Mr. W. C. Hawley 3 employed a stock solution of two pounds caustic potash, five pounds powdered alum, and ten quarts water, and used in the finishing coat three quarts of this solution in each batch of mortar containing two bags of cement. The mortar was mixed with two volumes of sand to one of cement and covered forty-eight square feet to a depth of about one-half inch. The extra Cost for materials and preparing solution was only about nine and a half cents per hundred square feet. With less than two parts sand to one cement, it was found the finishing mortar checked in setting. It was also found that any organic matter in the sand was softened by the potash, and an excess of potash caused checking, although an excess of alum had no deleterious effect.
The introduction of slaked lime in mortars designed to be water-proof is suggested by the fact that the permeability of mortar diminishes if water is allowed to percolate it for some time, the theory being that fine particles of cement and lime are dislodged by the passage of the water to form a deposit at or near the surface, and check the flow. This suggestion, however, needs experimental confirmation, since it seems quite possible that the introduction of a substance containing such a large proportion of water as does slaked lime, may increase the percentage of voids in the mortar, if not the permeability.
1 Trans. A. S. C. E., Vol. Ii, p. 128.
2 Trans. A. S. C. E., Vol. Ii, p. 129.
3 Journal New England water-Works Association, 1904.
The use of pulverized clay and pozzolanic materials for a similar purpose has been suggested. It has already been shown that moderate doses of clay have no deleterious effect on the strength of mortars for ordinary exposures. The action of the pozzolanic substances has been found by Dr. Michaelis and M. Feret to be not mechanical alone, but chemical, and the effect on the strength of the resulting mortar depends upon the exposure to which it is subjected, such admixtures being deleterious for mortars hardened in air.
The white deposit sometimes formed at the surface of brick and masonry walls is usually due to the filtration of water through the mortar, dissolving out salts of potash, soda, etc., and depositing these salts on the surface by evaporation or by the formation of sodic carbonate. The absorption of water from the atmosphere may also account for this deposit in some degree, especially near the sea. The same term is applied to a more harmful deposit, sulphate of calcium, which may be supplied by the filtrating water or may come from the cement, either from the addition of gypsum or from the fuel used in burning. The crystallization of this salt in the pores of the masonry at the surface may cause disintegration.
On the other hand efflorescence may be quite harmless, as when it is formed by washing out from the mortar an excess of hydrate of lime. A portion of the latter may then be changed to carbonate of lime near the surface of the wall and actually stop up the pores or voids, and prevent further filtration.
492. The discoloration of brickwork and fine masonry by efflorescence is sometimes serious. To ameliorate these conditions, the use of water-proof mortars, and careful pointing of the work, are precautions to be recommended. General Gill-more, in "Limes, Hydraulic Cements and Mortars," suggests the use of about ten pounds of animal fat to one hundred pounds of lime and three hundred pounds of cement; the object of the fat being to saponify the alkaline substance, the lime in form of paste serving only as a vehicle for the fat. A more practical method, however, would seem to be the application of soap and alum washes on the surface, or the use of soap and alum in the preparation of the mortar to be used near the face of the wall, and especially for pointing. The remedy to be adopted, however, will depend upon the cause of the efflorescence.
Pointing serves the double purpose of making the joint practically water-tight at the edges, and giving a finish to the face of the wall. If the edge of the joint is not well filled, moisture collects there either from the face or from seepage through the wall. Subsequent freezing or the crystallization of certain salts may spall the stones or loosen them from their bed.
In laying cut-stone masonry, the joints should be raked out for about two inches back from the face to be pointed. Pointing mortar should be prepared from fine sand and the best Portland cement. The proportion of sand should not exceed two parts by weight to one cement, and in the highest class work, equal parts of cement and sand are sometimes used. No advantage is gained, however, by using a mortar richer in cement than the one last mentioned. The use of fine sand and rich mortars are specified not only because such mortars are practically water-tight, but because they take a fine finish.
494. The tools required for pointing are a bent iron to rake out the joints (though this should be partially done while the mortar is green), a mortar board and small trowel, a calking iron and wooden mallet, a brush for moistening the joint, and one or more beading tools. After raking out the joint it is moistened by the brush, and the mortar, which is mixed quite dry, is filled in with the trowel. When enough mortar is in place to fill half the depth of the joint, it is tamped with the calking iron and mallet much as a ship's seam is calked with oakum. The joint is then filled to the face, and again tamped. The bead is then formed by running the beading iron back and forth over the joint. This beading iron is of steel with the handle parallel to, but some two or three inches out from, the line of the blade forming the bead. The blade is three to five inches long and "hollow ground" or finished with a smooth concave surface. Only such a length of joint is pointed at one operation as may be quickly carried to completion. The wall must be kept moist for some time after the pointing is done, and it should be protected from the direct rays of the sun, as fine cracks are very likely to appear in this rich, finely finished mortar. If possible, pointing should be done in moderate weather and must be entirely suspended in temperatures approaching the freezing point.