When installing stone materials with wet cement mortar, alkaline substances in the cement can migrate to the surface of the stone through its joints or capillaries. As the mortar dries, these substances can crystallize and leave a white powdery residue known as efflorescence.
Characteristics of Efflorescence
Efflorescence appears as white streaks that flow downward from the joints of the stone.
It seeps out from the coarser capillaries in the stone's surface, creating irregular white powdery patterns due to the varying densities of different stone areas.
Efflorescence can also seep out from cracks in the stone, following their direction and flowing downward.
The primary chemical composition of efflorescence includes calcium salts, such as calcium carbonate (an insoluble salt), calcium sulfate (a moderately soluble salt), and other calcium salts.
Causes of Efflorescence
The cement mortar is too diluted or mixed unevenly, causing the thin slurry to leak out from the joints and solidify on the stone surface.
The joints of the stone are not tightly sealed, allowing mortar to leak out and flow downwards.
Constructing a decorative facade at a high place and working at a rapid pace, resulting in excessive permeation pressure that forces the material out.
Rainwater or other water sources infiltrate the top or back of the stone surface, subsequently flowing out through the joints and carrying cement components with it.
The back surface is subjected to prolonged water pressure (e.g., in steps), causing water to slowly seep out from the front stone joints.
Using improper construction practices that lead to contamination or dripping of cement mortar onto the stone surface.
The causes of efflorescence can be categorized into the following types:
Primary efflorescence
Primary efflorescence, also known as initial efflorescence, occurs when cement slurry flows out from the joints or capillaries of the stone and solidifies on the surface. The primary chemical components are a mixture of calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3). This type of efflorescence, appearing within the first twenty days, is relatively loose and primarily composed of calcium hydroxide. It exhibits strong alkalinity and low hardness, with a Mohs hardness of approximately 1-2, comparable to talc or gypsum, which can be easily scraped off with nails and is mostly removable.
As exposure to air increases, calcium hydroxide gradually reacts with carbon dioxide (CO2) in the atmosphere, undergoing a carbonation reaction that converts it into calcium carbonate. This process reduces alkalinity and increases overall hardness, reaching a Mohs hardness of around 3, comparable to calcite, making it more difficult to remove. The main chemical reaction for primary efflorescence is as follows:
Ca(OH)2 + CO2 + H2O → CaCO3 (white solid) + 2H2O
Secondary efflorescence
Secondary efflorescence, or subsequent efflorescence, occurs in the later stages due to the environmental conditions.
One type of secondary efflorescence is when, over time, primary efflorescence reacts with acidic gases in the atmosphere, such as sulfur dioxide (SO2) and nitrogen dioxide (NO2), resulting in the formation of new salts, including calcium sulfate (CaSO4) and calcium nitrate (Ca(NO3)2).
This type exhibits lower alkalinity compared to primary efflorescence, and its hardness varies depending on the structure of the efflorescence. Its alkalinity can vary, with the outer layer typically being less alkaline than the inner layers, which may complicate removal with plain water. The primary chemical reaction can be represented as follows:
CaCO3 + H2SO4 + H2O → CaSO4·2H2O (white powder) + CO2 + H2O
There is often no clear boundary between primary and this type of subsequent efflorescence. In many cases, the surface layer of the efflorescence is primarily composed of hydrated calcium sulfate (gypsum, CaSO4·2H2O), while the middle layer is predominantly calcium carbonate, and the interior may still contain calcium hydroxide.
The second type of secondary efflorescence occurs when water (such as rainwater, seepage, or leaks) infiltrates the cement mortar layer on the back of the stone and seeps out through the joints or capillaries of the stone. This process can transport soluble components from the cement, such as calcium hydroxide (Ca(OH)2) and sodium hydroxide (NaOH), to the surface of the stone.
The alkaline substances being transported out can further react with carbon dioxide or acidic gases in the atmosphere, generating efflorescence similar to the primary efflorescence and the one described above. Notably, when the infiltrating water is acidic (for example, from acid rain), it can dissolve the calcium components in the cement, forming moderately soluble salts such as calcium sulfate and transporting them to the stone surface. As moisture evaporates from the stone surface, these moderately soluble salts crystallize, resulting in the formation of efflorescence.
