Thermal effects of activators on the setting time and rate of workability loss of geopolymers

A number of parameters influence the properties of both fresh and harden geopolymer concretes. Similar to the case of OPC concretes, the fresh geopolymer concrete must also exhibit desirable rheology and setting behaviours in order to be useful for field applications. These properties of geopolymer concrete cannot be controlled using traditional super plasticisers, water reducer or set retarders. Therefore, the significance of understanding alternative methods to adjust the properties of fresh geopolymer concrete is very high. Use of solid alkali activators in geopolymer mixes is less hazardous than the commonly used liquid activators and yields a cement like mix, where only water is added for its activation and hydration. The alkali activators used in solid form instead of a liquid form changes the kinetics of the reaction, as its dissolution occurs during the mixing process. This paper presents a study of the setting time and rate of slump loss of geopolymer binders based on fly ash and slag blends and activated with pentahydrate and anhydrous sodium metasilicate. Fly ash and slag were mixed at different ratios and activated with the two solid alkali activators mentioned above. Vicat needle test and calorimetric measurements were employed to determine the setting parameters and heat generation and sink effect of geopolymer mixes. The workability was measured using a flow table to examine the rate of slump loss. The calorimetric measurements showed that activation of the fly ash and slag blend by the pentahydrate activator is accompanied by negative heat flow (heat absorption) whereas anhydrous alkali activator dissolution is accompanied by release of heat. The additional heat released during the dissolution process accelerates the setting in anhydrous alkali activated geopolymers compared to hydrated alkali activated geopolymers. It was also observed that the strength of the geopolymers in later stage changes with changing calorimetric heat flow characteristics.