Isolation and Characterization of Strains used in Bacterial-Based Strategies for Accelerated Carbonation of Lime Mortars

Over the last century, lime has been quickly replaced by the uptake of Portland cement, mainly due to its faster hardening. Achieving earlier hardening in lime through faster carbonation is thus essential to help overcome one of limes limiting qualities. In this work, we isolated and selected strains suitable for use in lime mortars, and used bacterial suspensions to carbonate lime materials. An isolation campaign from a lime mortar wall returned two alkaliphilic isolates, Shouchella clausii and Shouchella patagoniensis. S. clausii was then further adapted to high pH (> 11) by adaptive laboratory evolution to produce a third strain. All three strains were then followed for a period of 14 days in serum bottles at pH 11 and gas composition of the headspace, intact/damaged cell populations and pH were measured. In parallel, lime mortar samples were incubated in a closed environment with bacterial suspension of the strains. The mortars were then tested at 7 and 14 days with thermogravimetric analysis to study the amount of carbonation through bacterial activity. Overall, S. patagoniensis produced more CO2, close to the estimated maximum CO2 uptake rate of lime, and carbonated the lime mortars faster and to a larger extent than the other strains. Finally, the bacterial suspensions were directly mixed with lime and the carbonation front was followed using analytical techniques. Once again S. patagoniensis led to faster carbonation. Overall, it was shown that bacterial-based strategies for accelerated carbonation of lime are a possibility despite the high alkalinity of the binder. ImportancePortland cement is the dominant binder used in most construction today, but until last century, lime was the ubiquitous construction material. The increase in use of cement has sprung from its higher strength and faster hardening, yet, lime still remain a relevant material, particularly in masonry structures and the built heritage. As such, novel lime materials are necessary to tackle some of the current limitations of lime, such as earlier hardening, which would not only make lime easier to work with but would also limit failure due to environmental conditions. As existing strategies to speed-up lime hardening have had limited uptake due to their reliance on expensive and often toxic chemicals, the need for novel solutions is in place. We show that bacterial-based strategies can help achieve a bio-based solution to go beyond the limitations of current strategies and open up new possibilities for microbial bioengineering in construction materials.