Mechanical and durability properties of blended OPC mortar modified by low-carbon belite (C₂S) nanoparticles

In this study, manually synthesised low-energy belite nanoparticles via a one-step method were employed into normal ordinary Portland cement (OPC) binding systems to improve properties and to reduce OPC carbon footprint. The mechanical and durability performance of the modified OPC mortar specimens were quantified and corresponding microstructural analysis was also conducted. Results showed that 5% dosage of the belite nanoparticles led to a significantly higher compressive strength of the modified OPC mortar, compared to the normal OPC counterpart without any belite additions. Besides, the carbonation depth and chloride penetration depth was reduced by up to around 43.5% and 28.1% respectively, compared to the corresponding values of the mortar without belite. However, when the content of belite increased to 15% and 20%, significant loss in mechanical strengths was observed. Worse still, the durability also exhibited no improvements or even worse behaviours evidenced by similar or greater carbonation depth and chloride penetration depth. These weaknesses are probably associated with the low hydration rate and possible agglomerations of belite nanoparticles in the OPC binding systems. Microstructural analysis reveals that belite nanoparticles refined the porous structure by reducing the average pore sizes but also led to higher porosity when higher dosage levels (10%, 15% and 20%) were applied. Finally, it was also found that belite nanoparticles could to some extent densify the microstructure of C–S–H gels due probably to their nanoscale particle sizes. This study thus provides guidelines for further improving the mechanical and durability properties of OPC-based binders through the use of belite nanoparticles synthesised via a simple method at a relatively low temperature.