Evaluating building stones: Physical-mechanical changes from high-temperature fire and water cooling

Area of Science:

• Geology and Materials Science
• Heritage Conservation Science
• Fire Engineering

Background:

• Natural stone heritage sites face significant risks from fire, which can degrade material properties and compromise structural integrity.
• Over three decades of research highlight the need for effective fire risk mitigation and restoration strategies for cultural heritage.

Purpose of the Study:

• To investigate the impact of high-temperature exposure on Portuguese heritage limestones (Lioz and grey Ançã stone).
• To provide insights for developing strategies to protect and preserve natural stone heritage structures against fire damage.

Main Methods:

• Simulated fire exposure at 200°C, 400°C, and 600°C, followed by water cooling.
• Analysis of morphological, physical, and mechanical properties using colorimetry (CIE L*a*b*), SEM-EDS, uniaxial compressive strength tests, Leeb D hardness, porosity measurements, ultrasound velocity, and thermogravimetric analysis (TGA).

Main Results:

• High-temperature exposure caused significant color changes (3-32%), microstructural modifications (cracks, porosity), and reductions in mechanical properties (compressive strength up to 33%, hardness up to 12.2%).
• Open porosity increased (70-289%), and ultrasound speed decreased significantly (up to 49%) after thermal cycling at 600°C.
• Mass loss was attributed to the release of retained and hydration water, and decomposition (H₂O, CO₂).

Conclusions:

• Limestones with higher toughness and compressive strength showed better resistance to fire-induced damage.
• The study identifies a knowledge gap in understanding limestone mechanical decay from fire, emphasizing the need for further analysis.
• Recommendations include increasing stone thickness to compensate for mechanical strength loss at high temperatures for heritage structures.