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The building sector is currently unsustainable in many respects mostly as a result of the material palette used. Bio-aggregate composites such as hemp-lime are high performance and sustainable to produce, meaning they offer a great opportunity to improve the sustainability of buildings in both construction and operation. The main current obstacles to the more widespread use of bio-aggregate composites is the conservatism of the construction industry and perceived risk of using alternative materials. Improving our understanding of what determines the properties of bio-aggregate composite will lower the perceived risk and help establish them within the sector. This thesis seeks to achieve an improved understanding of how the properties most relevant to industry, mechanical performance and thermal conductivity, are determined as a result of the inherent constituent and production variables of bio-aggregate composite. The level of understanding targeted is to put bio-aggregate composite on par with more established composite materials by enabling performance criteria led design through behavioural models. Specific focus is directed to the internal structure of the material which is anisotropic to a variable extent and so has a large and directional bearing on the behaviour. To assess the internal structure of bio-aggregate composites, a novel method was developed using two dimensional image analysis. The method is unique to the study and was able to provide a quantitative assessment of the degree of directionality within bio-aggregate composites allowing the impact of differing variables on the internal structure to be assessed. An extensive experimental program was undertaken to assess the impact of key variables on the internal structure, thermal conductivity, compressive strength and flexural strength of hemp-lime with a view to linking these behaviours to the production method via the internal structure. All tests were conducted bi-directionally to account for material anisotropy. The constituent variables considered were the aggregate particle size distribution and the ratio of aggregate to binding matrix while the production variables considered were the size of material layering, level of compaction and basic method of implementation, making the program reflective of the most easily specifiable aspects to industry. Results for the experimental program demonstrate that the material is highly anisotropic in its internal structure with the degree of directionality determined by both constituents and production. The properties of the composite were found to be bi-directional with variables having a directionally dependent impact on the behaviour as a result of affecting the structure. The findings from the experimental program indicate that there are serval opportunities for optimising the performance of the material within a walling context through manipulating the internal structure. Based on the experimental findings, a bi-directional model that accounts for the variable anisotropy of the internal structure was proposed and fitted to the experimental data gathered to predict compressive strength, flexural strength and thermal conductivity. The model was able to account for the directional impact of all variables considered with a close fitting to the experimental data sufficient for it to be a viable specification tool.