External sulfate attack on alkali-activated fly ash-blast furnace slag composite

Abstract

Reliable durability testing of alkali-activated binders, which might serve as a basis for the prediction of long-term field performance, is one of the main challenges towards broader commercial applications of AAB. This study investigated the influence of 5% Na2SO4 solution on mechanical and microstructural properties of alkali-activated binder based on fly ash and blast furnace slag (FA-BFS) composite. The influence of Na2SO4 solution was investigated during the period of 180 days and estimated by comparison with the reference samples simultaneously cured in a humid chamber and with the benchmark material CEM II. Strength, leaching (ICP-OES), pH, XRD, ATR-FTIR, and SEM/EDS analyses were performed. The Strength Loss Index was higher than 1 throughout the whole period of testing indicating strength increase during the exposure to sulfate attack rather than the loss of strength. Slower compressive strength development of the samples exposed to the sulfate solution in comparison with the reference samples was also observed. The exposure of alkali-activated binder to the sulfate solution led to a decrease in the Si/Al atomic ratio of binder matrix due to the leaching of Si, causing slower compressive strength development at the same time. The accelerated testing method used confirmed high resistance both of alkali-activated binder based on fly ash-blast furnace slag and CEM II to external sulfate attack and at the same time confirmed its ability to discriminate the resistance of different type of binders to sulfate aggressive environment.