Fiber Cement Board Raw Materials: Cement, Silica, Fiber and Additives

Fiber cement board raw material consists of four groups: Portland cement as the binder, ground silica sand or a mineral filler such as limestone, reinforcing fibers (mainly cellulose pulp), and small doses of process additives, all mixed with water. In autoclaved board, ground silica is a reactive ingredient and can make up as much of the mix as cement, or more.

This guide is about specifying and buying those materials. Fibers have their own guides, on cellulose fiber cement board and asbestos-free fiber cement board, so they are only summarized here. The focus is on what fills most of the board: cement, silica, fillers and additives. The plant steps are covered in the fiber cement board manufacturing process.

Fiber Cement Board Composition at a Glance

A fiber cement board is a cement-based matrix reinforced by fibers. NIOSH (2017) names cellulose fiber, sand or fly ash, cement and water as the principal ingredients of fiber-cement products. The European product standard EN 12467 (CEN, 2018) allows the matrix to be either cement or a calcium silicate formed by reacting siliceous and calcareous materials.

One published autoclaved formula shows how the groups are balanced (US 6,572,697, 2003):

The example values are one formula; the ranges are what the patent states for one preferred composition, not typical plant practice. Use them to see how the groups relate, then fix your own mix by trial with your own materials. In this patent example, silica is about twice the cement by mass. Laboratory autoclaved sheets in a 1995 CSIRO study used a different balance, ordinary Portland cement and silica at 1:1 (Ni, 1995), so published cement-to-silica ratios vary widely.

What Is Fiber Cement Board Made Of in Autoclaved and Air-Cured Production

The curing route sets the raw material list. In an autoclaved board, ground silica reacts with lime released by cement hydration to form calcium silicate hydrates, so silica is a binder ingredient, not a filler. The 1995 CSIRO thesis by Ni gives the numbers: research sheets were cured at 1.25 MPa steam pressure for 7.5 hours, the thesis notes that the autoclave reaction “is completed within 6 to 8 hours instead of 3 to 4 weeks required with air-cured products,” and it cites replacement of 40–60% of the cement by silica in autoclaved products.

Air-cured fiber cement is also made in the industry. Without the high-temperature reaction, ground quartz would stay unreacted, so air-cured formulas use much more cement, limestone or other fillers in place of ground sand, and synthetic fibers alongside cellulose. One published air-cured formula is 73–80% cement, 10–18% limestone filler, 3–4% unbleached softwood kraft pulp and 1.5–1.9% polyvinyl alcohol (PVA) fiber (US 11,773,023, 2023). The rest of this guide concentrates on the materials of autoclaved board.

Cement: The Main Fiber Cement Board Raw Material

Ordinary Portland cement is the standard binder, and its consistency matters more than its strength class. A published fiber cement patent states that “the hydraulic binder is preferably Portland cement” and prefers a fineness index of about 200–450 m²/kg, measured by the air-permeability method of ASTM C204 (US 6,572,697, 2003).

Research mixes reviewed in the journal Polymers (Balea et al., 2019) used Type I, Type II and Type V Portland cements and a Portland-limestone cement (CEM II/A-LL 42.5R). Each brings a different setting rate and sulfate resistance, so pick one that suits both the product and your local supply.

What to specify and check:

1. One cement type from one plant, fixed in the formula. A change of cement source changes setting and drainage on the sheet machine.
2. Fineness and its variation. Fine cement particles are held in the forming web by the fiber mat; coarser or finer lots change how much passes through with the water.
3. Setting time with your pulp. Check setting with the actual pulp in the mix, not only with neat cement.
4. Freshness and storage. Cement that has picked up moisture in storage hydrates partly before use. Silos, not open bags, are the norm on continuous lines.

If the formula includes fly ash or slag, count it as part of the binder system and test it with the cement, not separately.

Silica: Ground Sand in Autoclaved Board

In autoclaved fiber cement, ground silica is often the largest single ingredient. The same patent prefers ground silica sand as the aggregate, at the same 200–450 m²/kg fineness as the cement (US 6,572,697, 2003), and its example carries 54% silica against 28% cement.

Fineness matters because silica has to dissolve and react during a cycle of a few hours. Coarse quartz leaves more unreacted grains and less binder. Too fine a grind raises the energy used in the mill and can slow drainage on the board machine. Set the grind during trials, then hold it with particle size checks on every mill batch.

When you assess a sand source, look at:

• SiO₂ content and mineral form. Crystalline quartz is the usual basis; clay, feldspar and mica impurities change water demand and reaction.
• Variation over time. Sample several lots, and include the source you would fall back on.
• Grindability. It sets the size and running time of the wet ball mill.

Crystalline silica and dust control

Silica brings an occupational health duty with it. NIOSH (2017) reports that fiber-cement products can contain as much as 50% crystalline silica. Rules differ by country, so follow the dust limits of your own jurisdiction; as one example, the US OSHA construction standard sets a permissible exposure limit for respirable crystalline silica of 50 µg/m³ as an 8-hour time-weighted average (29 CFR 1926.1153). In the plant, silica is handled dry at unloading, storage and milling, so dust collection at those points belongs in the design from the start.

Fillers and Supplementary Materials

Fillers adjust density, cost, dimensional stability and surface quality, but each one changes water demand and strength. The main options in published sources are:

Fillers interact with density. A filler that lightens the board also makes it more porous, with more water absorption. How density changes the product is explained in our article on calcium silicate board density.

Process Additives

Process additives are used in small doses, but they decide whether the forming machine runs well. One patent lists silica fume, fire retardants, thickeners, pigments, colorants, plasticizers, dispersants, foaming agents, flocculating agents, water-proofing agents, organic density modifiers, aluminum powder, alumina trihydrate and polymeric resin emulsions among possible additives (US 6,572,697, 2003).

The ones a new plant usually deals with first:

• Flocculants. On a Hatschek or flow-on machine, fine cement and silica particles have to be held in the fiber web while water drains. Flocculants help bind them. A 2019 review notes that nanocelluloses can act “in a similar way to an anionic flocculant in the wet-end of the machine” (Balea et al., 2019).
• Dispersants and plasticizers. They reduce water demand in denser mixes; one research mix in the same review used a superplasticizer at about 1.6% of cement mass.
• Water-proofing agents and pigments. Usually for exterior or colored boards.

Additives interact with cement chemistry and with recycled process water. Introduce them one at a time in trials, and record dosage with the batch data.

Fiber, in Brief

Fiber is a small share of the mass but does two jobs: it forms the filter web that holds fines during forming, and it reinforces the finished board. A 2019 review gives about 10 wt.% as the maximum cellulose dosage in a Hatschek process with good fiber dispersion (Balea et al., 2019). A Coventry University thesis puts fiber in pulp-type processes at 10–20% of cement weight and reports that kraft fiber raised flexural strength up to about 8% of cement weight, beyond which strength fell while toughness kept rising (Khorami, 2011).

Autoclaved board relies mainly on cellulose pulp; synthetic fibers such as PVA are more common in air-cured formulas. All our lines are asbestos-free, and no new line in China uses asbestos today. Pulp grades, refining and the fibers that replaced asbestos are covered in our guides on cellulose fiber cement board and asbestos-free fiber cement board.

Water: The Raw Material Most Plants Forget

Water is used in greater volume than any solid, and most of it is recovered. A Coventry University thesis follows the water through Hatschek forming: a slurry water–cement ratio of about 3.5, falling to about 0.60 as a thin lamina forms and to about 0.34–0.36 after vacuum dewatering (Khorami, 2011). In other words, most of the water that enters the machine leaves the sheet again and returns to the recovery circuit.

Because the water is recirculated, dissolved salts and fine particles build up in it. Check the incoming water for chlorides and sulfates, and plan settling tanks and return pumps so that the recycled water stays within the limits your formula can tolerate.

A Raw Material Checklist for Fiber Cement Board Buyers

Before you sign for equipment, you should know which cement, sand, filler and pulp the plant will run on, because those choices size the silica mill, the pulping section and the press. A practical checklist for the samples you collect:

Then laboratory boards should be made at your target density and tested for bending strength, density, water absorption and moisture movement against the standard you will sell to, such as EN 12467 or ASTM C1186. On our reference designs, fiber cement board is made at 1.4–1.8 g/cm³; where your formula lands in that range affects the press you need.

FAQ

What is the typical fiber cement board composition?

There is no single composition. One published autoclaved formula is 28% cement, 54% ground silica, 7% cellulose, 4% metal hydroxide and 7% low density additive by dry mass. Laboratory autoclaved sheets in a CSIRO study used cement and silica at 1:1, a quite different balance. Treat such examples as a starting point, not a recipe.

Why is silica used in fiber cement raw materials?

In autoclaved board, ground silica reacts with lime released by the cement during steam curing, forming calcium silicate hydrates that bind the board. This lets producers replace a large share of the cement with silica; one study cites 40–60% replacement. Without an autoclave there is no such reaction, so air-cured formulas use limestone or other fillers instead.

Can fly ash be used in fiber cement board?

Yes. NIOSH lists sand or fly ash among the principal ingredients of fiber-cement products. Fly ash is not a simple swap for silica, because its composition and carbon content vary by power plant. Test ash from your actual source in laboratory boards and secure a long-term supply before building the formula around it.

How is fiber cement different from calcium silicate board in raw materials?

Fiber cement relies on Portland cement as its main calcium source, with ground silica in autoclaved grades. Calcium silicate board relies on lime and finely ground silica reacting in the autoclave, with less cement. See our guide to calcium silicate board raw materials for the lime, Ca/Si ratio and silica grind behind that product.

Planning a Fiber Cement Board Plant?

Raw material preparation, fiber pulping, flow-on or Hatschek forming and the press range for 3, 5 or 8–10 million m² a year are set out on our fiber cement board production line page. Send us your board types, target output and raw material information. For the lime-based product made on the same type of line, see calcium silicate board raw materials; for industrial insulation, see the calcium silicate plant setup guide.

Sources

• Laboratory evaluation of power shears for cutting fiber-cement siding, EPHB Report No. 358-21a (C. Qi, A. Echt) – NIOSH, 2017: https://www.cdc.gov/niosh/surveyreports/pdfs/358-21a.pdf
• STN EN 12467+A2:2018 (EN 12467:2012+A2:2018) Fibre-cement flat sheets – Product specification and test methods – CEN; Slovak Office of Standards, Metrology and Testing, 2018: https://normy.normoff.gov.sk/norma/127033/nahlad
• 29 CFR 1926.1153 Respirable crystalline silica (construction) – US OSHA, 2016: https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.1153
• US 6,572,697 B2, Fiber cement building materials with low density additives – US patent, 2003: https://patents.google.com/patent/US6572697B2/en
• US 11,773,023 B2, Methods for producing air-cured fiber cement products – US patent, 2023: https://patents.google.com/patent/US11773023B2/en
• Natural Fibre Reinforced Cement Composites, PhD thesis (Y. Ni) – Victoria University of Technology / CSIRO Division of Forest Products, 1995: https://vuir.vu.edu.au/15654/1/Ni_1995compressed.pdf
• Application of Natural and Synthetic Fibres as a Replacement for Asbestos Fibres in Cement Boards, PhD thesis (M. Khorami) – Coventry University, 2011: https://pure.coventry.ac.uk/ws/portalfiles/portal/42102466/Khorami2011.pdf
• Nanocelluloses: Natural-Based Materials for Fiber-Reinforced Cement Composites. A Critical Review (A. Balea, E. Fuente, A. Blanco, C. Negro), Polymers 11(3):518 – 2019: https://pmc.ncbi.nlm.nih.gov/articles/PMC6473712