A study on the synergistic effects of carbonation and hydration in the modification of filler particles to enhance early strength

The mechanical properties of backfill materials play a crucial role in promoting the efficient application of sustainable mining practices. To effectively enhance the mechanical properties of backfill materials, this study focuses on nano-silica-reinforced tailings cemented backfill. It investigates the effects of traditional solid powders and precursor solutions on the early-stage strength, microstructure, carbonation and hydration of the backfill. The results indicate that the uniaxial compressive strength of the backfill exhibits a trend of initially increasing and then decreasing with the addition of both types of nanosilica. Under the same curing period, compared to the nanosilica powder-modified backfill group at the optimal dosage (4%), the addition of nanosilica precursor solution at the optimal dosage (6%) resulted in a more significant improvement in the early strength of the backfill. Specifically, compared with the blank control group, the maximum compressive strength of the nano-silica powder-modified fillers increased by 27.68% and 19.43% after 3 and 7 days of curing, respectively, whilst that of the nano-silica precursor solution-modified fillers increased by 35.02% and 29.48% after 3 and 7 days of curing, respectively. Combining observations from scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TG-DTG) to examine the effects of nanosilica powder and precursor solution on the microstructure of the fillers at different curing ages, it was found that the microstructure of the nanosilica precursor solution-modified filler group was denser and more stable. Subsequent carbonation and cement hydration heat tests revealed that, as one of the raw materials in the nanosilica precursor solution is CO2, this group absorbed a certain amount of CO2 and reacted with Ca2+ within the filler to form a large amount of carbonation products. Furthermore, the extent of the cement hydration reaction was more intense than in the group modified with nano-silica powder. Within 72 hours, the maximum heat of hydration NS(s) and NS(aq) reached 47.76 J·g-1 and 55.70 J·g?1 respectively, representing increases of 3.60% and 16.62% compared to the blank control group; The maximum heat fluxes for the NS(s) and NS(aq) groups reached 0.19 mW·g-1 and 0.40 mW·g-1 respectively, whilst the maximum heat flux value for the blank control group was only 0.18 mW·g-1. This is attributed to the fact that the nano-silica precursor solution not only fully exploits the unique pozzolanic, filling and nucleation effects of nano-silica, but also effectively harnesses the synergistic effects of carbonation and hydration. This accelerates the cement hydration reaction and promotes the progression of the hydration process, thereby generating a large amount of carbonation and hydration products to fill the harmful pores within the matrix, forming a denser structure that is more conducive to the improvement of early strength. However, the tendency of nanosilica powder to agglomerate leads to an uneven distribution of hydration products, reducing the matrix density of the backfill and limiting the improvement in mechanical properties. These results indicate that an appropriate amount of nano-silica precursor solution is a promising admixture for enhancing the mechanical properties of backfill, providing both theoretical justification and experimental support for the rational application of nano-silica in mine backfill.