Eggshell Waste Strengthens Sand but Weakens Clay, Study Finds

Every year, households, restaurants and poultry processing plants around the world discard mountains of eggshells, most of which end up in landfills as part of the biodegradable waste stream. A new study from Bangladesh suggests that this humble calcium-rich refuse could find a second life beneath our feet, but only under very specific conditions. Researchers at the University of Asia Pacific and Stamford University Bangladesh have shown that powdered eggshell can dramatically improve the shear strength of sandy soils while simultaneously weakening clay, a split verdict that challenges the growing enthusiasm for eggshell powder as a universal, eco-friendly soil stabiliser.

Soil shear strength is one of the most fundamental parameters in geotechnical engineering, governing the stability of foundations, embankments, pavements and retaining walls. In developing countries, where locally available soils often display highly variable engineering properties, engineers frequently turn to stabilisers such as lime, cement and fly ash to strengthen weak ground. Yet these conventional binders carry a heavy carbon footprint and can be expensive or difficult to source in remote regions. That has driven a search for low-cost, locally available alternatives derived from biodegradable waste, and eggshell, which is composed of more than 95 percent calcium carbonate, has emerged as one of the most promising candidates.

The research team, led by MD. Moin Akon, set out to answer a question that most previous studies had sidestepped: does eggshell powder behave the same way in fundamentally different soils? They collected two contrasting materials from locations across Bangladesh. The first was a clay of intermediate plasticity, classified as CI under the Unified Soil Classification System, with a liquid limit of 46.72 percent and a plasticity index of 22.09, sourced from Matidali in the Bogra district. The second was a poorly graded sand, classified as SP, taken from Gabtoli in Dhaka. Both samples were extracted from depths of one to one and a half metres to minimise the influence of organic matter and surface desiccation.

The eggshells themselves were collected from households and restaurants in Dhaka, washed to remove the inner membranes, oven-dried for 24 hours at 105 degrees Celsius, crushed and sieved through a 1.18 millimetre mesh. The powder was then blended into each soil at three dosage levels, 8.5, 12 and 15 percent by dry weight of soil, a range chosen after preliminary mixing trials showed that lower contents produced negligible changes while higher contents caused segregation during specimen preparation. All treated specimens were sealed and cured for 28 days at 25 degrees Celsius, matching the standard benchmark used in comparable stabilisation studies.

The testing programme followed established ASTM standards. For the sand, direct shear tests under normal stresses of roughly 31 to 93 kilopascals, corresponding to shallow depths typical of road subgrades and earthwork fills, revealed a striking rise-and-fall pattern. The untreated Gabtoli sand exhibited an internal friction angle of 29 degrees, a value the authors attribute to the low confining stresses, the sub-rounded shape of its particles and its poor gradation, which together limit interlocking. When 8.5 percent eggshell powder was added, the friction angle climbed to a peak of 38 degrees, an improvement of about 31 percent. Maximum shear stress rose from 52 to 84 kilonewtons per square metre, an increase of roughly 62 percent. A similar value was recorded at 12 percent, but at 15 percent the friction angle fell back to 31 degrees, signalling that the benefit had been lost.

The clay told the opposite story. The untreated Matidali clay had an average undrained shear strength of 68.11 kilonewtons per square metre, corresponding to a stiff consistency. Adding just 8.5 percent eggshell powder caused that figure to collapse to 22.57 kilonewtons per square metre, a reduction of approximately 67 percent. At 12 percent the strength remained low at 24.99 kilonewtons per square metre, and even at 15 percent, where a partial recovery to 39.30 kilonewtons per square metre was observed, the treated clay was still about 42 percent weaker than the untreated soil. In other words, at no dosage did the eggshell powder restore, let alone improve, the clay’s original strength.

The authors propose that these contrasting outcomes stem from physical rather than chemical interactions, and they are careful to frame their explanations as working hypotheses, since no scanning electron microscopy, X-ray diffraction or energy-dispersive spectroscopy was performed. Eggshell powder is essentially calcium carbonate and, unlike lime or cement, it does not readily release reactive calcium hydroxide unless it is calcined or chemically activated. Without that pozzolanic reactivity, its effect on soil depends almost entirely on particle-scale physics. In sand, the finer eggshell particles appear to fill the voids between the larger grains, increasing the density of particle contacts and improving load transfer, while their irregular, angular morphology may raise sliding resistance at contact points in a sand whose own grains are sub-rounded. Beyond a critical fines content, however, the added particles begin to separate the sand grains rather than pack between them, disrupting the direct grain-to-grain contacts that carry load and eroding the strength gains.

In clay, the same physical additions appear to work destructively. Clay derives its undrained strength from electrochemical bonding between platelets and from the delicate fabric they form. Introducing coarser, non-cohesive calcium carbonate particles may break up that bonded fabric, while the increased specific surface area of the mixture can raise water demand and local pore water pressures under undrained loading, reducing effective stress. Crucially, without alkaline activation, the eggshell powder remains largely inert and cannot form the cementitious bonds that lime and cement create. The result is a filler that disrupts rather than reinforces the clay matrix, a conclusion consistent with earlier work showing that plain, non-activated eggshell powder offers limited or no benefit in high-plasticity clays and that benefits in expansive clays diminish beyond a few percent dosage.

The paired experimental design is what gives the study its force. Most published research on eggshell stabilisation has examined a single soil type in isolation, often treating the powder as a potential lime substitute for expansive clays, which makes it difficult to judge how widely positive results can be generalised. By testing a clay and a sand side by side under identical curing conditions, mixing protocols and dosage levels, the Bangladeshi team produced a directly comparable dataset demonstrating that the response to eggshell powder is governed by soil type. The authors also compared their findings with earlier studies, noting that improvements reported in soft Egyptian clay at 5 to 15 percent dosage, or in expansive clay at lower dosages, may reflect the weaker baseline of those materials, where the compaction and void-filling effect of the powder outweighs fabric disruption, whereas the stiffer intermediate-plasticity clay used here was more vulnerable to being disturbed.

The practical implications are pointed. For Bangladesh and similar deltaic regions dominated by fine-grained alluvial deposits and poorly graded river sands, eggshell powder shows genuine promise as a low-cost, locally sourced additive for improving sandy subgrades and fills, particularly for lightly loaded roads and earthworks, where a friction angle gain of this magnitude could meaningfully increase shear capacity. But the study issues a clear warning for fine-grained ground: used alone and without chemical activation, eggshell powder can actively degrade the performance of clay subgrades and should not be deployed as a standalone stabiliser for cohesive fills or embankments. The authors acknowledge limitations, including only two replicate specimens per dosage in the compression tests, a single 28-day curing period, and the absence of microstructural analysis, and they recommend consolidated triaxial testing, consolidation tests, pavement-focused California bearing ratio tests and evaluation of alkali-activated formulations as next steps. For now, the message is one of conditional optimism: the egg on your breakfast plate may indeed help build roads, but only where the ground beneath them is made of sand.

Subject of Research: Use of eggshell powder as a waste-derived additive to modify the shear strength of cohesive and cohesionless soils

Article Title: Eggshell as soil shear strength amplifier for cohesive and cohesionless soils

Article References: Akon, M. M., Shovon, K. M. M. I., Chowdhury, A. A., Ahmed, S., & Turja, A. S. (2026). Eggshell as soil shear strength amplifier for cohesive and cohesionless soils. Discover Soil, 3(1), Article 100. https://doi.org/10.1007/s44378-026-00251-8

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00251-8

Keywords: eggshell powder, soil stabilisation, shear strength, cohesive soil, cohesionless soil, geotechnical engineering, calcium carbonate, direct shear test, unconfined compression test, Bangladesh, sustainable construction, biodegradable waste