Physico-chemical changes in nano-silica and silica fume modified cement pastes in response to leaching

Leaching of calcium ions from cement pastes modified with silica fume and nano-silica, and the physico-chemical changes that this induces in the system are the focus of this paper. The pastes were subjected to slow (in deionised water) and accelerated (in 6M ammonium nitrate) leaching. Mass loss, increase in porosity and thermal analysis were used to quantify the physical and chemical changes in the system due to leaching. The modified cement pastes are found to exhibit better leaching resistance under both exposure conditions. It is shown that leaching in deionised water and cementing materials reaction are essentially coupled. When leached in ammonium nitrate, it is observed that the entire CH is leached from all the pastes, and pastes with very low initial CH contents suffer increased decalcification from C-S-H gel. It is shown that, when subjected to very aggressive leaching agents, a certain amount of CH acts as a buffer delaying C-S-H dissolution. The beneficial effect of nano-silica in controlling calcium ion leaching especially when subjected to accelerated leaching is brought out.