Sustainable Cement Utilization
[Enugu, Enugu State Nigeria]

White Cement White cement is a Portland cement CEM I made from specially selected raw materials, usually pure chalk and white clay (kaolin) containing very small quantities of iron oxides and manganese oxides. White cement is frequently chosen by architects for use in white, off-white or coloured concretes that will be exposed, inside or outside buildings, to the public's gaze.

Masonry Cement: Masonry cements, as their name suggests, are designed for use in masonry mortars for bricklaying, blocklaying, rendering and plastering work. They are generally mixtures of Portland cement CEM I plus selected mineral additions (e.g. limestone or hydrated lime) and chemical admixtures such as air-entraining plasticisers that form tiny bubbles of air in the mortar. Masonry cements are used with sands and water to produce workable, cohesive mortars that are freeze/thaw resistant in the fresh wet and hardened states.

Expansive Cements: Concretes, based on most cement types, tend to shrink in volume as they dry out. Expansive cements are designed to either compensate for this shrinkage or to lead to an overall increase in volume compared to the xxxi concrete when first placed. They tend to be mixtures of Portland and calcium sulphur aluminate clinkers, optimised for gypsum content.

Environmental Cements: In principle, environmental cements are of two types, neither of which conforms to any strict definition: those that are formulated to treat and encapsulate environmental residues and those designed to limit the environmental impact of manufacture in comparison with traditional cements by reducing energy usage, virgin raw materials and/or atmospheric emissions. The first type covers a range of compositions and properties, although many are Portland cement-based, they are specifically formulated to treat and remediate contaminated soils, sludges and wastes. The use of these cements in situ, produces a new construction product, for example a cement-bound soil, a practice/product that in time is set to replace the time expired dig and dump philosophy. The second type of environmental cements tend to be described by their manufacturers as eco-cements or low energy/low carbon cements and are differentiated by process, raw materials, properties and uses. There is wide range of these non-Portland 'alternative cements' and their potential is being monitored closely.

Non-Portland Cements: These products tend to embody less energy and have an inherently smaller 'carbon footprint' than most cements based on Portland cement clinker; this means that during manufacture comparatively less energy is used and less carbon dioxide is emitted. In principle, this makes such cements more sustainable and therefore potential candidates to eventually replace Portland cements in whole or in part.

However, whether such non-Portland cements will simply satisfy niche markets or occupy a more major role in construction depends crucially on the geographical/geological availability of the raw materials used in their manufacture. Limestone, the principal raw material in Portland cement clinker production is geologically abundant and hence it forms the basis for the 'global binder' Portland cement CEM I. By and large, the raw materials used to make non-Portland cements are less abundant and less widely dispersed. Examples of these disparate cements publicised as possible candidates to be general construction materials, are: calcium sulphur-aluminate based cements (e.g. CSA-bC2S variants) geopolymeric cements (alkali  aluminosilicates), magnesium-oxide based cements and C-Fix (hydrocarbon-based). Irrespective of the cement type, extensive research on long term performance will be required before widespread acceptance in general construction, or indeed structural applications, could be achieved. In addition, there are a number of non-Portland cements that have established niche markets, some examples of these are: calcium aluminate cement (high alumina) used for refractory purposes or its rapid hardening properties, magnesium oxychloride cement (Sorel cement) for flooring, magnesium phosphate cement for rapid repair of roads and airport runways, alkali activated slags and natural/prompt cement.

Portland cement is the most common type of cement in general use around the world, because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate which controls the set time, and up to 5% minor constituents (as allowed by various standards).

As defined by the European Standard EN197.1, "Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium content (MgO) shall not exceed 5.0% by mass." (The last two requirements were already set out in the German Standard, issued in 190

Works near Berrima, New South Wales, Australia. Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 °C xxxv for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as Low Heat (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3CaO.Al2O3) formed. The major raw material for the clinker-making is usually limestone (CaCO3) mixed with a second material containing clay as source of alumino-silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of these limestones can be as low as 80%. Second raw materials (materials in the rawmix other than limestone) depend on the purity of the limestone. Some of the second raw materials used are: clay, shale, sand, iron ore, bauxite, fly ash and slag. When a cement kiln is fired by coal, the ash of the coal acts as a secondary raw material.