Silicate-, aluminosilicate and calciumsilicate gels for building materials: chemical and mechanical properties during ageing

Silicate, aluminosilicate and calciumsilicate concretes, cements, and mortars were synthesized based on rice husk-bark ash, fly ash, slag and metakaolin. Alkali activation was done using sodium and potassium waterglass solutions. The hardening of the concretes and mortars was investigated in dependence on time by compressive strength measurements. The ageing of cement pastes was followed by infrared absorption spectroscopy. The infrared absorption peaks were evaluated in comparison to spectra obtained for silicate and aluminosilicate glasses and condensates from waterglass solutions. The increase in compressive strength of the materials at the beginning of ageing can be explained by the development of two main structural units on different time scales: a fast formation of silicate chain type units of considerable length and a slow formation of a silicate – and in presence of Al – aluminosilicate three-dimensional network enclosing the chains. The protection of the chains against destruction becomes crucial for long term high strength. Alkali activation of slag containing significant amounts of CaO leads to the formation of calcium silicate hydrate type phases and strongly enhanced mechanical strength.