Abstract
Despite lime mortars' performance and durability features as reflected in the existing old historic buildings, its 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 its flexibility and permeability in the current assessment Building Standards, with more emphasis on compressive strength (relative to Portland cement), an area where lime is considered naturally weak. However, these relegated features underscore limes’excellent performance and durability characteristics. The research therefore attempted to leverage these features through evaluation of synergised lime composites in order to facilitate development of improved and sustainable eco-friendly lime based mortars.Thus, considering the material's weaknesses, this study intended to integrate advantageous features of Time and cement’ as a composite on one part, and Time and Ground Granulated Blast Furnace Slag (Slag)’ on the other, at the expense of their known drawbacks. The study sought to evolve low carbon composite construction materials in forms of Time-cement' and Time-slag’ mortars with cement or slag serving as a partial replacement for lime in each case.The methodology involved mortars with the same Binder/Aggregate (B/A) mix ratio(1:3) using five different compositions of "cement-lime’ and ‘slag-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 air content and bulk density constituted the fresh state assessment parameters, mechanical characteristics, microstructural features, total water absorption as well as thermal conductivity, 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 pm) and (5 - 20 pm) into (10 nm - 2 pm) range. This alteration is associated with porosity reduction by up to 11 %. Significant improvements in the mechanical strengths of the composite (from 0.61 N/mm2 in lime mortar to 12.34 N/mm2 in the composite with highest cement content) is recorded. 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. As cement content is inversely proportional to porosity, low cumulative porosity across three low pore regions of Inter-Nano pores (1 - 10 ran), Super-Nano pores (10 - 100 nrn) and SubMicro pores (0.1-1 pm) of lime-cement composite is observed to be synonymous with low ‘compressive to flexural strength ratio' (fc/ff), thereby defining a measure of flexibility. Thus, composites would exhibit higher flexibility in the descending order of cement content. As cement addition leads to an increase in thermal conductivity of lime mortar, inverse relationship is established between the progressive cement content and the total water absorption of the mortar. Slag on the other hand has limited impacts on the PSD of lime mortar, with the increased porosity among composites. Its addition facilitates slight improvement in the compressive and flexural strengths of the mortar with the highest mechanical strength achievable in composite with equal lime-slag compositions. In spite of a small increase in the flexural strength, the composites are characterised with well pronounced deformation absorption tendencies. The higher porosity associated with slag hydration induces formation of micro-cracks, which in turn lowers thermal conductivity of the composite. Also, slag induces small but progressive reduction in total water absorption of lime mortar. Despite these impacts,none of the assessed parameters (porosity, mechanical strengths, thermal conductivity and total water absorption) are directly proportional to the increase in slag content.Review of the relevant existing building standards (BS EN 459-1 (BSI, 2015), BS EN459-2 (BSI, 2010). etc.) also shows a lack of necessary assessment parameters for such important inherent features of limes as flexibility and permeability.
Substitution of lime with cement or slag therefore has a significant impact on improvingperformance of lime mortar particularly, with respect to mechanical strengthdevelopments, thermal conductivity and water absorption. Despite the improvedcompressive strength, flexibility of lime is observed not to be lost completely in thecomposites. This is subject to the amount of lime substituted. Hence, relative to specificpurposes, lime revival needs to be encouraged and promoted in form of lime-cementand lime-slag composites. However, proper review of relevant building standardsremains a necessity for its overall technical acceptability.
| Date of Award | 2017 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Agnieszka Klemm (Supervisor) & Paul Baker (Supervisor) |