Effect of nano-metakaolin on establishment of internal structure of fly ash cement paste

Abstract

The study aims to enhance the viscosity and establishment of internal structure of fly ash (FA) cement paste through the addition of nano-metakaolin (NMK). Various tests, including apparent viscosity test, small amplitude oscillatory shear test, and calorimetric test, were conducted to investigate the effects of FA content, NMK content, superplasticizer content, and test temperature. The results showed that NMK increased the apparent viscosity of FA cement paste while weakening its shear thinning behavior. Specifically, incorporating 5 wt% NMK increased the apparent viscosity of cement paste containing 30 wt% FA by 60%. Furthermore, NMK accelerated the growth of storage modulus (G') and facilitated the rapid establishment of internal structure in FA cement paste, the optimal amount of NMK admixture in a 30 wt% FA cement paste is determined to be 5 wt%. However, the presence of superplasticizer hindered the increase in G' and delayed the establishment of internal structure in NMK/FA cement pastes. For cement paste containing 5 wt% NMK and 30 wt% FA, the growth rate of G' within 60 min decreased to 26% when 0.12 wt% superplasticizer was added compared to the cement paste without superplasticizer. Temperature had a significant impact on the establishment process of internal structure in NMK/FA cement pastes within 90 min but had minimal effect on the final state at 90min. For N5FA30 cement paste hydrated for 30 min and 90 min respectively, G' at 10 °C was approximately 7.3% and 66.7% compared to that at 40 °C. Moreover, a power function correlation was observed between G' and cumulative hydration heat per gram of cement (Hc) in NMK/FA cement pastes. The inclusion of superplasticizer decreased both G' and Hc in the cement paste. Notably, the influence of superplasticizer on G' was more pronounced than its impact on Hc during the hydration process.