Proposing a new sustainable approach for sand improvement using biologically-derived calcium phosphate cement

Bio-mediated soil improvement methods keep on gaining the attention of geotechnical engineers and researchers globally due to their nature-based elegance and eco-friendliness. Most prevalent bio-mediated soil improvement methods include microbially induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP). During their processes, the bacteria/free urease hydrolyzes the urea into ammonium and carbonic acid, which is accompanied by a considerable increase of alkalinity (about pH 9.0). The major problem associated with the above techniques is the release of gaseous ammonia that is extremely detrimental. Therefore, this study aims to propose a new sustainable approach involving lactic acid bacteria to facilitate the calcium phosphate mineralization for the strengthening of sand matrix. The major objectives of this investigation are: (i) to evaluate the urease activity of the lactic acid bacteria under different temperatures, pH conditions and additions of metal ions, (ii) to assess the treated sand matrix, (iii) to perform cost analysis. The outcomes indicated that Limosilactobacillus sp. could effectively facilitate the urea hydrolysis, hence increasing the pH from acidic to neutral and providing a desirable environment for the calcium phosphate to mineralize within the voids of the sand. The addition of 0.01 % Ni²⁺ in culture media was found to enhance the urease activity by 38.8 % and compressive strength over 40 %. A combined formation of amorphous- and whisker-like precipitates could bridge a larger area at particle-particle contact points, thereby faciliating a strong force-network in sand matrix. The mineralized calcium phosphate compound was found to be brushite. The cost herein for producing 1 L treatment solution was estimated to be about 2.5-folds and 11.8-folds lower compared to that of MICP and EICP treatment solutions, respectively. Moreover, since the treatment pH could potentially be regulated between acidic-neural range, it would greatly control the release of gaseous ammonia. With several environmental and economical benefits, the study has disclosed a new sustainable direction for sand improvement via the use of lactic acid bacteria.