Concrete is something that we take for granted. But, without cement and concrete modern life would not be possible.
Even in “modern” 6500 BCE, people living in what we now call Syria knew it was an essential material. The Romans used concrete widely in 600 BCE and by 200 BCE, they used it in almost all their construction. Many of those structures still stand proudly today.
Made from a mixture of volcanic ash, lime, seawater, and stone aggregate, the Romans cured concrete in wooden molds - much like our modern-day concrete block.
Then, the fall of the Roman empire and the onset of the Dark Ages brought an end to pozzolanic concrete until it was rediscovered in the 15th century.
In 1824, Joseph Aspdin patented a process for producing cement. He named it Portland cement on account of its resemblance to the Portlandite stone found on the English Isle of Portland.
The UK likes this kind of stuff. Another important ceramic product formed by firing clay is called Mullite. You guessed it, discovered on the Scottish Isle of Mull.
But I digress…
Portland cement is the material that holds aggregate together in our modern concrete.
Standards
Cement is one of the most standardized man-made materials on the planet.
It has been said that no matter where you buy cement, it’s going to be pretty much the same (provided it meets specification for its properties).
And this is a good thing. Our lives quite literally depend on the quality of cement used in construction.
Our lives also depend on the quality of the aggregate used in concrete.
Aggregate accounts for 70-85% of the mass of concrete - far more than the mass of cement. A failure of the aggregate could bring a bridge deck down on your head.
There are many different concrete standards. In the US, the prevailing standard is written by ASTM International (formerly known as the American Society for Testing and Materials), and many other standards in use around the world are modelled after the ASTM specifications.
The two primary ASTM standards for aggregates used in concrete are:
- ASTM C33 – Standard Specification for Concrete Aggregates
- ASTM C330/C330M – Standard Specification for Lightweight Aggregates for Structural Concrete
Other ASTM guidelines exist for special purpose aggregates, e.g., those used in concrete masonry units.
ASTM C33 - Concrete Aggregates
ASTM C33 establishes standards for both coarse aggregate – think “rock” – and fine aggregate – think “sand”.
Aggregates are classified by particle size as:
- Coarse - 14 different gradations, ranging from 100 millimeters all the way down to 30 microns, with tight particle size distribution required within those gradations
- Fine – a single classification with size requirement between 4.75 mm and 150 microns, with very stringent requirements on the particle size distribution within those boundaries
The standard lists twenty (yes, 20!) different tests and test methods that must be followed to establish physical properties and behavior, both before and after the aggregate is added to concrete.
The testing methods themselves are defined in other ASTM procedures, ranging from a simple bulk density measurement to something as complicated as the “Test method for Resistance of Concrete to Rapid Freeze-Thaw”.
The standard also differentiates aggregate for different intended use cases, including 12 different concrete classifications - ranging from bridge decks to foundation footings – and 3 different weathering classifications, depending on geography.
Since impurities can make a big difference to the performance of an aggregate, ASTM C33 even specifies what else can be in the mix, setting limits on organic matter, clay lumps, coal and lignite, and excess fine material.
This isn’t an exhaustive list, but hopefully it demonstrates the extent to which concrete aggregate standards run.
When we say we “grudgingly love” ASTM C33, it’s because the testing regimen it requires to qualify a concrete aggregate is complex and costly. The love comes from knowing that the bridge deck we mentioned earlier is likely to stay safely in place above our heads.
ASTM C330/C330M – Lightweight Aggregates For Structural Concrete
The requirements for lightweight concrete aggregate are no less stringent but, unlike ASTM C33, recognize that these aggregates may be naturally occurring materials (such as pumice) or manmade aggregates (such as perlite, ash, slag, or ceramic lightweight aggregates like expanded clay and shale).
All manmade lightweight aggregates are subjected to heat at some point in their journey to becoming concrete.
Ash and slag are byproducts of combustion and refining, while the production of perlite and expanded clay and shale is deliberately set up to yield a lightweight aggregate (LWA).
ASTM C330/C330M provides similar specifications to those found in C33 for things like size, gradation, and foreign contamination, but adds new, material-specific standards, including:
- Iron staining of concrete caused by slag dissolution
- Minimum bulk density (55 lbs/ft³)
- Maximum loss on ignition (LOI) from un-combusted materials
An important exception granted to expanded clay and shale aggregates is the elimination of an excess fine material requirement. Fines generated in these types of material are not deleterious, and can in fact be pozzolanic, bringing additional bonding power to the concrete.
The history of lightweight aggregates is shorter than those of concrete and cement, but they are no less important.
Virtually all concrete masonry units use a weight reduction agent - one of the reasons people still call them “cinder blocks”. Although masons are generally very strong people, even they are grateful for any weight reduction in concrete masonry units!
High-rise buildings, long span bridges, and geo-technical structures make widespread use of LWA to reduce weight in a structure.
LWA is also used in road surfacing. “Chip seal”, which is usually made from expanded clay or shale, virtually eliminates windshield breakage that is caused when denser aggregate is torn loose by tires. LWA also resists tire polishing much better than natural stone, giving a chip seal surface superior, non-skid properties.
In summary, lightweight aggregates are very important in our day-to-day life, allowing concrete mixes to achieve properties that would simply not be possible using dense aggregate.
Expanded Shale and Clay
Expanded Clay Expanded Shale
Expanded shale and clay destined to become lightweight aggregate is produced by mining clay or shale, subjecting it to a primary crushing, sintering in a rotary kiln, and then finishing it by crushing and screening to a specific gradation.
Starting with a dense clay, porosity is created by a rapid firing, where the surface seals before gases emitted by organics and low melting point substances within the clay can escape – a process known as bloating. The trapped gas bubbles form voids (porosity) that lowers the finished material’s specific gravity and bulk density.
The most important factors for its serviceability are strength, bulk density, and final particle size distribution. In general, large particles have a lower bulk density than fine particles. Blending of finished LWA can be helpful to achieving specific properties.
The real trick in making high performance LWA is controlling void size and distribution, so that low density is achieved without compromising strength. Almost any clay will bloat if you fire it at a high enough temperature, but not all will be strong.
We tip our hats to LWA manufacturers for performing this balancing act during a process that appears to be simple but is not.
Recycled and Repurposed Materials
Despite the stringent ASTM requirements governing concrete aggregates, recycled and repurposed materials are used to make perfectly good concrete.
A lot of concrete waste is generated during demolition, which turns out to be ideal material for crushing, washing, and use as coarse aggregate. More than 75% of “waste” concrete is repurposed in this manner.
Crushed concrete has proven to be a better aggregate than some virgin crushed rocks, with studies showing that recycled material improves concrete strength.
This is particularly valuable for concrete produced wherever there is a shortage of high-quality crushed stone – such as Coastal Florida, a particularly hot market for crush concrete aggregate.
Fines from quarrying operations are used as fine aggregate (sand) after careful screening and washing. Avoiding excessive fines is an important processing step when using this source of fine aggregate.
Another potential source for LWA production is waste minerals – an area in which IntoCeramics is well versed.
Over the past few years, we have leveraged our ceramic formulation and development background to successfully assess the potential of waste materials and determine viable pathways for repurposing them into LWA.
The worldwide production levels of concrete (10+ billion MT), crushed stone (50+ billion MT), and marine quality sand (50+ billion MT), clearly demonstrate the huge market opportunity for repurposing mineral waste streams.
Contact the IntoCeramics team for an assessment of your waste or excess materials.
Photo Credit: Concrete Aggregate - New Mexico State University
