External sulfate attack: comparison of several alternative binders

The aim of this work was to benchmark several binders by testing their capacity to resist external sulfate attack (ESA) under similar experimental conditions. Concrete samples were fabricated from seven different binders: two Portland cements, one blast furnace slag Portland cement, one super sulfated ground granulated blast furnace slag cement, one sodium carbonate alkali-activated slag cement, one metakaolin geopolymer and one calcium sulfoaluminate-belite cement. Mechanical strength measurements were used to characterize the concretes in the hardened state. Resistance to ESA was studied by measuring the longitudinal expansion of concretes submerged in a sulfate solution. In order to better understand the behavior of the binders in ESA, this study was completed by microstructural and mineralogical analyses carried out before and after attack by using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS), X-ray diffraction and thermodynamic calculations. It was found that ordinary Portland cement had high expansions (> 0.1%) due to the formation of excess ettringite caused by the reaction between aluminates and sulfates. Portland cement without C3A presented lower expansions but gypsum was found to be responsible for cracking at later ages. Alternative binders had low expansions, in the range of 0.01–0.03%, explained by the absence of C3A and portlandite, in addition to the formation of ettringite during hydration (case of ettringite binders) and the absence of calcium (case of the geopolymer-based metakaolin).