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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 19 significant improvement in the CO2 intensity of the product. One of the main concerns with the higher level of mineral addition has been the view that the active cement material has been "diluted" and therefore the cement containing mineral addition must perform at a lower level. is proposal has a number of embedded assumptions about how cement performs, and also suggests that there is a unique set of properties which guarantee the highest level of performance. e reality is that the physical and chemical nature of a cement requires various compromises to ensure that the product meets all of the requirements of the marketplace, and consequently there is no unique set of properties that provides a "perfect" product. It is within the boundaries of these physical and chemical limits that a cement can be optimised to allow the use of higher levels of mineral addition and still maintain parity performance in concrete. ere is of course an upper limit to how much mineral addition can be used before product quality is compromised, and this limit appears to be in the order of 15% -- at least with limestone mineral addition. While the benefits and potential effects of higher levels of mineral addition were debated at length during the development of the 2010 version of AS 3972, it is now possible to examine the effects of 7.5% limestone mineral addition in practice. ese early results are encouraging. ere is also an industry working group carrying out a large testing program, involving both laboratory and field work, to try to determine the practical upper limit for limestone mineral addition. Once this upper limit is known, AS 3972 will be amended (if necessary) to increase the allowable level of mineral addition to the "upper limit" value. is will then provide a cement that performs at the same level as cement containing 0-5% mineral addition, while minimising the CO2 footprint from the manufacture and use of our general purpose cements. 6.0 CONCLUSIONS is review of the principles and practices in the use of higher levels of limestone mineral addition in general purpose Portland cements has shown that despite the fairly extensive use of limestone as a mineral addition in cement in various jurisdictions around the world, there are still concerns being raised about the potential for the mineral addition to "dilute" the cement and lower its performance. e concerns about "dilution" require some assumptions to be made about the nature of commercial cements, how they react in concrete, and also about the reactivity of the mineral addition material, and particularly limestone mineral addition. It can be shown that there is no unique set of cement properties that guarantees the highest level of performance in all aspects, and that during manufacture, an optimum set of properties is sought that yields a cement that fits the needs of a particular market. ere are numbers of ways that cement performance can be optimised, and the use of higher levels of mineral addition does not compromise the ability to manufacture a cement that performs as well as one with lower levels of mineral addition, or no mineral addition -- at least for mineral addition levels up to about 15%. e cement hydration reaction does not always go to completion, particularly with low water:binder ratios, and the use of mineral addition can assist in improving concrete performance as measured in a variety of ways and as reported regularly in the literature. Limestone is also reported to undergo some direct reactions leading to a cementitious product of the calcium carbo- aluminate type. In the practical application of limestone mineral addition, several key properties are optimised to provide the required level of performance. ese include the psd and the SO3 level (including the DGD). In addition, the purity of the limestone is a key element in ensuring that the required level of performance of the cement is obtained. Data is presented showing early results after the implementation of a limestone mineral addition level of 7.5%. Cement test results are compared with those obtained from the previous cement containing 5% mineral addition at the same plant. e results are very encouraging, and suggest that once the 7.5% mineral addition level becomes the norm, the performance of these products will be at least equivalent to that from cements with lower levels of mineral addition. REFERENCES ASTM C150/C150M, 2009, Standard Specification for Portland Cement. Bentz, DP. et al, 2009a, "Limestone Fillers Conserve Cement, Part 1", Concrete International, p.41. Bentz, DP. et al, 2009b, "Limestone Fillers Conserve Cement, Part 2", Concrete International, p. 35. EN 197-1, 2011. Cement -- Part 1: Composition, specifications and conformity criteria for common cements. Hooton, RD. et al, 2007, "Portland-Limestone Cement: State-of-the-Art Report and Gap Analysis for CSA A 3000", University of Toronto, Cement Association of Canada, SN3053. NZS 3122, 2011. Specification for Portland and blended cements. Ranc, R. et al, 1991. "Durability of Cements with Fillers", Durability of Concrete Vol. II, Ed. V.M. Malhotra, Second International Conference, Montreal, Canada. Tennis, PD., omas, MDA. and Weiss, W.J. 2011, "State-of- the-Art Report on Use of Limestone in cements at Levels of up to 15%", PCA R&D Serial No. SN3148, Portland Cement Association. omas MDA. et al, 2010, "Field Trials of Concretes produced with Portland Limestone Cement", Concrete International, p.35. Tsivilis, S. et al, 2002, "An analysis of the properties of Portland limestone cements and concrete", Cement and Concrete Composites, 24, pp. 371-378.