Calcium silicate ('sandlime' and 'flintlime') bricks, subject to British Standard 187:1978, are made by mixing together lime and an inherently strong aggregate (sand or crushed siliceous or flint stone, or a mixture) with enough water to allow the mixture to be moulded under high pressure. The 'green' bricks are then subjected to high-pressure steam in an autoclave. In this process the lime reacts with silica to form hydrated calcium silicates, producing a durable strong and intimately bonded brick. The process at each works is highly mechanised and subject to rigid quality control at all stages. Because of this control and because no change of shape or size occurs when the bricks are hardened, both their strength and their dimensions are extremely regular.

Many pleasing colours are produced, ranging from natural (near white), with excellent light reflecting properties, through pastel shades to deep colours, in both smooth-faced and textured bricks. Multi-coloured and brindle facing bricks are particularly attractive for individual dwellings or larger housing projects.

Further reading from Wikipedia