Consequences of Colloidal Nanosilica Specific Surface on Its Performance in Concrete

Abstract Many results on performance of nanosilica in concrete have been reported by researchers in the domain of strength enhancement to reduction. A possible reason could be the vastly different specific surfaces of nanosilica materials used by different researchers. In this research, the effects of two colloidal nanosilicas, with specific surface areas of 80 m2/g and 500 m2/g on properties of concrete are investigated. The performance of these nanosilicas, which are specifically produced for use in concrete, are compared to that of microsilica. Additionally, for having a better understanding of the behavior of concrete in the presence of high specific surface area nanosilica, mortar specimens containing nanosilica with 500 m2/g specific surface area were also prepared and tested. Results show that the use of nanosilica particles causes a more severe reduction in workability of mixes compared to micro. The reduction in workability increased with increasing dosage and specific surface area of nanosilica. Compared to micro, the nanosilica materials resulted in higher strength enhancements relative to control at the age of 3 and 7 d. However, at 28 d the difference became small, and at 56 d the nanosilicas and micro had similar performances. Results of transverse electron microscopy (TEM) revealed that the state of nanosilica agglomeration with 80 m2/g is almost the same as microsilica; however, nanosilica particles with 500 m2/g are highly agglomerated compare with the other two materials because of the presence of large amounts of nanosilica. Scanning electron microscopy (SEM) results at 28 d also showed high levels of heterogeneity in the distribution of hydration products, which is the result of poor nanosilica dispersion; this phenomenon consequently decreased the compressive strength of specimens.