The Gray-Green Alchemy of Baycrete

In a hot Antioch yard in the summer of 2018, members of the North Bay Conservation Corps mixed a very strange cocktail. Here’s the recipe: marine-grade concrete is shaken with crushed oyster shells. Stir in native sand dredged from the Bay, gravel, and a whisper of admixture to hold it all together. This brew is “baycrete”: a material designed to create the base structure for man-made nearshore reefs and living shoreline projects, it can anchor habitat, slow waves, and invite life back into places hardened by fill and bulkheads. It’s roughly half concrete, half everything else. 

“It really is a field fitting each time,” says Marilyn Latta of the California State Coastal Conservancy, who has shepherded this hybrid material from idea to reality. “The folks running the concrete pour and the mixer, where they’re adjusting that mix, are not exactly recording those details.” But guided by a good bartender’s instinct, it always turns out basically right.

The recipe itself came together around 2010, from a group of scientists, engineers, and restoration practitioners searching for alternatives to pure concrete for a reef ball project at San Rafael. “There’s so much anthropogenic fill in the bay,” says Latta. “We were very interested to see if we could at least have a hybrid approach [using both] green and gray materials for the reef structure.” 

After being mixed and poured, each baycrete structure cures slowly in the sun, developing its own reef ball terroir. “It’s going to be influenced by the exact materials you got, the texture of the sand that happened to come,” says Latta. “The temperature varies slightly where you’re mixing” — the baycrete brewed in hot, dry Antioch cured much faster than in Inverness. Once cured, the baycrete reef balls and oyster blocks are hoisted into the Bay at sites like Giant Marsh, where they provide habitat, slow waves, trap sediment, and stabilize vulnerable marsh edges.

Baycrete is truly gray and green — concrete softened, seeded, and roughened. Olympia oyster shells, once burned in kilns to make the original concrete and mortar of early San Francisco, return, this time as natural fill, texture, and lure. The native oysters like to settle atop other oysters, so the shell in the mix likely makes the structures more attractive landing spots for larvae. The shell also contributes to the material’s rugosity: the measure of its surface roughness.

“Even if it’s [just] centimeters in that surface area, it gives little nooks and crannies — areas for moisture, or for critters to attach and get protection from predators or heat stress,” says Latta. In baycrete’s craggy skin, mussels, limpets, algae, crabs, and worms all find footholds. 

Fourteen years after the first baycrete reef balls were deployed near San Rafael, they remain remarkably intact. “We originally thought five to 10 years would be the design life,” Latta says. “But so far, they’re still holding up.”

Some of that staying power may come from what’s growing on them. As oysters, mussels, and algae accumulate, they create what Latta calls “biological glue”: a living encrustation that covers the structure and may be helping to hold the baycrete together.

It’s a kind of estuarine recursion: oyster shell, once charred into cement to build a city, is folded back into cement to rebuild marine habitat, where now a new cohort of bivalves secretes their own biological cement, adhering the whole system together again.