Effects of dry-wet circulation on cement-based composite filling materials

In recent years, cement-based composite materials have been widely used in mine filling, which can well solve the hidden danger of goaf collapse. However, when the water table and surrounding rock moisture content change, the filling materials will be in the process of dry and wet alternation, which will affect the long-term stability of the filling materials and goaf. In order to explore the influence of dry and wet cycles on the long-term stability of cement-based composite filling materials, taking water-cement ratio 4∶1 cement-based composites as the research object and using ETM mechanical test system, X-ray diffraction (XRD) and scanning electron microscopy (SEM) device, uniaxial compressive strength tests were carried out in the state of "water saturation" and "water loss" under different dry-wet circulation. The influence mechanism of dry-wet circulation was discussed by phase analysis and microstructure. The results show that as the number of dry-wet circulation increases, the loss rate increases gradually while the water content and bulk density decrease, the peak intensity first increases and then decreases, and the increase is as high as 9% under the saturated state. The water loss rate, water content and bulk density do not change much under the condition of "water loss", while the peak strength decreases from the initial state to up to 13.5%. The elastic modulus and residual strength of the two states show a downward trend. Through mechanism analysis, it is found that carbonation reaction is the main reason for material strength reduction in the "dry" process, while the CaCO₃ and other materials are converted into ettringite (AFT) and thaumasite (TSA) with some bearing capacity during the absorbing water process in "wet" process, which is the main reason for the strength recovery of materials. However, the recovery ability is limited, and the long-term dry-wet circulation will adversely affect the stability of cement-based composite filling material.