In wintertime or during spring and autumn the application of mortar and hydraulic binder based mineral render present the risk of carbonation that alters the appearance of mass pigmented decorative products.
This is a natural phenomenon related to the hydration reactions of hydraulic binders (cement and lime) generally used for preparing coloured mortars and finishes. The phenomenon of carbonation depends essentially on weather conditions at the time of application and during the hours and days following (setting and hardening phase of hydraulic binders).
The carbonation does not alter the performance characteristics of the product.
In cold and wet periods, a few days after applying the mass pigmented mineral plaster, whitish efflorescence can be seen. A crystalline film on the surface of the plaster generally forms this. The phenomenon is practically invisible on white plaster while it is visible on intense coloured finishes. The phenomenon occurs at temperatures below + 8°C and in the presence of high humidity. In addition, when the adverse conditions of temperature and humidity occur in the days following application.
In the setting and hardening phase the product releases an amount of lime (free lime) part of which is soluble in water. The excess water in the mixture and that not used by the hydration reaction of the cement tends to migrate outwards and brings with it the free lime in solution. When the water evaporates, the lime reacts with the carbon dioxide in the air and forms calcium carbonate that is visible on the surface. In good weather conditions, the crystals of calcium carbonate form beneath the surface of the plaster and therefore do not appear.
If the efflorescence are minimal they subside or disappear with time otherwise after having moistened properly the plaster they can be removed by the help of a slightly acidic wash (one part of hydrochloric acid and nine parts of water) accompanied by a vigorous brushing. After washing with the solution strong effervescence appear, it is then necessary to wash with clean water.
The carbonation that occurs when the cement is setting is not the only source of efflorescence on plaster. Soluble salts exist in more or less important quantities in building materials. A sufficient amount of water can dissolve and transport them to the surface of render. Once on the surface, the water evaporates and the salts deposit on the surface increasing in volume and generating efflorescence.