Direct evidence of pore structural effects on the compressive strength of lightweight cement slurry containing nano silica fume and hollow glass microspheres

Abstract

This paper presents the results of an experimental study about the pore structure of 7 kinds of density-controlled lightweight cement slurry (LWCS) containing hollow glass microspheres and nano silica fume. X-ray micro-computed tomography (XCT) was used to characterize the microscopic pore structure of the LWCS specimens blended with nano silica fume (NSF) by 0–30% on the aspects of plane porosity, total porosity, pore size, sphericity, specific surface area, and fractal dimension. Meanwhile, the mineralogical and microstructural characterization of the NSF-affected pastes after the curing regime were tracked analyzed by thermogravimetric analysis (TGA), XCT, and scanning electron microscopy (SEM). Subsequently, the relationship between pore parameters and compressive strength of LWCS specimens was established. The optimum amount of nano silica fume was determined based on compressive strength, pore structure characterization parameters, and weight loss. The results show that the incorporation of NSF into LWCS matrix contributes to the improvement of the compressive strength. The optimal addition content for NSF is 20%, which can maximize the compressive strength at all curing ages. The filling of available space, the promotion of heterogeneous nucleation of cement hydrates and the pozzonlanic reaction of NSF itself, which densifies and homogenizes LWCS microstructures, account for the effects of NSF on the strength and pore characteristics of LWCS. The pore structure characterization parameters of LWCS, including total porosity, large pores (>100 µm) volume, large pores and medium pores (50–100 µm) volume, pore size, fractal dimension, and specific surface area, are all negatively linearly correlated with compressive strength. The harmful effect of large pores and medium pores on compressive strength is more pronounced than that of tiny pores (< 50 µm). There is a good negative ternary linear fitting relationship correlation between the fractal dimension, the large pores volume and the compressive strength (R2 =0.89). In order to prepare LWCS with excellent strength, the pores with size greater than 50 µm should be controlled or eliminated if possible. Fractal dimension is a more accurate quantitative parameter to characterize the compressive strength than the other pore parameters.