Cow Bone Ash Transforms Weak Lateritic Soil Into Road-Ready Pavement Material

A laboratory study published in Discover Civil Engineering reports that cow bone ash, a calcium-rich powder produced by calcining slaughterhouse bones, substantially improved the strength, bearing capacity, and moisture behaviour of a Nigerian lateritic soil intended for pavement foundations. At the optimal dose of 6% by dry soil mass, the treated soil’s soaked California bearing ratio rose by up to 333%, its unsoaked bearing ratio by 167%, and its seven-day unconfined compressive strength by 103% compared with the untreated material.

The research team, led by Wasiu Oyeniyan with Mutiu Kareem and Afeez Bello, sourced natural lateritic soil from a borrow pit along the Ijagbo–Offa Erin bypass in Kwara State, Nigeria, excavating at a depth of one metre. Cow bones came from the abattoir at Sango Owode Market in Offa and were burned at 700 degrees Celsius for two hours in a muffle furnace, then ground and passed through a 75-micrometre sieve. The calcination temperature was chosen because the 600 to 700 degree range removes organic matter while keeping the apatite structure relatively amorphous, a condition considered favourable for pozzolanic-type reactions. The authors note that the effect of calcination temperature on ash reactivity was not specifically assessed in the study.

In its natural state, the soil was far from pavement-ready. Classified as A-2-6(2) under the AASHTO system and clayey sand (SC) under the Unified Soil Classification System, it had a liquid limit of 39%, a plasticity index of 15.4%, and a maximum dry density of 1.87 grams per cubic centimetre at an optimum moisture content of 13.7%. Its most striking weakness was moisture sensitivity: soaking collapsed its California bearing ratio from 24% to just 3%. X-ray diffraction and infrared spectroscopy identified kaolinite as the dominant clay mineral, alongside quartz and minor orthoclase, consistent with a mature kaolinitic laterite.

The cow bone ash itself proved chemically distinctive. X-ray fluorescence showed it was dominated by calcium oxide at 41.99%, with moderate silica at 20.40% and phosphorus at 13.27%. Notably, the sum of silica, alumina, and iron oxides reached only about 24.67%, well below the ASTM C618 thresholds for Class C or F pozzolans, meaning the ash is calcium-phosphate dominated rather than a classical siliceous pozzolan. The authors frame the comparison as context rather than formal classification, describing CBA as a calcium-rich additive that can promote pozzolanic-type reactions with reactive silica and alumina from the soil, forming calcium silicate hydrate and calcium aluminate hydrate gels.

Mixing the ash into the soil at 0, 2, 4, 6, 8, and 10% by dry mass reshaped both plasticity and compaction behaviour. The liquid limit fell by about 42.3% and the plasticity index by about 83.1% across the dosage range, reflecting flocculation and cation exchange between calcium ions and clay particles. The plastic limit behaved non-monotonically, dipping slightly at low doses, holding steady near 23.4% at 6% and 8% ash, and dropping to 19.9% at 10%. Compaction shifted in the opposite direction: maximum dry density declined 11.2%, from 1.87 to 1.66 grams per cubic centimetre, while optimum moisture content surged 89.8%, from 13.7% to 26.0%. The researchers attribute the density loss to the ash’s lower specific gravity of 2.10 versus 2.71 for the soil, which produces a lighter solid phase and a more open particle arrangement.

Strength results told a different story. Soaked CBR climbed from 3% to 13% and unsoaked CBR from 24% to 64% as the ash content rose to 10%, while seven-day unconfined compressive strength increased from 188.9 to 382.6 kilonewtons per square metre. The treated soils containing 6 to 10% ash exceeded typical subbase strength thresholds of roughly 200 to 300 kilonewtons per square metre for light to medium traffic roads. The authors caution, however, that because the specimens received no extended curing before bearing tests, the immediate strength gains cannot be attributed with confidence to secondary cementitious gel formation; physicochemical modification, cation exchange, flocculation, and filler effects from the fine ash particles are the more defensible mechanisms at low doses.

Hydraulic and volumetric measurements reinforced the picture of a denser, more stable material. Hydraulic conductivity, already low at 7.92 × 10⁻⁷ centimetres per second in the natural soil, fell by nearly 40% with treatment, dropping from 5.93 × 10⁻⁷ to 4.78 × 10⁻⁷ centimetres per second as ash content rose from 2% to 10%. Volumetric shrinkage fell even more dramatically, from 16.42% in the untreated soil to 5.15% at 10% ash, a reduction of about 69%. Lower permeability and shrinkage suggest reduced vulnerability to water infiltration, pore pressure buildup, desiccation cracking, and moisture-induced deformation, all of which matter for pavement layers in tropical climates with pronounced wet and dry seasons.

Statistical analysis lent rigour to the pattern. A one-way analysis of variance at the 95% confidence level found that ash content significantly affected the liquid limit, plastic limit, optimum moisture content, maximum dry density, soaked CBR, unconfined compressive strength, hydraulic conductivity, and volumetric shrinkage, but not the plasticity index or unsoaked CBR. The authors flag an important caveat: formal checks of ANOVA assumptions, such as normality of residuals and homogeneity of variances, were not performed, and post hoc pairwise comparisons were left to future work. All tests were run in duplicate, with mean values used in the analysis.

Microstructural imaging at the 6% optimum dose supported the mechanical findings. Scanning electron micrographs showed the untreated soil as a loosely packed fabric with interconnected pores, while the treated soil displayed a more compact particle arrangement with fewer and smaller visible macropores. X-ray diffraction of the treated soil showed intensified peaks for kaolinite, orthoclase, quartz, and haematite, which the authors link to secondary cementitious products filling soil pores. The team is careful to note that the SEM images lack the resolution or chemical evidence to identify specific reaction products such as calcium silicate hydrate gels, so the observed changes are interpreted primarily as particle packing, pore filling, and fabric modification rather than direct proof of new cement phases.

The study’s authors are candid about its limits. All experiments used a single lateritic soil from one location, one ash source and combustion condition, and short-term laboratory testing without wet-dry cycling, freeze-thaw exposure, prolonged soaking, or traffic simulation. The economic and environmental feasibility of large-scale ash collection, processing, and transport was not assessed, and field trials under real traffic and climate conditions remain necessary. Durability testing and comparative microstructural analysis at both 6% and 10% ash are planned for future research. Within those boundaries, the work positions cow bone ash as a promising, waste-derived alternative to carbon-intensive cement and lime stabilizers, one that could simultaneously reduce landfill pollution from slaughterhouse bones and strengthen the foundations of low- to medium-traffic roads in regions where both materials are abundant.

Subject of Research: Cow bone ash as a sustainable stabilizer for improving the geotechnical properties and microstructure of lateritic soil in road pavement applications

Article Title: Effect of cow bone ash on the geotechnical properties and microstructure of lateritic soil for road pavement applications

Article References: Oyeniyan, W. O., Kareem, M. A., & Bello, A. A. (2026). Effect of cow bone ash on the geotechnical properties and microstructure of lateritic soil for road pavement applications. Discover Civil Engineering, 3(1), Article 268. https://doi.org/10.1007/s44290-026-00661-1

Image Credits: AI Generated

DOI: 10.1007/s44290-026-00661-1

Keywords: cow bone ash, lateritic soil, soil stabilization, pavement subbase, California bearing ratio, unconfined compressive strength, hydraulic conductivity, volumetric shrinkage, microstructure, abattoir waste, sustainable construction, geotechnical engineering