There are many different types of rock used as aggregate. Hardness often varies even within the same classification of rock. For example, granite varies in hardness and friability (a measure of how easily a material crumbles) over a wide range of medium soft to hard.
The Mohs scale is frequently used to measure aggregate hardness. Values of hardness are assigned form one to 10. A substance with a higher Mohs number scratched a substance with a lower number-higher Mohs scale numbers indicate harder materials. The following table shows where some common minerals fall on the Mohs scale.
Mohs Scale
1- Talc
5- Apatite
9- Corundum
2- Gypsum
6- Feldspar
10- Diamond
3- Calcite
7- Quartz
4- Fluorite
8- Topaz
Most aggregates fall into the 2 to 9 range on the Mohs scale. Some commonly used aggregates measure this way on the Mohs scale:
Mohs Range
Description
Aggregates
8-9
Critically Hard
Flint, Chert, Trap Rock, Basalt
6-7
Hard
Some River Rock, Some Granites, Basalt, quartz, Trap Rock
4-5
Medium Hard
Some Granites, Some River Rock
3-4
Medium
Dense Limestone, Sandstone, Dolomite, Marble
2-3
Medium Soft
Soft Limestone
Aggregate hardness is one important factor when cutting concrete. Because hard aggregate dulls diamond more quickly, segment bonds on diamond saw blades generally need to be softer when cutting hard aggregate. This allows the segment to wear normally and bring new, sharp diamond grit to the surface. Softer aggregate will not dull diamond grit as quickly, so harder segment bonds are needed to hold the diamonds in place on the saw blade long enough to use their full potential.
Size of the Aggregate
the size of aggregate affects diamond blade performance. Large aggregates tend to make a blade cut slower. Smaller aggregates tend to make a blade cut faster. The most common standard sizes of aggregate are:
- Pea Gravel- variable in size, usually 3/8" or less in diameter
- ľ inch- sieved size of ľ' or less
- 1-1/2 inch- sieved size of 1-1/2 or less
