Influence of Water-to-Binder Ratio on Autogenous Shrinkage and Electrical Resistivity of Cement Mortar

This study investigates the effects of nano-metakaolin and fly ash contents, the water-to-binder ratio, and other factors on autogenous shrinkage, internal relative humidity, and resistivity. Hydration heat, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and other micro-testing techniques were employed to characterize the hydration process, phase composition, and pore structure of cementitious materials. The experimental results show that lower water-to-binder ratios lead to faster and more significant decreases in internal relative humidity within cement pastes. When nano-metakaolin and fly ash are combined, fly ash effectively mitigates the excessive autogenous shrinkage caused by nano-metakaolin under low water-to-binder ratios. Lower water-to-binder ratios result in faster resistivity growth in cement pastes. Specifically, when the water-to-binder ratio decreased from 0.35 to 0.30 and 0.25, the 28-day resistivity of nano-metakaolin–fly ash cement pastes increased by 8.08% and 7.33%, respectively. Additionally, the water-to-binder ratio has a relatively minor impact on the autogenous shrinkage and resistivity of fly ash cement pastes. Higher water-to-binder ratios accelerate the hydration rate and increase porosity, leading to the gradual coarsening of pore distributions. When the water-to-binder ratio increased from 0.25 to 0.35, the 28-day porosity increased by 50.31%. In hydration systems with lower water-to-binder ratios, internal relative humidity decreases more rapidly, pores become refined, capillary stresses increase, and autogenous shrinkage behavior becomes more pronounced. This research provides a practical foundation for studying the early-age autogenous shrinkage behavior of cementitious materials.