The influence of alkali-free and alkaline shotcrete accelerators within cement systems Influence of the temperature on the sulfate attack mechanisms and damage

摘要

The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al~(3+) and SO_4~(2-) added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca~(2+) supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO_4 attack, also occurred with Na_2SO_4 solutions and depending on the aluminate-sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 deg C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite.