Silica sand for sealants and joints: particle size, rheology and qualification

Direct answer: in a sealant, “silica” does not mean one single filler

To qualify silica sand, quartz flour or another silica-based filler for sealants and joints, start with the formulation rather than an isolated SiO₂ value.

Ground quartz can act as a particulate mineral filler. Fumed or precipitated silica, which is much finer and has a high specific surface area, can instead be used as an active filler, reinforcing agent or rheology modifier. All of these families may be described commercially with the word “silica”, but their behavior in a polymer is not interchangeable [3–6].

For the buyer, five questions structure qualification:

1. Which material family is actually required? Graded sand, quartz flour, fused silica, fumed silica, precipitated silica or a filler blend.
2. Which particle-size distribution can the formulation accept?
3. Which effect is required? Filling, cost and density adjustment, dimensional control, rheology, anti-sagging, reinforcement or another function.
4. Which chemical and moisture constraints come from the binder?
5. Which tests on the finished formulation actually validate the grade?

A quartz technical data sheet can characterize the raw material perfectly without proving by itself that the finished sealant will achieve its target viscosity, extrusion, anti-sagging, adhesion or movement class.

ISO 11600 and ASTM C920 illustrate this separation precisely: these references address the classification or performance of the finished sealant, not one universal mineral-filler recipe [1–2].

For the basic material-selection framework, also see our industrial silica sand guide and industrial quartz guide.

1. Silica sand, quartz flour and active silica: three levels not to confuse

Granular silica sand

Silica sand is a granular material rich in silica, generally dominated by quartz. In polymer formulations, relatively coarse fractions may be required where product geometry, texture, volumetric filling or a composite formulation calls for them.

The term “fine sand” is still insufficient for purchasing a filler. Two products carrying the same commercial description can have different particle-size distributions, fines contents, moisture and mineralogy.

Quartz flour or micronized silica

Quartz flour is produced by grinding and classifying a crystalline silica-bearing raw material. Its particle size is much finer than ordinary sand.

A Sibelco data sheet reviewed for one commercial silica-flour grade reports, for example, D10 = 3 µm, D50 = 20 µm and D90 = 57 µm, together with residue at 63 µm, moisture and XRF chemistry [5]. These figures are useful as an example of how a supplier characterizes a grade. They are neither a Minerallum recommendation nor a universal particle-size specification for sealants.

Fumed or precipitated silica

Fumed and precipitated silicas belong to a different functional logic. Evonik’s technical bulletin on sealants describes active fillers capable of strongly modifying mechanical and rheological properties, notably through their surface, filler–polymer interactions and filler–filler interactions [3].

In the systems discussed by Evonik, increasing active-filler content raises viscosity and can create a yield point that supports stability and anti-sagging; excessive filling can, however, reduce processability [3].

That function should not automatically be attributed to quartz flour simply because both materials contain SiO₂.

2. What does a silica-based filler actually do in a sealant?

A mineral filler can perform several functions at the same time. Their importance depends on the polymer - silicone, polyurethane, silane-terminated polymer, acrylic, epoxy or another system - and on the other ingredients.

Depending on the formulation, a filler may contribute to:

• increasing the solids fraction;
• adjusting density and volumetric yield;
• modifying viscosity and flow behavior;
• reducing settlement of other constituents;
• contributing to anti-sagging;
• influencing hardness, modulus and mechanical strength;
• modifying shrinkage and dimensional stability;
• influencing appearance, texture or opacity;
• modifying thermal conductivity or other properties when the system is designed for that purpose.

These effects are not guaranteed by the mineral name.

Research on polymer–silica composites shows that rheology depends not only on filler loading, but also on particle size, specific surface area, dispersion state and interfacial interactions [3,6].

Extender and functional filler are not absolute categories

In one formulation, quartz flour may be selected mainly to occupy volume and provide a consistent mineral phase. In another, its size distribution or interaction with the matrix may contribute more strongly to mechanical or rheological behavior.

Conversely, a very-high-surface-area silica may be added at low dosage specifically to build rheological structure.

The function should therefore be described by testing in the formulation, not only by the commercial family.

3. Particle size: why D10, D50 and D90 are more useful than “fine powder”

A description such as “0–100 µm quartz” does not show how particles are actually distributed. To compare two fillers, characteristic diameters and the complete curve are more informative.

• D10: 10% of the measured distribution is finer than this diameter.
• D50: median diameter of the distribution.
• D90: 90% of the measured distribution is finer than this diameter.

The exact basis - volume, mass or number - and the measurement method should be stated.

Why does the distribution change the formulation?

At the same filler loading, changing the distribution can alter:

• the total surface that must be wetted by the polymer;
• the apparent volume occupied;
• particle packing;
• the amount of resin needed to reach the same consistency;
• viscosity during mixing and extrusion;
• the risk of agglomeration or settlement;
• surface appearance.

Studies of polymer–silica systems clearly show that particle size and structure can modify viscosity and flow mechanisms [6].

It would nevertheless be incorrect to turn this relationship into a simplistic rule such as “finer is always better” or “finer is always more viscous”. Shape, distribution, surface, surface treatment, filler loading and polymer chemistry act together.

4. Shape, surface and dispersion: particle size does not tell the whole story

Two powders with the same D50 can behave differently.

Rheology also depends on:

• particle shape and angularity;
• distribution breadth;
• the presence of agglomerates;
• specific surface area;
• surface chemistry;
• any surface treatment;
• dispersion energy and sequence;
• interactions with the polymer and additives.

In fumed-silica-filled PDMS suspensions studied in Composites Part A, viscosity and modulus increased with filler concentration and specific surface area, while better dispersion and appropriate surface modification could reduce these values [6]. This result concerns a family of active silicas and a specific experimental system: it demonstrates the importance of interactions, not a quantitative law for every quartz flour.

For ground quartz, the supplier data sheet should therefore be complemented by a dispersion test in the actual binder.

5. Rheology: target the product’s flow profile, not one isolated viscosity

A sealant often has to meet contradictory requirements:

• remain mobile enough under shear to be mixed, pumped or extruded;
• remain stable at rest;
• limit settlement;
• resist sagging in a vertical joint when required;
• allow tooling and substrate wetting.

For that reason, one viscosity value without temperature, shear rate, measurement geometry and sample history poorly describes a non-Newtonian system.

Evonik explicitly distinguishes the effect of active fillers on viscosity and yield point, with the latter contributing to practical sealant stability [3].

ASTM C920 distinguishes, among others, P grades - pourable/self-leveling - and NS grades - nonsag/gunnable - and connects classification to finished-product tests [2]. This shows why the rheological target depends on use: a self-leveling product and a vertical sealant do not seek the same behavior.

What the filler buyer should ask the formulator

Before fixing a particle size or filler loading, identify:

1. application method;
2. manufacturing and application temperature range;
3. target viscosity or rheological profile;
4. anti-sagging requirement;
5. total filler loading;
6. other fillers and additives;
7. mixing and deaeration system;
8. extrusion or pumping constraints.

6. Moisture: especially critical when binder chemistry is sensitive to it

Moisture in a mineral filler is not merely a logistics figure.

In some reactive formulations, particularly water-sensitive one-component systems, moisture introduced by fillers can affect storage stability or trigger unwanted reactions.

Quarzwerke highlights the value of low-moisture silica fillers for long-term stability in silane-terminated adhesive systems [4]. Evonik also shows that, in certain moisture-sensitive alkoxy silicone sealants, the surface state of fumed silica influences water adsorption and formulation stability [3].

These references do not define one universal quartz moisture percentage. They show that the limit must be set by the chemistry of the system.

For procurement, specify:

• measurement method;
• drying temperature and duration where applicable;
• acceptance limit;
• packaging;
• protection after opening;
• storage conditions;
• lot-control frequency.

7. Chemical purity: useful, but rarely sufficient to predict the sealant

SiO₂ content documents silica richness. XRF analysis can also track Fe₂O₃, Al₂O₃, CaO, MgO, Na₂O, K₂O, TiO₂ or other constituents depending on the grade.

The Sibelco Q12 PO data sheet illustrates this type of control with published chemistry and physical parameters for one commercial grade [5].

For a formulator, chemistry may matter for:

• color and whiteness;
• compatibility with a purity requirement;
• stability of a system sensitive to certain impurities;
• lot-to-lot consistency;
• source comparison.

But high SiO₂ content cannot predict sealant viscosity, anti-sagging, adhesion or mechanical strength.

For a deeper explanation of chemistry versus mineralogy, see our XRF and XRD silica-sand dossier.

8. ISO 11600 and ASTM C920: what they say - and what they do not say

ISO 11600

ISO 11600:2002, confirmed in 2025, specifies types and classes of sealants used in building construction according to their applications and performance characteristics, together with the corresponding requirements and test methods [1].

ASTM C920

ASTM C920-18(2024) covers properties of single- or multicomponent cold-applied elastomeric sealants and classifies them by type, grade, class and use, among other categories [2].

The consequence for raw-material purchasing is essential:

Compliance of a finished sealant with a performance standard does not automatically create a normative specification for quartz, sand or silica flour.

The formulator may use a particular combination of polymer, plasticizers, fillers, active silicas, pigments, adhesion promoters and crosslinkers to achieve the required performance.

Quartz qualification therefore remains an internal formulation specification, even when the finished product is tested to ISO 11600 or ASTM C920.

9. Safety: crystalline quartz and amorphous silica should not be confused

Quartz is a form of crystalline silica. Fumed silicas are generally amorphous silicas. This mineralogical distinction matters for safety documentation.

NIOSH notes that respirable crystalline silica consists of particles containing crystalline silica that are small enough to penetrate deep into the lungs, and that dust-generating operations can create occupational exposure [7].

In a workshop handling dry quartz flour, safety should therefore not be inferred solely from the fact that the powder will later be incorporated into a wet or cured sealant.

The supplier dossier should include the applicable SDS, and the user should assess the actual operations: unloading, bag opening, transfer, dosing, mixing, cleaning and spill handling.

Regulatory exposure values vary by jurisdiction. They are not product-quality specifications.

10. How to build a useful purchasing specification

A robust specification separates filler characterization from formulation validation.

Raw-material parameter	Purchasing question	Why it matters
Mineral family	Crystalline quartz, fused silica, amorphous silica?	Prevents substitution of non-equivalent materials
SiO₂ and oxides	What composition and tolerances?	Chemistry, color, consistency
D10 / D50 / D90	What distribution and method?	Dispersion, surface, filling
Coarse fraction	What residue on the defined sieve?	Defects, texture, processing
Fines	What fraction below the defined threshold?	Surface, dust, rheology
Moisture	What method and limit?	Dosing, storage, reactive systems
Bulk density	Which method?	Logistics and volumetric dosing
Surface treatment	Present, absent, declared chemistry?	Compatibility and dispersion
Lot	How is it identified?	Traceability
SDS	Which classification and precautions?	Health, safety and compliance

The specification can also define test methods, certificate frequency and source-change procedure.

Do not copy one supplier data sheet as a universal standard

The Sibelco grade cited above [5] provides a useful example of measurable data: chemistry, D10, D50, D90, moisture, residue at 63 µm and bulk density.

Good practice is to reuse the structure of useful data, not the numerical values, unless formulation testing demonstrates that those values are actually the required limits.

11. Supplier qualification: a seven-step sequence

Step 1 - Define the polymer system

Identify silicone, PU, STP/SMP, acrylic, epoxy or another system, together with the cure mechanism.

Step 2 - Define the filler’s function

Separate volumetric filling, reinforcement, rheology control, anti-sagging, stability, texture and other objectives.

Step 3 - Define raw-material parameters

Set the data to control: mineralogy, chemistry, particle size, moisture, bulk density, surface treatment and critical contaminants.

Step 4 - Test in the real formulation

At minimum, measure the properties controlling manufacturing and application: mixing, dispersion, viscosity/rheology, extrusion, sag, storage stability and appearance.

Step 5 - Test the cured product

Depending on use: hardness, tensile properties, elongation, adhesion, cohesion, movement, aging, shrinkage or other relevant performance.

Step 6 - Link validation to the supplier lot

Retain raw-material and formulation results for the approved grade, with a retained sample where the quality procedure requires one.

Step 7 - Define change control

A change in deposit, grinding, classifier, surface treatment or particle-size range can justify reassessment even if the commercial name remains unchanged.

12. The most common purchasing mistakes

Confusing quartz flour with fumed silica

Both contain silica, but their structure, surface and function in the formulation can be radically different [3,6].

Specifying only “99% SiO₂”

That figure defines neither particle-size distribution, moisture, dispersion nor rheology.

Buying on D50 alone

Two powders with the same median can have different D10, D90, shapes, surfaces and coarse fractions.

Copying a particle-size value from a TDS

A TDS describes one grade. It does not prove that the grade suits your polymer, filler loading and equipment.

Using the sealant standard as the filler standard

ISO 11600 and ASTM C920 concern the finished product; they do not prescribe a universal quartz recipe [1–2].

Ignoring moisture in a reactive system

A small variation can be unimportant in one formulation and critical in another. Binder chemistry decides.

Qualifying one sample without locking consistency

An industrial formulation needs a reproducible grade. Qualification must be followed by lot and change control.

13. Request template to send the supplier

For technically comparable proposals, specify:

1. application: construction sealant, industrial joint, adhesive, compound or other;
2. polymer/binder;
3. required filler function;
4. accepted mineralogical nature;
5. chemical composition or critical impurities;
6. D10, D50, D90 or full particle-size curve;
7. coarse and fine fraction limits where required;
8. maximum moisture and method;
9. allowed or prohibited surface treatment;
10. color/whiteness if critical;
11. bulk density if used for dosing;
12. formulation tests to pass;
13. SDS and regulatory documentation;
14. lot certificate and frequency;
15. quantity, packaging and destination;
16. change-notification procedure.

This structure makes offers comparable on the same criteria and distinguishes documentary matching from real technical qualification.

Conclusion: buy a filler for a formulation, not a silica percentage

For sealants and joints, silica sand and quartz flour can be useful mineral fillers, but their selection cannot be reduced to SiO₂ or a D50.

The decision should connect silica type, particle-size distribution, shape, surface, moisture, dispersion, filler loading and lot consistency to the properties actually required from the product: processability, extrusion, anti-sagging, stability, appearance and performance after cure.

Fumed or precipitated silicas bring a different surface and rheology logic and should not be confused with ground quartz. ISO 11600 and ASTM C920, meanwhile, evaluate the finished sealant and do not create a universal filler specification.

The strongest qualification therefore works at three levels: characterize the raw material, test the formulation, then lock supply consistency.