Water and alkali salts in the hydrating and hardened green cement-based materials: Hydration process, moisture content and transport

Licentiate thesis, 2022

Reducing CO2 emissions in the production of cementitious binder is the most effective way to decrease the environmental impact of the construction industry, so a large amount of supplementary cementitious materials (SCMs) has been used in the green concrete. Both the SCMs and alkali salts in binders influence the hydration process and the structure in hardened cement-based materials. Experiments were performed to investigate the effects of them on the hydration reaction in fresh paste, the pore structure and moisture transport in hardened pastes because these properties determine the durability of concrete during its service life. The composition and morphology of hydration products were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical conductivity of hydrating paste was real-time monitored by a newly invented device to detect the structure change during hydration. An easy procedure was developed to determine the water distribution in paste. Electrical conductivity of the pore solution was calculated with the volume of evaporable water and chemical composition of the binders. The moisture transport in hardened pastes was measured by the new procedure and setup. The chloride migration in paste was measured by the rapid chloride migration method (RCM). The mercury intrusion porosimeter (MIP) was used to test the pore size distribution.

The results show that the precipitation of C-S-H is a nonclassical nucleation process. The initial structure building starts with the nucleation of primary globules. It grows by particle attachment and potassium salts influence not only the size of primary globule floc but also the packing orientation. A large increase in the heat release after the induction period may be due to the growing attachment rate of flocs instead of the dissolution of etch pits. The duration of induction period correlates to the size of primary floc. Al ion will change the size of floc to prolong the low-rate period, but alkali salts can mitigate the effect from it. A hypothesis regarding the dissolution of C3S and the nucleation of C-S-H within the near-surface region narrows the gap in the current theories.

The hydration reactivity of binders can be indicated by the evolution of electrical conductivity, formation factor and its growth rate in the hydrating pastes. The electrical properties of pastes are related to the setting, pore connectivity and volume of evaporable water. An increase in the water-binder ratio (w/b) lowers the electrical conductivity of pore solution due to the dilution of alkali concentration. However, it increases the connectivity of pore solution and reduces the formation factor of pastes. The blending of slag decreases the conductivity of pore solution and increases the formation factor. Fly ash induces a higher connectivity of pores at the early age owing to its lower reactivity compared to clinker, but the connectivity of pores in the fly ash paste is much lower than the plain pastes after long-term hydration (1 year). Limestone increases the connectivity of pore solution at the early age, but its filling effect becomes effective after a certain hydration age. The relationship between volume of evaporable water and formation factor can be well demonstrated by the extended percolation theory, and this provides theoretical basis for an in-situ detecting of evaporable water in pastes by electrical conductivity.

The procedure developed in this study can measure the moisture transport properties in both steady-state and non-steady state transport condition. The moisture transport coefficient in the hardened cement paste is RH dependent. The differences in RH dependency are due to discrepancies in the critical RH for percolation of liquid in pastes. The blended pastes have a more complex pore structure and lower concentration of alkali ions in pore solution, so the critical RH of the blended pastes is higher than that of OPC. The blending of fly ash and slag evidently reduce the moisture and chloride diffusivity in pastes due to its reduction effect in formation factor and pore connectivity. Formation factor is the major determinant for the moisture transport at a high RH interval, but porosity of small pores (middle capillary and mesopores) becomes the major determinant at a low RH. This study provides the meaningful data for the prediction and simulation of moisture and ion (e.g. chloride) transport in concrete during its service life with a continuous long-term hydration.