Effect of calcium aluminate cement on workability and compressive strength of fly ash geopolymer mortar cured at ambient temperature

Geopolymer concretes are made from alkali-activated industrial waste materials. Using geopolymers to replace Ordinary Portland Cement (OPC) as the binder of concrete not only reduces carbon dioxide emission, but also reduces the need for landfill. Most previous studies on class F fly ash based geopolymers were on properties of geopolymer cured upon heating. The heat curing procedure is readily achievable in a precast concrete site (factory) but considered as a barrier for cast in-situ applications. Calcium Aluminate Cement (CAC) has a rapid hardening property which is favourable for many practical applications. This paper aims to make class F fly ash based geopolymer concrete cured at ambient temperature through addition of CAC. Experiments show that geopolymer concretes can achieve desirable mechanical properties as well as workability using CAC as a small part of binder. A number of mixing parameters were found to influence the mechanical properties and workability. The Taguchi method was adopted to study the impact sequence of these parameters, including concentration of alkali solution (10, 12 and 14 M), activator/binder ratio (35, 40 and 45%) and CAC replacement ratio (5, 10 and 20%). A total of 9 mortar mixtures were prepared and analysed according to an L9 (3³) orthogonal array. Based on significance tests, three optimum mixtures were proposed.