Effect of alkaline activators and binder content on the properties of natural pozzolan-based alkali activated concrete

Abstract The stringent conditions are being imposed by the global community to reduce greenhouse gas emissions, will push the search for low carbon footprint alternative binders to ordinary Portland cement. In this pursuit numerous studies have been carried out to synthesize alkali activated concrete (AAC) utilizing industrial by-products, such as, fly ash that is rich in silica and alumina. Natural Pozzolan (NP) is one of such alternative cementitious materials having desirable chemical compositions that could be potentially used to develop AAC. The present work focuses on the effect of curing period, composition of alkaline activators and binder content on the properties of natural pozzolan-based AAC. A total of 15 AAC mixes were prepared by varying the sodium silicate to sodium hydroxide (SS/SH) by weight ratio from 2.0 to 2.75 and binder content between 350 kg/m 3 and 450 kg/m 3 . Fresh properties, such as, setting time of alkali activated pastes (AAP) and workability of concrete were determined. Compressive strength was measured on the specimens cured for 0.5, 1, 3, 7, 14 and 28 days in an oven maintained at 60 °C. For selected mixtures, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were utilized to study the morphology and mineral composition of the developed alkali activated paste (AAP), respectively. Additionally, FTIR was used to identify functional groups formed in the AAP. The results showed that the composition of alkaline activators and binder content have significant effect on the properties of AAC. Considerable strength gain was recorded after 3 days of curing and further curing up to 7 days improved the strength. However, continued curing up to 28 days marginally decreased the strength. Further, SS/SH ratio of 2.5 and binder content of 400 kg/m 3 , resulting in alkaline activators to binder ratio of 0.525, was found to be suitable for achieving superior strength and microstructural characteristics.