# Special Issue Advances in High-Performance of Eco-Efficient Concrete
## 4 Development and Evaluation of Nano-Silica Sustainable Concrete  
[PERSON] and [PERSON] [3] show that over the last decade, nano-materials made a major breakthrough in the concrete industry by providing the concrete with unique properties. Earlier studies have shown improvements in the early strength of concrete that can accelerate the construction process. In this study, 1% and 2% of nano-silica were added to concrete mixtures that contain 30% and 70% ground granulated blast-furnace slag (GGBS). Adding 1% of nano-silica to the 30% GGBS mixture showed an increase in the compressive strength by 13.5%, 7.8%, 8.1% and 2.2% at one day, three days, seven days and twenty-eight days, respectively. The 2% of nano-silica increased the 30% GGBS mixture's compressive strength less effectively, increasing by 4.3%, 7.6% and 4.9% at three days, seven days and 28 days, respectively, when compared to the 1%. On the other hand, adding 1% and 2% of nano-silica reduced the 70% GGBS mixtures' compressive strength. Moreover, nano-silica reduced the deformability of the mixtures significantly, which caused the increase in the Young's modulus. The flexural strength of the 30% GGBS mixtures had similar behavior as the 28-day compressive strength one. On the other hand, the flexural strength of the 70% GGBS mixtures increased as the nano-silica increased. The nano-silica addition improved the microstructure and the interface structure of the mixtures due to its high pozzolanic activity and the nanofiller effect, which is confirmed by RCPT results and SEM images. 5 Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers  
[PERSON] et al. [4] show that the acai (Euterpe oleracea) is a typical Brazilian fruit that is enveloped by natural fibers. This work investigated the effect of incorporating ground acai fibers (in natura and chemically treated with NaOH and HCl) in a 5-10 wt.% replacement of Portland cement on the rheology, hydration and microstructure of pastes. Rotational rheometry, isothermal calorimetry, X-Ray diffraction (XRD) and scanning electron microscopy (SEM) were performed to evaluate the cement pastes, in addition to SEM-EDS, FTIR, zeta potential, and XRD for fiber characterization. The results showed that the chemical treatment reduced the cellulose and lignin contents in acai fibers while increasing its surface roughness. The addition of 5% of fiber slightly increased the yield stress and viscosity of paste, whereas a 10% addition drastically increased these properties, reaching yield stress and viscosity values 40 and 8 times higher than those of plain paste, respectively. The incorporation of 5% in natura fibers delayed the cement hydration by about 2.5 days, whereas 10% in natura fibers delayed it by over 160 h. The chemical treatment significantly reduced this retarding effect, leading to a 3 h delay when 5% treatedfibers were incorporated. Overall, the combined NaOH/HCl treatment was effective for acai fibers' functionalization, and these fibers can be used in cementitious composites. Effect of Freeze-Thaw Cycles on Carbonation Behavior of Three Generations of Repeated Recycled Aggregate Concrete  
[PERSON] et al. [5] show in their paper that multiple recyclings of waste concrete has attracted widespread attention. This study presented the carbonation behavior of repeatedly recycled aggregate concrete (RRAC) used in a microfrozen region. The effects of freeze-thaw cycles on the carbonation depth of three generations of RRAC with a 25%, 75% and 100% replacement rate were evaluated. All RRAC specimens after different numbers of freeze-thaw cycles were rapidly carbonated for 28 d indoors to test the carbonation resistance of concrete. The results suggested that the carbonation depth of RRAC subjected to freeze-thaw cycles is higher than that in the non-freeze-thaw condition.