Calcium hydroxide

Calcium hydroxide, better known as slaked lime or hydrated lime, is one of the oldest and most discreet materials of human engineering. It is obtained by hydrating quicklime CaO (itself from limestone CaCO₃ calcination): CaO + H₂O → Ca(OH)₂, a violently exothermic reaction (-65 kJ/mol) familiarly called "slaking lime". This step doubles the solid's volume into a paste or a highly reactive dry powder, the base of myriad industrial and domestic uses.

Construction has been its core territory for 4,000 years. Mixed with sand, slaked lime forms a mortar that hardens slowly by reverse reaction with atmospheric CO₂: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. This carbonation process can take decades on thick walls and explains why Roman and medieval buildings grow stronger over time. The Romans discovered that adding pozzolana (volcanic ash) accelerated setting and enabled underwater curing — hence the still-standing ancient harbours like Caesarea Maritima. Modern Portland cement has replaced lime in most construction since 1850, but lime remains valued in heritage restoration (Notre-Dame, Loire Valley castles) because it "breathes" better and is compatible with old stones.

Outside construction, Ca(OH)₂ is a near-universal industrial pH regulator: acid effluent neutralisation (H₂SO₄ production, sewage treatment), flue-gas desulfurisation of thermal power plants (FGD wet scrubbing), pH adjustment of overly acidic agricultural soils (~10 Mt/yr in Europe for liming). In food, it intervenes in nixtamalisation (preparation of the masa for Mexican tortillas, which releases niacin from corn and prevented pellagra throughout Mesoamerican culture) and in cane sugar refining. Its very limited solubility (1.5 g/L) makes it a "slow" base that does not caustify skin immediately, unlike NaOH.