Abstract: In this study, Na₂SO₄ (Na alkali) and K₂SO₄ (K alkali) were added to adjust the total alkali content of low-heat Portland cement (LHPC), medium-heat Portland cement (MMPC) and ordinary Portland cement (OPC) to 0.8% and 1.2%. The effects of different alkali sulfates on autogenous shrinkage and dry shrinkage of cement-based materials with different mineral compositions were investigated. The mechanism of different alkali sulfates affecting the autogenous shrinkage and drying shrinkage of different cement-based materials was also revealed via microcalorimetry, mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR). Results indicate that 1) alkali sulfates promote the shrinkage of different cement-based materials. At the same content of alkali sulfate, LHPC has a low autogenous shrinkage rate and OPC has a high autogenous shrinkage rate; but as for drying shrinkage, OPC has the smallest rate and MHPC has the largest rate. 2) In different cement-based materials, the promotion effect of K alkali is higher than that of Na alkali and such promotion effect intensifies with the rise of alkali content. When alkali content is 0.8%, the promotion effect of K alkali is more than 1.1 times that of Na alkali; when alkali content reaches 1.2%, the promotion effect of K alkali is over 1.3 times that of Na alkali. Microscopic test also manifested a larger promotion effect of K alkali than Na alkali: K alkali promotes the hydration of cement, enhances the content of pores with a size smaller than 50 nm, and is related to the transfer of Al atoms into C-S-H chain.

Key words: alkali sulfates, cement-based material, shrinkage property, hydration process, pore structure, aluminum atom