Durability of ternary binders based on Portland cement, calcium aluminate cement and calcium sulfate

摘要

The applications of systems based on Portland Cement (PC), Calcium Aluminate Cement (CAC) and Calcium Sulphate (C$) are usually limited to indoor uses in the field of Building Chemistry because no durability data exist on the outdoor uses. These ternary binders have different phase compositions and microstructure than those of the products based purely on Portland cement. Whereas Portland cement is widely studied, there exists very little published data on the performance of these materials over time. This research work aimed at understanding the development of the microstructure of these ternary systems and identifying the degradation mechanisms under controlled atmosphere and under natural weathering. The influence of the formulation was thoroughly studied with respect to hydration mechanism, porosity and transport properties. SEM (BSE observations and EDS microanalysis) was the main analytical technique to understand the hydration mechanisms and was also coupled with XRD, TGA and 27Al NMR. Porosity was examined by solvent exchange and desorption isotherm. Transport properties were studied through oxygen diffusion, oxygen permeability and water sorption. In parallel, performance of different systems were tested with respect to the storage, carbonation, acid corrosion and external sulphate attack. Alongside with that study, the degradation mechanisms after 3 years of natural weathering were examined. The microstructural investigations of the cementitious matrix show that the hydration mechanisms of Portland-rich systems depend on the PC/CAC and CAC/C$ ratios but all present a delay of the silicates' hydration. The CAC-C$ rich binders lead to ettringite and AH3 formation. Despite different hydration mechanisms, Portland and CAC-C$ rich binder at similar porosity have similar transport properties. A first durability "map" was drawn with the data from accelerated ageing. It was found that binders containing portlandite provide superior resistance to carbonation but were more sensitive to sulphate attack, those with AH3 perform better in acidic media. The 3 years natural weathering reveals that samples underwent leaching and carbonation and the mechanical strengths remain the same from 1 to 3 years.