Mechanical strength and permeation properties of high calcium fly ash-based geopolymer containing recycled brick powder

Geopolymer is a ceramic-like material produced by the combination of alumino-silica source material and alkaline activator. High calcium fly ash is an emerging binding material for production of geopolymer while there is currently growing potential to use recycled clay brick as part of the binding material as clay brick is also inherently a ceramic-type material. For geopolymer made with different source materials, it is important to ascertain its compressive strength attainment. Hence, in this study, high calcium fly ash and brick powder from recycled brick were utilized as source materials to prepare geopolymer mortar for evaluation of its flowability, mechanical strength, absorption and sorptivity. The fly ash-based geopolymer mortars were produced with different concentrations of NaOH (6, 8 and 10 M) and Na2SiO3/NaOH ratios (1.5, 2.0 and 2.5). The optimum Na2SiO3/NaOH ratio of 2.5 was then selected to further study the effect of recycled brick powder content (0-20%) on the compressive strength, absorption and sorptivity. This study revealed that among the studied variables, the presence of brick powder is the major factor in reducing the flowability of geopolymer mixtures. The highest 28-day compressive strength of 44.2 MPa was obtained by incorporating 10% brick powder as fly ash substitute along with 10 M NaOH of activator. FESEM-EDS of the specimens indicated that brick powder replacement beyond 10% resulted in formation of inhomogeneous microstructure. Higher concentration of NaOH, though, increased the water absorption and sorptivity of the high calcium fly ash-based geopolymer mortars.