At the type location in Northern Ireland it occurs as an alteration of calc–silicate rocks by contact metamorphism of larnite–spurrite.

In Jebel Awq, Oman, it occurs as precipitates from an alkaline spring emanating from ultramafic bedrock.

In the Chelyabinsk coal basin of Russia it is produced by combustion of coal seams and similarly by spontaneous combustion of bitumen in the Hatrurim Formation of the Negev desert in Israel and the Maqarin area, Jordan.

[4][3] It occurs in association with afwillite, calcite, larnite, spurrite, halite, brownmillerite, hydrocalumite, mayenite and ettringite.

It was named portlandite because the chemical calcium hydroxide is a common hydrolysis product of Portland cement.