Granite weathering and stone deterioration in monuments and buildings in Oporto 

PhD thesis presented at University of Minho by Arlindo Begonha, 1997

Abstract
The purpose of this work is the study of stone decay in granitic monuments, specially those built with the Oporto granite.

The great thickness of the weathering profiles of the Oporto granite has always made difficult the use of the fresh rock. Stones of weathered rock were indeed systematically used in the monuments, since the exploitation and workability of the hard fresh rock was very difficult. In the present work, the study of the weathering process of the Oporto granite showed that it is basically a skeletal phenomenon, in which kaolinite and gibbsite are the final weathering products. The results of porosity, dry bulk density, ultrasonic velocity, uniaxial compressive strength, modulus of elasticity and strain in rupture showed the variation of these properties versus the weathering degree of the granite, allowing the determination of the correlations between these physical properties.

Optical microscopy, X-ray diffraction, scanning electron microscopy, infrared spectrophotometry, ionic chromatography, and plasma and atomic absorption spectrometry tests were used in order to characterize mineralogically and chemically the fresh and weathered rock as well as the stone pathologies in the monuments. Porosity, porosimetry, water absorption by capillarity, evaporation and permeability tests were carried out to study the porous media of the Oporto granite. The results of these tests showed that the weathered stones used in the monuments, characterized by a homogeneous and well interconnected porous network, decisively increased the rate of stone decay, specially in the present century by the impact of the atmospheric pollution and the consequent acidification and change in the composition of rain water. The evaporation tests showed the distinct behaviour between stones affected by granular disintegration and those affected by plates. In Oporto, air pollution, rain, the proximity of the Atlantic Ocean and the common use of Portland cement mortars are the most important sources of the soluble salts and fly ashes or other aerosol particles responsible for the genesis of stone deteriorations. Dissolution and crystallization cycles of the minerals of soluble salts are the direct cause of the formation of the pathologies. The Oporto granite does not interact chemically with the soluble salts, but it only allows their transfer and crystallization inside the porous network or on the surface of the stones.

The evolution of air pollution levels in Oporto in the last years and, the indiscriminate use of aggressive materials like Portland cement mortars in old and recent conservation and restoration works in many monuments and buildings do not seem to indicate the gradual decrease in the rate of stone degradation, but, on the contrary, to its acceleration. Therefore, it is urgent to establish the necessary measures that could prevent the relentless destruction of the cultural heritage of Oporto.

Theses abstracts