Dissolution behaviour of source materials for synthesis of geopolymer binders: A kinetic approach

Highlights

• Dissolution rate of geopolymer precursors is studied using a kinetic approach.

• The existing gap in release rates at far-from-equilibrium condition is addressed.

• Release rates affect the nanostructure of geopolymers and the properties of binder.

• Dissolution results have implications for the design of geopolymeric binders.

Abstract

Controlling the initial release rate of alumina and silica from source materials is known to have a significant effect on the nanostructure of geopolymer gel and its final mechanical properties. However, most of the studies regarding the solubility of source materials take an equilibrium approach, and there is a gap in understanding of the release rates at far-from-equilibrium conditions. In the present study, the initial dissolution rate of some geopolymer precursors is characterised. The liquid to solid ratios are designed to be sufficiently high to minimise precipitation of hydration products, and the effects of solution alkalinity and milling on dissolution rates are investigated. While fly ash and blast furnace slag particles seem to release Si and Al at approximately similar rates, metakaolin shows a distinctively higher release of Si from the very early time of dissolution. Increasing solution alkalinity increases the dissolution of source materials up to some point, and the greatest effect is observed on fly ash particles. The most interesting result of milling is observed on fly ash particles where the release rate of silica has become higher than alumina, while contrasting behaviour is observed in the non-milled fly ash system. The opposite behaviour is observed in the slag system where milling rapidly increases the release rate of Al while the release rate of Si is increased slowly.

Introduction

Geopolymers are low calcium binding materials that can convert aluminosilicate wastes such as fly ash, slag and metakaolin into strong cementitious binders. These alkali-activated binders are acid-, heat- and fire- resistant (Van Jaarsveld et al., 1997, Duxson et al., 2007a, Duxson et al., 2007b, Provis and van Deventer, 2009), and they can be utilised as a green alternative to Portland cement due to the carbon dioxide savings achievable in their production process (Phair, 2006, Duxson et al., 2007a, Duxson et al., 2007b). In the geopolymerisation reaction, an aluminosilicate source is usually activated by alkali silicate solutions to produce the tetrahedral geopolymeric network (Provis and van Deventer, 2007, Provis and van Deventer, 2009). The release rate of silicate and aluminate species from source materials is critical in controlling the synthesis process of geopolymers and the development of binding gel. A high initial dissolution rate of silicate is known to accelerate the conversion of aluminosilicate materials to geopolymers (Fernández-Jiménez et al., 2006, De Silva et al., 2007), while the availability of aluminium in the source materials, and its release rate, are known to control geopolymer gel properties (Fernández-Jiménez et al., 2006, De Silva et al., 2007).

During geopolymer formation, dissolution, hydrolysis and condensation reactions take place. Depending on the concentration of Si in the system, condensation reactions can occur between aluminate and silicate species, or between silicate species themselves. While the dissolution of various compounds from aluminosilicates causes a gradual increase in Al and Si concentration in solution, this will stabilise when the Al and Si concentrations reach equilibrium with respect to the solid/gel phases present (Antonic et al., 1993). Dissolution behaviour of aluminosilicate compounds is controlled by the relationship between the forward and backward reactions leading to equilibrium. Forward reactions are related to the action of solvent in breaking the surface bonds (SiOAl), and formation of soluble species, and, therefore, are dependent on pH. Backward reactions are related to the reaction between soluble species in solution, on or with the surface of dissolving solid, and hence the concentration and speciation of aluminium and silicon in solution are critical in determining the rate of the backward reaction (Antonic et al., 1993).

Many researchers have published investigations on the dissolution of aluminosilicate compounds, and possible leaching of elements from various sources (Wang et al., 1999, Praharaj et al., 2002, Popovic et al., 2005, Jankowski et al., 2006, Gitari et al., 2009, Izquierdo and Querol, 2012, Crundwell, 2014, Granizon et al., 2014, Müllauer et al., 2015, Simonova et al., 2015). However, most of the studies are conducted in equilibrium conditions where both forward and backward reactions are controlling the release of elements. Authors have previously examined the role of aluminate and silicate release rates in controlling geopolymer nano-structure and gel growth behaviour (Hajimohammadi et al., 2010, Hajimohammadi et al., 2011). In the early stages of the reaction, rapid release of alumina is shown to hinder the dissolution of silica particles, and more crystalline zeolite products and a more continuous gel microstructure is observed with slower alumina release rate (Hajimohammadi et al., 2010). In slowly released silicate systems, gel nucleation is observed to take place very close to the solid silica source particles, while nucleation happens in bulk regions in systems that initially contain dissolved silica. These differences in nucleation lead to a more chemically heterogeneous binder in slow silicate released systems (Hajimohammadi et al., 2011). Thus, understanding the initial dissolution rates of source material in far-from-equilibrium conditions is essential to understanding and controlling the behaviour of final geopolymeric binders.

In this paper, the dissolution rate of some geopolymer precursor materials is examined using a kinetic approach. The liquid to solid ratios were designed to be sufficiently high to minimise precipitation of hydration products. The dissolution behaviour of some geopolymer source materials is characterised, and the effects of pH and particle size distribution on dissolution behaviour are presented. These results have implications for the selection of geopolymer precursor materials and the mix design of geopolymer binders, and explain some of the behaviour of geopolymers in the literature.