Water-resistant and sustainable magnesium oxychloride cement-based strain-hardening composites with ground granulated blast furnace slag and coconut shell biochar

Magnesium oxychloride cement (MOC) is a sustainable alternative to ordinary Portland cement with a lower carbon footprint, whereas its inherent high brittleness, poor ductility, and weak water resistance severely limit its application. This paper develops a low-carbon and high-performance MOC-based strain-hardening cementitious composite (BC-MOC-SHCC) with improved short- and long-term water resistance through the introduction of ground granulated blast furnace slag (GGBFS) and coconut shell biochar. The fresh, physical, mechanical and microstructural characteristics, water resistance (up to 360 days), and carbon footprints of the developed BC-MOC-SHCC were investigated. The results indicate that the incorporation of an appropriate amount of GGBFS and biochar decreases the fracture toughness of MOC-SHCC. The optimised mix design (SL20B5) with 20% GGBFS and 5% biochar was achieved with compressive strength, tensile strength, and strain capacity reaching 78.8 MPa, 7.91 MPa, and 6.99%, respectively. The promoted production of phase 5 with GGBFS and biochar can be verified by XRD and TGA. The introduction of a proper amount of GGBFS and biochar also reduces the fracture toughness of BC-MOC-SHCC, facilitating multiple cracking capacity. After 30-, 180-, and 360-day water immersion, the SL20B5 mix exhibits the residual compressive strength of 70.6, 63.0 and 48.9 MPa, with corresponding retention ratios of 0.90, 0.80, and 0.62, with substantial improvement from the control group. The carbon footprint analysis suggests that introducing appropriate amounts of GGBFS and biochar is effective in carbon footprint reduction while maintaining improved or comparable performance. • Adding 20% GGBFS and 5% biochar increases strain capacity of MOC-SHCC by 18.9%. • MOC-SHCC of 20% GGBFS and 5% biochar retains 62% strength after 360-d water attack. • Incorporating GGBFS and biochar cuts embodied carbon of MOC-SHCC by up to 35.4%.