In keeping with its high basicity, tricalcium aluminate reacts most strongly with water of all the calcium aluminates, and it is also the most reactive of the Portland clinker phases. Its hydration to phases of the form Ca2AlO3(OH) leads to the phenomenon of "flash set" (instantaneous set), and a large amount of heat is generated. To avoid this, Portland-type cements include a small addition of calcium sulfate (typically 4-8%). Sulfate ions in solution lead to the formation of an insoluble layer of ettringite (3CaO • Al2O3 • 3CaSO4 over the surface of the aluminate crystals, passivating them. The aluminate then reacts slowly to form the AFm phase 3CaO • Al2O3 • CaSO4 These hydrates contribute little to strength development.