When mortar which has been immersed in water is transferred to dry air, a slight contraction may take place in volume, together with an increase in strength; while a transfer the other way may produce the opposite result; but no distortion of form or disinte gration of the mortar will take place in either case if the cement be of good quality.
Sometimes cement when made into mortar sets and hardens properly, and later, when exposed to the action of the atmosphere or water, becomes distorted and cracked or even entirely disinte grated. If the composrtion deviates hut slightly from the normal, this process of disintegration may not show itself for a considerable time and proceeds very slowly. It thus becomes an clement of considerable danger, as it is liable to escape detection in testing the cement.
The presence of small quantities of free lime in cement is doubt less one of the most common causes of disintegration in cement mortar. The lime being distributed through the cement in small particles is hydrated very slowly after the cement has set, causing, through its swelling during slaking, strong expansive forces on the interior of the mortar, and producing an increase of volume, loss of strength, and perhaps final disintegration.
Free magnesia in cement is supposed to act very much like free lime. The action of magnesia, however, is much slower than that of lime, and for this reason is a more serious defect. Specifications for Portland cement frequently limit the amount of magnesia that may be present in the cement.
Most Portland cements probably contain small amounts of the expansive elements, which when in very small quantity act with extreme slowness and perhaps produce no visible effect for several months after the use of the cement; then occurs a decrease of strength, which disappears with time. Cements which gain strength rapidly are quite apt to act in this manner, a depression in the strength curve occurring at from six months to one year after the mortar is made.
Cements for use in sea water should contain very little alumina. Some of the salts in the sea water attack these alumina compounds, causing disintegration of the cement and giving rise to expansive action which cracks and breaks up the work.
The presence of expansive elements in Portland cement is prob ably due to incomplete burning or lack of uniformity in the incor poration of the ingredients rather than to defective composition.
The fineness of the cement modifies the action of the free lime, as finely divided material will slake more quickly than coarse grains, and the lime is more apt to become hydrated before setting; or, if the cement be exposed before use, the lime in a fine state will sooner become air slaked.
18. Chemistry of LeChatelier was the first to explain the composition of Portland cement. He studied sec tions of clinker under the microscope, and examined the properties of the various compounds formed by the principal ingredients. He concluded 1 that the tricalcium silicate, 3CaO, Si00, is the only silicate that is really hydraulic, and that it is the essential active element in cement. In Portland cement he finds it to be the princi pal component, occurring in cubical crystals. It is formed by combination of silica and lime in presence of fusible compounds formed by alumina and iron.
" The dicalcitun silicate, 2CaO, Si00, possesses the singular property of spontaneously pulverizing in the furnace upon cooling. This silicate does not possess hydraulic properties and will not harden under water.
" There are various aluminates of lime, all of which set rapidly in contact with water. The most important is the tricalcium alu minate, 3CaO, A1003." Professor LeChatelier gives two limits within which the quantity of lime in Portland cement should always be found. These are, that the proportion of lime should always be greater than that represented by the formula and that it. should never exceed that given by the formula, The symbols in these formulas represent the number of equivalents of the substances present, not the weights.
Messrs S. B. and W. B. Newberry from a study of the compounds of silica and alumina with lime reached the following conclusions:' (1) Lime may be combined with silica in proportion of three molecules to one and still give a product of practically constant volume and good hardening properties, though hardening very slowly. With 3l molecules of lime to one of silica the product is not sound and cracks in water.