Effect of microbial biocementation on physicochemical And mechanical properties of mortar Made from portland cements

Abstract

Microorganisms in soil and water play a significant role in physico-chemical and mechanical properties as well as the durability of building materials. The microorganisms can either contribute to the improvement or deterioration of the materials. Beneficial microorganisms may deposit calcium carbonate in cement mortar or concrete through a process called microbial biocementation. These deposits exhibit binding properties for protecting and consolidating various building materials. Whereas the effect of Bacillus bacteria on fully hardened/cured mortar/concrete is well documented, the effect of such microorganisms on fresh mortar and concrete paste has not been fully investigated. Further, this study examined the microorganisms' biominerals, their chemical composition, and their role in the enhancement of nucleation on cement hydration. The Bacillus species under this study are commonly found in soil/water, are non-pathogenic and are urease active. Bacterial species, Lysinibacillus sphaericus, Sporosarcina pasteurii, and Bacillus megaterium were incorporated separately into the mortar-making mixing water at a concentration of 1.0 × 107 cells/mL. Mortar prisms with 0.5 watercement (w/c) ratio were cast using selected commercial Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC). Some prisms were then cured at room temperature in a microbial solution composed of bacteria, urea, and calcium acetate/calcium chloride, while others were cured in tap water. Lower normal consistency results from microbial mortar pastes than non-microbial pastes in both OPC and PPC were observed. This implied reduced water demand and improved workability. Initial and final setting time were generally lowered, with the OPC paste with Lysinibacillus sphaericus showing the highest reduction. The resultant chemical compounds formed in the mortar were analyzed using Scanning Electron Microscopy (SEM), powder X-ray Diffraction (XRD), and Fourier Transform Infrared (FTIR). Bavenite, Al2Be2Ca4H2O28Si9, and calcite, CaCO3, were found to be the resultant microbial cement hydration products. Compressive and flexural strength gain was observed after the 14th day of curing with the highest compressive and flexural strength gain observed at the 56th day of curing at 19.8 % and 37.0 % respectively for OPC mortars that had Lysinibacillus sphaericus. Rapid accelerated chloride and sulphate penetration tests were performed on the mortar prisms by exposing them to a media of 3.5 % by mass of sodium chloride and sodium sulphate separately for thirty-six hours using a 12V DC power source. The migration diffusion coefficient, Dmig, and apparent diffusivity coefficient, Dapp, for both the Cl1- and SO4 2- for mortar prisms were determined. Dapp was lowered from 3.5340 × 10-10 m2/s to 2.5449 × 10-10 m2/s and from 6.4810 × 10-10 m2/s to 4.5179 × 10-10 m2/s for Cl1- and SO4 2- respectively in PPC mortars that had Bacillus megaterium. After the 28th day of curing, water sorption change was determined across the mortar categories. Water sorption was lowered in the range of 47.8 % to 68.4 %. PPC mortars that had Bacillus megaterium exhibited a water sorptivity coefficient reduction from 0.0289 to 0.0093. The results show that the incorporation of the selected Bacillus species under this study improves the physico-chemical and mechanical properties of the test cements significantly