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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 27 1.0 DRIVERS OF LIMESTONE ADDITION To reduce energy consumption and greenhouse gas emissions, the cement industry both nationally and internationally is actively developing cements containing increased mineral additions. e body of scientific evidence on the benefits of mineral additions is significant. Yet, general penetration of these products has often been delayed through national standards restricting their use and consumer mistrust of performance of the final product. A major factor causing delay is the need to gain sufficient field evidence on concrete performance with materials outside existing standard mix designs and acceptance of liability should trial products fail. Mineral additions commonly accepted in worldwide standards include ground blast furnace slag, fly ash, pozzolans and limestone. Limestone is generally considered to be the poorest potential performer of the available suite of mineral additions and as such considerable effort is focused on developing portland-limestone cements that achieve the current general purpose portland cement performance. Considerable success has been achieved and portland-limestone cement is increasingly specified for general application around the world. 1.1 Green Building requirements Major construction projects and changes to building codes increasingly focus on building energy efficiency and "green" building materials. One such initiative is that of GreenstarTM, a program promoted by the Green Building Council of Australia who claim four million square metres of green-star certified space in Australia (Green Building Council, 2011). e system is points based: one point is scored for a 30% reduction in Portland cement, two points for a 40% reduction in Portland cement. Use of Portland-limestone cement can assist with achieving the reductions in general purpose Portland cement required to score points for the structure. 1.2 Product sustainability drivers Manufacture of Portland clinker and cement requires the expenditure of large quantities of energy. Typically the energy for a clinker kiln is in the form of combustible fuels, in the cement grinding process this is generally as electrical energy. Reduction in Portland clinker requirements through use of limestone results in two CO2 reduction mechanisms which lead to a more CO2 efficient cement: a) less CO2 is produced through the conversion of limestone to calcined lime and b) less CO2 is produced from the burning of fossil fuels to produce the same ultimate volume of Portland-limestone cement. From a cement milling perspective, while Portland-limestone cement needs to be ground finer to achieve equivalent performance the limestone component has a lower grinding energy requirement. Using the data in Figure 1 and assuming a move from the current 5% limestone addition rate at 400 m2/kg to 10% limestone addition rate at 450 m2/kg -- the change in total grinding energy is an increase of 22%. is is likely to be an over-estimation as the limestone will grind preferentially. In a limestone only system the energy change is 7%, the real value will logically fall between the two calculated figures. Other investigators have found "the replacement of about 15% by mass of the clinker by limestone reduces total CO2 emissions on average by 12% (Schmidt et al, 2010)", after Tennis et al, 2011. From a CO2 emission perspective, Portland- limestone cement is a positive step forward for the industry and the environment. 1.3 Durability perspective Generally cements in the mid 1900s were not as fine and did not contain as high a tricalcium silicate level as today s cement. While the current drive toward finer and faster reacting cement has been a boon for the contracting industry in the area of formwork stripping times and early strengths, these benefits have not been counterbalanced by impacts on the durability of the final concrete. Wischers records that in England in the 1960s a 35 MPa concrete would be made using 350 kg of cement and a water to cement ratio of 0.45 (Wischers, 1984). Fastforward to 1985 and Elevated limestone mineral addition impacts on laboratory and field concrete performance Martyn Compton -- QA and Technical Services Manager, Cockburn Cement Jason Chandler -- Technical Supervisor, Boral Concrete As part of initiatives to reduce carbon emissions, limestone mineral addition levels were increased to 7.5% in AS3972 on 22 October 2010. As part of the standard agreement process, a working group was established to determine if there was scope for further increase in limestone mineral addition level without impacting general purpose cement performance. is paper presents data from initial production scale trials of 10% and 12.5% limestone addition cement and provides field data experience with 7.5% limestone addition cement in the Sydney market. Strength and set time performance data shows equivalent performance is attainable at 10% limestone replacement, provided appropriate fineness adjustments are made by the cement manufacturer.