Without water cement cannot get hydrated. However, after the hydration process has been completed, the presence of water can dissolve the hydration product and degrade the structure. This is more true for water retaining concrete structures. Foe examole, in the case of dam, the hydrostatic pressure coupled with water permeability can leach out the hydration product in unprecedented rate. This article will discuss about leaching of hydration product focusing on water retaining structure.
Water can diffuse into the concrete and dissolve the hydration products and also aggregates. The dissolved material will be diffused out due to concentration difference. Soft water (water without low dissolved minerals) or water containing aggressive chemicals (such as acid, chloride) has higher leaching effect.
Halvorsen (1966) performed studies on lime leaching in water retaining concrete
structures. Halvorsen divides leaching into three types:
1) Leaching through very porous concrete,
2) Leaching from free concrete surfaces,
3) Leaching through cracks in concrete.
What can be leached out?
Depending upon the ionization capacity of water, Na, K and Ca are highly susceptible to leaching. Because Ca is the main constituents of hydration product (Alite and Belite) the strength of the concrete is reduced if leaching is substantial.
Moskvin (1980) demonstrated that leaching can be sustained, if leached water is evaporated, the seepage generally seals off, unless washed off again by external water such as rain.This is due to deposition of salt in the cracks and its subsequent carbonation.
Factors affecting leaching
- Water permeability
- The hardness of the water
- Total Ca in the concrete and the amount of Ca(OH)2
- Any additives that can bind lime
- Carbonation of Ca(OH)2
- The amount of carbonic acid which is free to attack the concrete.
The concentration of leached lime from concrete pore was studied by Moskvin (1980). The trend of lime leaching is shown in figure below. When the rate of flow is slow, the flow is saturated by Ca, amd as the flow rate increases, the leaching of Ca reaches static value because water has to diffuse into the pores to extract Ca from smaller pores which means diffusion process dominate when flow becomes sufficiently high.
The solubility of ions from cement matrix is a thermodynamic process which means it will vary depending on temperature and pressure. Thus, higher water temperature will help to leach out the Ca ions rapidly compared to cooler water. These dissolved ions can be oozed out either by convection (due to flow of water through permeable pores) or by diffusion towards the surface due to concentration difference. Obviously, leaching rate will be high in case of convection type of leaching due to high availability of water.
The CO2 can readily dissolve in water to form Carbolic acid H2CO3 lowering the pH of water. The rise in acidity of water can more easily dissolve the alkaline Ca(OH)2 and other calcium products of CSH matrix.
As discussed in another article, the leaching of Ca from the matrix can drastically reduce the strength of concrete (both compression and tension) and posing a serious risk for corrosion of reinforcement. Nevertheless, such kind of leaching phenomenon can be detected if observed properly- such observation should be done in dry period because we can detect the leached product (efflorescence product) on the surface on dry seasons only.
Core testing is one of the direct method to understand the extent of leaching (and also carbonation of concrete which generally occurs together).
A case study
Swedish Concrete Institute has published technical report (in 2001) on leaching performance of concrete. One of them is described below.
-In the concrete core that was taken out from 85 years old water tank of a filtration plant (constructed in 1906), it was found that about 10 mm of surface was damaged due to leaching. An intersting finding was that, there occured redistribution of Ca with flow of Ca from inner concrete towards the Ca lacking zone resulting in formation of secondary ettringite. It must be noted that in early days, the cement content in concrete used to be relatively higher than nowadays. Thus the leaching process could have consumed relatively higher amount of Ca. Nonetheless, we can guess that most of the leaching should have occurred due to diffusion process since the depth of leaching is relatively lower (only about 10 mm).