Calcium Silicate Board Density: Low, Medium and High Density Grades

Calcium silicate board density for building boards is about 0.6–1.2 g/cm³ (600–1,200 kg/m³) in one published patent background, and 0.7–1.2 g/cm³ in another. On our reference designs, calcium silicate board is made at 0.9–1.4 g/cm³, and fiber cement board at 1.4–1.8 g/cm³. Lighter boards are easier to cut and lift; denser boards are stronger and absorb less water.

For a plant owner, density is not just a line on a data sheet. It is a design input. The density you sell decides how much lightweight filler the formula carries, how hard the sheets are pressed, how much water the dryer removes and which customers the board suits. This article explains the density grades, what each one is used for, and how density is controlled from batching to drying. The step-by-step process itself is covered in our calcium silicate board manufacturing process guide.

Calcium Silicate Board Density at a Glance

Calcium silicate products cover a very wide density range, and only the middle of it is building board. A published patent background sorts calcium silicate boards by apparent specific gravity: heat insulating material at about 0.3 or less, fireproof coating materials at about 0.3–0.4, and fireproof building materials at about 0.6–1.2 (US 6,962,623, 2005). Another patent gives 0.7–1.2 as the general bulk specific gravity of calcium silicate boards (US 6,139,620, 2000).

The table shows why “calcium silicate” needs a qualifier. A light insulation product and a 1.2 g/cm³ partition board share the same chemistry but are made on different lines and sold to different buyers. If your project is industrial insulation, see calcium silicate insulation properties instead.

Low, Medium and High Density Calcium Silicate Board

There is no international standard that grades calcium silicate building board by density, so producers and buyers use working bands. (ASTM C656 does grade calcium silicate by density, but it covers structural insulating board, a different product; see the standards section below.) The bands below are our own working classification, built on the published ranges above, not taken from a standard.

The calcium silicate board range on our reference designs is 0.9–1.4 g/cm³, which covers the medium and high bands. Boards below 0.9 g/cm³ are described here as industry background.

Low density calcium silicate board

Low density calcium silicate board is attractive because installers handle less weight, but it is harder to make well. A published patent notes that reaching low density by conventional means needed “a considerable amount of light-weight aggregate,” which lowered the specific strength of the board (US 6,139,620, 2000). The same patent warns that when bulk specific gravity drops too far, strength can become insufficient for a building material. Low density is a formulation achievement, not just a lighter version of a standard board.

Medium and high density boards

Most building boards sit in the middle band. Above about 1.2 g/cm³, calcium silicate board starts to overlap with fiber cement, and the choice between the two becomes a question of binder, curing and market rather than weight. Our article on calcium silicate vs fiber cement board compares the two products, including their density.

What Density Means for Strength, Water and Heat

Density ties strength, water absorption and thermal behavior together. Higher density means fewer pores, which usually means higher strength and lower water absorption. Lower density means more pores, which usually means lower thermal conductivity and lower weight.

The first two findings below come from fiber-reinforced cement sheets, not calcium silicate board, but the pore logic is the same:

• Water absorption falls as density rises (fiber cement data). In a 1995 doctoral study at Victoria University of Technology and CSIRO, commercial fiber-reinforced cement sheets at 1.31, 1.40 and 1.62 g/cm³ absorbed 32.1%, 30.5% and 18.6% water; the densest was listed as a compressed sheet (Ni, 1995). The thesis explains that “as the density is lowered, the void volume increases with an associated potential increase in water absorption.”
• Fiber lowers density (fiber cement data). In the same study, autoclaved laboratory sheets fell from 1.62 g/cm³ at 2% fiber to 1.00 g/cm³ at 22% fiber, and air-cured sheets were denser than autoclaved ones (Ni, 1995).
• Porosity lowers thermal conductivity (calcium silicate board). A NIST study of two calcium silicate boards of different densities linked their thermal conductivity to porosity and pore size, measured from room temperature to 400 °C (Do, Bentz and Stutzman, Journal of Building Physics, 2007).

No single density is “best.” A ceiling board and a wet-area lining need different densities, so decide which band your market buys before the line is configured.

How Density Is Set in the Plant: Formulation

Formulation sets the density range a board can reach; pressing and curing then fine-tune it within that range. The main formulation levers are:

1. Lightweight aggregates and fillers. They lower density but also lower strength, so they are dosed with care (US 6,139,620, 2000). One fiber cement patent lists mica, calcium carbonate, wollastonite and organic density modifiers among its additives, and defines low density additives as materials with a loose bulk density of about 0.8 g/cm³ or less (US 6,572,697, 2003).
2. Fiber content. More cellulose fiber means a lighter, tougher board, as the CSIRO data above show. Fiber also changes drainage on the forming machine.
3. Binder chemistry. The lime–silica ratio and silica fineness decide how much tobermorite forms in the autoclave, and so how much strength the board keeps at a given density. Our guide to calcium silicate board raw materials explains the Ca/Si ratio and silica grind.
4. Water. The solids content of the slurry and the water left in the green sheet affect the final pore volume.

Change one lever and the others move. A formula made lighter with extra filler may need more fiber or a finer silica to keep its strength. That is why the density target has to be written into the formula brief, not added after the equipment is chosen.

How Pressing and Curing Change Density

Pressing raises density; leaving the board unpressed keeps it light. That is the simplest rule in board production, and it drives one of the largest equipment decisions on the line.

Pressing

After forming, green sheets can be stacked between steel templates and pressed. A calcium silicate board patent notes that when a molded body is pressed before the hydrothermal reaction, “the bulk specific gravity increases and more labor is required” (US 6,139,620, 2000). In the fiber cement literature reviewed by Ni (1995), most reported pressures were about 2–3 MPa, with one very high stack compression pressure of about 20 MPa; flexural strength rose with casting pressure without a loss in fracture toughness.

On our reference designs, presses of 5,000, 7,000, 10,000 and 14,000 tonnes are matched to board density and output. Tell the supplier which densities you plan to make, because the press follows that decision.

Curing and drying

Calcium silicate board is cured in an autoclave, where lime and silica react into calcium silicate hydrates. The crystals that form give the board its strength at a given density, so the autoclave cycle and the formula are set together.

Drying then removes free water. Density is usually quoted on dry board, so check whether any figure you are given is a dry or an as-delivered value. A board at delivery carries some moisture and weighs more than its dry density suggests.

Density, Weight per Square Meter and Freight

Density becomes weight per square meter, and that is what installers, framing designers and freight planners work with. The table converts common densities into mass per square meter for three thicknesses; the lb/ft³ values (1 g/cm³ = 62.43 lb/ft³) and masses are our arithmetic on dry density.

A 9 mm board at 1.6 g/cm³ weighs about 60% more per square meter than the same board at 1.0 g/cm³ (our estimate from the table: 14.4 versus 9.0 kg/m²). For an export plant, that difference shows up in freight per square meter, pallet design and forklift capacity. The top of the table is fiber cement territory: conventional fiber cement building sheets are about 1.2–1.7 g/cm³ according to a patent background (US 6,572,697, 2003). The full product comparison is in our calcium silicate vs fiber cement board article.

How Standards Treat Calcium Silicate Board Density

The main building board standards require density to be measured and reported, but none of them fixes one density for every board. ASTM C656 is the exception that proves the point: it does grade by density, but for a different product.

• EN 12467 (CEN) covers fiber-cement flat sheets, including sheets whose matrix is a calcium silicate formed from siliceous and calcareous materials. Apparent density is one of its requirements (clause 5.4.2) and test methods (clause 7.3.1), and it is measured on the external dimensions of the sample, so pores are included (EN 12467:2012+A2:2018). Check the current edition for how a declared value and its tolerance are to be stated. Our EN 12467 guide explains its categories and classes.
• ASTM C1186 (ASTM International) lists density among the physical requirements for flat fiber-cement sheets, alongside flexural strength (ASTM C1186-26). Check the current edition for the values and the test method.
• JC/T 564.1-2018 (China) covers non-asbestos calcium silicate boards. According to its foreword, the apparent density is stated by the supplier in the quality document, and it remains an exit-factory inspection item. Boards are classed A, B and C by use, with flexural strength grades R1 to R5. Older Chinese density classes may still appear in supplier documents, so check which edition a supplier is quoting.
• ASTM C656 (ASTM International) covers calcium silicate structural insulating board for general thermal insulation, fire-resistive and marine bulkhead uses, up to 927 °C. It has Type I (760 °C) and Type II (927 °C), and Grades 1 to 8 by typical density, from 224 kg/m³ (Grade 4) up to 961 kg/m³ (ASTM C656-17(2023); check the current edition). It is not a specification for partition or ceiling board, so do not quote its grades for a building board.

The practical consequence: your customer’s specification, not the standard alone, will usually name the density. Ask for it early.

Choosing a Density Range Before You Configure the Line

Choose your density range before you choose the press, the filler handling and the dryer. Settling it first avoids paying for capacity you will not use, or discovering that the line cannot reach the board your market wants.

1. List the products and markets. Ceilings and fire linings point to lower density; wet areas, coated substrates and exterior use point to higher density, and possibly to fiber cement.
2. Fix the density band and the thickness range together. Mass per square meter, not density alone, decides handling and freight.
3. Check that your raw materials can reach the target. A sand or lime that works at 1.2 g/cm³ may lose too much strength at 0.9 g/cm³, so trial boards should be made at the densities you intend to sell, not only at a convenient middle value.
4. Tell the supplier the densest product. Press tonnage follows the density target and output, so the densest board on your list matters most.
5. Plan the laboratory. Density, bending strength and water absorption should be checked on every batch, because density drift is the earliest sign of a formula or forming problem.

If fire performance is part of the product plan, test boards at the intended density. See calcium silicate board fire rating for how fire classification works.

FAQ

What is the density of calcium silicate board?

Published patent backgrounds give about 0.6–1.2 g/cm³ for calcium silicate building boards used as fireproof building materials, and 0.7–1.2 g/cm³ as the general range. On our reference designs calcium silicate board is made at 0.9–1.4 g/cm³. Calcium silicate insulation is a different product and much lighter, around 0.3 g/cm³ or less.

What is low density calcium silicate board used for?

Low density calcium silicate board, below about 0.9 g/cm³, is used in the industry for ceilings, fire-protection linings and light partitions, where low weight and easy cutting matter more than maximum strength. It needs a carefully designed formula, because adding lightweight aggregate to reduce density also reduces strength, as published patent literature notes.

Is higher density calcium silicate board better?

Not in every use. Higher density usually brings higher strength and lower water absorption, as published research on fiber-reinforced sheets shows, but it also adds weight and usually raises thermal conductivity. Match the density to the application: ceilings and linings favor lighter boards, wet areas and coated substrates favor denser ones.

Does ASTM C656 apply to calcium silicate building board?

No. ASTM C656 covers calcium silicate structural insulating board for thermal insulation, fire-resistive and marine bulkhead uses, with eight density grades. Partition and ceiling boards are normally specified to EN 12467, ASTM C1186 or national standards such as JC/T 564.1 in China. Check which standard your customers actually name.

How is calcium silicate board density measured?

Density is calculated from the dry mass of a sample divided by its volume. EN 12467 measures apparent density on the sample’s external dimensions, so pores are included. Check that test results state whether the board was dried first, because moisture in an as-delivered board makes it appear denser than its dry value.

Planning a Calcium Silicate Board Plant?

Board sizes, density ranges, press tonnages and the forming options for 3, 5 or 8–10 million m² a year are set out on our calcium silicate board production line page. Tell us the standard your market uses and whether the boards are for interior or exterior use: it decides the formulation, the density and the press you need. If you are also weighing an industrial insulation product, the calcium silicate plant setup guide covers that line.

Sources

• US 6,962,623 B2, Hardened calcium silicate having high strength (background section) – US patent, 2005: https://patents.google.com/patent/US6962623
• US 6,139,620 A, Calcium silicate board and method of manufacturing same – US patent, 2000: https://patents.google.com/patent/US6139620A/en
• US 6,572,697 B2, Fiber cement building materials with low density additives – US patent, 2003: https://patents.google.com/patent/US6572697B2/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
• Microstructure and Thermal Conductivity of Hydrated Calcium Silicate Board Materials (C. T. Do, D. P. Bentz, P. E. Stutzman), Journal of Building Physics 31(1), 55–67, DOI 10.1177/1744259107079020 – NIST, 2007: https://www.nist.gov/publications/microstructure-and-thermal-conductivity-hydrated-calcium-silicate-board-materials
• 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
• ASTM C1186-26 Standard Specification for Flat Fiber-Cement Sheets – ASTM International, 2026: https://store.astm.org/c1186-26.html
• ASTM C656-17(2023) Standard Specification for Structural Insulating Board, Calcium Silicate – ASTM International, 2023: https://store.astm.org/c0656-17r23.html
• JC/T 564.1-2018 Fiber reinforced calcium silicate boards – Part 1: Non-asbestos calcium silicate boards – Ministry of Industry and Information Technology of China, 2018 (standard text not freely available online; check the current edition)