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Moisture Penetration & Leaching

The cement paste in concrete contains water-soluble compounds such as portlandite (calcium hydroxide). If the concrete is in an environment where moisture is able to migrate through the cement paste such as concrete in a retaining wall the porosity of the concrete will gradually become depleted in water-soluble compounds resulting in an increase in the porosity of the cement paste and progressive weakening of the concrete. In severe conditions the rate of deterioration can accelerate as the increase in porosity that results from leaching makes the cement paste more permeable and more susceptible to leaching.

Petrography can be used to diagnose porosity changes due to leaching.