Posted on May 27, 2021
LC3 is a low-carbon alternative to ordinary Portland cement (OPC). It is also an increasingly practical option. In LC3, calcined clay (CC) and limestone reduce the reliance on energy-intensive clinker.
There are many advantages:
Kaolin is available around the world. Kaolin from local sources reduces transportation costs.
It is possible to produce calcined clay from lower grade clay.
Calcined clay does not need the high kiln temperatures associated with clinker production.
Calcined clay increases concrete’s strength by reducing porosity.
Credit a chemical reaction between the aluminates in the calcined clay and the calcium carbonate in the limestone for the reduced porosity. Also, the addition of some gypsum reduces agglomeration that would otherwise occur.
The calcined clay comes from heating Kaolinite to about 650-750 degrees Celsius. The search is on for alternative ways to fuel the calcination process. For example, F.L. Smith has come up with its HotDisc combustion device. As it rotates, technicians feed it with various alternate fuels. Producers use the device to replace up to 80 percent of the fossil fuels ordinarily required.
LC3: History and Definition
The term calcination comes from the Latin term calcinare which means “to burn lime.” Ancient Romans used burnt lime to bind various substances together. Crushed rock, volcanic ash and pulverized brick were common choices. They called the final product “opus caementicium.”
The idea of using calcined clay as a pozzolan dates to 2004. In that year, Professors Scrivener (EPFL) and Martirena (UCLV-Cuba) contemplated the possibilities. The following year, they launched their first research project. From 2009-2012, a second project focused on a ternary blend cement combining calcined clay and limestone. Their work revealed the tremendous potential of calcined clay as a supplementary cementitious material (SCM).
At high temperatures, limestone breaks down to lime and carbon dioxide. However, the process is energy-intensive. It requires substantial radiant heat transfer using coal or petroleum coke. This means the process generates large amounts of CO2. In fact, traditional calcination accounts for an estimated four percent of global carbon emissions.
Calcined clay reduces dependence on traditional calcination. It is an SCM offering many advantages:
Good supplies, often local
Control over color
Pilot lab guides full-scale plant development
Opportunity to use alternative fuels
Chloride resistance extends bridge life spans
Carbon emissions more important every year
Capital costs are less than opening a new clinker line
All SCMs make concrete more sustainable. Fly ash and slag are the two most popular options. However, supplies are sometimes insufficient and/or costs are not competitive. Ternary blends are often preferred. Natural pozzolans are of interest, but consistency may be an issue.
Widespread, systematic closing of coal-fired power plants reduces fly ash supplies. Some producers now obtain their fly ash from landfills. Locally sourced fly ash is sometimes in short supply. Also, since fly ash is a powder, it requires more sophisticated handling. Producers must invest in pneumatic conveying systems and specialized silos.
Granulated blast furnace slag (GBFS) is another popular SCM. However, sources are often quite distant from cement plants. As a result, the price of slag may not always be competitive.
In light of the challenges facing common SCMs, calcined clay becomes a more attractive alternative.
Calcined Clay Products Reduce Carbon Emissions
Concrete incorporating calcined clay is more and more common. Bloomberg Green reports on an updated Colombian concrete plant now producing LC3. The Argos plant uses clay mined at a site just 10 miles away. Processing occurs in a new, more efficient kiln. The updates reduce carbon emissions by about 50 percent. They also reduce energy consumption by 30 percent.
Italy's Cementir Holding launched a new gray cement product to begin the new year. FutureChem, reduces CO2 emissions by 30 percent compared to ordinary Portland cement (OPC). Gains are due to replacing clinker with 35 percent calcined clay and limestone. The Italian cement producer says strength and quality remain competitive. They’re also using the FutureChem formulation in two ultra-high performance white cements. Cementir and its Danish partner Aalborg want to expand the use of calcined clay to more cement formulations.
Study Confirms Viability of Impure Calcined Clay
One study confirmed that even impure CC met both the physical and chemical requirements of ASTM C618-19 for natural pozzolans. Strength at 28 days was compatible with regular concrete. The CC blend also reduced chloride penetration and mitigated the alkali-silica reaction (ASR). Drying shrinkage compared favorably to that of traditional concrete.
A pavement-grade mixture with 20 percent calcined clay met key requirements. It achieved the required slump as well as fresh and hardened air content. Compressive strength was about 15 percent less than that of the OPC control. Researchers found it was possible to compensate by reducing water content.
The use of calcined clay does affect the workability of the concrete. Superplasticizers help, but there are limits. In some applications, mixes can tolerate as much as 30 percent calcined clay.
Finally, newer ternary blended cements also use lower-quality overburden traditionally considered waste. The increased use of dolomite-rich crushed limestone and low-grade clay further reduces the amount of clinker required.
The Pennsylvania Aggregates and Concrete Association (PACA) tracks industry developments at SpecifyConcrete.org. The goalie to inform those in the industry as well as the general public. To learn more about the use of LC3 concrete and other more sustainable mixes, please contact us.