Belitic calcium sulfoaluminate () cement is a more environmentally friendly alternative to ordinary Portland cement because of its lower clinkering temperature and reduced limestone demand. However, the carbon dioxide ((2)) sequestration potential of cement under accelerated carbonation has not been systematically quantified. In this study, the carbonation behaviour, carbon dioxide uptake and performance evolution of mortars exposed to 20% carbon dioxide, 80% relative humidity and 30 degrees C for up to 28 days after 1 h of initial air curing were investigated. The degree of carbonation (DoC), derived using thermogravimetric analysis (), increased significantly from 5.49% at 3 h to 24.51% at 28 days, while the DoC of the air-cured specimens remained below 3.36%, confirming limited natural carbonation. Accelerated carbonation initially reduced the compressive strength of the mortar due to ettringite destabilisation but promoted subsequent strength recovery, with 28-day strengths comparable to those of the air-cured samples. and X-ray diffraction confirmed progressive ettringite consumption and the formation of monocarboaluminate, hemicarboaluminate and calcium carbonate phases. Microstructural observations revealed early pore refinement followed by the development of a dense carbonate-rich matrix. Overall, the cement demonstrated high carbon dioxide sequestration capacity under accelerated carbonation while maintaining later-age mechanical performance, highlighting its potential as a 'low-carbon' binder.
Concretes incorporating moderate-heat Portland cement (MHC) and Portland–slag cement (PSC) are widely used in hydraulic engineering due to their low temperature rise and excellent long-term strength. However, understanding of the hydration and carbonation behaviours of these cements is incomplete. In this investigation, long-term calorimetry was combined with thermodynamic modelling to quantify hydration degrees, pore-solution evolution and phase transformations. A predictive model for the hydration degree of MHC was established, giving a stable-state value of 87.9%. Thermodynamic simulations revealed that the saturation indices (SIs) of different calcium silicate hydrate end-members correlate positively with the portlandite SI and with the concentrations of OH− and silicon, but negatively with calcium concentration. The addition of slag led mainly to the consumption of portlandite, which was completely depleted at a slag content of 60%. Lower slag contents promoted calcium silicate hydrate (C-S-H) with a higher calcium/silicon ratio (CSR), whereas higher slag contents reduced the C-S-H content, the CSR and the pH of the pore solution. Carbonation directly lowered the paste alkalinity and caused decalcification of portlandite, C-S-H gel and ettringite to form calcium carbonate, amorphous silica and microcrystalline aluminium hydroxide. Although PSC offers considerable potential for low carbon dioxide and mass concrete applications, its lower alkalinity may impair carbonation resistance. When the pH of the paste pH was reduced to 9, MHC exhibited 31.8% superior carbonation buffering capacity compared with PSC.
Spent cathode carbon (SCC) is a hazardous solid waste generated from aluminium electrolysis, which poses challenges for safe disposal and resource recovery. In this study, the use of SCC as an alternative fuel in cement kiln co-processing and its effects on clinker mineralogy, hydration kinetics and mechanical performance were investigated. The results showed that SCC incorporation enhanced clinker burnability, promoted the formation of alite (C3S) and tricalcium aluminate (C3A) over belite (C2S) and ferrite (C4AF), and induced a progressive M1→M3 transformation of C3S. Higher substitution ratios enlarged alite crystals while reducing belite crystal sizes. Regarding hydration, SCC substitution prolonged the induction period and led to a non-monotonic variation of the reaction rate during the acceleration stage. Compressive strength was improved with SCC substitution ratios below 20% (with the optimum being 20%), whereas a marked decline occurred at 30% substitution. These findings confirm that controlled SCC substitution enhances clinker performance while offering a sustainable route for hazardous waste management.
Advancements in sustainable cement technology rely on the use of supplementary cementitious materials to enhance performance and reduce environmental impact. Mineral additives such as fly ash (FA), blast-furnace slag, silica fume and limestone are commonly used to lower clinker content, refine microstructure and improve durability. Among these, calcium sulfoaluminate (CSA) expansive agents are widely applied in shrinkage-compensating concretes, grouts and repair mortars, as they induce controlled early expansion that mitigates shrinkage and cracking. This study investigates the effect of CSA on the hydration and mechanical behaviour of cementitious systems containing FA and metakaolin (MK). The binders consist of CEM II cement with 12% limestone filler, combined with FA and MK, and are analysed using X-ray diffraction, thermogravimetric analysis, isothermal calorimetry and cement Gibbs energy minimisation software modelling. Results show that CSA promotes early ettringite formation and accelerates hydration, increasing overall product formation but slightly reducing C–S–H content and compressive strength while enhancing the flexural strength. The combined use of CSA, FA and MK yields balanced reactivity, with early acceleration from CSA and later pozzolanic activity, achieving strengths comparable to the reference system.
