Characteristics of water stains
Water stains occur when hygroscopic substances penetrate the stone, resulting in moisture marks on the surface that are difficult to dry naturally. The main characteristics include:
Visible moisture stains: To the naked eye, there appear to be traces similar to wetness.
Resistance to drying: These moisture stains remain wet or are difficult to dry completely. Even when exposed to sunlight or heated, the moisture stains persist.
Water stains are one of the most commonly observed defects in stone materials, and due to their challenging nature to remedy, many refer to them as "stone cancer."
Classification of water stains
Based on their causes, water stains can generally be categorized into the following types:
Water stains primarily composed of alkali-silica gel
Water stains formed by hygroscopic salts and alkali-silica gel
Water stains coexisting with alkali-silica gel and moisture sources
Imprints similar to water stains from dried moisture
Alkali-silica gel from cement
It has been observed that using stones with a high water absorption rate in conjunction with wet cement mortar for wet lay increases the likelihood of water stains occurring. The specific characteristics include:
Large area stains: When the stone is unprotected, the resulting water stains often appear in large, contiguous areas.
Visible brush marks: If the protective agent is applied unevenly on the surface or bottom of the stone, the water stains typically show clear brush marks and distinct boundaries.
Edge stains: When the protective agent is not applied to the sides of the stone, or when edge cutting damages the protective layer, water stains tend to appear at the edges, radiating from the edge toward the center.
Water stains can be simulated in a laboratory under conditions that mimic large-area wet lay of stone. The steps are as follows:
Prepare a box measuring 160 mm × 160 mm × 25 mm.
Line the box with a plastic film measuring 240 mm × 240 mm.
Add cement mortar with a cement-to-sand mass ratio of 1:2.5, mixing in an appropriate amount of water until uniform.
Place the stone (150 mm × 150 mm × 20 mm) face down, ensuring the cement mortar reaches about half the thickness of the stone. Fold the edges of the plastic film over the stone surface and seal the perimeter with plastic tape.
For granite that has high porosity, water stains will appear on the experimental samples made using the above method after 7 days of placement.
Analysis of samples taken from the surface of the stone indicates that cement components have migrated from the bottom to the surface layer, where they solidify within the material. Among these components, gel-like substances account for over 70%, primarily consisting of alkali-silica gel formed from the cement.
The alkali-silica gel has a network structure with a specific surface area of 200 to 600 m2/g, and the size of the gel pores ranges from 1.5 to 3.0 nm, which is only an order of magnitude larger than water molecules. These gels exhibit strong hygroscopic properties and a certain degree of water permeability (with a permeability coefficient of approximately 10-4 cm/s), occupying the pores in the surface layer of the stone. The water is adsorbed in the extremely fine pores of the gel, appearing translucent and altering the light reflectivity of the surface, giving the impression that the stone is wet.
Hygroscopic salts from various sources
During the wet lay process and after the installation of stone, hygroscopic salts from various sources can accumulate on the surface layer of the stone through the migration of water. These sources include:
Use of highly alkaline or low-quality cement: The oxides in the cement aggregate, such as Na2O and K2O, have an alkali content ranging from 0.6% to 1.5%. When mixed with water, they generate soluble alkalis like NaOH and KOH. Some of these react with the active silica (SiO2) in the aggregate to form alkali-silica gel, while others react with dolomite powder in the aggregate to produce sodium carbonate and calcium hydroxide. Excess NaOH and KOH can precipitate from the pores of the stone.
Improper use of cement additives: Certain additives, such as early-strength agents and antifreeze agents, can introduce soluble salts. Typical examples include NaCl, CaCl2, and triethanolamine.
Soluble salts from substrate materials: Soluble salts from the underlying materials, such as soil and sand, can migrate upward with capillary water. When the moisture evaporates, these salts remain on the surface layer of the stone.
Use of strong acids for cleaning: When strong acids are used to clean rust stains or cement stains from the stone surface, they react with the contaminants to form hygroscopic soluble salts. For instance, hydrochloric acid used to clean rust or cement stains can generate hygroscopic salts like iron chloride and calcium chloride. It is important to note that many stones are treated with strong acids before leaving the factory, allowing some hygroscopic salts to enter the stone's pores. During the wet lay of cement mortar, these salts can combine with alkali-silica gel to form water stains.
Use of low-quality protective agents: Some low-quality water-based protective agents, particularly those with strong alkalinity, can easily form hygroscopic alkalis such as NaOH and KOH.
Environmental pollution: Acid rain and acid mist can react with various metal oxides and carbonates, such as calcium carbonate and magnesium carbonate, as well as other alkaline chemicals present in the stone. This reaction results in the formation of soluble salts.
Experiments have demonstrated that after the wet lay of stone, excess alkalis from the cement hydration reaction, generated salts, soluble salts introduced by various additives, hygroscopic salts formed from acid cleaning of the stone, and soluble salts produced by acid rain and acid mist pollution can all migrate to the surface layer of the stone under the influence of capillary water, along with alkali-silica gel.
Hygroscopic salts, which include deliquescent salts, have a lower vapor pressure. When the vapor pressure of these salts is lower than the humidity in the air, they will absorb moisture, potentially leading to wet stains. For example, table salt (sodium chloride) is hygroscopic and can absorb moisture in humid environments. And salts formed from strong acid cleaning of rust or cement stains often exhibit significantly greater hygroscopicity than table salt.
Additionally, the pore sizes in stone are very small, typically ranging from 0.1 to 50 μm. The surfaces of these pores exhibit significant hydrophilicity, resulting in strong capillary moisture absorption. From a physicochemical perspective, the essence of the capillary water absorption phenomenon in stone pores is that the pores generate negative additional pressure. This pressure drop within the pores follows the Laplace equation: Pw-Pa=2ycos(π-θ)/r where Pw is the pressure within the pore, Pa is the atmospheric pressure, y represents the surface tension of the liquid, θ is the contact angle, and r is the radius of the pore.
From this equation, it is evident that:
The smaller the pore radius (r) in the stone, the greater the additional pressure and the stronger the capillary moisture absorption.
The more hydrophilic the wall of the stone (the smaller the contact angle (θ), the greater the additional pressure and the stronger the capillary moisture absorption.
Therefore, the capillary pores in the stone and the hydrophilic pore walls are significant reasons for the stone's ability to automatically adsorb water.
The hygroscopicity of the salts, combined with the moisture absorption property of the stone's pores, means that, regardless of external weather conditions, water on the surface layer of the stone is always difficult to dry. Even if the surface of the stone is temporarily dried, the presence of salts and pores will lead to moisture absorption again as the humidity in the air increases. This results in persistent dampness.
Effects of moisture and capillary water migration
One scenario involves stone that is directly or indirectly connected to a moisture source during wet lay process. For example, if there is damp soil beneath or behind the stone, or if the groundwater level is high; if there are nearby water features (such as fountains) or frequent water seepage (such as in bathrooms); or if there is water accumulation on the roof or poor drainage, it is common to see noticeable water stain marks on the stone surface. The primary reason for this is that, due to the connection with the water source, gel-like substances in the mortar and soluble salts can continuously migrate through the stone's pores with capillary water and accumulate on the stone's surface as moisture evaporates. Even with various drying treatments applied to the stone surface, the water stains will not dry due to the constant supply of capillary water.
Another scenario occurs when excessive amounts of water are used or come into contact with the stone during installation and maintenance. For instance, if the stone is wet before installation or has been exposed to rain; if the cement mortar is too diluted or excessive during application; or if a large amount of water is used to wash the stone during the curing period before the cement has fully set, or if the stone is submerged in water. The excess moisture present in the stone, cement, or joints will carry more alkali-silica gel and hygroscopic salts to the surface layer of the stone as it migrates outward and evaporates, exacerbating the water stains.
Other substances in the pores of the stone surface
When stone remains in a damp state for an extended period, the moist surface easily attracts dust. Over time, contaminants can infiltrate the stone's pores, and once the moisture evaporates, they leave behind water stain imprints.
Additionally, when certain liquid substances, such as adhesives, pollutants, or low-quality protective agents, flow onto the stone surface and seep into the pores, they can create imprints similar to water stains.
According to the standard definition of water stains, the stains resulting from this cause do not qualify as true water stains. This is because the substances forming these imprints are not necessarily hygroscopic. Therefore, in many cases, stains of this type are relatively easier to remedy.
