Degradation of cement pastes by organic acids

Agriculture produces effluents, like liquid manure and ensilage effluents, that cause serious environmental problems. In order to limit this pollution, manure needs to be stored in water-tight silos often made of concrete. Manure contains organic acids which constitute a severe chemical threat for concrete. This research aims to analyze the degradation mechanisms of cement-based materials stored in organic acids. The results are used to identify the composition parameters of binders that influence durability. Ordinary and blended Portland cement specimens were immersed for 18 weeks in a mixture of 5 organic acids found in liquid manure at a pH of 4. Physical, chemical and mineralogical modifications in the pastes were explored by water intrusion porosity tests, electron microprobe and X-ray diffraction analyses. Analyses were run from the sound to the altered zone. The altered depths and the mass loss variations of the samples were monitored over time. The degradation of the matrix occurs by almost total decalcification, the vanishing of the crystallized or amorphous hydrated phases and the probable formation of a silica gel, which limits the kinetics of further degradation. In the altered zone, the anhydrous silico-calcic grains are chemically modified but C(4)AF and slag grains keep their initial composition.