PH reduction through amendment of cement mortar with silica fume enhances its biological treatment using bacterial carbonate precipitation

Abstract

Biological treatment of concrete has recently been the subject of research as an environmentally friendly technique for increasing its durability. One of the drawbacks with this method is that the very high pH environment in concrete retards the activity of even the alkalophilic bacterial species used in this treatment process. In the present study, reduction of the pH in concrete by replacement of 20 % of cement with silica fume was considered as a means of enhancing the biological treatment process. For this purpose, mortar specimens not amended (normal) and amended with silica fume were subjected to bacterial treatment using Sporosarcina pasteurii ATCC 11859 cells, which produce calcium carbonate in cement mortar specimens through enzymatic hydrolysis of urea. In order to test the extent of the enhancement effect, some mortar specimens containing artificial cracks were also used in the experiments. The findings of the present study showed that the biological treatment of silica fume amended mortar specimens results in lower formation of bacterial biofilm but meaningfully higher precipitation of calcium carbonate, in the form of calcite, aragonite and vaterite, compared to normal specimens. Irrespective of the curing age and the presence or absence of artificial crack, the compressive strength of silica fume amended mortar specimens was always meaningfully higher than the corresponding value for normal mortar specimens. In further tests, the value of sorptivity coefficient for 28 days old biologically treated mortar specimens not amended and amended with silica fume was 42–48 % and 57–64 % lower, respectively, than the corresponding value determined for untreated specimens.