Diatomite and Na-X zeolite as carriers for bacteria in self-healing cementitious mortars

The use of bacteria in the self-healing process of cementitious composites has been recently widely considered. In this study, diatomite and the Na-X zeolite were proposed as carrier materials impregnated by the Bacillus subtilis endospores. The effect of the impregnation method on a bacterial survival rate (BSR) and cement composites modification method on the strength parameters, was investigated. Additionally, the influence of modification and healing method on the flexural and compressive strength recovery and surface crack repair ratio, was assessed. Test results indicated higher BSR for a vacuum impregnated than immersed samples (up to 31% after 28 days of storage for the impregnated Na-X zeolite). The BSR in the self-healing mortar may be promoted by local pH reduction in the vicinity of carrier particles, especially diatomite (pHPZC = 4.6). Mortars containing medium dissolved in the mixing water showed an increase in the flexural and compressive strength (up to 10.4% and 18.0%, respectively). The addition of bacteria immobilized on the both carrier materials caused a decrease of strength properties (to 16.6%), while the series containing both bacteria and the medium attained the strength properties similar or better than the reference series and the series with carrier materials alone. Series containing bacteria immobilized on carrier material and with medium showed a self-healing ability, which was confirmed by the compressive strength recovery and surface repair ratio of 32.6% and 92.6%, respectively. The increased precipitation of CaCO3 was confirmed by the SEM and EDS analysis.