Influence of hydrated lime on mechanical and shrinkage properties of alkali-activated slag cement

Alkali-activated slag (AAS) cement activated by water glass (WG) has a high compressive strength, however, its high shrinkage restricts its popularization and application. In the present paper, hydrated lime (HL) was added to the AAS cement activated by WG. The influence of HL content on the mechanical properties, autogenous shrinkage and drying shrinkage was studied. The influence mechanism was discussed based on the results of hydration percentage, pore structure and microstructure of AAS paste by chemical binding water content measurement, nitrogen sorption, XRD, SEM and thermogravimetric analyses. The results show that the incorporation of HL affects the mechanical properties and increases the autogenous shrinkage, especially the early development in mechanical properties and autogenous shrinkage, while it reduces the drying shrinkage of AAS system. With the increase of HL content from 0 to 12%, the mechanical strength of AAS mortar first increase, then decrease, and reach the maximum value when the HL content is 5%. Compared with the specimen without HL, the flexural strength and compressive strength of the specimen with 5% HL at 3 days increase by 21.0% and 31.3%, respectively. With the increase of HL content from 0 to 12%, the autogenous shrinkage of AAS paste gradually increases. Compared with the specimen without HL, the autogenous shrinkage of the specimen with 12% HL increase by 58.0% and 41.4% at 1 day and 7 days respectively. This can be attributed to the fact that the addition of HL promotes the hydration of AAS cement, especially the hydration within 1 day. With the increase of HL content from 0 to 10%, the drying shrinkage decreases more. Compared with the specimen without HL, the drying shrinkage of the specimen with 10% HL decreases by 43.0% and 28.5% at the age of 28 days and 180 days respectively. This is due to the formation of more crystal-like hydration products and the reduction in the number of pores, especially the mesopores.