Influence Of Mineral Admixtures On Calcium Leaching Of Hardened Cement Paste

Cement-based materials which are frequently exposed in aggressive environment such as soft water or acid water easily suffer calcium leaching of hydrated products. This process would lead severe physical or mechanical damage on concrete structure. Leaching of cement-based materials is a diffusion controlled process and can be influenced by compositions and microstructures of materials and surrounding environments. Considering the obvious improvement of mineral admixtures in leaching of cement-based materials, this thesis studied the effects of mixing amounts of different mineral admixtures and curing ages on leaching of hardened cement pastes (HCP). And the X-ray computerized tomography (X-CT) technology was used to study the leaching process. The main contents and results of this paper are as following:1. The influences of fly ash (FA) amounts and curing ages (10 months and 3 years) on leaching of HCP were studied.6M ammonium nitrate solution (NH4NO3) was used as the leachant for accelerating the leaching process. Gravimetric analysis, X-CT method and ultrasonic test technique were respectively used to measure the mass losses, the leaching depths and the relative dynamic elastic modulus of HCP before and after leaching. The results show:the mass loss of HCP decreases as FA amount increasing; the leaching depth is minimum when the FA amount is 30%, which is the optimal mixing amount; X-CT method is in accordance with gravimetric analysis in evaluating mass losses of HCP; excessive FA may make the relative dynamic elastic modulus of leached HCP decrease. Longer curing ages reduces the mass losses and leaching depths of HCP.2.The influences of mixing amounts of silica fume (SF) or slag (SL) on leaching of HCP in 6M NH4NO3 solution were studied. It is found that the leaching depth is minimum when the mixing amount of FA is 30%, and the leaching depths decrease with the SL amounts increasing. The mass loss of HCP decrease with the amount of SF or SL increased. The relative dynamic elastic modulus of the HCP after leaching are higher when increased SF or SL is mixed. Effects of water to binder ratios (w/b) on leaching of HCP were also studied, and the results show that the leaching depth and mass loss decrease with the w/b of HCP increased.3. On the basis of the imaging principle of X-CT, the arbitrary grayscale values (GSV) and the specially defined CT numbers of the main phases of cement-based materials were calibrated using five different materials including air, deioned water, silicon, aluminum and gypsum. The calibration results provide basis for further quantitative application of X-CT research in cement-based materials.