Portland cement is a material that architects and designers who deal in sustainable construction love to hate. But until recently, the market has seen few sustainable alternatives, particularly for structural applications. That’s changing. Among the products aiming to fill the gap is a new masonry unit from Napa, Calif.–based Watershed Materials, which is led by rammed-earth construction evangelist David Easton. The modular Watershed Block is made of fused soil and rock fragments including quartz, feldspar, clay, and other minerals. The hollow-celled unit contains half the cement of typical concrete blocks but weighs up to 30 percent more. EcoBuilding Pulse talked with Easton about the new block and how it could make masonry construction more environmentally friendly.
What is your goal for the Watershed Block?
We’re trying to eliminate our dependence on energy-intensive Portland cement and develop a series of formulas that allow us to recreate the original Roman cement, in which a suite of volcanic ashes were blended with lime and rock fragments to create this amazingly durable concrete. Portland cement is a fantastic product because it can glue sand and gravel together to make enormously strong finished products—7,000 to 10,000 psi. Roman cement can’t attain those strengths but there are very few applications for which you need something to be that strong.
For architects and designers working with a client, what’s the selling point?
You take one of the most common building components in the world and you make it better—more resource-conserving, energy-efficient, geographically appropriate, completely recyclable, and with a smaller carbon footprint. And it’s structure and finish, which is really efficient. Your masons lay it up and it’s done. There’s no Tyvek, there’s no painting, there’s no lathing on the outside. It’s very compelling if you have a short project timeline. We’ve worked hard to develop a series of formulations that make the block look like raw rock.
Can you explain the science behind those formulations?
Cement includes limestone that has been mined, crushed, heated, and powderized and its bonding can be improved with the addition of a blend of other materials. We’re trying to identify those naturally occurring materials and blend and size them for use with the activators that they’re mixed with, such as sodium hydroxide and sodium silicate. We then apply hydraulic pressure and high-frequency vibration. The process comes from rammed-earth technology, which showed that compression wasn’t enough. You need all of the particles to be bouncing against each other within the matrix to create density through grain-to-grain contact. 
How does that process affect manufacturing costs?
That’s one of the elements that we are trying to resolve. We’re trying to develop a technology that mirrors concrete-block production but with an exceptionally lower carbon footprint. Our first-generation Watershed press is a slow-cycling machine and so the cost of making each block at this stage is high. It’s labor-intensive. We’re pricing it comparably to all the other expensive concrete blocks, including dyed, split-face, and honed-face blocks. We’re beginning to invest in improving the technology so the machine will have a shorter cycle time. The cost of our raw materials is considerably less than the cost of the raw materials for a typical concrete block, so all we have to do is reach production efficiency, and then we think we will have a very compelling price point.
How long does it take to make one block right now?
Twenty seconds, or three per minute. I’d like to get 20 blocks per minute. Then we’d have a machine that’s as fast as the big concrete-block machines in use today.
From where do you source your materials?
Our plant is adjacent to a quarry that provides 80 percent of our raw aggregates. We intentionally located next to this quarry because we knew that it had a byproduct that fit our business philosophy and that it couldn’t sell as a premium product. A lot of our formulas have 70 percent to 80 percent of this particular quarry’s material. For the remaining 10 percent or 15 percent, which affects the color of the block, we’ll go as far as 30 miles from the plant.
How far will you ship the blocks to fill an order?

It’s available everywhere but I don’t think it’s responsible to take something heavy like this and ship it [far away]. You could make an argument that while trying to develop an appreciation and a wide distribution for a product, you would be willing to put it on train cars and take it a long distance. But in the long run, it’s not consistent with our philosophy of resource conservation and carbon-dioxide reduction.

What are some of the challenges to getting the supply chain to understand the technology?

The initial blocks use ordinary Portland cement to provide a level of assurance that we think the market needs now. Unlike concrete blocks, which are essentially the same everywhere you find them, the Watershed Block requires deeper science. Without Portland cement as the universal glue, the manufacturer must be more careful in its formulations. The sweet spot is a blend in which the natural minerals do most of the work and the carbon footprint is as small as possible.
What’s the status of your work with the National Science Foundation (NSF) to develop a block containing no Portland cement?
Watershed was awarded a Phase I grant from the NSF in 2012 to research the feasibility of producing a building block that met the performance standards of ASTM C-90 without the use of Portland cement. That research confirmed our hypothesis [of creating blocks that achieve the strength requirements without the use of Portland cement] and so we have applied for a Phase II grant which, if awarded, will support the initial commercialization phase of taking the zero-cement block to market. The formulation is proprietary, but we anticipate having that block on the market by mid-2015.
Where does a product like the Watershed Block fall within the realm of rammed earth?
We see the block as a logical evolution of monolithic rammed earth. Building with blocks is less expensive than cast-in-place wall systems because of the absence of formwork and the decreased mobilization expenses. On difficult jobsites or where the structural designs dictate greater wall strength, blocks can be much more efficient. Additionally, rammed earth is a specialty trade. There aren’t very many qualified installers and that’s going to scare off a contractor or a banker. It look 15 to 20 years for [the market to accept] rammed earth and while I don’t think it’s going to take that long for the Watershed Block, it does have to penetrate the consciousness of the architecture and design community.
Where are you launching the first blocks?
We’ve decided on the [San Francisco] Bay Area. I know the architectural community there. People have trust in what we’re doing and that’s what it’s going to take to get started.

Note: This Q&A has been edited and condensed for clarity.