Cédric Patapy

This paper discusses the performance of calcium sulpho-aluminate (CSA) cement and a Sulphate-Resisting Portland Cement (SRPC) with a fly ash (FA) additive (i.e., a SRPC + FA binder system) in a ‘live’ sewer environment; it deepens the understanding of their deterioration mechanisms by using a laboratory test for simulated sewer conditions. It also studies the role of an iron-based additive (‘Hard-Cem^®’, HC) in improving the performance of SRPC + FA concrete under a biogenic acid attack. The performance of 0.4 w/b concrete specimens of the three binders (CSA, SRPC + FA, and SRPC + FA + HC) with calcite aggregates in sewer exposure was assessed by visual observation, measurements of mass and thickness changes, and microstructural analysis for approximately 25 months. The laboratory test, i.e., the Biogenic Acid Concrete (BAC) test, was used to study the deterioration mechanisms of these binders in terms of leaching solution pH and standardised cumulative leached calcium and aluminium. The results indicate that CSA concrete had improved performance in the sewer environment, showing no mass loss and only about one-third of thickness lost in the SRPC + FA concrete over a 25-month exposure period in the sewer environment. The BAC test results complemented the field observations. The iron-based additive in sewer concrete slightly reduced mass loss, likely due to its better resistance to abrasion and erosion, but not due to any chemical influence, since it does not participate in hydration or dissolution reactions. The findings imply that CSA cement may represent a suitable alternative binder for concrete sewer construction. They also suggest that a surface hardener has limited benefits, except when it is under abrasive conditions. Further investigation is required, especially since CSA contains high amounts of sulphate, the effect of which is not well understood.

The biodeterioration of cementitious materials in sewer networks has become a major economic, ecological, and public health issue. Establishing a suitable standardized test is essential if sustainable construction materials are to be developed and qualified for sewerage environments. Since purely chemical tests are proven to not be representative of the actual deterioration phenomena in real sewer conditions, a biological test–named the Biogenic Acid Concrete (BAC) test–was developed at the University of Toulouse to reproduce the biological reactions involved in the process of concrete biodeterioration in sewers. The test consists in trickling a solution containing a safe reduced sulfur source onto the surface of cementitious substrates previously covered with a high diversity microbial consortium. In these conditions, a sulfur-oxidizing metabolism naturally develops in the biofilm and leads to the production of biogenic sulfuric acid on the surface of the material. The representativeness of the test in terms of deterioration mechanisms has been validated in previous studies. A wide range of cementitious materials have been exposed to the biodeterioration test during half a decade. On the basis of this large database and the expertise gained, the purpose of this paper is (i) to propose a simple and robust performance criterion for the test (standardized leached calcium as a function of sulfate produced by the biofilm), and (ii) to demonstrate the repeatability, reproducibility, and discriminability of the test method. In only a 3-month period, the test was able to highlight the differences in the performances of common cement-based materials (CEM I, CEM III, and CEM V) and special calcium aluminate cement (CAC) binders with different nature of aggregates (natural silica and synthetic calcium aluminate). The proposed performance indicator (relative standardized leached calcium) allowed the materials to be classified according to their resistance to biogenic acid attack in sewer conditions. The repeatability of the test was confirmed using three different specimens of the same material within the same experiment and the reproducibility of the results was demonstrated by standardizing the results using a reference material from 5 different test campaigns. Furthermore, developing post-testing processing and calculation methods constituted a first step toward a standardized test protocol.