Improving the reactivity of fly ash

As discussed above, and as with ggbs, using fly ash as a partial replacement of Portland cement leads to lower strength and slow strength gain at early ages because of the slow rate of the pozzolanic reaction between the silica in fly ash and Ca(OH)₂. It is important to find methods of improving the reactivity of fly ash as that will subsequently lead to use of higher replacement levels; this is especially important in HVFA concrete where a significant amount of the fly ash remains unreacted. Three methods are commonly used to improve the reactivity of fly ash (section 2.4), these are briefly explained below.

Mechanical treatment: Chindaprasirt et al. (2005) studied the effect of fly ash fineness on compressive strength and pore size of blended cement pastes. They utilised two types of fly ash with different size ranges, a coarse one with median particle size of 19.1µm and a finer one with median particle size of 6.4µm. They found that the cement paste, with 40% fly ash replacement level and water/binder ratio of 0.35, made of the finer fly ash had a higher 90-day compressive strength (78.5MPa) compared to that made with the coarser fly ash (61.4MPa). In the same study, it was also reported that both the pore size distribution and the average pore

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diameter of the paste decreased with increasing fly ash fineness. In another study it was reported that the rate of both the hydration reaction and pozzolanic reaction increased with increasing fly ash fineness which can explain the increase in compressive strength reported by the previous study (Chindaprasirt et al., 2007).

Similar conclusions were drawn by other studies (Bentz et al., 2011; Lawrence et al., 2005; Karim et al., 2011; Payá et al., 1995; Payá et al., 1996; Payá et al., 1997).

Chindaprasirt et al. (2007) noticed that the chloride penetration depth, by partial immersion in 3% NaCl solution, in the concrete was reduced by the use of finer fly ash. A similar trend was observed with the RCP test, with lower total charge passing for mixes with finer fly ash which could be due to the denser matrix of the paste providing improved resistance to chemical penetration.

Thermal treatment: Payá et al. (2000) looked at the strength development of ground fly ash in mortar mixes cured at different temperatures. Increasing the curing temperature led to an increase in early-age compressive strength. Mortar mixes with 30% fly ash replacement cured at 20, 40, 60 and 80°C achieved 3-day compressive strengths of 15.9, 24.5, 28.9 and 32.2 MPa respectively. Though all these strengths were lower compared to that of the reference Portland cement mix cured at those temperatures, this is indicative that increase in temperature, as a catalyst, increases the rate of both the hydration reaction and subsequently the rate of the pozzolanic reaction of fly ash which has been reported by other studies (Hanehara et al., 2001;

Narmluk & Nawa, 2011). However a different trend was observed with the 28-day strength where the strengths of the mortar mixes were 34.4, 47.3, 37.2 and 31.9 MPa respectively. Increasing the curing temperature from 20 to 40°C led to an increase in the 28-day compressive strength however further increase of curing temperature to 60 and 80°C led to a reduction in the 28-day compressive strength.

A reason for this could be that due to the increased rate of reaction, the cementitious hydrates are rapidly and less uniformly formed leading to a weaker matrix hence lower compressive strength. Also curing at such high temperatures could result in micro-cracking in the concrete matrix reducing overall strength.

Results obtained by Narmluk & Nawa (2011) show that for a paste mix with 50%

fly ash replacement (water/binder ratio 0.25) the degree of hydration reaches levels above 0.8 (at 1000 hours) when cured at 20 and 35°C whereas for the paste cured at

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a higher temperature of 50°C, the degree of hydration seems to plateau at 0.8 after 100 hours.

Chemical treatment: As with ggbs, the reaction rate of fly ash can be improved by increasing the alkalinity of the pore solution (refer section 2.4.1.5). Over the past 20 years, with the goal of achieving higher replacement levels and the introduction to HVFA concrete, interest in looking at improving fly ash reactivity through chemical activation has increased. In their study, Saraswathy et al. (2003) concluded that the use of chemically activated fly ash yielded better results compared to using mechanically and thermally activated fly ash. Various chemicals have been used as activators such as high concentrations of Ca(OH)2 (Ma et al., pozzolanic reaction resulting in higher compressive strengths at early ages. In their study Fernández-Jiménez et al. (2006) compared two concrete mixes, one made with ordinary Portland cement and one made with alkali-activated fly ash (the activator used was a mixture of 85% NaOH + 15% Na2SiO3). The mechanical strength and shrinkage properties of concrete mixes were compared and their results show that the rapid strength gain of alkali-activated fly ash concrete of about 50MPa at 1 day compared to 10MPa for the Portland cement mix. However it is worth noting that the alkali-activated fly ash mix obtained its maximum strength at 1-day and the strength rise thereafter was very slow (58MPa at 28 days) whereas the ordinary Portland cement mix had more noticeable strength gain up to 28 days (30MPa) as they gain almost 90% of their maximum strength only after 28 days.

The shrinkage properties of the alkali-activated fly ash mix was better that the reference Portland cement mix, where at 90 days, the values were 0.01 and 0.09%

respectively. As with alkali-activated slag (section 2.4.1.5), the performance of mixes made with alkali-activated fly ash depend on many factors such as the nature of the activator (Fernández-Jiménez Palomo, 2005 Komljenović et al., 2010), activator concentration (dosage) (Guo et al., 2010; de Vargas et al., 2011; Criado et al., 2007) and curing temperature (Palomo et al., 2004; Guo et al., 2010).

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In addition to the factors mentioned above, there are still concerns and problems which need to be addressed with regards to concrete made with alkali-activated cement such as larger drying shrinkage and higher carbonation rate compared to Portland cement concrete (Shi et al., 2006). Also there is little literature available about the effects of chemical admixtures on the properties of concrete made with alkali-activated cements as most chemical admixtures currently available on the market are mainly for Portland cement-based mixes and do not seem to work well with alkali-activated cement concrete (Collins & Sanjayan, 1999; Puertas et al., 2003; Shi et al., 2006; Bilim et al., 2013).