What is Roman Cement™?

Roman Cement™ is a hydraulic cement material composed of Portland cement and one or more SCMs and more particularly includes novel, particle size optimized binary and ternary  blends of Portland cement and pozzolan (e.g., fly ash, natural pozzolan, or slag) or other supplementary cementitious materials (SCMs) engineered to perform the same as or better than 100% ordinary Portland cement (OPC) in concrete.

Particle size optimized Roman Cement™ blends can be used in instead of 100% OPC with the following benefits:

  • Lower material and energy costs of production
  • Lower capital cost to produce Roman Cement™ compared to the same quantity of OPC
  • Green (reduced CO2 and other pollutants) (carbon credits and LEED qualified material)
  • Sustainable (reduced fossil fuel consumption and greater use of waste materials)
  • Less permeable and more resistant to chemical degradation (e.g., ASR, sulfate attack and carbonation)

Binary and ternary Roman Cement™ blends can perform similar to, or even better than, pure OPC at a given water to cement ratio. Some binary and ternary Roman Cement™ blends are "Plug and Play™" because they can be used as general purpose cement and substituted 1 for 1 for OPC in concrete with little or no redesign of standard mix designs. Plug and Play™ and other binary and ternary blends of Roman Cement™ are both general purpose and specialty cements that permits both low cost, general purpose concrete and also structural concrete to include less Portland cement with higher substitution levels of pozzolan and other supplementary cementitious materials (SCMs) without having to engineer the concrete for high SCM substitution.

Roman Cement™ also refers to concrete made using binary or ternary blends of Roman Cement™, such as Plug and Play™ blends of Portland cement and SCM. Roman Cement™ concrete, also called Roman Concrete™, can include particle size optimized binary or ternary Roman Cement™ blends, together with aggregates, water, and chemical admixtures.

Is Roman Cement™ what the Romans used?

No. The Romans used a pure pozzolanic cement made from volcanic ash (pozzolana) and quicklime. Portland cement was not used by the Romans because it was not invented until modern times. Roman concrete has withstood the test of time, with the Pantheon and aqueducts still standing after 2000 years. However, pure pozzolanic cement develops strength too slowly to be suitable in modern building practices and is mainly used to repair ruins.

Roman Cement™, as described herein, is not pure pozzolanic cement but a hybrid cementitious material that includes binary and ternary blends of Portland cement and one or more SCMs. Roman Cement™ is not only a range of  cementitious compositions but also a methodology for particle size optimizing Portland cement and SCMs for use with each other. This permits high SCM substitution levels while still providing high early strength, as required by modern building practices and codes.

The Portland cement fraction is designed to hydrate rapidly and provide high early strength. The SCM fraction reacts with and chemically stabilizes excess lime released during hydration of Portland cement to yield concrete that is more chemically balanced, durable, and resistant to chemical attack than concrete made using pure OPC. In that way, Roman Cement™ concrete made using particle size optimized binary or ternary Roman Cement™ blends more closely resembles ancient Roman concrete than concrete made using pure OPC. For this reason we sometimes refer to Roman Cement™ concrete as Roman Concrete™. Roman Concrete™ is less expensive to manufacture than concrete of similar strength made using 100% OPC. Roman Concrete™ is also "greener" than ordinary concrete because it has smaller carbon and energy footprints and can include industrial waste products such as fly ash and slag, as well as finely ground post-consumer glass, which contains mostly pozzolanic glassy silica.

How does Roman Cement™ differ from current industry practices?

OPC has been well optimized for use by itself, and has been since its discovery over 150 years ago, but is not well optimized  for SCM replacement. Consequently, concrete can only include modest quantities of fly ash before losing much of its early strength. Concrete that includes high SCM substitution levels must typically be engineered using large quantities of chemical admixtures to meet early and later strength requirements.

Fly ash, slag and other SCMs are slow reacting compared to Portland cement. When fly ash is used in unmodified form, the industry practice is to either use smaller amounts of fly ash (e.g., less than 20%) to preserve strength in normal concrete or higher amounts (e.g., 20-50%) in thick mass pours (e.g., bridges and dams) to control heat of hydration and prevent excessive internal heat build-up. To preserve strength at high fly ash replacement levels, water reducers must be added to greatly reduce the water to cementitious material ratio (w/cm) (e.g., from 0.44 to 0.20).

For general use concrete, the industry has attempted to "fix" fly ash by grinding it more finely to, and thereby increase reactivity, often by inter-grinding with Portland cement. However, blended cements constitute only about 1-2% of the U.S. market, typically cost more than OPC, and therefore provide little environmental or economic benefit.

Roman Cement™ balances out the strength-reducing effect of SCMs and increases replacement levels by optimizing the particle size distribution (PSD) of the Portland cement fraction to optimize its reactivity and ensure that none remains unhydrated over time. Roman Cement™ unleashes the vast strength producing reserves contained in Portland cement by providing an optimized narrow distribution of Portland cement that completely hydrates within about 28 days. By itself, a steep distribution of Portland cement particles as used in  Roman Cement™ would ordinarily increase water demand and reduce workability to a high degree, which is why this material has never been used as general purpose cement. 

Roman Cement™ remedies this problem by combining the narrow PSD Portland cement fraction with one or more particle size optimized SCM fractions which complement the PSD of the Portland cement fraction. This broadens out the particle size distribution of the overall blend, which increases particle packing density, decreases water demand, and increases workability of the Roman Cement™ blends.

For example, in a binary Roman Cement™ blend a  "fine" narrow PSD Portland cement fraction is combined with a "coarse" SCM fraction. The "coarse" SCM replaces the "coarse" Portland cement particles missing from the "fine" narrow PSD Portland cement. 

In a ternary Roman Cement™ blend, a narrow PSD Portland cement fraction of medium fineness is prepared without "ultra-fine" and "coarse" cement particles and then combined with "ultra-fine" and "coarse" SCM fractions that replace the "ultra-fine" and "coarse" cement particles missing from the narrow PSD Portland cement fraction. The result is a blended cement product with high SCM substitution that develops strength similar to 100% OPC at all ages.

The runner analogy

In track and field, there are sprinters and long distance runners. They are born that way. It's in their genes.

Sprinters have thick, fast-twitch muscles that provide tremendous off the block speed but quickly tire out. Long distance runners have lean, slow-twitch muscles with extended endurance but are not as fast as sprinters and never can be.

To maximize team performance, each runner ideally trains to maximize his/her own natural abilities. It would not make sense to try and make sprinters into long distance runners and long distance runners into sprinters. Yet that is analogous to how the cement industry uses OPC and fly ash.

Cement manufacturers almost universally produce OPC having a relatively broad particle size distribution ("PSD") (e.g., between about 1-60 m) in an attempt to strike a balance between the competing effects and demands of reactivity, rate of strength development, water demand, inter particle spacing, paste density, porosity and shrinkage. The inclusion of coarse cement particles that never fully hydrate is by design to prevent problems that occur when cement is ground too finely. But the inclusion of coarse cement particles is only "necessary" because cement manufacturers optimize OPC for use with itself, without regard to how OPC behaves when blended with SCMs.

Because OPC contains substantial quantities of coarse particles that can never fully hydrate, OPC continues to have a large, latent store of untapped reactivity and strength potential that, through 150 years of optimization, have been deliberately sacrificed. In this way, OPC is like a sprinter that is held back and forced to run long distance, thereby falling far short of its true potential.

Fly ash and other SCMs are slow reacting and analogous to long distance runners because they continue to react and develop strength over time. By grinding fly ash more finely, the industry attempts to make fly ash and other SCMs into being better sprinters, with limited success. The industry therefore attempts to use "sprinters" and "long distance runners" in much the same way rather than maximize the performance of each, with poor and often unpredictable results.

In contrast to the industry practice, Roman Cement™ puts Portland cement, fly ash and other SCMs to their highest respective uses. Portland cement is allowed to do what it does best, which is to react more quickly and burn itself out earlier to maximize its contribution to 1-28 day strength. Fly ash and other SCMs are allowed to do what they do best, which is to provide less reactive particles that increase packing density of the blend, react slowly and reduce agglomeration of the Portland cement particles so as to lower the water demand and improve flow in fresh concrete, and then continue to react continuously and steadily to provide additional 7-28 day strength (and beyond). The result is a "team" that uses the best of what it has.

Has the Roman Cement™ technology been proven?

Roman Cement™ has been subjected to rigorous testing by the National Institute of Standards and Technology (NIST), which has confirmed the science and ability of binary blends of Roman Cement™ to equal or exceed the strength of 100% OPC at various fly ash replacement levels and constant w/cm of 0.35 (volumetrically normalized to account for the lower density of fly ash).

For example, 65:35 and 80:20 blends of Portland cement and fly ash were designed to have virtually the same strength at 1 to 28 days as 100% OPC at the same w/cm, as shown in Chart 1 below. We call these blends "plug and play™" because they can essentially replace an equal volume of OPC, which greatly simplifies their use in existing concrete with little or no redesign.

Chart 1

High strength 80:20 blends were also designed that exceeded the strength of 100% OPC at the same w/cm, as shown in Chart 2 below. Such blends would permit a concrete manufacturer to add additional amounts of fly ash, other pozzolans, or even inert fillers and still maintain or exceed the strength of 100% OPC. Roman Cement™ therefore provides even greater flexibility for self blending by end users than OPC because it can be engineered (under ASTM C-595 and/or C-1157) to meet a wider variety of strength specifications and permit greater variability in the amount of fly ash or other SCM that can be used, while maintaining strength, compared to conventional methods.

Chart 2

In addition to the NIST testing of binary Roman Cement™ blends, researchers at South China University of Technology tested several ternary blends of Portland cement and SCM that are very similar to ternary blends of Portland cement and SCM described in issued patents owned by Roman Cement, LLC. The research and test results are described in the following publications: Zhang et al., “A new gap-graded particle size distribution and resulting consequences on properties of blended cement,” Cement & Concrete Composites, 33 (2011) 543–550; Zhang et al., “Efficient utilization of cementitious materials to produce sustainable blended cement,” Cement & Concrete Composites, 34 (2012) 692–699.

The publications cited above show that several ternary blends which included 75% SCM and 25% Portland cement performed similar to a control cement containing 100% OPC and better than a control blend containing an inter-ground mixture of 75% SCM and 25% Portland cement. The SCMs used in each of the ternary blends that were tested included one or more of fly ash, limestone, ground granulated blast furnace slag, and steel slag. The publications further demonstrate that using a narrow PSD cement with complementary-sized SCM fractions to broaden the overall PSD of the ternary blend is based on sound and proven science.

Highly particle packed Roman Cement™ blends improve upon previous versions of Roman Cement™ by selecting particle size distributions for the cement and SCM fractions that yield a high degree of particle packing. This reduces the amount of water required to fill the interstitial voids between the particles and frees up more water to lubricate the particles and provide desired flow.

For example, in a typical cement binder having a particle packing density of about 50%, the volume of void spaces between the cement particles is about 50%. That means that, in 1 litre of cement binder, there is essentially 1/2 litre of solid particles and 1/2 litre of void spaces between the solid particles. One half litre of water is therefore required to fill the void spaces. An additional quantity of water, or water of convenience, must then be added on top of the 1/2 litre to lubricate the cement particles and create the desired flow.

By way of comparison, in a highly particle packed Roman Cement™ blend having a particle packing density of 75%, the volume of void spaces between the particles is only 25%. That means that, in 1 litre of this cementitious binder, there is essentially 3/4 litre of solid particles and only 1/4 litre of void spaces between the solid particles. One quarter litre of water is therefore required to fill the void spaces. An additional quantity of water, or water of convenience, is then added in an amount sufficient to lubricate the cement particles and create the desired flow. The total amount of water required to yield the desired flow is therefore greatly reduced in a highly particle packed cement blend as compared to an ordinary cement binder. This, in turn, reduces the water to cement ratio for a cement paste of a given flow, which increases strength.

Advantages of Roman Cement™ over conventional practices

Concrete companies in the U.S. typically self-blend and do not purchase blended cement, which constitutes only about 1-2% of the cement market in the U.S. and can cost even more than OPC (because blended cement is marketed as a "premium product"). Inter-grinding cement and fly ash is typically how cement companies attempt to maximize the strength of blended cements. However, inter-ground cement-fly ash blends at 30% substitution (i.e., 70:30 blend) cannot match the 28 day strength of 100% OPC according to data published by cement companies themselves.

Self-blending requires each concrete company to re-engineer its own mix designs for SCM usage. Concrete having high volume fly ash (e.g., 30%+) can be engineered by specialty concrete companies for use in thick slab pours to reduce heat of hydration and/or by using expensive chemical admixtures that greatly reduce the w/cm. Unfortunately, there are approximately 8500 concrete plants in the United States, each with dozens of unique mix designs that cannot be transferred from one plant to another due to variability in raw materials (e.g., rock, sand and cement). In non-engineered concrete, fly ash can dramatically reduce early strength and increase set time. This may explain why an estimated 50% of all concrete in the U.S. contains no fly ash.

Is Roman Cement™ patented?

The U.S. Patent and Trademark Office (PTO) granted the following patents covering binary blends of Roman Cement™, also referred to as "First Generation" Roman Cement™: US Pat No 7,799,128, which was fast-tracked under the Green Technology Pilot Program initiated Dec 2009, US Pat No 7,972,432, and US Pat No 8,323,399. Patents in countries outside the U.S. have also been issued covering binary blends of First Generation Roman Cement™.

We have other US and foreign patents pending relating to various aspects and improvements of Roman Cement™, including the following patents covering ternary blends of Roman Cement™, also referred to as "Second Generation" Roman Cement™: US Pat No 8,377,201, US Pat No 8,414,700, and US Pat No 8,551,245. Second Generation Roman Cement™ improves upon First Generation Roman Cement™ in several key areas discussed above, including reduced water demand, grinding and shipping costs, and permeability.

Highly particle packed Roman Cement™ blends, also referred to as “Third Generation” Roman Cement™, are described in International Patent Publication Number WO 2013/059339.

Roman Cement™ reduces marginal cost and capital expenditures

Substituting Portland cement with fly ash, natural pozzolan, limestone, or other SCM reduces cost. OPC typically costs about $80-120/ton retail, while fly ash typically costs about $10-60/ton retail. The cost of natural pozzolan can be lower than fly ash. The cost of limestone is very low (e.g., less than $5/ton). Even assuming an additional cost of $5/ton for the Portland cement fraction of Roman Cement™, substituting 20-40% of Portland cement with an equivalent amount of fly ash or other lower cost SCM yields a product that marginally costs about 10-30% less to produce than OPC.

Roman Cement™ can be used instead of OPC in single silo operations that currently do not use fly ash. In this way, small "mom and pop" operations can immediately begin using cement-SCM blends without having to invest in an additional silo (as is required when companies self-blend).

Finally, Roman Cement™ can be implemented by cement plants that wish to increase production capacity without investing hundreds of millions of dollars to build new kilns and downstream clinker processing equipment to increase clinker production. For example, substituting 33% of Portland cement with one or more SCMs would effectively increase production capacity by 50% (e.g., 1 million tons of cement clinker would produce essentially 1.5 million tons of Roman Cement™ at a substitution level of 67:33).

Roman Cement™ is good for the environment and yields better concrete

Roman Cement™ can make use of vast quantities of waste fly ash and slag that continue to accumulate in alarming quantities. Less than 20% of fly ash is actually used in concrete. Most is discarded into holding ponds and landfills. Roman Cement™ can beneficially use the vast untapped oversupply of fly ash and slag. According to a recently published paper, China produces about 85 million tons of steel slag annually, and the total amount of deposited steel slag is about 400 million tons.

Fly ash, when stored in impoundment ponds, can lead to this.

When used properly in concrete, fly ash reduces green house gases, energy consumption, and yields better concrete.