Sand-lime bricks also known as Calcium Silicate Brick are an excellent alternative to traditional clay bricks, particularly in regions where high-quality clay is not readily available. These versatile bricks are also commonly used for decorative purposes in construction. While sand-lime brick are widely utilized in countries like Russia and Germany, their use in India has been limited, with a few notable projects in Kerala showcasing their potential.
What are Sand Lime Bricks?
Sand-lime bricks are made from a mixture of slaked lime and fine siliceous sand. The process involves molding the mixture under mechanical pressure and hardening it under steam pressure. The primary constituents of these bricks include:
- Sand (88-92%)
- Lime (8-12%)
- Coloring Pigment (0.2-0.3%)
It’s important to note that the water used in manufacturing must not contain more than 0.25% soluble salts or organic matter. Sea water is entirely unsuitable for this purpose.
Manufacturing Process of Sand Lime Brick
- Material Selection:
- Sand used should be free of clay or mica, passing through a 20-mesh sieve.
- Three-fourths of the sand should be retained on a 100-mesh sieve, and larger grains must be removed or crushed.
- Lime Selection:
- High-calcium lime is preferred for faster hardening over brown or dolomitic lime.
- The lime content in the mixture should be between 4-10%.
- Mixing:
- The sand and lime are often mixed dry using a tube mill and tempered in a pug mill.
- In some cases, quicklime is slaked before mixing, and the wet mixture is ground in a wet-and-dry grinding mill to achieve an intimate blend.
- Water, constituting 3-5% of the mix, is added to achieve a semi-dry consistency.
- Molding:
- The mixture is then molded into bricks using powerful presses capable of exerting a compression of 30 to 60 N/mm².
- Autoclaving (Hydrothermal Treatment):
- The bricks are stacked on cars and placed into autoclaves, where they are subjected to steam pressure between 8 to 16 kg/cm² for 6-12 hours.
- During this process, a chemical reaction occurs between the lime and sand, forming a strong bond. This step, known as autoclaving, eliminates the need for burning or drying the bricks.
Standard Dimensions
According to IS: 4139, sand-lime brick should have dimensions of 190 × 90 × 90 mm and 190 × 90 × 40 mm, with allowable tolerances of up to 3 mm in length and 2 mm in width and height.
Advantages of Sand Lime Bricks
- Strength and Durability:
Sand-lime bricks are known for their high strength and hardness, making them ideal for load-bearing structures and durable construction. - Uniform Color and Texture:
These bricks exhibit a consistent and aesthetically pleasing appearance, with uniformity in both color and texture, which can enhance the overall look of buildings. - Efflorescence-Free:
The raw materials used in sand-lime bricks, particularly the absence of soluble salts, ensure that these brick are free from efflorescence. This helps maintain their appearance over time without unsightly white stains. - Precision in Size:
Sand-lime bricks are manufactured with high dimensional accuracy. This means that masonry work can be done with very thin layers of mortar, resulting in faster construction, reduced material usage, and a cleaner finish. - Aesthetic Appeal:
Thanks to their pleasing natural appearance, sand-lime brick often do not require plastering or additional finishing, which can help reduce both material costs and construction time.
Disadvantages of Sand Lime Bricks
- Unsuitable for High-Temperature Applications:
Sand-lime bricks tend to disintegrate when exposed to prolonged high temperatures, making them unsuitable for use in furnaces or any other high-heat environments. - Poor Abrasion Resistance:
Due to their relatively low resistance to abrasion, these bricks are not ideal for paving or flooring applications, where continuous wear and tear is expected. - Limited Water Resistance:
Sand-lime bricks have a lower resistance to water penetration compared to other types of bricks, making them less suitable for structures exposed to constant moisture, such as foundations or damp areas.
Physical Properties of Sand Lime Bricks
- Compressive Strength:
Sand-lime bricks, when laid flat, have a compressive strength ranging between 7.5 to 20 N/mm² (as shown in Table 1). The compressive strength of any individual brick should not be more than 20% below the minimum average strength specified for that class of brick. - Modulus of Rupture:
The modulus of rupture, or the brick’s ability to withstand cross-bending forces, should be greater than 1 N/mm², ensuring a degree of flexibility under load. - Water Absorption:
A good-quality sand-lime brick should not absorb more than 18% of its weight in water after boiling for 5 hours, indicating its resistance to moisture ingress. - Bulk Specific Gravity:
The bulk specific gravity of sand-lime bricks typically falls between 1.8 and 2.0, indicating a balance between weight and strength. - Drying Shrinkage:
The average drying shrinkage of sand-lime bricks should not exceed the values provided in Table 2. This ensures minimal contraction when drying, which reduces the risk of cracking or warping during installation.
Comparison of Sand Lime Brick vs. Clay Brick
While both sand-lime and clay bricks are widely used in construction, each has distinct characteristics that make them suitable for different applications:
- Surface and Shape:
Sand-lime brick offer a smoother surface and more uniform shape than clay brick. This gives them a more refined and aesthetically pleasing appearance, especially for visible facades. - Efflorescence-Free:
Sand-lime brick are free from efflorescence, which prevents the appearance of white stains often seen on clay brick surfaces due to salt deposits. - Color Uniformity:
Sand-lime brick exhibit uniform color, making them ideal for projects where a consistent finish is desired. - Strength:
Both types of bricks offer satisfactory strength, though sand-lime bricks may be preferred for specific applications where dimensional accuracy and surface smoothness are critical. - Durability in Weather:
Although sand-lime brick have many advantages, they may not be as durable in extreme weather conditions compared to high-quality clay brick. When exposed to harsh outdoor environments, some sand-lime brick may show reduced durability, especially if they are not manufactured with precision. - Frost Resistance:
Clay bricks generally offer better frost resistance compared to sand-lime bricks. Sand-lime bricks, if not carefully manufactured, can suffer damage from freeze-thaw cycles, making them less ideal for areas prone to severe winters.
Conclusion
Sand-lime bricks provide numerous benefits, including strength, uniformity, and an aesthetically pleasing finish. Their precision in size and smooth surface make them a popular choice for many construction projects. However, they may not perform as well as clay bricks in extreme weather conditions or frost-prone areas. Therefore, choosing between sand-lime and clay brick depends largely on the specific requirements of the project, with careful consideration of environmental factors and intended use.