Effect of flue gas temperature on NO2 adsorption by aged recycled concrete Waste: DRIFTS, TGA and BET study

Abstract

Removing NO2 from cement kilns can have tremendously beneficial effects on the environment and human health. Sequestering NO2 in demolished concrete is an innovative, cost-effective, and sustainable approach to remove NO2 flue-gas from cement kilns and other industrial plants to minimize their environmental impact. Another notable advantage of this approach was signified by our recent discovery of NO2 sequestered Recycled Concrete Aggregate (NRCA) acting as a corrosion inhibitor when recycled backed into new concrete. This paper focuses on NO2 sequestration by the waste concrete at elevated temperatures that are representative of those found in the cement kilns flue gas exhaust. The gas-phase uptake experiments were performed for 1, 2, and 20-years old concrete samples to reflect the variable age of NRCA. The mechanistic studies of NO2 adsorption to concrete surfaces at 27 °C, 150 °C, and 250 °C temperatures that simulate flue gas temperatures were obtained by time-resolved Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTs) and gas-phase analysis. The results showed that NO2 sequestration increased with an increase in temperature. Most importantly, the 20-year-old concrete still had significant uptake capacity, which shows that aged waste concrete can be used to reduce air pollution and then recycled back into new concrete structures to prevent corrosion. These findings were also supported by TGA, BET, and XRD results. The XRD data indicated a presence of alite, belite, ettringite, portlandite, and dolomite, where the increased fractions of portlandite and hydrates were correlated to higher NO2 uptake. Moreover, the BET results indicated notable changes in the microstructure of the concrete at elevated temperatures, which also contributed to changes in the NO2 uptake capacity of concrete.