Mechanical properties and chloride penetration resistances of very-low-C₃A cement based SC-UHP-SFRCs incorporating metakaolin and slag

Cementitious mix with very-low-tricalcium aluminate (C₃A) cement or VLAC (C₃A ≤ 3%) as the primary binder requires lower water to binder ratio to attain a target flowability than that with moderate-C₃A cement or MAC (6% < C₃A ≤ 10%). Hence, better mechanical performance, as well as good flowability, can be achieved for the mix with VLAC. Utilizing such beneficial features of VLAC, self-compacting ultra-high performance steel fibre reinforced concretes (SC-UHP-SFRCs) have been synthesised in this study. In the marine environment, however, VLAC based composite is not likely to exhibit a chemical chloride binding capacity as good as that of a MAC based composite. This is because the insufficient C₃A content in VLAC limits the formation of Friedel's salt. Therefore, the chloride penetration resistance of VLAC based SC-UHP-SFRC has been aimed to be investigated in this study. The potentials of supplementary binders having higher alumina (Al₂O₃) contents than VLAC, such as metakaolin and ground granulated blast-furnace slag (GGBS), to improve the chloride penetration resistance, as well as the mechanical properties, of VLAC based SC-UHP-SFRC have also been aimed to be studied. Results indicate that 40% replacement of silica fume with metakaolin increases the 28-d flexural strength of SC-UHP-SFRC with VLAC as the primary binder by 35.7%. 30% replacement of VLAC with GGBS increases the 28-d flexural strength by 65.6%. The level of chloride penetration resistance of VLAC based SC-UHP-SFRC with metakaolin or GGBS can be ‘extremely high’, after 216 d of exposure to cyclic drying and wetting with 10% NaCl solution. X-ray diffractometry (XRD) confirms the formation of more Friedel's salt in the mix with metakaolin or GGBS.