Watershed Materials A saw-cut half cylinder test sample of northern California fabricator Watershed Materials’ prototype geopolymer masonry unit, which uses natural clay in place of cement.

Getting the energy-intensive Portland cement out of masonry units has long been the goal of sustainable building-product manufacturers and researchers. So far, the most promising alternatives—among them fly ash and blast-furnace slag—are byproducts of other complex industrial processes. But that could soon change. New research from the National Science Foundation (NSF) with Napa, Calif.–based fabricator Watershed Materials shows promise for cement-alternative binders made of minerals derived from natural clay. In recent testing, their novel concrete material reported compressive strength of 7,000 psi—double that of typical concrete—while resisting water and chemical erosion.

The alternative binder relies on geopolymerization, a chemical reaction in which an industrial byproduct or natural material (in this case, the minerals found in clay) is combined with an alkali (such as lime or lye), and allowed to harden at a lower temperature than typical concrete masonry units and on par with that of other geopolymer cures, rendering the final product durable and water-resistant. 

“We’re … trying to coax a cement-type reaction out of natural clay,” says David Easton, president and co-founder of Watershed Materials and a longtime rammed-earth construction specialist. Watershed Materials is working with NSF grants—$150,000 awarded in 2013 and $750,000 in 2014—to find a scalable way to make blocks using the process. He couldn't say how much the blocks might cost, other than that greater adoption of cement-alternative binders (including fly ash and slag) would help to reduce production costs across the supply chain.

Watershed Materials Measuring the liquid limit of clay minerals used in one of Watershed Materials’ geopolymer mixes. Understanding each mineral component's plasticity is key to determining the extent of change it will undergo during the fabrication process.

"Masonry has been around forever and is a fantastic way to build because it's durable, lasts for a long time, and is very safe if it's built properly," Easton says. But instead of relying on the current energy-intensive cement, he adds, the creation of greener binders can help the industry "develop a new respect for an ancient building material and process."

Encouraging architects and builders to take up novel construction materials is a challenge, and one that the Watershed team is already broaching with its eponymous masonry unit launched last year. The hollow-cell Watershed Block is made of fused rock and soil fragments and uses half the cement of typical concrete. The ingredients list, however, is coming in second to the block's aesthetic among buyers in northern California, where Watershed has its manufacturing facility and does most of its business. "Green products are very attractive to the marketplace but they can't really absorb the significantly higher price tag," Easton says. "So we're selling the Watershed Block today in northern California based on its beauty—a physically attractive masonry block."

The company wants to have a cement-free masonry unit on the market within 18 months. As with the existing product, the team faces few restrictions on sourcing the raw materials for its forthcoming geopolymer unit. "It's everywhere," he says. "Clay, gravel, it's one of the infinite materials on the face of the planet." That mixture of broken rock and sand often contains the reactive substance required for the geopolymer formula.

Watershed Materials Curing temperatures for the geopolymer units are relatively low. Waste heat from hydraulic motors used by the production machinery can be reclaimed to raise ambient temperatures for curing, supplemented by solar heating where necessary.

Easton and the team at Watershed Materials aren't the only ones rethinking concrete on a molecular level. Researchers at Purdue University recently fortified concrete with cellulose nanocrystals from wood fiber, requiring project teams to use less material to achieve their desired effect. In Norway, manufacturer ReforceTech is adding polymer-coated basalt fibers to concrete to eliminate the need for rebar in structural applications and cladding. And companies like CTS Cement Manufacturing Corp. are making calcium sulfoaluminate cement, whose clinker mix of limestone, bauxite, and gypsum requires less limestone than usual, lowering the material's embodied energy.