Improving freeze-thaw resistance of alkali-activated slag by admixtures

Abstract

Alkali-activated material (AAM) is one of the most attractive alternatives to ordinary Portland cement (OPC). Recently, more and more research interests are devoted to durability performance. In cold areas, freeze-thaw resistance is a crucial durability factor for concrete. However, the understanding of the freeze-thaw resistance of AAMs is minimal. Accordingly, the subject of this thesis is drawn forth. The main aim of this study was to investigate the effects and mechanisms of using admixtures (SAP and AEA) to improve the freeze-thaw resistance of alkali-activated slag (AAS).
Regarding the general properties, such as workability and mechanical properties, it was found that the influence of adding SAP or AEA is acceptable. Then, the air-void systems created by SAP and AEA were reconstructed and characterized by the micro-CT scan. It was found that both SAP and AEA successfully entrained air voids into AASM, while the characteristics of the resultant air-void system were quite different. Further analysis on the surface condition of AASM undergoing 28-day sealed curing or preconditioning procedures in the CDF test, and the ASTM C672 test revealed that AASM shows a considerable surface microcracking potential. Samples sealed for 28 days showed a considerable extent of microcracking due to autogenous shrinkage. Samples that underwent 14-day drying after 7/14 days’ curing showed significant microcracking due to autogenous shrinkage and drying shrinkage. With the addition of SAP, the cracking caused by autogenous shrinkage was minimized, but the cracking caused by drying was only slightly reduced. With the addition of AEA, there was only a minor improvement in surface integrity.
The freeze-thaw resistance of AASM with or without the addition of SAP or AEA was investigated in the form of surface scaling damage by a modified CDF test. Plain AASM showed poor surface scaling resistance mainly attributed to the pre-existing surface microcracking. The addition of SAP and AEA successfully improved the surface scaling resistance of AASM. The improvement was more significant by adding SAP due to the effect of improving surface integrity. Based on the SAP mixtures, a good correlation was found between the SAP air-void system and the resultant surface scaling resistance. The final cumulative surface scaling generally decreased with higher entrained air content, denser air voids distribution, and smaller air voids size. A preliminary logarithm model was developed for the relationship between the reduction in surface scaling and the equivalent water/binder ratio and the air-void system. Finally, upscaling tests on the concrete scale also found that the addition of SAP improved the surface scaling resistance of AASC.
Overall, in this study, the freeze-thaw resistance of AAS was successfully improved by the SAP and AEA. The effects of adding SAP/AEA and the related factors on the selected properties of AASM were revealed. The mechanisms behind these observations were also understood and generated a series of further research possibilities. It is hoped that these findings and observations can give some guidance to the industrial applications of AAS and some inspiration to the scientific community.