Effects of sodium doping on carbonation behavior of α-CS

In our previous work, a method of stabilizing high-temperature polymorphs (α-CS) by rapid cooling was reported to improve the carbonation activity of ultra-low calcium CO2 sequestration binder (LC-CSB). This paper further investigates the effect of sodium (Na) doping on the properties of α-CS minerals, such as burnability, mineral composition and microstructure. In addition, the carbonation reactivity, carbonation hardening properties, carbonation products and microstructure of α-CS with different sodium doping were also characterized by pH and conductivity, XRD, TGA, FT-IR, SEM-EDS. The mechanism of the effect of sodium doping on the structure and carbonation activity of α-CS minerals was revealed. The results show that the incorporation of sodium significantly improved the burnability of α-CS, lowered the sintering temperature by 180 °C and formed a new phase of Na2O·2CaO·3SiO2(NC2S3). As sodium doping increased, the early carbonation hardening properties were slightly lower than those of pure α-CS, but there was a significant increase in compressive strength and carbonation degree in the later stages. This can be attributed to the fact that sodium doping increases the alkalinity of the paste and delays the dissolution of calcium ions from the α-CS. Furthermore, in the presence of sodium ions, metastable vaterite crystals can be stabilized in addition to calcite. The above results provide potential applications for the low-temperature sintering of alkali-containing solid waste into high carbonation activity LC-CSB. •The incorporation of sodium significantly improves the burnability of α-CS.•A new phase of Na2O·2CaO·3SiO2 is formed by sodium doping.•The alkali-containing solid waste can be upcycled into high carbonation activity LC-CSB.