Evaluation of the resistance of CAC and BFSC mortars to biodegradation : laboratory test approach

Biodeterioration of cementitious materials in sewer networks is a major concern for health and economic reasons. Essentially, it is due to the biological oxidation of H2S into H2SO4 leading to a local progressive dissolution of the cementitious matrix and the precipitation of expansive products likely to provoke cracks. However, it is widely known that CAC has a better performance in such environments but the mechanisms are not very well understood. Nevertheless, previous studies focused mainly on measuring the mass loss of the specimens accompanied with little information on the chemical alteration of the cementitious matrix. This study aims to compare the performance of CAC and BFSC mortars in sewer conditions using laboratory test (BAC-test). Leaching kinetics were evaluated by concentrations measurements of cementitious cations in the leached solutions and of sulphate production by the microorganisms. Moreover, SEM observations coupled with EDS analyses allowed the identification of the chemical alteration of the cementitious matrix

Laboratory test to evaluate the resistance of cementitious materials to biodeterioration in sewer network conditions

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

Electrical characterization of sipos mesa power transistors electric field repartition in the collector-base junction

SIPOS mesa power transistors with large negative bevel angle, obtained by chemical etching, have been electrically tested especially using Scanning Electron Microscopy (S.E.M.) in Electron Beam Induced Current (E.B.I.C.) mode. Multiplication curves can be obtained in the space charge region when the beam scans over the reverse biased collector-base junction which allows the determination of the electric field repartition as function of reverse bias (when a multiplication effect occurs).Des transistors de puissance largement biseautés, fabriqués en méthode mesa par attaque chimique et recouverts de SIPOS ont été testés électriquement en utilisant le microscope électronique à balayage en mode courant induit. Des courbes de multiplication ont pu être obtenues dans la zone de charge d'espace de la jonction collecteur-base polarisée en inverse lorsque le faisceau balaye perpendiculairement le plan de la jonction. Ceci permet la détermination de la répartition du champ électrique dans la zone de charge d'espace pour un champ supérieur à 105 V/cm

Accelerated test design for biodeterioration of cementitious materials and products in sewer environments

WOS:000372185700006International audienceA new test method for evaluating the resistance of cementitious products and materials to biogenic acid attack has been developed. It consists of inoculating pipes with a highly diverse microbial consortium (from an urban wastewater treatment plant), and trickling a feeding solution containing a safe and soluble reduced sulfur source over the inoculated surface in order to select a sulfur-oxidizing activity. A sulfur substrate, thiosulfate, was used in the feeding solution, which facilitated the monitoring of the bacterial activity, and of the leaching of cementitious ions. The functioning of the test in terms of selection of sulfur oxidizing microorganisms, acid and sulfate production, and degradation mechanisms occurring in the cementitious materials has been validated previously. This paper aims (i) to evaluate the reproducibility and repeatability of the test when the source of the sludge used for the inoculation is changed and (ii) to optimize the test design. The following changes were carried out with a view to intensifying the biological and chemical reactions: change of sulfur substrate (tetrathionate instead of thiosulfate), increased sulfur flow rate at the surface of the biofilm, and increased temperature. The results highlight good reproducibility of the test: the change in the inoculum and in the sulfur substrate led to the same phenomena in terms of (i) transformation of reduced sulfur species into sulfate and production of acid and (ii) degradation mechanisms of the cement linings. Moreover, the change of the sulfur substrate combined with the reduction of the exposed surface locally intensified the alteration kinetics. In contrast, in the experimental conditions, the increase in temperature did not seem to have any positive influence on the microbial activity

Numerical modelling of sulfuric acid attack on OPC and CAC materials

A reactive transport model was developed using HYTEC to simulate sulfuric acid attack on cementitious materials. The model was tested on ordinary Portland cement (OPC) and calcium aluminate cement (CAC) matrices using two sulfuric acid solutions at pH 3 and 1 for 100 days. The main results were that the cumulative leached Ca2+ per initial total Ca2+ was controlled by diffusion mechanisms as it increases linearly versus the square root of time. CAC exhibited a better performance, compared to OPC, at pH 3 while it suffered more deterioration at pH 1. Ettringite precipitation was observed in the decalcified zone with gypsum formation detected on surface only at pH 1. Moreover, the analyses of the solid phases’ profiles after 100 days revealed that the dissolution of AH3 in CAC material accounted for the severe deterioration of the matrix at pH 1. Moreover, an optimization of the numerical model to represent the associated laboratory test – BAC-test [Peyre-Lavigne et al. 2015, 2016] – using a 1D model was validated. Furthermore, a brief comparison of the results obtained by two different numerical models (Aquasim and HYTEC) was presented to highlight the main differences – such as database, reactions of hydrated phases and the diffusion in the porous medium – between the two approaches

Controlling the microbial competition between hydrogenotrophic methanogens and homoacetogens using mass transfer and thermodynamic constraints

International audienceThe reduction of CO2 allows the synthesis of platform molecules for the chemical and energy industry. Anaerobicmicrobial consortia contain homoacetogenic microorganisms (HAC) capable of reducing CO2 to acetate. How-ever, one of the obstacles to their use is the understanding and management of their functional diversity. Inparticular, managing the competition between HAC and hydrogenotrophic methanogens (HM) that convert CO2into methane is crucial to selectively produce acetate.This study contributes to bring new knowledge on the competition between HAC and HM. This microbialcompetition is encountered in numerous anaerobic systems, and it is necessary to know how to manage it. In thissense, mass transfer between the gas phase where the substrates are located, and the liquid phase which containsthe microbial catalysts, as well as kinetic and thermodynamic aspects of biological reactions have been inte-grated in this work. The microbial competition between HM and HAC was studied in successive batches. Theeffect of temperature between 25 ◦C and 35 ◦C was investigated, as well as different states of mass transferlimitation in the system.A clear effect of temperature between 25 ◦C and 35 ◦C on the outcome of the competition between HM andHAC was highlighted, as well as the effect of mass transfer limitation. Enrichment of Acetobacterium homoace-togens and elimination of hydrogenotrophic methanogens was possible at 25 ◦C without mass transfer limitation.Acetate product selectivity was of 100% by the end of the enrichment period in successive batches. On thecontrary, under mass transfer limitation, and/or at 35 ◦C, Methanobacterium hydrogenotrophic methanogenswere promoted with 100% of methane selectivity by the end of the enrichment. This study contributes to identifyspecific process parameters influencing the selection of HAC over HM and should help in the design of experi-ments depending on the target product from H2/CO2. Furthermore, the results obtained could be applied tocontinuous acetate producing systems, at a larger scale, and they can also be useful in other gas fermentationsystems and processes