When evaluating silica fume quality, most buyers focus on SiO₂ content and LOI. But particle size distribution (PSD) is equally important — it tells you how fine the material actually is, how uniformly the particles are distributed, and how the silica fume will behave in a concrete mix. This article explains the five key parameters you will encounter on a laser diffraction PSD report, and what each one means in practice.

What Is Particle Size Distribution?

Particle size distribution describes the range of particle sizes present in a powder sample and the proportion of material at each size. For silica fume, this is typically measured by laser diffraction — a technique where a beam of laser light is scattered by particles in suspension, and the scattering pattern is used to calculate the size distribution. Instruments like the Malvern Mastersizer are widely used for this purpose.

The result is not a single number but a curve — and from that curve, several summary statistics are extracted. The most important ones are D10, Д50, D90, Д[3,2], and D[4,3].

Understanding these figures helps you assess pozzolanic reactivity, predict superplasticizer demand, and compare materials from different suppliers on a like-for-like basis. For a broader overview of how silica fume is evaluated, see our complete guide to silica fume testing.

D10, Д50, D90 — Percentile Values

D10, Д50, and D90 are all percentile values read from the cumulative volume distribution curve. They share the same logic: the number after the “Д” tells you what percentage of the total sample volume is made up of particles smaller than that size.

D10 — The Fine End

D10 means that 10% of the particles by volume are smaller than this value. It represents the finest fraction of the material. In silica fume, a low D10 is a positive sign: the finest particles have the highest surface-area-to-volume ratio, react most rapidly with calcium hydroxide, and are most effective at filling the smallest voids in the cement paste matrix.

For well-produced silica fume, D10 typically falls in the range of 0.05–0.10 μm.

D50 — The Median Particle Size

D50 is the median: exactly half the material by volume is finer than this size, and half is coarser. It is the most commonly cited particle size figure and the primary benchmark used when comparing silica fume products across suppliers.

For context, ordinary Portland cement has a typical D50 in the range of 10–20 μm. Quality silica fume sits in the range of 0.10–0.20 μm — roughly 100 times smaller. This size difference is what gives silica fume its ability to fill the spaces between cement grains and densify the interfacial transition zone (ITZ).

A D50 below 0.15 μm is generally considered good for high-performance and ultra-high-performance concrete applications.

D90 — The Coarse Tail

D90 means that 90% of particles are finer than this value. Only the coarsest 10% of the sample exceeds this size. D90 is particularly useful for quality control: a high D90 relative to D50 suggests either a broad size distribution or the presence of agglomerates — particles that have clumped together during densification or storage and have not been fully broken apart.

The ratio (D90 − D10) / D50 is sometimes called the “span” and gives a quick measure of how wide or narrow the distribution is. A lower span means a tighter, more uniform distribution, which is generally preferred for predictable mix design behavior.

For quality silica fume used in high-performance concrete, D90 is typically below 0.5 мкм.

Summary Table: D10 / Д50 / D90

Д[3,2] and D[4,3] — Mean Diameters

While D10, Д50, and D90 describe specific points on the distribution curve, Д[3,2] and D[4,3] are calculated averages that summarize the entire distribution in different ways. They use different mathematical weightings, which is why they give different values for the same sample — and why understanding which one to use matters.

Д[3,2] — The Surface-Weighted Mean (Sauter Mean Diameter)

Д[3,2] is calculated by weighting each particle according to its surface area. Smaller particles have a much larger surface area relative to their volume, so D[3,2] is pulled strongly toward the fine end of the distribution.

This makes D[3,2] the most relevant average for processes that are governed by surface area: pozzolanic reaction rate, adsorption of water-reducing admixtures (PCE superplasticizers), and early-age strength development. When you want to understand how reactive a silica fume is, Д[3,2] is the more meaningful mean.

Д[3,2] is always smaller than D[4,3] for the same sample. For quality silica fume, Д[3,2] typically falls in the range of 0.10–0.15 μm. It is also directly related to the specific surface area of the material — the finer the D[3,2], the higher the surface area per gram. For more on how specific surface area is calculated and why it matters, see our FAQ on specific surface area of silica fume.

Д[4,3] — The Volume-Weighted Mean (De Brouckere Mean)

Д[4,3] is calculated by weighting each particle according to its volume. Because volume scales with the cube of the diameter, larger particles contribute disproportionately to this mean. Как результат, Д[4,3] is sensitive to the presence of coarse particles or unbroken agglomerates — even a small number of large particles can raise D[4,3] noticeably.

Д[4,3] is the “стандартный” mean that most laser diffraction instruments report by default, and it is what most suppliers refer to when they quote an “average particle size.” It is the appropriate value to use when modeling bulk behavior in a suspension — for example, when predicting how silica fume will disperse in mix water, or estimating the effect of particle size on concrete workability.

For quality silica fume, Д[4,3] typically falls in the range of 0.15–0.25 μm.

The Relationship Between D[3,2] and D[4,3]

Because D[3,2] weights toward surface area (fine particles) and D[4,3] weights toward volume (coarser particles), Д[4,3] is always larger than D[3,2] for the same sample. The size of the gap between them tells you something useful about the distribution:

• A small gap (Д[4,3] / Д[3,2] close to 1.0) indicates a narrow, uniform distribution with few coarse outliers.
• A large gap suggests a broad distribution or the presence of agglomerates pulling D[4,3] upward.

For well-produced silica fume, a D[4,3] / Д[3,2] ratio in the range of 1.3–1.8 is typical. Ratios significantly above 2.0 may warrant investigation into de-agglomeration quality or storage conditions.

Summary Table: Д[3,2] vs D[4,3]

How to Use These Values When Buying Silica Fume

When requesting PSD data from a supplier, look for the following:

• D50 below 0.20 мкм. This confirms the material is genuinely sub-micron and suitable for high-performance concrete. A D50 above 0.30 μm may indicate poor de-agglomeration or an inferior grade of material.
• D90 below 0.60 мкм. A high D90 suggests agglomerates or broad distribution. For UHPC and reactive powder concrete, target D90 below 0.50 мкм.
• Consistency across batches. Request PSD reports from multiple production lots. D50 and D90 should remain stable. Drift in D90 over successive batches often signals storage or processing issues.
• Measurement conditions stated. Always check what dispersant was used and whether ultrasonic pre-treatment was applied. Results measured in water versus ethanol, or with versus without sonication, are not directly comparable.

PSD data should be read alongside the full COA — SiO₂ content, LOI, moisture, and Strength Activity Index. For guidance on silica fume fineness requirements under ASTM C1240, as well as how the micro-filler filling effect depends on particle size, refer to our related guides.

Densified vs. Неуплотненный: Does Physical Form Affect PSD?

One common source of confusion is the difference between densified and undensified silica fume PSD results. Химически, both forms are identical. But densification — a mechanical process that compacts loose silica fume into granules or pellets for easier handling — can affect how PSD is measured.

If a densified sample is not properly dispersed before laser diffraction measurement, agglomerates may be detected as large particles, inflating D90 and D[4,3]. This is a measurement artifact, not a true property of the material. A well-conducted PSD test applies appropriate ultrasonic energy to break apart agglomerates before measurement, so that the result reflects primary particle size rather than agglomerate size.

When comparing PSD reports for densified and undensified silica fume, always confirm that the same measurement protocol and dispersant were used. For more on the practical differences between these product forms, see our article on the difference between densified and undensified silica fume.

Резюме

Particle size distribution parameters are not just technical footnotes — they are direct inputs into mix design, reactivity prediction, and supplier qualification. Here is a quick reference:

• D10: The finest 10% of particles. A low value indicates a highly reactive fine fraction.
• Д50: The median. The primary benchmark for comparing silica fume fineness across suppliers.
• D90: The coarse tail. A high value may signal agglomeration or poor process control.
• Д[3,2]: Surface-weighted mean. Most relevant for reactivity and admixture behavior.
• Д[4,3]: Volume-weighted mean. Most relevant for bulk suspension and workability prediction.

Henan Superior Abrasives provides PSD documentation — including D10, Д50, D90, Д[3,2], and D[4,3] — for our уплотненный а также неуплотненный silica fume grades on request. Contact our technical team to request a sample pack with full test data before placing an order.

О Хенане Верхнем Абразивах (HSA)
Henan Superior Abrasives is a China-based manufacturer and global exporter of silica fume (микрокремнезем), карбид кремния, and related industrial minerals, supplying concrete producers and construction materials companies across 30+ страны. HSA produces densified and undensified silica fume powder compliant with ASTM C1240 and EN 13263, available in grades from 85% к 96% SiO₂. For product documentation or samples, contact our export team.