What is mortar carbonation? Is it desirable? Why do some mortars carbonate faster than others?
Carbonation occurs when calcium hydroxide from hydrated lime or the hydration of portland cement in the mortar combines with carbon dioxide in the air to form calcium carbonate. Carbonated mortar is no longer alkaline and therefore, no longer provides protection for embedded steel. Carbonation is not a concern when masonry does not contain steel. Carbonation occurs fastest in porous mortars with low percentages of portland cement and lime. Very dry or very wet mortars carbonate much more slowly than do mortars where the relative humidity is about 50%.