Spatiotemporal evolution of water and pore structure in cement-based material containing SRA by LF NMR

Abstract

Shrinkage-reducing agents (SRA) have been extensively applied in cement-based materials for overcoming the shrinkage-induced cracks. The shrinkage behavior of cement-based materials is closely linked to evolution of water at different states and pore structure, meanwhile the shrinkage mechanism of SRA has not been clarified yet. In this study, the cement-based materials with different additions of SRA were prepared. The T2 relaxation spectra during the hydration and drying process were measured via low-field nuclear magnetic resonance (LF NMR), so as to explore the effects of SRA content on the water evolution and pore structure characteristics in cement-based materials. Meanwhile, the effects of SRA content on mechanical properties and drying-shrinkage deformation were analyzed by measuring the flexural strength, compressive strength and drying-shrinkage ratio. The results show that, the interlayer space (representative of T2 relaxation time) decreases, and then the water flows from gel pore to interlayer space during the hydration process. SRA can improve the contents of gel and adsorpted layer water and pore water. The increases became more obvious with the increase in SRA content, while the influencing degree of SRA weakened gradually as the curing age increased. Moreover, SRA significantly increased porosity and most probable pore diameter of cement-based materials, and the increases became particularly obvious at a larger addition. The shrinkage deformation of cement-based materials can be reduced after the addition of SRA. This is due to the fact that the addition of SRA lowered the evaporation of free water in cement mortar. So, the mortar maintained a high humidity in the hydration process, and simultaneously the pore size becomes thicker, causing the decline in shrinkage stress.