Mechanical tests on silica sol- An introductory study on material characteristics for silica sol
This report is part of the project Non-cementitious low-pH injection grout for smaller fractures that is a sub-project under the main project Injection grout for deep repositories. The project is financed by SKB (Sweden), Posiva (Finland) and NUMO (Japan). The aim of these laboratory experiments is to increase the knowledge of the characteristics of silica sol. These experiments can be seen as an introductory study of the mechanical aspects of silica sol and should be followed up with measurements on issues that are surveyed in this report. The different aspects of the characteristics for silica sol that have been studied are:
- Drying shrinkage
- Flexural strength
- Compressive strength
- Youngs modulus (Modulus of elasticity)
- Shear strength
All of the tests were made with the same silica sol that consisted of 35% by weight SiO2 and the accelerator were CaCl2 with a concentration of 2.9%. The pH was around 10 and the silica sol was aluminium modified. The test samples were moulded with a ratio between the silica sol and the accelerator of 8:1 by weight. A total of 117 test samples were moulded.
The test samples were stored in four different environments. In a climate room that had a temperature of 8°C three environments were created with humidities of 75%, 95% and 100%. These environments were created to imitate the environment in the rock. The fourth environment was 20°C and 50% relative humidity, which is a standard environment for concrete shrinkage tests.
The results show that the strength of silica sol increases over a long time. The increase in strength depends on the humidity that silica sol is exposed to. Lower humidity results in a more rapid strength increase. Though in low humidities silica sol also have a tendency to shrink.
The development of the compressive, flexural and shear strength seems to be rather attuned for each storage environment. The strength of silica sol increases over a long time. The increase of strength depends on the humidity that silica sol is exposed to. Less humidity will give a faster strength increase. Though in low humidities silica sol also have a tendency to shrink.
With the use of Mohr-Coulomb failure criteria a value of the friction angle could be estimated for silica sol. Generally the friction angle increased with time, from around 20º after three weeks to around 50º after six months. It seems that silica sol reasonable well could be described by Mohr-Coulomb failure criteria.
To imitate the effect of drying shrinkage of silica sol in a grouted fracture, an experiment with water pressure on one side and air on the other side of a silica sol grouted borehole was conducted. The hypothesis is that some shrinkage will occur near the tunnel due to the lower moisture content in the air than the water in the fractures. The result of this experiment was that the specific flow decreases over time. Calculations were done to verify the crack width that causes the flow of water. The magnitude of the width of this crack are 1-14 μm, this should be compared with the originally width of the grouted borehole of 86 mm.