Sodium Silicate

Sodium silicate, commonly known as water glass or liquid glass, is an important inorganic compound with the chemical formula Na₂SiO₃ (though its composition can vary depending on the ratio of sodium oxide to silicon dioxide). It is a colourless or slightly cloudy viscous liquid or white solid that dissolves readily in water to form an alkaline solution. Sodium silicate is one of the most versatile industrial chemicals, extensively used in detergents, adhesives, construction, textiles, water treatment, and as a chemical intermediate. Its unique properties—such as high alkalinity, adhesive strength, and ability to form glassy films—make it indispensable in both traditional and modern industries.

Chemical Composition and Structure

Sodium silicate is a sodium salt of silicic acid and belongs to a family of compounds with varying SiO₂:Na₂O ratios. The ratio determines its properties and applications.
General formula: Na₂O·nSiO₂ (where n typically ranges from 1 to 4).
Key properties:

• Molecular formula (common form): Na₂SiO₃
• Molar mass: 122.06 g/mol (anhydrous)
• Appearance: White powder (solid) or viscous, syrup-like liquid (aqueous solution)
• Density: 2.4 g/cm³ (solid); 1.2–1.5 g/cm³ (solution)
• Solubility: Highly soluble in water; insoluble in alcohol and organic solvents
• pH: 11–13 (strongly alkaline)
• Melting point: Approximately 1088°C (depending on composition)

Chemically, sodium silicate consists of a network of silicon-oxygen tetrahedra (SiO₄) linked together by shared oxygen atoms, with sodium ions balancing the negative charge of the silicate anions. The degree of polymerisation varies depending on the ratio of silica to sodium oxide, influencing viscosity, solubility, and reactivity.

Preparation and Manufacture

Sodium silicate is primarily produced by fusing silica (SiO₂) and sodium carbonate (Na₂CO₃) or sodium hydroxide (NaOH) at high temperatures (around 1100–1200°C).
Chemical reaction: Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂↑
The molten product is then cooled and dissolved in water under pressure to form liquid sodium silicate. Alternatively, hydrothermal methods are used where silica sand reacts with sodium hydroxide in autoclaves at high pressure and temperature.
The choice of raw materials, reaction conditions, and SiO₂:Na₂O ratio determines the grade of sodium silicate produced. Liquid forms are used in detergents, adhesives, and coatings, whereas solid forms serve as precursors in chemical manufacturing.

Historical Background

The use of silicate-based materials dates back to ancient civilisations, where natural forms of sodium silicate were used in pottery glazes and glassmaking. The first synthetic sodium silicate was produced in the early 19th century. In 1818, German chemist Johann Nepomuk von Fuchs prepared and described the substance, coining the term “water glass” due to its glassy appearance when solidified.
By the late 19th and early 20th centuries, sodium silicate became a commercially significant material used in waterproofing, fireproofing, and textile processing. Its versatility expanded during the industrial revolution, establishing it as a cornerstone in modern chemical industries.

Physical and Chemical Behaviour

Sodium silicate behaves as a strongly alkaline substance due to the dissociation of hydroxide ions in aqueous solution. The solution can react with acids to precipitate silica gel, a property widely utilised in coatings, adhesives, and catalysts.
Reaction with acid: Na₂SiO₃ + 2HCl → 2NaCl + H₂SiO₃↓ (silicic acid, which polymerises to form silica gel)
When exposed to air, sodium silicate gradually absorbs carbon dioxide and forms a solid silica layer, enhancing its utility as a sealant or coating material. It also reacts with metal salts to produce insoluble silicates, a principle used in water softening and corrosion control.

Industrial and Technical Applications

Sodium silicate is an exceptionally versatile compound, used across diverse industrial sectors due to its ability to act as an adhesive, binder, detergent builder, and corrosion inhibitor.
1. Detergent and Cleaning IndustrySodium silicate serves as a key ingredient in detergents and cleaning agents.

• Acts as a builder, enhancing detergent performance by softening water and preventing redeposition of dirt.
• Provides alkalinity, aiding in the breakdown of fats and oils.
• Prevents corrosion of metal surfaces in washing machines and dishwashers.
• Used in industrial cleaning formulations for degreasing and descaling.

2. Construction and Building MaterialsIn construction, sodium silicate plays a critical role in enhancing strength, durability, and resistance to moisture.

• Concrete hardening: Applied to concrete surfaces to form a silica gel layer that fills pores, improving hardness and water resistance.
• Cement additive: Improves setting properties and enhances bonding in mortars and plasters.
• Fireproofing and sealing: Used in refractory materials, coatings, and sealants for heat resistance.
• Soil stabilisation: Injected into soils to solidify loose sand, creating stable foundations for construction.

3. Adhesives and BindersSodium silicate acts as a low-cost binder for paperboard, wood composites, and casting moulds. It is also used in corrugated box manufacturing and pelletisation processes for minerals and ceramics. When dried, it forms a rigid, glassy film with strong adhesion properties.
4. Water TreatmentIn water treatment, sodium silicate functions as a corrosion inhibitor and coagulant aid.

• It forms protective silica coatings on metal pipes, preventing corrosion.
• Aids in coagulation by destabilising colloidal particles.
• Reduces scaling and deposition in boilers and cooling systems.

5. Textile and Paper IndustrySodium silicate has long been used in bleaching and dyeing processes. It stabilises hydrogen peroxide in textile bleaching and improves the brightness of paper by acting as a sizing agent.
6. Foundry and Metallurgical ApplicationsIn foundries, sodium silicate is employed as a mould binder for sand cores and moulds. Upon exposure to carbon dioxide, it hardens rapidly, forming a durable casting structure:Na₂SiO₃ + CO₂ → Na₂CO₃ + SiO₂ (solid binder)
This property enables precise casting with minimal defects, making it valuable in steel and iron industries.
7. Automotive and Engineering UsesSodium silicate is used as a sealant and adhesive in automotive repairs. It acts as a radiator sealant and is sometimes used in “liquid glass engine repair” processes to seal microcracks in engine blocks.

Role in Environmental and Green Technologies

Sodium silicate contributes significantly to environmental protection and sustainability.

• Wastewater treatment: Facilitates removal of heavy metals and suspended solids.
• Flue gas treatment: Helps capture particulates and acid gases from industrial emissions.
• Soil remediation: Used to immobilise heavy metals in contaminated soils.
• Carbon capture: Silicate solutions can chemically bind carbon dioxide to form stable carbonate minerals.

Additionally, sodium silicate is integral to eco-friendly detergents and green cement formulations, supporting global sustainability initiatives.

Applications in Fireproofing and Insulation

Sodium silicate solutions are used to make fire-resistant coatings and insulating materials. When applied to fabrics, wood, or paper, it forms a glassy film that resists ignition and slows combustion. In combination with mineral fillers, it produces lightweight insulating panels and refractories used in furnaces and kilns.

Medical and Pharmaceutical Uses

While not a pharmaceutical ingredient itself, sodium silicate has indirect applications in healthcare. It is used in controlled drug release systems, adhesive medical bandages, and as a binding agent in tablet manufacturing. In dentistry and orthopaedics, silicate-based materials derived from sodium silicate contribute to bioactive glass formulations that support bone regeneration.

Laboratory and Analytical Uses

In laboratories, sodium silicate serves as a precursor to silica gel, a widely used desiccant and chromatography medium. It is also employed in chemical synthesis, as a buffering agent, and for glass cleaning and preservation. Limewater and sodium silicate are used together in certain qualitative tests and precipitation reactions involving calcium and magnesium ions.

Safety and Toxicology

Sodium silicate is strongly alkaline and must be handled with caution. Contact with skin or eyes may cause irritation or burns, and inhalation of dust or vapour can irritate the respiratory tract. Ingestion may lead to nausea or gastrointestinal distress due to alkalinity.
Safety guidelines:

• Use gloves, goggles, and protective clothing during handling.
• Store in corrosion-resistant containers away from acids.
• Avoid discharge into the environment without neutralisation, as high alkalinity can harm aquatic life.

Despite its caustic nature, sodium silicate is considered environmentally benign once neutralised, and its degradation products—sodium and silica—are naturally abundant and non-toxic.

Economic and Industrial Significance

Sodium silicate represents a multi-billion-dollar global industry, with applications spanning construction, chemicals, detergents, and paper manufacturing. Major producers include China, the United States, Germany, and India. The global market continues to expand due to increased demand for sustainable materials, high-performance adhesives, and eco-friendly detergents.
Its role as an intermediate compound for producing silica gel, zeolites, and silicate-based ceramics further enhances its economic importance. The compound’s low cost, chemical stability, and adaptability make it a fundamental material in both heavy and fine chemical industries.

Emerging and Advanced Applications

Research continues to expand sodium silicate’s role in innovative technologies:

• Geopolymers: Used in synthesising alkali-activated materials that serve as low-carbon alternatives to Portland cement.
• Nanotechnology: Source material for producing silica nanoparticles and coatings.
• Energy storage: Investigated as a component in sodium-ion batteries and solid electrolytes.
• Catalysis: Serves as a support matrix for heterogeneous catalysts in petrochemical processes.

These emerging applications demonstrate sodium silicate’s evolving relevance in the context of sustainability and advanced materials science.