What are the possible implications of the heating of the structural stonework of Notre-Dame cathedral during the fire?

Question

Yesterday, Notre Dame cathedral caught fire. The roof and the central spire were destroyed and some of the stonework was subjected to high temperatures for an extended period.

What are the likely implications for the strength of the individual stones, the walls and other structures?

Answer 1

Concrete can be significantly damaged at 600F but specifics are highly variable according to the Portland Cement association. It depends on the ratio of cement to aggregate , sizes of aggregate, composition of the aggregate, heating/cooling rate, duration, etc. Specific information on sandstone and limestone is even more difficult to find. Rock can even crack during the fire depending on moisture content and how fast it is heated. I expect it will be many weeks or months before objective information is available.

Answer 2

There is a specific temperature range that causes calcination of limestone.

$$\text{CaCO}_3 + \text{Heat} = \text{CaO} + \text{CO}_2$$

The temperature range of reaction is 840°C-900°C and is used for creating cement in a kiln. Adding oxygen and sulphur dioxide causes limestone to become gypsum... which is structurally significantly weaker than limestone from which the old cathedral is built. Above 900°C the carbon dioxide emissions accelerate beyond permissible emissions levels.

Effectively, superheating limestone causes chemical reactions that result in physical changes of the structural material blocks. Exfoliation is a likely result with degradation of surfaces reaction to other chemical compounds in the fire itself. If sulphur compounds were present then there is a risk that the stone may have been changed into another type of rock to varying depths.

Add water to carbon dioxide and it creates carbolic acid which may attack limestone.

Carbonic acid, hydrochloric acid and acetic acid are some acids that react with limestone, causing it to dissolve. Each of these acids reacts with limestone in different ways. Limestone is made up mainly of calcite that is the chemical compound calcium carbonate.

Carbonic acid, which is a weak acid, forms when rainwater and carbon dioxide in the air react with one another. When carbonic acid comes into contact with limestone or calcium carbonate, it can cause it to dissolve over a long period of time. In nature, this reaction leads to the formation of caves.

Source: https://www.reference.com/science/acids-dissolve-limestone-8485a52922d6ebe1

Answer 3

Two factors that are positive.

• Masonry, grout, concrete are some of the best fire resistant materials.
• Exposure to the hottest flames has been avoided by the fact that geometry of the building and its long hallways mostly acted as a ventilation manifold to feed the flames with fresh oxygen, and kept the walls awash with cool air.
• They have some of the best experts in preservation and rehab of these world heritage structures.
• It's incredible and offensive even why safety measures, fire sprinklers, alarms were not installed especially during the construction or failed to work.