Cement, Energy and Environment

Quality & Standards ALKALIS IN CEMENT This article first appeared in International Cement Review in December 2009 and is reprinted with kind pennission of Tradeship Publications Ltd, United Kingdom. Email: info@CemNet.com Website: www.CemNet.com Issues relating to alkalis in Portland cement often arise in the context of maximum allowable alkali contents to avoid the possibility of the alkali-silica reaction. This involves some particularly reactive aggregates combining with alkalis and other constituents of the cement to form a disruptive gel some time (generally years) after concrete has been placed. Figure 1 shows a section from a sample of concrete which has suffered from alkali-silica reaction examined using a scanning electron microscope. The effects of the gel formation can clearly be seen in the cracking of the concrete. Arthur Harrison, UK In order to avoid these reactions many concrete codes prohibit the placing of concrete containing high levels of alkali when reactive aggregates may be used. These limits may be expressed as maximum weight per cent of alkali per cubic metre of concrete or as maximum percentage alkali in the cement, or both. However, the presence of alkalis in cement is not necessarily harmful but, in moderate quantities and in a an appropriate form is regarded as beneficial in assisting early strength development and improving water demand. What are alkalis? Cement is by its nature an alkaline material. The primary constituent is calcium oxide, which impacts a pH of the order of 13 to 14 to freshly mixed concrete. This has significant safety implications and one of the most common health problems associated with the use of Portland cement is that of alkali burns. In the cement industry, however, the term, 'Alkalis' generally refers to the oxides of sodium and potassium, respectively Na20 and K20. For most purposes relating to cement, K20 and Na20 are regarded as interchangeable. For this reason , they are normally considered together as, 'Sodium equivalent' or, 'Na20 '. The sodium equivalent is arrived at by correcting the weight per cent of K 2 0 to a stoichiometric equivalent of Na20 and then adding it to the actual Na20 weight per cent. This in practice means that the Na20e = (K20 x 0.66) + Na20 . Alkalis are provided to the cement kiln feed from raw materials and to a lesser extent from fuels. The source of alkalis is most commonly clay or shale, but they can also be present in some limestones, sands and frequently in the various types of alternative raw materials, which are used in cement plants, such as fly ashes, foundry sands, waste iron oxides, etc. Apart from the fly ashes, which frequently contain high alkali levels, generally these materials will be of lower alkali content than the clays and shales. Waste fines from quarries, which wash or saw granitic products, will frequently contain high proportions of alkali because of the presence of alkali feldspars in the rock. Feldspars are alumino- silicate crystals which are the primary source of alkali from the Earth. Granites and similar igneous rocks have been eroded by rivers and washed to the oceans to be deposited as clays and shales which have subsequently been uplifted by earth movements and exploited for purposes such as cement making. Where the days have been deposited in areas on land where forests grew, they are found interbedded with coal seams and when the coal is fired in the cement kiln, the clay forms the ash which becomes incorporated in cement clinker. 18