Belite hydraulic activity

Dorn and Adams (1983) have described the various etch rates of alite and belite in relation to hydraulic activity. A blue color on alite after a 15-second nital etch was said to represent an active alite. 

We reported earlier the first-principles study on the crystal structure and hydraulic activity of P- and y-form belites (Sakurada et al. 2009a, 2009b). In this chapter, we present our DFT simulation results on the study of hydraulic properties of cement phases. Ca-Ca, Ca-0, and Si-0 bond lengths are chosen as a yardstick for making reliable prediction on the hydraulic activity of p- and Y-C2S phases. Moreover, to find a change in crystal structure of P-C2S on substitution of the strontium atom or barium atom for a calcium atom, a periodic boundary condition has been applied on a large supercell (504 atoms, a x 3, b x 3, c x 2) to remove any spurious interactions. 

The mechanism of Si-O chemical bonding was analyzed for covalent and ionic bond orders in p- and Y-C2S. Each bond order was evaluated by an overlapping population calculated by the DV-Xa molecular orbital method (Xiuji et al. 1994). There are slight differences in computed covalent and ionic bond orders obtained for both dicalcium silicates. However, Xiuji et al. (1994) demonstrated that the differences in hydraulic activity between p- and Y-C2S do not arise from the difference of Si-O chemical bonding. Further investigation will be required to establish the relationship between these differences in the crystal structure to hydraulic activity of belite. 

The longer bond should be advantageous to split the Ca-O bond, resulting into dissolution of calcium atoms into pure water. This hypothesis causes a contradiction to hydraulic activity between P-C2S and C3S, as tabulated in Table 13.3. lost et al. (1977) pointed out that the differences in Ca-O bond length between the clinker compounds exhibit very little effect on its ability to react faster or slower with water rather the connection of CaO polyhedra by common faces influences hydraulic activity. There is a connection of CaO polyhedra by common faces in P-C2S. In contrast there is a connection of polyhedra by common edges and comers in y-C2S. This means that only Ca-O bond lengths provide less information on the hydraulic activity of the belites. 

Crystallographic analysis of dicalcium silicates are conducted using DPT simulations to predict the hydraulic activity of pure p-Belite, as well as those doped with strontium or barium atoms. The results of this analysis are summarized as follows ... 

---