Sewer concrete subjected to biogenic acid corrosion: analysis of concrete deterioration phases using QEMSCAN

Department of Civil Engineering, University of Cape Town, South Africa

^* Corresponding author: bkrali003@myuct.ac.za

Abstract

QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) was used to analyse the evolution of cementitious phases of sewer concrete after biogenic acid attack. Two concrete mixes; a blend of Sulphate Resisting Portland Cement and Fly Ash (SRPC+FA) and Calcium Sulpho-Aluminate (CSA) cement, with calcite and siliceous aggregates, were subjected to a field sewer environment, with a maximum H2S gas concentration of 100 ppm, for about two years. After exposure, both concretes experienced thickness losses, with CSA indicating mass gain and SRPC+FA mass loss. QEMSCAN analysis results indicated that SRPC+FA was rich in calcium silicate hydrates in the intact zone, Mg-silicate in the transition zone, and Al-silicate, gypsum, and quartz in the deterioration layer. CSA contained ettringite and portlandite in the intact zone, gypsum and aluminate phases in the deterioration zone, while the transition zone was challenging to observe due to the superimposition of ettringite before and after the attack. It was concluded that, despite CSA showing a lower corrosion rate than SRPC+FA, it exhibited severe deterioration under QEMSCAN. Therefore, QEMSCAN is a potential tool for preliminary analysing of concrete microstructural phases under biogenic acid attack. However, it should be incorporated with other microstructural analysis tools.