As the world works towards reaching the United Nation’s target of Net Zero by 2050, one solution could be found in combining the soil and concrete beneath our feet.
Cement manufacture contributes as much as 9% of global anthropogenic carbon dioxide emissions with forecasted cement and concrete use set to increase demands on energy and materials and exacerbate greenhouse gas emissions.
To reduce carbon emissions, and to improve sustainability of infrastructure materials used in major projects in Australia, a solution could be found in shifting Australia’s cement manufacture to lower clinker-factor cement blends, in which cement is partially replaced with locally sourced calcined clays. Cement clinker is formed from cooking clays intermixed with limestone and other silicates at temperatures exceeding 1200°C and used to form modern cement by milling with fresh limestone and gypsum. This intensive process is a big contributor to the cement industry’s overall energy consumption and greenhouse gas emissions.
On the other hand, calcined clays have multiple benefits besides replacing a portion of cement including reducing the embodied carbon content of the resulting material and lowering our reliance on traditional supplementary cementitious materials that are often imported.
To achieve this target Australia will require large quantities of local material. Melbourne has large quantities of reclaimed soils from remediation works, excavated soils from tunnelling and construction as well as quarry fines from aggregate processing, which can be suitable for use as components in cements and concretes.
Giving industrial waste a new purpose
After intensive research, in 2021-2022 IFM undertook a field trial using heat treated soils (classified as industrial wastes as they originated as soils contaminated with per- and polyfluorinated alkyl substances, or PFAS) in concrete. While the mix design was focused on using the heat-treated soils to replace fine aggregate in concrete, up to 17% of the coarse aggregate could also be replaced. Both field-cored and lab-cured samples indicated that the amended concrete had as good or better compressive strengths, indirect tensile strengths and comparable shrinkage to the control concrete.
Workability was the only real hurdle experienced during the field trial. During the pour of the amended heat-treated soil amended concrete, ambient temperatures exceeded 35°C, well above optimum temperatures for concreting. However, this was controlled by increasing the water to binder ratio to about 0.5 and using a water reducer.
Despite this, the concrete containing heat-treated soil compacted well, with less evidence of pug-holes and aggregate separation than the reference concrete.
A major outcome of the research on heat-treated soil was the prospect for some pozzolanic activity – that is reaction of the calcined material with portlandite produced by cement hydration – of the clay-sized fines fraction, particularly when cooling could be achieved via non-quenching means.
Under normal operations with organo-contaminated soils, the heat-treating process is followed by a rapid quenching in which a water spray is used. Quenching effectively diminishes any activation of clay minerals achieved by the heating process, but re-heating enables recovery of pozzolanic activity, as observed by consumption of portlandite. In collaboration with the industry partner non-quenched samples were supplied and found to have appreciable pozzolan activity.
In this case mortars incorporating heat-treated soil (replacing 100% of fine aggregate and 30% of cement) were found to exceed 90% of the control strength. Similar results were achieved at 40% cement replacement levels if a water reducer was also used.
A strong correlation between cement replacement level and portlandite consumption (above the amount expected from dilution of cement hydration) was observed, underscoring pozzolan reactivity by the heat-treated soil.
Solutions for diverting soils and excavation spoils from landfill
Our research team expanded its work, taking a closer look at the excavated soil materials from Victoria’s Big Build to determine if alternative local materials, otherwise destined for landfill or low-value uses, could be used to increase the sustainability of concrete.
An excavated soil material, provided by the client, had a mineralogy dominated by the clay mineral smectite. Following Australian Standards for strength activation index testing on samples heated to 600°C or 700°C, compressive strengths increased, enabling classification of the material as Type 1 manufactured pozzolan, the highest for manufactured supplementary cementitious materials.
These promising results enabled the client to incorporate spoil material into their construction design to improve sustainability outcomes of major projects. It also enabled us to embark on a more complete assessment of fines materials from the metropolitan Melbourne area.
Quarry fines – an untapped resource?
As part of an on-going PhD research project, we’ve investigated processed quarry fines for their clay mineralogy and pozzolanic activation on heating.
Currently, most quarry fines, waste soils and spoils have limited uses and are stockpiled leading to other environmental issues associated with erosion, runoff, weeds and pests and associated costs to producers.
The clay minerals smectite, kaolinite and illite were found to be present in all materials but differed in which was dominant. Material A was dominant in smectite, material B was co-dominant kaolinite and smectite and material C was co-dominant illite and kaolinite.
Optimal calcination temperatures, following Australian Standards for pozzolanic activity, were found to depend strictly on the dominant clay minerals present, and these class 1 manufactured SCMs attained excellent strengths exceeding the control mortar strength.
When calcined for two hours at optimum temperatures mortar mixes containing as much as 40% cement replacement achieved > 90% of the control strength, and continued strength improvement for at least 56 days.
Pozzolanic activity as indicated by consumption of portlandite was the reason for these excellent strengths. Thus, while the exact mineralogy of the soil, spoil or fines material was proven to not be the limiting factor, optimising the calcination temperature for the specific mineralogy is one key factor.
Reducing embodied carbon in concrete
With appropriate mixes, the total clinker factor of cement products using these reactive fines can be reduced to 0.5. The technology thus enables replacement of half of the cement content in mortars with these types of reactive fines to produce mortars that have equivalent or greater strengths than reference mortars.
Optimal calcination temperatures are at least 250°C lower compared with other non-traditional supplementary cementitious materials being developed for low embodied carbon cement products.
Given the results achieved thus far, if calcined at optimal temperatures (generally ≤800°C) the embodied carbon content associated with processing is as low as 15-20% that of current cement production.
Thus replacing 50% clinker with quarry fines (and other minerals) reduces the equivalent overall per unit strength carbon footprint of concrete by at least 40%.
For Australia to reach the United Nation’s Net Zero target by 2050, we need to rethink where we source materials and how we process the large amounts of cement and concrete needed for our built environments. Repurposing industrial wastes, excavation spoils and quarry fines have strong promise for helping us transform our environmental burdens into sustainable solutions.
About the author: Associate Professor Will P Gates is an internationally recognised clay mineralogist with 30+ years’ experience and expertise in applications of advanced spectroscopy to clay minerals and other poorly crystalline materials, as well as developing functional materials. With Deakin University’s Institute for Frontier Materials, he has focused on the chemistry and reactions of cements and supplementary cementitious materials, primarily with calcined clays where he holds two preliminary patents. Much of his recent research is industry-funded and focused on recovery of value and critical metals from legacy tailings in SE Australia, tailings stabilisation and alternative uses for tailings in remediation and construction. He has recognised national (e.g. Vic EPA, Environmental Protection Australia) and international (e.g., South Africa, U.S.A.) expertise in landfill barrier function and design for handling new and emerging contaminants (e.g. microplastics, PFAS, phthalate esters).