A recent study published in Construction and Building Materials explored the potential for recycling infra-lightweight concrete (ILC) elements into recycled lightweight concrete aggregates (RLCAs) through mechanical processing and screening. The goal was to use these RLCAs to produce recycled ILC (RILC) with properties similar to the original ILC.
Study: Renewable construction with lightweight concrete – Reclaimed recycled material systems with CO2-absorption. Image Credit: Parilov/Shutterstock.com
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Background
Reusing construction and demolition waste is key to fostering circularity in the construction industry. However, recycling lightweight concrete, which contains lightweight aggregates, differs from the process used for ordinary concrete. While recycled concrete aggregates (RCA) from lightweight concrete can be incorporated into structural concrete in limited amounts, they cannot be used to produce new lightweight concrete.
ILC is specifically designed to balance density, strength, and thermal conductivity. Although its mechanical properties are similar to lightweight aggregate concrete, it differs in being a ready-mixed, cast-in-place material. Its monolithic structure is particularly suited for recycling and reuse, as it eliminates the need for material separation—a common challenge in multi-layer construction. Given these advantages, this study proposed a method to recycle and reuse ILC, aligning with circular economy goals and CO2 reduction strategies.
Methods
To produce RLCA, larger ILC elements (4×2×0.5 m) were first manually broken down and then mechanically crushed using a laboratory jaw crusher. The resulting material was screened into five particle size categories: <1 mm (RLCAfine), 1–2 mm (RLCA1–2), 2–4 mm (RLCA2–4), 4–8 mm (RLCA4–8), and 8–16 mm (RLCA8–16). Key physical properties of RLCA, including particle density and water absorption, were assessed using the pycnometer method.
Next, the impact of the recycling process on RLCA characteristics—such as density, water absorption, and particle strength—was analyzed to determine their influence on concrete performance. To evaluate mechanical properties, compressive strength tests were conducted on 100×100×100 mm cubic samples after 28 days of curing.
Using the RLCA characterization data, RILC was developed with a mixed design incorporating up to 54 % RLCA by volume. Fresh concrete properties were analyzed, with plastic shrinkage monitored using a shrinkage cone and a laser beam for contactless deformation measurement. After 28 days, the compressive strength of cylindrical specimens (300 mm in height, 150 mm in diameter) was tested, with stress-strain behavior recorded via displacement sensors. The modulus of elasticity was also determined at this stage.
Finally, the CO2 absorption capacity of RLCA was examined using thermogravimetric analysis (TGA) and Fourier-transform infrared (FTIR) spectroscopy to assess its potential for carbon sequestration.
Results and Discussion
Upon evaluation, RLCAs exhibited an agglomerate structure due to the crushing process, which also broke down the original lightweight aggregates. Water absorption and density were found to be critical factors influencing RLCA integration into RILC.
The particle densities of RLCA were approximately three times higher than those of the original lightweight aggregates in the same size group. Water absorption rates varied from 27 % to 60 % by mass, with oven-dry conditions producing higher absorption values than laboratory conditions.
The study confirmed that RLCA quality and strength remained consistent across different batches. Notably, the strength of RILC was not strictly dependent on that of ILCOrigin, suggesting that the reuse of recycled material is not necessarily constrained by its original properties.
RILC derived from RLCA demonstrated comparable strength, elasticity modulus, and thermal conductivity to ILCOrigin. Although RILC showed a notable 31.6 % increase in dry density, its thermal conductivity increased only slightly, preserving nearly the same performance characteristics as the original material. Importantly, monolithic wall elements constructed entirely from RLCAs exhibited the same functional properties as those made from ILCOrigin.
Regarding CO2 absorption, accelerated carbonation of RLCA led to significant CO2 uptake, with smaller particle sizes exhibiting faster initial carbonation rates. After 10 days in a controlled environment with 0.5 % CO2 concentration, the maximum CO2 uptake ranged from 123 to 138 kg/t of RLCA. This corresponded to 64–68 % of the estimated total CO2 absorption potential and recaptured about 30 % of the greenhouse gas emissions from ILCOrigin.
Conclusion
This study provided a comprehensive assessment of the recycling potential of lightweight concrete, particularly ILC, which is often excluded from conventional recycling methods. The researchers successfully processed RLCAs from ILC and used them to produce RILC with comparable properties to ILCOrigin.
A key finding was that monolithic wall elements could be built entirely from RLCAs without compromising performance. This method promotes material circularity, reduces CO2 emissions, and validates the structural feasibility of recycled lightweight concrete.
The researchers recommend further studies on different types of lightweight concrete, optimization of RLCA carbonation, and life cycle assessments to enhance environmental benefits.
Journal Reference
Haller, T., Scherb, S., Beuntner, N., & Thienel, K.-C. (2025). Renewable construction with lightweight concrete – Reclaimed recycled material systems with CO2-absorption. Construction and Building Materials, 466, 140339. DOI: 10.1016/j.conbuildmat.2025.140339, https://www.sciencedirect.com/science/article/pii/S0950061825004878
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