Characteristics of cement mortar with nano-SiO2 particles

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Abstract

The properties of cement mortars with nano-SiO2 were experimentally studied. 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-SiO2 particles in order to make high-performance concrete. The experimental results show that the compressive strengths of mortars with nano-SiO2 particles were all higher than those of mortars containing silica fume at 7 and 28 days. It is demonstrated that the nano-particles are more valuable in enhancing strength than silica fume. In addition, the continuous hydration progress was monitored by scanning electron micrograph (SEM) observation, by examining the residual quantity of Ca(OH)2 and the rate of heat evolution. The results of these examinations indicate that nano-scale SiO2 behaves not only as a filler to improve microstructure, but also as an activator to promote pozzolanic reaction.

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 re-engineer 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. Hui Li [1] investigated cement mortars with nano-SiO2 or nano-Fe2O3 to explore their super mechanical and smart (temperature and strain sensing) potentials. The Fuji Chimera Research Institute (2002) addressed functional applications of SiO2 in nano scale. However, up to now, research performed over the years has been largely aimed at achieving high mechanical performance with cement replacement materials in micro size. Lu and Young [3] obtained 800 MPa strengths on compressed samples, and Richard and Cheyrezy [4] developed Reactive Power Concretes (RPCs) ranging from 200 to 800 MPa and fracture energies up to 40 kJ m−2. The development of an ultra-high strength concrete was made possible by the application of DSP (Densified System containing homogeneously arranged ultra-fine Particles) with superplasticizer and silica fume content. In view of these advances, the aim of this study is to investigate the influences of nano-SiO2 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 plausible to add nano-SiO2 of a high purity (99.9%) and a high Blaine fineness value (60 m2/g) in order to improve the characteristics of cement mortars.

Section snippets

Materials and methods

Cementitious materials used were ordinary portland cement (OPC), silica fume powder (SF) and nano-SiO2 particles (NS). Their pertinent chemical and physical properties, as provided by the manufacturer, are given in Table 1. Scanning electron micrographs (SEM) and powder X-ray diffraction (XRD) diagrams of silica fume and nano-SiO2 are shown in Fig. 1, Fig. 2.

Compressive strength

Compressive strengths after 7 and 28 days are shown in Table 3. It can be seen that the compressive strength was developed in mortars containing nano-SiO2 particles in every case higher than that of control cement mortars. The difference in the strength development of the mortars can be attributed to pozzolanic reaction. As mentioned above, nano-particles are thought to be more effective in pozzolanic reaction than silica fume. Also, the nano-SiO2 would fill pores to increase the mortar

Conclusions

Based on the results of compressive strength test, it is expected that nano-scale SiO2 behaves not only as filler to improve mortar cement microstructure, but also as a promoter of pozzolanic reaction. To verify these mechanisms predicted, we have analyzed the hydration process using three types of cement mortars. Results are consistent with the strength enhancement expected. Therefore, it is effective to add nano-SiO2 particles to cement mixtures for introducing high-performance to concrete.

Acknowledgement

The authors acknowledge the financial support from the Korea Ministry of Construction and Transportation.

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