Slate
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Constituent and Colour:
The mineral constituents and its chemical properties determine the colour of the slate and whether it is unfading or semi-weathering or weathering (i.e. colour fastness on exposure to the weather). Surface colours can be uniform or have contrasting hues. Basic slate colours are blue black, blue grey, mottled purple, green and red.
- Unfading: indicates that the slate will retain its original colour on exposure to the weather
- Semi-weathering: indicates that the slate will have some 10-20% that may turn brown in colour on exposure to weather
- Weathering: indicates that the slate will have more than 50% that may turn brown in colour on exposure on weather [1]
Physical Properties:
| Stone group | Metamorphic |
| Common rock forming mineral group | Calcareous |
| Composition | It was formed in layers, and, as metamorphism occurred, resulted in a stone that splits readily into thin slabs, which are strong, and durable (with high tensile strength). Slates also contain large quantities of colourless mica in small, irregularly shaped scales. |
| Porosity (%) | 0.4-5 |
| Water Absorption (%) | 0.1-1.7 |
| Density | 2.6-2.9 gr/cm3 |
| Compressive Strength | 68.95-161 MPa |
| Modulus Rupture | 27-88.95 MPa |
| Specific Gravity | 2.74-2.82 |
| Fire behaviour | Fireproof |
Source: Terry Kennedy; Roofing Instant Answers; McGraw-Hill, 2002 [1]
Nowadays, building industry has found alternative to natural slate, which cost less and lighter. It includes those with fibre cementand resin or polymer base.