Natural analogues of Portland cement can provide insights into carbonation processes operating over timescales of interest to performance assessments of CO2 storage. Two such sites in Northern Ireland (Scawt Hill and Carneal Plug) have been investigated, where carbonation reactions of naturally-occurring cement minerals can be observed. Here, partially hydrated larnite-rich nodules have been reacting with atmospheric CO2 or dissolved bicarbonate ions at low temperatures over the last 10-20 thousand years. This has produced rims of carbonate minerals around hydrated low-temperature calcium silicate (CSH) minerals enclosing residual cores of primary high-temperature metamorphic calcium silicates and calcium aluminates (dominated by larnite, and often accompanied by wollastonite, spurrite, paraspurrite, brownmillerite, bredigite, andradite-grossular) with carbonation proceeding progressively from the outer margins of the nodules. Calcite and scawtite are the dominant secondary calcium carbonate minerals, but vaterite and aragonite have also formed. The carbonation produces dense, low-porosity carbonate rims. This is associated with a reduction in volume, accompanied by shrinkage and microfracturing of the underlying residual poorly crystalline CSH gel and its silica-rich alteration product, which has created significant secondary porosity in the altered material. Although some secondary calcium carbonate reaction products may partially mineralise the fractures, they do not seal the fractures completely, allowing further ingress of CO 2/bicarbonate. That said, the ingress rate has not been sufficient to completely carbonate the nodules, even after several thousand years. Uncertainties remain in terms of quantifying carbonation reaction rates and the CO2/HCO3 – flux, and further work is needed to understand these potentially very useful analogues.