Adequacy of the ASTM C1240 Specifications for Nanosilica Pozzolans

Background: Nano-silica is a new pozzolan that is boosting the field of nanomaterials and can be used effectively in the concrete industry. ASTM C1240 is used to qualify silica fume pozzolans to be used in concrete and for marketing purposes. Objective: The present study aims to assess the adequacy of ASTM C1240 in qualifying nanosilica to be used in concrete and to introduce a modified procedure to measure the pozzolanic activity of nanosilica used with cementitious composites. Methods: The effects of various particle sizes and dosages on the compressive strength of blends in standard mortar were investigated. The correlation between the chemical reactivity and the compressive strength were also investigated using TGA and XRD analyses. The criterion conceived was validated at two ages: 7 days and 28 days. Results and Conclusion: The strength activity index results revealed that the following standard procedure ASTM C1240 is not applicable for nanosilica due to the high surface area and great pozzolanic reactivity of SiO2 nanoparticles, so a lower cement replacement percentage should be used. This study suggests that it should not be more than 7%. The pozzolanicity testing results confirm that a nanosilica replacement level of more than 7% could not be effective in transforming further Portlandite into a C-S-H or other major cementing compound, leading to a “false negative” for nanosilica pozzolans.