Reaction and the Solid Reaction Products in the System Lime-Silica Gel-Water at Ordinary Temperature

Abstract

Calcium silicate hydrates, which are one of the principal bonding materials in mortars and concretes, are formed by puzzolanic reactions or hydrations of calcium silicate anhydrates.This paper is a report of an investigation of the reactivity of silica gel in lime solution, and deals with electronographic morphology, X-ray and electron diffraction analyses of solid phases.The initial C/S mol. ratios ranged from 0.50 to 3.50, and the solution was kept at the constant temperature of 20°±1°C for 95 days giving occasional agitation by a magnetic stirrer.For comparing the solid reaction products with the hydrated solid phases of calcium silicates in clinker, ordinary portland cement and β-C2S were subjected to similar tests.The results obtained are summarized as follows:(1) More than 90 days seem to be necessary before the system lime-silica gel-water reaches to neary stable equilibrium. CaO combines rather rapidly in the first week with the velocity decreasing with time. The effect of C/S mol. ratios in starting mixtures on these of the reaction products came to the front on long term experiments, for example, about 60 days.(2) C/S mol. ratios of the solid reaction products were in the range from 0.45 to 1.24 which showed a rapid increase up to about 0.8, the point of inflection, turned then to a slow increase even if an appreciable amount of CaO remained in the liquid phase. The principal factors governing the reaction were the initial C/S mol. ratio and the time of reaction.(3) The solid phases thus obtained were exclusively the aggregates of semi-transparent, thin, flexible films or foils being quite likely as having a gell-like structure as it was proved by the observation under electron microscope. The crystals often curled up to rolls, and gave the lattice spacing calculated from the pattern of electron diffraction usually smaller than that estimated by X-ray analysis. It is likely that this kind of difference is due to the shrinkage of crystals produced by the electron bombardment and also by high vacuum in electron microscope. Difference of C/S mol. ratios of the initial mixture did not give any recognizable influence on the properties of the solid reaction products. It is very likely that the variation of the C/S mol. ratios of solid reaction products is caused by the adsorption of CaO but not by the formation of solid solution. The filmy crystals may be the earlier stage of the crystalization of mono-calcium silicate hydrate (low temperature modification).(4) X-ray analysis showed the systematic absence of h k l for h+k odd. The reflection of (220) was abnormally strong and its peak profile traced by Geiger-counter diffractometer was generally unsymmetrical, which probably shows the two dimensional mistaked structure.(5) X-ray analyses showed the presence of calcium silicate hydrates in portland cement pastes (W/C=50%) cured 3-7 days. The observation under electron microscope of the paste cured 35 days revealed the existence of the semi-transparent filmy, and also fibrous, or needle-like crystals whose appearances were able to draw a distinction from ettringite. It is highly probable that these are calcium silicate hydrates, although it was impossible to discriminate between mono and dicalcium salts.(6) According to the results of X-ray analysis and of the observation under electron microscope the hydrated solid phase of dicalcium silicate in large amount of water is monocalcium silicate hydrate identical with that formed in the system lime-silica gel-water. It is also filmy crystals whose C/S mol. ratio was <1.38.