Understanding the synergetic effect of SAP and nano-silica on the mechanical properties, drying shrinkage and microstructures of alkali-activated slag/fly ash-based concrete

Superabsorbent polymers (SAP) have been commonly used to mitigate the shrinkage of alkali-activated materials through internal curing, but may cause some strength loss in the materials. To overcome this issue, nano-silica (NS) is used in this paper. The synergetic effect of SAP and NS on the mechanical properties and drying shrinkage behavior of slag/fly ash-based alkali-activated concrete (AAC) was investigated. The corresponding hydration reaction kinetics and microstructures were simultaneously examined to reveal the shrinkage-mitigating mechanism. The results revealed that, the incorporation of SAP induces a compressive strength deterioration of AAC, but mitigates the drying shrinkage of AAC, with the reducing amplitudes of 12.4-32.7 % at 56 d. The internal relative humidity (IRH) of 83 % is deemed as a critical node for the internal curing by SAP, and the additional moisture can maintain the IRH of AAC above 74 %, which improves the geopolymerization reaction around the SAP particles and reduces the proportion of capillary pores. The NS and SAP show a synergetic strengthening impact on the mechanical properties, drying shrinkage and microstructures of AAC, with the pores left by SAP water release filled and compensated by NS. In addition, the induction hydration period and its delay effect due to SAP can be reduced by NS, which decreases the initial and final setting times of AAC. From the results obtained in this work, AAC shows better mechanical performance and shrinkage-mitigating effect with the SAP and NS content of 0.2 % and 2 %, respectively. The research outcomes provide a new possibility for the drying shrinkage control without strength loss of AAC.