The durability of fly ash geopolymer and alkali-activated slag concretes

摘要

The implementation of sustainable development in civil engineering society has led to the use of new materials with low environmental impact. Traditionally Ordinary Portland cement (OPC) is the primary material used in the production of concrete. However, the manufacture of OPC has led to environmental concerns over the energy needed to produce the material, the depletion of the quarried resources and the production of CO2. This has led to the use of fly ash and slag, waste products, as cement replacement materials due to their characteristic pozzolanic and latent hydraulic properties. Recent research has shown that it is possible to develop concrete based solely on fly ash and slag activated directly by alkali solution, without the presence of OPC, known as fly ash geopolymer and alkali-activated slag (AAS). A major benefit is that the greenhouse gas emissions produced by fly ash geopolymer and AAS are reduced compared to those of OPC, which depends on the limestone calcination process and produces around 5% of worldwide greenhouse emissions. In this research, the mechanical and durability properties of fly ash geopolymer and AAS concretes have been studied. The study has focussed on the relationship between durability and the mechanical properties of fly ash geopolymer and AAS concretes over the long term. The micro-structure was also studied to support the analysis. The existing concrete codes and standards (Australian Standard, and Concrete Institute of Australia recommended practice for fly ash geopolymer concrete) have also been studied to assess their validity when predicting material properties for these new materials. The results show that fly ash geopolymer and AAS concretes exhibit a comparable compressive strength, lower modulus of elasticity but higher tensile strength compared to OPC concrete. In terms of mechanical properties, the short term behaviour of AAS concrete is better than fly ash geopolymer concrete, however fly ash geopolymer concrete showsa better performance in the long term. The AAS concrete exhibits a reduction in strength over time due to the development of micro-cracks which leads to inferior strength in the long term, while the fly ash geopolymer concrete strength shows an improvement in strength with age due to the slow formation of the geopolymeric network. In terms of durability properties (permeation, resistance to chloride and carbonation), AAS concrete demonstrates a better performance compared to fly ash geopolymer concrete. However in the longer term the growth of the micro-cracks with time raises a question about the long term performance of AAS concrete. The existing Australian Standard should not be applied to AAS concretes due to the reduction in performance over time. However in the case of fly ash geopolymer concrete, the standard may conservatively be applied to the prediction of tensile strength but not to the prediction of modulus of elasticity which the standard over-predicts.