Classification and Milling Increase Fly Ash Pozzolanic Reactivity

Abstract

Upcycling and reclaiming of low quality or stored coal combustion fly ashes could enable to tap into a voluminous resource of supplementary cementitious materials (SCMs) for low-carbon blended cements. Low reactivity fly ashes are usually either too crystalline or too coarse. Beneficiation treatments for coarse fly ashes comprise size classification or milling processes to extract or produce fine size fractions of higher pozzolanic reactivity. This article compares the effect of size classification and milling treatments on the reactivity of a siliceous fly ash (FA). The intrinsic chemical reactivity is assessed using the R3 heat release test method. The results showed significant increases of 57 and 40% for fine classified and milled fly ash compared to the initial fly ash, respectively. In addition heat release and portlandite consumption were measured for blended cements with 30 wt.% Portland cement replacement by the fly ashes. Both test results are combined to calculate the degree of reaction of the fly ashes over time in blended cement. The results demonstrate a strong effect of particle size on fly ash reactivity and degree of reaction. It is shown that increasing the inherent reactivity of fly ashes is an effective way of both accelerating compressive strength gain and enhancing late age strength with fine classified fly ashes reaching equivalent strength as neat Portland cement by 28 days and attaining a strength activity index of 137% by 90 days.

Recent developments in reactivity testing of supplementary cementitious materials

“…Using heat release in the R 3 test, 40 to 57% increase in reactivity was quantified for size-classified or milled Class F fly ash. Phase composition had only a secondary effect on the reactivity [33]. However, this finding may not hold true for some Class C fly ashes.…”

“…A number of publications have used reactivity test outputs to predict the properties of cement paste, mortar, and concrete, or have discussed the potential of doing so [8][9][10][12][13][14][16][17][18][19][20][32][33][34][35]. We do not attempt to cover all publications or their results in detail, but present important findings and their implications.…”

“…Reactivity test outputs could be used as inputs in modeling; doing so through carefully validated models can allow for the prediction of concrete strength and durability properties with minimal experimental effort [45][46][47]. Physical, chemical, mechanical, and thermal processing/modification of SCMs is becoming increasingly common and reactivity tests can provide mechanistic insights into how such processing affects SCM behavior [33,[48][49][50].…”

Pilot Scale Production of Precast Concrete Elements with Wood Biomass Ash

“…Moreover, the addition of WBA resulted in increased peaks, especially in paste blends, in which WBA2 and WBA5 were used. It is known that the addition of pozzolans increases the effective ratio of water to Portland cement and provides an additional surface for nucleation and growth of hydrates, leading to an increase in the heat of hydration [ 74 , 75 ]. Compared to the other samples, WBA5 had a significant amount of pozzolanic oxides, which may have contributed to the increase in heat of hydration.…”

Rheological behaviour, setting time, compressive strength and microstructure of mortar incorporating supplementary cementitious materials and nano-silica

“…With the development of nano-materials, a noval replacement of FA, named fly ash cenosphere (FAC), might be an option, which is produced by the dusty plasma separation technology on the fly ash. 37,38 The particle of FAC is spherical much finer particle than normal FA, which is close to the size of SF. Therefore, FAC has the morphological effect, improving the workability of cement-based mixture.…”