Influence of gas impurities (SO₂ and NO_X) in flue gas on the carbonation and hydration of cement compacts

Flue gas with a typical CO2 concentration of 10-20 % is found to be potentially utilized for accelerating the curing process of cement compacts. However, the competitive effect of acid gas impurities (i.e. SO2 and NOX) contained in flue gas on carbonation behaviors remains unclear. In this work, multi-component gases (500 ppm SO2, 500 ppm NO2, 10 % O-2, and 74.9 % N-2) with a CO2 concentration of 15 % are simulated to study the carbonation reactivity of cement compacts after 2 h of curing. The influence of subsequent water curing on strength and microstructure development over time is also further investigated. The experimental results show that after exposure to CO2, the highly soluble acidic SO2 and NOX gases can considerably reduce the pH of cement compacts, and thus mitigate the carbonation efficiency. The dissolved SO2 can potentially compete with CO2 for calcium-bearing ion combination, leading to a reduction of similar to 30 % in compressive strength. However, this inferior performance can be compensated by the progressive hydration reaction in subsequent water curing. The findings from this study highlight the essential consideration of gas impurities in flue gas CO2 curing, in particular their impact at an early age. Additionally, there is a need to explore the durability characteristics and interactions of gas impurities during high-temperature curing.