The role of Ca(OH)2 in sulfate salt weathering of ordinary concrete

• Na2SO4 salt weathering can occur in the calcium sulfoaluminate (CSA) cement concrete.
• Ca(OH)2 absence in CSA cement paste plays the key role in Na2SO4 crystals occurrence.
• Na2SO4 crystallization cannot occur in Portland cement paste due to Ca(OH)2 existence.

In the present paper, a micro-analysis of sulfate salt weathering of calcium sulfoaluminate (CSA) cement paste and concrete was conducted to offer reference values and to elucidate the sulfate salt weathering of ordinary concrete. The test results showed that Na2SO4 crystals could be clearly identified in CSA cement paste and concrete, causing the failure of the paste and concrete in the evaporation zone. Based on the surface physicochemical theory of crystallization in a pore, a solution film between the pore wall and the salt crystal causes crystal growth and can be formed due to the inert relationship between sulfates and CSA cement hydration products without the effect of Ca(OH)2. However, after the inevitable chemical reaction between the sulfates and hydration products of Portland cement due to the existence of Ca(OH)2, the solution film is not formed, and Na2SO4 crystallization cannot occur in the Portland cement concrete.