Alkali-activated natural pozzolan/slag binders: limitations and remediation

To address issues related to fast setting time of high-calcium-based alkali-activated binders and low-calcium-based alkali-activated binders' need for curing at elevated temperatures, this study aimed to develop hybrid low-calcium- and high-calcium-based binders with satisfactory fresh and hardened properties. This study was divided into two main phases: (i) production of alkali-activated natural pozzolan/slag pastes that can provide similar properties – particularly setting time – to those of Portland cement paste; (ii) confirmation of the pastes’ performance through their application in mortar and concrete mixtures. Different strategies including using various set-retarding admixtures, reducing the sodium hydroxide concentration and varying the combination of alkaline activators were considered to achieve this objective. The use of different chemical admixtures including commercial retarders, malic acid, sodium phosphate, phosphoric acid, sodium gluconate, a lignosulfonate-based admixture and gypsum, as well as a varying combination of alkaline activators, rendered fruitless to produce alkali-activated natural pozzolan/slag binders with acceptable setting time, flowability and strength. Optimising sodium hydroxide concentration was found to be the most effective method for achieving the expected performance of natural pozzolan/slag binders activated with sodium hydroxide and sodium silicate. The performance of mortars and concretes made with the resulting alkali-activated natural pozzolan/slag binders was promising. They achieved similar fresh properties and strength and had superior transport and durability properties to those of reference Portland cement mixtures.