Utilization of basic oxygen furnace slag in geopolymeric coating for passive radiative cooling application

Basic oxygen furnace slag (BOFs) is difficult to reutilize because it contains excessive free lime, and thus causes serious expansion. For this reason, how to reuse BOF slag has turned out to be an imperative issue in order to meet the concept of a circular economy. The key intention of this research work is to develop a new way to reutilize BOF slag, which due to its high emissivity in the 8-13 μm wavelength range, can be used as a sustainable, passive radiative cooling material. Passive radiative cooling, without the consumption of any energy, achieves the cooling of a surface by reflecting the sunlight and radiating the heat throughout the outer space (not absorbed by the atmosphere). BOF slag is used as a radiative cooling material in geopolymeric coating. This coating possesses an emissivity of 0.95 within the range of 8-13 μm and also has high conductivity, but its gray appearance absorbs too much heat. Therefore, by improving the situation through a double-layer structure, a temperature drop of 5.9 °C was reached compared to non-coated concrete under simulated sunlight, simultaneously with a low heating rate and high cooling rate. Besides, the binding strength between the geopolymeric coating and Portland cement concrete is comparable to two commercial organic paints. It is highly probable that the utilization of BOF slag in geopolymeric coating is energy saving and also feasible for passive radiative cooling applications. Hence, it can greatly decrease indoor temperature and improve the comfort of people living in buildings.