Bio-enhanced lime mortars under atmospheric CO₂ variability : implications for heritage conservation and sustainable construction

Abstract

The increasing atmospheric CO₂ levels in urban environments significantly influence lime mortar carbonation kinetics and mechanical properties, presenting critical implications for heritage restoration applications. This study investigates the performance enhancement of organically modified lime mortars (OMLMs) under controlled CO₂ conditions: ambient (400 ppm) and accelerated (1000 ppm). Four mortar formulations were synthesized using hydrated lime, siliceous aggregate, and two organic additives, kadukkai (Terminalia chebula) and jaggery, compared against an unmodified reference composition. Comprehensive characterization techniques were performed using uniaxial compressive strength, quantitative X-ray diffraction with Rietveld refinement, thermogravimetric analysis, and mercury intrusion porosimetry following 56-day controlled carbonation. Results demonstrated that OMLMs exhibited enhanced mechanical performance, with jaggery specimens showing optimal development. Under accelerated CO₂conditions, organic additives yielded 33 % compressive strength improvements relative to reference mortars, with improved pore structure refinement and crystallinity indices. Correlative multimodal characterization revealed that organics facilitate secondary hydration product nucleation and calcium oxalate formation concurrent with carbonate precipitation. Microstructural characterization demonstrated porosity reduction to 27.7 % in mixed mortar under accelerated conditions vs 36.8 % for natural reference. Mineralogical analysis confirmed the formation of stable weddellite phases contributed to enhanced durability through matrix densification. This research provides scientific validation for traditional methodologies while demonstrating the potential of OMLMs for enhanced performance in high CO₂ urban environments, offering sustainable solutions for both heritage restoration and contemporary construction applications.