Effect of sulfate attack on the composition and micro-mechanical properties of C-A-S-H gel in cement-slag paste: A combined study of nanoindentation and SEM-EDS

Sulfate attack is a common type of corrosion that results in the deterioration of cementitious materials, but its attacking mechanism on the micro-mechanical properties C-A-S-H gel is still not fully understood. This paper utilizes combined techniques of XRD, nanoindentation and SEM-EDS to investigate the composition and mechanical properties evolution of C-A-S-H gel in cement-slag paste under sulfate attack. It is found that the pozzolanic reaction of slag depletes portlandite and produces AFm, which can act as a buffer for the ettringite formation during sulfate attack. Sulfate ions can adsorb on C-A-S-H gel and cause the its compositional decalcification and dealumination. Slag incorporation can decrease the sulfate adsorption capacity of C-A-S-H gel and moderates its compositional change under sulfate attack. The composition change of C-A-S-H gel induced by sulfate attack is accompanied with its micro-mechanical properties’ degradation, and this degradation is a result of its increasing porosity during decalcification and dealumination but not related with its original composition. The incorporation of slag reduces the decalcification and dealumination of C-A-S-H gel in the paste induced by sulfate attack, thus improving its micro-mechanical properties stability in sulfate attacking environment.