Abstract

Sound and durable concrete mixtures are essential in environments where microbial induced corrosion (MIC) is a concern. In this study three concrete mixtures prepared with different binders (ordinary portland cement (OPC), calcium aluminate cement (CAC) and alkali-activated cement (AAC) were petrographically examined after exposure to laboratory-controlled MIC conditions. Analytical techniques included reflected light microscopy, polarized and fluorescent transmitted light microscopy (FLM) and scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer (SEM/EDX). SEM/EDX analyses obtained from the same areas provided information on chemical and mineralogical alteration, while quantitative image analysis with FLM affords an opportunity to quantify the variation in capillary porosity of the paste within alteration zonation. The microscopic observations indicate that the change in normalized capillary porosity coincide broadly with different alteration zones, which are in turn dependent on the type of binder. The OPC concrete shows the greatest deterioration where the paste is largely replaced by gypsum and high capillary porosity is lined to leaching of a calcium. The formation of ettringite densifies the paste and lowers the capillary porosity inboard of the gypsum zone in the OPC concrete. The CAC and AAC mixtures show better resistance to leaching than the OPC mixture, which explains their superior resistance to MIC. These findings are consistent with independent macroscopic observations that indicate the OPC mixture deteriorated more rapidly and severely than the CAC and AAC mixtures.