“Those who cannot remember the past are condemned to repeat it.” – George Santayana, The Life of Reason (1905)
In our case, oh how we wish that were true.
When Santayana wrote these words, he was thinking of the wars, famine, and destruction that have plagued mankind in a relentless, repeating cycle for thousands of years.
But we also forgot – quite literally – many of the advances in materials and associated benefits that humanity has achieved throughout history.
Burning libraries in Alexandria tends to promote forgetfulness. Just saying.
History is full of wonderful materials that were used in ancient times but then lost for centuries after civilizations such as the Roman Empire fell.
The ancient Romans built many massive structures that still stand.
Built in 128 CE, primarily of concrete, bricks, stone, and mortar, the Pantheon still stands proud in Rome and is one of the most visited tourist attractions in the Eternal City.
It is in better condition than many concrete buildings one-fortieth of its age.
This article in the Smithsonian magazine titled Why the Pantheon Hasn't Crumbled explains why it has survived this long, but I’ll sum it up like this: Roman architects, engineers, and people were really smart!
Understanding the Pantheon
The Pantheon is topped by a 142-foot concrete dome that doesn’t contain any reinforcing steel.
No building commission in the modern world would approve the plans for this structure (and nor would any architect submit them).
Yet, the Pantheon has stood strong for almost 2,000 years. How can this be?
The answer is both simple and complex – materials.
What we know today as building cement, used in concrete and mortar, is Portland cement, which wasn’t invented until the late 19th century – after which it rapidly became accepted for use in all types of construction.
Roman cement contained a combination of lime (calcined from limestone), sand, and volcanic ash.
An objective material analysis shows that Roman cement-based concrete was superior in many ways to today’s Portland cement-based concrete, having:
- Superior corrosion resistance, particularly to salt and saltwater
- Higher ultimate strength
- Development (in many cases) of interlocking crystalline needle-shaped structures that reinforce the structure and grow over time to strengthen the cement paste
Other silica-rich materials were also found to work well as the Roman equivalent of cement additives, and gypsum was used as an alternative source of lime (calcium).
We now refer to these materials as pozzolans.
The Fall and Rise of Pozzolans and Supplementary Cementing Materials (SCMs)
Back to history and the downside for forgetting the good stuff.
When the Roman empire fell, so did the magic of pozzolanic cement and the concrete made using it.
The secrets of producing these wonderful materials remained lost for centuries, only gradually being rediscovered during the 14th Century.
By 1670, a major canal in southern France was constructed using pozzolanic concrete.
In 1849, Le Monier demonstrated the efficacy of steel reinforced concrete, followed in 1894 by the invention of Portland cement. At that point it was “Katy, bar the door!” for concrete usage around the world.
Today, concrete is the most widely used building product in the world.
Can you imagine a world without Portland cement and concrete? Nor can I.
Yet the world of cement has many problems, despite Portland cements being over ten-times stronger today than when they were first introduced.
Like many construction materials, concrete use is increasing while coming under pressure due to environmental concerns.
Cement production is both energy intensive and a large source of carbon emissions.
And construction that uses concrete is:
- Subject to corrosion and weathering
- Overengineered to compensate for potential failures (excessive safety factor)
- Sensitive to the aggregates used
- Sensitive to the amount of water used in placement
- Sensitive to curing conditions
SCMs (or pozzolans) were introduced to address these issues. They include:
- Fly ash from coal furnaces
- Blast furnace slag
- Silica fume (micro-silica)
- Metakaolin (lightly calcined kaolin clay)
- Volcanic ash (just like the Romans), including pumice
- Metallurgical slag
SCMs have enabled concrete production with lower Portland cement content.
Fittingly, analysis has shown the critical calcium/silica ratio of pozzolanic cement to be almost identical to that used by the Romans!
The first major use of SCM concrete was in a tunnel spillway at the Hoover Dam in the early 1930s.
Between 1948 and 1952, SCM concrete was used extensively in the massive Hungry Horse dam in Glacier National Park.
Today, concrete using SCMs is applied widely across the building world for structures, road construction, and large civil projects.
Are Pozzolans the Answer?
One might assume that, by reducing the amount of Portland cement used in large-scale construction, SCMs solve the associated energy and carbon footprint challenges.
Not so fast.
The supply of fly ash – the most frequently used pozzolan – is shrinking rapidly as coal generated power plants are shuttered. Consequently, the price of suitable fly ash is rising, and its long-term availability is in question.
Furnace slag, which is also widely used, has a carbon footprint that often exceeds that of the cement it displaces (steel production creates almost 1.5x the carbon emissions of Portland cement production).
Nevertheless, since both fly ash and furnace slag are waste products from other industries, it can be argued that their re-use still makes environmental sense - provided they are still available.
Manufactured pozzolans tend to be more expensive than mined pozzolans, such as pumice or volcanic ash, and are currently still more expensive than fly ash or slag.
To make matters worse, supplies of sand and coarse aggregates are running short in many parts of the world.
For example, the lack of suitable sand has stopped highway construction in Vietnam and led them, along with other Asian countries, to ban exports.
So What Next?
Research and development into SCMs and better concrete technology is now a hot sector, with growing recognition that reducing the use of traditional materials will be essential to our sustainable future.
Reduction in the volume and weight of concrete structures is considered essential.
Logical outcomes will be:
- Increased use of SCMs
- Increased use of lightweight aggregate (LWA)
- Improved engineering designs
IntoCeramics has extensive experience working with SCMs and LWA, including micro-silica and expanded aggregates.
We can determine whether your surplus or waste material can be repurposed into a valuable raw material for the new world of cement and concrete.
When in Rome…
