Mix lime with the volcanic ash of the Bay of Naples, and the mortar will bind rubble into a solid mass that hardens even under the sea.
Vitruvius describes a powder found near Baiae and Vesuvius which, mixed with lime and rubble, makes piers that set hard in the sea. He explains it by the fires under the volcanoes, which have dried the earth. The modern explanation is chemical: the reactive silica and alumina of the volcanic ash combine with the lime to form compounds that harden without drying: a hydraulic cement.
Pozzolana made Roman concrete strong enough for vaults and domes and durable enough for harbours; recent studies of Roman marine concrete have found that it continues to grow crystals in its pores over the centuries. The name survives in “pozzolanic” cements.
In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.
load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g
Build an arch of wedge-shaped stones (voussoirs) whose joints all point towards the centre of the curve: each stone can then fall only by forcing its neighbours apart, and so the ring holds itself up once the last stone, the keystone, is in place.
joints ⟂ intrados (radial to the centre)
An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.
H ≈ w·s² ⁄ 8f (uniform load w per metre, span s, rise f)
Trace the path of the resultant force through the stones from crown to foundations. If a line can be found that stays everywhere within the masonry, the arch will stand; where it touches an edge, a hinge will open, and four hinges make it collapse.
An arch cannot support itself until it is complete. Build it on a timber centring shaped to the intrados, strike the centring only when the keystone is set and the mortar has taken, and reuse it from bay to bay, which is one reason to keep the spans of an arcade equal.
In a row of arches the thrusts of neighbouring arches cancel over each intermediate pier, which may therefore be slender; at the ends of the row nothing balances them, so the outermost piers must be made wider or buttressed.
Cite this page Chris Paton, “Pozzolana: a mortar that sets under water”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/pozzolana. Text, drawings and models licensed under CC BY-NC 4.0.