A sustainable solution for the global construction industry can be partial substitution of Ordinary Portland Cement (OPC) by use of supplementary cementitious materials (SCMs) sourced from industrial end-of-life (EOL) products that contain calcareous, siliceous and aluminous materials. Candidate EOL materials include fly ash (FA), silica fume (SF), natural pozzolanic materials like sugarcane bagasse ash (SBA), palm oil fuel ash (POFA), rice husk ash (RHA), mine tailings, marble dust, construction and demolition debris (CDD). Studies have revealed these materials to be cementitious and/or pozzolanic in nature. Their use as SCMs would decrease the amount of cement used in the production of concrete, decreasing carbon emissions associated with cement production. In addition to cement substitution, EOL products as SCMs have also served as coarse and also fine aggregates in the production of eco-friendly concretes.

The potential for self-healing of concrete via biomineralization processes in which microorganisms influence mineral precipitation is promising. To embed microorganisms within a cement-based material, key challenges are to find a microorganism that can tolerate the highly alkaline conditions, survive the mixing process, and remain viable with limited access to nutrients. The focus of this work is to determine the metabolic state of unencapsulated Sporosarcina pasteurii, inoculated vegetatively, in a cement-based matrix over time and to examine its ability to remediate internal cracks and reduce porosity. Viable S. pasteurii was found in hardened mortar samples that were as old as 330 days, and 48% of the viable cells detected were vegetative. A greater fraction of the inoculated cells remained viable in mortar as compared to cement paste, which is promising because mortar is a better representation of the composite nature of concrete than cement paste. Furthermore, as compared to neat paste and neat mortar, addition of the vegetative cell culture to bacterial paste and bacterial mortar resulted in reduced porosity. Bacterial mortar also demonstrated increased strength recovery as compared to neat mortar. The reduction in porosity and increase in mechanical regains demonstrated by the bacterial mortar suggest improved durability and service life for bioconcrete as compared to traditional concrete.

Recent research in the field of concrete materials showed that it might be possible to develop a smart cement-based material that is capable of remediating cracks by Microbial-induced calcium carbonate precipitation (MICP). The early remediation of microcracks enables the design of cement-based systems with an elongated service life with a sustainable approach. However, the main challenge of the application is to extend the viability of the cells against the restrictive environment of cement-paste. These cells have to tolerate the highly alkaline conditions of cement paste, survive the mixing process, and remain viable even when access to nutrients is limited. This paper summarizes a novel study undertaken to investigate the self-healing efficiency of Sporosarcina pasteurii (S. pasteurii) cells immobilized on zeolite and sepiolite minerals having the same particle size. This manuscript reports an extensive experimental study to understand the factors influencing the efficiency of immobilization barriers, such as composition and reactivity. To obtain the bio-additive, the bacterial cells were immobilized without nutrients and additional nutrients were only provided during the curing stage after crack initiation. Screening of the healing process was done with ultrasonic pulse velocity (UPV) testing and stereomicroscopy. Further evaluation on performance was done by evaluating the decrease in water absorption capacity. The healing precipitate was characterized through Environmental Scanning Electron Microscope (ESEM) and Fourier-Transform infrared spectroscopy (FTIR). With this approach, the cracks on mortar surface were sealed and the water absorption capacity of the so-called self-healed mortar decreased compared to its counterpart cracked mortar samples. Sepiolite was found to be a more suitable bedding for the microorganisms compared to zeolite, therefore samples containing sepiolite exhibited a higher performance in terms of crack healing. The results showed that while vegetative cell immobilization on locally available materials is a simple and economically feasible approach the healing capacity of bacterial cells can be hindered due to the reactivity of the mineral.

Cracks in cement-based materials are one of the main factors affecting the durability of structure. Recent research in the field of concrete materials showed that self-healing in cement-based systems can be achieved by triggering biogenic calcium carbonate (${\mathrm{CaCO}}_3$ ) precipitation. The goal of this study is to establish a comparative evaluation of the use of sepiolite, bentonite, and diatomaceous earth (DE) as an immobilization barrier of Sporosarcina pasteurii (S. pasteurii) cells to trigger self-healing in cement-based systems. For the first time in the literature, this study will provide insight into the use of natural minerals, such as bentonite and sepiolite, as protective carriers for vegetative S. pasteurii cells in cement-based materials and present a comparative evaluation of factors influencing crack healing, such as the microstructure and composition of immobilization barriers. A two-phase self-healing bioadditive was obtained by immobilizing vegetative S. pasteurii cell samples on natural porous minerals with or without the use of required nutrients. Then the samples were cracked by a three-point bending test, and the healing process was screened via stereomicroscope imaging and ultrasonic pulse velocity (UPV) testing after subjecting the cracked samples to 28 days of moist curing. Flexural cracks induced in mortar samples were filled with biogenic precipitate. Relatedly, the water absorption capacity of the samples was decreased in mortar samples containing bacterial cells, the nutrients were added in the curing solution. Fourier transform infrared spectroscopy and scanning electron microscopy analyses showed that calcite was the predominant polymorph of ${\mathrm{CaCO}}_3$ sealant in cracks.