The effect of bacteria-based self-healing system on the performance of geopolymer pastes and mortars

In this study, the effects of Bacillus subtilis addition on various performance properties of geopolymer paste and mortar samples produced using ground blast furnace slag (GBFS) were investigated. Within the scope of the pre-tests, a total of 10 groups of samples were analyzed, taking into account the parameters such as different sample content and curing mediums. After completion of pre-tests, the sample content to be used in the main experiments was determined. In this study, in which Bacillus subtilis was used as a healing agent, bacteria were produced at two different concentrations (107 and 109 CFU/mL). Bacteria/binder ratio was also selected as 1, 2 and 3 %. For geopolymer composites activated only by using Na2SiO3, the total liquid/binder ratio was kept constant as 0.4 and 0.55 for geopolymer paste and mortar samples, respectively. All of samples, in which bacterial liquid cultures were added directly to the fresh mixtures, were cured under laboratory conditions for the first 7 days, and at the end of the 7th day, they were left in three different curing mediums (precipitation medium, water and ambient conditions). Many mechanical, permeability and durability tests were carried out to examine the effects of bacterial concentration, bacterial dosage and curing conditions on fresh and hardened geopolymer composites. The obtained findings revealed that the microbial-induced self-healing mechanism significantly increased the performance of geopolymer composites. Optimum values in terms of bacterial dosage were determined as 1% and 3% for geopolymer paste and mortar samples, respectively. The results also showed that in case of the addition of bacteria directly to the mix, efficient self-healing can only be achieved with a suitable curing medium containing urea and calcium. On the other hand, all the experimental results obtained were confirmed by microstructural observations. SEM images of all group samples thought to be capable of self-healing showed significant CaCO3 formations. Similarly, the results of the EDS analysis revealed that the three main elements (Ca, C and O) that point to the formation of CaCO3 are located in high atomic ratios in these sample groups. In addition, XRD analysis results confirmed the presence of prominent calcite peaks in the self-healing geopolymer mortar samples.