Thaumasite-background and nature in deterioration of cements, mortars and concretes

摘要

Thaumasite has been shown to form at lowtemperatures, particularly 0-5℃,as a non-binding calciumcarbonate silicate sulphate hydrate under conditions ofdestructive sulphate attack. Formation of thaumasite arisesgenerally from calcium silicate hydrate C-S-H andCa2+,CO32-,SO42-,CO2 and water, or from ettringite, with which ithas often been confused in the past. Conversion of the maincementitious binder C-S-H into the non-binder thaumasite is adestructive form of sulphate attack. Greater awareness of thepotential problems that thaumasite can cause has arisen with theincreased use of limestone fillers in cements, the commonemployment of limestone aggregates in concrete and theintroduction of Portland limestone cements, together with therealisation that structural foundations of buildings are, onaverage, below ambient temperature and (more often than notwith on-and above-ground construction)are within the optimumtemperature range for thaumasite to be formed. Instances havebeen found in specific studies of large quantities of thaumasitebeing formed in foundation concretes with no evidence for anystructural damage above ground level. It is important to be awareof the propensity of thaumasite to form at low temperatures formix designs, so as not to encourage any destructive sulphateattack by thaumasite to arise. This means utilising lowwater/cement ratios for workable mortars and concretes, so as togive reduced permeability. This will prevent, or at least suitablyhinder, ingress of destructive ions and water, so that the potentialfor destructive sulphate attack by formation of thaumasite is notactually realised in practice. 1999 Elsevier Science Ltd. All rights reserved.