Cement, Energy and Environment

Sulphur content Table-1 shows typical chemical analyses of some of the most common clay minerals. While the alkali content of some of them is high, there is no figure quoted for sulphur and indeed sulphur is not a normal constituent of clay minerals. In the early Earth, before erosion and redeposition of the crustal rocks, sulphur-containing crystals were only found in what are termed basic igneous rocks, that is those containing little in the way of alkali. As discussed below it is important in cement clinker to ensure that the correct balance is achieved between alkali and sulphate. It is frequently the case, however, that sulphur and alkali arrive in the same clay or shale deposits by different routes. While the alkalis are removed from granitic rocks by erosion and end up in clay minerals, the sulphur is also eroded from the basic rock types. When the sulphur reaches the sea, it remains dissolved in the seawater until such time as local evaporation causes precipitation of sulphate minerals, typically calcium sulphates. Where these deposits are forming in a regior where clay minerals are also being washed from eroding mountains, then the resulting rock will contain both the alkali clays and a source of sulphate. While this simple scenario illustrates why the two are frequently found together, it also illustrates that the availability of alumina and silica in a clay should not be the only consideration when selecting raw materials, the presence of alkali will need to be balanced against the availability of sulphate for the cement kiln if the quality of the cement clinker is to be optimized. There are three reasons why alkalis must balance with Table-1: chemical analyses of clay minerals Kaolinite Montomorillonite Illite Glauconite Si02 45.4 51.1 43 52.6 AI203 38.5 19.8 29 5.8 Fe203 0.80 0.83 2.27 17.9 MgO 0.08 3.22 1.32 3.40 GaO 0.08 1.62 0.67 0.12 Na20 0.66 0.04 0.13 0.18 K20 0.14 0.11 7.47 7.42 Source: "Petrology of the Sedimentary Rocks" JT Greensmith sulphate in cement clinker. Firstly, the type of sulphate which forms will affect the solubility of the sulphate phase when water is added to cement. It is necessary to dissolve sulphate as soon as possible in order to retard C 3 A hydration and reduce water demand in concrete as much as possible. Additionally, the rapid dissolution of alkali into mixing water raises the alkalinity of the pore fluid and accelerates alite hydration and hence strength development. The second reason is that, in the absence of sulphate, alkali will become incorporated into the other clinker minerals, most notably in the C 3 A. Tricalcium silicate is a crystal which takes on a cubic form in clinker, but when high amounts of impurity are present the crystal lattice is first distorted and then, in the case of alkali impurity the crystal is actually modified to a different form, known as orthorhombic, or prismatic, C 3 A. The practical significance of this is that the reactivity changes. This has two main effects on the quality of cement made with the clinker. Firstly, retardation of C~ is the reason that gypsum is added to cement at the grinding stage. The workability of concrete is dependent on the consistency of the reaction between C 3 A, water and sulphate provided by gypsum. A change in reactivity of C 3 A will affect the workability of concrete and produce unpredictable setting times. The second factor is that the action of plasticizers on cement grains depends on the adherence of the admixture to the incipient hydration products. The first of these to appear are those of the cubic C 3 A which react very rapidly with water. Experience has shown that those cements made from clinker with relatively high alkali contents, but little sulphur, are those which have poor response to the commonly used water reducing additives. This is ascribed to the lack of suitable hydration products to anchor the plasticizer in place. The third reason that alkalis and sulphate need to remain in balance concerns the stability of the kiln and the possibility of forming harmful buildup. Alkali without sulphate will act as a lime substitute, increasing the basicity of the mix and making combination of lime with silicates more difficult. The presence of sulphate counteracts this effect, providing an alkali sulphate liquid phase which is less viscous than the main clinker liquid and makes combination , and therefore kiln control, easier. Both alkalis and sulphate are volatile in the burning zone and a cycle will be established in the kiln system. Condensation will ideally occur onto the feed particles as they come through the kiln and bring the volatiles 19