All you wanted to know about roofing slate and you were afraid to ask

Some questions (and answers) about roofing slate

by Victor Cárdenes

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NSA 2016 Annual Meeting, March 3-6 Philadelphia

• What is a roofing slate? (from a geological point of view)

• Who many types of roofing slates exist?

• What about pyrite?

• What about carbonates?

• What about ribbons?

• Some remarks about standards

• How are the slates from Europe/China/Brazil?

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TWO DIFFERENT TERMS!

- ROOFING SLATE: Commercial term, refers to any rock able to be split and used for roofing

- SLATE: Geological term, refers to a specific type of rock

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What is a roofing slate?

Not all slates can be used for roofing, and not all roofing slates are slates

Roofing slates are metamorphic rocks

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MUDP

RES

SU

RE

TE

MP

ER

ATU

RE

TIM

ESLATE

PHYLLITE

LOW-GRADE SLATE

SCHIST

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CLEAVAGE 1) Sedimentation 2) Lithostatic compression

3) Tectonic compression

a) b)Cleavage

Mud

Sand

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Cleavage allows splitting

Pneumatic chisel

Hammer and chisel

Alignment of the mica minerals

0.1 mm

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Cleavage vs. flexural strength

Mayen, Germany

Minas Gerais, Brazil

Otta, Norway

Penrhyn, WalesValentia, Ireland

Lugo, Spain

Cle

avag

e

Flexural strenght

203000

405800

608700

MPaPSI

0 6001100

400750

200400

°C°F

Pressure vs. Temperature

META

MO

RPH

ISM

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Who many types of roofing slates exist?

Low-grade slate Slate

Phyllite Schist

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10

TONS Millions $

Low-grade slate25.0%

Slate73.0%

Schist1.0%

Phyllite1.0%

Low-grade slate38.0%

Slate60.0%

Schist1.0%

Phyllite1.0%

Estimation of the total volume of roofing slate production depending on the type of rock

Source: Personal data and United Nations COMTRADE Database

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Roofing slates are also classified by their color

Three color families: - Black Gray (B) - Green (G) - Red-Purple (R)

Mayor mineral components • Quartz • Mica • Chlorite Accessory components (<2%) Iron sulphides (pyrite): B, G Organic matter (carbon): B, g Iron oxides (hematite): R

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The combination of the rock type and the color family establishes the classification for roofing slates

Roofing slate lithotypes

LOW GRADE SLATES

0SLATES

1SCHISTS

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PetrographiccodeColor

code

BLACK-GRAYB

GREENG

PURPLE-REDR

B0Black-gray low grade slates

B1Black-gray slates

Black-gray phyllites

B2 B3Black-gray schists

G0Green low grade slates

Green slatesG1 G2

Green phyllites

Green schistsG3

R0Purple-red low grade slates

R

Purple-red slates1 R2

Purple-red phyllitesR3

Purple-red schists

PHYLLITES

2

Octahedron

“Dollar”

Fossils

Framboids

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What about pyrite?

Iron Sulphides: Pyrite (FeS2) and Pyrrhotite (FeS1-x)

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Sometimes, these minerals might cause trouble

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How works oxidation?

SO2 H O2

Oxidation is triggered by weather conditions

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Oxidation grades according to EN 12326-2

T1 T2

pyrite cube aggregate

pyrrhotized shell

pyrite cubeT3

Initial stage Color change Rust marks Corrosion

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Estimation of the incidence of oxidation

T1

T2

T3

60%

40%

<0.1%

Oxidation is mainly an aesthetical problem

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Type of slate and oxidation

Three color families: - Black Gray (B) - Green (G) - Red-Purple (R)

Mayor mineral components • Quartz • Mica • Chlorite Accessory components (<2%) Iron sulphides (pyrite): B, G Organic matter (carbon): B, g Iron oxides (hematite): R

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Has oxidation any solution?

So far, different solutions have been tested

Cleaning and application of protective coating

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Has oxidation any solution?

So far, different solutions have been tested

Application of coating treatment during the fabrication process

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Ankerite: CaFeCO3

Calcite: CaCO3

Siderite: FeCO3

Dolomite: MgCaCO3

What about carbonates?

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Gypsification (Fading) CaCO3 + SO2 CaSO4 2H2O

1x

10x

100x

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Calcite Gypsum

SO2 H O2

Gypsification is also triggered by weather

33% increase

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Slaty cleavage

Gypsum

Detachment

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Fading Calcite and dolomite white fading (CaCO3 ) (MgCaCO3)

Ankerite and siderite red fading (CaFeCO3 ) (FeCO3)

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Sedimentation

River network

A) Normal mud sedimentation B) Occasional sand sedimentation C) Mud sedimentation again

What about ribbons?

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1) Sedimentation 2) Lithostatic compression

3) Tectonic compression

a) b)Cleavage

Mud

Sand

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4 ft.

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Some remarks about standards

EN 12326 ASTM C 406 IS 6250 GB/T 18600

1999 1932 1981 2009

European Union USA India China

Mechanical behavior Flexural strength

Water Absorption

Permeability

Freeze-Thaw

Thermal cycle

Acid exposure

Carbon and carbonate content

Petrography

(*)Qualititative classification

COUNTRY

Sampling

Formatting

Weathering behavior

Petrological characterization

YEAR

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European Standard EN12326

- 13 test methods - Developed from former

national standards - CE mark - Petrology

Petrographic examination

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American Standard ASTM C406

- 3 test methods - Lifespan - No oxidation tests

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Indian Standard IS 6250

- 6 test methods - Inspired in the former BS - Last revision 1981!

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Chinese Standard GB/T 18600

- 5 test methods - Inspired by other standards - Quality and lifespan

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Thank you for your attention Any questions?

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