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Calcium silicate

Calcium silicate is a class of inorganic compounds consisting primarily of (CaO) and (SiO₂), with common stoichiometries including CaSiO₃ (), Ca₂SiO₄ (), and Ca₃SiO₅ (), often occurring in hydrated forms such as (C-S-H). These materials are typically white or off-white, free-flowing powders that are insoluble in , possess a of approximately 2.9 g/cm³, and exhibit high with melting points exceeding 1500°C. Commercially produced by reacting (calcium hydroxide or oxide) with or other silica sources through processes like or autoclaving, calcium silicate is valued for its low thermal conductivity, fire resistance, and chemical inertness. In the construction industry, it serves as a primary component in , where tricalcium and dicalcium silicates contribute to the reactions that develop concrete's strength and durability. High-temperature insulation boards and pipe coverings made from calcium silicate provide and thermal barriers in industrial settings, outperforming alternatives due to their non-toxic nature and mechanical stability. As a food additive, calcium silicate functions as an anticaking agent to prevent clumping in powdered products like , spices, and , with regulatory limits set at no more than 2% by weight of the food (or 5% in ) to ensure safety and efficacy. In biomedical applications, its bioactive properties—releasing calcium and silicate ions that promote and mineralization—make it suitable for dental cements, scaffolds, and endodontic materials that support regeneration. Additionally, it finds use as a reinforcing filler in polymers, rubbers, and ceramics, enhancing mechanical properties while maintaining environmental compatibility.

Properties

Chemical properties

Calcium silicates encompass a family of compounds characterized by varying stoichiometric ratios of calcium, , and oxygen, primarily represented by formulas such as (wollastonite, a metasilicate with a 1:1 Ca:Si ratio) and Ca₂SiO₄ (larnite, an with a 2:1 Ca:Si ratio). In these structures, Ca²⁺ cations form ionic bonds with anions, such as [SiO₃]²⁻ in metasilicates or [SiO₄]⁴⁻ in orthosilicates, where the -oxygen framework provides the anionic component through covalent Si-O bonds balanced by electrostatic interactions with calcium. These ionic interactions contribute to the overall stability of the compounds, with the anions exhibiting partial covalent character that influences reactivity. Compositional variations occur across different forms, including tricalcium silicate (Ca₃SiO₅, denoted as C₃S with a 3:1 Ca:Si ratio) and dicalcium silicate (Ca₂SiO₄, C₂S with a 2:1 Ca:Si ratio), which can exist as solid solutions incorporating impurities like phosphates or aluminates. These solid solutions allow for deviations from ideal , affecting properties such as reactivity, where C₃S exhibits higher calcium content and thus greater susceptibility to compared to C₂S. Calcium silicates demonstrate notable reactivity with acids, undergoing dissolution to release calcium ions and form silicic species; for instance, wollastonite reacts with hydrochloric acid according to the equation CaSiO₃ + 2HCl → CaCl₂ + H₂SiO₃. Similarly, dicalcium silicate dissolves as Ca₂SiO₄ + 4HCl → 2CaCl₂ + SiO₂ + 2H₂O, highlighting the breakdown of the silicate framework under acidic conditions. Their solubility is pH-dependent, with minimal dissolution near neutral pH but increasing significantly in acidic environments (pH < 7) due to protonation of silicate anions, while in alkaline conditions (pH > 10), solubility rises owing to the formation of soluble silicate species. For hydrated forms like calcium silicate hydrates (C-S-H), solubility is governed by a solubility product constant (Ksp) approximately on the order of 10⁻⁷ to 10⁻⁸, indicating low but measurable dissolution in aqueous media, with values varying based on Ca/Si ratio and crystallinity.

Physical properties

Calcium silicate exhibits a range of physical properties depending on its specific form, such as anhydrous minerals like wollastonite (CaSiO₃) or dicalcium silicate (Ca₂SiO₄), and hydrated gels like calcium silicate hydrate (C-S-H). These properties influence its handling, processing, and utility in industrial applications. Anhydrous forms generally display higher densities and thermal stability compared to hydrates, which are more porous and lower in density. The density of anhydrous calcium silicates typically ranges from 2.9 to 3.1 g/cm³, as seen in with a specific gravity of approximately 2.9. Hydrated forms, such as the C-S-H gel prevalent in pastes, have lower densities of 2.2–2.6 g/cm³ due to their amorphous, nanoporous structure. These variations arise from the incorporation of water and the resulting gel-like morphology in hydrates. Melting points for calcium silicates are high, reflecting their nature. For example, dicalcium silicate (Ca₂SiO₄) has a melting point of approximately 2130°C, though it undergoes incongruent melting, decomposing into (CaO) and a phase rather than melting congruently. In contrast, (CaSiO₃) melts at around 1540°C. In terms of appearance, calcium silicate commonly presents as white to off-white powders in synthetic or ground forms, while mineral variants like occur as fibrous or acicular (needle-like) crystals. These materials have a Mohs of 4.5–5.5 for mineral forms, providing moderate suitable for processing into fine particles. Thermal conductivity varies significantly with density and . Porous insulating forms of calcium silicate, often used in high-temperature applications, exhibit low values of 0.05–0.1 W/m·K at ambient to moderate temperatures (e.g., 0.06 W/m·K at 200°C). Dense variants, however, show higher conductivity, typically 1–2 W/m·K, due to reduced air voids. Industrial calcium silicate powders are characterized by micron-sized particles, often ranging from 2–20 μm in size, forming aggregates that enhance reactivity in processes like hydration or mixing. The morphology includes acicular or platy shapes in wollastonite-derived powders, contributing to their flowability and packing efficiency.

Occurrence and production

Natural occurrence

Calcium silicates are primarily encountered in nature as minerals formed through metamorphic and metasomatic processes in the Earth's crust. The most common form is wollastonite (CaSiO₃), which develops in skarn deposits associated with contact metamorphism, where siliceous limestones or dolomites react with intrusive igneous rocks. Another notable mineral is rankinite (Ca₃Si₂O₇), which occurs in zones of contact metamorphism, particularly in altered limestone xenoliths within igneous intrusions. These minerals are frequently associated with other calc-silicate and phases in limestone-derived deposits, including , , and , reflecting their formation in silica- and calcium-rich environments. , for instance, often appears intergrown with these minerals in tactite or assemblages, while rankinite is found alongside larnite and other high-temperature calc-silicates in localized reaction zones. Major global deposits of are located in , the (particularly ), , , and , with accounting for over 70% of production. Annual world production of wollastonite reached approximately 1,010,000 metric tons in 2023 (estimated 1,100,000 in 2024), primarily extracted from these metamorphic terrains. Rankinite occurrences are rarer and more localized, with significant sites in Ireland (Scawt Hill) and ( and Isle of Muck). The formation of these calcium silicate minerals typically involves , where silica-rich hydrothermal fluids interact with rocks under elevated temperatures of 400–800°C, leading to the replacement of carbonates by silicates. This process releases volatiles like water and , facilitating the crystallization of in skarns and rankinite in high-temperature contact zones. Rare occurrences of calcium silicates extend beyond terrestrial settings, including high-pressure variants in lunar meteorites formed as condensates in impact melt pockets, and in volcanic ejecta as products of rapid cooling in high-temperature environments.

Industrial production

Calcium silicate is industrially produced through several methods tailored to achieve specific forms, such as hydrated or phases, with scalability and purity as key considerations for commercial applications. The primary method is , where (CaO) or is reacted with silica sources like sand or in an aqueous . This process occurs in autoclaves at temperatures of 150–200°C and pressures of 8–15 bar for several hours, yielding hydrated calcium silicates such as (Ca₅Si₆O₁₆(OH)₂·4H₂O). Another common approach is high-temperature , particularly for calcium silicates used in production. A mixture of and silica is heated to 1200–1500°C in rotary kilns, where decomposes to , which then reacts with silica to form clinker phases like dicalcium silicate (2CaO·SiO₂, or C₂S) and tricalcium silicate (3CaO·SiO₂, or C₃S), following the simplified equation CaCO₃ + SiO₂ → CaSiO₃ + CO₂ after initial . Precipitation from aqueous solutions provides a route for amorphous or gel-like calcium silicates, involving the reaction of with under controlled conditions (typically 9–11) at ambient or mildly elevated temperatures. This method allows precise control over and morphology, suitable for specialty applications. Industrial byproducts serve as sustainable feedstocks for calcium silicate production; for instance, , rich in calcium silicates, is processed through grinding, , and classification to isolate and purify the material for reuse. Similarly, dust can be leached and reacted to recover calcium silicate phases. Recent advancements since 2020 include energy-efficient microwave-assisted synthesis, which accelerates reactions in solid-state or hydrothermal setups, reducing required temperatures by 20–30% and shortening processing times compared to conventional heating, thereby lowering energy use in producing phases like .

Structure

Crystalline forms

Calcium silicates exhibit several anhydrous crystalline polymorphs, primarily (CaSiO₃) and the dicalcium silicate (C₂S, Ca₂SiO₄) phases, each characterized by distinct atomic arrangements that influence their and . These structures consist of silicon-oxygen tetrahedra coordinated with calcium cations, forming frameworks that range from chain-like to isolated units. The polymorphs differ in , , and , with transitions between forms occurring at specific temperatures. Wollastonite adopts a triclinic with P1, featuring infinite single chains of SiO₄ tetrahedra that share vertices and run parallel to the b-axis, linked by Ca²⁺ ions in irregular polyhedra. This chain motif, known as a pyroxenoid , repeats every three tetrahedra, providing structural rigidity while allowing flexibility in occurrences. The unit cell parameters are approximately a = 7.94 , b = 7.32 , and c = 7.07 , reflecting the low symmetry of the triclinic system. In contrast, larnite, the β polymorph of C₂S (β-Ca₂SiO₄), possesses a monoclinic structure with P2₁/n, stable at high temperatures above approximately 620°C. Its framework comprises isolated SiO₄ tetrahedra interspersed within a network of interconnected Ca polyhedra, forming a glaserite-like arrangement that promotes thermal stability. The unit cell dimensions are a = 5.5051(3) , b = 6.7551(3) , c = 9.3108(5) , and β = 94.513(4)°, accommodating four formula units (Z = 4). Tricalcium silicate (C₃S, Ca₃SiO₅), known as and the main component of clinker, exhibits multiple polymorphs depending on temperature and impurities, including three triclinic (T1, T2, T3), three monoclinic (M1, M2, M3), and one rhombohedral (R) form. The structure consists of isolated SiO₄ tetrahedra coordinated by Ca²⁺ ions in a complex framework. The high-temperature rhombohedral polymorph adopts R3m, while lower-temperature forms are typically monoclinic or triclinic. Polymorphic transitions in C₂S significantly affect material reactivity; for instance, the α and α′ forms convert to the β phase upon cooling through 675–750°C, enhancing compared to the less reactive γ polymorph stable below this range. This β form, metastable at , exhibits higher reactivity in applications like due to its distorted tetrahedra, which facilitate mobility during reactions. X-ray diffraction (XRD) patterns serve as key identifiers for these polymorphs; wollastonite, for example, displays characteristic peaks at 2θ ≈ 29.9°, 25.4°, and 50.9°, corresponding to its triclinic chain structure and enabling phase discrimination in mixtures. Defect structures in these crystals, such as vacancies and cation substitutions, play a critical role in modulating electrical properties; in di-calcium silicate, lattice defects like oxygen vacancies contribute to ionic conductance by enabling mobility, as determined through measurements of electrical resistivity. Substitutions, such as minor divalent cations replacing Ca²⁺, can introduce localized charge imbalances that further influence conductivity without disrupting the overall framework.

Hydrated structures

Calcium silicate hydrate (C-S-H) is the principal hydrated phase in cementitious materials, characterized by a poorly crystalline, gel-like structure that incorporates into its framework. This structure consists of layered Ca-O-Si sheets formed by layers linked to chains, primarily in the form of dreierketten units with Q² environments, separated by interlayers containing molecules and additional calcium ions. The typical of C-S-H in systems approximates Ca1.7SiO3.7·nH2O, reflecting a Ca/Si molar ratio around 1.7, though this can vary between 0.7 and 2.0 depending on synthesis conditions. Related crystalline phases include tobermorite variants, which serve as structural models for C-S-H due to their layered architectures. The 11 Å tobermorite features a basal spacing of approximately 1.13 nm and the layer formula [Ca4Si6O16(OH)2]·nH2O with n ≈ 4, while the 14 Å variant (plombierite) has an expanded interlayer with n ≈ 7–8, accommodating more water. These phases exhibit zeolite-like channels within the framework, formed by the cross-linking of silicate tetrahedra, which facilitate and water mobility. C-S-H forms primarily through pozzolanic reactions, where amorphous silica reacts with in aqueous environments to yield the hydrated : SiO2 + Ca(OH)2 → C-S-H + H2O. This process generates a nanoscale porous with gel pores typically ranging from 2 to 5 nm, contributing to the material's high surface area and water retention capacity. The thermal stability of hydrated structures is limited, with dehydration occurring progressively between 100°C and 500°C, resulting in loss of interlayer water, shrinkage, and eventual structural collapse into denser, phases. Spectroscopic techniques provide key insights into bonding; 29Si NMR reveals dominant Q² sites indicative of chain-like polymerization, while () spectroscopy identifies Si-O-Ca stretching modes around 950–970 cm−1 and Ca-O near 300–500 cm−1, confirming the connectivity in the Ca-O-Si framework.

Applications

Construction and cement

Calcium silicate phases are fundamental constituents of Portland cement clinker, where tricalcium silicate (C₃S, also known as ) typically comprises 50–70% of the clinker and is primarily responsible for the early strength development during . Dicalcium silicate (C₂S, or ) accounts for 15–30% and contributes to the long-term and later-age strength gain, as its slower allows for sustained structural enhancement over time. These phases form the hydraulic matrix essential for concrete's mechanical properties in applications. The hydration of C₃S is a key process in cement setting, represented by the reaction C₃S + H₂O → C-S-H + Ca(OH)₂, which produces (C-S-H) gel and while evolving significant heat, approximately 500 J/g. This drives the initial and final setting times, with standards requiring a minimum initial set of 45 minutes and a maximum final set of 375 minutes (6 hours 15 minutes) using the Vicat to ensure workability and prevent premature stiffening. The resulting C-S-H gel densifies over time, leading to typical compressive strengths of 40–50 at 28 days in standard mortars. Modifications to standard formulations, such as the addition of , are used to produce low-heat cements by reducing the C₃S content and overall clinker proportion, thereby minimizing heat evolution and thermal cracking in pours. itself was first patented in 1824 by Joseph Aspdin, who developed a resembling , and modern compositions adhere to standards like ASTM C150 for consistent performance in construction.

Thermal insulation

Calcium silicate is widely utilized in high-temperature applications due to its low thermal conductivity and structural integrity. It is commonly produced in the form of rigid coverings and block boards, which provide effective heat retention in settings. These materials typically exhibit densities ranging from 240 to 350 kg/m³ (15 to 22 lb/ft³), enabling lightweight yet durable suitable for service temperatures up to 650°C. The manufacturing process involves autoclaving a mixture of silica (from sand or similar sources) and () under high pressure and temperature, typically around 180–200°C, to form fibrous (C-S-H) structures. This hydrothermal reaction yields a porous, interlocking network of fibers with diameters ranging from 0.1 to 1 μm, contributing to the material's low and insulating properties. The resulting is molded into sections or blocks, cured in the for several hours, and then dried to produce the final product. Performance characteristics include a thermal conductivity of approximately 0.06 W/m·K at mean temperatures around 200°C, increasing to about 0.20 W/m·K at 650°C, which supports efficient heat management in elevated-temperature environments. Additionally, these insulations demonstrate minimal shrinkage, typically less than 2% after 24 hours of exposure at 800°C, ensuring dimensional stability under thermal cycling. In industrial applications, calcium silicate is employed to line boilers, furnaces, and in plants, where it minimizes loss and protects surrounding structures from excessive temperatures. Its non-combustible nature and resistance to chemical further enhance its suitability for these demanding conditions. As an environmentally beneficial option, calcium silicate insulation serves as an asbestos-free alternative, developed and widely adopted following and regulations that banned asbestos-containing materials in pipe and block insulations due to risks. This shift has allowed for safer, compliant use in high-temperature systems without compromising performance.

Fire protection

Calcium silicate materials are widely employed in passive fireproofing applications, particularly in the form of boards and preformed blocks, to protect structural elements and enclosures from fire exposure. These materials provide a non-combustible barrier (Class A1 rating) that maintains structural integrity during high-temperature events, preventing the spread of flames and heat transfer to underlying substrates. Unlike active suppression systems, calcium silicate relies on its inherent thermal stability and insulating properties to delay fire progression, making it suitable for use in fire doors, partition walls, ceilings, and structural steel encasements. The primary fire protection mechanism of involves its low thermal conductivity, which limits heat transmission, combined with an endothermic during heating. Upon exposure to temperatures above 100°C, hydrated forms such as (C-S-H) and associated (Ca(OH)₂) undergo , absorbing significant heat energy while releasing that further insulates and dilutes combustible gases. This endothermic reaction helps maintain lower temperatures on protected surfaces without contributing to fire intensity, as the material remains stable up to 1200°C without melting or emitting toxic fumes. In structural steel applications, boards are applied as encasements to prevent the steel core from exceeding 540°C—the critical where structural strength is significantly compromised—for durations up to 120 minutes under standard conditions. These systems are tested to ensure the protected retains load-bearing capacity, with typical configurations achieving 2–4 hour fire resistance ratings. For fire scenarios common in industrial settings, spray-applied or trowelable calcium silicate-based formulations, such as those meeting UL 1709 rapid-rise fire tests, provide equivalent protection by forming a dense, heat-resistant layer on beams, columns, and vessels. Fire performance of calcium silicate fireproofing is rigorously evaluated under standards like ASTM E119 for load-bearing assemblies, which simulates cellulosic fires in building construction, and UL 1709 for rapid fires in environments. These tests measure the time to failure based on ( rise limited to 250°C average or 325°C maximum on the unexposed side) and structural integrity criteria, confirming calcium silicate's reliability in maintaining compartmentation and occupant safety.

Environmental remediation

Calcium silicate serves as an effective neutralizing agent in the treatment of (), a highly acidic generated from activities that contains elevated levels of and sulfates. The primary mechanism involves the incongruent dissolution of calcium silicate (CaSiO₃), which releases calcium ions (Ca²⁺) into solution according to the reaction CaSiO₃ + 2H⁺ → Ca²⁺ + H₂SiO₃, thereby consuming protons and elevating the while leaving behind an amorphous silica-rich residue. This process facilitates the of metal hydroxides and oxy-hydroxides, such as iron and aluminum compounds, which act as sinks for trace contaminants including , , , and . In laboratory experiments using nearly pure (a form of CaSiO₃) with natural at an initial of 2.1, the increased to 3.5 over extended contact times (15–80 days), with significant retention of (up to 49.8 ppm) and near-complete removal of (>99%). Similarly, modified calcium silicate at dosages of 1 g/L has raised from 5.6 to 8.84 in samples, reducing acidity from 183 mg/L to 22 mg/L as CaCO₃ equivalent. Recent studies as of 2025 have also explored its use in adsorbing (H₂S) from industrial emissions, leveraging calcium silicate hydrate's surface properties for gas capture in environmental control systems. Optimal dosage rates for neutralization typically range from 1–5 g/L, determined through jar test protocols that simulate mixing and settling conditions to assess adjustment and metal removal efficiency without excessive reagent use. These tests involve incremental additions of calcium silicate to samples under controlled , followed by measurement of , , and metal concentrations to identify the minimum effective dose that achieves target levels (often 6–8) and precipitates >90% of key metals like iron and . Steel slag, rich in calcium silicates, has been evaluated in such protocols, demonstrating sustained production up to 80 mg/L over 12 hours of contact. In beyond AMD, calcium silicate excels at adsorbing due to its high surface area and the presence of (Si-OH) groups on the silica components, which provide negatively charged sites for cation binding via electrostatic attraction and surface complexation. For instance, (CSH) derived from fly ash removes up to 85% of Pb²⁺ from single-metal solutions at neutral , with maximum capacities exceeding 200 mg/g through where released Ca²⁺ is replaced by Pb²⁺. The groups enhance selectivity in acidic conditions by deprotonating at higher , facilitating adsorption of metals like Pb²⁺, Cu²⁺, and Zn²⁺ while minimizing interference from competing ions. Field implementations of calcium silicate-based systems, particularly using steel slag as a permeable reactive barrier (PRB), have been deployed at sites since the early 2000s to passively treat . These PRBs consist of trenches filled with granular steel slag, allowing to flow through and undergo neutralization and metal without active pumping. At sites like those investigated in the U.S. and , steel slag PRBs have achieved >95% removal of (e.g., , , Zn) and pH increases to near-neutral levels over multi-year operations, with maintained above 10⁻⁴ cm/s to prevent . Byproduct management from these treatments includes the formation of (H₂SiO₃) gels during dissolution, which can be further processed for silica recovery to enhance . The residual silica-rich phases, often in gel form, are separated via or and subjected to acidification or to yield high-purity amorphous silica for industrial reuse, reducing waste disposal needs and recovering valuable materials from the remediation process.

Agriculture and fertilizers

Calcium silicate serves as an effective soil amendment in agriculture, particularly for delivering bioavailable () to crops. It enhances silicon nutrition, which is crucial for plants like that benefit from Si supplementation to bolster structural integrity and physiological functions. Application of silicon-calcium fertilizers has been shown to increase yield by approximately 20-21% through improved root development, uptake, and , while also enhancing disease resistance against pests and pathogens such as rice blast and sheath blight. In addition to silicon provision, calcium silicate exhibits a liming effect by slowly dissolving in to release calcium ions (Ca²⁺), which neutralize acidity and elevate in acidic fields. This is particularly valuable in regions with low pH soils, where typical application rates range from 1 to 2 tons per to achieve desired pH adjustments of 0.5 to 1 unit. The material's in acidic conditions facilitates gradual Ca²⁺ supply without rapid pH spikes, supporting sustained . Common forms of calcium silicate used in fertilizers include powder, derived from natural mineral deposits, and slag-based products from industrial byproducts, both offering slow-release characteristics. These forms provide nutrients over 2 to 3 years, minimizing the need for frequent reapplication and promoting long-term . In tropical soils, such amendments reduce aluminum () toxicity by complexing Al³⁺ ions and improving growth, as demonstrated in field studies from and during the 2010s, where supplementation alleviated Al and boosted in acidic environments. Since 2019, calcium silicate has been recognized under the Fertilising Products as a permitted liming material, allowing its inclusion in fertilizers containing oxides, hydroxides, carbonates, or silicates of calcium for agricultural use across member states.

Other uses

Calcium silicate serves as a filler in -based sealants and , where its fibrous structure enhances mechanical reinforcement and cohesive strength, contributing to better on various substrates. , a crystalline form of calcium silicate, specifically improves to hydrophilic surfaces in adhesive formulations, while the inherent weather resistance of matrices is maintained or augmented by such fillers for outdoor applications. In pharmaceutical applications, precipitated calcium silicate is incorporated into formulations to neutralize gastric acidity and adjust in the , providing relief from acid-related conditions like . Its neutral and absorbent properties help stabilize the formulation and facilitate the release of active ingredients in the . As a designated E552, calcium silicate functions as an anti-caking agent in powdered foods such as table salt, , and dry mixes, preventing clumping by absorbing excess moisture. It holds (GRAS) status from the FDA, permitting its use in amounts not exceeding those necessary to achieve the intended effect, typically up to 5% in dry formulations. This application leverages its high and fine to maintain free-flowing characteristics without altering or . In biomedical applications, calcium silicate's bioactive properties—releasing calcium and silicate ions—promote , mineralization, and regeneration, making it suitable for dental cements, scaffolds, and endodontic materials. Recent advancements as of 2025 include its use in systems and improved classifications of calcium silicate-based cements for clinical use. Emerging research since 2020 explores and related composites in lithium-ion batteries, particularly as components in solid-state electrolytes to enhance ionic conductivity and stability. Hybrid amorphous-crystalline structures, derived from abundant silicates like , achieve lithium-ion conductivities up to 1.42 × 10⁻⁴ S/cm, supporting safer, higher-performance batteries through improved interfacial with cathodes. These developments focus on scalable production from waste materials, addressing limitations in traditional liquid electrolytes for next-generation , including applications in electric vehicles. In cosmetics, calcium silicate acts as an absorbent in powder formulations like face powders and dry shampoos, where controlled particle sizes (typically 10–30 μm) optimize absorption to control sebum and mattify skin. Products such as Florite PS-10 exhibit high absorption capacities of around 434 g/100 g, enabling effective oil sequestration without compromising texture or spreadability. Sebum-absorbing variants with absorption exceeding 200 ml/100 g are formulated for long-wear makeup, reducing shine and improving product .