Chloride binding behaviors of metakaolin-lime hydrated blends: Influence of gypsum and atmospheric carbonation

•Gypsum addition promotes the formation of monosulfoaluminate and ettringite is favored in an even higher gypsum addition.•Carboaluminate and monosulfoaluminate can transform to Friedel’s salt in a chloride-rich environment.•Delayed-ettringite is formed during exposing metakaolin-lime with gypsum to chloride solution.•Carbonation reduces chloride bound in chloroaluminate as well as those absorbing on C-S-H gels.•Thermodynamic calculation and experiment demonstrate consistent results. Metakaolin-lime (1:1 by mass) blends are prepared to simulate the hydration products of metakaolin in cement and to clarify the effects of gypsum and carbonation on their chloride binding behaviors. The bound chloride content, hydration products and mechanisms were investigated by titration, X-ray diffraction, thermogravimetry and thermodynamic analysis. The results show that formation of stratlingite (C2ASH8, St) and carboaluminate are favored in gypsum-free blend, while monosulfoaluminate and ettringite are preferred in the gypsum-containing blends. During chloride exposure, St demonstrates good stability. Kuzel’s salt (C4As0.5ClH12, Ks) is kinetically favored at 3 h in gypsum-containing blends and it converts to Friedel’s salt (C4ACl2H10, Fs) with continuous exposure. Chlorides can replace the sulfates in monosulfoaluminate (C4AsH12, Ms) and Ks to eventually form Fs and release sulfates that can further react with Ms to form delay-ettringite. The thermodynamic calculation and experiments demonstrate consistent results related to the phase conversion. Addition of 4 wt% and 8 wt% gypsum reduce the bound chloride content up to 11%. During carbonation, chlorides bound by Friedel’s salt and C-S-H gels can reverse to free chlorides due to the decrease of pH and reduced calcium ion concentration in exposure solution. In a deeply carbonated system, ettringite does not exist.