Durability of alkali-activated mortar in sewage environment

Abstract

Concrete is the most consumed material which is used in almost every construction project such as roads, building, hydraulic structures, bridges. Alkali-activated binders are considered as a new trend in concrete technology. The primary difference between these materials and conventional ordinary Portland cement (OPC) is that they are free from clinker which has a high carbon footprint. Alkali-activated binders are a potential alternative in the field of sustainable infrastructure development. Three different experimental programs were considered to assess the performance of low-calcium fly ash based geopolymer mortar (FA-GPm) and alkali-activated slag based mortar (AASm) in comparison with commercially used sulphate resistant Portland cement (SRPCm) and calcium aluminate cement mortars (CACm). Firstly, specimens were placed in Manly wastewater treatment plant in Sydney for an exposure period of 24 months and data loggers were installed to monitor the variations in environmental conditions. Then, two laboratory experiments were carried out aiming to simulate the deterioration mechanism observed in natural conditions. The first experiment involved the conventional approach of immersing mortars into 1.5% sulphuric acid solution. Whereas the second experiment is a new test simulating microbially induced corrosive environment by inoculating acidophilic and neutrophilic sulphur oxidizing microorganisms namely A. Thiooxidans and T. Intermedius, respectively. Changes in physio-mechanical properties such as mass, porosity, strength, dry bulk density, loss in alkalinity, neutralization depth and surface pH were estimated. Microstructural analysis techniques such as scanning electron microscopy (SEM), energy dispersive X-Ray (EDX), X-Ray diffraction (XRD), fourier transform infrared (FTIR) and Raman spectroscopy were used to assess the matrix degradation, estimate the deterioration depth, highlight the difference in corrosion mechanisms and to develop simplified models based on data collected from in situ exposure for service life prediction. Results showed that the performance of CACm was far superior as compared to other binders and that high slag content has a negative impact on the resistance of alkali-activated binders. Furthermore, the microbiological experiment depicted almost similar corrosion as that observed in natural sewer environment indicating its suitability to be used as a testing method to simulate corrosion in natural sewer environment.