Sodium Silicate Grades: Modulus, Baumé, Liquid, Solid and Powder

Modulus Describes the Silica-to-Alkali Balance

Sodium silicate does not have one fixed composition. It is commonly written as:

Na₂O·nSiO₂

The value of n is the modulus. It shows how much silica the product contains relative to sodium oxide.

A lower-modulus grade contains more Na₂O relative to SiO₂. It is therefore more alkaline and usually dissolves more easily. A higher-modulus grade contains more silica relative to Na₂O. It has lower alkalinity and behaves differently during drying, bonding, gel formation and reactions with acids or metal salts.

Modulus does not tell the buyer whether the product is concentrated or dilute. A modulus of 3.2 describes the relationship between SiO₂ and Na₂O, not the amount of water in a liquid product.

The basis of the ratio also needs attention. Many suppliers state modulus as the molar ratio of SiO₂ to Na₂O, while some commercial specifications use the weight ratio. These values are close but not identical:

Molar ratio = weight ratio × 1.032

For example, a 3.20 weight ratio corresponds to a molar ratio of about 3.30. A purchase specification should therefore state whether the requested modulus is based on weight or moles. Nippon Chemical and PQ both distinguish these two ways of expressing sodium silicate composition.

Labels such as alkaline grade and neutral grade give a general indication of composition, but they do not replace the actual ratio. Product codes also vary between manufacturers. A Grade N, Grade 40 or Grade 3 from one supplier does not automatically match a similarly named product from another.

Baumé Measures Liquid Density, Not Modulus

Liquid sodium silicate contains Na₂O, SiO₂ and water. Its concentration is often expressed through degrees Baumé, written as °Bé.

A higher Baumé reading means a denser solution. For one sodium silicate composition, increasing the solids content normally raises both density and °Bé. The industry also converts Baumé to specific gravity using:

Specific gravity = 145 ÷ (145 − °Bé)

At 20°C:

Baumé does not show the silica-to-soda ratio. A 42°Bé liquid can have a high modulus or a lower modulus. It only tells the buyer how dense the solution is at the stated temperature.

For example, OxyChem Grade 40 combines several separate limits:

The Baumé value identifies the solution density, while the ratio identifies its chemical balance. Neither value can replace the other.

Temperature must also appear beside a Baumé specification. Sodium silicate solutions expand as they become warmer, so the measured density falls. PQ data show that the same liquid can give different Baumé readings at 10°C, 21°C and 60°C. A requirement such as “42°Bé” is incomplete unless the test temperature is known.

Viscosity needs its own specification as well. It changes with modulus, solids content and temperature. A more concentrated liquid is generally thicker, but equal Baumé values do not guarantee equal viscosity when the products have different moduli.

Liquid, Solid and Powder Grades Are Used Differently

The physical form does not automatically define the modulus. Similar silica-to-soda ratios can be supplied as liquid, solid glass or powder. The main difference is how the user handles and introduces the sodium silicate.

Liquid sodium silicate

Liquid sodium silicate is an aqueous solution supplied at a controlled modulus and concentration. It is ready for pumping, dilution or direct addition.

This form suits continuous processes that already handle liquids, including adhesives, foundry binders, grouting systems, mineral processing and the production of silica derivatives. It avoids an on-site dissolution step, but the buyer also transports and stores the water contained in the solution.

A liquid grade should normally state:

• SiO₂/Na₂O ratio and its basis
• Na₂O and SiO₂ content
• Total solids
• Baumé or specific gravity at a stated temperature
• Viscosity at a stated temperature
• Insoluble matter and iron limits when relevant

Solid sodium silicate

Solid sodium silicate is the glass produced by fusing silica sand with soda ash. It is commonly supplied as lumps, cullet or coarse granules. PQ describes furnace-produced glass briquettes as a product that can be sold directly or dissolved to make sodium silicate solutions.

Solid glass contains little or no added water, which reduces the amount of water shipped with the product. However, coarse high-modulus glass does not turn into a usable solution as quickly as liquid sodium silicate. Industrial users commonly dissolve it with hot water and pressure before use.

The solid form therefore suits plants that have dissolution equipment or use sodium silicate glass as a production feedstock. A lower purchase or freight cost has little value if the plant cannot dissolve the material at the required rate.

Sodium silicate powder

“Powder” is not precise enough for a purchase order.

A supplier may use the term for finely milled anhydrous sodium silicate glass. Another may use it for spray-dried or hydrous instant sodium silicate. Both are dry products, but they can have very different bulk density, moisture content and dissolution speed.

A milled solid powder can retain the composition and high density of the original glass. One commercial neutral powder, for example, specifies a SiO₂/Na₂O weight ratio of 3.1–3.2, 22–24% Na₂O, 75–77% SiO₂ and a bulk density of 1,350–1,450 g/L.

Spray-dried hydrous powder contains retained water but normally disperses and dissolves more readily. It fits dry blends such as powdered cleaners, refractory mixes and some construction formulations where adding a liquid would disturb the product’s water balance.

The powder specification should therefore include:

• Production type: milled glass or spray-dried powder
• Modulus
• Moisture or loss on ignition
• Soluble solids
• Particle-size distribution
• Bulk density
• Dissolution conditions and dissolution time

Powdered sodium silicate should also not be confused with sodium metasilicate. Sodium metasilicate has a modulus close to 1.0 and is sold in anhydrous or hydrated forms. It provides much higher alkalinity than the common 2.0–3.3 modulus sodium silicate grades. The two materials may share similar names, but they are not direct substitutes.