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-UHPSFRC 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.

Automated summary

Cementitious mix with very-low-tricalcium aluminate (C3A) cement as the binder can achieve good mechanical performance and flowability. However, its chloride penetration resistance in marine environment is not as good as moderate-C3A cement. Therefore, the addition of supplementary binders can improve the chloride penetration resistance and mechanical properties of low-tricalcium aluminate cement.