A Treatise On The Principles And Practice Of Dock Engineering

ACTION OF SEA-WATER UPON CONCRETE. 129 compact shell and the poorer kernel should be made in one operation. Where this is not possible, and the shell is added subsequently, numerous iron ties should be used. “2. From the chemical point of view, cements or hydraulic limes, rich in silica and as poor as possible in alumina and ferric oxide, should be used, for aluminate and ferrate of lime are not only decomposed and softened rapidly by sea-water, but they also give rise to the formation of double compounds, which in their turn destroy the cohesion of the mass by producing cracks, fissures, and bulges. The salts contained in sea-water, especially the sulpbates, are the most dangerous enemies of hydraulic cements. The lime is either dissolved and carried off by the salts, and the mortar thus loosened, or the sulphuric acid forms with it crystalline compounds as basic sulphate of lime, alumino-sulphate and ferro-sulphate of lime, which are segregated forcibly in the mortar, together with a large quantity of water of crystallisation, and a conséquent increase in volume results. The separation of hydrate of magnesia is only the visible but completely innocuous sign of these processes. The magnesia does not in any way enter into an injurions reaction with silica, alumina, or ferric oxide, it is only displaced by the lime from its solution in the shape of a flocculent, slimy hydrate, which may be rather useful in stopping the pores, but can never cause any strain or expansion, even if it subsequently absorbed carbonic acid. The carbonic acid, whether derived from air or water, assists the hydraulic cement as a preservative wherever it cornes into contact with the solid mortar. It could only loosen the latter if present in such an excess that bicarbonate of lime might be formed. “ 3. The use of substances which render the mortar, at any rate in its external layers, denser and more capable of resistance. Such substances are—- “(a) Sesquicarbonate of Ammonia (from gas liquor) in all cases where long exposure to the air is impossible. Such a solution applied with the brush, or as a spray, and then allowed to dry, converts the hydrate of lime into carbonate of lime. The latter is not acted upon by the neutral sulphates present in sea-water. It must be repeated that it is a decidedly erroneous opinion that the texture of otherwise sound cements is injured by the action of carbonic acid; on the contrary, it renders them more capable of resistance, except in the above-mentioned case, which is extremely rare, when bicarbonate of lime is formed and goes into solution. “ (/?) Fluosilicates, among which magnesium fluosilicate is most to be recommended. The free lime is converted into calcium fluoride and silicate of lime, and, in conjunction with the liberated hydrate of magnesia, these new products close the pores of the mortar. Both salts are sufficiently cheap to be used on a large scale. “ (7) Last, not least, Barium Chloride. Two or three per cent, of the weight of the cement is dissolved in the water with which the conerete is mixed. This forms perfectly insoluble barium sulphate with the sulphates