Impact of changing microstructural compositions of lime based mortar on flexibility: Case study of sustainable lime-cement composites

Abstract

Lime mortar, an age-long building material is primarily popular for its flexibility, the basis of its ability to accommodate masonry deformation, hence durability. However, lime's characteristic delayed setting/hardening time, low mechanical strength and poor internal cohesion often characterised by volumetric changes have put its use into decline. These shortcomings have therefore relegated relevance of this fundamental flexibility feature which underscores limes' excellent performance and durability characteristics. The research therefore attempted to leverage this feature through evaluation of synergised lime composites, using cement as a partial replacement for lime. This is with a view to integrating advantageous features of 'lime and cement' as a composite, at the expense of their known individual drawbacks. The methodology involved mortars with the same Binder/Aggregate (B/A) mix ratio (1:3) using five different compositions of 'cement-lime' binders (i.e. 1:1, 1:2, 1:3, 2:1 and 3:1). The research focused on comparative evaluations of each composition in both fresh and hardened states, with the latter covering twelve-month curing period. While Water/Binder ratio constituted the fresh state assessment parameters, mechanical characteristics and microstructural features were evaluated in the hardened state. Results of the investigation show that progressive addition of cement significantly changes pore size distribution (PSD) of lime mortar from predominant pore sizes between (0.5 - 5 μm) and (5 - 20 μm) into (10 nm - 2 μm) range. This alteration is associated with porosity reduction by up to 11%. Significant improvements in the mechanical strengths of the composite is recorded as both the compressive and flexural strengths of the composite with 75% of cement is 18 and 6 times higher respectively, compared with the reference mortar. However, progressive addition of cement is proportional to the E-value of the composite (with a clear linear relationship), leaving a negative impact on the flexibility. Nonetheless, all the composites investigated exhibit elastic behaviours relative to the basic lime mortar. In particular, composite with cement addition up to 33% of the binder compositions exhibits deformation tendencies under compression. However, mortars with higher cement compositions (i.e. above 33%) would strain linearly until failure occurs suddenly with minimal deformation. Substitution of lime with cement therefore has a significant impact on the microstructural compositions of lime mortar, and subsequent improvement on the performance of the composite. Despite the improved mechanical strengths, inherent flexibility of lime is maintained though negatively impacted, subject to the amount of lime substituted. Hence, relative to specific purposes, lime revival can be promoted in form of sustainable lime-cement composites.