Characterization of isolates used in bacterial-based strategies for accelerated carbonation of lime mortars.

Portland cement largely replaced hydraulic lime over the past century because of its rapid hardening. Achieving earlier hardening in lime through faster carbonation is thus essential to help overcome one of lime's limiting qualities. Here, we isolated two alkaliphilic bacteria, Shouchella clausii and Shouchella patagoniensis, from a lime mortar wall. S. clausii was then further grown in high pH (>11) by adaptive laboratory evolution to acclimate a third isolate. Bacterial suspensions of all three isolates were followed for 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 isolates and analyzed with thermogravimetric analysis after 7 and 14 days to quantify carbonation. S. patagoniensis produced more CO₂, close to the estimated maximum CO₂ uptake rate of lime, and carbonated the lime mortars to a larger extent than the other isolates. Finally, the bacterial suspensions were directly mixed with lime. A linear and homogeneous carbonation of the paste samples was measured compared to water-based pastes, and the development of Liesegang patterns was observed upon phenolphthalein spreading. All this indicated that the organic addition altered the carbonation dynamics of the material, although bacteria did not accelerate carbonation relative to media alone and inhibited it relative to water-based paste. Still, a relationship between bacterial activity, CO₂ emission, and carbonation rate was established, but practical aspects of bacterial delivery into lime must be addressed.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 remains 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 could be a viable option to go beyond the limitations of current strategies, but limitations are in place.