Calcium Silicate Board Raw Materials: Silica, Lime, Cement and Fiber

Calcium silicate board raw material comes down to four groups: a silica source (usually ground quartz sand), a lime source (quicklime or slaked lime, often with some cement), reinforcing fiber (usually cellulose pulp) and fillers. In the autoclave, lime and silica react into tobermorite-type calcium silicate hydrates, so their purity, fineness and Ca/Si balance largely decide board strength.

This article is about buying and specifying those materials. The plant steps that turn them into board are covered in our calcium silicate board manufacturing process guide. Here the questions are different: how much lime does your silica need, how fine is fine enough, and what should you test before committing to a local quarry or lime kiln?

Calcium Silicate Board Composition: What the Board Is Made Of

A finished board is mostly calcium silicate hydrate binder, with unreacted silica, fiber and fillers held in it. It is made from a dilute slurry, so water is also a process material, though most of it leaves during forming, pressing and drying.

Published patents show how widely the composition can vary, and also what producers treat as the limits of each group.

These are examples from individual patents, not a recipe. They describe different products at different densities. Treat them as a map of what has been done, then fix your own mix by trial. On the lines we supply, calcium silicate boards typically run at 0.9–1.4 g/cm³, and a target in that range changes how much filler and fiber the formula can carry and how hard the board is pressed.

The Ca/Si Ratio: How Much Lime Your Silica Needs

The Ca/Si molar ratio is the first number to fix in a calcium silicate formula. Tobermorite has a Ca/Si ratio of about 0.83 and xonotlite 1.0; tobermorite is ordinarily synthesized at about 180 °C and xonotlite at about 200 °C (Noma, Kyushu University, 1998).

The most directly useful study for board makers is by Sakiyama, Oshio and Mitsuda (2000). They molded lime, quartz and cellulose fiber to 1,000 kg/m³ at Ca/Si ratios of 0.6, 0.8 and 1.0, using quartz of 2.6, 8.4 and 15.5 µm average particle size, and autoclaved the boards at 180 °C for 1–80 hours. Three results matter to a producer:

1. Short cycles favor fine quartz and low Ca/Si. These gave the highest early strength and a finer microstructure.
2. Long cycles change the answer. With longer treatment, the coarser quartz at Ca/Si 0.8 gave the highest strength, linked to better tobermorite crystallinity.
3. Overcuring reduces strength. After 80 hours, tobermorite decomposed into gyrolite at Ca/Si 0.6 and 0.8, and into xonotlite at 1.0, with lower strength and a coarser microstructure.

So raw material specification and autoclave cycle have to be settled together. A silica grind that suits a long cycle may not suit a short one.

Converting the molar ratio into a batch

Plants batch by weight, so the ratio has to be converted. The figures below are our own stoichiometric estimates for pure materials, using molar masses of 60.08 (SiO₂), 56.08 (CaO) and 74.09 (Ca(OH)₂).

Real materials are not pure. As an illustrative estimate, take 100 kg of sand with 95% SiO₂, a quicklime with 85% available CaO and a target of 0.83. The sand supplies 95 kg of SiO₂, which calls for about 74 kg of CaO, or about 87 kg of that quicklime.

Two corrections follow. If the formula includes cement, count the calcium and silica it brings, using its actual analysis. And use available CaO, not total CaO, because lime that has already carbonated will not take part in the reaction in the same way. This is why a total-oxide figure on a lime supplier’s data sheet is not enough on its own.

Silica Source: Quartz Sand and Its Alternatives

Quartz sand is the common choice because it is widely available and can be ground in a wet mill to a controlled size. The alternatives react differently, and “more reactive” does not always mean “better.”

Noma’s thesis cites work showing why amorphous silica needs care: C-S-H formed from amorphous silica at a Ca/Si of about 0.8 was stable and converted to tobermorite only very slowly (Mitsuda et al., cited in Noma, 1998). A silica that dissolves faster may produce binder sooner, yet produce less of the crystalline phase you want in the same cycle.

How fine should the silica be?

Noma compared two quartz sands at 190 °C. The finer one formed C-S-H, then 1.1 nm tobermorite, then xonotlite. The coarser one first formed α-C₂SH, which then converted to C-S-H and finally a small amount of xonotlite. The thesis concludes that the reaction “highly depends on the rate of dissolution of the siliceous materials.”

Fineness therefore belongs in the purchase and mill specification, not just the operator’s notes. One patent sets the silicate material at a Blaine surface area of at least 3,000 cm²/g, and preferably 7,000 cm²/g or more (US 6,464,771). That is one producer’s specification for one product. Measure particle size on every mill batch, and fix the target during trials with your own sand.

Lime Source: Quicklime, Slaked Lime and Reactivity

Lime is usually bought as quicklime or as slaked (hydrated) lime. Quicklime brings more CaO per tonne but needs slaking at the plant. Slaked lime is simpler to handle but is already diluted by combined water. The commercial comparison depends on local supply and freight.

Purity alone does not predict behavior. Baltakys and Prichockiene (2010) reacted two limes, one 99.96% CaO and one 96% CaO, with amorphous silica at 175 °C:

• At Ca/Si 0.83, the C-S-H made with the purer lime only partly recrystallized into 1.13 nm tobermorite even after 72 hours. With the less reactive 96% lime, tobermorite was dominant by 24 hours.
• At Ca/Si 1.0, the more reactive lime gave both tobermorite and xonotlite after 72 hours. The less reactive lime gave tobermorite and traces of gyrolite, with no xonotlite identified.

These were unstirred laboratory suspensions, not board slurries, and they used a hydrated silica rather than quartz. The practical lesson still carries over: lime reactivity changes which crystals form, and a change of lime kiln can change your board even when the analysis looks acceptable. Keep one approved source where possible, test every new source before use, and track its reactivity lot by lot.

Cement in Calcium Silicate Board

Many building boards add Portland cement alongside lime. It supplies calcium as it hydrates and gives the sheet some strength before it reaches the autoclave. US 6,464,771 lists ordinary Portland cement among its lime materials.

Cement also changes the sums and the risks:

• It carries its own silica, alumina and sulfate, so include its analysis in the Ca/Si calculation.
• Sugars from pulp can interfere with it. Global Cement’s overview notes that cellulose fibers can contain sugars and other organics that increase cement setting time, and that kraft pulps release much less sugar, so pulp choice and cement are linked.
• A high-cement mix can drift toward fiber cement. The difference between the two boards is largely one of formulation. See calcium silicate vs fiber cement board for how that choice affects the product, and the standards it falls under.

Reinforcing Fiber: Why Cellulose Dominates

Cellulose pulp is the main fiber in the board patents cited above, and it is the fiber specified for our asbestos-free board lines. In the slurry it forms the filter mat that retains fine lime and silica during forming. In the finished board it bridges cracks and gives toughness.

The purchasing details belong in their own guides. Pulp grades, refining and freeness are covered in cellulose fiber cement board, and the fibers that replaced asbestos in asbestos-free fiber cement board. For calcium silicate board, one extra check applies: the fiber has to survive the autoclave. Before you specify a synthetic fiber, confirm that it keeps its properties at your curing temperature and in a lime-rich environment.

Fillers, Gypsum and Additives

Fillers adjust density, cost and dimensional stability, but they compete for water and can weaken the binder if overdosed. Three published findings show the trade-offs:

• Gypsum is not neutral. Adding 0–20 mass% gypsum to a lime–quartz mix formed hydroxyl ellestadite, which delayed strength gain. Later in the cycle it decomposed into tobermorite and anhydrite, giving a finer microstructure and higher strength (Sakiyama et al., 2000). US 6,464,771 favors about 0.5–10% gypsum dihydrate and says it improves carbonization resistance.
• Aluminum salts can raise strength, up to a point. US 5,851,354 adds alunites, alums or aluminum sulfate at 2–20% of solids. When applied between layers at more than 50 g/m², a layer can form in which cracks develop on drying and flexural strength falls.
• Lightweight fillers change the water balance. Perlite and similar materials reduce density, but check how the moisture behavior changes. The same principle is discussed for fiber cement in our article on fiber cement problems.

A filler chosen to make the board cheaper can also affect its fire performance, which most buyers of this product care about. Check the finished board against the requirements in calcium silicate board fire rating after any substantial formula change.

Testing Calcium Silicate Board Raw Material Before the Line Is Configured

Board density, strength and dimensional stability depend on your local sand, lime, cement and fiber, so we ask for samples to be tested before the line configuration is fixed. A useful program has four stages:

1. Characterize. Run oxide analysis on sand, lime and cement; mineral identification on the sand; particle size at the intended grind; and available CaO and reactivity on the lime.
2. Calculate. Set a starting Ca/Si from the oxide data and your target density, then write the mix on a dry-solids basis.
3. Trial. Make laboratory boards across a small range of Ca/Si and silica fineness, and autoclave them for several cycle lengths. Sakiyama’s work shows why one cycle is not enough.
4. Verify. Measure density, bending strength, moisture movement and, ideally, crystal phases by X-ray diffraction. Then compare the results with the standard you will sell to. In China that is JC/T 564.1-2018 for non-asbestos calcium silicate boards.

The results also feed the equipment design. They show whether you need a lime slaker or can dose hydrated lime directly, how hard the silica mill must work, and how long each autoclave cycle will be, which affects the number of autoclaves. It is much easier to change these on paper than after the equipment is built.

Sample at least several lots over time, including from the source you would fall back on. A single selected sample tells you what a quarry can do at its best, not what it will send you each month.

FAQ

What is calcium silicate board made of?

Calcium silicate board is made of a silica source, usually ground quartz sand, a lime source such as quicklime or slaked lime, often some Portland cement, reinforcing fiber, usually cellulose pulp, and fillers such as perlite, wollastonite or mica. New boards are normally asbestos-free. Steam curing in an autoclave reacts the lime and silica into a calcium silicate hydrate binder, mainly tobermorite.

What is the typical calcium silicate board composition?

There is no single composition, since it depends on density and use. One patented lightweight board uses 17–50% calcareous material, 15–45% siliceous material, 2–8% fiber and 5–40% fillers by weight of solids (US 5,851,354). Use published ranges as a guide only, then set your own mix by trials with your local materials and your autoclave cycle.

What Ca/Si ratio is used in calcium silicate board?

Tobermorite has a Ca/Si molar ratio of about 0.83, so formulas aimed at tobermorite often work near that value. A published board study tested 0.6, 0.8 and 1.0 at 180 °C, and the best ratio depended on quartz fineness and cycle length (Sakiyama et al., 2000). Confirm your own ratio by trial, counting the calcium and silica supplied by any cement.

Can fly ash replace quartz sand in calcium silicate board?

Fly ash is named as a siliceous material in board patents, so it can work. It is not a simple swap. Its composition and carbon content vary by power plant, and it brings alumina and other oxides into the reaction. Test the ash from your actual source for strength and moisture movement, and secure a long-term supply before you design the formula around it.

Do calcium silicate board and calcium silicate insulation use the same raw materials?

They share the same basic chemistry, with silica, lime and fiber, but they are different products made on different lines. Insulation is much lighter and is designed for high service temperatures, so its crystal phases and formulation differ. See calcium silicate insulation properties for how that product is specified.

Planning a Calcium Silicate Board Plant?

See how silica and lime preparation, fiber pulping, batching and forming are arranged for 3, 5 or 8–10 million m² a year on our calcium silicate board production line page. Our calcium silicate insulation plant setup guide covers a different product, but its approach to raw material testing and planning is a useful reference.

Sources

• Sakiyama, Oshio and Mitsuda, Influence of Quartz Particle Size and Ca/Si Ratio on Strength of Autoclaved Calcium Silicate Board, Journal of the Society of Inorganic Materials, Japan, 7(287), 2000: https://www.jstage.jst.go.jp/article/mukimate2000/7/287/7_287_278/_article/-char/en
• Sakiyama, Oshio and Mitsuda, Influence of Gypsum on the Hydrothermal Reaction of Lime-Quartz System and on the Strength of Autoclaved Calcium Silicate Product, Journal of the Society of Inorganic Materials, Japan, 7(289), 2000: https://www.jstage.jst.go.jp/article/mukimate2000/7/289/7_289_685/_article/-char/en
• H. Noma, Solid-State NMR Investigation on Crystallization of Xonotlite from Poorly-Crystalline Calcium Silicate Hydrate, doctoral thesis, Kyushu University, 1998: https://catalog.lib.kyushu-u.ac.jp/opac_download_md/25091/noma.pdf
• K. Baltakys and E. Prichockiene, Influence of CaO reactivity on the formation of low-base calcium silicate hydrates, Materials Science-Poland, 28(1), 2010: https://www.materialsscience.pwr.edu.pl/bi/vol28no1/articles/ms_28_2008_681baltakys.pdf
• US 6,464,771 B1, Cured calcium silicate object with high strength: https://patents.google.com/patent/US6464771
• US 8,197,642 B2, Inorganic board and method for manufacturing the same: https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/8197642
• Ask Corp., US 5,851,354 A, Method of manufacturing calcium silicate board: https://patents.google.com/patent/US5851354A/en
• Cape Boards and Panels Ltd, US 4,132,555 A, Building board: https://patents.google.com/patent/US4132555A/en
• A. Saunders and E. Davidson, “Cement Boards 101”, Global Cement Magazine, December 2013: https://globalcement.com/magazine/articles/825-cement-boards-101
• JC/T 564.1-2018 Fiber reinforced calcium silicate boards – Part 1: Non-asbestos calcium silicate boards (standard listing): https://www.chinesestandard.net/PDF/English.aspx/JCT564.1-2018