The failure process of mortars during sulfate attack

Abstract

External sulfate attacks on concrete structures maycause serious damage and it has attracted a widerange of attention from numerous researchers overthe past decades. However, many studies have beenconcentrated on the sample which has been alreadydestroyed. This paper investigated the entiredeterioration process of mortars that wereimmersed in Na2SO4 solution containing 3 gSO42-/Land 33.8g SO42-/l at 20 oC up to 600 days. The studyon time-varying regularity of expansion, cracks,compressive strength and mineral phases wasinvestigated. Back scattered electron image wasused to further examine the evolution ofmicrostructures of the mortars during the attackprocess. The results showed that damage process ofmortars can be described as induction stage,surface damage, bulk damage and then completelydamage stage. Fine ettringites that were formed inrestricted spaces, approximately 2-5 ?m, result insurface damage. At the bulk damage stage,cracking was the main characteristic of mortarwhich leads to obvious expansion. In this stage,some large ettringite crystals (>20?m) were justdeposited in the formed cracks. At the later stage,gypsum can be easily formed at interfacialtransition zones as the consumption of calciumhydroxide, which mainly contributed to completelystrength failure rather than expansion.