Hydraulicity of lime plasters from Teotihuacan, Mexico: a microchemical and microphysical approach

Hydraulicity is a well-known characteristic of mortars and plasters from the Roman, Late Roman, and Medieval periods, while the use of hydraulic mortars in pre-Columbian Mesoamerica and Central Mexico in particular, has not received equal attention. To address this issue, in this paper, we discuss the problem of hydraulicity and suggest a method to study it in Mesoamerican plasters. To prove the effectiveness of this method, we selected plaster samples from the archaeological site of Teotihuacan (Mexico), the main city of Central Mexico during the Classic period. The characterisation of plaster samples confirmed that, similar to other Teotihuacan plasters, they are made of two layers: the outermost layer (enlucido), composed of a mixture of lime and volcanic glass shards, and the underlying layer locally called firme, consisting of crushed volcanic scoria (tezontle) mixed with a mud-based binder. The compositional characterisation of the plasters confirmed their hydraulicity and the combination of microchemical (energy dispersive spectrometer (EDS) microchemical analysis), microphysical (Vickers microhardness), and petrographic (optical microscopy) measurements demonstrated that it was not the presence of glass shards in the external plaster layer which produced hydraulicity, as we originally thought. Instead, this was due to the reactivity of the tezontle, present in the lower layer. This suggests that the small thickness of the enlucido layer, in addition to save the amount of lime used, increased its performance, promoting chemical reactions with the tezontle in the contact layer and that the glass shards played another role in the mixture, rather than producing hydraulicity.