrainment additive for fiber cement sheets and panels. The test was conducted using a mixer (KitchenAid 325) with 500 ml of highly alkaline processing water from a fiber cement Hatschek manufacturing process and 4 grams of the selected AEA being mixed in a bowl at gear 8 of the mixer for 10 minutes. The foaming ability of the AEAs was measured by measuring the height of the resulting foam from the base of the mixing bowl.

(22) TABLE-US-00001 TABLE 1 Foam test for AEA in highly alkaline fiber cement processing water. AEA type Commercial Name Detailed Description of AEA Foaming (mm) Wood MB-VR Sodium vinsol resin 137.2  resins Polychem VR Sodium vinsol resin 100   Super Air Plus Powder VR no foam Petroleum Polychem AE Substituted benzene sulfonate (Na salt) 73.75 (unstable) salts NASA HS 90 Sodium benzene sulfonate 118.13 Micro Air Sodium olefin sulfonate no foam Airen S Benzenesulfonic Acid (Na salt) 85   Fatty acid Eucon Air 40 Tall oil. PCMC. Glycol ether no foam salts Hydrophobe Complex carbohydrate, polyalcohols, etc. no foam Synthetic Chemical Sodium lauryl sulfate no foam detergents

(23) As shown in Table 1, the fatty acids salt and the synthetic detergent AEAs were deactivated in highly alkaline fiber cement process water with no air bubble formation. Sodium vinsol resin and sodium benzene sulfonate successfully produced foam from the highly alkaline fiber cement processing water.

(24) A cement hydration test was conducted using differential calorimeter method on cement slurry having a water to slurry ratio (w/s ratio) of 0.4. The test showed that sodium benzene sulfonate was a strong cement hydration retarder with a 0.5% by weight addition to cement, which resulted in the cement not setting within 72 hours. On the other hand, sodium vinsol resin had little effect on cement hydration. Since the process water for fiber cement product has a high in pH due to its constituents, additional laboratory tests were conducted and it was found that sufficient and controllable levels of entrained air were difficult to attain with most concrete AEAs.

(25) To further assess why the fiber cement process water prevented most concrete AEAs from foaming, the following tests were performed using Micro Air for sodium olefin sulfonate, which did not foam with the fiber cement processing water, and sodium vinsol resin. The test and measurements are the same as in the fiber cement processing water test except the amount of AEA was reduced by 50% to 2 grams of the AEA in question.

(26) TABLE-US-00002 TABLE 2 AEA foaming ability in different alkaline water AEA Type Solution Foaming (mm) Micro Air Sodium olefin Fresh water (pH 7) Good (73 mm) sulfonate Micro Air Sodium olefin NaOH water (pH 13) Good (73 mm) sulfonate Micro Air Sodium olefin Ca(OH).sub.2 water (pH 13) No foaming sulfonate (35 mm) MB-VR Sodium vinsol Fresh water (pH 7) Good (76 mm) resin MB-VR Sodium vinsol NaOH water (pH 13) Good (76 mm) resin MB-VR Sodium vinsol Ca(OH).sub.2 water (pH 13) Good (76 mm) resin

(27) From the results of Table 2, it appears that the presence of lime in the process water deactivates the Micro Air AEA from foaming. Accordingly for an AEA to potentially work in the fiber cement articles, the AEA needs to work in high pH, and possibly high Ca.sup.+ ion concentrations.

(28) Aerated Fiber Cement Sheets Using Sodium Vinsol Resin as an Additive

(29) Fiber cement building sheets made by the Hatschek process with the addition of sodium vinsol resin was conducted and a separate trial with additions of hollow microspheres for comparison. It was found that 0.5% concentration of sodium vinsol resin addition to the formulation slurry for fiber cement produced the same fiber cement building sheet density as a formulation using 3% of hollow microspheres. The product performance of fiber cement building sheets using sodium vinsol resin as an additive is shown in Table 3:

(30) TABLE-US-00003 TABLE 3 Fiber cement building sheet performance comparison Sodium vinsol resin as percent of total Moisture Movement solids in slurry MoR ILB (Pre-Carb/Post-carb) Density 0% (control) 10.1 1.7 0.18/0.53 1.28 0.1% 9.7 1.43 0.18/0.53 1.25 0.5% 9.4 1.4 0.18/0.51 1.23

(31) A separate study using fiber cement building sheets produced by the Hatschek process and using 2% sodium vinsol resin produced an average fiber cement product density 1.15 g/cm.sup.3. Further scanning electron microscope (SEM) analysis of the resulting fiber cement with aeration shows the air bubble particle size in the fiber cement building sheets to range between 20 μm to 100 μm. In addition, SEM analysis also shows that although the morphology of the air bubbles appear to be irregular, possibly due to the nip pressure, the entrained air bubbles remain unbroken and isolated in fiber cement sheet without the presence of air channels, which might occur due to excessive vacuuming forces. Also analysis of the fiber cement building sheets found the overall air pockets accounted for about 9.2% of the board by volume with the entrained air distributed evenly throughout.

(32) A comparison of pore distribution in low density fiber cement products that was made by sodium vinsol resin and ceramic microsphere as low density additive with the same density of 1.15 g/cm.sup.3 are shown in FIG. 5. As shown, in some embodiments, fiber cement products incorporating aerating agents show a percent increase in pore volume in certain pore size ranges. In some implementations, there is about 5 to 30% volume increase in pores ranging from 0.01 to 0.1 microns in diameter. In another implementation, there is about 10 to 23% volume increase in pores ranging from 0.01 to 0.1 microns in diameter. As can be seen, in some embodiments, the methods disclosed herein may be used to increase the pore volume of pores in the range of about 0.01 to 0.1 microns by at least 5%, and preferably by at least 10% up to about 30%. In yet another implementation, the fiber cement products incorporating aerating agents show an average pore diameter of greater than 20 microns, preferably between 20 to 100 microns, constituting at least 50% of the volume of the pores and at least 3% of the volume of the fiber cement product. Furthermore, an addition of 0.5% sodium vinsol resin to total slurry solids by weight was found to produce about a 4% decrease in product the post autoclave density of the fiber cement building sheet and about a 7% decrease in saturated MoR when compared to a control fiber cement building sheet. For the moisture expansion property, building sheets using sodium vinsol resin showed parity to the control building sheets. To understand how the addition of sodium vinsol resin compares to spheres in fiber cement products made by the Hatschek process, we compared the density reduction and performance with 0.5% sodium vinsol resin sampled from this trial and microsphere addition trial. From the analysis, for density reduction 0.5% of sodium vinsol resin to the total solids is equivalent to 3-4% of microspheres addition to the same. In addition Modulus of Rupture (MoR) performance as measured in according to ASTM C 1185, Clause 5 in wet condition shows parity between the two fiber cement building sheets.

(33) Some preferred embodiments also control the size of bubble introduction into the fiber cement slurry. Since smaller bubbles have higher surface tension than larger bubbles, it was found that bubble size within the range of 20 to 100 microns is best to survive the Hatschek process. The process can be engineered by using a high slurry agitation or high slurry pumping rate or introduction of a designated high shear pump for bubble formation.

(34) Sacrificial Fillers

(35) Other preferred embodiments provide a method of manufacturing aerated fiber cement products by careful selection of suitable sacrificial fillers that will be configured to disintegrate during the Hatsheck process, leaving air pockets in the matrix of the end product. A range of potential sacrificial fillers were tested for the Hatschek fiber cement process. The test results are shown in Table 4. The filtration density reduction percentage was calculated by comparing the filter cakes obtained from the vacuum filtration of cement/silica slurry with and without a sacrificial filler. The pad press density reduction percentage was calculated from fiber cement pads produced from a Wabash hydraulic press machine (model PC-75-4™) using 28 ton/ft.sup.2 compression pressure. From the results, it was found that only a few selected materials are suitable for aeration purposes.

(36) TABLE-US-00004 TABLE 4 Testing of Sacrificial Fillers as potential additives for aerating fiber cement building sheets Density Reduction Under Under Variable (addition) Addition Level filtration Pad Press Acrylic copolymer 0.70% −7.2%  — encapsulating blowing agent Polyacrylamide water 0.30% −3.7%  — absorbent polymers Carbopol (swelling 0.50% −3.8%  — polymer) PVA (PVOH)   1% — — Organic Blowing Agent 0.40%  −4% — (Benzene Sulphohydrazide (BSH)) Organic Blowing Agent 0.80% −12% — (Benzene Sulphohydrazide (BSH)) Aluminum Powder 0.55% −12.5%.sup.  −4% Ammonium Nitrate 1.50% −8.5%  — Ammonium Sulfate 1.50% −11% — NaHCO.sub.3 1.5% Filter cake  −9% — 2% Pad Press −3.3%.sup.  NaHCO.sub.3   6% −32% −6% Starch 1.5% Filter cake −4.5%  — 2% Pad Press −1.9%.sup.  Starch   8% −28% −8% Ammonium Bicarbonate 1.5% Filter cake −13% — 1% Pad Press −4.9%.sup.  Ammonium Bicarbonate   6% −42% −11%  Urea   2%  −7% −2%

(37) FIG. 6 illustrates a process 600 for forming a low density, aerated fiber cement article using the Hatschek process. The process 600 is designed to entrain air in a fiber cement matrix such that the air voids formed therein are uniform, predictable and can survive the harsh conditions of the Hatschek process. In this embodiment, air is entrained in the fiber cement panel to form the air voids during the Hatschek process. The process 600 begins with Step 610 in which one or more aerating agents is vigorously mixed with an aqueous slurry comprising water, cellulose fiber, silica, cement and additives, including low density additives. In one implementation, the aqueous slurry contains about 8 to 15% solids by weight, with the cement accounting for about one-third of the total solids by weight. In some embodiments, the aqueous slurry can be mixed by a shear pump at a speed of greater than 450 rpm or preferably greater than 500 rpm. The water to cement ratio for the aqueous slurry can be around 20-35. The aerating agent is preferably vinsol resin, more preferably sodium vinsol resin, comprising about 0.3 to 2 wt. % of the aqueous slurry. The inventors have surprisingly found that aerating agents, when present in this weight percent range, provide desirable buffering and other properties in the fiber cement matrix. The aerating agent can be added directly to the aqueous slurry in this embodiment. In some implementations where microspheres or other low density additives are typically used to reduce density of the fiber cement article, aerating agents can be added in combination with microspheres to reduce the amount of microspheres needed, hence reducing the cost of the material. In one implementation, the aerating agents comprise about 0.1-0.2 wt %, preferably 0.13 wt. % of the fiber cement formulation and microspheres comprise about 0.1 wt. % of the fiber cement formulation.

(38) The process 600 continues with Step 620 in which the aqueous slurry containing the air bubbles is delivered to a feed sump and pumped to a plurality of vats. Each vat has its own agitators to ensure a consistent slurry mixture. The fiber cement material in the aqueous slurry in the vats is then deposited as thin fiber cement films on a plurality of sieve cylinders that are rotated through the slurry in the vats in Step 630. The thin fiber cement films are then layered on top of each other to produce a thicker fiber cement layer in Step 640. The process 600 produces the thicker fiber cement layer from the thin fiber cement films by the use of a specialized felt-based belt that both drives the rotating sieve cylinders through the slurries in the vats and picks up the thin fiber cement films deposited on the rotating sieve cylinders.

(39) The rotating sieve cylinders can be rotated by the bottom run of the specialized felt-based belt and filter the slurry through a sieve screen attached to the sieve cylinder. As the slurry filters through this sieve screen a thin film layer of fiber cement material is deposited on the surface. The thin film of fiber cement material has a thickness of between 0.20 to 0.45 mm (20-450 μm) and contains between 60%-75% solids (with a water to cement ratio of 1.4 to 2.2 when cement comprises 30% of the solids). The excess water from the slurry then passes through the sieve screen as filtrate and exits from the end of the sieve cylinder, enabling recovery and recirculation of the slurry solids in the vat. As the apertures of the sieve screen are typically between 0.30 to 0.50 mm (300-500 μm) all non-fibrous material that is significantly smaller than 300 μm is either washed through the sieve screen or caught by the cellulose fibers that form a mesh over sieve screen. Accordingly entrapment of small non-fibrous materials depends on the formation of an initial filter layer of cellulose fibers on the surface of the sieve screen.

(40) The fiber cement film formed on the surface of the sieve screen on each sieve cylinder is then transferred upon contact to the outer surface of the specialized felt-based belt. This transfer process takes place because the felt is less porous than the sieve screen. As the specialized felt-based belt passes over each successive vat in the series it picks up a corresponding series of sequential layers of fiber cement films from the associated sieve cylinders and builds the thicker fiber cement layer. Thereafter the thicker fiber cement layer passes over a vacuum box positioned along the top run of the belt to remove the water and moisture still present in Step 650. Given the thickness of the thicker fiber cement layer and the water content present the vacuum settings can range from 18 to 25 inches of mercury.

(41) The thicker fiber cement layer then passes between a tread roller, which also provides the driving force for the specialized felt-based belt, and an adjacent accumulation or “size” roller in the form of a relatively large diameter drum. The tread and size rollers are positioned such that further water is pressed out of the thicker fiber cement layer while it is transferred to the size roller by a mechanism similar to that by which it was previously transferred from the sieve cylinders to the belt. The pressure in the nip between the tread and size roller, also referred to as the nip pressure, is anywhere between 350 psi to 1000 psi depending on the processing requirements. Preferably, the pressure is selected so as to not remove a minimal number of air bubbles in the fiber cement layers. The size roller accumulates a number of thicker fiber cement layers according to the number of turns allowed before the laminated sheet is cut off. Thus, the formation of a fiber cement sheet with multiple thin substrate layers is achieved by allowing a larger number of turns before cutting the film. In the cut off process in Step 660, a wire or blade is ejected radially outwardly from the surface of the size roller to cut longitudinally the cylinder of laminated fiber cement material that has cumulatively formed on the surface of the roller.

(42) Once cut, the sheet of laminated fiber cement material peels off the size roller to be removed by a run-off conveyor. The material at this stage has the approximate consistency of wet cardboard, and therefore readily assumes a flat configuration on the run-off conveyor. To complete the process at the wet end, the felt is cleaned as it passes through an array of showers and vacuum boxes, before returning to the vats to pick up fresh layers of film. It will be appreciated that the quality and characteristics of laminated sheet material produced from the process depends on a wide range of variables associated with the slurry formulation and the various settings at the wet end of the machine. Further down the process line this material, known as “green sheet”, is roughly trimmed to size at a green trim station using high pressure water jet cutters, after which it proceeds as individual sheets to a stacker. At the stacker, the green sheets are picked up by vacuum pads and formed with interleaving sheets into autoclave packs. After partial curing, and optionally a further compression process to increase density, the green sheets are loaded into an autoclave unit for final curing under elevated temperature and pressure conditions in Step 670. In the autoclave, a chemical reaction occurs between the raw materials to form a calcium silicate matrix which is bonded to the cellulose reinforcing fiber. This process is undertaken in 160-190° C. at saturated steam pressure for 8 to 12 hours. At its completion, the sheets emerge fully cured, ready for accurate final trimming, finishing and packing in Step 680. The resulting product is an aerated, low-density fiber cement panel or sheet with multiple overlaying substrate layers. The fiber cement panel has air bubbles evenly distributed throughout the fiber cement matrix. The air bubbles are preferably closed-cell and have an average diameter ranging from 10 to 100 μm. In some embodiments, over 80% of the air bubbles are closed-cell. In some other embodiments, over 90% of the air bubbles are closed-cell.

(43) FIG. 7 illustrates another process 700 for forming a low density, aerated fiber cement article using the Hatschek process. In process 700, instead of adding the aerating agent directly to aqueous fiber cement slurry, the aerating agent is first mixed with water, and optionally other additives, using a high shear pump to form a foam mixture in Step 710. The foam mixture is then added to and mixed with the aqueous cement slurry in Step 720. Premixing the aerating agent with water can result in more uniformly distributed air bubbles once the foam mixture is added to the aqueous slurry.

(44) FIG. 8 are SEM photos comparing an aerated low-density fiber cement sheet made in accordance with the methods described herein with a control low-density fiber cement sheet made with microsphere additives. As shown in FIG. 8, the size of the air bubbles in the aerated fiber cement sheet are controlled maintained in the range of between 10 to 150 μm, which is comparable to the size of the microspheres. However, the aerated fiber cement sheet is less costly to manufacture as it does not require the addition of microsphere or other additives to create uniformly distributed air voids in the fiber cement matrix.

(45) The foregoing description of the preferred embodiments of the present disclosure has shown, described and pointed out the fundamental novel features of the inventions. It will be understood that various omissions, substitutions, and changes in the form of the detail of the apparatus, system, and method as illustrated as well as the uses thereof, may be made by those skilled in the art, without departing from the spirit of the invention.