High Performance Reinforced Hemp-Lime Nanocomposite Construction Materials

Abstract

The aim of the project is to develop high performance, lightweight, durable, and environmentally friendly construction materials. Construction materials should have high compressive and flexural strength, low porosity, low thermal conductivity, low shrinkage and high water vapour permeability (breathability). The following materials were selected to achieve this aim; lime was selected as the base matrix material with hemp (fibres and shives) and wood glue (Poly vinyl acetate, PVAc). Specially selected nanomaterials were used as fillers. The properties of the developed material were used to design an eco-friendly wall consisting of a central 'Core' which will be the load bearing element, highly insulative layers with low thermal conductivity 'Insulators' and outer rendering materials for enhanced aesthetics purpose and breathability 'Renders'. The research conducted in this project enabled a number of different construction materials to be developed, each exhibiting their own characteristics to enable the aims and objectives of the project to be achieved. A summary of the findings is as follows: A load bearing wall requires relatively high compressive and flexural strengths of about 5 MPa and 4.0 MPa, respectively or higher. The 'Core' material designed consisted of 10 wt. % hemp fibres, 12 % PVAc/L, 4 wt. % nZnO (nanozinc oxide) and lime (NHL5, which its quantity was 1 kg for each batch of 4 samples for the whole project) and prepared using air curing method. The compressive strength was 17.7 MPa and the flexural strength was greater than 7.0 MPa, which were the highest results of strength throughout the project. The same mentioned mixture (10 wt. % hemp fibres, 12 % PVAc/L, 4 wt. % nanozinc oxide of lime) was cured using 'Oven-drying', the strengths in compression and flexure were still considerable, being 10 MPa and 4 MPa respectively which were more than the minimum limit of loadbearing material. This material, therefore, due to its high compressive strength, used as the 'Core' load bearing element of the proposed wall in the absence of a timber framework. The 'Insulator' was developed using a water removal 'Solvent exchange' technique and the mixture was 20 wt. % hemp shives, 12 % PVAc/L, 4 wt. % nanozinc oxide and lime. The thermal conductivity was 0.06 W/mK, much lower than that of pure lime which was 0.16 W/mK. The 'Insulator' will be applied in two layers, one on either side of the 'Core'. The 'Render' was developed using lime and 4 wt. % nanozinc oxide by wt. of lime and cured via air curing. It possessed a low porosity (18 %) in comparison to that of pure lime (36.4 %) and low thermal conductivity,  (0.13 W/mK) in comparison to pure lime 0.16 W/mK cured by solvent exchange. Shrinkage was lowest in a Render material containing 4 % wt. nZnO, averaging 750 microstrain (μs) compared to the control sample (lime only) of 2428 μs. Chopped fibres, PVAc and nanozinc oxide were used for the first time with lime and no other examples of this exist in the literature (in the best knowledge of the researcher). Water vapour permeability (breathability), which is a beneficial property for construction materials was generally enhanced by using nanomaterials and the optimum breathability was achieved by adding 2 wt. % nanoclay to lime. The results achieved were used to design an eco-friendly wall in accordance with the Building Regulations. The U-value target was 0.18 W/m2K and the results show that a decrease in thickness of 40 mm could be achieved by using optimum materials V developed in this project in comparison to traditional hemp shives/Lime walls, in addition to eliminating timber studding which is normally required to provide support to non-loadbearing lime based walls.