Mechanical and Transport Properties of Alkali-Activated Natural Pozzolans as Sustainable Binders
OPC production accounts for 5 to 7% of the global carbon dioxide (CO2) emissions, contributing to global warming. The desire to reduce CO2 emissions and produce more durable concrete has given impetus to search for new binders. It is suggested that alkali-activated natural pozzolans have the potential for use as a sustainable replacement for OPC in concrete.
The current study presented herein evaluated fresh, mechanical, and transport properties of alkali-activated natural pozzolan mortars containing various concentrations of sodium hydroxide solutions as an alkaline activator. To this aim, alkali-activated natural pozzolan mortars were made with concentrations of sodium hydroxide (NaOH) of 2.5, 5, 7.5, 10 and 12.5 molar (M) and by using various solution-to-binder ratios (s/b ratios) of 0.50, 0.54, and 0.58 with a fine aggregate-to-binder ratio of 2.
The produced mortar samples were sealed-cured for 3 hours at 60°C, and then de-molded and cured at 80°C until testing at different ages of 1, 3, and 7 days. Another group of samples were cured in a curing room for 28 days. Various tests were conducted on the alkali-activated natural pozzolan mortar samples including flow spread, compressive strength, flexural strength, absorption, void content, and rapid chloride migration.
The 12.5 M 0.58 s/b ratio mixture achieved the highest overall compressive strength after seven days of sealed curing. The sealed curing environment was found to be most conducive to strength gain, as the exposed condition led to dehydration within the samples and the moisture condition did not allow for full removal of excess water, reducing bond formation. As the NaOH concentration increased, the compressive and flexural strengths increased, while the flow, void content, and chloride migration decreased. Increasing the s/b ratio reduced compressive and flexural strengths and increased flow, void content, and chloride migration.