Effect of aggregate size on water distribution and pore fractal characteristics during hydration of cement mortar based on low-field NMR technology

The influence of aggregate size (AS) on the hydration of cement mortar is very important for improving the quality and stability of cement mortar. This study aims to study the effect of AS on water distribution and pore structure characteristics on the early-age hydration of cement mortar by low-field NMR technology. The hydration degree and fractal dimension of the hydration process were calculated by the T2 spectrum. The water distribution models at different times and locations during the hydration process were established based on the MRI bitmap data. The variation law of water volume at different locations was studied. The results indicate that: In the process of hydration, some of the water is consumed by the hydration reaction, and some of the water oozes out. The water in the large pores flows to the small pores under the action of hydrostatic pressure. AS affected the uniformity of the cement mortar at the initial state, and the gradation of the sand could improve the uniformity. The smaller AS was, the more complex the pore water distribution and changed during hydration. The bleeding mass was directly proportional to the porosity and inversely proportional to the specific surface area of particles. The effect of porosity on bleeding mass was more significant than AS. Pore fractal dimension was negatively correlated with aggregate diameter. In the initial stage, when the aggregate composition became complex, the change of fractal dimension with time also became complex. The fractal dimension increased gradually with the increase of hydration time. The gel pores (G) did not have fractal characteristics from beginning to end, capillary pores (C) and transition pores (T) had fractal characteristics after a particular time of hydration reaction. Although the air-voids (A) had fractal characteristics from beginning to end, they were not continuous.