Concrete quality depends heavily on the aggregates used in the mix. Poor-quality or unsuitable aggregates can lead to weak concrete that cracks, crumbles, or fails to set properly. When mixing concrete on-site, avoid aggregates that contain excessive amounts of clay, silt, organic materials, salt, or chemical contaminants. Additionally, aggregates that are too soft, too porous, or have improper particle shapes can significantly reduce concrete strength and durability.

Using proper aggregates is crucial since they make up 60-75% of concrete's volume. These materials provide structural strength, determine workability, and affect the long-term performance of concrete structures. The wrong aggregates can lead to expensive repairs, dangerous structural failures, and shortened lifespans of concrete installations. Understanding which aggregates to avoid helps ensure your concrete projects remain strong and durable for decades.

Aggregates That Should Be Avoided When Making Concrete

1. Organic Materials and Soil Contaminants

Organic materials are among the most problematic contaminants in concrete aggregates. These include:

• Topsoil with organic matter

• Plant material (roots, leaves, grass, etc.)

• Wood chips or sawdust

• Humus or decomposed plant material

• Peat or other organic soils

When organic materials are present in concrete aggregates, they create serious problems. Organic matter decomposes over time, leaving voids in the concrete that weaken its structure. These materials also interfere with the cement hydration process, preventing proper bonding between cement paste and aggregates.

Even small amounts of organic materials can significantly reduce concrete strength. Just 0.1% organic content can decrease strength by up to 20%. To check for organic materials, perform a simple colour test: 

• Mix the aggregate with a sodium hydroxide solution

• If the solution turns darker than a standard colour, organic impurities are present.

Clay and silt particles cause similar issues in concrete mixes. These fine materials coat aggregate surfaces, preventing proper bonding with cement paste. They also increase water requirements, leading to higher shrinkage and more cracking in the finished concrete. Aggregates with more than 3% clay or silt content should be washed before use or avoided entirely.

2. Chemical Contaminants and Reactive Aggregates

Chemical contaminants can react with cement or reinforcing materials, causing long-term deterioration of concrete. Avoid aggregates containing:

• High chloride content (salt-contaminated sand and gravel)

• Sulfates from industrial waste or contaminated soils

• Reactive silica that can cause an alkali-silica reaction (ASR)

• Pyrite or other iron sulfide minerals

• Industrial waste contaminants

Salt-contaminated aggregates are particularly problematic in reinforced concrete. The chlorides from salt accelerate the corrosion of steel reinforcement, causing it to expand and crack the surrounding concrete. This is why beach sand should never be used in concrete without thorough washing to remove salt.

Alkali-silica reaction (ASR) occurs when certain forms of silica in aggregates react with alkaline cement, forming a gel that absorbs water and expands. This expansion creates internal pressure that cracks concrete over time. Common reactive aggregates include certain quartzites, cherts, volcanic glasses, and some sandstones. ASR damage typically takes years to appear, but it can severely compromise concrete structures.

3. Weak, Porous, or Poorly Shaped Aggregates

The physical characteristics of aggregates directly impact concrete performance. Avoid aggregates with these undesirable physical properties:

• Soft or friable materials (less than 4 on the Mohs hardness scale)

• Highly porous or absorbent materials

• Flat or elongated particles

• Weathered or deteriorated rock

• Aggregates with high freeze-thaw susceptibility

Soft aggregates like shale, some sandstones, and weathered rock crush easily under pressure, creating weak points in concrete. The commonly used Los Angeles abrasion test measures aggregate hardness; materials with abrasion losses exceeding 40-50% should be avoided in structural concrete.

Porosity affects both strength and durability. Highly absorbent aggregates (water absorption above 2-3%) require more mixing water, leading to higher shrinkage and lower strength. They also trap water that can freeze and expand in cold climates, causing internal damage to the concrete.

4. Improperly Graded or Sized Aggregates

Aggregate size distribution (gradation) plays a crucial role in concrete performance. Well-graded aggregates contain particles of various sizes that pack tightly together, minimising voids that must be filled with cement paste. This improves concrete density, strength, and economy. Single-sized aggregates create excessive voids, requiring more cement paste and resulting in concrete that's less economical and more prone to shrinkage.

Problems occur with:

• Single-sized aggregates (uniform gradation)

• Excessive fine or coarse material

• Gaps in certain size ranges

• Unwashed manufactured sand with too many microfine particles

• Aggregate mixtures that don't meet gradation standards

Too many fine particles increase water demand and shrinkage, while too many coarse particles reduce workability and create honeycombing (voids). For standard concrete, fine aggregates (sand) should generally have particles ranging from 0.075 to 4.75 mm, while coarse aggregates typically range from 4.75 to 19 or 25 mm.

The ideal aggregate mixture follows standard gradation curves established by organisations like ASTM or EN. These standards ensure the right distribution of particle sizes for optimal concrete performance. Testing aggregate gradation before mixing concrete helps avoid problems related to improper size distribution.

5. Environmentally Dangerous or Contaminated Materials

Some aggregates contain hazardous materials that pose environmental or health risks:

• Asbestos-containing materials

• Aggregates with high heavy metal content

• Materials contaminated with petroleum products

• Industrial byproducts without proper testing

• Aggregates from brownfield sites without remediation

These materials can release toxins during concrete placement or over time as concrete weathers. Additionally, they may:

• Interfere with the concrete setting

• Reduce durability

• Causes unexpected chemical reactions

Always source aggregates from reputable suppliers who test their materials for contaminants.

Recycled concrete aggregate (RCA) and other recycled materials can be excellent sustainable options, but they require proper screening and testing. RCA may contain gypsum from attached drywall, which can cause sulfate attack in new concrete. It might also contain materials like brick or ceramics that don't perform well in concrete.

When using recycled or industrial byproduct aggregates, ensure they meet the same quality standards as natural aggregates and have been tested for potential contaminants or reactive materials.

Bottom Line 

Using the right aggregates is fundamental to producing quality concrete. By avoiding organic materials, chemical contaminants, physically weak materials, poorly graded aggregates, hazardous substances, and temperature-sensitive materials, you can significantly improve concrete strength, durability, and longevity. Always test or request testing documentation for aggregates before using them in important concrete projects.

When your project demands concrete that stands the test of time, the choice of supplier matters as much as the materials themselves. At Pro-Mix Concrete, our specialised aggregate selection process includes multi-stage testing that exceeds industry standards. Our obsessive attention to detail is why structures built with Pro-Mix Concrete consistently outlast and outperform the competition.

Request your free aggregate consultation and project assessment today! Schedule your project with our online booking system for priority service.