Assessment of Bacillus subtilis and Bacillus licheniformis as Agents Against External Sulfate Attack on Cementitious Materials

Bacteria in concrete has been studied as an additive to repair microcracks and reduce permeability, as well as increase compressive strength. Within the broad spectrum of bacteria, two types promise to be effective agents against external sulfate attack: (i) Bacillus subtilis, which could indirectly prevent the entry of sulfates through the mechanism of sealing by calcium precipitation, and (ii) Bacillus licheniformis, which could encapsulate the sulfates that enter by diffusion and prevent the consequences of the pathology, such as expansion and loss of strength. This research evaluates the impact of B. subtilis and B. licheniformis on the performance of cementitious mixes against external sulfate attack, measuring compressive strength, expansion, permeability, and effects on the microstructure. Results show that both bacteria can produce compressive strength improvements of up to 20% at 28 days and 50% at 180 days. Moreover, in the presence of sulfates, improvements of up to 90% can be observed over control mixes. However, this result should be carefully evaluated because although B. licheniformis produces better results in the long term, it results in lower strength in the presence of sulfates in the short term. At the same time, B. licheniformis significantly reduces expansion against external sulfate attack, decreasing it by up to 80%, because it generates less ettringite and gypsum. Thus, B. licheniformis is an effective agent against external sulfate attack. Based on the results, it is estimated that both bacteria can be used to improve performance; however, care must be taken with concentration, which affects homogeneity or generates negative effects. In particular, it is noteworthy that calcium carbonate loss was observed from the mixes due to continuous curing and that calcium precipitation can generate negative effects against sulfates in the long term.