Characteristics of Cement Mortar with Nano-SiO^sub 2^ Particles

The amorphous or glassy silica, which is the major component of a pozzolan, reacts with calcium hydroxide formed from calcium silicate hydration. The rate of the pozzolanic reaction is proportional to the amount of surface area available for reaction. Therefore, it is plausible to add nano-SiO^sub 2^ particles (NS) to make high-performance concrete. The compressive strengths of cement mortar were evaluated at various water-cementitious material ratios (w/cm). Five different w/cm were used, including 0.23, 0.25, 0.32, 0.35, and 0.48 and four contents of NS, 3, 6, 9, and 12% by weight of cement. The compressive strengths of cement mortar with the addition of silica fume were also evaluated at a w/cm of 0.35 to compare with mortar containing nano-SiO^sub 2^ particles and three contents of silica fume were: 5, 10, and 15% by weight of cement. The experimental results show that the compressive strengths of mortars with NS were all higher than those of mortars containing silica fume at 7 and 28 days. It was demonstrated that the nano-particles were more valuable in enhancing strength than silica fume. This paper also analyzes some available examinations to monitor the hydration progress continuously, such as SEM observation, residual quantity test for Ca(OH)2, and the rate of heat evolution. The results of the examinations indicate that the SiO^sub 2^ in nano scale behave not only as a filler to improve the microstructure, but also as an activator to promote pozzolanic reactions.

Keywords: compressive strength; pozzolanic reactions; silica fume.

INTRODUCTION

Recently, nano technology has attracted considerable scientific interest due to the new potential uses of particles in nanometer (10-9 m) scale. The nano-scale size of particles can result in dramatically improved properties from conventional grain-size materials of the same chemical composition. Thus, industries may be able to reengineer many existing products and to design new and novel products that function at unprecedented levels. There are few reports on mixing nano-particles in cement-based building materials.1-6 Li et al.1 investigated cement mortars with nano-SiO^sub 2^ or nano-Fe2O3 to explore their super mechanical and smart (temperature and strain sensing) potentials. The Fuji Chimera Research Institute4 addressed functional applications of SiO^sub 2^ in nano scale. Up until now, however, research performed over the years has been largely aimed at achieving high mechanical performance with cement replacement materials in micro size. Lu and Young5 obtained an 800 MPa (116,030 psi) strength on compressed samples, and Richard and Chyrezy6 developed reactive powder concretes with compressive strength (RPCs) ranging from 200 to 800 MPa (29,007 to 116,030 psi) and fracture energies up to 40 kJ m-2. The development of an ultra-highstrength concrete was made possible by the application of a densified system containing homogeneously arranged ultrafine particles (DSP) with high-range water-reducing admixture and silica fume content. In view of these advances, the aim of this study is to investigate the influences of nano-SiO^sub 2^ in cement mortars. The amorphous or glassy silica, the major component of a pozzolan, reacts with calcium hydroxide formed from the hydration of calcium silicates. The rate of the pozzolanic reaction is proportional to the amount of surface area available for reaction. Therefore, it is feasible to add nano-SiO^sub 2^ of a high purity (99.9%) and a high Blaine fineness value (50 m2/g [244,077 ft2/lb]) to improve the characteristics of cement mortars.

RESEARCH SIGNIFICANCE

The nano scale of particles can result in dramatically improved or different properties from conventional grain-size materials of the same chemical composition. Accordingly, the use of nano-SiO^sub 2^ in particle form with 99.9% of SiO^sub 2^ in nano scale was considered in this paper to compare with the influences of silica fume in micro scale in cement mortar.

EXPERIMENTAL PROCEDURE

Materials

The addition of finely ground solid materials to portland cement is an established practice in modern cement technology. Silica fume has been used either as a partial replacement for cement or as an additive when special properties are desired. The most frequently specified property has been a minimum silicon dioxide (SiO^sub 2^) content, with values ranging from 85 to 95% being typical and moisture content with a maximum of 3.0%, and a specific surface area with minimum of 20 m2/g (97,630 ft2/lb) at a bed porosity of 50%. Accordingly, silica fume used in this study was in powder form with 95% of SiO^sub 2^ and a particle size of 0.1 µm to compare with the influences of nano-SiO^sub 2^ in cement mortar. A cement paste is made up of small grains of hydrated calcium silicate gels, nanosized individual pores, capillary pores, and large crystals of hydrated products. There should be room for nanophase materials to improve the properties of pure cement paste.7 In view of the abovementioned, nano-SiO^sub 2^ in particle form with 99.9% of SiO^sub 2^ and in particle size of 40 nm was used. The cementitious materials used are ordinary portland cement (OPC), silica fume powder (SF), and nano-SiO^sub 2^ particles (NS), their pertinent chemical and physical properties, as provided by Sanghyeon nanophase material company in Korea, are given in Table 1. Particle morphologies and x-ray diffraction (XRD) diagrams of silica fume and nano-SiO^sub 2^ are shown in Fig. 1 and 2.