By Jay Fox

In a recent Component Spotlight hosted by Passive House Accelerator on April 22, PROSOCO Concrete Flooring Specialist and industry veteran Bruce Ferrell walked attendees through one of the most consequential and least understood shifts in the construction industry: the transition from traditional Portland cement to low-carbon alternatives classified under ASTM C-595. Drawing on decades of hands-on experience rolling out solutions for these new cement types (including nearly 17 years with the company), Ferrell delivered a session that was equal parts chemistry lesson and field report, covering the properties of blended hydraulic cements, what makes them different from their predecessors, the issues they're causing on job sites, and how the industry is adapting.

Why the Industry Is Moving Away from Portland Cement

The motivation is straightforward. Traditional Portland cement, classified under ASTM C-150, is created by heating limestone and other materials up to 1,500°C (2,700°F) in huge kilns to form clinker. Clinker is then pulverized and blended with gypsum and other ingredients. Depending on how the kiln is fueled, the process can be extremely energy intensive. Moreover, cooking limestone boils out embedded CO₂ and releases it into the atmosphere.

On account of how it’s produced and the fact that concrete is the number one building material in the world, concrete accounts for between 7% and 8% of all human-generated CO₂ emissions. Any realistic plan to reduce carbon emissions must address concrete, the largest elephant in the room.

Currently, about 70% of new construction uses some form of low-carbon cement. Some rely on supplementary cementitious materials (SCMs), pozzolans designed to reduce the carbon intensity of cement mixes, and replacing the cooked limestone used in Portland cement with raw limestone. Each of these new “recipes” for cement is covered by ASTM C-595.

However, Ferrell notes that traditional ASTM C-150 Portland cement is still available, but much of it is now imported from China, India, Turkey, and Vietnam, and is more prevalent in regions in close proximity to major ports. In fact, some teams on the East and West Coasts may find themselves using C-150 Portland cement without realizing it.

What's Actually in These New Cements?

ASTM C-595 covers four types of blended hydraulic cement, each with a distinct composition.

Beyond these base formulations, Ferrell outlined a growing inventory of materials being introduced into cement mixes:

• Fly ash (a byproduct of coal combustion)

• Bottom ash (a coarser, heavier byproduct of coal combustion with more heavy metals)

• Ground glass

• Electronic blast-furnace slag

• Mine tailings

• Calcined clays

• Biochar (a form of charcoal produced by heating organic material in a low oxygen environment)

The chemistry of the raw limestone itself also varies by region. The best deposits in the United States are in the Ohio River Valley, where one finds a variety of limestone (dolomitic limestone) composed primarily of calcium magnesium carbonate rather than calcium carbonate. Dolomitic limestone is harder and less absorbent, and cement made from this type of limestone performs differently from other sources on the Blaine test, which measures particle fineness.

What Teams Are Seeing in the Field

As Ferrell noted, the transition has not been seamless. Cement producers initially believed that the new types were equal replacements for ASTM C-150. This has not been the case. There are differences. Moreover, producers are not offering a universal alternative. Many project teams have no idea what they're getting unless they ask their producers directly.

The consequences are showing up across the country. Ferrell outlined the placement issues first: unpredictable set times that leave crews guessing when they can put trowels on slabs; color uniformity problems where just a few degrees of temperature change between pours results in visible differences; poor floor flatness or floor levelness; delamination; and delays in passing early strength tests. The final product may also be less aesthetically pleasing due to longer cure times and reduced workability.

Post-placement, the problems compound. Slabs may fail to meet owner specifications for levelness, color, or aesthetics. Latent issues can surface after the HVAC comes on, requiring remediation after the building is sealed in. The floors tend to be more porous, meaning accidental stains penetrate deeper and are harder to remove. As a specific example, Ferrell described a data center where crews ran power trowels on the slab for an extra 20 minutes to an hour to achieve the desired finish. The heat and friction brought hydrophobic polymers to the surface, causing water to exude from the concrete and to bead on the surface.

For high-performance building practitioners, there is an additional concern: the increased presence of limestone powder can interfere with the adhesion of control layers—a critical issue when airtightness and moisture management are non-negotiable.

How the Industry Is Adapting

The American Concrete Institute (ACI), the American Society of Concrete Contractors (ASCC), and ASTM International are working to move beyond anecdotal evidence and provide empirical data and actionable strategies to assist in the transition. These agencies are working with government, academic institutions, and private industry to fund studies into how net-zero solutions for cement production perform, since Ferrell cautioned that not every solution performs as advertised. "There's a trial and error for what works and what doesn't work," Ferrell said.

He also raised a concern that should resonate deeply with the Passive House community: "The goal (of eliminating emissions from cement production) is good, but durability is not high enough on the priority list." A building should last 80 to 100 years, and Ferrell expressed uncertainty about whether some recent concrete construction will meet that standard.

PROSOCO's Solutions for the ASTM C-595 Era

PROSOCO is helping lead the industry's response with a comprehensive lineup of products for surface preparation, densification, protective treatments, and maintenance. These products have been evaluated and refined specifically for ASTM C-595 applications.

For surface preparation

• PreKlean removes residual salts and laitance from newly installed concrete floors.

• SafEtch offers a safe alternative to traditional acidic compounds for softening and etching concrete surfaces.

• The Sure Klean line handles tilt-up and off-form concrete cleaning, while Cure & Seal Remover and Wax & Cure Remover strip films from vertical and horizontal surfaces.

• Oil & Grease Stain Remover is a poultice cleaner for embedded oil and grease.

• Consolideck Concrete Floor Restorer is a general-purpose restoration cleaner.

For protection

• Consolideck Concrete Protector SB is a solvent-based water, oil, and stain repellent.

• LSGuard provides a glossy sealer finish.

• Saltguard delivers water and salt barrier protection for outdoor applications.

For densification

PROSOCO also offers densifiers formulated specifically for ASTM C-595 cements.

• DensiKure cures and densifies new concrete in a single step.

• BD1 is a high-performance silicate blend designed for Type 1L and other ASTM C-595 surfaces.

• Blended Densifier is a concentrated formulation for steel-troweled and burnished floors.

The Bottom Line

Low-carbon cement is here to stay, and for good reason. Emissions reduction is essential. However, the transition from carbon-intensive Portland cement to its low-carbon alternatives will demand new knowledge, new coordination, and new products tuned to the realities of ASTM C-595 chemistry. Ferrell's session made one thing clear: understanding what's in your cement is no longer optional.

To learn more, watch the full Component Spotlight recording on YouTube, visit PROSOCO's website, or explore their concrete flooring product line to find the right solutions for your next project.