Meso- and micro-structures and residual compressive strengths of high-strength concrete (HSC) and mortar (HSM) with blast-furnace slag (BFS) replacement ratios of 0% - 55% were experimentally obtained after 20℃ - 800℃, respectively. The results indicated that the meso-crack started to appear after 400℃ and grew into the main crack of HSC and the net-like crack of HSM with the increasing temperature. Calcium hydroxide could dehydrate above 400℃ and calcium carbonate could decarbonate above 600℃ in paste. The micro-morphology of paste became more and more loose from 400℃ to 800℃. Residual compressive strengths of HSC and HSM reduced as the temperature rose, except for the strength recovery of HSM after 400℃. The compressive strength of HSC had a reduction with the increasing BFS replacement ratio at 20℃, but HSC with 18% BFS had the highest residual compressive strength above 200℃. An increasing BFS replacement ratio could reduce the residual compressive strength of HSM after the temperatures of 20℃ - 600℃. Equations for estimating residual compressive strengths of HSC and HSM as well as the relationship between them were proposed with the temperature and the BFS replacement ratio as two primary parameters.
Silicate-based surface impregnations have received much attention in the protection of concrete against carbonation. However, the understanding of their behavior is still not widely understood. In the present study, protective performance and mechanism of the silicate-based surface impregnations against carbonation were experimentally investigated through the carbon dioxide absorption and the scanning electron microscopy (SEM) observations. Results obtained lead to an exploration hypothesis that the silicate-based surface impregnation products were effective in enhancing the resistance to carbonation due to the synergy effect of pores-blocking effect and the carbon dioxide absorption through the chemical reaction.
This study evaluates the influence of aluminum sulfate (AS) on hydration and properties of cement pastes. The hydration behavior of cement paste contributed by AS was identified via isothermal calorimetry, X-ray diffraction (XRD), scanning electron microscope (SEM) and helium pycnometry. Setting time, compressive strength and pore structure of accelerated cement paste were also evaluated. Results indicate that the addition of AS can reduce setting time significantly and increase compressive strength of cement paste. Moreover, hydration process of accelerated cement paste can be divided into four stages according to absolute volume change measured by helium pycnometry, which are dissolution-crystallization stage, induction stage, rapid shrinkage stage and structure compacting stage. More nano-scale pores (pore diameter smaller than 10 nm) are generated in structure compacting stage, resulting in the “fake” absolute volume expansion. The rebound rate of shotcrete can be reduced with the increase of AS.