Performance of sewer pipe concrete mixtures with portland and calcium aluminate cements subject to mineral and biogenic acid attack

The paper reports on the performance of a series of sewer pipe concrete mixtures and cementitious lining mixtures in acid environments. Binder types based on ordinary portland cement (OPC) and calcium aluminate cement (CAC) were used, with both acid-soluble and acid-insoluble aggregates and various supplementary cementitious materials (SCM). One series of tests subjected the mixtures to pure mineral acid (hydrochloric acid, pH = 1), using a specially designed dynamic test rig. The other series of tests involved monitoring specimens placed in a live sewer under very aggressive conditions induced by acid-generating bacteria. Under mineral acid attack on concretes with conventional dolomite aggregates, OPC/silica fume concretes displayed best performance, attributed to their densified microstructure coupled with substantially improved ITZ. CAC concretes with dolomite aggregate did not perform any better than similar OPC specimens under these conditions, primarily because of their higher porosity. However, with concretes using synthetic alag(TM) aggregates in mineral acid testing, CAC/alag(TM) mixtures performed exceptionally well due to their homogeneous microstructure, inferred absence of an ITZ, and slower dissolution and finer size of alag(TM) aggregate particles. The dynamic acid test was able to reveal differences in physical and chemical interactions between constituents in concrete mixes. Under biogenic acid conditions in the sewer, CAC concretes clearly outperformed OPC concretes. This is ascribed to the ability of CAC to stifle the metabolism of the acid-generating bacteria, thereby reducing acid generation. Thus the effects of neutralisation capacity and stifling of bacterial activity need to be distinguished in designing concrete mixtures to provide good acid resistance. Relative rates of dissolution of binder and aggregates are also important in overall performance, with uniform rates preferable in order to avoid aggregate fallout.