The relationship between chloride resistance and microstructure of alkali-activated flyash-slag concrete

The relationship between chloride resistance and microstructure of alkali-activated flyash-slag concrete

Due to the threat of global warming, the development of sustainable construction and building materials has attracted extensive attention. Alkali-activated materials (AAMs) are considered as a promising candidate to ordinary Portland cement (OPC). Among all AAMs, the ones based on Class F fly ash (FA), granulated blast furnace slag (GGBFS) and their blends are most intensively investigated due to the huge annual production of these two precursors. For the long-term application of alkali-activated fly ash-slag (AAFS) in chloride-containing environments, it is indispensable to understand its chloride resistance.

Although the chloride resistance of AAFS has been studied for decades, there are still four issues deserving to be further studied: 1. Most previous studies were limited to short-term tests, which may be not suitable for AAFS and cannot show long-term results; 2. The influence mechanism of different parameters on chloride resistance is still not well understood; 3. The existing data is insufficient to establish the long-term prediction model; 4. The measurement of AAFS modified by Mg-based admixtures is not completely accurate. In view of the above four issues, corresponding research has been carried out in this project.

In Chapter 3, AAFS concretes with different parameters (GGBFS content, w/b, Na2O content, SiO2 content and s/a) were designed. The chloride resistance of them was investigated by natural chloride diffusion test (NCD) in Chapter 4. The results show that higher GGBFS content, lower w/b, higher Na2O content and lower SiO2 content generally improved the chloride resistance of AAFS concretes, while s/a ratio had no obvious effect.

The corresponding microstructure (including phase evolution and pore structure) was analysed in Chapter 5. Compared with the type of reaction products, the quantity of reaction products seemed to be more important for the chloride resistance of AAFS concrete. There was a good correlation between chloride resistance and volume of capillary pores. Besides, the threshold pore diameter also had a certain effect on chloride resistance.

The long-term chloride diffusion in different AAFS concretes and the corresponding time to corrosion initiation were predicted in Chapter 6. According to the prediction results, correctly formulated AAFS concretes (50%-70% GGBFS content, 0.45 w/b, 6% Na2O content and 6% SiO2 content) might successfully extend the initiation phase for decades or even more than 100 years.

In Chapter 7, The effects and mechanisms of different Mg-based admixtures were investigated. In AAFS, MgO could fill capillary pores and lead to the formation of more Friedel's salt. Mg-Al-CO3 LDH and CLDH seemed to result in more air voids, but the enhanced chloride binding had a greater positive impact on chloride resistance.

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