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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 17 types and properties, changes in intermediate products like raw meal and clinker and changes in plant. Importantly, there is no single, universal set of cement properties that suits all markets at all times. ere is a multitude of variables that continually require change and process and product optimisation is an ongoing activity in cement manufacture. Further, certain properties that are beneficial in some respects are detrimental in others. For example, increasing cement fineness will generally increase strength performance at all ages, but may negatively impact key properties like mortar shrinkage and concrete handling characteristics. At all times, a cement manufacturer is involved in optimising their product to yield one that meets market needs -- not just maximising a single aspect of performance. In the situation where higher levels of mineral addition are used, product optimisation is required to obtain the best results, and a cement manufacturer has considerable "room to move" to amend the properties of cement to achieve the needs of the market and, as is required in this situation, to match the performance of products with lower levels of mineral addition, or with no mineral addition at all. 2.2 Cement hydration Implicit in the concern about mineral additions "diluting" cement is the assumption that cement in concrete is fully hydrated in all cases and therefore more mineral addition must reduce strength performance. It is clear from the work of Bentz et al (Bentz et al, 2009a; Bentz et al, 2009b) and others that for high performance concrete in particular, where low water:binder ratios are used, full hydration never occurs. In these cases, the "dilution" of cement particles by mineral addition allows the cement present to more fully hydrate with a resultant higher performance per unit weight of actual cement. In addition, it is claimed ( omas et al, 2010; Hooton et al, 2007; Tsivilis et al, 2002) that the fine limestone particulates act to provide nucleation sites that accelerate the hydration process, again assisting the efficiency of the cement hydration reaction. ese mechanisms which counter the higher mineral addition levels mean a lower actual "cement" content and hence, an effectively higher water:cement ratio. at the proposed mechanisms leading to higher cement efficiency when mineral addition materials are used have real outcomes in terms of concrete performance is illustrated by good strength performance and improved concrete porosity and lower permeability -- at least for limestone levels of up to 15%, based on a variety of work reported by Ranc et al (1991). 2.3 Limestone reactivity Various authors report evidence of the participation of limestone, albeit in a fairly minor way, in hydration reactions and in the formation of cementitious products -- generally considered to be calcium carbo-aluminates. ere is some debate about the level of contribution of these products to concrete performance, though some authors ( omas et al, 2010; Ranc et al, 1991; Hooten et al, 2001) claim it is significant. Importantly, it appears that these reactions are more likely in the presence of higher levels of aluminate species such as is the case where either or both fly ash and slag are used as supplementary cementitious materials. 3.0 PRACTICAL ASPECTS IN THE USE OF LIMESTONE MINERAL ADDITION Almost all of the cement commercially produced in Australia in recent years has contained mineral addition, generally limestone. e use (or potential use) of 7.5% limestone mineral addition represents an incremental change -- not a paradigm shift in composition. Nevertheless, there are some practical considerations required to ensure that the aim of producing a product of equivalent performance to cements containing 0-5% limestone is achieved. ese considerations are the same as those that would be made with any process or compositional change. Specifically, in order to achieve equivalent performance in terms of strength, shrinkage and setting times, consideration needs to be given to several key properties including: (a) particle size distribution, (b) limestone purity, and (c) optimum SO3 level. 3.1 Particle size distribution e particle size distribution (psd) of cement is an important criterion in relation to performance, driven primarily as a result of its effect on the rate of cement hydration. Basic measures of psd used in cement manufacture are Fineness (Index) expressed as a surface area (m2/kg); Residue (% retained on a 45 micron sieve) and 3-32 micron size fraction. Inter-grinding extra limestone mineral addition with cement generally results in the softer limestone being concentrated in the finer fractions of the cement product (Tennis et al), and in order to counter this, the overall Fineness of the cement generally needs to be increased to maintain equivalent strength performance. With the minor change from 5% to 7.5% limestone, recent experience has shown that an approximate increase in Fineness (Index) of about 20 m2/kg is required to achieve equivalent strength performance, this equating to about 0.5% decrease in the Residue value. e impact on the 3-32 micron fraction is minimal, probably reflecting the fact that the extra limestone reports to the sub-3 micron fraction, and the slightly higher Fineness results in a minor increase in the 3-32 micron fraction. ese slight changes need to be considered in relation to the level of variability in "Fineness" over time for any cement milled to a Fineness (Index) value of 350 m2/kg to 400 m2/kg. e range of "Fineness" over (say) a month s production for such a product would be expected to be about ± 20 m2/kg around the mean Fineness (Index) value. 3.2 Limestone purity Limestone can differ in purity due to variations in the deposit. Most cement plants have stockpiles of "high-grade" limestone on site to amend raw meal chemistry as part of their normal operation. e purity of this high-grade limestone can be >95% calcium carbonate. Within AS 3972, a limit of >80% calcium carbonate is set for the purity of the limestone to be used as a mineral addition without the need for testing