Cement, Energy and Environment

To understand why the process of cement hydration is important to the cement maker it is useful to have a recapitulation of the mechanism of C 3S hydration. Tricalcium silicate comprises some 50-60 per cent of Portland cement. It is this component which provides the bulk of the reactive material to hold cement-based product together. It does this by undergoing a metamorphosis from a dry crystalline solid to a disordered, hardened gel– like material in which about half of its volume is water. c3s formation In the cement kiln C 3 S forms at about 1450"C. The initial c 3s crystal is of rhombohedral structure (ICR August 2008) , which then inverts on cooling, usually to monoclinic form (M1 or M3). Most C3S is stabilized to some extent by the presence of impurities in the crystals and does not invert further to the triclinic form. Even the lower temperature triclinic modification, however, would not be the end of the story for a pure crystal. If pure C3S is cooled very slowly it will eventually decompose to the materials from which it was formed, namely C2S and free calcium oxide. C3S is not inherently stable at room temperature and fast cooling of the crystals freezes a certain amount of disorder into the crystals at lower temperatures. In cooling the C3S as quickly as possible down from 1450"C the manufacturer is improving the reactivity of C 3S by increasing its potential to decompose when mixed with water. Hydrated calcium silicate In the presence of water instability becomes apparent as crystals of c 3s begin to dissolve when the surface is attacked. CaO and Si0 2 are able to dissolve into the mixing water for concrete, mortar or grout. The solubility of both these oxides is, however, very low and reprecipitation into a more stable material happens rapidly. At the surface of the C 3S grain there are three units of CaO to each Si02. The gel-like material which eventually forms from c 3s and water has a ratio of CaO to Si0 2 of about 2:1, so there is an excess of CaO available in the water. This excess, with water, forms a crystalline material known as calcium hydroxide. In the initial stages the hydrated calcium silicate, known as C-S-H because it has no strictly defined structure nor composition, forms on the site of the dissolving c 3s crystal, forming a ghost or relict. Figure 1 is a scanning electron micrograph of a polished section of concrete . The lightest areas are the unreacted grains of cement. Surrounding the grains is a light grey shell C-S-H which has formed in the same location as the C3S crystal as it dissolved away. The excess CaO, having a slightly greater solubility, travels into the water-filled space in the newly-mixed cement paste and precipitates there as calcium hydroxide. However, the C-S-H material has a larger volume than c 3s and so also extends outwards into the water-filled space. While this reaction is going on with C 3S, the reactions concerning C 3A and sulphate in water, as described in October's issue of ICR, are also occurring. Into the water-filled space therefore, is being precipitated C-S-H, calcium hydroxide and the calcium sulpha-aluminate known as ettringite. As ettringite is starved of sulphate it converts to monosulphate and these three phases, C-S-H, calcium hydroxide and monusulphate are the three which form the bulk of the cement paste in concrete . In addition , some 15-20 per cent of the cement clinker consists of C 2S. This phase does not react as fast as C 3S but begins to dissolve, releasing CaO and Si0 2, into the mixing water which is not capable of retaining large quantities of either oxide in solution. As with C3S the reaction product precipitated in the paste is C-S– H. The ratio of CaO to Si0 2 is similar to the starting composition of C 2S so the production of calcium hydroxide is not increased by c 2s hydration , although the 20