Durability testing and characterisation of alkali activated materials with fibre reinforcement

Abstract

Over the years, alkali activated materials (AAM) have gained interest as a sustainable alternative to conventional concrete made with ordinary Portland cement (OPC) as OPC production is a major contributor to CO₂ emissions in the construction sector. The need of concrete in the construction sector continues to grow, and therefore, the environmental impact is a legitimate concern. AAM-concrete has shown to be a promising alternative to OPC-based concrete. Along with the elimination of OPC it gives the possibility to incorporate waste streams to become an even more sustainable alternative. In this research the aim is to investigate the durability of alkali activated materials with basalt fibre (BF) reinforcement together with the incorporation of local waste streams such as wood waste ash (WWA). During the research the mechanical properties of test specimens were assessed by compression strength, residual flexural tensile strength (RFTS) and freeze-thaw (FT) testing on mixes with three different ratios of BFs. The strength development is assessed by compression testing after curing for 1, 7 and 28 days. During this period a logarithmic strength development was found with the highest strength increase during the first 7 days of curing with no significant influence of the BF reinforcement. However, the reinforced specimens showed clear crack-bridging properties. During RFTS testing the same conclusion was found. The prisms without fibres failed entirely during testing, but showed similar load at limit of proportionality as the prisms reinforced with fibres. This shows that the initial resistance against failure is delivered by the AAM-matrix itself, after which the fibres contribute to higher load resistance after the initial crack in the reinforced prisms. For both the compression and residual flexural tensile strength testing, the results were mainly influenced by the air content. The compressive strength results confirmed that the air content was more of vital factor in the compressive performance than the BF content. Resulting in overall better results with lower air contents. The FT test was performed according to SN-CEN TS12390-9, which is standardly used for OPC-concrete at UiT. Unfortunately, the sample preparation is not suitable for AAM-concrete as the silicone sealing failed between 14 and 28 days of FT exposure for the majority of the specimens with some of them failing before completing 14 days of exposure. Due to time constrain, no repetition of FT testing was performed. To examine durability of AAM-concrete further research is needed with modified sample preparation techniques. It was found that AAM-concrete achieved high early age compression strength with a maximum of 54 MPa on day one and 91 MPa on day 28 after casting. There was a negligible influence of BF except for the crack bridging properties. During the flexural testing all mixes showed similar LOP, but the 10,8BF gave the highest RFTS across all CMOD levels confirming the positive influence of high fibre content on the post-cracking performance of AAM-concrete.

Key words - Alkali activated materials (AAM), Durability, Compressive strength, Residual flexural tensile strength (RFTS), Freeze-thaw resistance (FT), basalt fibres (BFs), wood waste ash (WWA)