Sodium silicate is one of the most important soluble alkali silicates used in industrial applications, due to its combination of chemical and physical properties, such as alkalinity, film-forming ability, binding power, adhesiveness, and the formation of inorganic structures after water removal.
Often referred to as “liquid glass,” sodium silicate exhibits particularly interesting behavior during drying. An initially liquid solution can gradually transform into a solid, rigid, and predominantly amorphous film as water is removed and silicate species come into close proximity.
This phenomenon is often described as vitrification. However, from a materials science perspective, it is more accurate to understand the process as a combination of concentration, dehydration, condensation, and the formation of an inorganic siloxane (Si–O–Si) network, the degree of development of which depends on the chemical composition, the SiO₂/Na₂O molar ratio, the amount of water, the drying conditions, and the presence of modifiers.
The classic literature on the chemistry of silica and sol-gel processes provides the foundation for understanding these phenomena at the molecular level. Among the fundamental references are *The Chemistry of Silica* by Ralph K. Iler and *Sol-Gel Science* by Brinker and Scherer.
What is sodium silicate?
Chemically, sodium silicate is a system composed of species derived from silica (SiO₂) and sodium oxide (Na₂O), typically supplied commercially in the form of an aqueous solution. The product’s composition can be characterized, among other parameters, by the SiO₂/Na₂O molar ratio. This ratio has a significant influence on properties such as alkalinity, solubility, solution stability, film formation rate, and the mechanical behavior of the dried material.
In the aqueous solution, the silicate species are hydrated and distributed throughout the liquid medium. The presence of sodium imparts alkalinity and contributes to the stabilization of the silicate species in solution. When water is removed, a progressive transformation of the system occurs: the concentration of the species increases, molecular mobility decreases, and association and condensation processes between silanol-containing groups are favored.
How does the vitrification of sodium silicate occur?
The formation of the solid film can be understood in several stages.
Evaporation and Concentration
The first phenomenon observed during drying is the evaporation of water. As water leaves the system, the concentration of silicate species increases. Consequently, the average distance between them decreases, and the viscosity of the system increases. This progressive increase in viscosity is fundamental to the material’s transformation from a liquid solution to an increasingly rigid structure. The material ceases to behave like a conventional liquid and enters a region of high concentration, in which interactions between the silicate species become predominant.
Condensation of silicate species
As the system becomes more concentrated, silanol (Si–OH) groups can participate in condensation reactions. In simple terms, two silanol groups can react to form a siloxane bond:
Si–OH + HO–Si → Si–O–Si + H₂O
The formation of Si–O–Si bonds promotes the gradual construction of a three-dimensional inorganic network. It is precisely this structural evolution that explains much of the increase in stiffness observed during drying. The resulting material does not necessarily exhibit an ordered crystalline structure. On the contrary, the silica network that forms is predominantly amorphous in nature, a characteristic associated with glassy materials.
Why can the silicate film be extremely rigid?
The high rigidity of dry silicate is related to the formation of a continuous network of Si–O–Si bonds. Silica has a structure based on tetrahedral units of silicon coordinated by oxygen. During condensation, these units can progressively connect to form a three-dimensional network. The higher the degree of connectivity in this network, the greater the material’s stiffness tends to be. However, this same characteristic that provides hardness and strength can lead to a significant technological limitation: brittleness.
A highly cross-linked inorganic network has a limited capacity to accommodate deformations. When subjected to stresses resulting from shrinkage during drying, temperature variations, or mechanical forces, the film may develop microcracks and subsequently exhibit cracking.
The importance of the SiO₂/Na₂O molar ratio
One of the most important parameters in the formulation of sodium silicates is the molar ratio of SiO₂ to Na₂O. This ratio directly influences the structure of the species present in the solution and, consequently, the material’s behavior during drying. In general:
- lower SiO₂/Na₂O ratio: higher alkalinity and greater relative presence of sodium;
- higher SiO₂/Na₂O ratio: higher proportion of silica and a tendency toward the formation of more extensive silicate structures.
A relative increase in silica content favors the formation of more polymerized species and a more developed inorganic network during drying.On the other hand, a relative increase in the proportion of sodium can help keep silicate species smaller and more stabilized in the alkaline environment, slowing down certain condensation processes.
Thus, there is no single sodium silicate composition that is ideal for all applications. The composition must be selected according to the desired balance between solubility, alkalinity, drying rate, hardness, water resistance, adhesion, and flexibility.The material provided highlights this influence of the molar ratio and links silicates with a higher silica content to faster vitrification and a greater tendency toward brittleness.
Why does sodium silicate crack?
Cracking should not be interpreted simply as a chemical defect in the product. It is, to a large extent, a consequence of the stresses that develop during the transition from a liquid/hydrated phase to a solid and progressively more rigid structure. During drying, the following processes occur simultaneously:
- evaporation of water;
- concentration of silicate species;
- increase in viscosity;
- approximation of structural units;
- condensation of silanol groups;
- formation of Si–O–Si bonds;
- volumetric shrinkage;
- increase in film stiffness.
The problem arises when the shrinkage caused by water loss exceeds the film’s ability to accommodate the deformation. In this scenario, internal stresses are generated. If these stresses exceed the film’s local mechanical strength, microcracks may form, which subsequently develop into the phenomenon known as cracking.
Therefore, brittleness is not associated solely with “rapid drying,” but with the interaction between drying kinetics, shrinkage, the development of the inorganic network, and the film’s mechanical properties.
How can we reduce the fragility of the film?
Modifying the formulation can be used to reduce the tendency to crack. The goal is not simply to prevent the formation of the silicate network, but to control its density, continuity, and ability to accommodate stresses. Among the strategies described in the technical literature are:
Glycerin
Glycerol (glycerin) has multiple hydroxyl groups capable of forming interactions—especially hydrogen bonds—with species containing hydroxyl groups present in the system. Thus, it can act as a matrix modifier, increasing the film’s ability to accommodate deformations and contributing to moisture retention. The reference document identifies glycerin as one of the plasticizers used to increase the flexibility of the silicate film.
Sorbitol
Sorbitol also has multiple hydroxyl groups and can alter the intermolecular interactions within the system. In addition to its plasticizing effect, its ability to interact with water can influence the film’s state of hydration during and after drying.
Polyethylene glycol — PEG
Polyethylene glycol (PEG) can be used as a modifier, introducing a more flexible organic component into the system. This approach may be particularly useful when seeking to modify mechanical properties without completely eliminating the characteristics of the inorganic matrix.
Water-soluble polymers and polymer emulsions
Another possibility is the use of environmentally compatible polymers, such as certain water-soluble polymers, PVA, CMC, or acrylic systems. The combination of a rigid inorganic phase and a more flexible organic phase can result in a hybrid structure capable of dissipating some of the mechanical stresses. The background document describes this approach as the formation of an interpenetrating or hybrid polymer network structure, in which the organic chains can help hinder the propagation of microcracks.
The Role of Wastewater
Water plays a fundamental role in the behavior of the silicate film. It is necessary, however, to distinguish between free water—which can evaporate during drying—and water that is strongly bound to or retained by the structure. Hygroscopic materials and certain organic modifiers can promote moisture retention, altering the balance between stiffness and deformability. Maintaining a certain degree of hydration can reduce the tendency toward an excessively rigid and anhydrous structure, although this may also increase sensitivity to water depending on the formulation. For this reason, reducing cracking cannot be achieved simply by indefinitely increasing water retention. It is necessary to strike a balance between flexibility, mechanical strength, dimensional stability, and water resistance.
Lamination does not mean “altering the silica”
A conceptually important point is that the plasticization of silicate does not necessarily involve chemically modifying the fundamental structure of silica. In most modified formulations, the desired effect is to alter the way inorganic units interact and how stresses are distributed within the material. Polar molecules, such as glycerol, can interact with hydroxyl groups in the matrix and act as spacers or modifiers of molecular mobility. Organic polymers, on the other hand, can form a continuous or partially interpenetrating phase, providing additional mechanisms for energy dissipation. This approach is particularly important in the development of coatings, adhesives, binders, and inorganic-organic hybrid materials.
Plasticizer Concentration: Why Can Too Much Be Harmful?
The use of a plasticizer should be treated as a formulation variable, not as a fixed rule. The reference material specifies an initial range of 1% to 5% glycerin for laboratory or experimental testing aimed at reducing the tendency to crack. However, this range should not be interpreted as a universal specification. The final behavior depends on factors such as:
- solids concentration;
- SiO₂/Na₂O ratio;
- water content;
- film thickness;
- drying rate;
- temperature;
- relative humidity;
- substrate;
- type and concentration of plasticizer;
- chemical compatibility between components.
Adding too much plasticizer can reduce hardness, alter water resistance, increase stickiness, or significantly change the drying time. Therefore, the formulation must be optimized experimentally.
Sodium silicate as a hybrid system
The combination of alkali silicates and organic components represents an important strategy for developing materials with properties that lie between those of purely inorganic and purely organic systems. Silicates provide characteristics such as:
- high hardness;
- inorganic nature;
- thermal resistance;
- alkalinity;
- film-forming ability;
- binder action.
Organic components, in turn, can contribute to:
- flexibility;
- tenacity;
- energy dissipation;
- retraction control;
- reduction in the propagation of microcracks.
The result is a hybrid matrix whose performance can be adjusted to suit the application.
Vitrification versus crystallization
Another important aspect is to distinguish between vitrification and crystallization. In crystallization, atoms or ions arrange themselves into a periodic and ordered structure. In the formation of a glassy phase, the structure exhibits local order but does not necessarily have long-range periodic organization.
In the case of silicate films, the formation of an amorphous network of silicate units is responsible for a significant portion of the dry material’s properties. Therefore, technically, it is more appropriate to speak of the formation of an amorphous or glassy silicate phase than to imagine that the solution simply “turns into glass” in the same way as occurs in conventional fused glass manufacturing.
Formulation Control: The Balance Between Hardness and Flexibility
The main technological challenge in formulating a sodium silicate film is finding the right balance between properties that naturally compete with one another. A very rigid film may exhibit:
greater hardness → lower deformability → greater risk of cracking.
A film that is excessively laminated may exhibit:
greater flexibility → lower hardness → a possible reduction in water resistance and mechanical strength.
Therefore, the development of an effective formulation must take the system as a whole into account. The choice of the SiO₂/Na₂O ratio, the control of solids content, the amount of water, the applied thickness, and the selection of modifiers are interdependent variables.
Conclusion
The so-called vitrification of sodium silicate is the result of a series of physicochemical phenomena that occur during the concentration and drying of the solution.The removal of water progressively brings the silicate species closer together and promotes condensation processes involving silanol groups, leading to the formation of a network of Si–O–Si bonds. The development of this network causes an increase in viscosity, a loss of mobility, and the formation of a predominantly amorphous solid film.The same structure responsible for the film’s hardness and strength can, however, result in high brittleness. Shrinkage associated with drying and the development of internal stresses can lead to microcracks and cracking.
Formulation engineering allows for the control of this behavior. The SiO₂/Na₂O molar ratio, drying kinetics, water content, and the use of modifiers such as glycerin, sorbitol, PEG, CMC, PVA, or certain acrylic polymers can significantly alter the structure and mechanical properties of the film.Thus, the development of sodium silicate-based products should not be based solely on the idea of “making the silicate vitrify,” but rather on controlling the formation of the inorganic network and the stresses generated during drying.
Thus, it can be said that for cements and alkali-activated systems: choose alkaline sodium silicate, as it has higher alkalinity and reactivity. For adhesives: choose neutral sodium silicate Thus, it can be said that for cements and alkali-activated systems: choose alkaline sodium silicate, as it has higher alkalinity and reactivity. For adhesives: choose neutral sodium silicate to promote the formation of a film with greater cohesion. For adhesives that require flexibility: the ideal choice is silicate combined with a plasticizer or compatible polymer, such as glycerin, sorbitol, PEG, CMC, or PVA, to reduce cracking.
There is no single “best” silicate: the choice depends on the application and must also consider solids concentration, Na₂O content, viscosity, water content, and drying conditions. This understanding allows for the development of more stable formulations, with a better balance between hardness, adhesion, strength, flexibility, and crack resistance, suited to different industrial needs.