Microwave curing cement-fly ash blended paste

Abstract

The embodied carbon dioxide (ECO2) of Portland cement (PC) is about 850 kg/t and consequently the global cement manufacture accounts for 7–9% of the CO2 produced by mankind annually. On the other hand, the ECO2 of fly ash (FA) is only 0.1 kg/t. Therefore, by replacing more PC with FA, significant reduction in the ECO2 can be achieved. However, higher level of PC replacement with FA can seriously reduce the early strength of concrete cured at room temperature, causing concerns in industrial applications. Although thermal curing can effectively increase their early strength, it is an energy intensive process. In this paper, the potential of using microwave as a low-energy alternative to cure a FA/PC blend with 50% PC replaced by FA was investigated and compared with those cured under room temperature and by thermal curing methods respectively. Firstly, the best regime for microwave curing was determined to be ‘A × 1 + B × 1 + C × 16′ with total curing duration of 5.29 h and the compressive strength reached 47.9 MPa. Following this, experiments on the compressive strength and characterisation on the microstructure of 50/50 FA/PC blend were conducted. Results show the early-age strength of 50/50 FA/PC blend can be significantly increased with both 60 °C thermal curing and microwave curing with no degradation in compressive strength being observed up to 28 days. The pore structure of hardened paste at 28 days was better under microwave curing than under thermal curing. Compared with conventional 60 °C thermal curing, microwave is a more environmentally friendly alternative heating source for curing FA/PC blend. These findings provide a reference for using microwave curing to manufacture low-carbon concrete by using 50% fly ash to replace cement.