Strength development of alkali-activated fly ash produced with combined NaOH and Ca(OH)2 activators

• The use of combined NaOH and Ca(OH)2 alkaline activators with fly ash were studied.
• Increasing the amount of Ca(OH)2 the compressive strength decreased over time.
• SEM/EDS studies showed the formation of two different aluminosilicate gels.
• The gels are responsible for the mechanism of strength development.

Alkali-activated matrices can attain compressive strengths of the order of 30–120 MPa, primarily depending on the mix formulation. The objective of this study is to evaluate compressive strength development by testing, at different ages, fly-ash-based mortars alkali-activated with activating solutions containing varying proportions of a combination of NaOH and Ca(OH)2. The NaO/SiO2 ratio was constant for all samples (N/S = 0.3). Three different CaO/SiO2 molar ratios of the total quantity of CaO to the total quantity of SiO2 present in the mix were studied; specifically: 0.05 (C/N = 0.033) – M5; 0.15 (C/N = 0.370) – M15; and 0.25 (C/N = 0.700) – M25. The M25 mix attained compressive strength of 30 MPa at 7 days. However, after periods of 28 and 91 days, M25 compressive strength had decreased to 22 MPa and 16 MPa respectively. The M15 matrices exhibited similar compressive strength results. In contrast, the M5 mix exhibited increasing compressive strength over time. The SEM micrographs of M5 and M25 matrices showed the presence of two different aluminosilicate gels: the M5 sample developed a massive aluminosilicate gel over time, while the M25 sample began to exhibit a spongy gel at 28 days, resulting in a weaker material. Therefore, the reduction in compressive strength appears to be related to increasing amounts of CaO (higher C/S and C/N) for the alkali-activated matrices tested in this study.