Reconstructed stone is made from an aggregate of crushed stone, cement and water. The stone is crushed so that the maximum size of the particles is 6 mm and it is mixed with cement in the proportions of 1 part cement to 3 or 4 parts of stone. Either portland cement, white cement or coloured cement may be used to simulate the colour of a natural stone as closely as possible. A comparatively dry mix of cement, crushed stone and water is prepared and cast in moulds. The mix is thoroughly consolidated inside the moulds by vibrating and left to harden in the moulds for at least 24 hours. The stones are then taken out of the moulds and allowed to harden gradually for 28 days.
Well made reconstructed stone has much the same texture and colour as the natural stone from which it is made and can be cut, carved and dressed just like natural stone. It is not stratified, is free from flaws and is sometimes a better material than the natural stone from which it is made. The cost of a plain stone, cast with an aggregate of crushed natural stone, is about the same as that of a similar natural stone. Moulded cast stones can often be produced more cheaply by repetitive casting than similar natural stones that have to be cut and shaped.
A cheaper, inferior, form of cast stone is made with a core of ordinary concrete, faced with an aggregate of crushed natural stone and cement. This material should more properly be called cast concrete.
The core is made from clean gravel, sand and Portland cement and the facing from crushed stone and cement to resemble the texture and colour of a natural stone. The crushed stone, cement and water is first spread in the base of the mould to a thickness of about 25 mm, the core concrete is added and the mix consolidated. If the stone is to be exposed on two or more faces the natural stone mix is spread up the sides and the bottom of the mould, This type of cast stone obviously cannot be carved as it has only a thin surface of natural looking stone.
As an alternative to a facing of reconstructed stone, the facing or facings can be made of cement and sand pigmented to look somewhat like the colour of a natural stone.
Cast stones made with a surface skin of material to resemble stone do not usually weather in the same way that natural stone does, by a gradual change of colour. The material tends to have a lifeless, mechanical appearance and may in time tend to show irregular, unsightly dirt stains at joints, cracks and around projections.
Reconstructed stone is used as an ashlar facing to brick or block backgrounds for both solid and the outer leaf of cavity walls and as facings.