A Crystalline Hydrated Magnesium Silicate formed in the Breakdown of a Concrete Sea-wall

Abstract

DURING an investigation of a serious breakdown of a concrete sea-wall, a small amount of a finely divided soft white deposit has been found which gives an X-ray diffraction pattern not unlike that of 4CaO.Al₂O₃.13H₂O. However, the chemical composition of the material, after allowing for 23 per cent impurity of calcium carbonate and 3 per cent of silica, can be expressed ideally as 4MgO.SiO₂.8.5H₂O. This is thought to be the first reported occurrence of a crystalline hydrated magnesium silicate formed at ordinary temperatures in the breakdown of a cement. Steopoe¹ has given indirect evidence of the formation of such a compound through the action of magnesium hydroxide on silica gel present in cement mortars decomposed by sulphate attack. The material reported on here has probably formed in a manner similar to that discussed by Steopoe. The magnesium hydroxide arises from the breakdown of calcium sulpho-aluminate through the presence of soluble magnesium salts derived from the sea-water². The calcium sulpho-aluminate is, as is well known, a common product of reaction between calcium sulphate solutions and calcium aluminate. Silica gel can be expected to form readily from the poorly crystallized hydrated calcium silicates of the set cement under the decomposition conditions.