Efflorescence (chemistry)

Efflorescence is the process in which a hydrate—a compound containing chemically bound with water molecules—spontaneously loses water. This happens when the pressure of the water inside the hydrate is greater than the pressure of water vapor in the air. Efflorescent materials are hydrated compounds that spontaneously lose water into the atmosphere. People also use the term efflorescence to describe the process of a hydrate losing water and then depositing a crystalline material onto a porous surface after all the water has evaporated. The material left behind is often a salt, as hydrates commonly include salts.

Background

In chemistry, a solution is a mixture of one or more solutes dissolved in a solvent. A solute is any substance that dissolves in a solution to produce a homogeneous mixture, which is a mixture where the composition is uniform. A solvent is the substance in which the solute dissolves. Mixtures contain a higher percentage of solvent than of solute. Many types of solutions exist. Although solvents are most commonly liquids, they can also be gases, liquids, or solids. Solvents can also be solids, liquids, or gases. A common everyday solution is sugar mixed with water. The solution is made by mixing the solid sugar into the water until the sugar dissolves. That particular solution is called an aqueous solution because it contains water.

A hydrate is a substance that is similar to an aqueous solution, but it is a chemical compound in which the water molecules are chemically bonded with the dissolved material. Hydrates include molecules of water in their crystalline structures. Hydrates can be created from aqueous solutions. They also commonly include salts.

People can remove the water from most hydrates by heating them. After heating, the remaining substance is called the anhydrous compound. The anhydrous compound is the substance that originally bonded with the water to create the hydrate. Many anhydrous compounds are soluble in water.

People can remove water from hydrates to create anhydrous compounds. They can also add water to anhydrous compounds to create hydrates. For example, copper sulfate pentahydrate is an example of a hydrate. This material has a blue crystalline structure. Its chemical formula is CuSO₄·5H₂O. The H₂O in the chemical formula shows that hydrate includes water. When this material is heated, the water escapes from the crystals. Because of the chemical change, the blue crystals transform into a white powder. The hydrate can then be reformed by adding water to the white powder.

Overview

Efflorescence occurs when a hydrate loses water spontaneously, which means the water evaporates without heating. When the water evaporates, the hydrate is changed so that the water is removed from the compound. Once the water is removed, only the anhydrous compound is left behind. Some hydrated compounds spontaneously lose water into the atmosphere. These compounds are referred to as efflorescent because the process that occurs when they lose water is efflorescence.

Efflorescence is affected by the humidity in the air. Efflorescence occurs only when the water vapor pressure in a hydrate is greater than the water vapor pressure in the atmosphere. In other words, the chance of efflorescence occurring increases as the humidity of the environment decreases. Efflorescence is more likely to occur in relatively dry environments than it is in relatively moist environments. It will occur until an equilibrium is reached where the pressure of the water vapor in the hydrate is the same as the pressure of the water vapor in the environment.

Deliquescence is the opposite of efflorescence. Both phenomena are often discussed together because they are often related. Deliquescence occurs when a substance absorbs water from the atmosphere. For deliquescence to take place, a material has to absorb so much water from the atmosphere that it dissolves into the water. These materials will create hydrates when they absorb enough water. Deliquescent substances are substances that absorb water vapor from the air. These materials absorb so much water that they dissolve and form aqueous solutions. Hygroscopic substances are similar in that they absorb water from the atmosphere, but they do not absorb enough water to create an aqueous solution. Deliquescence is affected by humidity in a similar way to efflorescence.

The term efflorescence comes from the French word meaning “to flower out.” This definition comes from the deposits of salts and other materials that can occur on porous surfaces because of efflorescence. The term efflorescence is commonly used to describe the process that occurs when an aqueous solution, usually containing salts, migrates through a porous material and deposits salts or other solutes on the material’s surface.

Concrete that is curing can get white crystals on the exterior. When the concrete is curing, salts and minerals that naturally occur in the mixture can be dissolved by water and form aqueous solutions. These solutions can migrate through the concrete. Since the salts and minerals are part of the solution, they move as well. They can move to the exterior or the concrete. As the concrete cures, water evaporates from the hydrates or solutions. As it evaporates, the solutes or the anhydrous compounds are all that remains. These will often develop into white crystals on the concrete’s surface. Efflorescence in curing concrete usually does not damage the structure because the salts and minerals that were dissolved and deposited were naturally occurring inside the material. However, efflorescence can be harmful to materials when dissolved salts or other materials are moved into a material via an aqueous solution.

People also commonly use the term efflorescence to refer to the substance left behind on the surface of materials by the efflorescence process. Efflorescence commonly occurs on concrete, masonry, and stone surfaces as these materials are porous and allow for the movement of aqueous solutions. Researchers have focused on reducing efflorescence in sustainable concrete materials such as geopolymers and alkali-activated cement systems. Researchers have also used artificial intelligence systems to detect efflorescence damage on concrete and masonry surfaces.

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