The Effect of Biochar on the Properties of Alkali-Activated Slag Pastes

Abstract

This paper examines the influence of biochar on the properties of alkali-activated slag pastes using two activator solutions, namely NaOH and Na2CO3. The biochar demonstrated different absorption kinetics in the mixture of slag and the two activator solutions. The pastes with biochar showed a delay in the heat flow peak, compared to the pastes without biochar, but the cumulative heat release in these pastes at later hours was increased, compared to the pastes without biochar. It was found that the use of biochar reduced autogenous shrinkage in the pastes and the reduction in autogenous shrinkage was more pronounced in the alkali-activated slag with NaOH, compared to Na2CO3. The void structure of the pastes was investigated using x-ray micro-computed tomography. It was found that refined pore structure due to reduced effective solution/slag in the pastes with biochar was able to compensate for the decreasing effect of biochar voids on compressive strength. The electrical resistivity was shown to be lower in the pastes with biochar.

The Effect of Hydrogels with Different Chemical Compositions on the Behavior of Alkali-Activated Slag Pastes

“…It is seen that the pastes with hydrogels showed a relatively shorter initial setting time, but their final setting time appeared to be increased compared to that of the paste without hydrogels, except in the case of the paste with H-d, which experienced a shorter final setting time. A prior investigation also demonstrated that AAS activated with NaOH and containing biochar as an internal curing agent had a longer final setting time compared to AAS without biochar with the same water/slag [ 58 ]. A possible reason for this behavior could be the lower availability of Na + in the pastes with hydrogels, as a portion of the activator solution is taken up by the hydrogels and is not readily accessible to the slag for reaction.…”

Development of early age autogenous and thermal strains of alkali-activated slag-fly ash pastes

“…The autogenous shrinkage of AASF pastes is much higher than that of OPC pastes (Jensen and Hansen, 1999). The chemical shrinkage and finer pore structure are certainly responsible for the change in magnitude (Cartwright et al, 2014;Prabahar et al, 2021;Vafaei et al, 2020). For OPC, some swelling may be observed (as is the case for our mix) just after setting, while there is no swelling for the AASF.…”

“…The initial one corresponds to the early dissolution of the FA and BFS and occurs already during the mixing and is thus not recorded entirely (Fang et al, 2018;Prabahar et al, 2021;Vafaei et al, 2020). The second one corresponds to the acceleration of the formation of the C-N-A-S-H gel (Fang et al, 2018;Prabahar et al, 2021;Vafaei et al, 2020), the period in between is called "induction period" (Fang et al, 2018). Interestingly, the second peak occurs later (the induction period lasts longer) and is more pronounced for pastes with higher S/B and higher M s .…”

“…As explained in the introduction section, the development of the autogenous shrinkage is associated to the reaction process (formation of finer or more refined capillary pores (Prabahar et al, 2021;Vafaei et al, 2020)) and thus the heat release. In Figures 6A,B, the development of the autogenous shrinkage is plotted according to the heat release.…”

“…However, AAMs typically possess crucial shortcomings, most notably, they often exhibit large shrinkage strains which can be up to seven times larger than that of OPC (Bakharev et al, 2000). The high shrinkage may be mitigated when using biochar, superabsorbent polymers or lightweight aggregates (Ye and Radlińska, 2017;Vafaei et al, 2020;Prabahar et al, 2021). When restrained, this shrinkage causes the build-up of tensile stresses that can result in early-age cracking and thus in severely reduced mechanical and durability performance (Bendimerad et al, 2020;Serdar et al, 2020).…”