Role of pore structure on resistance to physical crystallization damage of calcium sulfoaluminate belite (CSAB) cement blends

The resistance to physical sulfate damage was tested for a variety of cementitious mixtures including ordinary portland cement, calcium sulfoaluminate belite (CSAB) cement, a CSAB-fly ash blend and an air-entrained CSAB. Mortar bars were exposed to DI water or 5 % sodium sulfate solutions under cyclical wet-dry conditions. The expansions of the specimens were measured after each wetting cycle. A water sorptivity test, thermogravimetric analysis, x-ray diffraction analysis, mercury intrusion porosimetry method, and micro-computed tomography were applied to study differences in expansion and cracking in the mixtures. The results showed that the incorporation of fly ash into the CSAB matrix reduced cumulative pore volumes and slightly increased micropore sizes - both of which contributed to failure of the CSAB-fly ash systems. Use of the air-entraining agent in CSAB produced more, smaller, and more uniform pores, but did not influence the performance of CSAB. Both the CSAB and air entrained CSAB mixtures showed good resistance to physical sulfate damage. Samples with average pore diameters <15 nm prevented infiltration of sulfate ions and slowed the onset of salt crystalization damage.

Automated summary

This study tested the resistance to physical sulfate damage of various cementitious mixtures, finding that the incorporation of fly ash reduced cumulative pore volumes and slightly increased micropore sizes, leading to failure; the use of an air-entraining agent produced more, smaller, and more uniform pores without affecting the material's performance; both CSAB and air-entrained CSAB mixtures showed good resistance to physical sulfate damage.