Effect of dosage of sodium carbonate on the strength and drying shrinkage of sodium hydroxide based alkali-activated slag paste

This study investigated the properties of alkali-activated slag (AAS) pastes with NaOH and a blend of NaOH and Na2CO3 as the alkaline activator. The effects of five different Na2CO3 to NaOH molar ratios (0:10, 0.9:10, 2.5:10, 5.7:10 and 15:10) and three different Na2O contents (4%, 6% and 8%) in the alkaline activator on the setting time, mechanical performance and drying shrinkage were tested. The water to slag ratio was 0.35 by weight. The results showed that both the Na2CO3 to NaOH molar ratios and Na2O contents are major factors influencing the fresh and hardened pastes. Higher Na2CO3 to NaOH molar ratios yielded a longer setting time and higher compressive strength. The Na2O content of 6% was more sensitive to the properties compared with 4% and 8% Na2O. When the Na2O content was 4%, the drying shrinkage increased as the Na2CO3 to NaOH molar ratio increased; however, when the Na2O content increased to 6% and 8%, the drying shrinkage decreased as the Na2CO3 to NaOH molar ratio increased. The main hydration products were C-S-H(I), hydrotalcite, calcite and gaylussite. There was a strong relationship between the drying shrinkage and pore structures of AAS. Consequently, the Na2CO3 to NaOH molar ratio of 15:10 and Na2O content of 6% can be regarded as the optimum mix proportion.