Using bacteria for early-age strength improvement of concrete

Development of early age strength of concrete structures is a crucial parameter in in-situ concrete casting affecting the time and therefore the cost of the construction process. A hindering factor for expanding the use of low-carbon cement is the initial lower strength gain that these materials present in comparison to Portland cement. Nonetheless, employing low-carbon cementitious materials is key for achieving sustainability in the construction industry. An unconventional way for improving the early age strength of concrete is by using bacteria. Several studies have demonstrated the enhancement of strength in concrete by the addition of bacteria cells. These results were attributed to the precipitation of calcite that is induced by the metabolism of the microbes. Here, a different approach to the phenomenon is attempted. In this paper, both live and dead bacteria were added directly in cement mortars in different concentrations. The effect of the bacterial addition on the hydration of cement pastes and the strength properties of mortars at 3, 7 and 28 days were studied. An increase in the strengths of most of the mortars containing bacteria was reported, which was more pronounced for the samples containing dead cells. No additional calcite formation was detected in the samples disproving earlier theories. Furthermore, the hydration rate of the cement was not significantly affected by the addition of the bacteria, either live or dead, suggesting that it is also not a nucleation effect. Microstructural analysis with Scanning Electron Microscopy (SEM) and Thermogravimetric analysis (TGA) were applied for interpreting the strength results and other hypotheses for the strength improvement are suggested. The addition of bacteria appears to be a cheap and environmental-friendly solution for enhancing the properties of low-strength sustainable cement-based materials, thus encouraging their wider use in constructions.