Compressive Strength of Chemically and Mechanically Activated Aluminosilicate systems.

Grizelda du Toit1, 2, Elizabet M. van der Merwe2, Elsabé P. Kearsley3, Mike McDonald1 and Richard A. Kruger4 1Afrisam SA, C/o Main Reef & Elias Motsoaledi Roads, Roodepoort, Johannesburg, 1724; Departments of 2Chemistry and 3Civil Engineering, University of Pretoria, Lynnwood Road, Pretoria, 0002; 4Richonne Consulting, PO Box 742, Somerset Mall, 7137, South Africa CONFERENCE: 2015 World of Coal Ash – (www.worldofcoalash.org) KEYWORDS: fly ash, hybrid cement, chemical activation, mechanical activation ABSTRACT The aim of this study is to evaluate the reaction products, performance and suitability of activated, high fly ash containing cement blends in an effort to reduce CO2 emissions by reducing clinker factors; and to optimally utilize South African fly ash in blended cements. Due to the high energy demand and the emission of greenhouse gasses during clinker production, it is common practice to utilize appropriate supplementary cementitious materials (SCMs) to offset environmental impact. These materials are usually industrial by-products that must otherwise be stockpiled or disposed. The purpose of this research is to produce hybrid-alkali activated aluminosilicate systems containing up to 70% fly ash, by means of both mechanical and chemical activation of siliceous coal fly ash. Three siliceous fly ashes produced from coal combustion at the same power station, differentiated by fineness and/or particle shape (due to mechanical activation), have been chemically activated by addition of varying amounts of alkali activator. These specimens were subjected to physical testing, including both mortar and concrete compressive strengths at different curing ages. Combined activation of fly ash indicated significant improvements in hybrid cements with regards to compressive strength in mortar and concrete specimens, especially in early age strength development.

2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN - May 5-7, 2015http://www.flyash.info/

INTRODUCTION Blended cements and concrete containing high volumes of SCMs, have become a pertinent research topic in recent years. The reason for the increased interest in gaining more knowledge and a better understanding of this subject matter is driven by the cement industries’ need to produce more environmentally friendly products. It is well known that clinkering, a key step in cement production is energy intensive (kiln temperatures of up to 1450 °C), and produces large amounts of greenhouse gasses which are emitted into the atmosphere. The production of blended cements provide an alternative option of improving the environmental impact, by making use of suitable by-products like coal fly ash that would otherwise be disposed or stockpiled1-3. The particular SCMs under consideration in this specific study, is siliceous coal fly ash from a coal-fired power station in South Africa, Due to the country’s significant coal reserves the generation of electricity (and fly ash), is dominated by coal-fired power stations. Eskom, one of the top utilities in the world, is the state-owned power utility and supplies approximately 95% of the electricity consumed in the country. In 2011, Eskom used 125 million tons of coal and produced 36.2 million tons of ash from their coal-fired stations. Currently, two new supercritical coal-fired 6 x 794 MW (gross) power stations are being constructed, and are the largest ever ordered by Eskom. Once these utilities are completed, Eskom will generate an estimated 45 million tons of coal ash per year. Blended cements The production of blended cement is the predominant application of fly ash in the South African building and construction industry, with 72% of the SCM volume being fly ash. The domestic market for coal ash for non-cementitious applications is very small4. The current South African National Standard (SANS 50197-1:2013) specification for the composition and conformity criteria of common cements, was adopted from the European Standard (EN 197-1:2011). It lists 27 common cements, of which eight allow for the addition of siliceous fly ash, and two for calcareous fly ash (not available in South Africa). The maximum level of replacement of clinker with fly ash is for CEM IV/B cement, which allows up to 55% fly ash5. This study aims to go beyond the specified 55% level of clinker replacement, and investigates the activation and performance of blended cements containing substantially higher amounts (up to 70%) of fly ash. These formulations are also known as hybrid cements6. An understanding of the activation mechanism and physical performance of such activated hybrid aluminosilicate systems, as well as characterization of the reaction products, will provide much needed and valuable information towards the production of low carbon footprint cementitious products.

When used in blended cement systems, the effect of the pozzolanic reaction between fly ash and portlandite is delayed. The end result being that the early strength gain is compromised and the influence of the pozzolanic reaction is only observed at later ages of curing. Cement activation As is demonstrated in this investigation the strength development characteristics are significantly improved by subjecting the fly ash to a combination of mechanical (milling of the ash) and chemical (Na2SO4) activation. These hybrid cements were water cured at room temperature. The early age hydration of hybrid cement activated by means of Na2SO4 as an alkali activator has been studied before7,8. Depending on reaction conditions and composition of the raw material used these authors postulate that the following principal reactions may occur during early hydration:

Na2SO4 + Ca(OH)2 → 2 NaOH + CaSO4.xH2O (1) CaSO4.xH2O + C3A → ettringite (Aft) (2)

The presence of SO4

2- enhances the initial dissolution of alite as well as the formation of ettringite, resulting in a denser matrix and an increase in early compressive strength7. There is however uncertainty regarding the structure and composition of the main reaction products of these hybrid cements. It is generally accepted that the main reaction product is a tetrahedrally coordinated Na2O.Al2O3.SiO2.nH20 gel (N-A-S-H) or a (N,C)-A-S-H-type gel, where the calcium (C) content is determined by the local availability of calcium. It is the cross-linked nature of the N-A-S-H-type gel which is believed to be responsible for the increase in early strength1,7,9. It has been reported that mechanical activation of fly ash leads to increased reactivity, especially when the median particle size (d50) is reduced to less than 5-7 µm; the critical particle size for silicates below which mechanical activation begins to manifest itself10-12. Jueshi et al13 proved that the combination of grinding and the addition of Na2SO4

produced higher compressive strength compared to any single method of activation for lime-fly ash systems. MATERIALS AND METHODS Materials Three different fly ash products were used in this study. These are:

an unclassified ash (d50 ~ 50-60 µm), a very fine classified ash (d50 < 5 µm), and a mechanically activated (milled) residue (d50 < 5-7 µm) of the unclassified

ash.

The cement used contained 8% limestone and had a density of 3.14 g/cm3. The chemical and mineralogical composition of the fly ash samples and cement are presented in Tables 1 and 2. From the compositional analysis, it is evident that all three fly ash samples contain a low amount of volatiles, which is evident from the low loss on ignition (LOI) values. The South African National Standard on fly ash (SANS 50450-1:2014), adopted from EN 450-1:2012, specifies LOI not greater than 5% by mass. High loss on ignition values due to unburnt carbon, may lead to numerous issues in concrete mixes for example higher water – and activator demands. Table 1. Chemical composition of raw materials (wt. %).

Unclassified

fly ash Classified

fly ash Mechanically activated

unclassified fly ash Cement

LOI 0.85 1.23 1.52 3.72 SiO2 54.06 55.21 54.54 20.22 Al2O3 34.78 31.15 30.38 4.54 CaO 4.58 4.86 6.10 65.14 Fe2O3 3.09 3.61 3.83 2.54 MgO 1.27 1.14 1.20 1.71 K2O 0.73 0.62 0.63 0.47 Na2O 0.23 0.17 0.17 0.11 TiO 1.76 1.57 1.54 0.42 Mn2O3 0.03 0.03 0.03 0.14 P2O5 1.01 0.57 0.55 0.09 SO3 0.40 0.28 0.31 2.73 Total 102.79 100.44 100.80 101.83

Table 2. Mineralogical composition of raw materials (wt.% normalized).

Unclassified

Fly Ash Classified Fly Ash

Mechanically activated unclassified Fly Ash

Cement

Anhydrite - - - 2.26 Belite - - 1.04 7.51 Alite - - 1.41 48.00 Brownmillerite - - - 14.26 Calcite - - - 7.47 Gypsum - - - 0.92 Hematite 1.35 0.30 0.81 - Mullite 36.09 26.32 29.02 - Quartz 14.84 3.93 11.49 1.21 Amorphous 47.73 69.45 56.24 18.38 The fly ash samples consist predominantly of an amorphous alumina silica glass phase, which is higher in classified ash than in unclassified ash. The two major crystalline

phases are mullite, the most abundant, and a lesser amount of quartz. After milling the unclassified ash both the mullite and the quartz decreased with a concomitant increase in the amorphous phase. It is believed that the small amount of belite and alite evident in the mechanically activated fly ash was due to contamination during the milling process. Commercially available Na2SO4 powder (99% purity) was used in all the chemically activated mixes, and was added in powder form directly to each mix. The Na2SO4

content is reported as a percentage of the cementitious mass. Analytical Methods The oxide composition (except SO3) of the raw materials was determined by fused bead analysis on XRF equipment (PANalytical Axios Cement). In order to produce the fused bead, 1 g of sample was mixed with 5g of fluxing agent (100% Li2B407) and fused at 1050 °C. The fine powdered samples were heated to 1050 °C and the mass loss used to determine loss on ignition (LOI). Following the de-carbonation, all samples were manually pulverized. The SO3 content was determined by combustion in a LECO S-144DR instrument. XRD measurements were carried out by using a PANalytical X’Pert Pro powder Diffractometer an X’Celerator detector and variable divergence- and fixed receiving slits, with Fe filtered Co-Kα radiation (λ=1.789Å). The phases were identified using X’Pert Highscore plus software. After addition of 20% Si (Aldrich 99% pure) and milling in a McCrone micronizing mill, the samples were prepared for XRD analysis using back loading preparation method. The relative phase amounts (weight %) were estimated using the Rietveld method (Autoquan Program). The respective particle size distribution analyses were determined utilizing a Mastersizer 2000 laser particle size analyzer fitted with a Scirocco 2000 sample handling unit from Malvern. Scattered light data were recorded for 25 seconds and 25 000 measurement snaps. Size data collection was performed within the mandatory obscuration in the range of 10-20%. To study morphology of the fly ash samples, images were obtained with a Zeiss Ultra SS (Germany) field emission scanning electron microscope (FESEM), operated at an acceleration voltage of 1 kV under high-vacuum conditions. The FA powder was mounted on double-sided carbon tape by dipping carbon stubs into the samples. Excess material was removed by gentle blowing with compressed nitrogen. Heat of hydration was measured using a TAM Air microcalorimeter from TA Instruments, at 25 °C and 0.5 water:binder ratio.

Blends for all of the physical test work (mortar and concrete) consisted of 70% of the relevant fly ash product and 30% Portland cement. The blends were chemically activated by adding 1%, 3% and 5% Na2SO4 respectively. Mechanical activation of unclassified fly ash was carried out in a laboratory ball mill (Lab 43) in 20 kg batches, until a d50 < 5 µm was achieved (4 hours). The stainless steel balls used as the milling media consisted of approximately 50% of 15 mm and 40 mm each. The material to media ratio of 1:9.9 was maintained throughout. Preparation, mixing and compressive strength testing of all mortar blends were done as per SANS 50196-1: Methods of testing cement Part 1: Determination of strength. Setting time was determined according to Part 3: Determination of setting times and soundness. Compressive strength testing of all concrete blends were done as per SANS 5863:2006 Compressive strength of hardened concrete. Consistency was determined according to SANS 5862-1:2006 Consistence of freshly mixed concrete – slump test. All the concrete mixes were prepared at a 0.67 water:binder ratio, and contained 90 kg/m3 cement and 210 kg/m3 fly ash product. Sets of two 100 mm cubes were crushed for compressive strength purposes. RESULTS AND DISCUSSION Effect of mechanical activation on particle size distribution (psd) and morphology of coal fly ash Even though the chemical and mineralogical composition of the unclassified ash did not change drastically after mechanical activation, significant changes were observed regarding the physical nature of the ash. It is clearly evident from Table 3 and Fig. 1, that mechanical activation reduces the particle size. One can also see the similarities in mean particle size for classified and mechanically activated ash at both d10 and the median size d50, however, as can be observed from the d90 value, the milled ash has more coarse particles and therefore a broader particle size distribution than the classified ash. Table 3. Particle size analysis indicators for the three fly ashes (µm).

Unclassified

fly ash Classified

fly ash Mechanically activated

unclassified fly ash d10 4.44 0.53 0.55 d50 52.46 3.07 4.79 d90 224.22 6.67 19.01

Figure 1. Particle size distribution for the three fly ashes. The differences in the morphology of the three fly ash products are also clearly evident from the scanning electron microscopy (SEM) images portrayed in Fig. 2.

Figure 2. Scanning electron micrographs for the 3 fly ash products used. From left to right: unclassified fly ash, fine classified fly ash, and milled unclassified fly ash. Setting time and heat of hydration Table 4 lists the setting time of the three different fly ashes at the corresponding sulfate additions. The setting time reduces with increasing sulfate addition for all three chemical activated fly ash blends. It is also worth noting that the results for the mortars containing unclassified ash shows a trend of shortened setting time after being subjected to mechanical activation. The classified ash blends have significantly prolonged setting times compared to the corresponding milled and unclassified blends.

Table 4. Mortar setting times (minutes).

0% Na2SO4 1% Na2SO4 3% Na2SO4 5% Na2SO4

Unclassified fly ash blend 386 351 276 260

Classified fly ash blend 712 612 526 406

Milled unclassified fly ash blend 351 316 218 230

The cumulative heat (J/g) versus time, and heat flow (J/g.h) versus time profiles for the respective samples, taken for the initial 48 hours (early age hydration), as well as at 23 days are shown in Figures 3-4. The maximum heat flow and total heat values are presented in Table 5. For all the graphs, the following abbreviations are used: sodium sulfate (SSF), unclassified fly ash (U-FA), classified fly ash (C-FA) and milled unclassified fly ash (MU-FA).

Figure 3. Heat flow rates, 48 hours and 23 days.

Figure 4. Cumulative heat, 48 hours and 23 days.

Table 5. Calorimetric data.

Maximum heat flow (J/g.h) Cumulative heat (J/g) 0% Na2SO4 5% Na2SO4 0% Na2SO4 5% Na2SO4

Flow Hours Flow Hours 48 hrs 23 days 48 hrs 23 days Unclassified fly ash blend

3.53 17.14 4.42 12.27 85.78 178.45 103.80 217.25

Classified fly ash blend

3.56 28.00 4.17 14.19 81.13 172.59 113.18 199.59

Milled unclassified fly ash blend

2.97 17.21 5.97 9.79 98.22 218.56 158.42 242.89

It is clearly evident from Fig. 3 that the addition of sulfates to the hybrid cement blends increases both the rate of hydration, as well as the peak heat evolution intensities for all of the represented scenarios. When they investigated the effect of sulfate addition on hybrid cements, Donatello et al7 concluded from similar calorimetric data that in the presence of SO4

2-, the initial dissolution of alite is enhanced, setting times shortened and early compressive strength increased. The cumulative heat graph (Fig. 4) shows that combined activation not only increases the rate of hydration, but also continues to have the highest heat output up to 23 days. This result agrees with combined activation blends also producing the highest mortar compressive strength, especially at early curing ages. Compressive strength of mortar bars With increasing sulfate addition, mortar compressive strength for the unclassified ash blends (Fig. 5) indicates an increase in strength for all curing ages up to 90 days. However, at 180 days of curing, the strength decreases with increasing sulfate addition. At 3% sulfate addition, there is a 3.2% drop in strength between 90 days and 180 days, and a 16.8% drop at 5% sulfate addition for the same two curing ages. The classified fly ash blends (Fig. 6) also show that, up to 90 days, increasing the level of sulfate increases the compressive strength. In contrast to the unclassified ash, the strength at 180 days is not negatively affected with increasing sulfate addition. However, at 3% sulfate addition, there is a 3.6% drop in strength evident from 90 days to 180 days.

Figure 5. Mortar compressive strength for unclassified fly ash blends.

Figure 6. Mortar compressive strength for classified fly ash blends. The blends consisting of both mechanically and chemically activated fly ash (Fig. 7), followed a similar strength trend as the normal unclassified fly ash blends up to 28 days. Regardless of the amount of Na2SO4 added, the combination of chemical activation and milling was the most effective approach and enhanced mortar strengths up to 90 days.

This finding also correlates with work done by Jueshi et al13 on lime-fly ash blends up to 28 days of curing, where the combination of grinding and addition of sulfate gave higher strength than any single activation method investigated. After 90 days, mortar strength appears to reach a plateau when the amount of Na2SO4 exceeds 1%.

Figure 7. Mortar compressive strength for milled unclassified fly ash blends Compressive strength concrete As expected from the different morphologies for the three different fly ashes, the addition of mechanically activated unclassified fly ash with its irregular particle shape, substantially reduced the workability of the concrete (Table 6). Mixes changed from being very flowable for unclassified ash and classified ash, to less workable for the milled ash blends. Table 6. Concrete slump retentions (mm).

0% Na2SO4 1% Na2SO4 3% Na2SO4 5% Na2SO4

Unclassified fly ash blend 150 155 160 170

Classified fly ash blend 160 170 175 170

Milled unclassified fly ash blend 70 70 75 75

Fig. 8 to 10 portray the concrete compressive strength results for the respective curing ages. It can be seen that the same trend is evident for concrete as for mortar. Irrespective of the amount of chemical activator added or fineness of fly ash used, the

samples always indicate an increase in concrete compressive strength, with an increase in chemical activation up to 28 days of curing when compared to the respective control samples. It is only when samples reach the 90 day curing mark and onwards, that results become erratic and deviate from the above mentioned trend for both of the unclassified ash blends. However, unlike the findings for the mortar testing, the fine classified fly ash samples still produce definite increases in compressive strength with an increase in chemical activation up to 180 days of curing. Regarding the chemical activation of unclassified fly ash (Fig. 8), little activation takes place at 24 hours for different sulfate additions when compared to the control sample.

Figure 8. Concrete compressive strength for unclassified fly ash blends. If the fine classified fly ash is considered (Fig. 9), the activation effect is evident for all five curing ages presented on the graph. It is possible that the consistent enhancement in strength may be due to the very fine nature of the classified fly ash, which can result in enhanced “filler effect” in the concrete. The filler effect of the fine spherical particles can result in improved particle packing and workability, as well as the provision of additional nucleation sites on the surface of the fly ash for the cement hydrates (seeding effect), and the increase of effective water-to-cement ratio (w/c), when the water-to-solid ratio is kept constant whereby the hydration of cement is promoted14.

Figure 9. Concrete compressive strength for classified fly ash blends. The concrete strength for mechanically and chemically activated unclassified fly ash blends are presented in Fig. 10. Considering the previous two fly ash blends discussed which included only chemical activation, one can clearly see the benefit, especially at early strength ages, which the combined activation of the ash provides.

Figure 10. Concrete compressive strength for milled unclassified fly ash blends.

SUMMARY AND CONCLUSIONS The findings of the investigation on the use of combined activation techniques for hybrid cements, and their effect on compressive strength, can be summarized as follows:

- Increased sulfate addition, as well as mechanical activation, reduces setting time of mortar blends, which is due to increased hydration and reaction rates, especially at early curing ages. This finding correlates well with the faster heat flow rates and higher cumulative heat from the calorimetric data, as well as increased early compressive strength.

- Combined activation of hybrid cement produced the most enhanced compressive

strength in mortar up to 90 days of curing, and up to 28 days of curing for concrete mixes. This activation method is especially effective for early ages (1 day to 28 days) of both mortar and concrete compressive strength.

- Mechanical activation of the unclassified fly ash significantly reduces the

workability of concrete mixes.

- Enhanced filler effect from chemically activated, classified fly ash results in continuous concrete compressive strength gain over all five curing ages presented.

RECOMMENDATIONS These results form part of an on-going PhD study, and the following recommendations will still be included as part of future investigations:

- A repeatability study on mortar and compressive strength gain for combined activation of hybrid cement systems.

- Reporting on specimens cured for 365 days to assist with the determination of

optimum activation conditions.

- Complete durability studies on the concrete mixes and include results for 365 days of curing.

- Investigate the reason for the plateau/drop in strength at later curing ages.

- In depth study on the characterization of the reaction products formed during combined activation of hybrid cements.

ACKNOWLEDGEMENTS The author acknowledges the financial aid and study opportunity from AfriSam (South Africa) (Pty) Ltd., and also for making their laboratory resources and equipment available. Ms Wiebke Grote (University of Pretoria) is acknowledged for performing the XRD analyses, and the University of Pretoria Laboratory for Microscopy and Microanalysis for assistance with SEM. REFERENCES [1] Férnández-Jiménez, A., Sobrados, I., and Palomo, A. Hybrid cements with very low OPC content. International Congress on the Chemistry of Cement ICCC XIII, Madrid, 2011, p. 142. [2] Shi, C., Férnández-Jiménez, A., and Palomo, A. New cements for the 21st century: The pursuit of an alternative to Portland cement, Cement and Concrete Research, 2011, 41, pp. 750-763. [3] Lothenbach, B., Scrivener, K., and Hooten, R.D. Supplementary cementitious materials, Cement and Concrete Research, 2011, 41, pp. 1244-1256. [4] Kruger, R.A. Coal ash in South Africa: Production, Utilization and Research. South African Coal Ash Association, 2013, pp. 1-17. [5] South African National Standard, SANS 50197-1, SABS standards devision, 2013, p. 15. [6] Garcia-Lodeiro, I., Férnández-Jiménez, A., and Palomo, A. Variation in hybrid cements over time. Alkaline activation of fly ash-portland cement blends, Cement and Concrete Research, 2013, 52, pp. 112-122. [7] Donatello, S., Férnández-Jiménez, A., and Palomo, A. Very high volume fly ash cements. Early hydration study using Na2SO4 as an activator, J. Am. Ceram.Soc, 2013, 96, pp. 900-906. [8] Pacheco-Torgal, F., Labrincha, J.A., Leonello, C., Palomo, A., and Chindaprasirt, P. (Eds.) Handbook of alkali-activated cements, mortars and concretes, Woodmead publishing, 2015, pp. 35-42. [9] Palomo, A., Alanso, S., Férnández-Jiménez, A., Sobrados, I., and Sanz, J. Alkali activation of fly ashes. A NMR study of the reaction products, J. Am. Ceramic. Soc., 87, pp. 1141-1145.

[10] Kumar, S., and Kumar, R. Mechanical activation of fly ash: Effect on reaction, structure and properties of resulting geopolymer. Ceramics International, 2011, 37, pp. 533-541. [11] Temuujin, J., Williams, R.P., and Van Riessen, A. Effect of mechanical activation of fly ash on the properties of geopolymer cured at ambient temperature. Journal of Materials Processing Technology. 2009, 209, pp. 5275-5280. [12] Balaz, P. Mechanochemistry, Nanoscience and Minerals Engineering, Springer Publication, XIV, 2008. [13] Jueshi, Q., Caijun, S., and Zhi, W. Activation of blended cements containing fly ash, Cement and Concrete Research, 2001, 31, pp. 1121-1127. [14] Deschner, F., Winnefeld, F., Lothcenbach, B., Seufert, S., Schwesig, P., Dittrich, S., Goetz-Neunhoeffer, F., and Neubauer, J. Hydration of Portland cement with high replacement of siliceous fly ash, Cement and Concrete Research, 2012, 42, pp. 1389-1400.