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Aims To determine the effect of direct embedment of Bacillus megaterium into Portland pozzolana cement mortars on water sorptivity and diffusivity coefficient of sulphate ions. Methods and Results Prisms with a water/cement ratio of 0·5 were prepared by blending Portland Pozzolana cement with the requisite volume of a B. megaterium (microbial) solution whose concentration was 1·0 × 107 cells per ml. Mortar prisms of 160 mm × 40 mm × 40 mm were fabricated for this study. Mortars cured for 28 days were exposed to 0·2465 mol l−1 Na2SO4 solution using accelerated ion migration test method for 36-h session using a 12V DC power source. Sulphate ion concentration was then determined through the ingressed mortar at 10 mm interval. A minimum water sorption gain of 0·61% was observed on the prism prepared with and cured in microbial solution. A maximum of 0·0289 and a minimum of 0·0093 water sorptivity coefficients were exhibited by the control prism and microbial prisms, respectively. The microbial prisms exhibited the lowest apparent diffusion coefficient (Dapp) of 4·5179 × 10−11 m2 s−1. Conclusions Direct incorporation of B. megaterium in mortar preparation, curing or both regimes significantly retarded water sorption and lowered sulphate ion ingress. The inclusion of this bacterial in the mortar further complements the pozzolana pore structure benefits. Significance and Impact of the Study This novel B. megaterium bacteria which can survive and cause biocementation within hydrating cement mortar when not encapsulated would result in a green innovation. Once adopted and applied in real-life scenario, it would promote construction of durable, safe, resilient and affordable shelter.

Most of concrete structural failures are attributed to poor workmanship and poor engineering designs. Some microorganisms present in sewer systems can degrade the concrete and/or mortar. Concrete failures due to microbial attack has not attracted much attention especially in developing countries such as Kenya. This study investigated the effect of Thiobacillus intermedius bacteria on the performance of Ordinary Portland Cement (OPC). Preparation of test mortar prisms was done using the bacterial solution as either mix water, curing water or both. The control mortar prisms were prepared and cured in distilled water. Compressive strength test was done after 7th, 28th, 56th and 90th days of curing respectively. Results showed significant drop in compressive strength for the mortar prisms prepared and cured in bacterial solution as compared to the control mortar samples. Soundness and normal consistency increased significantly for the bacterial treated cement paste as compared to the control sample. Scanning Electron Microscopy (SEM) analysis showed severe damage on the bacterial treated cement mortar. This was characterized by formation of deleterious expansive products like ettringite and gypsum. Control mortar sample exhibited even formation of hydration products within the pore system.