Performance optimization of cement-modified bajra mortar for sustainable heritage masonry conservation in Nepal

Abstract Traditional Bajra mortar, widely used in Nepal’s Malla- and Rana-era monuments, is highly compatible with historic masonry but exhibits low mechanical strength and limited documented performance, restricting its use in modern heritage conservation. This study investigates the mechanical performance of eight mortar mixes spanning cement, cement–surkhi, lime, and Bajra-based systems, with particular focus on enhancing Bajra mortar through a 10% addition of cement in lime-based mortar along with indigenous additives, namely surkhi (brick powder), chaku (concentrated jaggery), and maas (black lentil paste). All mixes were evaluated for workability, water absorption, and 35-day compressive strength following IS 2250:1981, ASTM C1403-00, and IS 6932 (Part VII):1973, respectively. Compressive behaviour was further validated using finite element analysis in ABAQUS based on the Concrete Damaged Plasticity model calibrated with the Kent–Park constitutive relationship. The incorporation of chaku and maas increased compressive strength from 2.10 to 3.14 MPa (≈ 50%), while the 10% cement-modified mix (1 L:2Su:0.05 Ma:0.4Ch:0.1 C) achieved the strength of 4.21 MPa, representing a 34.24% improvement over conventional Bajra mortar, along with a 47.9% reduction in water absorption. Numerical predictions agreed well with experimental results, with differences ranging from 3.81 to 7.72%. Life-cycle and cost analyses showed that the 10% cement-modified bajra mortar retained an embodied carbon of 316.49 kg-CO₂/m³, which was 53.66% lower than that of conventional cement mortar, while reducing masonry construction costs by approximately $8.32 per cubic meter. These findings demonstrate that limited cement addition, combined with traditional additives, produces a stronger, more durable, breathable, low-carbon, and cost-effective mortar suitable for sustainable heritage conservation in Nepal.