Calcium Silicate Board Manufacturing Process

Calcium silicate board is made from lime, a finely ground siliceous material such as quartz sand, cellulose fiber and water, often with some cement and fillers. The slurry is formed into sheets, stacked and usually pressed, pre-cured, then cured in saturated steam in an autoclave (one patent gives 150–200 °C), where lime and silica react into calcium silicate hydrates. The boards are then dried, sanded and trimmed.

The autoclave is the step that defines the product. Fiber cement board can be air-cured or autoclaved; calcium silicate board cannot skip the autoclave, because the binder only forms there. This article follows the process step by step, explains what the plant has to control at each stage, and shows where a calcium silicate board line differs from a fiber cement line built on similar equipment.

What Calcium Silicate Board Is, and What It Is Not

EN 12467, the European standard for fiber-cement flat sheets, allows two kinds of matrix: cement, or a calcium silicate formed by a chemical reaction of a siliceous and a calcareous material, reinforced by fibers (EN 12467). Calcium silicate board is the second kind. Its strength comes mainly from calcium silicate hydrates formed in the autoclave, not from the hydration of Portland cement.

Two other products share the name and are often confused with it:

• Fiber cement board is cement-based, reinforced with cellulose and sometimes synthetic fibers, and may be air-cured. Its process is described in Fiber Cement Board Manufacturing Process, and the two products are compared in Calcium Silicate Board vs Fiber Cement Board.
• Calcium silicate insulation (high-temperature blocks and pipe sections for industrial plants) is a much lighter product, molded in steel molds by filter pressing rather than formed on a sheet machine. It is made on a separate calcium silicate insulation board and pipe section line.

This article covers the building board: the sheets used for partitions, ceilings, linings and decorative substrates.

The Calcium Silicate Board Manufacturing Process in Ten Steps

Steps 5, 6 and 9–10 look much like a fiber cement line. Steps 1, 2 and 8 are where calcium silicate board needs its own equipment and its own process control.

Raw Materials for Calcium Silicate Board

The chemistry needs two reactants, a calcium source and a silica source, plus fiber to hold the sheet together during forming and to give the finished board toughness.

• Calcareous material. Slaked lime or quicklime (Ask Corp., US 5,851,354). Cape Boards’ patent defines the calcareous component as any material containing more than 30% CaO, lime being the usual example (Cape Boards, US 4,132,555). Many producers also add some Portland cement, which supplies lime as it hydrates and gives green strength before the autoclave.
• Siliceous material. Silica sand, diatomaceous earth or fly ash (Ask Corp., US 5,851,354). Quartz sand is the most common and must be ground fine to react in the time the autoclave allows.
• Fiber. Cellulose pulp is the main fiber. Patents also list polypropylene, vinylon and glass fiber (Ask Corp., US 5,851,354).
• Fillers. Perlite, wollastonite, mica, talc, calcium carbonate and gypsum are used to adjust density, dimensional stability and cost (Ask Corp., US 5,851,354).

Published proportions give an idea of the balance. One patented formulation for lightweight board uses 17–50% calcareous material, 15–45% siliceous material, 2–8% fiber and 5–40% inorganic fillers by weight of solids (Ask Corp., US 5,851,354). A Cape Boards example pre-reacts 100 parts lime with 85 parts quartz to make tobermorite (Cape Boards, US 4,132,555). Neither is a general recipe.

Why the raw material test comes first

How much calcium silicate hydrate forms, and how well it crystallizes, depends on the purity, fineness and reactivity of your local lime and silica. Lime from one kiln and sand from one quarry will not behave like the materials a supplier tested at home. That is why we ask for samples of local sand, lime, cement and fiber before the line configuration is fixed: board density, strength and dimensional stability all depend on them.

Preparing the lime

If the plant buys quicklime, it needs a slaker to hydrate it into lime milk under controlled temperature and water ratio, with dust extraction on the feed. Hydrated lime can be stored in silos and dosed dry or as a slurry. Either way, the lime has to be consistent from batch to batch, because its reactivity drives the autoclave reaction.

Milling the silica

Quartz sand is ground in a wet ball mill into a fine slurry and stored in agitated tanks. Fineness is the key control. Coarse silica reacts only at the surface, leaving unreacted grains and weaker boards. Over-grinding costs power and can slow drainage on the board machine.

Pulping the fiber

Cellulose pulp is broken up in water and refined, as on a fiber cement line. Refined fibers trap fine lime and silica particles on the felt during forming. Without them, much of the fine material would drain away with the water.

Mixing, Forming and Pressing the Sheets

Lime, silica slurry, refined fiber, cement and fillers are weighed and mixed into a dilute slurry, then pumped steadily to the board machine. Both common machines build the board in thin wet layers on a moving felt and wind them onto a forming roll until the target thickness is reached:

• Flow-on forming applies the slurry continuously onto the felt, dewaters it by suction and winds it onto the roll (Nichiha, US 8,043,461).
• Hatschek forming uses rotating sieve cylinders in vats, each passing a thin film to the felt. Patents for calcium silicate board name the Hatschek sheet machine as a standard forming method (Ask Corp., US 6,139,620).

On the board lines we supply, both machines are available, and flow-on forming is the more common choice: the equipment is simpler, energy and running costs are lower, it tolerates more variation in raw materials, and it uses less water. The machines themselves are explained in The Hatschek Process for Fiber Cement Sheets.

Stacking and pressing

When the layers on the roll reach thickness, the wet sheet is cut off, laid flat and stacked between steel templates. The templates keep each board flat through pressing and curing.

Pressing is optional but common. It removes water, raises density and strength, and reduces moisture movement. Patent literature notes that pressing after molding raises bulk density (Ask Corp., US 5,851,354). On our reference designs, presses of 5,000, 7,000, 10,000 and 14,000 tonnes are matched to board density and output. Lightweight ceiling and lining boards may be pressed lightly or not at all; high-density boards for wet areas or heavy fixing need more pressure.

Pre-Curing, Autoclave Curing and Drying

Pre-curing

Before the stacks enter the autoclave, the green sheets need enough strength to survive steam heating without cracking or delaminating. One patent sets a minimum for this primary cure: curing above 15 °C, with (curing temperature − 15 °C) × curing time of at least 120 °C·h (Ask Corp., US 5,851,354). In other words, warmer pre-curing shortens the wait. Pre-curing space and template count are sized together.

Autoclave curing

In the autoclave, saturated steam raises the boards to a temperature where dissolved lime and silica react to form calcium silicate hydrates. Tobermorite, a crystalline calcium silicate hydrate, is the main binder in most autoclaved calcium silicate products (Connan, 2007). Cape Boards’ patent names tobermorite and xonotlite as the preferred crystal forms in calcium silicate board (Cape Boards, US 4,132,555).

Published curing conditions:

The right cycle depends on the silica fineness, the lime-to-silica balance and the board thickness. Thick, dense boards need more time for heat and steam to reach the core evenly.

On our reference designs, autoclaves are fitted with steam recovery: exhaust steam from one autoclave heats the next, saving about 25–30% of the steam input. The boiler and autoclaves set much of the plant’s energy use, so this is worth comparing between proposals.

Demolding, drying and sanding

After the autoclave, boards are separated from the steel templates. Boards that stick are a frequent cause of breakage, so our reference designs include an automatic board release unit after autoclaving. Templates are cleaned and sent back to the stacker.

Autoclaved boards are wet. They are dried to the moisture content the market and the standard expect; boards packed wet can shrink or bow after installation. Calcium silicate boards are then usually sanded on one or both faces to a calibrated thickness, trimmed to size and inspected. Our reference designs check boards on the line, so defects are caught before packing. A UV coating line can be added for decorative stone or wood finishes.

Board Sizes, Density and Standards

On our reference designs, the standard board is 2440 × 1220 mm, with 2400 × 1200 and 3660 × 1220 mm also made. Volume production covers 4–24 mm. On the same designs, calcium silicate board is made at 0.9–1.4 g/cm³, lighter than fiber cement board at 1.4–1.8 g/cm³. Patent literature gives 0.7–1.2 as the general range of bulk specific gravity for calcium silicate boards (Ask Corp., US 6,139,620), with lightweight grades below that.

The standard the boards must meet should be fixed before the line is configured:

• EN 12467 covers fiber-cement flat sheets, including the calcium silicate matrix, for internal and external wall and ceiling finishes. Sheets are grouped into categories A to D by exposure and five classes by bending strength. It does not cover sheets for fire protection purposes (EN 12467).
• JC/T 564.1-2018 is China’s standard for non-asbestos fiber-reinforced calcium silicate boards, used for walls, ceilings, partitions, ship partitions and composite insulation panels. It sets flexural strength grades R1 to R5 (JC/T 564.1-2018).

Boards sold as fire protection need fire testing under the system of the target market, because EN 12467 excludes them. That affects the formulation and density, so tell your supplier early if fire-rated products are planned.

Calcium Silicate Board vs Fiber Cement Board on the Same Line

Both boards can be made on the same type of line with different formulations and curing settings. The differences that matter for equipment are these:

A plant that plans both products should size the silica mill, lime preparation and autoclaves for the calcium silicate side, because that is the more demanding of the two.

FAQ

What are the main raw materials of calcium silicate board?

A calcareous material (quicklime or slaked lime, often with some cement), a siliceous material (usually finely ground quartz sand, sometimes diatomaceous earth or fly ash), cellulose fiber and water. Fillers such as perlite, wollastonite or mica adjust density and stability. Modern boards are asbestos-free, and China’s JC/T 564.1-2018 is written specifically for non-asbestos calcium silicate boards.

Why must calcium silicate board be autoclaved?

Because the binder forms in the autoclave. In saturated steam at autoclave temperatures (published patent ranges are 150–200 °C), lime and silica react into calcium silicate hydrates, mainly tobermorite. At room temperature, lime and quartz react far too slowly to make a usable board. Fiber cement, whose strength comes from cement hydration, can be air-cured instead.

What autoclave temperature and time are used for calcium silicate board?

Published patent ranges are 150–200 °C in saturated steam for 5–20 hours, with 170–190 °C for 8–12 hours preferred, and examples at 180 °C for 10 hours (Ask Corp., US 5,851,354). The right cycle for your plant depends on silica fineness, the lime-silica balance and board thickness, and is set during trials.

What is the density of calcium silicate board?

On our reference designs, calcium silicate board is typically 0.9–1.4 g/cm³. Patent literature gives 0.7–1.2 as the general bulk specific gravity range, and one lightweight-board patent targets about 0.5–0.8. Density is set mainly by the formulation, fillers and pressing, so choose the target density before selecting the press.

Is calcium silicate board the same as calcium silicate insulation?

No. Building board is denser and is used for partitions, ceilings and linings. Calcium silicate insulation is a much lighter product for high-temperature industrial pipes and equipment, molded by filter pressing in steel molds, and it needs a different line. The two should not be quoted from one equipment list.

Planning a Calcium Silicate Board Plant?

See how lime and silica preparation, forming, pressing and autoclaving are configured 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, insulation board and pipe sections, but its planning sequence from raw material tests to commissioning is a useful reference.

Sources

• CEN, EN 12467:2012+A2:2018 Fibre-cement flat sheets – Product specification and test methods (SIST adoption listing): https://iteh.es/catalog/standards/sist/e6248861-5e94-4617-a934-1a3c85c01195/sist-en-12467-2013a2-2018
• 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
• Ask Corp., US 5,851,354 A, Method of manufacturing calcium silicate board: https://patents.google.com/patent/US5851354A/en
• Ask Corp., US 6,139,620 A, Calcium silicate board and method of manufacturing same: https://patents.google.com/patent/US6139620A/en
• Cape Boards and Panels Ltd, US 4,132,555 A, Building board: https://patents.google.com/patent/US4132555A/en
• H. Connan, Novel autoclaved fibre-cements utilising clay-brick waste, PhD thesis, University of Technology Sydney, 2007: https://opus.lib.uts.edu.au/handle/10453/49102
• Nichiha Corp., US 8,043,461 B2, Method for manufacturing fiber reinforced cement board: https://patents.google.com/patent/US8043461B2/en