Towards a better understanding of historic mortar production\u2014burning experiments on calcareous nannofossils

Abstract

Calcareous nannofossils are < 30 µm sized calcitic fossil remains of single-celled marine photoautotrophic algae. Carbonate-rich sedimentary rocks, containing these fossils, are used as raw material for lime-based mortars. The carbonates (CaCO3) are heated up to approx. 900 °C. The burning process, which causes the thermal decomposition of CaCO3 into CaO (= quicklime) and CO2, destroys the calcitic fossils. Surprisingly, remains of these calcareous algae were recently encountered in historic mortars and mortar-based materials. To gain a better understanding of the behaviour of calcareous nannofossils during the calcination procedure, four sample sets were heated to nine temperature levels (100 °C, 300 °C, 500 °C, 600 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C). For each sample set, the initial and the heated material of all nine temperature levels were analysed with respect to its nannofossil content and preservation by using settling slides. Our results show a decrease of absolute abundance and preservation from 500 °C onwards; rare nannofossils are preserved up to 900 °C. Changes in the relative abundance of individual species document that certain taxa are more heat resistant than others. This pattern is explained by different crystal sizes and forms of the relevant taxa. Differences in the calcareous nannofossil assemblages, observed in the raw material and in the mortar produced from it, can be used to estimate the temperature reached during quicklime production.