Effect of the early dissolution of Al-phases in reactive industrial wastes, on the setting times and rheology of sustainable binders

Abstract

Aluminium-containing minerals in industrial wastes are relatively common, and they have potential as pozzolans or geopolymer precursors for the production of sustainable cements. The availability of reactive Al plays an important role in the early formation of calcium aluminate hydrate or aluminosilicate gel on hydration, thereby influencing setting time and rheology. Setting times are closely related to the transport and placement of mortars and concretes. Excessive or insufficient setting times and a poor rheology can distress the construction process. Four industrial wastes are studied including alum sludge (AS), ground granulated blast-furnace slag (GGBS), fluid catalytic cracking catalyst (FCC) and fly ash (FA). They all contain Al and are significantly vitreous. The early dissolution of Al3+ in saturated lime solutions was quantified with colorimetric methods, and the relationships between dissolution rate, rheology and setting times drawn. The results indicate that the variation in Al3+ concentration in the early solution causes considerable differences in setting times. The aluminate wastes (AS) dissolve fast and reach equilibrium in solution within minutes, setting very quickly. On the contrary, the silico-aluminate wastes (FCC, FA) slowly dissolve for hours or even days, taking much longer to set. The results evidenced that there is a close relationship between the early Al dissolution, setting and rheology: too rapid Al dissolution in some of the AS materials caused flash setting and unworkable pastes. Furthermore, the initial setting time of the FA pastes (25hours) coincides with the time at which Al begins to dissolve from FA (after 24h of reaction). The paper concludes that the organic matter content, the nature of the Al phases and the amount of silica are the main variables that control the early Al3+ dissolution in SCMs. Raising silica and organic matter content in the SCMs lower the Al dissolution rate. Al present as disordered oxyhydroxides (i.e. AS) readily contributes significant Al3+ into the solution, whereas when the Al is present as silicates (mullite in the FA and zeolite in FCC), dissolution is limited and slow, and setting takes much longer.