Life cycle assessment of recycled construction materials : methodology framework development and results evaluation

Abstract

The demand for green construction materials and products is burgeoning globally due to the shortage of natural materials and the associated environmental consequences. In addition, the sustainable management of rapidly increasing waste materials in Hong Kong is urgent due to the shortage of and the associated environmental burdens on landfills. With the promotion of environmental protection in the construction industry, the mission to achieve more sustainable use of resources during the production process of construction materials/products is becoming increasingly important. In this study, the environmental evaluation of valorized waste materials and their utilizations were evaluated using life cycle assessment (LCA) techniques, with the aim to identify a resource-efficient solution for waste materials as well as construction sustainability. LCA, a holistic approach for evaluating the environmental impacts of a product or system, was adopted in this study. In order to select sustainable aggregates, a comparative environmental consequence of recycled aggregates production from construction and demolition (C&D) waste and waste glass, and natural aggregate production from virgin materials was assessed using LCA approach. Environmental performance of concrete paving and partition wall blocks prepared with natural and recycled materials were also evaluated by LCA. Environmental impacts of different types of cement produced in Hong Kong were assessed based on their strength class, and several potential sustainable strategies were proposed for impacts reduction. To promote sustainability in the concrete industry, various supplementary cementitious materials (SCMs), such as fly ash, blast furnace slag and silica fume, have been used to replace cement in the production of concrete. The nature of these waste materials has been changed from wastes to by-products according to international conventions. As such, their environmental impacts are required to be reallocated consequently in LCA. However, the choice of the impacts distribution procedure for LCA of concrete incorporating SCMs is a methodological challenge. This study critically evaluated different approaches with a case study on concrete production in Hong Kong. Based on the findings, a decision matrix was provided to select the most suitable approach which can ensure appropriate allocation approaches of the by-products. The assessment of social sustainability has seldom been carried out during the selection of materials due to the lack of available social life cycle assessment (S-LCA) tools. This study developed a new single score based methodology, namely a social sustainability grading model, for assessing the social sustainability performance of construction materials. A case study on construction materials was conducted to illustrate the implementation of the method using case-specific first-hand data. On the basis of the collected data, assumptions made and the defined system boundaries, the findings of the study showed the use of recycled aggregates reduced 49-59% of total environmental loads when compared that of natural aggregates. The results also demonstrated that eco-blocks consumed 26-32% lower energy, and emitted 17-20% lower greenhouse gases compared to the natural blocks. In addition, concrete partition wall blocks prepared with concrete slurry waste and recycled aggregates achieved higher sustainability than the conventional ones. The LCA results demonstrated that ordinary Portland cement production had high environmental impacts as a result of the use of associated raw materials and burning of fossil fuels. Significant environmental impact reductions associated with cement production can be achieved by the use of glass powders as part of the raw materials to substitute clinker, and bio-fuel produced from wood wastes as a co-fuel with coal. The system expansion can be the preferred approach over allocations for assessing the environmental impacts of concrete/concrete products produced with SCMs. Based on the proposed S-LCA method and sustainability index for conducting S-LCA, recycled aggregates from waste materials scored higher (about 31-34%) social sustainability scores than the imported natural aggregates. In addition, recycled aggregates and natural aggregates achieved 'sustainable' and 'neutral' rating sustainability levels, respectively. The results of this research can be used as a guideline for sustainable waste management and resourceful utilization of wastes, a basis for the selection of sustainable construction materials, a means of a comprehensive environmental and social sustainability assessment of construction materials and products, as well as a reference to improve the sustainability performance in the construction industry.