Efflorescence of microwave-heated alkali-activated cement synthesized with ultrafine coal combustion ashes

Abstract

Using coal combustion ashes to synthetize alkali-activated cements (AACs) has significant advantages in the sustainable development of coal industry and exploiting cementitious materials for special occasions. Microwave heating and ultrafine precursors have been reported to improve the reaction extent that would greatly influence the strength development at the early ages of AACs, however, few researches focused on their influence on efflorescence, which is caused by the excessive unreacted alkaline in AACs diffusing to its surface, and reacting with CO2 in the air to form white carbonate deposits on the surface, and efflorescence may lead powder and peel on the AAC surface and gradually lose strength from the outside to the inside. In order to investigate the influence of microwave heating and ultrafine precursors on efflorescence, AAC samples adopting normal and ultrafine coal combustion ashes, and heated with steam and microwave respectively were synthetized. Additionally, the properties including geo-polymerization products, pore structure and moisture transport process which directly affect efflorescence were also compared. The results revealed that AACs heated with microwave exhibited faster and more serious efflorescence, especially for precursors with more amorphous phase and higher geo-polymerization reactivity, which can be attributed to the weak crystallization of reaction products improving the instability of sodium ions, and a less compact microstructure dominated absolutely by capillaries that promotes moisture movement, and also the transmission of sodium ions. Faster efflorescence tendency also revealed on AACs synthetized with ultrafine coal combustion ashes, however, their damage was significantly lower than that of microwave heating, and the increased moisture transport rate caused by the unitary capillaries distribution concentrated at 30 nm–100 nm may be the key reason. These results enrich the research on the application of coal combustion ashes and the influencing factors of AAC efflorescence.