INFLUENCE OF SOLUTION pH ON SLAKE DURABILITY AND CHEMICAL WEATHERING OF GRANITE: AN ASSESSMENT USING CIA AND PIA INDICES

Abstract

The durability of rock materials under environmental conditions is governed by a combination of mechanical disintegration and  chemical weathering processes. This study investigates the influence of solution pH on the slake durability behaviour of granite collected from the Bundelkhand region of central India. Repeated wetting-drying cycles were performed following ISRM standards using slaking media with pH values of 3, 5, 7, 9, and 11. The chemical composition of the rock samples and molar proportions of key oxides were computed to evaluate chemical weathering trends. Two widely accepted geochemical indices - Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) - were used to quantify the degree of alteration during successive slaking cycles. The results indicate that CaO exhibits the highest mobility, particularly under acidic conditions, signifying preferential leaching of Ca bearing minerals, while Al₂O₃ remains relatively immobile and becomes residually enriched. CIA values ranging from approximately 72 to 90 indicate moderate to intense weathering, with increasing trends in later cycles reflecting progressive alteration. Consistently high PIA values ( > 85) highlight dominant plagioclase weathering and the transformation of feldspar minerals into clay-rich phases. Mildly acidic conditions (pH = 5) were found to promote sustained and systematic chemical weathering, whereas extreme pH environments accelerated reactions but resulted in higher variability.