• Aboulela, Amr

Abstract

The biodeterioration of cementitious materials in sewer networks is a major concern for health and economic reasons. Mainly, it is due to the biological oxidation of H2S into H2SO4 leading to a progressive dissolution of the matrix and the precipitation of expansive products likely to provoke cracks. In the literature, it is established that calcium aluminate cement (CAC) has a better resistance in such environments than Portland cement (PC). The better performance of CAC was mainly linked to the high aluminum content of this material and to the physico-chemical properties of the cement matrix (mineralogical phases, porosity, etc.). In addition, recent studies revealed the potential role of iron in the resistance of materials in such environments, however, without clarifying this role.This thesis aimed to (i) study the behavior and the deterioration mechanisms of different low-CO2 cementitious materials (e.g. calcium sulfoaluminate cement (CSA) and alkali-activated slag (AAS)) exposed to a microbial attack in sewer conditions; (ii) evaluate the impact of partially substituting PC with mineral additives, rich in aluminum and/or iron on the resistance of the materials; (iii) understand the mechanisms of interactions between the cement matrix, the aggressive environment and the biofilm; (iv) use and optimization of a numerical model, based on the coupling of materials’ chemistry and the transport of species in solution, to include biological transformations of sulfur species.The experimental work was carried out in laboratory conditions using an accelerated test. This test made it possible to create favorable conditions for the development of a sulfur-oxidizing activity at acidic pH on tetrathionate (S4O62-) substrate and to evaluate the behavior of various materials (based on PC, CAC, CSA and AAS).A performance indicator (PIeqOH) was developed by evaluating the leaching of chemical elements/compounds (in particular calcium, aluminum, iron, magnesium and sulfate which defines the neutralizing capacity of the materials). This performance indicator was used to establish a classification of materials according to their resistance to sulfuric acid attack in these exposure conditions.In the experimental conditions, Acidithiobacillus and Thiomonas genus were the main bacteria identified, with the latter being able to disproportionate tetrathionates in absence of oxygen. The disproportionation of tetrathionate was identified as responsible for the presence of elemental sulfur precipitation on the materials' surface.Certain CSA-based materials have shown better behavior than Portland cements in the experimental conditions. This resistance was mainly attributed to the formation of aluminum hydroxide and its chemical stability in acidic environments. Moreover, the mineralogical form of aluminum-bearing phases is critical regarding their resistance to the acid attack and the simple increase of the aluminum content by non-reactive mineral additives was shown to not improve the performance of the materials.Iron, non-reactive initially, remained relatively inactive during the very aggressive microbial attack. Neither the iron concentration nor the form in which it was present showed any impact on the development of microbial activities. Moreover, no improvement of the resistance of cement-based materials by iron enrichment has been obserbed maybe in relation with the absence of new Fe-bearing hydrated phases.In the conditions of the BAC test, the deterioration mechanisms of alkali-activated slag materials were similar to those of Portland cement based ones, and consisted of a strong decalcification of C-A-S-H, leading to the formation of an aluminosilicate gel on the surface.

Topics

• impedance spectroscopy
• mineral
• surface
• compound
• Carbon
• phase
• Oxygen
• Magnesium
• Magnesium
• aluminium
• reactive
• crack
• chemical stability
• cement
• precipitation
• leaching
• iron
• porosity
• Calcium
• atomic absorpion spectrometry