Salt, sun and the Cape: a hard place to leave anything outdoors

Three forces, working on different things

A durable memorial in Cape Town has three specific things to survive, and it is worth separating them, because they attack different parts of a material.

Salt is carried inland on the wind as fine aerosol and settles on every surface. On a porous material it is drawn in with moisture, then crystallises as it dries — and crystallising salt exerts real pressure inside a pore. Where it reaches steel, it does far worse: chloride is what breaks down the protective chemistry around rebar and starts corrosion.

Ultraviolet does nothing to a mineral surface and a great deal to an organic one. Paints chalk, tint, and eventually lift. Anything relying on a surface coating for its colour is on a clock in this light.

Thermal cycling is the quiet one. A hundred times a year a piece heats through the day and contracts through the night. Every material moves; the question is whether the parts of an assembly move together or against each other.

What each force finds

Steel-reinforced concrete offers salt an easy path: a porous matrix, and steel to corrode once the chloride arrives. This is why coastal concrete spalls, and why it does so on a timescale of decades rather than centuries.

Painted or coated finishes offer ultraviolet an organic layer to break down. Once the coating goes, the substrate is exposed and the piece needs recoating rather than cleaning.

Assemblies that mix materials with very different expansion behaviour — a steel fixing rigidly set into stone, for instance — give thermal cycling something to work against. Over enough summers, something has to give.

What survives it

GFRC meets each of the three at the point where it is strong. Its matrix is dense and polymer-modified, so absorption is low and there is comparatively little for salt to get into. It contains no steel at all, so chloride has nothing to corrode. Its colour is mineral oxide carried through the body, so ultraviolet has no organic film to attack.

This is not a coincidence of our specification. It is the reason the material was developed: GFRC was created for building facades in exactly these conditions, and the fifty-year service record behind it was earned outdoors.

Two things that help any material

Drainage. Standing water is the multiplier on every one of these forces. A base that sheds water, and ground that does not pond against it, does more for longevity than any material choice.

A rinse. Salt that is washed off is salt that never gets drawn in. Clean water and a soft brush when you visit is genuinely most of the maintenance any outdoor memorial needs, whatever it is made from — the care guide covers the rest.

What we do differently for a coastal site

The material is the same; the detailing is not.

We specify a heavier seal on exposed coastal work and suggest a shorter reseal interval — nearer eight years than ten. We pay closer attention to how water leaves the piece, because a detail that holds a film of salty water in one place will show it eventually. And we avoid fixings that trap moisture in a joint, which is part of why the parts are dry-assembled rather than bedded in mortar.

If a memorial is going in within sight of the sea, tell us at the drawing stage rather than after. It changes small things, and small things are what a century tests.

A note on lichen

Lichen and algae will find any stone or concrete surface in a damp, shaded cemetery, and there is no material that prevents it. On a sealed GFRC surface it sits on top and washes off, rather than rooting into the material.

Some families ask us to remove it. Others prefer it, and we understand why. It is not damage.