Pore Structure as a Determinant of Flexibility in Sustainable Lime-Cement Mortar Composites

Abstract

Flexibility of Lime Mortar resulting from its good permeability feature through its relatively large interconnected pore structure makes it popular as an old sustainable building material. This characteristic feature enhances its capability to suppress masonry deformation. However, its mechanical strength is weak. To address this shortcoming, cement is added to form a stronger composite. This study, therefore, evaluates the pore structure of the resulting lime-cement composite, with a view to assessing impact and sustainability of the lime’s flexibility in the overall performance of the composite. Adopting equal mix ratio (1:3) of Binder/Aggregate (B/A), mortars were prepared using lime as a lone binder, as well as other five different compositions in the ascending and descending order of lime and cement binders (i.e., 1:1, 1:2, 1:3, 2:1 and 3:1). Each composition was assessed in fresh state using Water/Binder ratio, Air Content and Bulk Density. For the hardened state assessments, mechanical characteristics and microstructural features evaluated through six (6) and twelve (12) month curing periods respectively, were considered. Results of the investigation reveal that low cumulative porosity (i.e., summation of the porosities) across the three pore regions of Inter-Nano pores (1 – 10 nm), Super-Nano pores (10 – 100 nm) and Sub-Micro pores (0.1 – 1 µm), for each of the mortars, is observed to be synonymous with low cement content, low Modulus of Elasticity, low (fc/ff), and thus, high flexibility. This observation is supported by high linear coefficient of correlation (R2 = 0.89) for the equation describing both ‘cumulative porosity’ (within the referenced three pore ranges) and cement content. Thus, it could be inferred that despite cement addition with resulting alteration in the pore structure of the composite, presence of lime would retain flexibility property of the composites, with more flexibility in the descending order of cement contents. Considering the impacts of pandemic climate change, growing energy costs and human activities on the environment, this study therefore attempts to promote revival of an old but partially abandoned sustainable lime based mortars, with a view to facilitating improved lime mortar performances, conservation of energy resources and overall protection of the environment.

Sustainable Hybrid Lime Mortars for Historic Building Conservation: Incorporating Wood Biomass Ash as a Low-Carbon Secondary Binder

“…Several studies [86,90,91] suggest that the relation between compressive and flexural strength is proportional to the modulus of elasticity in tests lasting from 1 to 6 months, assuming that a low ratio between compressive and flexural strength (f C /f F ) correlates with a low modulus of elasticity. Similarly, the lower the modulus of elasticity values and the lower the f C /f F ratio, the higher the flexibility of the mortar is likely to be.…”