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Pozzolans and Admixtures – How Can We Use These to Our Best Advantage?

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Pozzolans and Admixtures – How Can We Use These to Our Best Advantage? POZZOLANS AND ADMIXTURES – HOW CAN WE USE THESE TO OUR BEST ADVANTAGE? A DURANT, C H BIGLEY, N B MILESTONE Callaghan Innovation (formally Industrial Research Ltd) ABSTRACT Concrete has traditionally been made using Portland cement as the cementitious binder. However, these days there is pressure to encompass a ‘green’ portfolio of building materials to obtain the ‘green star tick”. Overseas, cement blends utilising pozzolans such as blast furnace slag and pulverised fuel ash are now the norm. Their use provides benefits in strength and durability. The wide variety of Portland clinkers made previously to cope with environmental conditions have now been largely replaced with these blends. New Zealand does not have these materials available unless they are imported, so cement replacement is difficult and expensive and misuse of SCMs as occurred in Australia has not occurred. We have large deposits of natural pozzolans but their use has been limited despite work by Roy Kennerley and recently, Warren South. Some have been successfully used for particular jobs such as pumicious diatomite (Waikato dams), pumicite (Ohaaki cooling tower) and Microsilica 600 (2nd Manapouri tunnel) but there has not been a wholescale take up of new technology, even though a NZ Standard for Portland pozzolan cement has been in place since 1974. This paper describes our recent work with a fine waste pumice and Microsilica 600 (MS600) where tangible benefits can be obtained from their use. Up to 30% cement replacement can be made with pumice before the effects of the replacement are noticed with a drop off in compressive strength. A similar effect was noted with MS600. By following how cement reacts by measuring the rate of heat output with an isothermal calorimeter we find that while pumice addition reduces heat output as the pozzolanic reaction is slow, hydration is not slowed with MS600 and there is no reduction in heat. The usual explanation of the pozzolanic reaction is the siliceous additive reacts with Ca(OH)2 formed from hydrating cement, but we find increasing evidence that MS600 also reacts with the C-S-H so there is not the usual delay in strength build up. One of the issues when using a pozzolans is usually water demand. The use of a water reducer is almost essential if high replacement levels are used. However, the addition of admixtures to the two cements commonly available in New Zealand can give different results, even though the two cements behave similarly when admixtures are not used. Understanding why this occurs could help advance the use of cement replacements in New Zealand. INTRODUCTION Manufacture of Portland cement generates considerable CO2 emissions. There is now a push towards ‘green concrete’ in which these emissions can be reduced, with one of the simplest being the use of siliceous supplementary cementitious materials (SCMs). Siliceous additives to cement have been used for many years to provide a composite binding product which usually is more durable than plain Portland cement. Romans used the volcanic ash from Pozzoli mixed with slaked lime (Ca(OH)2) to form concrete but there is evidence to suggest the use of cementitious binders is much older. The pozzolanic reaction is the chemical reaction that occurs between calcium hydroxide, portlandite or (Ca(OH)2, and a reactive silica in a reaction that can be depicted in cement nomenclature as CH + [S] + H → C-S-H The amorphous calcium silicate hydrate formed is indistinguishable from C-S-H derived from calcium silicate hydration and helps fill up void space making the product stronger and less permeable. The reaction is usually slower than Portland cement hydration and requires longer moist curing to go to completion. Overseas, the pozzolans currently used have been largely based on fly-ash or blast furnace slag with silica fume also being used. None of these are readily available in New Zealand and natural pozzolanic materials have been considered. However, despite work by Kennerley and Clelland (1959), Smith (1973), and more recently South and Hinzack (2001), the natural pozzolans available in New Zealand have not been widely used. Some have been successfully used for particular projects such as the Whirinaki pumicious diatomite (Waikato dams), pumicite (Ohaaki cooling tower) and Microsilica 600 (MS600) (2nd Manapouri tunnel). Kennerley (1959) showed the Whirinaki diatomite was suitable as a pozzolan although the diatomite content decreased as the deposit was worked while Chisholm (1997) showed that MS 600 would function well as a pozzolan. However, there has not been a whole scale take up of new technology, even though a NZ Standard for Portland pozzolan cement has been in place since 1974 (NZ3123, updated in 2009). Perhaps one of the challenges in using natural pozzolans has been their uniformity, something that can be an issue as the product is usually ‘dug up’ rather than manufactured. Pozzolans are usually defined by their ‘pozzolanic activity’, a term that can be difficult to define. The ASTM tests C311 and C593 measure requirements for pozzolans but they are time consuming and certainly not suited for quality control. Milestone (1978) suggested a rapid dissolution test with NaOH which correlated well with lime pozzolan mortar strength tests for diatomaceous pumicite. Similar tests have been devised by workers such as Donatello et al. (2010), and Gava and Prudêncio, (2007). What pozzolans do we have available in New Zealand? Huntly fly ash has been available in small quantities for some time but the plant was not designed to collect ash as a pozzolan and supply has been both intermittent and variable in composition. Nevertheless, trials conducted both in New Zealand and Australia showed it was suitable provided the retardation was taken into account when using Huntly coal. Holcim have previously imported Gladstone fly ash and we have obtained samples of several Australian ashes, some ostensibly ‘better’, based on glass content (Hyroc, Mt Piper). We have some diatomite deposits such Ngakura and Waikaukau identified by South (2009), several different pumice and pumicite deposits, some of which have been used intermittently, hydrothermally altered silica (Microsilica 600) as well as pulverised recycled glass. We have examined all of these for reactivity using a variety of tests. Over the last few years we have been fortunate with several intern students working at IRL who have addressed the reactions of pozzolans, particularly as to how their reactivity can be assessed. This paper is a compilation of the work of Aurelian Boyer, Anne-Hélène Puichaud and Loriane Magneron from Ecole Nationale Supérieure de Céramique Industrielle (ENSCI), Limoges, France, and Putri Fraser from Victoria University of Wellington, all of whom have made a valuable contribution to our understanding of how pozzolans behave. While our primary aim has been to assess their use for geothermal cementing, we have examined rapid ways of assessing the reactivity of a range of potential pozzolans and studied longer term effects at moderate temperatures for some. We have also followed hydration reactions of cement using an isothermal calorimeter which measures heat output from hydrating cement against time. It is also ideal for measuring the effect admixtures have on hydrating cement. This paper presents some of our findings. EXPERIMENTAL Materials Holcim Ultracem was used as the primary cement for when cement blend mixes were made. Both Ultracem and Golden Bay GP cement have been studied in isothermal calorimetry runs. XRF analyses of the cements along with the pozzolans investigated are shown in Table 1. Gladstone, Hyrock, Chinese, and Mt Piper fly ashes, silica flour and a fine waste glass (Stevensons) were used along with samples of pumice from International Processors and Microsilica 600 from Golden Bay Cement. Ultra GBGP Pumice Hyrock MS600 Mt Glad- Chine Silica cem Piper stone se flour SiO2 21.06 23.25 67.71 66.40 87.60 70.59 53.67 49.77 99.-73 Al2O3 4.30 4.29 17.47 22.00 4.31 22.30 26.12 36.92 0.13 Fe2O3 2.10 2.12 2.27 6.03 0.59 1.65 9.76 4.36 0.02 TiO2 0.21 0.15 0.15 0.98 1.16 0.87 1.29 1.37 - MgO 0.94 0.96 0.07 0.55 <0.02 0.21 1.33 0.60 - CaO 66.38 65.58 0.65 1.03 0.32 0.32 3.39 2.40 - Na2O 0.21 0.23 2.55 0.22 0.14 0.29 0.48 0.28 - K2O 0.52 0.50 3.07 1.01 0.49 1.98 1.43 0.86 - SO3 2.62 2.37 < 0.01 0.03 0.25 0.01 0.09 0.07 - MnO 0.21 0.07 0.06 0.09 0.03 0.01 0.14 0.05 - P2O5 0.10 - < 0.01 0.16 0.05 0.09 0.81 0.29 - L.O.I 0.94 0.93 5.77 0.52 4.89 1.70 1.40 2.87 - I. R. 0.01 Methodology Methods for determining pozzolanic activity i Dissolution in 0.5 M NaOH 1g of pozzolan was boiled for 3 minutes with 0.5 M NaOH, rapidly cooled and filtered through a 0.45µm membrane filter, dried and weighed and the amount of material dissolved calculated. Investigations were also made with artificial pore water (0.5M OH-) made up with varying ratios of Na+/K+ along with the ratios in the local cements. ii. Reaction with Ca(OH)2 1.00 g of each pozzolan sample was reacted with 1g of Ca(OH)2 at 20 and 40°C in 25ml water for 7 and 28 days. Samples were filtered through a 0.45µm membrane and the Ca(OH)2 content determined by thermogravimetry.
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