## 1 Introduction  
The use of pozzolanic materials in concrete has several benefits. Additional calcium-silicate-hydrates (C-S-H) are generated by the pozzolanic reaction, and the fine pozzolanic particles fill the voids in the cement paste. As a result, the cement matrix becomes denser, and the durability of the concrete is enhanced by decreasing its permeability [1]. Fly ash (FA) and silica fume (SF) are well known pozzolanic materials, and numerous studies have been conducted to study the mechanical and chemical properties of concrete blended with these pozzolanic materials. Recently, nanosilica (NS) has been introduced, and some researchers have tried to use it as an alternative to fly ash and silica fume.  
Nanotechnologies have also been applied to concrete technology. Several nanomaterials have been examined for their ability to enhance concrete properties [2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. Fe\({}_{2}\)O\({}_{3}\) nanoparticles and carbon nanotubes have been used as sensors inside the cement paste [2]. TiO\({}_{2}\) can be applied for self-cleaning concrete [11]. Nanosilica is a pozzolanic material with a very large surface area, which can enable further growth of the C-S-H [8; 9], even with a very small dosage, and which guarantees a much faster reaction than fly ash or silica fume [12]. In this study, nanosilica was blended with concrete to increase its carbonation resistance. In addition, the degree of reaction of the nanosilica was calculated and compared with those of fly ash and silica fume.  
This study focused on the carbonation resistance of pozzolan blended concrete. For this, rapid carbonation tests were performed. The relative carbonation resistance for each pozzolan blended concrete can be compared using the rapid carbonation test results. However, these tests are not sufficient to analyze the carbonation phenomenon in-depth, therefore further microstructural analysis of the cement matrix was needed. Several material characterization techniques have been used for cement hydration analysis. For example, NMR, Thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscope (TEM), nano-indentation, and micro-computed tomography have been widely used to analyze the cement hydration kinetics and the microstructure of hardened cement [13; 14; 15; 16; 17]. Using NMR spectra, the chemical bonds in the matrix can be observed. In the case of cement paste or concrete, \({}^{29}\)Si MAS NMR is usually used and can identify the silicate chains in the cement matrix. By comparing the silicate chains in different cement or concrete mixtures, the differences in the microstructure of the specimens can be explained. In this study, the conversion fraction and degree of carbonation were investigated in-depth using \({}^{29}\)Si MAS NMR analysis.  
This paper presents the carbonation resistance of three pozzolanic blended concrete mixtures including fly ash, silica fume, and nanosilica. To eliminate the effect of mechanical properties to the carbonation, all tested mixtures were intentionally designed to have similar mechanical properties by adjusting the replacement ratio of pozzolanic materials with cement content. Rapid carbonation tests were conducted, and the carbonation depth was measured. \({}^{29}\)Si NMR experiments were conducted to obtain the conversion fractions for each of the pozzolan blended concretes. The degree of carbonation was also observed from the NMR results. From the results, it can be found that the nanosilica blended concrete showed the best carbonation resistance. In addition, the nanosilica blended concrete showed the highest conversion fraction among the specimens, based on the \({}^{29}\)Si MAS NMR analysis results.