Cement, Energy and Environment

increasing quantity of C-S-H maintains the increase in strength even after the usable c3s has been exhausted. The development of strength in cement paste depends primarily on the simple exercise of filling up the space which was initially occupied by water with hydration product. Hydration reaction Identifying the nature of the hydration reaction products has been made a little less difficult in the past few decades with the availability of the electron microprobe, which can perform a chemical analysis on a volume of material measured in a few cubic microns. An electron beam is fired onto the surface of a flat-polished piece of cement paste. The beam penetrates a few microns into the surface and excites x-rays of different energy depending on the elements present. A spectrum of energies from the excited volume is produced and the energy peaks relating to specific elements identified. Figure 2 is a simulated bulb of excitation shown in section with the x-rays generated moving randomly through the material. Those which emerge from the top of the specimen are collected, analysed and compared to standard materials of known composition. The resulting chemical analysis relates to a volume determined by the voltage of the electron beam and the ease of penetrability of the material. For cementitious materials it is typically about 51Jm. Interpretation of the chemical analyses once collected requires and understanding of the three phases being precipitated in the cement paste described above. A useful manner of presenting 0.70 0.60 0.50 AFm 0.40 AliCe 0.30 0.20 0.10 H 0.00 0.00 0.20 the data is demonstrated in Figure 3. This is a plot of the atom ratios of three of the main elements being precipitated , namely, aluminium silicon and calcium. The X-axis measures Si/Ca and the Y-axis AI/Ga. In order to collect the data the electron beam was positioned over a large number of sites of hydrated phases in a cement paste. The X-axis shows the ratio of silicon to calcium for each spectrum collected. In the case of calcium hydroxide this ratio would be very low and the position of calcium hydroxide crystals when hit by the electron beam would be closed to zero on both axis. The bulk of the .. . -·- ... 0.40 s~ 0.60 0.80 cement hydrates will be C-S-H and most of the plotted points " will be largetr this composition, however; as described above, the material forming in the water filled space of the cement paste will be a very fine mixtu re of C– S-H, CH (Calcium Hydroxide) and (in a young paste) ettringite or (in a paste more than a few days old) monosulphate. The monosulphate phase is also known as AFm, a recognition of small amounts of iron oxide signified by the F, aluminium oxide, the A and monosulphate. The ettringite phase is known as AFt for a similar reason but with the t representing trisulphate. 21