Abstract
The high autogenous shrinkage of alkali-activated materials made from slag and fly ash is recognised as a major drawback with regard to the use as construction materials. In this study, metakaolin was introduced into the alkali-activated slag-fly ash (AASF) paste to mitigate the autogenous shrinkage. The shrinkage mitigation mechanism of metakaolin was explained by studying the influences of metakaolin on the microstructure, shrinkage related properties, and mechanical properties of AASF paste. It was found that adding metakaolin could significantly reduce the chemical and autogenous shrinkage of AASF paste. This shrinkage mitigation is accompanied by a decrease in the alkalinity of AASF paste pore solution, a reduced drop in internal relative humidity, and an increase in porosity of AASF paste. Moreover, the incorporation of metakaolin does not change the type of the reaction products, but greatly delays the formation of the reaction products of AASF paste. The addition of metakaolin, above 5% of the binder, results in lower 28-day compressive and flexural strength of AASF paste.
Highlights
Alkali activated materials (AAMs) are promising as a desirable alternative to ordinary Portland cement (OPC)
The paste with more MK showed lower heat release. These results indicate the presence of MK can significantly reduce the early-age reaction rate and slightly reduce the total reaction degree of activated slag-fly ash (AASF) paste
There is a part of the autogenous shrinkage that is irrelevant to the internal relative humidity (RH), especially in the acceleration period [24]. This finding was confirmed by the results reported in [60], which showed a limited mitigating effect of internal curing on the autogenous shrinkage of AASF in the very early age
Summary
Alkali activated materials (AAMs) are promising as a desirable alternative to ordinary Portland cement (OPC). A wide range of aluminosilicate materials that are reactive in alkaline environments can be used as precursors to synthesise AAMs [4,5], among which, blast furnace slag and coal fly ash are the most widely utilised industrial by-products. Two types of fly ash have been used for AAMs production, namely high-calcium. Compared with Class C fly ash, Class F fly ash (reactive CaO 10%), due to its wider availability, is more intensively studied in the world and the AAM paste synthesized from Class F fly ash usually exhibits better volume stability [7]. Alkali activators with the modulus (SiO2/Na2O) in the range of 0.5–1.5 are found to promote the formation of a dense microstructure, resulting in high strength for slag and fly ash-based AAMs [9]
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