One route to more sustainable concrete construction is to use a 'greener' cement. Dr Bill Price reports on a new alternative to classic Ordinary Portland Cement, which is now available in the UK.
Two years ago the new British Standard BS EN 197-1:2000 potentially opened up the UK market to a wide range of European-style cements. Portland-limestone cement (PLC), which can contain up to 35% finely ground limestone, is one of the most recent of these to become available - but there is still some confusion among specifiers and customers over its properties and application.
With the current emphasis on reducing greenhouse gas emissions, however, PLC could be a very attractive option for the environmentally aware, as its production requires significantly less energy than traditional cements.
Cutting the energy consumption of cement manufacture by substituting another material for part of the clinker content before grinding is a well established option used by cement companies all over the world - although in the UK the more normal practice has been to add the alternative material actually in the concrete mixer.
Availability is usually the key factor in the choice of alternative material. Ground granulated blastfurnace slag and pulverised fuel ash have been used for decades and have the advantage of reactivity - latent hydraulic properties - which means they form compounds during the hydration process which add strength and durability to the hardened cement matrix.
But reactivity is not the only way that an alternative to Portland cement clinker can contribute to concrete quality. As the French first demonstrated, cements which contain up to 35% ground limestone can produce concretes which are at least as strong and durable as traditional mixes, despite the undeniable fact that limestone (CaCO 3) does not react chemically during the hydration process.
Instead, some of the limestone particles act as nuclei for the cementitious reaction, promoting faster, more complete hydration. The rest act as void fillers and pore blockers, densifying and ultimately strengthening the concrete matrix.
Other proven benefits include reduced bleeding of the fresh concrete and a finer surface finish. The only restriction in current codes on the use of PLC is in aggressive sulphate ground conditions, where the limestone constituent, could be vulnerable to attack. Otherwise PLC can be treated exactly the same as a CEM I cement - the new name for OPC.
CEM Is could also contain ground limestone as one of the permitted 'minor additional constituents', defined as 'specially selected inorganic materials', up to 5% of which can be added to the clinker before grinding.
This seems to have caused some confusion in the market place, where traditionally there has been a clear division between 'pure' OPC and its blended alternatives. Now there is CEM I with up to 5% limestone, then CEM II/A-L with a limestone content of between 6% and 20%, and CEMII/B-L, which may contain up to 35%.
A further complication comes from the use of an LL suffix instead of just L. This indicates the ground limestone used is of exceptional purity. The two grades of PLC currently available in the UK, in both bags and bulk, are CEM II/A-L and CEM II/A-LL which comply with the requirements of strength class 42.5.
On site, PLC concretes behave normally. Setting time is much the same as with CEM I cements, so is colour. The same admixtures may be used, although dosages may vary. Users, therefore, can benefit from a high quality concrete, with the added bonus of knowing that they are doing their bit to help reduce CO 2emissions and combat global warming.
Dr Bill Price is a senior research manager with the British Cement Association.