Influence of Ca2+ on Early Degradation of Cast-In-Situ Mortar Induced by Sulfate-Magnesium Multiple Combined Attack

Early degradation of cast-in-situ concrete induced by multiple internal-external sulfate combined attacks significantly affects the development of concrete strength. An experimental study regarding the effects of Ca²⁺ on the early degradation of cast-in-situ mortars subjected to internal-external sulfate and magnesium combined attacks is investigated in this paper. In particular, a specific method for accurately simulating the degradation of cast-in-situ structures was proposed in this experiment. Physical properties (including weight, size changes, and porosity), mechanical properties (including flexural strength and compressive strength), sulfate concentration, and microstructural properties were monitored during 28 days of immersion. The results show that an internal sulfate and magnesium combined attack (ISA-IMA) obviously retards the development of early strength and accelerates the degradation induced by external sulfate attack (ESA). The diffusion path of sulfate ions from outside is blocked by flake-shaped magnesium hydrates, delaying the penetration of external sulfate attacks. However, it is far from neutralizing the strength loss induced by an internal magnesium attack (IMA) at an early age. Premixed excessive Ca²⁺ would improve the strength development and pore structure of concrete or mortar, enhancing durability against corrosive conditions.