The research, published in the journal Discover Geoscience, is notable for connecting what happens at the scale of individual mineral grains to the performance of a roadbed a driver can actually feel. Led by Mohammed Abdulkareem Adisa of Kwara State University, the team combined laboratory geotechnical testing with X-ray diffraction and scanning electron microscopy to trace exactly how recycled concrete changes the internal architecture of tropical laterite. The work addresses a gap in the existing literature: most previous RCA-soil studies have examined temperate-region soils or engineered fills, while lateritic soils, which are quartz-rich, iron-oxide-bearing and relatively low in clay minerals, have received far less attention despite being the dominant subgrade material across Nigeria and neighboring countries.

The lateritic soil for the experiments was excavated from a pit along the access route to the University of Ilorin’s permanent campus in Kwara State, an area of humid tropical climate receiving roughly 1,200 millimeters of rain annually. The soil proved to be a medium-plasticity, fines-dominated material, classified as AASHTO A-7-6 and USCS CL, essentially a brown lateritic lean clay with sand. Its specific gravity was 2.72, its liquid limit 42.5 percent, and its plasticity index 18.3. The recycled concrete aggregate, by contrast, was recovered from demolished reinforced concrete slabs and beams at a construction site in the same state, crushed in a laboratory jaw crusher and sieved to particles between 4.75 and 20 millimeters. Gradation testing showed the RCA to be a well-graded gravel, a sharp complement to the poorly graded, fines-rich laterite.

X-ray diffraction revealed the mineralogical story behind the pairing. The laterite produced a sharp, high-intensity quartz peak near 26.6 degrees two-theta, together with minor feldspar peaks and clear signals from hematite, the iron oxide responsible for the soil’s reddish-brown color. Weak kaolinite reflections indicated that clay mineral content was low. The RCA diffractogram was distinctly different, dominated by quartz but also containing anorthite, a calcium-rich feldspar, and albite, phases consistent with the cementitious chemistry of demolished concrete. These calcium-bearing minerals hint at the possibility of secondary cementitious or pozzolanic reactions when the aggregate meets clay-bearing soil, although the authors are careful to note that any such chemical contribution was not independently verified in this study.

The mechanical results were unambiguous. In Standard Proctor compaction tests, the untreated control soil achieved a maximum dry density of 1.98 grams per cubic centimeter at an optimum moisture content of 9.5 percent. As RCA was added in five-percent increments, density climbed steadily: 2.01 at five percent, 2.04 at ten percent, 2.09 at fifteen percent, and a peak of 2.11 grams per cubic centimeter at twenty percent RCA, while the optimum moisture content fell to just 8.0 percent. The improvement arises because coarse, high-specific-gravity RCA particles occupy voids within the clay structure, forming a stiffer, better-interlocked skeleton that requires less water for optimal packing, since the crystalline, silica-rich recycled material absorbs less moisture than plastic clay particles.

Bearing capacity followed the same trajectory. In California Bearing Ratio tests, the untreated soil recorded 58 percent unsoaked and 30 percent soaked. At twenty percent RCA, those figures rose to 74 percent unsoaked and 41 percent soaked, a substantial gain in the load-bearing performance that governs pavement design. Scanning electron microscopy provided the visual explanation: micrographs at 500-times magnification showed large, angular RCA fragments with rough, porous surfaces bearing adhered lateritic fines, while images at 1,000 times revealed fine particles from the hematite-rich soil and RCA dust partially filling the intergranular voids between rigid quartz and feldspar grains. The result is a dense, mechanically interlocked fabric with reduced pore volume and enhanced particle-to-particle friction, which translates directly into the improved density and strength measured in the laboratory.

Intriguingly, the benefits did not continue rising indefinitely. At twenty-five percent RCA, performance reversed: maximum dry density slipped to 2.08 grams per cubic centimeter, optimum moisture content jumped to 11.0 percent, and both soaked and unsoaked CBR values declined. The authors attribute this reversal to several compounding mechanisms. Beyond twenty percent, the proportion of coarse and mortar-adhered RCA particles exceeds what the remaining clay-silt fraction can effectively bind, disrupting the continuity of the fine matrix. Excess RCA particles begin contacting one another directly, producing a coarser, more open skeleton that traps voids the fines can no longer fill. Meanwhile, the old adhered cement mortar on recycled particles is inherently porous, so higher dosages raise the water demand sharply, promoting segregation during compaction. The lesson is that more recycled material is not always better; dosage matters, and there is a genuine optimum.

The environmental and economic case may prove the study’s most consequential contribution. In a screening-level assessment normalized to one tonne of treated material, a conventional five-percent cement stabilization scheme was estimated to emit 52.7 kilograms of CO2, whereas the optimized twenty-percent RCA mixture emitted only 3.6 kilograms, an indicative reduction of roughly ninety-three percent within the system boundary considered. Material costs also favored RCA, with improvements running nine to forty-six percent cheaper per tonne than cement-based methods across the tested dosages, and the twenty-percent blend offering the best balance of performance gain against cost. The authors stress these figures are preliminary, based on industry-average emission factors and assumed transport distances, and recommend a full life-cycle assessment with site-measured data before the results feed into specifications or certification decisions.

The researchers are equally candid about the study’s limits. Testing covered immediate strength only; long-term durability under repeated wetting-drying cycles typical of tropical climates was not assessed, nor was the potential for slow strength gain from residual cementitious phases in the RCA through extended curing. No leachate analysis was performed, leaving open the question of whether recycled concrete might introduce sulfates, chlorides or other contaminants to groundwater. And because the findings apply to one specific quartz-rich, low-kaolinite laterite, they may not transfer directly to more plastic or clay-rich tropical soils. Future work, the team suggests, should include unconfined compressive strength, triaxial shear, resilient modulus, permeability and durability testing.

Even with those caveats, the study lands at a moment of urgent need. Construction and demolition waste constitutes an estimated twenty to fifty percent of municipal solid waste in industrialized nations, with China alone generating nearly 2.36 billion tonnes in 2020, and conflict zones such as Ukraine adding more than 100 million tonnes of rubble to the global burden. Turning even a fraction of that stream into road subgrade material, at a dosage that measurably improves strength while slashing emissions and cost, offers a concrete demonstration of circular-economy principles applied beneath our wheels. For Nigeria and other rapidly urbanizing tropical nations, where infrastructure demand and demolition waste are both rising sharply, the message is that the strongest foundation for new roads may already be lying in the rubble of the old ones.

Subject of Research: Stabilization of tropical lateritic subgrade soil using recycled concrete aggregate for sustainable road construction

Article Title: Compaction, CBR, and microstructural performance of lateritic soil stabilized with recycled concrete aggregate

Article References: Adisa, M. A., Omolara, K. E., Maurel, H. T., Tomiwa, A. H., Ndane, S. P., Amao, A. O., & Kayode, Y. A. (2026). Compaction, CBR, and microstructural performance of lateritic soil stabilized with recycled concrete aggregate. Discover Geoscience, 4(1), Article 347. https://doi.org/10.1007/s44288-026-00718-9

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00718-9

Keywords: recycled concrete aggregate, soil stabilization, lateritic soil, CBR, compaction, XRD, SEM, circular economy, sustainable roads, CO2 emissions, geotechnical engineering, construction and demolition waste