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Concrete In Australia : March 2012
18 Concrete in Australia Vol 38 No 1 for clay content or Total Organic Carbon -- this limit being consistent with the requirements given in EN 197 (2011). e impact of changes in limestone purity is more a function of the "impurities" than of the limestone material itself. Potential impurities include clays which, for both chemical and physical reasons, may cause problems related largely to increased water demand and its consequent effects. As is the case with any additive, adherence to requirements of the Standard limits any detrimental effects. It is also prudent to carry out trials with the proposed material using a known control to ensure that any changes due to the use of a new material can be both understood and quantified. 3.3 SO3 optimisation SO3 optimisation of cement is carried out routinely from time to time to ensure that a balance of performance between strength, mortar shrinkage and setting time is being optimally achieved. While no significant changes are expected due to the presence of a slightly higher proportion of limestone mineral addition, the potential changes to psd, and consequently to the rate of cement hydration, would generally provide sufficient reason to reassess the optimum SO3 level. However, it is not simply the SO3 content that is at issue, but at least as significantly, the degree of gypsum dehydration (DGD) is the critical property. It is therefore necessary to ensure that a constant DGD is maintained during the SO3 optimisation test program. In terms of the potential impact on strength, mortar shrinkage and setting times, the DGD and SO3 contents are likely to be much more significant than the relatively minor change in allowable mineral addition content that has been recently approved. 4.0 PRACTICAL OUTCOMES While the experience in Australia with 7.5% limestone mineral addition is quite limited at this time, it is worth reflecting on the outcome for one plant that now has about three month s experience with 7.5% limestone mineral addition. Comparative performance for both cement and concrete test results for two products, a Type GP and a Type SL, are shown above. 4.1 Cement performance ese results show some of the expected behaviour with a slightly higher level of mineral addition, in particular: • higher level of fineness (and lower residue) with the higher limestone Type GP product • the 3-32 micron fraction is not significantly changed • slightly lower 28-day mortar strength reflecting the "dilution" of the cement in this particular test • slightly improved concrete performance at all ages • slightly higher mortar shrinkage at 28-days. Importantly, feedback from the marketplace suggests that no change in concrete strength or setting time performance has been noted in the field when using the cement containing the higher level of mineral addition. 5.0 DISCUSSION e basic concept of using mineral additions like limestone in Portland cement is now well accepted in many parts of the world. It has been shown that cements containing limestone levels below about 15% can have similar or equivalent performance to cement containing 0-5% limestone. Acceptable performance can be achieved as well as a Tables 1(a) and 1(b). Cement performance -- 5% vs 7.5% limestone -- GP and SL. (a) Type GP: Limestone level (%) Fineness (m2/kg) Residue (%) 3-32 micron (%) Mortar strength (MPa) 3d 7d 28d 5% 368 3.1 67 38.9 51.4 63.7 7.5% 388 2.7 68 39.2 51.0 62.5 Table 2. Concrete performance -- 5% vs 7.5% limestone -- GP and SL. Limestone level (%) Fineness (m2/kg) Residue (%) 3-32 micron (%) Mortar strength (MPa) Mortar shrinkage (microstrain) 3d 7d 28d 5% 333 6.6 65 36.1 49.4 62.2 614 7.5% 337 5.8 64 35.9 48.6 60.4 622 Limestone level (%) Type GP strength (MPa) Type SL strength (MPa) 3d 7d 28d 3d 7d 28d 5% 27.4 36.1 52.6 24.9 33.7 50.2 7.5% 29.0 37.1 55.0 24.8 33.5 51.9 (b) Type SL: