Cement

The early use of lime mortar has already been mentioned. Both the Greeks and the Romans often mixed with the lime and sand certain volcanic deposits really consisting of fused silicates and aluminates. The resulting product was much stronger than ordinary mortar and resisted the action of water, either fresh or salt. It was thus the forerunner of the modern hydraulic cements. The Greeks used Santorin earth and the Romans pozzuolana from Puteoli, near Naples. Failing these, powdered tiles or pottery formed good substitutes. Trass, from the Rhine valley, has similar properties.

Smeaton in England, from 1756 onwards, and Vicat in France, a little later, experimented with hydraulic limes. These were obtained by burning argillaceous limestone. The product, after burning, set hard under water. In 1827 the original patent for Portland cement was taken out by Joseph Aspdin, a bricklayer of Leeds and Wakefield, in Yorkshire. He proposed to grind lime and clay finely with water, and then to dry and calcine until carbon dioxide was entirely expelled. The name was given from a fancied resemblance to Portland stone.

In Aspdin's process the temperature employed was too low to yield the substance now known as Portland cement. This, since the setting up of the first British Standard Specification in 1904, is a carefully standardised product not subject to the variations in quality which characterise the natural, Roman, or rock cement formed by simply calcining siliceous limestone to just below sintering point.

Natural pozzuolanic substances are not cements, but, when mixed with lime, they form hydraulic mortar without further heating. Artificial pozzuolanic substances include various ignited argillaceous materials and certain blast-furnace slags.