Rheological characterization of low-calcium fly ash suspensions in alkaline silicate colloidal solutions for geopolymer concrete production

The rheological behavior of low-calcium fly ash suspensions in activating solutions of colloidal silica and alkali hydroxide is investigated. The study is aimed at relating the rheological behavior of alkali-silicate activated low-calcium fly ash (AAF) suspensions with Portland cement paste suspensions. The yield stress and the viscosity of the AAF suspensions increase with the alkalinity of the colloidal solution, which is due to changes in the surface charges on fly ash particles with the change in the ionic medium. Increasing the alkalinity results in a less negative zeta potential of fly ash in the alkaline solution of colloidal silica and an increase in the yield srtess of the AAF suspension. The thixotropic behavior in AAF suspensions is associated with structure breakdown to a finely dispersed suspension of particles produced by shearing. Energy measurements indicate a very slow change in the internal particle structure with age at room temperature. A comparison of AAF suspensions is presented with suspensions of Portland cement in water, which are proportioned for similar physical flow characteristics and yield stress. AAF suspensions have a larger solid fraction than the cement suspensions in water of comparable yield stress. The zeta potential of cement particles in water is less negative when compared to fly ash in alkaline-silicate solutions. The AAF suspensions of comparable yield stress exhibit a significantly higher viscosity than the cement suspensions in water. Cement paste and AAF suspensions exhibit a rate dependent yield response. Cement paste suspensions exhibit a threshold strain rate for minimum yield stress. In AAF suspensions there is a continuous decrease in the yield stress at lower strain rates. The thixotropic behavior in cement paste is influenced by chemical ageing which produces a rapid recovery of yield stress. In comparison, there is very little ageing at room temperature in the AAF suspensions and a very slow recovery of yield stress after shearing. (C) 2019 Elsevier Ltd. All rights reserved.