THERE are hardly any materials used by the engineer, architect, or builder, on which so much depends as upon mortar and concrete.

There are differences of opinion on many points connected with the preparation and use of these materials, and there is still much prejudice existing in favour of exploded notions and of old-fashioned ideas.

These prejudices are the more difficult to overcome, because the old-fashioned methods of preparing mortar and concrete were, as a rule, less troublesome than those of more recent introduction.

In order to clear the way for a proper understanding of this important subject, it will be well, first, to explain the meaning of some of the commonest terms used in connection with it.

Terms In Use

The natural Limes and Cements used for building are produced by the calcination of limestones or other calcareous minerals, the effect of which is to drive off the carbonic acid and moisture they contain.

Calcination is heating to redness in air.

Quicklime or Caustic Lime is the resulting lime as left immediately after calcination.

Slaking is the process of chemical combination of quicklime with water. This gives rise to various phenomena which will be more particularly described hereafter. (See p. 146.)

Slaked Lime is the substance remaining after slaking, and is chemically known as the "hydrate of limey" 1

1 Calcium hydrate. The ordinary chemical nomenclature has been adopted throughout these Notes as being more familiar to readers generally than the new nomenclature. The modern names are given in footnotes.

Setting is the hardening of lime which has been mixed into a paste with water.

This is quite a different thing from mere drying. During drying the water in the paste evaporates, but no setting action takes place.

Hydraulicity

Lime or cement is said to be more or less hydraulic, according to the extent to which paste or mortar made from it will set under water, or in positions where it is free from access of air.

Limestones and other minerals from which limes and cements are produced differ greatly in their composition, ranging from pure carbonate of lime,1 such as white chalk or marble, to stones containing from 10 to 30 per cent of clay, in addition to other foreign constituents, such as magnesia, oxide of iron,2 etc.

As the properties of the resulting lime or cement depend very greatly upon the composition of the stone from which it is prepared, it will be instructive briefly to note the characteristics of the most common constituents of such stones before proceeding further; especially distinguishing those which produce hydraulicity from those which have not that effect.