A frequent case of the failure of masonry is the disintegration of the mortar in the outside of the joints, although this does not take place to such an extent in buildings as in engineering works. "Ordinary mortar - either lime or cement - absorbs water freely, common lime mortar absorbing from 50 to 60 per cent, of its own weight, and the best Portland cement mortar from 10 to 20 per cent., and consequently they disintegrate under the action of the frost. Mortar may be made practically non-absorbent by the addition of alum and potash soap. One per cent., by weight, of powdered alum is added to the dry cement and sand and thoroughly mixed, and about 1 per cent, of any potash soap (ordinary soft soap made from wood ashes is very good) is dissolved in the water used in making the mortar. The alum and soap combine and form compounds which are insoluble in water. These compounds are not acted upon by the carbonic acid of the air, and add considerable to the early strength of the mortar and somewhat to its ultimate strength."* The alum and soap are comparatively cheap and can be easily used.
Plaster of Paris, which is sulphate of lime, when added to either lime or cement mortar in quantities not exceeding 5 per cent., accelerates the setting and also increases the early and the ultimate strength of mortar. Lime mortar to which plaster of Paris has been added is called gauged mortar. Selenetic cement, an artificial cement much used in England, is made by combining plaster of Paris and hydraulic lime, in the proportion of three pints of the plaster to a bushel of unslaked lime. The addition of the plaster of Paris to lime appears to increase the strength of the mortar from two to three times.
*"Treatise on Masonry Construction," Baker.
Sugar has been employed for centuries in India as an ingredient of common lime mortar, and adds greatly to the strength of the mortar.
An addition of sugar or syrup equal to one-tenth of the weight of the unslaked lime, to lime mortar, adds 50 per cent, to the strength of the mortar and will cause the mortar to set more quickly. The addition of sugar to lime mortar is especially beneficial when used in very thick walls, as the lime mortar thus placed never becomes fully saturated with carbonic acid.
Sugar added to Rosendale and Portland cement mortars in the proportion of 1/8 to ¼ per cent, in weight of the cement, increases the strength of the mortars about 25 per cent.
As the combination of sugar and lime is soluble in water, sugar should not be added to mortar that is to be used under water.
Freezing does not appear to injure lime mortar if the mortar remains frozen until it has fully set. Alternate freezing and thawing materially damages the strength and adhesion of lime mortar, and as this is generally what happens when mortar is laid in freezing weather, it is much the safest rule for the architect to specify and see that no masonry shall be laid with lime mortar in freezing weather. "Mortar composed of 1 part Portland cement and 3 parts of sand is entirely uninjured by freezing and thawing, mortar made of cements of the Rosendale type, in any proportion, is entirely ruined by freezing and thawing."*
Salt in Mortar. - When it is desired to use natural cement mortar in freezing weather the mortar should be mixed with water to which salt has been added in the proportion of one pound of salt to eighteen gallons of water, when the temperature is at 320 F., and for each degree of temperature below 320 add three additional ounces of salt. Mortar mixed with such a solution does not freeze in ordinary winter weather, and hence is not injured by frost.
*Trans. Am. Soc. of C. E., Vol. XVI., pp. 79-84.
Builders sometimes advocate the addition of lime to Rosendale cement mortar in cold weather to warm it. The heating effect of the lime, however, would not be appreciable, as heat is generated in lime only when it slakes. If cement of the Rosendale type must be used in freezing weather, the only safe way of using it is by the addition of salt, as described above, otherwise the mortar will be completely ruined by freezing.
Change of Volume in Setting. - Cement mortars diminish slightly in volume in setting in air and expand when under water, but the expansion and contraction is not sufficient to injuriously affect building construction.