The term gravel refers to pieces of rock and minerals that measure more than 2 millimeters in diameter or length. In addition, the term covers debris or rock clasts that are the size of grains, pebbles and blocks.
The gravel has several implications according to the application that it is going to be given, for example:
Types of gravel
Gravel in sediments and sedimentary rocks
In sedimentology and in the study of sedimentary rocks, gravel corresponds to sediments, clasts or debris that measure more than 2 millimeters in diameter. If this gravelly material were to cement and lithify, it would form sedimentary rocks known as conglomerates and also breccias..
As is natural, the conglomerates and breccias are formed by pieces of gravel.
Here you can also differentiate the gravel of fluvial and deltaic environments
Gravel as building material
As a construction material gravel it is very important because it can be used as ballast or fill for roads and as a base for the construction of buildings, bridges, houses and a variety of civil works.
In addition, gravel is an additive or primary component of concrete.
In this case, it is important that the gravel meet certain physical properties and characteristics that make it useful as a material for civil engineering.
Some of these properties are:
- Stable chemical composition (generally between 45% to 52% Silica SiO2)
- Resistance to shear stresses (pieces of fresh stones or rocks) and that are not affected by fractures or veinlets.
- Stable mineral composition (generally associated with basalts, andesites, and volcanic rocks)
- It should not contain many mica minerals such as biotite or muscovite.
Regarding the size, the following can be named:
- Fine gravel: from 2 to 8 millimeters
- Medium gravel: from 8 to 12 millimeters
- Coarse gravel: Greater than 12 millimeters
Crushed gravel for concrete
Generally, when a gravel deposit is counted to exploit it or extract it in a quarry, the gravel material usually has a wide variation in size, so it is necessary to process and crush it to leave it as homogeneous as possible.
This with the objective of homogenizing the mixture in the concrete and achieving the appropriate weight in the mixture.
Characteristic:
- Size between 5mm to 38mm
- The weight of the gravel in the concrete should be around 85%
- It must be of a stable composition: related mainly to basalts and andesites, sometimes dacites and diorites.
Properties and characteristics
Gravel size
By definition, gravel is all those materials or parts of rocks and minerals that are more than 2 millimeters in diameter or length.
Chemical and mineralogical composition
Because gravel is parts and rocks of any type of pre-existing rock mass, it makes the whole a material of gravel have a varied chemical and mineralogical composition.
As now you know the gravel They are practically pieces or parts of pre-existing rocks, so they can have various compositions.
However, we can mention that the rocks that commonly appear to make up gravel materials are: basalts, andesites, schists, gneiss, slates, quartzites, granites, diorites, gabbros, rhyolites, however, it can be any type of rock.
But suppose we observe that we observe that most of the gravel corresponds to fragments of basalt rocks, so we could say that the chemical composition is mafic or is between 45% and 52% silica.
As for the minerals in particular, the most resistant ones also appear, such as quartz, zircon, olivine, garnets, epidote, etc.
Of course, the most resistant to wear and transport almost always appear.
| Sediment components | |
| MINERALS | |
| Quartz | Monocrystalline and polycrystalline quartz |
| feldspars | Potassium feldspars and plagioclase |
| clay minerals | Kaolinite, illite, smectite, chlorite, sepiolite |
| glauconitic minerals | Mineral blend including glauconite |
| micaceous minerals | Muscovite, sericite and biotite |
| carbonate minerals | Calcite, dolomite, siderite, ankerite. |
| evaporites | Gypsum, anhydrite and others |
| Heavy minerals (not opaque) | Large number of typical rock-forming minerals with specific gravity greater than 2.9. |
| Heavy (opaque) minerals | Hematite, limonite, goethite, magnetite, ilmenite, pyrite, marcasite |
| Zeolites | Common products of volcanic glass alteration (zeolites) |
| ORGANIC AND BIOGENIC MATERIAL | |
| calcareous skeletal material | Calcareous fossils (foraminifera) |
| siliceous skeletal material | siliceous fossils |
| Phosphatic material (phosphorites) | Includes apatite |
| Organic material | Palynomorphs, Charcoal Types, and Kerogen |
| ROCK FRAGMENTS | |
| Igneous rocks | Intrusive, plutonic, and volcanic igneous rocks. (granites, gabbros, basalts, rhyolites, diorites, peridotites, andesites, etc.) |
| Metamorphic rocks | All kinds of metamorphic rocks (quartzite, schist, gneiss, amphibolites, marble, etc.) |
| Sedimentary rocks | All kinds of sedimentary rocks (sandstones, shales, siltstones, limestone, etc.) |
| CEMENTING MATERIAL | |
| silicates | Mainly quartz, chalcedony, opal, feldspars and zeolites |
| carbonates | Calcite, dolomite, siderite, aragonite. |
| iron oxides | Hematite, limonite, goethite |
| Sulfates | Anhydrite, gypsum, celestine, barite |
Hardness
The hardness of this material will depend on its chemical and mineralogical composition, for example, if the gravel material is rich in quartz, its average hardness would be equal to quartz on the Mohs scale, in this case 7.
Shape, roundness, sphericity and sorting of gravel grains
The shape of the materials gravel to depend above all on the type of transport they have had and the geological environment from which they come.
Gravel from fluvial environments
For example, the gravel that comes from fluvial environments generally tends to have spherical, round and disc-like shapes.
The roundness usually varies between angular to well rounded, it all depends on how far that sediment has traveled.
The draw in this case is usually very bad because there are always very fine sediments such as clay, silt or sand mixed with the gravel.
Gravel from deltaic environments and fans
For example, the gravel that forms in deltaic or fan environments, on the other hand, is generally a wavy to very angular subnetwork, because they do not have much transport.
Here the draw is usually equally bad because there is a large amount of mixed sediment.
Color
The color of the gravel will depend on the type of rock it comes from and how weathered or fresh it is.
For example, a gravel that comes from andesites or basalts will be dark gray tones.
If that same gravel is oxidized it will have a reddish color.
The colors of the gravel can be seen mainly in fluvial environments or river gravels.
Uses and applications
- Usually gravel is used and applied as a construction material
- For filling roads, railway bases, bases for buildings and civil works.
- It is an important compound of concrete.
- It is exploited in construction materials quarries, in Ecuador it is called La chispa.
- In sedimentology it indicates the sedimentary and geological environment from which it comes.