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

cem GBGP Pumice Hyrock MS600 Mt

Piper Glad-stone

Chinese

Silica 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.

iii. Use of mortar blends. Standard mortar blends were made up with 0, 10, 20, 30, 40 and50% additions of a ground pumice. Plasticiser was needed for additions above 30% to obtain sufficient workability. These were tested for compressive strengths at 28 days.

Isothermal calorimetry A TAM III isothermal calorimeter from TA Instruments supplied by AlphaTech running at 40 °C was used. Approximately 1.6g from a 100g sample was used in the sealed glass vials and data recorded for 24 hours. RESULTS Alkali solubility Results for solubility in boiling 0.5M NaOH for 3 minutes are shown in Fig 1. The most reactive pozzolan tested is MS600 followed by Pozz52 (a ground pumice) and a Chinese fly ash. Another form of pumice, Pum 52, is also reasonably reactive. The Australian fly ashes prove to be little more reactive in this test than a fine quartz flour with Gladstone the most reactive. Milestone (1978) showed that the time of boiling needs to be optimised for different types of pozzolans so the time may not be ideal. Apart from MS600 which is very fine, the other reactive pozzolans contain significant amounts of aluminium which may be a key factor in determining a pozzolan’s reactivity. In many fly ashes the aluminium is combined as mullite which is not reactive and this would appear to be the case for the Hyroc and Mt Piper flyashes. For both the Chinese and Gladstone flyash there is more aluminium present than can be accounted for in the mullite as determined by XRD so it must be incorporated in the glass.

Figure 1: Alkali solubility with 0.5M NaOH solution boiled for 3min

Mixed alkalis The pore solution in hydrated cement is not pure NaOH but a mixture of KOH and NaOH with KOH predominating. Changing the alkaline extraction solution gives different results for the most reactive pozzolan, MS600 as shown in Figure 2.

Figure 2: Comparison of amount of MS600 dissolved in different solutions (0.5M)

It is also clear from Fig. 2 that 3 minutes of boiling is not sufficient time for MS600 to be completely dissolved as 10 minutes boiling gives a greater amount dissolved for all pozzolans tested and an optimum boiling time needs to be established. A mixture of K/Na gives different results depending on the ratios so it could be expected that different cements may have different initial reactions with the same pozzolan. The use of admixtures may change the rate at which a pozzolan reacts as many admixtures are sodium salts. Reaction with solid Ca(OH)2 As the pozzolanic reaction is usually thought of a lime/silica reaction, experiments were conducted with Ca(OH)2. Results at 20 and 40 °C are shown in Figs. 3 and 4 respectively for 1 and 4 weeks reaction. Apart from Mt Piper and Hyrock the 4 weeks samples show increased amounts of Ca(OH)2 reacted, indicating the pozzolanic reaction is not rapid and continues after a week. For both Mt Piper and Hyrok it is unclear why the results are anomalous but could be due to adsorption of Ca onto the surface of the flyash particles rather than a true pozzolanic reaction as strength measurements do not reflect this additional reaction. There is little spread compared to the results from NaOH dissolution suggesting a different reaction occurs. Normally we would not expect quartz flour to react at room temperature. At the increased temperature of 40°C the results show increased reaction and follow the strength results with MS600 being the most reactive pozzolan.

Figure 3: Reactivity with Ca(OH)2 solid at 20°C for 1 and 4 weeks

Reactions with pumice Compressive strength of paste samples with various additions of pumice are shown in Fig 5. If the strengths are normalised based on the amount of cement in the binder, we get an indication of the contribution the pumice is making to compressive strength. This is shown in Fig 6. It shows the contribution increases as time of curing increases.

Figure 4: Reactivity of pozzolans with Ca(OH)2 solid at 40°C

Figure 5: Compressive strengths of cement pastes with pumice additions

Figure 6: Contribution of pumice to compressive strengths

Examining how much reaction has occurred with the pozzolan can be estimated by determining how much Ca(OH)2 remains by thermogravimetry. Results for 28 days curing at 20°C are shown in Figure 7 with results for 40°C shown in Figure 8. The Ca(OH)2 content is shown by the size of the weight loss at 440°C peak. Ca(OH)2 remains in all samples at 20°C but has fully reacted in the 40 and 50% additions at 40°C.

Figure 7: Thermogravimetry results after 28 days in water at 20°C

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Isothermal Calorimetry When we follow the hydration of the two NZ cements by isothermal calorimetry which measures the heat output with time, we find at 40°C they appear similar in their hydration pattern (Fig 9). But introduction of either a pozzolan or an admixture can cause the hydration to behave differently in the first day of hydration (Figs 10 and 11).

Figure 9: Isothermal calorimetry of GB and Holcim cements at 40°C

Figure 10: Effect of admixtures

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Figure 11: Effect of flyash additions

DISCUSSION The pozzolanic reaction is considered to occur between the Ca(OH)2 and reactive silica. However, as Ca(OH)2 is only sparingly soluble with a saturated solution only being 0.02M in concentration the pore solution of concrete (typically pH >13 so [OH- ] is around 0.5M) is largely due to Na+ and K+ hydroxides which will influence at least the initial reaction with an active silica and this is indeed what happens. The alkali silicates formed are soluble, unlike Ca silicate (hydrate), so will not contribute to early strength until there is available calcium from the cement hydration. As we can see from Fig. 2, Na+ and K+ ions have different effects on the initial reaction of pozzolanic silicas and this is carried through to different combinations of the two elements. This means that cements available in New Zealand which have different alkali contents are likely to behave differently with any pozzolan in the initial reaction leading to early strength development, although the final strength may not be very different. Both Holcim and GB GP cements have similar alkali contents and behave similarly in isothermal calorimetry. The use of admixtures, particularly those containing sodium salts, will affect the initial reaction of a pozzolan. The different pozzolans appear to react at different rates but it is not clear which factors determine whether one is better than another. It has long been thought that it is the glass content in fly ashes that is important. However, Mt Piper ash contains considerately more glass than Gladstone, yet in all the tests we have conducted, Gladstone performs better. Thus it is likely to be the composition of the glass that determines ultimate reactivity and this may not be readily measured via a rapid test. Ultimately results from any rapid test will need to be calibrated against concrete tests that have been cured for at least 28 days. Part of the reason why the initial reaction with pumice (and it may apply to fly ash) is slow is that it contains significant amounts of Al which become incorporated in to the calcium silicate hydrate but require a counter cation, usually K+, in order for that to occur. This removes alkalis from the pore solution reducing the pH and slowing the dissolution so the reaction with Ca(OH)2 is slower and certainly flyash retards. (The retardation caused by Huntly flyash was due to boron). Addition of pumice provides an additional binder and higher strengths up to 20% addition. Even though the strengths for additions greater than 20% show a falloff in strength, there is still a significant contribution from the pozzolanic reaction as shown in Fig 6. (It should be pointed out that all samples were made with a w/b ratio of 0.55 as the higher levels were difficult to mould.) In practice a water reducer would be used or additional water added. It is

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surprising that not all the Ca(OH)2 has reacted but this is probably because overall the internal pH has dropped and reaction with the pozzolan is slow. Isothermal calorimetry gives a good way of following the hydration of both blended cements and the addition of admixtures. It quickly shows differences between different cements as well as different mineral additives. The retardation of flyashes is clearly shown in Fig 11. CONCLUSIONS Adding a pozzolan such as MS 600 or pumice to a cement blend reduces the amount of cement needed without sacrificing strength at levels up to around 20% addition. Beyond this amount the water demand requires the use of plasticisers. MS 600 reacts quickly whereas the reaction of pumice is slower and heat output is reduced. Even at high addition levels, a pozzolan contributes to strength. What determines pozzolanic activity is not well defined. Assessing pozzolanic activity of different pozzolans using a rapid test is difficult. The initial reaction is influenced by the amount and type of alkalis present. The difference in reactivity between the two cements can be pronounced if admixtures are used. REFERENCES Chisholm, DH, (1997), “Performance characteristics of concrete incorporating a natural amorphous silica”, ACI SP171 271-296 S. Donatello, M. Tyrer, C.R. Cheeseman, (2010), “Comparison of test methods to assess pozzolanic activity”, Cem. Concr. Comp. 32, 121–127 Gava, G. P. and L. R. Prudêncio, (2007), “Pozzolanic activity tests as a measure of pozzolans' performance. Part 1”, Mag. Concr. Res., 59 (10), 729 –734 Kennerley, R.A. and Clelland, J., (1959), “An investigation of New Zealand Pozzolans: Part 1. Chemical tests and tests on mortars; Part 2: Physical tests on concrete”, DSIR bulletin 133 Kennerley, R.A (1959), “Evaluation of Whirinaki Diatomite Processed for Use as Pozzolan”, Dom Lab Report 2016 Milestone, N.B., (1978) “A Rapid Method for estimating the Reactivity of Pozzolanic Materials”, CD Report 2273 Smith, LM. “Pozzolanic materials and NZS.3123”, Trans. N Z Inst. Eng., 4, (1), 37-54 (1977) W.South and I Hinzack, (2001) “New Zealand Pozzolans-An Ancient Answer to a Modern Dilemma”, ACI SP 202, 97-110, South, W., “A study of the compressive strength and drying shrinkage of cementitious composites prepared using natural pozzolans”, PhD Thesis, University of Woolongong, (2009)