Sustainable biocementation of sandy soils: boosting MICP performance with metakaolin and waste tire textile fibers

The stabilization of sandy soils is a key challenge in geotechnical engineering due to their low cohesion and compressive strength. Microbially Induced Calcite Precipitation (MICP) has emerged as a promising bio-based technique, though its effectiveness can be limited by brittleness and low tensile resistance. This study investigates the improvement of MICP-treated sandy soils through the combined incorporation of metakaolin (MK) and waste tire textile fibers (WTTFs). Sandy soil samples were treated using MICP with varying MK contents (0%, 3%, 6%, 9%, and 12%) and cementation solutions of different molarities (1.5, 2.0, 2.5, and 3.0 M), and then cured for 3 and 7 days. All results represent short-term mechanical improvement after 3 and 7 treatment days only; no long-term durability tests were conducted. The optimum mix of 3% MK with a 2.5 M cementation solution applied over 7 days yielded the highest unconfined compressive strength (UCS), stiffness, and energy absorption. This optimal combination was further reinforced with WTTF at 0.5%, 0.75%, and 1.0% by weight, with a fiber length of 10 mm. A fiber content of 0.75% resulted in significant increases in UCS, indirect tensile strength (ITS), and ultrasonic pulse velocity (UPV), without compromising ductility. Microstructural analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed improved calcite crystal formation, denser interparticle bonding, and more uniform crystal distribution on surfaces. The combined use of MK and WTTF accelerated calcite precipitation and improved mechanical behavior through physical filling and nucleation effects of MK along with fiber bridging of WTTFs. Compared to MICP alone, the optimized mixture of 2.5 M and 7 injection cycles achieved up to 655% higher UCS and 176% greater UPV. These findings highlight the potential of integrating industrial waste materials into bio-cementation processes to enhance soil strength in poorly graded sands, though further validation on other soil types is needed before broader application. This is the first study to demonstrate the combined use of MK and WTTF in MICP-treated sandy soils.