Workability, autogenous shrinkage and microstructure of alkali-activated slag/fly ash slurries: Effect of precursor composition and sodium silicate modulus

Abstract

The alkali-activated slag/fly ash system is used as a cementitious material to instead of ordinary Portland cement and has good application prospects for recycling various solid wastes and low carbonization in the construction field. In this work, we aim to reveal the effects of changes in precursor composition and sodium silicate modulus in alkali-activated slag/fly ash slurry on strength and autogenous shrinkage, and make a comparative study about workability, autogenous shrinkage and microstructure. The test results showed that the engineering performance of alkali-activated cementitious materials mainly depended on the constituent materials and their ratios. With the increase of fly ash content, the slump was enhanced, the setting time was extended, the strength was slightly reduced, and the autogenous shrinkage was significantly mitigated. For the reference slurry (n = 1.2), the 28 d autogenous shrinkage value of S10F0 was 6064.28 μm/m, the 28 d autogenous shrinkage value of S9F1 was 1516.67 μm/m, and the shrinkage value was reduced by 75%. And with the increase of sodium silicate modulus, the slump was enhanced, the setting time was increased, the early strength was reduced, the later strength developed faster, and the influence on the sample autogenous shrinkage was smaller. The reaction products were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). With the increase of fly ash content, the calcite phase was observed in the reacted products, the flocculent components and many unreacted fly ash spheres in diameter were observed, and there were many micro-pores in the hardened solid particles. Combined with autogenous shrinkage and mercury intrusion porosimetry analysis, it could be discovered that when the modulus of sodium silicate increased, the capillary pore volume increased, and the total porosity and pore size decreased significantly, when the modulus of the activator increased from 1.2 to 2.0, the porosity decreased by 44.36%. making the autogenous shrinkage and strength increase. When the activator modulus was 1.2 and the content of fly ash was more than 30%, the content of NaOH in the solution was higher, which accelerated the early reaction rate, resulting in more pores inside the slurry, larger pore size, smaller capillary pore volume, and expansion of alkali-activated material slurry volume. This study may shed valuable insights to the improvement of autogenous shrinkage by adjusting precursor composition and sodium silicate modulus.