Calcium Silicate (Sand-Lime) Bricks vs Clay Bricks

Calcium silicate (sand-lime) bricks are pressed from sand and lime and hardened in saturated steam at about 200 °C; clay bricks are formed from clay or shale and fired at around 1,000 °C. Sand-lime bricks leave the press at their final size and, in the EPDs compared here, carry less carbon per tonne, but they shrink over time and need closer movement joints than clay brickwork.

This comparison is written for investors and plant managers choosing between the two plants, and for anyone specifying either product. It covers raw materials and process, energy and carbon, strength and movement, the cracking problems calcium silicate brickwork is known for, standards and markets, and how to decide which plant fits your site.

Calcium Silicate vs Clay Bricks at a Glance

Sources for each row are given in the sections below. The movement values are the ranges in Eurocode 6 (EN 1996-1-1), as reproduced by Poland’s Building Research Institute ITB (Jarmontowicz and Sieczkowski, 2016); check the current edition of EN 1996-1-1 and its national annex before you design with them.

Raw Materials and Process: Autoclaving vs Firing

Calcium silicate bricks

A calcium silicate brick needs only sand, lime and water. On the lines we supply, lime is 8–12% of the mix and the rest is sand. After mixing, the quicklime slakes in reactors; the units are then pressed and steam-hardened. Germany’s sand-lime research association puts hardening at around 200 °C for roughly four to eight hours, with the time set by the unit’s shape (Eden, ZKG International, 2019). On our lines, curing runs at about 1.4 MPa and 195 °C for about 7 hours.

In the autoclave, silica from the surface of the sand grains reacts with the lime to form calcium silicate hydrates, which bind the grains together. The brick leaves the press at its final dimensions, because nothing in curing makes it shrink. That makes close dimensional tolerances easier to hold. For the full process, see the sand-lime brick manufacturing process.

Clay bricks

Clay bricks take a much wider range of raw materials. China’s standard for fired common bricks, GB/T 5101-2017, covers bricks made mainly from clay, shale, coal gangue, fly ash, construction waste, river silt and sludge (GB/T 5101-2017 record). The clay is ground, tempered with water, formed, dried and fired; in the United States most bricks are extruded by the stiff-mud process. The Brick Industry Association reports drying shrinkage of about 2–4% and firing shrinkage of about 2.5–4%, so dies and cutters are oversized to compensate, and firing takes about 10 to 40 hours (BIA Technical Note 9).

Firing temperatures are an order of magnitude higher than in an autoclave. The UK brick industry’s sector EPD says vitrification “normally takes place at around 1000°C” (BDA EPD, BRE Global); on the lines we supply, firing is typically 950–1,050 °C, up to 1,150 °C. For the steps, see the clay brick manufacturing process.

Energy and Carbon: What the EPDs Show

Environmental product declarations (EPDs) give comparable cradle-to-gate figures (modules A1–A3) when they follow EN 15804. Two public examples:

Sources: ITB EPD 634/2024; BDA UK Clay Brick EPD. The BDA EPD covers 46 UK sites making 99% of UK brick output.

Read these figures with care:

• Where the carbon comes from differs. In the H+H EPD, raw material supply (module A1), which includes the lime, accounts for 131 of the 214 kg CO₂e, about 61% by our arithmetic. Lime is made by burning limestone, which releases CO₂ from the stone as well as from the fuel. In a clay brick, the carbon comes largely from firing: the kiln fuel, and any carbonates in the clay.
• The grids and years differ. The Polish EPD uses an electricity factor of 0.685 kg CO₂ per kWh; the UK EPD reflects UK energy in its own reference years.
• Per tonne is not per wall. Calcium silicate units are dense; a hollow clay block puts far less material into a square meter of wall. Compare per m² of the wall you will actually build.
• Plant figures differ from averages. A clay plant firing on gas with waste-heat drying and a sand-lime plant buying lime from a distant kiln will both differ from these averages.

For a new plant, the practical reading is this: a calcium silicate plant’s footprint depends heavily on the lime it buys, while a clay plant’s depends on its kiln and fuel. See clay brick firing temperature for how the firing curve drives fuel use.

Performance: Strength, Moisture and Movement

Strength

Both products cover a wide strength range, so compare classes, not product names. China’s GB/T 11945-2019 grades autoclaved sand-lime bricks from MU10 to MU30 (GB/T 11945 record). A 2020 study of three Polish calcium silicate units of 1,638–1,840 kg/m³ measured dry compressive strengths of 26.6–39.6 MPa (Garbalińska et al., Materials, 2020).

Moisture matters more than many buyers expect. The same study found that saturation reduced the compressive strength of the three calcium silicate units by about 31–37%. EN 772-1 allows for this with conversion factors: 1.0 for air-dry units or units at 6% moisture, 0.8 for oven-dried units and 1.2 for units tested after immersion, as reported in that study. When you compare strength claims, ask how the units were conditioned.

Moisture and thermal movement

This is the clearest technical difference between the two. Eurocode 6 gives these ranges for long-term moisture movement and thermal expansion (Jarmontowicz and Sieczkowski, 2016):

Negative values mean shortening. Clay brickwork tends to grow after firing as the units take up moisture; calcium silicate brickwork shrinks as it dries, and it also moves more with temperature.

Calcium Silicate Brick Problems: Cracking and How to Avoid It

Most reported calcium silicate brick problems are cracks, and most cracks come from treating sand-lime brickwork like clay brickwork. A wall that shrinks and moves more with temperature needs more frequent joints. UK warranty guidance that refers to PD 6697:2019 gives movement joint spacing of 7.5–9 m for calcium silicate brickwork, against 12 m for clay brickwork, with 15 mm joints for clay and 10 mm for calcium silicate (LABC Warranty, 2024).

What causes cracking, and what prevents it:

1. Joints spaced for clay. Applying clay spacing to a shrinking wall concentrates the shrinkage at windows, doors and changes of height. Follow the spacing for calcium silicate, or your national code.
2. Mixing the two materials in one panel. A clay brick that expands and a sand-lime brick that shrinks pull against each other. Separate them with a movement joint or detail the junction for differential movement.
3. Laying wet units. A unit laid wet shrinks as it dries in the wall, which adds to the long-term shrinkage. Keep pallets covered on site.
4. Ignoring temperature. Long, sunlit walls in hot climates move more with calcium silicate than with clay, because of the higher expansion coefficient.

None of this makes calcium silicate a weaker product. It makes it a different one: the designer and the bricklayer need to know which brick they are using. For a producer, this belongs in the technical literature you give to customers.

Standards and Markets

Check the current edition of each before fixing a product specification (ASTM C73-23; EN 771-2:2011+A1:2015; ASTM C62-23).

Market acceptance varies by country. In Poland, calcium silicate units held 15.1% of the wall material market in 2017, up from 9% a decade earlier, against 39.0% for AAC and 28.6% for ceramic units, according to figures cited by Garbalińska et al. (2020). In the UK, by contrast, the clay brick industry’s EPD covers 46 sites making 99% of the country’s brick output. A market where builders, codes and bricklayers know only clay brick will need product literature, test reports and detailing guidance before calcium silicate sells.

Calcium Silicate or Clay Brick Plant: Which Fits Your Project?

The product comparison decides the market; the raw materials decide the plant. Use these factors:

A calcium silicate plant can also make autoclaved fly ash bricks with the same presses and autoclaves, if batching is designed for both. See autoclaved fly ash bricks.

FAQ

Are calcium silicate bricks better than clay bricks?

Neither is better in general. Calcium silicate bricks have precise dimensions, high strength classes and, in the EPDs compared here, lower carbon per tonne. Clay bricks expand rather than shrink, move less with temperature, come in many hollow formats and are accepted almost everywhere. The right choice depends on the wall, the climate, the local standard and what builders know.

Why do calcium silicate bricks crack?

Mostly because the wall shrinks. Eurocode 6 gives calcium silicate masonry a long-term moisture movement of −0.4 to −0.1 mm/m, and a higher thermal expansion than clay. If movement joints are spaced as for clay brickwork, or units are laid wet, the shrinkage shows up as cracks. UK guidance spaces joints at 7.5–9 m for calcium silicate.

Can calcium silicate and clay bricks be used in the same wall?

Not in the same panel without detailing. Clay brickwork tends to expand over time while calcium silicate brickwork shrinks, so the two pull apart at the junction. Where both are used, separate them with a movement joint or have the designer detail the junction for differential movement, following your national masonry code.

Do calcium silicate bricks need firing?

No. They are hardened in an autoclave in saturated steam at about 200 °C, where lime and silica react to form calcium silicate hydrates. On the lines we supply, curing runs at about 1.4 MPa and 195 °C for about 7 hours. There is no kiln and no firing shrinkage.

Which takes less energy to make, calcium silicate or clay bricks?

In the two EPDs compared here, the calcium silicate units used about 970 MJ of non-renewable primary energy per tonne and the average UK clay brick 2,430 MJ, cradle to gate. The autoclave runs at about 200 °C against about 1,000 °C in a kiln, but lime production adds energy upstream. Compare per m² of wall and for your own fuel and lime supply.

Planning a Brick Plant?

Our autoclaved brick production line page sets out the press, autoclave and boiler configuration for sand-lime and fly ash bricks. If you have a clay deposit, compare it with our fired clay brick production line.

Sources

• W. Eden, Current research findings on calcium-silicate unit production technology, ZKG International 5/2019 (Forschungsvereinigung Kalk-Sand e.V.): https://www.zkg.de/en/artikel/zkg_Current_research_findings_on_calcium-_silicate_unit_production_technology-3355302.html
• Brick Industry Association, Technical Notes on Brick Construction 9: Manufacturing of Brick (December 2006): https://gobrick.com/media/file/9-manufacturing-of-brick.pdf
• R. Jarmontowicz and J. Sieczkowski, Odkształcalność muru pod wpływem wilgoci i temperatury (Deformability of masonry under moisture and temperature), Przegląd Budowlany 3/2016, Table 1 (values of EN 1996-1-1): https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-52342706-69d7-4519-8a98-f3c46c7d9b82/c/Jarmontowicz_Odksztalcalnosc_3_2016.pdf
• H. Garbalińska, J. Strzałkowski and A. Stolarska, Moisture Influence on Compressive Strength of Calcium Silicate Masonry Units, Materials 13(17), 3817 (2020): https://pmc.ncbi.nlm.nih.gov/articles/PMC7504377/
• LABC Warranty, Guidance on the Use of Movement Joints, TS-3174-3.00-050324 (March 2024): https://www.labcwarranty.co.uk/hubfs/Blog/Blog%20PDFs/LABCW%20-%20Guidance%20on%20the%20use%20of%20Movement%20Joints.pdf
• ITB, Type III EPD No. 634/2024, H+H Calcium Silicates, H+H Polska Sp. z o.o.: https://itb.pl/wp-content/uploads/2024/07/ITB-EPD_634_HH-Polska-Sp.-z-o.o.-HH-Calcium-Silicates.pdf
• Brick Development Association, EPD for UK Clay Brick, BREG EN EPD No. 000002 (BRE Global, issued February 2019, expired February 2024): https://asbp.org.uk/wp-content/uploads/2021/10/UK_EPD_UK_Clay_Brick_202402.pdf
• GB/T 5101-2017 Fired common bricks, standard record: https://www.chinesestandard.net/Related.aspx/GBT5101-2017
• GB/T 11945-2019 Autoclaved sand-lime solid brick and solid block (replaces GB 11945-1999), standard record: https://www.chinesestandard.net/Related.aspx/GB11945-1999
• ASTM C73-23 Standard Specification for Calcium Silicate Brick (Sand-Lime Brick): https://store.astm.org/c0073-23.html
• ASTM C62-23 Standard Specification for Building Brick (Solid Masonry Units Made from Clay or Shale): https://store.astm.org/c0062-23.html
• EN 771-2:2011+A1:2015 Specification for masonry units – Part 2: Calcium silicate masonry units (iTeh Standards abstract): https://iteh.es/catalog/standards/cen/820e5c2e-548e-4ca6-8123-6e069f095b96/en-771-2-2011a1-2015