Sustainability and Factory-Made Cements
Integration of sustainability into all its operations now sets the UK cement industry's agenda. For cements to remain viable, their embodied energy and carbon footprint must be reduced over time without jeopardising product performance. The increasing availability of cements such as CEM II types for use in concrete, mortar and grout plus the continued production of niche Masonry cements for use only in mortar, will help the industry to meet its social and environmental obligations and achieve necessary economic objectives.
Until now, Portland cement CEM I, of strength classes 42,5 or 52,5, has been the 'traditional' cement in the UK, although it is the least sustainable type given its high proportion of cement clinker. Greener, more sustainable 'non-CEM I' cement solutions incorporating lower proportions of clinker are now generally available in both bulk supply and packed in bags. Use of these non-CEM I factory-made cements should become more and more widespread as prudent specifiers include a cement's sustainability credentials within their specification criteria.
Factory-Made Composite Cements
Used here, the expression, 'factory-made composite cements' means any cement that comprises Portland cement clinker combined (interground or blended) with one or more additional inorganic constituents plus an optimised amount of set-regulator (gypsum). The additional constituents are selected from materials such as power station fly ash, blastfurnace slag and limestone, all already familiar to UK concrete, mortar and grouting practice. Therefore, in terms of the British/European standard for common cements, BS EN 197-1, factory-made composite cements, are collectively, types: CEM II, CEM III, CEM IV and CEM V i.e. any 'non-CEM I' cement specified in the standard. Such a use of the term aligns with the UK concrete sector's traditional understanding but is wider than is implied by the standard itself wherein two particular cements, CEM II/M and CEM V, include the word 'composite' in their names. In general, the appropriate use of factory-made composite cements delivers performance equal to that of concrete containing CEM I cement and under certain conditions, can improve on the durability performance achieved.
Furthermore, in the case of use in masonry mortars within the generality of exposure conditions, factory-made composite cements can achieve the same level of performance on a one-to-one basis as a CEM I cement within the same traditional volume mix proportions.
Portland Cement CEM I
Formerly known as ordinary Portland cement (OPC), CEM I is manufactured to conform to British Standard BS EN 197-1. CEM I is the cement that has been most commonly used throughout the world in civil engineering and building works. Concretes and mortars made using CEM I are versatile, durable and forgiving of poor construction practice. In addition, specific properties can be enhanced by altering either the cement-making recipe or the size of the particles and so producing different cements. However, CEM I is the least sustainable type and use of alternatives is in the ascendancy.
The traditional sulfate-resisting cement used in the UK has been sulfate-resisting Portland cement (SRPC), conforming to BS 4027. SRPC is a special type of CEM I cement manufactured to contain a high content of iron oxide in order to limit the amount of the mineral phase tricalcium aluminate (C3A) and thereby increase its sulfate resistance. Additionally, SRPC is normally a low alkali cement which benefits concrete in resisting the alkali silica reaction (ASR). However, it is not the only sulfate-resisting cement available. Various factory-made composite cements are also sulfate-resisting including the generally available CEM II/B-V type of Portland-fly ash cement containing at least 25% of fly ash. Such CEM II/B-V cements are permitted for use in the same wide-range of sulfate exposure conditions as is SRPC and are also low in reactive alkalis. Moreover, SRPC is a type of CEM I cement with a high clinker content, it is no longer manufactured in the UK and is becoming more difficult to source. Consequently, greener sulfate-resisting composite cements will continue to grow in importance.
Rapid Hardening Portland Cements
Rapid hardening versions of CEM I cements are available. The average particle size is smaller in these cements and they gain strength more quickly than do ordinary CEM I types. They generate more heat in the early stages and can be useful in cold weather concreting. However, their principal use is in manufacturing precast concrete units where the high early strength of the concrete permits quick re-use of moulds and formwork.
Cements described as either rapid-setting or extra rapid hardening may be mixtures of CEM I and a non-Portland cement such as calcium aluminate or calcium sulfoaluminate and will tend to both set and harden (gain strength) very quickly.
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 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.
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 concrete when first placed. They tend to be mixtures of Portland and calcium sulfoaluminate clinkers, optimised for gypsum content.
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 cover 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 a wide range of these non-Portland 'alternative cements' and their potential is being monitored closely.
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 sulfoaluminate-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.