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 CO2) or alternatively under N2 atmosphere (in the absence of CO2). The structural transformations have been detected with 29Si MAS NMR spectroscopy. Heating converts the crystalline Q3 phyllosilicates into the amorphous Q3am phase. The Q3am phase is partially converted by lime in the presence of CO2 into amorphous Q2am inosilicates, while the Q3 phase remains unchanged. In the absence of CO2, lime converts swiftly the Q3am phase (and more slowly the Q3 phase) into the crystalline Q2 inosilicates and Q1 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 Q2am phase.