Rheological behavior and key properties of metakaolin and nano-SiO2 blended fibrous self-compacting concretes

Abstract

The basic aim of the study herein is to experimentally determine and evaluate the influences of nano-SiO2 and metakaolin incorporation on the rheological and workability characteristics as well as mechanical properties of fibrous self-compacting concretes (SCCs). For that purpose, two SCC series were designed at a constant total binder content of 550 kg/m3 and a water-to-binder (w/b) ratio of 0.37. The first SCC series were manufactured as nonfibrous while the second series was produced as a steel fiber reinforced at a volume fraction of 0.5 %. In all SCC mixtures, 30 % of the total binder content was fly ash by weight. In both series, the cement (70 % of the total binder content) was partially replaced with metakaolin (0 % and 10 %) and nano-SiO2 (0 %, 2 %, and 4 %) by weight. In this way, a total of 12 SCC mixtures were designed and manufactured. The ICAR rheometer was employed to carry out the rheology measurements of the mixtures and the modified Bingham model was applied to describe the rheological behavior. Fresh characteristics of mixtures were investigated in terms of slump flow diameter, T50 slump flow time, V-funnel flow time, and l-box height ratio and flow times. In addition, the fundamental mechanical properties like compressive strength, elastic modulus, and splitting tensile strength were determined at the ages of 28 and 90 days. The plain SCC mixture had the slump flow diameter, T50 slump flow time, V-funnel flow time, and l-box height ratio of 750 mm, 2.29 s, 8.3 s, and 0.96, respectively. Metakaolin and nano-SiO2 replacements and steel fiber addition resulted in lower self-compactibility with up to 7% reduction in slump flow diameters. Especially the steel fiber particles played a blocking role and, thereby, caused a decrease in the flowability and passing ability of the mixtures by about 8–10 %. A similar effect was observed in the rheology measurement. The mixtures involving nano-SiO2, metakaolin, and/or steel fiber behaved more cohesively and viscously. On the other hand, the 28-day (90-day) compressive and splitting tensile strengths and modulus of elasticity of plain SCC mixture were respectively 67.5 (76.4) MPa, 3.95 (4.32) MPa, and 37.9 (39.4) GPa. Both metakaolin and nano-SiO2 replacements led to a significant increase in the strength performance of the mixtures, especially at a later age. However, it is hard to state whether steel fiber addition has a distinct and systematic effect on the compressive strength and modulus of elasticity. In contrast, it was clearly observed that steel fiber incorporation remarkably enhanced the mixtures’ tensile performance, resulting in more than 20 % and about 17–24 % increases in 28- and 90-day splitting tensile strengths, respectively.