Alkaline treatment of clay minerals from the Alhambra Formation: Implications for the conservation of earthen architecture

Clay–water interactions result in damage and loss of earthen architecture. Natural and artificial additives were traditionally added to earth in order to increase its water resistance and mechanical strength. More recently, portland cement, ethyl silicates and synthetic resins have been applied to consolidate earthen structures, however often with limited success. Here, in an effort to design a more effective procedure for the in situ consolidation of earthen architectural remains, the alkaline reaction of clay-rich earth used in the construction of the Alhambra (Spain) was studied. Alhambra Formation clays were treated with diluted Ca(OH)2, NaOH and KOH solutions. The most sensitive smectite fraction was rapidly destroyed after alkaline treatment. Furthermore, NaOH and KOH treatments resulted in the formation of interstratified illite–smectite, the partial destruction of both expandable and non-expandable clays, and their transformation into poorly-crystalline alkali aluminosilicates and calcium silicate hydrates, the latter two exhibiting cementing properties. These preliminary results suggest that alkaline activation might be efficient in stabilizing earthen architecture, as well as other clay-rich building and ornamental materials by reducing the intracrystalline swelling capacity of smectites, limiting osmotic swelling of both expandable and non-expandable clays, and producing gel-like silicate cements. However, further research and field tests will be necessary to study the effectiveness of the treatment in situ.