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Concrete In Australia : March 2012
28 Concrete in Australia Vol 38 No 1 that same grade of concrete would now be made with 250 kg of cement (Aitcin, 2000). While structurally these products are identical from an engineering design perspective, from a microstructural and hence durability perspective they are chalk and cheese. We need to reconsider concrete not in the context of strength alone but rather view porosity and permeability as integral requirements for accurate representation of concrete performance. e improved microstructural benefits of Portland-limestone cement may well provide some durability benefits, but rather than simply increasing the fineness to achieve Portland-like performance, the beneficial wisdom of history may indicate that we need to revisit total cement contents for long term durability requirements. 2.0 INTERNATIONAL USE OF LIMESTONE: CEMENT STANDARDS While EN197-1 (2000) formalised the acceptance of limestone in European cements there has been a long history of limestone mineral addition use in Europe. Spanish standards permitted up to 10% limestone in cement as far back as 1960 and France adopted limestone use of 35% in 1979 (Moir et al, 2003). All of the cement types defined in EN197-1 may contain up to 5% limestone. Higher limestone levels of between 6% and 20% are permitted for cements with the designations CEM II/A-L and CEM II/A-LL. Over the period 1999 to 2004 use of Portland-limestone cement in Europe doubled from 15% to 30% market share (Tennis et al, 2011). A review undertaken by Pandey & Sharma (2002) of cement standards showed 39 countries permit mineral additions to cement in amounts up to 20%, with several specifically mentioning the use of limestone including China and the former USSR. New Zealand has permitted 10% limestone mineral addition since 2009 (NZS3122:2009 Amendment 1). Australian standard AS3972 has permitted 5% limestone mineral addition since the 1991 revision, and in 2010 this level was increased to 7.5%. A development program is currently under way to further increase this level provided performance hurdles from the premixed concrete, governmental and engineering communities are attained. 3.0 LIMESTONE ADDITION IMPACTS ON PERFORMANCE Limestone is sometimes claimed to be an inert filler. However, recent work has shown limestone contents up to 15% may actually increase early-age strength through a combination of: • Improved particle packing (Sprung & Siebel, 1991). • Increased rate of cement hydration (Vuk et al, 2001) (Bonavetti et al, 2000). • Production of calcium carboaluminate (Voglis et al, 2005) (after Tennis et al, 2011). 3.1 Particle size distribution and microstructure impact Perhaps counter-intuitively, adding a finely ground powder such as limestone can improve the particle size distribution on the cement and actually result in reduced water demand as overall porosity of the system is reduced. Limestone has a lower bond work index than clinker and hence requires less energy to grind to the same fineness as shown in figure 1. When limestone and clinker are inter-ground the overall particle size distribution is widened on the finer end of the particle size distribution resulting from the more easily ground limestone increase in the -20 μm particle fraction. is can lead Figure 1. Grindability of clinker and limestone modified from Opoczky, 1992. 0 20 40 60 80 100 120 140 160 0 100 200 300 400 500 600 Gr indabilit y in kJ/ kg Surface area in m 2/kg Clinker grindability in kJ/kg Limestone grindability in kJ/kg