Abstract
A sustainable solution for crack maintenance in geopolymers is necessary if they are to be the future of modern green construction. This study aims to develop self-healing biogeopolymers that could potentially rival bioconcrete. First, a suitable healing agent was selected from Bacillus subtilis, Bacillus sphaericus, and Bacillus megaterium by directly adding their spores in the geopolymers and subsequently exposing them to a precipitation medium for 14 days. Scanning electron microscope with energy-dispersive X-ray (SEM-EDX) analysis revealed the formation of mineral phases for B. subtilis and B. sphaericus. Next, the effect of biochar-immobilization and co-culturing (B. sphaericus and B. thuringiensis) on the healing efficiencies of the geopolymers were tested and optimized by measuring their ultrasonic pulse velocities weekly over a 28-day healing period. The results show that using co-cultured bacteria significantly improved the observed efficiencies, while biochar-immobilization had a weak effect, but yielded an optimum response between 0.3–0.4 g/mL. The maximum crack width sealed was 0.65 mm. Through SEM-EDX and FTIR analyses, the precipitates in the cracks were identified to be mainly CaCO3. With that, there is potential in developing self-healing biogeopolymers using biochar-immobilized spores of bacterial cultures.
Highlights
Geopolymers have become a promising greener alternative to concrete, due to their low carbon footprint and excellent mechanical, and chemical properties
The geopolymers with B. subtilis, B. sphaericus, and B. megaterium were observed after 14 days of immersion in the precipitation medium to select a suitable healing agent that could be used
Upon closer inspection using an optical microscope, trace amounts of mineral-like structures were observed on the crack surfaces of the geopolymers with B. subtilis and B. sphaericus
Summary
Geopolymers have become a promising greener alternative to concrete, due to their low carbon footprint and excellent mechanical, and chemical properties. They can be produced from a reaction involving an aluminosilicate source, which can come from waste byproducts like coal fly ash, and an alkaline solution that can induce the geopolymerization process. Being a cementitious material like concrete, they are still vulnerable to crack formation. This is undesirable, as it can cause the loss of structural integrity when geopolymers are used as materials of construction.
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