Why do structural clay products have high compressive strength?

Structural clay products, such as brick and tile, derive their high compressive strength from their unique microstructure, which is formed during the firing process in a kiln.

When these products are fired at temperatures typically between 1,050° and 1,100° C, the clay or shale ingredients contain impurities that melt to form a glass phase. This glassy phase acts as a binder, effectively bonding the coarser filler particles—usually crushed quartz—together. As the material cools, crystalline mullite needles grow into this glassy matrix. The resulting structure consists of large secondary particles embedded within a fine-grained matrix of mullite and glass, containing a substantial volume of pores.

The presence of both large and small particles within this microstructure allows the finished products to bear relatively heavy loads without fracturing, which is the primary property that qualifies them for use in building construction.1