An NMR study of the role of coir fibers in the hydration and drying of cement paste at early age

Natural fibers such as coir fibers have been increasingly explored as reinforcement in the cementitious composite to promote sustainable and economical construction. However, the in-situ application of this composite presents challenges due to the complex interplay of water behaviors, involving the hydrophilicity of natural fibers, the hydration of cement and the evaporation due to drying. Although these water-related processes play a crucial role in achieving the desired properties, they have been relatively underreported in the literature. This study aims to investigate the role of coir fibers in the water distribution and pore size distribution during the early age (initial 3 days) of cement paste hydration under drying conditions. Accordingly, Nuclear Magnetic Resonance (NMR) methods involving the moisture profiling and transverse relaxation are used to monitor the water distribution and pore size distribution in time but also in space. The samples are made by adding water-saturated coir fibers to the cement paste at different dosages (0, 1%, 2% and 4%). The effects of drying and coir fiber dosages on the samples are evaluated. The results show that drying leads to a moisture gradient and a larger transverse relaxation time T2, i.e., a coarser pore structure near the drying surface. By adding the saturated coir fibers, the moisture gradient caused by drying is decreased and spread out along the sample. As the coir fiber content increases, the water content along the sample and the total water content of the sample increase. As a result, the samples with coir fibers exhibit a lower T2, i.e., a denser pore structure near the drying surface. Coir fibers are shown to eliminate the adverse effect of drying on cement hydration through three effects, including the internal curing, improved water transport and crack control. The results from this study reveal the promising potential of coir fibers for in-situ construction applications.