Interaction between clay and lime in “cocciopesto” mortars: a study by 29Si MAS spectroscopy

The structure of historical “cocciopesto” mortars (utilized in Venice and more generally in the Mediterranean area) have been mimicked with samples of phyllosilicate rich (by 58%) clay heated at different temperatures (between 500 and 700 °C) and treated with lime over a 5-month seasoning period in the air (in the presence of CO 2) or alternatively under N 2 atmosphere (in the absence of CO 2). The structural transformations have been detected with 29Si MAS NMR spectroscopy. Heating converts the crystalline Q 3 phyllosilicates into the amorphous Q 3am phase. The Q 3am phase is partially converted by lime in the presence of CO 2 into amorphous Q 2am inosilicates, while the Q 3 phase remains unchanged. In the absence of CO 2, lime converts swiftly the Q 3am phase (and more slowly the Q 3 phase) into the crystalline Q 2 inosilicates and Q 1 sorosilicates. As the conditions encountered by historical “cocciopesto” mortars during their secular seasoning are better mimicked by clay treated with lime in the air, the hydraulic properties of the former material are temptatively attributed to the presence of the Q 2am phase.