Many concrete customers in industrial applications are turning to a new family of nonportland cements to increase the life of their physical plant and cut repair costs and downtime. Based instead on pozzolanics, these cements create concrete with a durability that outperforms conventional concrete in laboratory tests and in the field. When repair is necessary, their rapid set times bring facilities back to service faster.
Severe environments
Concrete is usually thought of as a durable building material. But in some severe environments, portland cement-based concrete is not tough enough. Acids present in petrochemical production, food processing, and wastewater steadily eat away at portland cement concretes in floors and containment structures until they become unusable. Extreme heat of metal processing eventually decomposes the cement paste.
Users of these high-demand applications are searching for new mixes, admixtures, and coatings that make concrete last longer in corrosive and high-temperature conditions. And they have a strong economic incentive. Concrete deterioration can make processes inefficient in the short run, and eventually force concrete repairs or replacement. The cost of repair can be substantial, but the indirect cost of stopping production to make repairs is worse. The facility loses output and revenue, schedules may be missed, and the effect ripples through the entire operation.
A variety of methods have been used to improve the durability of conventional concrete made with portland cement. Unfortunately, these alternatives have their limits. Adding inert, fine-particulate minerals, such as fly ash or silica fume, is a common practice intended to increase durability for portland cement concrete that comes into contact with corrosive liquids and gases. These minerals fill the capillaries within the concrete crystal structure, which reduces the penetration of fluids and slows their attack, but does not stop it.
Roger Simons is vice president and owner of Widgeon Construction in Orange, Texas, a concrete contractor that does a lot of work in industrial facilities. In highly acidic environments, he says, “In the past, they would use 5000 psi portland cement concrete with added fly ash to try to make the structures more durable, but it was really futile. Within a year, you would have already started seeing potting and the face of the concrete would have been eaten up.”
Another alternative is to put a chemically resistant coating, usually an epoxy, over the exposed concrete surfaces. These modern coatings stand up well to corrosives, but the cost is so high that use is restricted to a few areas considered most vulnerable. Also, it can be difficult to achieve and maintain an effective seal.
In all of these cases of chemical deterioration, the weak link in the mix is the portland cement. Durable aggregates typically are available if needed. However, the paste produced by portland cement is composed principally of calcium hydroxide and calcium silicate hydrates. These break down in the presence of corrosive and caustic chemicals—even when using a Type II or Type V portland cement.
“You can try to protect concrete with fines and coatings,” says Mike Byrne, a longtime consultant to major petrochemical companies, “but eventually the chemicals reach the portland cement and, when they do, it still breaks down.”
Measures to resist heat run into similar roadblocks. The calcium silicate hydrates of portland cement decompose when exposed to temperatures above 250° F (120° C). Some minerals improve concrete’s heat resistance by increasing the density of the paste, but as with chemical corrosion, the calcium silicate hydrate is still vulnerable. As the cement paste loses strength, eventually both it and the aggregates it is intended to bind with begin falling off in pieces.
Pozzolanic cement
The new industrial cements are the product of materials company, Ceratech Inc. They consist of entirely different compounds, more like the pozzolanics used in the ancient Roman Coliseum and aqueducts that are still standing today. Ceratech cements are 95% fly ash, combined with chemical activators that give them a robust cementing action. This chemistry replaces all of concrete’s portland cement with recycled material, something that greatly decreases the concrete’s carbon footprint.
Most importantly for industrial applications, these cements create a paste entirely out of fine, inert materials. The concrete is denser, without the interconnected capillaries of conventional concrete, making it resistant to both chemicals and heat.
This cement in a typical mix can achieve a 28-day compressive strength of about 8000 psi. All the cements in this company’s line offer greater chemical and heat resistance than portland cement, but some are formulated specifically to increase one property or another. Concrete made with the most chemically resistant product (known as Kemrok) maintained its material mass and compressive strength sharply longer than portland cement concrete in a series of exposure tests with sulfuric, nitric, acetic, and hydrochloric acids. Concrete with the most heat-resistant cement (Firerok) withstands sustained temperatures of 570° F (300° C) and intermittent temperatures of 1850° F (1000° C).
The relatively rapid set time of this cement is a plus in repair situations. According to Ceratech’s president Jon Hyman, “Depending on the product used, 24-hour compressive strengths of concrete mixes range from 2100 to 4800 psi. One of the great things about this cement chemistry is that it produces both high early and high ultimate strengths without extra measures. If we can get a million-dollar-per-week wastewater plant or industrial production facility online a day or two faster, the payoff is huge.”
The company offers their cements in bags, in supersacks, and in bulk. Asked about placing concrete with these cements, contractor Simons says, “The pours with Kemrok went very well. It is easy to work with, no problem.”
Applications
Chemical processing company Gulf Sulphur Services Ltd., chose Kemrok for replacement of concrete trenches that hold molten sulfur for extended periods in their terminal in Galveston, Texas. Manager Tony Worthen says, “You need a durable concrete that withstands hydrogen sulfide exposure in the vapor zone and thermal shocks in the trenches. A lot of products will do these things individually, but we never found anything that did both as well as Kemrok.”
During the sulfur trench replacement work, the company also had used their traditional Type V portland cement with microsilica mix for some trench sections. “After nine months, we compared the Kemrok section to the one next to it repaired with Type V portland cement,” says Worthen. “The surface already was falling off the Type V product, but the Kemrok concrete looked like the day we poured it.”
Precasters have begun to develop wastewater containment structures made with the new cements. The traditional approach in this application is to apply coatings over the concrete surfaces that will be most exposed to high-sulfate water. But with pozzolanic cements they can create a monolithic product that is resistant to sulfates everywhere, eliminating concerns over the cost and quality of the coating work.
The sustainability of the new cements is a side benefit. They use 95% recycled material (fly ash) and 5% renewables (curing activators). “Besides keeping coal ash out of landfills, our products are entirely carbon neutral,” says Hyman. “We don’t burn fossil fuels and release carbon dioxide the way portland cement does.” Greener concrete is a plus in today’s sustainable-construction market. “We like being able to work with a ‘green’ product that offers superior performance,” says Worthen.
But users are quick to point out the primary reason for choosing these new cements is durability. “We wanted to be environmentally responsible and still meet our service and reliability requirements,” Worthen adds, “and in fact it looks like we will beat them.” Gulf Sulphur plans to replace the rest of its trenches with concrete made with Kemrok.
Dr. Pieter VanderWerf ([email protected]) has researched and advised companies on new construction products for over 20 years. He is president of consulting firm Building Works Inc. and a professor at Boston College.
