Investigation of Sulfate-Driven Deterioration in Cementitious Binding Materials Using Microstructural, Nanomechanical, and Chemical Characterization

Cementitious material is a heterogeneous composite material at all length scales, and the deterioration of cementitious material due to sulfate attack initially takes place at microstructural level. The chemical and microstructural changes of the reaction products, when exposed to sulfate environments, become significantly complex and are inherently associated with multiple length-time scales with multiphysical aspects. For better understanding of the deterioration of cementitious material due to sulfate attack, this research aimed systematic laboratory tests and characterization in a multiscale aspect. Toward that end, the sulfate-driven deterioration due to MgSO4 in three binding materials (Type I OPC, Type V OPC, and fly ash-based geopolymer) were investigated by examining nanomechanical properties along with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy at different levels of sulfate exposure. The nanoindentation results were related with SEM-EDX analysis and explained in light of the microstructural and chemical degradations. The comparison of mechanical-morphological-chemical characterization provides insights into the degradation of cementitious materials under the progressive sulfate attack. In addition, the effect of sulfate attack on macroscopic behavior including changes in compressive strength and dimension were studied. The experimental investigation attempted in this study further confirmed that the decalcification of C-S-H gel is one of main causes of reduction in the macroscopic properties in cement concrete mixtures when they face MgSO4 solution attack. The C-S-H is gradually degraded, and it appears that calco-magnesium silicate hydrate (C,M)-S-H is formed due to the replacement of calcium with magnesium. The results also demonstrate that the degradation in macroscopic and microscopic properties of Type V cement under MgSO4 solution is similar to that of Type I cement. On the other hand, the fly ash-based geopolymer showed an insignificant sign of deterioration with MgSO4 attack. It appears that the migration of Mg2+ into the microstructure did not cause significant chemical degradation, while it helped increase in elastic modulus of sodium alumina silicate hydrate (so-called N-A-S-H) gel in the geopolymer. The multiscale characterization attempted in this study could help more fundamentally understand how the material-specific degradation driven by sulfate occurred in different types of cementitious binding materials