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Concrete with chert aggregate is a composite material consisting of a hardened cement paste matrix binding together various sizes of aggregate, including chert. Chert, a cryptocrystalline sedimentary rock composed of silicon dioxide (SiO2), can be problematic in concrete due to its reactivity with the alkaline pore solution in the cement paste (Alkali-Silica Reaction, ASR) and its susceptibility to freeze-thaw damage, especially if porous. The appearance of the concrete will vary depending on the cement color, water-cement ratio, and the specific characteristics of the chert and other aggregates used.
How to Identify
- Color
- Variable, typically gray to off-white for the cement matrix, with chert aggregate appearing in various colors (gray, brown, black, white, red, green) depending on its origin. The overall color is a mix of the cement paste and the aggregate colors.
- Luster
- Dull to earthy for the cement matrix; chert aggregate can range from dull to waxy or vitreous when freshly fractured.
- Texture
- Rough, granular, and heterogeneous. The cement paste is fine-grained, while the chert aggregate pieces are typically angular to sub-rounded and range in size from sand to gravel. The texture is often described as clastic or fragmental within a fine matrix.
- Crystal Form
- Not applicable for the composite material as a whole. Chert itself is cryptocrystalline (microcrystalline quartz), meaning individual crystals are too small to be seen without magnification. The cement paste is amorphous to microcrystalline.
- Cleavage
- Not applicable for the composite material. Chert exhibits no cleavage but has a conchoidal fracture.
- Geological Environment
- Not a naturally occurring rock. It is an engineered material produced in construction environments, batch plants, and on construction sites. Chert, as a natural aggregate, originates from sedimentary environments, often found in limestone or chalk formations, or as nodules and beds within other sedimentary rocks.
Key Facts
- Hardness: Variable; cement paste is Mohs 3-4, chert aggregate is Mohs 7. Overall effective hardness is a composite.
- Specific Gravity: Variable, typically 2.2 - 2.4 for hardened concrete. Chert itself has a specific gravity of 2.58 - 2.64.
- Crystal System: Not applicable for the composite. Chert is cryptocrystalline (quartz, trigonal system).
- Color: Gray (matrix) with various colors of chert aggregate.
- Luster: Dull to earthy (matrix), dull to waxy/vitreous (chert).
- Transparency: Opaque (concrete). Chert can be translucent to opaque.
- Fracture: Irregular to conchoidal (for chert aggregate), irregular (for cement paste).
- Cleavage: None (for both cement paste and chert).
- Composition: Hydrated calcium silicates, calcium aluminates, and calcium sulfoaluminates (cement paste) + Silicon dioxide (SiO2) in cryptocrystalline form (chert aggregate) + other inert aggregates (sand, other gravels).
Quick Check
- Color: Gray matrix with variable colored aggregate (often light gray, brown, or black)
- Luster: Dull to earthy (matrix), dull to waxy/vitreous (chert aggregate)
- Streak: White (for both cement paste and chert)
Physical Characteristics
- Crystal Habit: Not applicable for the composite. Chert is cryptocrystalline, forming nodules or beds.
- Cleavage Type: None
- Fracture Type: Irregular to conchoidal (chert), irregular (cement paste)
- Tenacity: Brittle
- Luster Type: Dull to earthy (matrix), waxy to vitreous (chert)
Formation
Concrete with chert aggregate is formed by mixing Portland cement, water, and aggregates (sand and gravel, where the gravel component includes chert) in specific proportions. The cement and water react through hydration to form a hardened paste that binds the aggregate particles together. Chert, a microcrystalline quartz, is incorporated as a coarse aggregate.
Usage
Primarily used as a construction material for foundations, pavements, bridges, buildings, and other infrastructure where concrete is typically employed. However, its use is often avoided or mitigated in regions with freeze-thaw cycles due to the potential for alkali-silica reaction (ASR) and freeze-thaw damage.
Age Distribution
Modern (since the invention of Portland cement, early 19th century, with widespread use from the late 19th century onwards)
Where to Find
Construction Sites
Anywhere concrete is being poured or has been used for infrastructure, buildings, or pavements. Chert-bearing concrete is more common in regions where chert is an abundant and economically viable aggregate source.
Quarries and Aggregate Plants
Sources of chert aggregate for concrete production. These locations will supply the raw materials, not the finished concrete.
Demolition Sites
Where old concrete structures are being removed, providing samples of aged concrete that may contain chert aggregate.
Finding Tips
Visual Inspection
Look for angular to sub-rounded, often light-colored (but can be dark) aggregate particles embedded within a gray cement matrix. Chert often has a distinctive conchoidal fracture when broken.
Hardness Test
Chert is hard (Mohs 7), so it will scratch glass and steel. The cement paste is much softer (Mohs 3-4). This difference can be felt or observed when scratching the material.
Acid Test (for Chert identification)
While not for the concrete itself, a drop of dilute hydrochloric acid (HCl) will not react with chert (SiO2), distinguishing it from limestone (CaCO3) aggregates which would effervesce. Be cautious with acid.
Observe Deterioration
In older concrete structures, look for signs of ASR (Alkali-Silica Reaction) such as map cracking, gel exudation, and pop-outs, which are indicative of reactive aggregate like chert.
Similar Rocks
Concrete with Limestone Aggregate
Portland Cement Concrete with Calcareous Aggregate
Also known as: Limestone concrete
Concrete with Granite Aggregate
Portland Cement Concrete with Granitic Aggregate
Also known as: Granite concrete
Asphalt Concrete
Bituminous Concrete
Also known as: Asphalt, Blacktop
Scientific Classification
- Mineral Class
- Not a mineral. It is an artificial composite material.
- Group
- Artificial Construction Material
- Crystal System
- Not applicable for the composite. Chert (quartz) is trigonal.
- Chemical Formula
- Variable, primarily (CaO)x(SiO2)y(H2O)z for cement paste, and SiO2 for chert.
- Composition
- Calcium silicate hydrates, calcium hydroxide, calcium aluminate hydrates (from cement hydration); cryptocrystalline quartz (chert); other silicates and aluminates from sand and other aggregates.
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