What is stone deterioration?
Stone deterioration refers to the various phenomena that affect the appearance, intrinsic quality, and functional use of stone as a building and decorative material. These issues arise due to a range of natural and human-induced factors, resulting in surface manifestations such as rust stains, water marks, salt efflorescence, organic discoloration, oil stains, reduced surface gloss, fading, color enhancement, delamination, and powdering.
Stone material deterioration can be prevented, mitigated, and managed through scientific methods and technical interventions, with a focus on prevention. Current technological advancements enable the effective elimination of most forms of deterioration when appropriate remediation measures are implemented. While some types of deterioration remain challenging, ongoing scientific progress and industry collaboration are expected to provide solutions over time.
Remediating stone deterioration
The remediation of stone material deterioration involves employing suitable technical measures to eliminate the adverse effects of deterioration, thereby restoring the stone's appearance, properties, and functionality to a state comparable to that before the deterioration occurred. The technology for remediating stone deterioration encompasses three main components:
Diagnostic techniques: This includes cause analysis and sample testing. Accurate diagnosis is a prerequisite for effective remediation.
Remediation techniques: This involves methods such as chemical cleaning and specialized physical cleaning (including particle jet cleaning, steam cleaning, and laser cleaning). The most appropriate treatment method is often the most cost-effective.
Preventive techniques: This includes post-cleaning protective measures or isolating sources of contamination.
Factors contributing to stone deterioration
Accurate diagnosis of stone deterioration is essential for effective remediation. The factors contributing to stone deterioration are numerous and can be broadly categorized into two main aspects:
Internal factors: These encompass the inherent qualities and properties of the stone itself, including its structural composition and chemical characteristics, which can predispose it to deterioration.
External factors: These pertain to the conditions associated with the stone's processing and the environment in which it is used. This includes influences from extraction, processing, installation, and the surrounding environment that can contribute to the deterioration of the stone.
Main internal factors
The pores in stone
Stone is generally classified as a porous material, with its pores originating from primary fissures formed during lithification and secondary fissures developed through geological changes.
For instance, granite, an igneous rock primarily composed of quartz, feldspar, and mica, has solidification points of approximately 1720°C, 1290°C, and 900°C, respectively. As magma cools and solidifies into rock, the various mineral components crystallize at different temperatures due to the gradual decrease in heat. Quartz crystallizes first, followed by feldspar, and finally mica. This sequence leads to a phenomenon known as magmatic segregation, resulting in a granular structure and the formation of pore spaces between the grains.
Similarly, marble, which is classified as both a sedimentary and metamorphic rock, is primarily composed of calcium carbonate. During geological sedimentation and recrystallization, internal stresses can cause the formation of fissures and granules.
Most other natural stones exhibit similar granular and porous structures.
The pores in stone can be broadly categorized into three levels based on pore size distribution:
Micro-pores: Pore radius is less than 10 nm (10-8 m). These pores primarily allow gaseous moisture to pass through, with minimal capillary transport due to their small size.
Capillary pores: Pore radius ranges from 10-7 m to 10-4 m. In this range, capillary action is significant, enabling the drawing of moisture and other liquids into the stone and facilitating their movement and diffusion.
Macro-pores: Pore radius exceeds 10-4 m. These larger pores can facilitate the movement of significant volumes of liquid, allowing for liquid transport through mechanisms other than capillary action.
The porous structure of the stone is a primary contributor to various forms of stone deterioration. The naturally occurring micro-cracks and pores in stone predominantly fall within the capillary pore range, allowing for the adsorption of various liquids, fine dust, and contaminants. Over time, these substances can gradually penetrate from the stone's surface into its interior, triggering a series of secondary physical, chemical, and biological changes. This process can lead to a reduction in the decorative effect of the stone in mild cases, while severe cases may shorten the stone's lifespan.
Additionally, the distribution of pore sizes and the morphology of the pores also influence the types of deterioration. For instance, granite has a high permeability rate due to its interconnected pores, which can lead to the formation of "water marks" on the stone. In contrast, limestone tends to have a higher proportion of closed pores, which is associated with the phenomenon of "powdering" in the stone.
The pore characteristics of stone vary significantly, with key indicators including:
Porosity (= pore volume/sample volume × 100%),
Permeability coefficient (also known as hydraulic conductivity, defined as the unit flow rate under a unit hydraulic gradient, indicating the ease with which fluids can pass through),
Water absorption rate (= absorbed water mass/sample mass × 100%).
In general, stone with higher porosity, permeability coefficients, and water absorption rates are more prone to deterioration.
The chemical composition of stone
Compared to other building materials, stone exhibits a much more complex variety of types and chemical compositions. Commonly referred to as marble, granite, slate, etc., these are not specific minerals or rocks but rather commercial classifications of various stone types. Certain stones have chemical compositions that are prone to reacting with substances in the external environment, leading to alterations or erosion of the stone. For example:
Iron content and rust stains: The iron content in stone is directly related to the formation of rust stains. Most stones contain varying degrees of iron compounds, which can undergo oxidation reactions when exposed to humid air, resulting in yellow or brown rust stains. The higher the iron content in the stone, the more susceptible it is to rust stain formation.
Calcium compounds and powdering/spalling: The calcium content in stones (such as marble, dolomite, and limestone) is directly associated with phenomena like powdering and spalling. Most natural marbles primarily consist of carbonates, such as calcium carbonate and magnesium carbonate, along with small amounts of alkaline oxides. In humid environments, calcium carbonate and magnesium carbonate can react with acidic gases in the air, such as sulfur dioxide and nitrogen dioxide, producing calcium sulfate and calcium nitrate. This alters the mineral structure of the marble, leading to a loss of luster, color fading, and powdering in mild cases, while severe cases can result in weathering and spalling of the stone's surface.
Calcareous or argillaceous cement in sandstone: The calcareous or argillaceous cement in sandstone is closely related to the phenomena of yellowing or blackening in the stone.
Common external factors
There are numerous external factors that contribute to stone deterioration, primarily arising from the following aspects:
Moisture – a mediating factor
Almost all chemical reactions or physical processes occurring within the stone are related to water. For instance, moisture can cause the iron components in the stone to rust, and the migration of water can lead to the leaching of calcium compounds from the cement through joints to the stone surface, resulting in efflorescence. However, pure and stable water does not damage stone. For example, some inscriptions in stable water environments have remained well-preserved for hundreds or even thousands of years. Water often serves as a primary medium for other destructive factors, making it an essential consideration when analyzing stone deterioration. It is also one of the main external causes that should be prioritized in the prevention of stone deterioration.
Infiltration of pollutants – physical causes
Many pollutants can infiltrate stone materials through liquids such as water or oil, including:
Aqueous substances: Such as tea, coffee, cola, soy sauce, soup, fruit juice, and waste.
Organic materials: Leachates from wood, straw, cardboard, cigarettes, etc.
Oily and oil-soluble substances: Such as motor oil, cooking oil, gasoline, solvents, and tar.
Adhesive materials: Including pressure-sensitive adhesives, strong glues, marble adhesives, and grout.
Pigmented materials: Such as paint, markers, ink, color powders, crayons, dyes, and printing inks.
Transfer of colored substances: From items like colored fabrics, colored paper, and bleached clothing.
Chemical or biologically active substances
Some pollutants are associated with significant chemical changes and can cause chemical erosion. Additionally, certain organisms secrete acidic substances to absorb specific minerals from the rock, leading to biological erosion. This includes:
Iron materials: Such as sawdust, rust from iron cans, welding sparks, iron components, and other iron objects.
Atmospheric pollution: Environmental pollutants and acid rain can contaminate and corrode stone materials.
Biological growth: The proliferation of organisms such as lichens, mosses, and molds can contribute to the deterioration of stone.
Natural environmental conditions
Wet-dry cycles: The fluctuations in air and surrounding humidity over time cause frequent wet expansion and dry contraction in the stone.
Freeze-thaw cycles: Cold winter temperatures can lead to the formation of ice within or around the gaps in the stone, resulting in expansion and potential cracking.
Salinity and alkalinity: Soluble salts can penetrate the pores of the stone with water and crystallize near the surface, causing expansion.
Wind and sand: Wind-blown sand can erode the surface of the stone, leading to wear and loss of material over time.
Improper methods and poor management
Extraction: Pollution from expansion agents, explosives, and sand.
Cutting: Contamination from iron sawdust, iron filings, and machine oil.
Grinding and Polishing: Pollution from lubricants and coolants.
Transportation: Contamination from markers and iron filings.
Installation: Pollution from concrete, joint fillers, and isolating materials.
Usage: Contamination from pigments, cooking oils, and moss.
Maintenance: Pollution from cleaning agents, waxes, and protective agents.
