CEMENT and CONCRETE MINERAL ADMIXTURES

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An admixture is broadly defined as something that is mixed together. Many of the additional materials that are being used in cement and concrete may be considered as admixtures according to this broad definition, since they are mixed together either with cement or concrete. However, more explicit terms are being used in cement and concrete terminology. An admixture, as defined by the ACI Committee 116, is ‘a material other than water, aggregates, hydraulic cement, and fibre reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that is added to the batch before or during mixing’. Materials of similar nature are called additions if they are incorporated into hydraulic cements during manufacture either by intergrinding or blending (ACI 116,2005). Both of these terms embody a very wide range of materials some of which are commonly used while others have only limited applications. Concrete admixtures are customarily categorised into two major groups as chemical admixtures and mineral admixtures. Chemical admixtures include air-entraining, water-reducing, high-range water-reducing, retarding and accelerating agents which are water-soluble compounds. They are added to concrete in very small amounts. The scope of chemical admixtures is much wider than that is given here. Since this book deals with commonly
used mineral admixtures in cement and concrete, the reader may be referred elsewhere (Mailvaganam and Rixom, 1999; Paillére, 1995; ACI 212, 2010) for chemical admixtures. Mineral admixtures are finely ground insoluble mineral matter added to concrete in large amounts. Although the terms cement replacement materials (CRM), supplementary cementitious materials (SCM) or complementary cementitious materials are sometimes synonymously used for mineral admixtures; none of them thoroughly explain the function of mineral admixtures in cementitious systems. Mineral admixtures may be used to
partially replace fine aggregate or as another main constituent in concrete, too. Only few of them have limited hydraulic activity of their own, in order
to be called cementitious. The main reasons for using mineral admixtures in concrete either as a direct constituent or as combined with portland cement (PC) are to enhance the workability of fresh concrete and the durability of hardened concrete. In fact, they affect almost every property of concrete and these will be discussed in the following chapters individually for the common mineral admixtures. Besides technical reasons, economic and ecological reasons are equally
or sometimes more significant in using mineral admixtures in cement and concrete. Manufacturing of PC is an energy-intensive process. About 110 kWh electric energy and about 4 GJ fuel energy is consumed to produce 1 ton of PC. On the other hand, roughly speaking, 1 ton PC production leads to 0.9 tons of CO2 emission. Approximately half of this originates from the decomposition of the calcareous raw material for PC during the burning process.

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Therefore, the use of mineral admixtures in cement and concrete results in reduced energy consumption as well as lower greenhouse gas emissions since they do not require a clinkering process. Furthermore, most of these mineral admixtures are industrial by-products which when not used are wastes causing serious environmental pollution. Mineral admixtures may be subdivided into three categories as (1) materials of low or no reactivity, (2) pozzolans and (3) latent hydraulic materials. Those in the first category include materials like ground limestone, dolomite, quartz and hydrated lime. They are primarily used to improve the workability of fresh concrete, especially those with insufficient fine aggregate contents. Such concretes are prone to segregation and bleeding. Therefore, their coarse aggregate- and water-holding capacities (cohesiveness) should be increased by increasing the fines content. A pozzolan is defined in the ACI Committee 216 report (ACI 116, 2005) as ‘a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties’.

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