Fly ash geopolymers: effect of admixtures on fresh and hardened properties

One of the most important challenges for the cement industry is to find sustainable solutions to mitigate environmental footprint of its activities. Geopolymers are particularly attractive for this purpose; the use of waste as precursors, along with a room temperature curing, makes these materials low-polluting binders potentially suitable for sustainable building products. The lack of information on effective admixtures is limiting the practical acceptance of geopolymer concrete. The purpose of this paper is to study the influence of different superplasticizers, commonly used in Portland cement concrete technology, on properties of fly-ash based geopolymers. First, second and third generation superplasticizers (i.e., lignin-, naphthalene-, melamine-, polycarboxylic ether, acrylic based superplasticizers) have been used for the preparation of pastes and mortars. Two different amounts of admixture were tested: 0.6wt% and 1.0wt% by mass of binder. Among the investigated admixtures, the polycarboxylic ether based superplasticizer is the most effective. With a dosage of 1.0wt % by mass of fly ash it can be achieved an increase in workability of both geopolymer pastes and mortars without compromising the final strength of hardened material.