The amount of alum and soap to be used is limited practically to alum equal to 1i per cent of the water and soap equal to 3 per cent, because it is impossible to dissolve more than about 3 per cent of hard soap in cold water. Of course, if it is desired to use greater amounts, the soap may first be dissolved in hot water which may afterwards be mixed with a larger portion of cold water. The above amounts of alum and soap will produce 3 per cent of dry water repelling compound; and this is enough to render any reasonably dense concrete practically, if not absolutely, water-tight. In a series of tests by the author,* 1.3 per cent of the alum and soap compound incorporated in a cement mortar having 23.8 per cent of voids reduced the percolation to one third of that of similar, un treated mortar; in other words, alum and soap compound equal to about one twentieth of the voids in a lean and porous mortar stopped two thirds of the percolation. This seems to show that the alum and soap compound in mortar or concrete acts like oil on the wires of a moderately fine sieve, i.e., prevents percolation chiefly by its water-repelling properties, rather than as a mere void-filling material.
The addition of the alum and soap weaken the mortar, 2 per cent of the compound decreasing the strength about 20 per cent, varying a little with the method of storage.
Instead of using alum and soap as above, it is better to use aluminium sulphate (sometimes, but improperly, called alum) and soap. The aluminium sulphate is cheaper than alum, and only two thirds as much of it is required to produce substantially the same amount of void-filling material as the alum and soap. The best proportions are 1 part aluminium sulphate to 3 parts of hard soap. As in the preceding case an excess of either ingredient does no harm, although an excess of soap is better than a deficiency.
Lime and Soap. Lime and soap combine to form calcium soap, a finely divided water-repelling compound; and hence another method of rendering mortar or concrete waterproof is to incorporate lime and soap in it. The proper proportion is unslaked lime 1 part and hard soap 12 parts; and, since it is impossible to dissolve more than about 3 per cent of hard soap in cold water, the amounts to be used in practice are unslaked lime 0.25 per cent and hard soap 3 per cent of the weight of the water, and these amounts will give 2.7 per cent of void-filling compound. These quantities will make any reasonably good concrete absolutely water-tight. Before the water containing the soap and lime is used, it should be stirred to mix the ingredients and to keep the precipitate in suspension. Calcium soap is a product in the manufacture of candles; and hence if a con siderable amount of concrete is to be waterproofed, as in the con struction of a long aqueduct, it might be wise to buy the calcium soap and add it directly to the cement.
Calcium soap is formed, if an excess of soap is added in the alum and soap compound, provided the cement contains any free lime— as it usually does. Apparently calcium soap is the essential element of several proprietary waterproofing compounds.
Proprietary Compounds. There are upon the market several proprietary compounds to be mixed with the concrete to make it impervious. Some are liquid to be added to the water used in mixing the concrete; some are powders to be mixed with the dry cement; some are sold mixed with the cement ready for use; and some are not sold but are applied by the proprietors. Only a com paratively small proportion of each is required to make ordinary mortar or concrete waterproof. It is not proved that they are any more effective than some of the means previously described.
Other Ingredients. There are several other materials that may be used to make concrete impervious; but none of them is as cheap or as effective as those previously mentioned. Among these are: alum alone, aluminium sulphate alone, wax, resin, paraffin, stearic acid, and oil emulsion.
Alum and lye are sometimes recommended; but cement is some times destroyed by contact with alkaline water, and therefore the lye may injure the cement.
Alum and Soap Washes. The oldest, the best known, and probably the most effective method of rendering set concrete water proof is to apply alternate washes of soap and alum solutions. This is the well-known Sylvester method of making masonry im pervious. The alum and soap combine, and form an insoluble compound in the pores of the concrete. The alum solution is made by dissolving 1 pound of alum per gallon of water, and the soap solution by dissolving 2.2 pounds of hard soap per gallon of water. For information concerning the necessary accuracy of proportions, see § 373. The concrete should be clean and dry, and not colder than about 50° F. The soap wash should be applied boiling hot, but the alum solution may be 60° to 70° F. when applied. One wash should be put on and allowed to dry—usually for 24 hours,— when the other is applied. The solutions should be well rubbed in, but care should be taken not to form a froth. Two pairs of coats of the above solutions will usually make any fairly good concrete practically impervious under moderate pressures; and eight pairs of coats have made leaky concrete impervious under a head of 100 feet.