Design - How old is my mortar? Can you date it?

Dating mortar accurately is very difficult unless it is obviously something like a black ash mortar which was prevalent in the 19th and early 20th century.

You can tell phasing by looking at different mortars in the same building or group of buildings but attributing a date to the mortar itself is very expensive and full of complications.

If the mortar has a wood/ charcoal content it may be suitable for radiocarbon dating (although this can yield results much older than the actual mortar).

They have successfully completed a study of dating mortar in medieval churches in Finland using accelerator mass spectrometry radiocarbon dating of non-hydraulic mortar. Atmospheric carbon dioxide is fixed in the carbonate formed during the hardening of lime mortar at the time of construction, which in principle makes it ideally suited for 14C dating. To produce building lime, limestone is heated to at least 900 °C to liberate carbon dioxide and produce quicklime (calcium oxide, CaO). The quicklime is then slaked with water to form calcium hydroxide (Ca(OH)2) or building lime, which is mixed with aggregates or filler (sand) and water to form mortar.

Calcium hydroxide in mortar reacts with atmospheric carbon dioxide forming calcium carbonate (CaCO3), as the binder in the hardened mortar. The 14C content of a mortar sample can thus in principle give a measure of the time elapsed since the time of hardening. However, this is fraught with difficulty; the mortar may contain old limestone, either as remains from incomplete conversion into calcium oxide in the burning process or from sedimentary carbonate in the aggregate, yielding apparent ages that are too old due to this form of contamination. Conversely, delayed hardening in thick walls or later recrystallization of the carbonate incorporating younger carbon dioxide can lead to dates that are too young.

The best way to analyse the date of a building is not by its mortar but by its architecture.