Consolidation of clay-rich earthen building materials: A comparative study at the Alhambra fortress (Spain)

Abstract

Earth has been an important construction material throughout history. Being a relatively fragile material, it can undergo extensive weathering depending on prevailing climate conditions, consequently requiring constant maintenance and often consolidation. The latter is still one of the most challenging tasks in the conservation field. Here we compare the effectiveness of a conventional consolidant (ethyl silicate) with several more recently introduced consolidants (nanolime and nanosilica) and consolidation methods (alkaline activation and bacterial biomineralization). Improvements in weathering resistance, water drop absorption, contact angle, and water vapor transmission, as well as mechanical strength (drilling resistance and compressive strength) of laboratory-prepared rammed earth mock-ups and a rammed earth wall at the Alhambra (Granada, Spain) were evaluated and related to treatment-induced compositional and textural changes. A detailed analysis of advantages and shortcomings of each consolidant was performed and modifications to current application protocols are proposed to optimize the efficacy of conventional and novel consolidation treatments.