A crude extract from jack beans (Canavalia gladiata) is demonstrated to be an effective source of urease enzyme for biocementation via enzyme-induced carbonate precipitation (EICP). Test tube tests of crude and purified extracts from jack beans, jack bean meal, soybeans, and watermelon seeds show that the crude jack bean extract results in the highest unit yield, defined as urease content per initial mass of source material, among these four plant sources. The efficacy of EICP using crude jack bean extract for biocementation was compared with the efficacy of three commercially available enzymes by biocementation of a granular soil. Unconfined compression tests on the granular soil specimens subject to biocementation via EICP demonstrated that the crude extract and the less purified commercially available enzyme were actually more effective than commercially available highly purified urease enzymes at enhancing soil strength, an effect attributed to the presence of complementary proteins in the less purified enzyme sources. The simple technique used to produce the crude extract from jack beans significantly lowers the cost of EICP, eliminating a major barrier to practical applications, including infrastructure construction and environmental protection.

Three different batches of “standard” Ottawa 20-30 sand were biocemented using enzyme-induced carbonate precipitation (EICP). The specimens from each batch were treated using the same biocementation procedure. The unconfined compressive strength (UCS) of the treated specimens varied from 0.22 to 1.6 MPa depending on the batch of sand used to prepare the specimens. These results show that sand of the same predominant mineral composition (e.g., $>98\%$ silica), of the same grain size distribution, and from the same geological formation but from different quarries within the formation do not necessarily respond in the same manner to biocementation. Hence, standard sands for mechanical geotechnical testing should not be considered a priori to be a standard sand for biogeotechnical testing.

The mechanical properties of calcium carbonate minerals formed by enzyme-induced carbonate precipitation (EICP) were studied using nanoindentation. Two types of precipitates were considered: (i) a “baseline” precipitate, synthesized via urea hydrolysis in an aqueous solution of urease enzyme, urea, and calcium chloride; and (ii) a “modified” precipitate, synthesized from a similar solution, but with the inclusion of nonfat dry milk. While both precipitates predominantly comprised calcite, X-ray diffraction and Raman spectroscopy indicated broader peaks in the modified precipitate, implying differences in the crystal structure of the two precipitates. Both precipitates were polycrystalline and had a higher average indentation hardness (H) and a lower indentation modulus (M) compared with the values for single calcite crystals reported in the literature. The ductility of the precipitates was quantified by the ratio M/H. The modified precipitate had a higher average M/H, implying greater ductility. The increased ductility of the modified precipitate results in higher resistance to crack propagation. In sands biocemented using the modified EICP solution, the increased ductility of the precipitate, in addition to preferential precipitation at interparticle contacts, may contribute to relatively high unconfined compressive strengths at low carbonate contents.

Calcium-based chemical stabilizers are traditionally used for the stabilization of weak subgrade soils. However, these traditional stabilizers incur significant environmental costs which make them less desirable. As a result, alternative materials based on recycled waste and industrial byproducts are becoming popular. Geopolymers (GP) are a new addition to this list that are gaining traction because of their environmental benefits. GPs improve the soil by precipitation of a polymer gel as a result of mixing an alkaline activator (activator) with an aluminosilicate source (precursor). The effectiveness of GP and the improvement in engineering performance of GP-stabilized soils is contingent on sound GP synthesis parameters. This study investigates the effectiveness of GP-based treatments in improving weak soil and evaluates the significance of various parameters in contributing to the efficacy of GP treatment. As a result, a group of eight GP mixes was designed with a range of values for controlling parameters namely water/solid, activator/precursor, Si/Al and cation/Al ratios. A fat clay was treated by the application of these GP mixes at a dosage of 10% by dry weight. Improvements in mechanical strength were evaluated through unconfined compression strength testing. This was followed by statistical inference on laboratory strength data using analysis of variance (ANOVA) and post hoc tests. Ultimately, parameter importance was quantified by the random forest (RF) regression model. The results indicated that GP-based treatments enhanced the strength of untreated soil. Additionally, GP with higher aluminosilicate content and sufficient activator performed better. Overall, this study provides insight into the relative contribution of various GP synthesis parameters to the performance of GP-stabilized subgrade soils.