Interaction of water with supplementary cementitious materials: Hydration mechanism, microstructure and moisture transport

Doctoral thesis, 2023

Supplementary cementitious materials (SCMs) offer a sustainable solution to reduce carbon emissions from the production of cement and concrete. This dissertation explores the impact of SCMs and the related additives on the hydration process of cementitious materials, which can affect their microstructure and transport properties. Water is involved in the whole life of the cementitious materials thereby determining the hydration, microstructure and durability. Advanced techniques were employed in this study to investigate the impact of additives on the hydration of C3S, examining microstructure refinement by SCMs and its relationship to transport processes, and assessing changes in water dynamics. A device was designed to continuously monitor the effect of SCMs on early hydration, and it was subsequently updated to monitor the hardening process of concrete containing SCMs.

Results show that the dissolution theory fails to explain anomalous hydration of tricalcium silicate at high water to solid ratio. A new hypothesis in this study proposes that calcium silicate hydrate (C-S-H) primarily nucleates within the near-surface region, and this hypothesis bridges the gap between dissolution and protective layer theories. The designed device performs well in monitoring water interaction with SCMs. The evolution of electrical conductivity in hydrating pastes closely relates to chemical reaction processes and can be classified into four stages. The growth rate of the formation factor indicates the reactivity of different binders. Blending SCMs refines the pore structure, decreases pore connectivity and results in a higher formation factor. SCMs affect the pore structure of, the phase assemblage and water dynamics. The mesoscale pore structure in pastes with SCMs can be well indicated by water vapour desorption isotherms, but ion effects on water vapour equilibrium pressure must be considered when calculating pore size distribution. A novel approach works well in evaluating the hydration degree of SCMs by use of water vapour sorption and thermodynamic modelling. Thermoporometry and broadband dielectric spectroscopy effectively characterise moisture distribution and dynamics in hcps, respectively. SCMs have limited effects on the dynamics of structural water, primarily influencing water dynamic in small gel pores and interfacial polarization. The first drying process decreases the volume of unfrozen water (< ~2.4 nm) under various levels of relative humidity. Gel pores coarsen significantly during the drying between 75 % and 50 %.

Change of microstructure alters the transport of moisture and chloride in hcp. The decrease in both moisture transport coefficient and chloride migration coefficient induced by SCMs is notably more significant in hcp with a higher water to binder ratio. The modified moisture transport in blended systems is primarily due to pore structure refinement, specifically the reduction in pore connectivity. Both the formation factor and porosity of small pores determine the moisture transport properties of hcp, with the formation factor being more significant at high RH and the porosity of small pores being more significant at low RH. The effect of SCMs on chloride is also due to the decrease in pore. A simplified model based on the formation factor of hcp can be used to estimate the chloride migration coefficient for the blended pastes and mortars.

The upgraded device provides a reliable non-destructive monitoring of concrete performance. Formation factor and ultrasonic pulse velocity are reliable indices for concrete strength; however, formation factor exhibits the optimal performance. This study provides insights into the mechanism of how water interacts with cementitious materials and a new non-destructive monitoring method to promote the application of SCMs in sustainable concretes.