Understanding the Impact of ${\mathrm{KNO}}_3$ on the Carbonation Behavior of $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$

$\gamma$-dicalcium silicate ($\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$) has received little attention in engineering applications due to its low hydraulicity, but it exhibits high carbonation reactivity and rapid strength gain with the added benefit of ${\mathrm{CO}}_2$ absorption, which demonstrates a vast potential for application in building materials. Included herein is an investigation of the promotion impact concerning a potassium nitrate (${\mathrm{KNO}}_3$) solution on the carbonation behavior of $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$, including the degree of carbonation (DOC), microstructure, and mechanical properties. Results show DOC increases when ${\mathrm{KNO}}_3$ is introduced into the system, and as the concentration of ${\mathrm{KNO}}_3$ solution rises, DOC can reach as high as 60%, compared with 49% without ${\mathrm{KNO}}_3$; The porosity of the system is reduced by 37%; The compressive strength after 24 h of accelerated carbonation, with the ${\mathrm{KNO}}_3$ concentration of $2\mathrm{mol}/\mathrm{L}$, reaches 90 MPa—34% higher than that of the blank group. The presence of ${\mathrm{KNO}}_3$ solution contributes to retaining water—the dominant position for carbonation—in the sample and reducing evaporation, thus prolonging the carbonation time. The increased volume of carbonation products densifies the whole system as the reaction occurs. A barrier to $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$ dissolution, the amorphous silica layer wrapping on the surface of grains, is destroyed due to the introduction of ${\mathrm{K}}^{+}$, facilitating the continuous dissolution of silicate. The incorporation of ${\mathrm{KNO}}_3$ results in a larger crystal in size and more regularity in shape. Prolongation of the reaction duration allows for more adequate crystal growth, resulting in compacts with superior mechanical properties.

Limestone is the raw material for Portland cement manufacturing, with excellent mechanical property and resistance to ultra-high and low temperature and erosion. If a building material with its composition and structure similar to limestone could be prepared, the infrastructure construction and major engineering projects could be applied in wider regions. $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$ is a kind of calcium silicate minerals that can react with CO₂ and produce a binding effect. The main binder is calcium carbonate which is also the main component of limestone. This principle is helpful for the design and preparation of a new kind of high-performance engineering material. The investigation of the carbonation reaction of $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$ allows the preparation of products with excellent mechanical properties and good durability, such as aerated concrete and fiber panels using $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$ as the main cementitious material. It will be also beneficial for the revitalization of industrial solid wastes such as magnesium slag and ladle furnace slag, whose main component is $\gamma \text{-}{\mathrm{C}}_2\mathrm{S}$. The present study was carried out in order to enable a more complete carbonation reaction and continue to investigate the mechanism of the excellent properties, ultimately establishing the basis for application expansion of this new material.