The first factor that determines the capacity, product quality and, above all, the operating cost of a crushing and screening plant is the rock itself. The same crusher can consume, in a few weeks on a siliceous quartzite, wear parts that would last for months on a soft limestone. That is why crusher selection, plant design and the spare parts budget all start with a correct understanding of rock properties.
In this guide we look at the main rock properties that determine crushability and abrasiveness, the laboratory tests, the behaviour of common rock types, and the effect of these properties on crusher selection and wear cost.
What Are Crushability and Abrasiveness?
Crushability describes how much energy is needed to reduce a rock to a given size. Abrasiveness shows how quickly the rock wears the metal surfaces it contacts (jaws, mantles, blow bars, screen wire). These two properties are independent of each other: some rocks break easily but are very abrasive (some sandstones, for example), while others are hard to break but relatively mildly abrasive.
Main Rock Properties and Tests
| Property / Test | What Does It Measure? | Use |
|---|---|---|
| Uniaxial compressive strength (UCS) | The rock’s resistance to failure under compression (MPa) | Crusher type and capacity estimate |
| Point load index | Practical strength estimate | Quick assessment on site |
| Los Angeles (LA) abrasion test | Resistance of aggregate to fragmentation under impact and abrasion | Aggregate quality; an indication of impact crusher suitability |
| Bond crushing work index | Energy required for crushing (kWh/t) | Crusher power and capacity calculation |
| Bond grinding work index | Energy required for grinding | Mill sizing |
| Abrasiveness tests (e.g. LCPC, Bond abrasion index) | Tendency to wear metal surfaces | Estimating wear part life and cost |
| Silica (SiO₂) / free quartz content | Proportion of abrasive minerals | Wear and dust (silica) risk |
| Density | Weight per unit volume | Calculating capacity in tonnes |
| Moisture and clay content | Stickiness and tendency to block | Need for feeders, screens and washing |
The results of these tests should be assessed together, not in isolation. For example, a rock with high UCS but low silica can be crushed economically in a cone crusher, while a medium-strength but highly siliceous rock can create very high wear costs in an impact crusher.
Common Rock Types and Crushing Behaviour
| Rock | General Strength | Abrasiveness | Crushing Note |
|---|---|---|---|
| Limestone | Low–medium | Low | Ideal for impact crushers; good cubical product |
| Dolomite | Medium | Low–medium | Impact or cone crusher; fines content should be considered |
| Marble | Medium | Low | Crushes easily with impact crushers |
| Basalt | High | Medium | Jaw + cone crusher; shape improvement with a VSI at the tertiary stage |
| Andesite | Medium–high | Medium | Cone-crusher-based circuits |
| Diabase | High | Medium–high | Cone crusher; wear part selection is important |
| Granite | High | High (quartz) | Jaw + cone crusher; high wear cost in an impact crusher |
| Quartzite | Very high | Very high | Compression crushing (jaw, cone); an impact crusher is generally not economical |
| River gravel | Variable | Usually high (siliceous) | Cone crusher and VSI; washing required |
Note: Rocks with the same name can show very different properties in different quarries. The assessments in the table are general; laboratory tests should be carried out before a final decision.
Effect on Crusher Selection: Impact or Cone?
Rock properties are decisive, particularly at the secondary crushing stage, in the choice between an impact crusher and a cone crusher.
- Impact crushers give a high reduction ratio and an excellent cubical product in low-abrasion rocks such as limestone, dolomite and marble. In rocks with a high silica content, however, blow bars and apron liners wear quickly. See our impact crusher guide for details.
- Cone crushers work at lower wear cost in hard and abrasive rocks (basalt, granite, diabase, quartzite) thanks to the compression crushing principle. Particle shape is improved by the correct setting and choke-fed operation. Our cone crusher guide covers the details.
- Jaw crushers are suitable for almost every rock type at the primary stage; wear is managed through jaw plate material and profile (jaw crusher guide).
- VSI crushers are used for shape improvement and manufactured sand production; in abrasive rocks the rock-on-rock (rock shelf) configuration reduces wear (VSI guide).
Estimating Wear Cost
Wear cost is usually tracked as wear part cost per tonne. The following approach can be used for an estimate:
- The rock’s abrasiveness test result and silica content are determined.
- The experience of plants working with similar rocks and manufacturer data are compared.
- The expected life for the selected crusher type and wear part material is estimated in tonnes.
- Once the plant is running, the estimate is updated by measuring actual life.
Choosing the wear part material (manganese grade, martensitic steel, high chrome, ceramic inserts) to suit the rock can change the total cost significantly. We cover this in detail in our wear part materials guide.
Moisture, Clay and Stickiness
Moisture and clay content determine plant performance as much as the rock’s mechanical properties. Clayey, wet material:
- Bridges in hoppers and chutes and disrupts flow in feeders (hopper guide).
- Sticks in the crushing chamber, reducing capacity and causing packing in cone crushers.
- Blinds screen surfaces and may require anti-clogging screen wire or polyurethane panels (screen surface selection).
- Lowers product quality and requires washing (sand screw washer).
For this reason, scalping with a grizzly feeder ahead of the primary crusher and removing the clayey fines is a common practice.
Sampling and Test Plan
Representative samples are essential for sound decisions. Samples should be taken from different areas of the quarry, at different depths and from different rock zones. Drill cores and surface samples should be evaluated together. Especially for large investments, pilot-scale crushing tests provide valuable information on both capacity and product quality. For how rock variability is handled in quarry planning, see our quarry planning guide.
Common Mistakes
- Selecting the crusher on capacity alone and ignoring abrasiveness.
- Generalising the results of a single sample to the whole quarry.
- Choosing an impact crusher for siliceous rock for “better particle shape” without accounting for wear cost.
- Discovering the clay and moisture problem only after the plant has been built.
- Not measuring wear part life in tonnes.
Field Example
Suppose an impact crusher was chosen for secondary crushing at a quarry and an excellent cubical product was obtained at first. As the quarry advances, however, siliceous bands within the limestone increase and blow bar life becomes noticeably shorter. Abrasiveness tests confirm that the new zone is much more abrasive. As a solution, the blow bar material is changed to a ceramic-insert grade and material from the siliceous zone is blended into the feed. In the longer term, a switch to a cone crusher at the secondary stage plus a tertiary VSI is evaluated. This example shows that rock variability must be monitored throughout the life of the operation.
Frequently Asked Questions
Is hard rock always more abrasive?
No. Abrasiveness depends mostly on the content of hard minerals (especially quartz). A high-strength rock that contains no quartz can be relatively mildly abrasive.
Is the Los Angeles test sufficient for crusher selection?
Not on its own. The LA value is important for aggregate quality; for crusher selection it should be assessed together with strength, abrasiveness and silica content.
Why is the Bond work index important?
It allows the energy required for crushing and grinding to be estimated; it is basic data for motor power and capacity calculations.
Should wear parts be changed when the rock changes?
If wear behaviour has changed significantly, a different material or profile should be considered; tonnage-based records make this decision easier.
Wear Part Recommendations by Rock Type
| Rock Group | Jaw Crusher | Cone Crusher | Impact Crusher |
|---|---|---|---|
| Limestone, dolomite, marble (low silica) | Mn18 jaw plate | Mn18 mantle–concave | Martensitic or high-chrome blow bar |
| Basalt, andesite (medium abrasive) | Mn18 / Mn22 | Mn18Cr2, Mn22 | Martensitic or ceramic insert; wear cost should be closely monitored |
| Granite, quartzite, siliceous gravel (highly abrasive) | Mn22, TiC inserts depending on the application | Mn22 or alloyed manganese | Usually not economical; ceramic insert in special cases |
| Recycled concrete (with rebar) | Mn18 | — | Martensitic or manganese (because of the metal) |
The table is general guidance; the final selection should be made together with the crusher model, feed size and test results. For wear parts with the right metallurgy for your crushers, see our jaw crusher spare parts, cone crusher spare parts and impact crusher spare parts pages. For details of blow bar materials, see our blow bar guide. Whatever the alloy, correctly seated liners last longer: our own Aurox backing compound supports cone and gyratory liners evenly.
Example Laboratory Test Plan
- Representative samples are taken from different zones of the quarry (surface and core).
- Rock type and mineralogy are determined by petrographic examination.
- Compressive strength and point load tests are carried out.
- Aggregate quality is assessed with Los Angeles, Micro-Deval and water absorption tests.
- Wear cost is forecast with an abrasiveness test and silica analysis.
- Bond crushing and grinding work index tests are carried out if required.
- Clay and moisture content are determined, along with a methylene blue test.
- The results are mapped by quarry zone.
Glossary
- UCS: Uniaxial compressive strength (MPa).
- LA coefficient: Los Angeles resistance to fragmentation; a lower value means a more durable aggregate.
- Abrasiveness index: A value showing the rock’s tendency to wear metal surfaces.
- Bond work index: Energy required for crushing or grinding (kWh/t).
- Petrography: Microscopic examination of a rock’s minerals and texture.
- Free silica: Crystalline silica minerals such as quartz within the rock.
More Questions
Do rock properties change within the same quarry?
Yes. Because of bedding, fault zones and the degree of weathering, properties can vary considerably; the test programme should therefore be updated as the quarry advances.
Can an impact crusher be used on rock with high silica content?
Technically yes, but the wear cost can be very high. The decision should be based on a calculation of total cost per tonne.
How long do rock tests take?
It depends on the type of test and the laboratory’s workload; for large investments the test programme should be built into the project schedule early.
CSP Mühendislik Support
CSP Mühendislik provides engineering support on crusher and screen selection by rock properties, wear part material recommendations and plant design. You can find wear parts with the right metallurgy for your crushers on our crusher spare parts, cone crusher spare parts and impact crusher spare parts pages.
