How to Identify a Natural Cement: Case Study of the Vassy Church, France

Natural or Roman cements were the first modern cements to be industrially produced at the beginning of the 19th century in Europe. Used equally by engineers for their hydraulic properties, and by architects for their aesthetic qualities, they were massively employed for façade decoration or as cast-stone elements for masonries. This cultural heritage, even if it is abundant, is relatively unknown and needs now to be clearly identified and restored. Because of lack of knowledge and data on these cements, they are often identified as Portland cement or hydraulic lime, or hydraulic lime mixed with gypsum or Portland cement. Furthermore, heterogeneous calcination and varied quarried stone, which strongly modify their properties, make their characterizations more difficult. This paper presents a case study of the Vassy Church, with a focus on the binder identification of mortars. The Vassy church, built in 1859 by Gariel, a Vassy cement producer, is located in Burgundy, France. On the church, ochre-coloured mortars, were used either indoors, outdoors, as renders, as pointing mortars, as “run in situ” mortars, applied on stone or brick. Different types of mixes were employed, from rich to poor mortars. The testing protocol of those analysis firstly consisted of clinker grain analysis on polished sections (with and without Borax etching) by optical (OM) and scanning electron microscopy (SEM) observations, coupled with EDS analysis. Secondly, hydrated phases were characterised on mortar fractures, through SEM observations coupled with EDS analysis. Finally, crystallised phases were identified by XRD analysis of the binder in powder samples. Results of these analysis show several clinker morphologies and compositions, and different types of hydrates phases, according to the carbonation state. The presence of Gehlenite in very significative proportion, and C–A–S–H phases as a matrix in all of the samples, the absence of portlandite and the strong presence of ettringite in non-carbonated areas clearly indicate that the binder used to manufacture the cement mortars of the Vassy church was a natural cement, with a high sulfate and aluminate content. Furthermore, the coexistence of unburnt, burnt and over-burnt anhydrous grains reveal a production by a heterogeneous firing process. These results show important differences in terms of microstructure between natural cements and the other common hydraulic binders like Portland cement or lime.