Calcination Optimisation of Corncob Ash for Sustainable Cementitious Applications: A Pathway to Low-Carbon Construction

Abstract

The construction sector faces pressure to decarbonise while addressing rising resource demands and agricultural waste. Ordinary Portland cement (OPC) is a major CO₂ emitter, yet biomass residues are often open-burned or landfilled. This study explores corncob ash (CCA) as a sustainable supplementary cementitious material (SCM), examining how calcination conditions influence pozzolanic potential and support circular economy and climate goals, which have not been adequately explored in literature. Ten CCA samples were produced via open-air burning (2–3.5 h) and electric-furnace calcination (400–1000 °C, 2 h), alongside a reference OPC. Mass yield, colour, XRD, XRF, LOI, and LOD were analysed within a process–structure–property–performance–sustainability framework. CCA produced in a 400–700 °C furnace window consistently achieved high amorphous contents (typically ≥80%) and combined pozzolanic oxides (SiO₂ + Al₂O₃ + Fe₂O₃) above the 70% ASTM C618 threshold, with 700 °C for 2 h emerging as an optimal condition. At 1000 °C, extensive crystallisation reduced the expected reactivity despite high total silica. Extended open-air burning (3–3.5 h) yielded chemically acceptable but more variable ashes, with lower amorphous content and higher alkalis than furnace-processed CCA. Simple industrial ecology calculations indicate that valorising a fraction of global CC residues and deploying optimally processed CCA at only 20% OPC replacement could displace 180 million tonnes CC waste and clinker avoidance on the order of 5–6 Mt CO₂ per year, while reducing uncontrolled residue burning and primary raw material extraction. The study provides an experimentally validated calcination window and quality indicators for producing reactive CCA, alongside a clear link from laboratory processing to clinker substitution, circular resource use, and alignment with SDGs 9, 12, and 13. The findings establish a materials science foundation for standardised CCA production protocols and future life cycle and performance evaluations of low-carbon CCA binders.

1. Introduction

One of the driving focuses behind the growing emphasis on environmental sustainability is the mounting evidence of the detrimental impacts associated with conventional construction materials and practices, alongside a global push toward carbon neutrality. This commitment was reiterated at the recent United Nations Framework Convention on Climate Change (UNFCCC) Conference of Parties (COP28) [1], where global leaders pledged to achieve net-zero emissions by the mid-21st century and limit global temperature rise below 1.5 °C [2]. In parallel, the United Nations Environment Programme underscored the urgent need to reduce global greenhouse gas emissions by 7.6% annually from 2020 to 2030 to remain on track with this target [3]. The construction industry, facing escalating infrastructure demands, is simultaneously under pressure to mitigate its environmental footprint. Concrete, the second most consumed material after water [4], plays a pivotal role in this challenge due to the high carbon intensity of Portland cement production, which alone accounts for roughly 8% of worldwide CO₂ emissions [5]. These environmental pressures, compounded by rising material costs and the depletion of natural resources, have catalysed the global search for sustainable supplementary cementitious materials (SCMs) derived from agricultural and industrial waste. The strategic deployment of SCMs has the potential to replace up to 50–100% of Portland cement clinker in some applications, offering the possibility of reducing global CO₂ emissions by as much as 1.3 gigatons annually [6,7].

Agricultural waste valorisation has emerged as a promising pathway for developing sustainable construction materials while simultaneously addressing waste management challenges [8,9]. Among various agricultural residues, corncobs (CCs) present a particularly attractive resource due to their abundance, driven by global corn production exceeding 1.2 billion tonnes annually [10]. Despite this availability, CCs are often left to decompose in fields or are openly burned, leading to environmental pollution and missed opportunities for resource recovery [11]. Beyond its technical viability, the sustainable potential of CCA spans several dimensions. Its application as a partial cement replacement not only reduces cement consumption and the associated CO₂ emissions but also offers an environmentally responsible solution for agro-waste disposal [12]. Furthermore, incorporating CCA into concrete production reduces the embodied energy of construction materials, contributing to more sustainable building practices [13]. From an economic perspective, CCA presents a globally viable option, as it is derived from a widely available and low-cost agricultural by-product, making it a cost-effective alternative to conventional cement in many contexts.

CCA, produced through the burning of CC, has demonstrated potential as an SCM owing to its pozzolanic properties, which stem primarily from its high silica content. During hydration, the calcium hydroxide (CH) released from the reaction of cement with water can react with the silica and alumina present in CCA, forming additional calcium silicate hydrate (C–S–H) gels, thereby enhancing the hardened properties of concrete [14]. According to Pandey and Kumar [15], this pozzolanic reaction typically occurs more slowly than the primary hydration of OPC, affecting both the rate and heat evolution during the early stages of hydration. Preliminary studies have shown that when properly processed, CCA can positively influence both the strength development and durability of concrete [16]. However, the relationship between key processing parameters, such as calcination temperature, duration, and their influence on the resulting chemical structure and elemental composition, remains insufficiently understood, particularly in terms of how these variables govern the pozzolanic reactivity of CCA.

Literature evidence reveals that although CC varieties tend to exhibit similar structure and compositional characteristics [17], the processing methodology and conditions play a crucial role in determining the chemical composition and pozzolanic reactivity of the resulting CCA as SCM. These differences ultimately influence the strength and performance of the resulting concrete, an aspect that many published experimental works have not adequately recognised or accounted for. Despite CCA’s potential, research findings regarding its effectiveness remain inconsistent. For instance, studies by the authors of [18,19,20] reported notable improvements in concrete performance when incorporating CCA. In contrast, other investigations, such as those by Anjaneyulu [21] and Abdul-Manan [22], observed negligible or no enhancement in compressive strength at replacement levels of 5% and 40%, respectively, after a standard 28-day curing period. This inconsistency is further reflected in studies [16,23,24,25] that found that CCA incorporation generally led to reductions in compressive strength. Similarly, workability assessments produced conflicting results: Adesanya and Raheem [16] reported a reduced slump, whereas Kamau et al. [26] observed an increased slump with CCA usage. Additionally, Ahangba and Tiza [27] noted that substituting 10% cement with CCA extended the setting time from 258 to 277 min, while Owolabi et al. [28] found that increasing CCA content decreased both workability and compressive strength, although strength improved over extended curing periods. Interestingly, findings by Shakouri et al. [29] suggested that while CCA may interfere with early cement hydration, it can help reduce chloride ion permeability, potentially improving long-term durability.

These varying results reported for OPC replacement with CCA in concrete mixtures clearly warrant further investigation. The contradictory findings across multiple studies highlight unresolved questions regarding the fundamental mechanisms by which CCA influences concrete properties. Although existing research, such as the comprehensive review by [30], has provided valuable insights into general strength assessment, it primarily offers outcome-based evaluations, with limited emphasis on how processing conditions affect material behaviour. Similarly, the work of Okeke et al. [31] contributed to identifying optimal CCA replacement levels in concrete yet did not explore in detail how different processing techniques might influence these values across varying environmental and application contexts. In addition, ref. [32] presented a systematic review of CCA in construction, investigating a wide range of uses; however, it did not address how specific calcination parameters impact material performance. This gap is particularly important, as the physicochemical and mineralogical properties of CCA are highly sensitive to production methods [33], which ultimately govern its effectiveness as an SCM. While testing procedures and concrete production follow established international standards, such as ASTM C109 [34] for compressive strength and ASTM C191 [35] for setting time, there are currently no internationally recognised standards governing the processing of CCA itself. This lack of standardisation explains the variability in experimental results across the literature, hindering meaningful comparisons and limiting the confidence of industry stakeholders in the use of CCA as a reliable SCM.

Literature evidence consistently demonstrates that CCA performance is highly dependent on processing conditions. For instance, Suwanmaneechot et al. [18] found that incineration temperatures ranging from 100 °C to 600 °C yielded CCAs with varying silica contents and pozzolanic activities, with optimal reactivity occurring only within specific thermal thresholds. Similarly, Zareei et al. [36] showed that burning duration plays a significant role in determining amorphous silica content, a key factor in pozzolanic behaviour. This variability in reported optimal conditions is widely viewed as a major barrier to the broader adoption of CCA as a reliable SCM. For engineering professionals and concrete producers, the use of any SCM requires the assurance of consistent and predictable performance. However, existing research offers limited clarity on the exact production parameters necessary to ensure such consistency [37]. This challenge underscores the urgent need for a systematic and evidence-based investigation to establish robust production guidelines, ultimately facilitating the transition of CCA from an experimental material to a viable mainstream alternative in sustainable cement systems.

The present study is a part of a broader investigation into CC as an alternative building and construction material and responds to these gaps by experimentally investigating CCA within an extended process–structure–property–performance–sustainability framework. Using a single, well-characterised corn variety, it pursues three linked objectives. First, it generates a controlled set of CCA samples by varying the calcination environment (electric furnace vs. open air), peak temperature (400–1000 °C), and burning duration (2–3.5 h) and benchmarks them against OPC. Second, it establishes an experimentally grounded calcination window and proposes straightforward quality control markers (mass yield, colour, LOI/LOD, amorphous fraction and oxide chemistry) that can be applied in both laboratory and low-resource settings. Third, it embeds these findings in an extended conceptual framework, using simple material-flow and scenario calculations to relate optimised CCA production to clinker substitution potential, waste management benefits, and alignment with the UN SDGs. While existing SCM optimisation studies typically evaluate limited calcination conditions or focus primarily on either chemistry or reactivity proxies, the specific contribution of this study is a single dataset that compares open-air and furnace calcination across multiple temperatures and durations using similar screening metrics. This enables the identification of a practical processing window for producing chemically compliant, low-volatiles CCA, presenting a consolidated decision-oriented basis as a reference condition for subsequent performance and durability validation and accessing sustainability gains in the cement and concrete sector.

1.1. Biogenic SCM Calcination Optimisation

Recent investigations into biomass ash pozzolans have substantially expanded the understanding of optimal calcination parameters for various agricultural residues, providing a crucial context for CCA processing. Studies examining rice husk ash have demonstrated that calcination temperature profoundly influences pozzolanic reactivity, with amorphous silica content, specific surface area, and particle fineness serving as primary determinants of performance [38,39]. Systematic investigations by Onyenokporo et al. [40] and Zaffar et al. [41] revealed that rice husk ash calcined at 600 °C for 2 h exhibited optimal pozzolanic activity, achieving strength activity indices of 86.5% and 101.2% at 7 and 28 days, respectively, while temperatures exceeding 600 °C promoted crystallisation that diminished reactivity. The significance of post-calcination processing has been emphasised by research demonstrating that grinding duration critically influences pozzolanic activity, with extended mechanical grinding proving more effective than manual processing [41]. For sugarcane bagasse ash, studies have established similar temperature-dependent behaviour patterns. Bayapureddy et al. [42] reported that bagasse ash processed through calcination at 600 °C for 2 h, followed by ball milling, resulted in material with enhanced pozzolanic properties characterised by a 12.3% increase in amorphous phase content. Research by Prabhath et al. [43] demonstrated that industrial sugarcane bagasse combustion at temperatures between 1000 °C and 1300 °C produced ash requiring secondary calcination at 600 °C to reduce loss on ignition values below 6% and improve pozzolanic material content to 70%, with post-heating, sieving, and grinding identified as critical enhancement methods. Subsequent investigations have confirmed that controlled recalcination at 600 °C combined with ultrafine grinding produces pozzolanic sugarcane bagasse ash that reduces total accumulated heat and portlandite content while refining pore structure [44]. Wheat straw ash research has reinforced the critical importance of temperature control. Memon et al. [45] conducted systematic investigations calcining wheat straw at temperatures from 500 °C to 800 °C for 2 h, establishing that 600 °C represented the optimal temperature, with higher temperatures transforming amorphous silica into crystalline forms that reduced pozzolanic efficiency. Their findings revealed that ash calcined at 600 °C required 120 min of grinding to achieve a 48% increase in the Blaine surface area. Multiple subsequent studies have converged on approximately 600 °C as the optimal wheat straw ash calcination temperature, with evidence showing that exceeding this threshold promotes silica crystallisation that diminishes pozzolanic performance [41]. For corn-based biomass specifically, research on corn stalk ash by Memon et al. [46] identified 500 °C as optimal, achieving pozzolanic activity of 96.8%, Fratini calcium oxide reduction of 93.2%, and Chapelle activity of 856.3 mg/g. X-ray diffraction analysis confirmed that silica remained amorphous at lower temperatures while crystallising at elevated temperatures, corroborating the narrow temperature window observed for other agricultural ashes [46]. These collective findings from biomass ash investigations underscore the critical importance of precise temperature control for maintaining amorphous silica content and maximising pozzolanic reactivity in agricultural waste-derived SCMs.

1.2. Conceptual Framework

The present work is grounded in the process–structure–property relationships that underpin the performance of SCMs in cementitious systems. Contemporary SCM theory emphasises that the reactivity and performance of a given material are governed not only by its bulk oxide composition but also by its amorphous content, glass structure, and microstructural features developed during processing [47,48]. Within this framework, calcination conditions (temperature, duration, and atmosphere) are key process variables that control the transformation of lignocellulosic biomass into a reactive siliceous ash. These variables regulate the removal of organics, the volatilisation of alkalis, and the formation or suppression of crystalline silica phases, all of which directly influence pozzolanic reactivity and durability performance in blended cements [49,50].

Agricultural residues such as rice husk, sugarcane bagasse, and other biomass ashes are now recognised as a distinct class of “biogenic SCMs” with behaviour that is similar in principle, but not identical, to conventional industrial pozzolans such as coal fly ash and natural pumice [50,51]. Systematic reviews show that these materials typically exhibit a relatively narrow processing window in which amorphous silica is maximised, residual carbon is minimised, and deleterious crystalline phases remain limited [47,49]. Outside this window, underburning leads to high loss on ignition and poor reactivity, whereas overburning promotes crystallisation of silica and reduces the capacity of the ash to consume calcium hydroxide in cement pastes. This dual sensitivity to combustion severity and feedstock heterogeneity explains much of the variability in performance reported for biomass ashes across the literature [50,51].

From a conceptual standpoint, the present study can be interpreted in a multiscale framework (an extended process–structure–property–performance–impact chain) linking calcination conditions to the sustainability performance of CCA in cementitious systems. In this framework, the thermal treatment of CC is viewed not only as a materials-processing step, but as the first stage in a chain that connects process variables to microstructure, engineering properties, concrete performance, and ultimately sustainability outcomes in the built environment. In this chain, the process step comprises the choice between open-air and controlled furnace calcination and the associated combinations of temperature and time. These processing routes determine the resulting structure of the CCA in terms of amorphous/crystalline balance, glass network connectivity, and residual alkali content. The structural state then governs key properties, including chemical reactivity, volatility (LOI/LOD), and compatibility with Portland cement. These properties, in turn, condition the expected performance of the ash when used as an SCM in concrete (strength development, durability, and workability), and ultimately the impact at the system level in terms of clinker reduction and associated CO₂ savings [52]. By systematically varying calcination conditions and quantifying the resulting oxide chemistry and amorphous content, the present work operationalises this theoretical chain for CCA and provides empirical evidence for the location and breadth of the optimal processing window.

In addition, the study contributes to ongoing efforts to integrate microstructural understanding with sustainability assessment of binders. Emerging work on the LCA of cementitious systems shows that clinker substitution with well-characterised SCMs can significantly reduce embodied greenhouse gas emissions and other environmental burdens, provided that durability is not compromised [52,53]. The optimisation of calcination parameters presented here therefore forms a necessary precursor to robust LCA and multi-criteria decision-making exercises that compare different SCM strategies at structural and system scales [53,54].

Table 1. Conceptual process–structure–property–performance–sustainability framework for CCA in cementitious systems.

Level	Main Variables in This Study	Typical Indicators Used Here	Expected Implications for CCA-Based Concrete (Conceptual)
Process	Calcination environment (furnace/open-air); peak temperature; burning duration	Residual mass fraction; colour; qualitative temperature control	Controls extent of devolatilisation and oxidation; sets boundary conditions for ash yield and basic quality.
Structure	Phase assemblage; degree of amorphousness; distribution of major and minor oxides	XRD patterns (amorphous hump vs. crystalline peaks); XRF oxide spectra	Determines amount of reactive silica/alumina, availability of Ca, and presence of potentially harmful alkalis.
Property	Pozzolanic potential; compatibility with cement; durability-related chemistry	SiO2 + Al2O3 + Fe2O3; approximate amorphous fraction; LOI and LOD; K2O and Na2O levels	Governs capacity to consume portlandite, influence on water demand, risk of alkali–silica reaction, and other durability issues.
Performance	Strength development; permeability; shrinkage; resistance to chemical attack	Not measured directly in this work; inferred qualitatively from chemistry and amorphous content	Anticipated effects on compressive strength, long-term durability, and service life of blended cements and concretes.
Sustainability	CO2 emissions; resource efficiency; waste management; economic viability	Qualitatively linked to clinker substitution potential and CC utilisation	Potential reduction in clinker-related emissions, diversion of agricultural waste from open burning, and support for low-carbon construction pathways.

summarises this conceptual chain and clarifies how the measurements reported in the present work occupy the central “structure” and “property” layers, while also providing the evidence base for future performance- and sustainability-oriented studies.

In the current study, only the process, structure, and selected property components of the chain were experimentally measured. Specifically, process variables (calcination environment, temperature, and duration) were controlled or recorded, and structure/chemistry were quantified using XRD (phase evolution and semi-quantitative amorphous trends), XRF (oxide composition), and LOI/LOD with mass-yield observations. By contrast, concrete performance outcomes (e.g., compressive strength, permeability, ASR expansion, or service-life extension) and system-level sustainability impacts (e.g., LCA-based CO₂ reductions) are not directly measured here and are discussed only as conceptual implications based on established SCM literature. Accordingly, any references to performance in this paper should be understood as potential performance inferred from the measured indicators, and the experimental validation of mechanical and durability performance is identified as a priority for future work.

1.3. Theoretical Framework of Surface Chemistry and the Reactivity Principle

The present work is not only an experimental characterisation of CCA but also an application of several established theoretical frameworks from cement chemistry, glass science, durability modelling, and sustainability science. Together, these frameworks justify the chosen variables (temperature, time, environment) and measured indicators (amorphous content, oxide chemistry, LOI/LOD, alkali levels) and provide a basis for interpreting CCA as SCM with potential sustainability benefits.

1.3.1. SCM Reactivity and Dissolution–Precipitation Mechanisms

Current understanding of SCM behaviour views pozzolanic reaction primarily as a dissolution–precipitation process. In a high-pH pore solution, reactive amorphous aluminosilicates dissolve, releasing silicate and aluminate species that subsequently precipitate with calcium to form additional C–S–H, C–A–S–H, and related hydrates [47,48]. The kinetics and extent of this reaction are strongly governed by the amount and structure of the amorphous phase, bulk oxide composition (especially CaO, SiO₂, Al₂O₃ and Fe₂O₃), particle size and surface area, and the alkali and sulphate environment in the pore solution.

Within this general framework, SCMs are often positioned along a spectrum from reactive, low-calcium pozzolans (e.g., silica fume, rice husk ash) to latent hydraulic, calcium-rich SCMs (e.g., blast furnace slag) [47]. Biogenic ashes such as CCA typically fall toward the low-calcium, high-silica end of the spectrum, where their contribution to strength and durability is controlled by the accessibility and solubility of amorphous silica and alumina rather than by intrinsic hydraulicity. Adopting this framework for CCA has two main implications for the present study. First, it motivates a strong focus on the amorphous fraction and combined pozzolanic oxides (SiO₂ + Al₂O₃ + Fe₂O₃) as primary indicators of potential reactivity. Second, it underscores the importance of calcination conditions in determining whether the CCA behaves as a reactive pozzolan or as an essentially inert filler. The experimental programme therefore emphasises systematic variation of temperature and duration, XRD-based assessment of amorphous content, and XRF-based oxide quantification so that CCA can be positioned within the broader SCM reactivity landscape.

1.3.2. Thermodynamic and Kinetic Descriptions of Amorphous–Crystalline Transformations

The evolution of CCA phase assemblage during calcination is governed by a balance between thermodynamic driving forces and kinetic constraints. At lower temperatures, combustion of organics and devolatilisation dominate, leading to the formation of a largely amorphous silica-rich matrix. With increasing temperature and residence time, nucleation and growth of more stable crystalline polymorphs such as quartz, cristobalite, and other silicates become thermodynamically favourable. A complementary concept is Ostwald’s step rule, which states that phase transformations often proceed through a sequence of metastable states rather than directly through the most stable phase [55]. For silica-rich glasses, this implies that an intermediate temperature window may exist in which metastable amorphous or poorly ordered phases are dominant, before more stable crystalline silica forms at higher temperatures or longer times. The Ostwald frameworks suggest that an optimal calcination regime for CCA will be one where combustion and glass formation are largely complete, but the kinetics of crystallisation are still slow enough that a high fraction of amorphous silica is retained.

This theoretical picture provides the rationale for mapping the processing window explored in this study (400–1000 °C, 1–3 h). The experimental data are therefore interpreted within this thermodynamic–kinetic framework.

1.3.3. Alkali–Silica Reaction and Durability Framework

This theoretical framework centres on the alkali–silica reaction (ASR), a deleterious expansion mechanism that occurs when reactive silica in aggregates reacts with alkalis in the pore solution to form a swelling gel. Recent reviews [56,57] describe ASR in terms of three interacting factors: (i) availability of alkalis (Na⁺ + K⁺) in the pore solution, (ii) reactivity of silica present in aggregates or SCMs, and (iii) transport conditions (moisture, temperature, restraint). SCMs can mitigate ASR by consuming portlandite, reducing pore-solution alkalinity, and densifying the microstructure, but they may also exacerbate ASR if they introduce additional reactive silica or alkalis without sufficient binding. CCA is distinctive in that it combines highly reactive amorphous silica with relatively high K₂O contents compared to many other SCMs. Within the ASR framework, this duality creates both an opportunity and a risk. On the one hand, the pozzolanic consumption of Ca(OH)₂ and pore refinement could reduce alkali mobility and mitigate ASR. On the other hand, the total alkali contribution of the binder system may increase if CCA with high K₂O is used at high replacement levels, especially in combination with reactive aggregates. For this reason, the present study tracks K₂O and Na₂O contents explicitly and discusses the resulting alkali equivalent in light of ASR theory, even though direct ASR testing is beyond its scope. The aim is to provide a chemically informed basis for setting prudent replacement levels and for identifying when additional mitigation measures (e.g., low-alkali cement, lithium admixtures, non-reactive aggregates) might be warranted.

2. Research Methodology

The experimental study focused on utilising the Sammaz-16 corn variety to produce CCA. This hybrid variety was selected for its favourable agronomic benefits and uniform physicochemical characteristics. These advantages ensured that the raw material used for experimentation was consistent, facilitating reliable and reproducible comparative analysis. Prior to processing, the CCs were cleaned of debris and were air-dried (23 ± 2 °C for 72 h) to constant mass. Initial feedstock moisture was not independently measured by oven-drying; therefore, the reported ash yields are presented on an air-dry basis [58]. To achieve optimal calcination conditions, a series of controlled experiments were conducted, subjecting CC samples to different burning conditions. Two distinct calcination approaches were investigated, as illustrated in Figure 1. The first approach involved the use of an electric furnace capable of reaching temperatures up to 1100 °C with a power output of 1.8 kW. In contrast, the open-air burning method was carried out at a farmhouse location to simulate a more traditional and accessible means of calcination, particularly for most settings where high-tech equipment might not be available. Open-air samples were placed in a crucible outdoors; temperature varied between 200 and 550 °C (no control); comparisons here are indicative only.

Open-air burning was included to represent low-resource production conditions, but no continuous temperature–time profile was recorded during these burns. Unlike the electric furnace route (fixed setpoint and dwell), open-air combustion progresses through transient flaming and smouldering phases with strong spatial and temporal temperature gradients that depend on wind, pile geometry, oxygen access, and ash/char insulation. Fire-science literature shows that smouldering biomass typically occupies a broad thermal band (often on the order of several hundred degrees Celsius) and can alternate with short-lived higher-temperature flaming episodes, meaning that a single equivalent setpoint is not physically meaningful without logging [59,60]. Therefore, the open-air results in this study are not interpreted as directly temperature-comparable to furnace conditions. Instead, they are discussed as an outcome-based reference route, and any comparisons with furnace samples are made using measured endpoint proxies (mass yield, LOI/LOD, oxide compliance, and XRD hump/peak evolution). To provide a thermal-history context despite the lack of logging, Section 3.1.1 introduces an effective thermal-severity estimate derived by mapping open-air LOI/LOD values onto the controlled furnace LOI/LOD trends. This method was essential for understanding how local and traditional practices influence CCA properties and their viability as an SCM. By incorporating both controlled and uncontrolled calcination techniques, this study aimed to provide a holistic evaluation of CCA production under different environmental conditions.

2.1. Preparation and Experimental Design

Ten CCA samples were prepared, with varying temperature and duration conditions, to assess their impact on the ash properties. The first experimental phase focused on open-air burning, where four CC samples were burned for durations of 2 h, 2.5 h, 3 h, and 3.5 h, respectively. Given the inherent difficulty in controlling temperature during open-air burning, the primary variable under investigation was the duration of combustion. A key consideration was energy efficiency: excessive burning time could lead to unnecessary energy consumption and potential degradation of the ash’s desirable properties, thus undermining our sustainability objectives. Furthermore, the choice of starting from 2 h was substantiated by the literature evidence of Vásquez et al. [61] that the carbonisation process of CC is within two hours of heating. To further assess the influence of temperature on CCA characteristics, a second set of six CC samples underwent controlled calcination in an electric furnace. These samples were subjected to a fixed duration of two hours at temperatures of 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, and 1000 °C, respectively. The temperature selection was informed by prior research, particularly the findings of Aprianti et al. [62], who extensively reviewed SCM production from agricultural waste materials. The decision to maintain a two-hour burning duration was further substantiated by previous studies [25,63], as well as investigations into the thermal decomposition of lignocellulosic biomass, the principal constituent of CC [64,65,66]. These studies indicate that the most significant thermal decomposition of lignocellulosic components occurs within the first two hours of heating, ensuring a high degree of thermal conversion with minimal energy expenditure. Each reported value represents a single measurement on a representative sample from each calcination batch. Standard practice in materials characterisation studies of this scope often reports single measurements when the focus is on comparative trends rather than certification.

2.2. Analytical X-Rays

All prepared CCA samples were analysed in comparison with OPC as a control specimen. Several analytical techniques were employed to characterise the physicochemical and mineralogical properties of CCA.

2.2.1. X-Ray Diffraction

XRD 600. X-ray diffractometer equipped with a copper (Cu) anode source, operating in a theta/2-theta geometry to ensure accurate diffraction data collection. The instrument was configured with a fixed divergence and receiving slit setup and operated in a pre-set scan mode for stepwise data acquisition. The specific diffractometer used was a MiniFlex 300/600+ model with a goniometer radius of 240 mm. The copper anode had a K-Alpha1 wavelength of 1.54060 Å. Approximately 500 mg of finely powdered CCA was placed into a standard XRD sample holder, carefully levelled to achieve a smooth and uniform surface suitable for diffraction analysis. The scan range was set between 5° and 70° 2θ, with a step size of 0.0200°, and each complete scan lasted 40 min.

The amorphous fraction of each CCA sample and the OPC reference were estimated using a semi-quantitative peak/hump area method rather than full Rietveld refinement with an internal standard. This choice was motivated by the exploratory nature of the study and by the availability of a consistent set of scans over a wide range of calcination conditions. Similar peak-area based approaches have been used in screening studies of natural pozzolans and biomass ashes to distinguish relative amorphous contents when full quantitative phase analysis is not feasible [67,68]. For each diffractogram, a smooth background curve was first fitted to the 5–70° 2θ range using the instrument software and checked manually to ensure that it followed the baseline outside the main Bragg peaks. Crystalline peaks were then identified and fitted with pseudo-Voigt functions. The total integrated intensity was separated into (i) the sum of fitted crystalline peak areas and (ii) the residual broad hump area between approximately 15 and 35° 2θ, which corresponds to scattering from disordered silica-rich glass. The amorphous fraction was then approximated as

$$f_{\mathrm{amorphous}}={\displaystyle \frac{A_{\mathrm{hump}}}{A_{\mathrm{hump}}+A_{\mathrm{crystalline}}}}\times 100\%$$

where $A_{\mathrm{hump}}$ is the integrated area of the background-subtracted hump and $A_{\mathrm{crystalline}}$ is the sum of the integrated areas of all resolved crystalline peaks in the scanned range. Because no internal standard was added, this procedure does not provide absolute phase content, but it yields a consistent relative measure of amorphousness across samples processed on the same instrument under identical conditions. On the basis of repeat fits on selected scans and a comparison with typical values reported for similar materials quantified by Rietveld methods (e.g., fly ash and biomass ash; [69,70]), the uncertainty on individual amorphous fractions is estimated to be ±10 percent. The absolute percentages reported therefore represent semi-quantitative indicators of amorphous content, while the trends and relative differences between calcination conditions are regarded as more robust. The resulting diffraction patterns were recorded and analysed to identify the key mineral phases present, providing insight into the pozzolanic potential of the CCA and confirming its suitability for use as an SCM.

2.2.2. X-Ray Fluorescence (XRF) Analysis

XRF analysis was performed to determine the elemental composition of CCA, adhering to ASTM C618-19 standards [71]. The study utilised a Panalytical Epsilon 3 XRF spectrometer (Panalytical, Almelo, The Netherlands). Each CCA sample (4–5 g) was ground to a particle size below 75 μm, dried at 105 °C for 2 h, and mixed with boric acid (9:1 ratio) before being pressed into pellets under 20–30 tons of pressure. The XRF instrument was calibrated using reference materials conforming to DIN EN ISO 12677:2013-02 [72], and the results were reported as oxide weight percentages. To explicitly evaluate alkali contribution and ASR-related risk, the sodium oxide equivalent (Na₂Oeq) of each ash was calculated from the XRF results using

$${\mathrm{Na}}_2{\mathrm{O}}_{\mathrm{e}\mathrm{q}}(\%)={\mathrm{Na}}_2\mathrm{O}(\%)+0.658\times {\mathrm{K}}_2\mathrm{O}(\%)$$

where the factor 0.658 accounts for molecular-mass differences between Na₂O and K₂O as commonly used in cement and concrete durability practice [73]. The reported Na₂Oeq values represent total alkalis by composition; the fraction that becomes available in pore solution depends on binding, dissolution kinetics, and hydration products.

2.3. Loss on Ignition (LOI) and Loss on Drying (LOD) Determination

LOI analysis involved heating approximately 1 g of CCA in a high-temperature furnace at 1025 °C for one hour in an air atmosphere to ensure complete combustion of organic matter and carbonates. The weight loss before and after ignition was used to calculate LOI. Additionally, LOD was assessed by drying CCA at 105 °C for two hours, representing its inherent moisture content. Both LOI and LOD were calculated using the following formula:

$$\mathrm{L}\mathrm{O}\mathrm{I} \mathrm{o}\mathrm{r} \mathrm{L}\mathrm{O}\mathrm{D}={\displaystyle \frac{{\mathrm{W}}_0-{\mathrm{W}}_1}{{\mathrm{W}}_0}}$$

where W₀ = initial weight; W₁ = final weight.

These analytical techniques collectively provided comprehensive insights into the physicochemical characteristics of CCA, ensuring its suitability as an SCM in cement-based applications. Data were analysed to determine the influence of calcination parameters on CCA properties and key metrics compared against OPC benchmarks.

3. Results and Discussion

3.1. Physical Analysis

3.1.1. Open-Air Burning Using Proxy Thermal-Severity Estimates

A limitation of the open-air route is the absence of a recorded temperature–time history, which prevents direct one-to-one comparison with furnace setpoints. However, the present dataset includes outcome variables that are strongly linked to combustion severity and burnout (particularly LOI (residual carbon/organics), LOD (residual moisture), and mass yield), and these can be used to contextualise open-air results relative to the controlled furnace series.

Table 2. Proxy-derived effective thermal-severity estimates for open-air calcination.

Open-Air Condition	LOI (%)	LOD (%)	EFT from LOI (°C)	EFT from LOD (°C)	Interpretation
2 h	10.20	6.80	<400	<400	Very low severity: incomplete burnout and drying; unreliable SCM quality.
2.5 h	8.50	3.60	~433	~546	Moderate severity: drying improves faster than burnout; still variable.
3 h	6.20	2.50	~552	~725	Mixed severity: moisture removal suggests hot episodes, but burnout remains oxygen-limited in zones.
3.5 h	4.81	2.10	~611	~775	Highest severity among open-air runs: approaches low-LOI region, but still heterogeneous.

shows that as open-air duration increases from 2 h to 3.5 h, LOI decreases from 10.20% to 4.81% and LOD decreases from 6.80% to 2.10%, indicating progressively more complete drying and oxidation. To provide an interpretable (but explicitly non-instrumented) thermal history context, we define an effective furnace temperature (EFT) as the furnace setpoint that would yield a similar LOI (or LOD) under the study’s 2 h dwell conditions. EFT is computed by linear interpolation between the two bracketing furnace points:

$$T_{\mathrm{E}\mathrm{F}\mathrm{T}}\left(x\right)=T_1+{\displaystyle \frac{\left(x_1−x\right)}{\left(x_1−x_2\right)}}(T_2-T_1)$$

where $x$ is LOI (or LOD) of the open-air sample and $\left(T_1,x_1\right)$, $\left(T_2,x_2\right)$ are the adjacent furnace measurements. Because open-air burning has different heating rates, oxygen access, and cooling histories than furnace runs, EFT should be interpreted as a severity-equivalence index, not as a measured peak temperature.

From Table 2, the following can be deduced: First, the open-air series spans a broad severity envelope rather than a single temperature. Second, EFT(LOI) and EFT(LOD) diverge at longer durations, which is expected in open burns: LOD can drop rapidly during brief hot episodes, whereas LOI depends strongly on sustained oxygen availability and mixing, so carbon burnout may lag even when sections of the bed reach high temperatures. This behaviour aligns with combustion literature describing alternating flaming/smouldering regimes and spatially heterogeneous temperatures in biomass fuel beds [74].

Table 3. Combustion parameters to obtain CCA samples.

Burning Conditions	Weight of Crucible (g)	Weight of CC and Crucible (g)	Weight of CC Sample (g)	Weight of CCA and Crucible (g)	Weight of CCA Sample (g)	Percentage Mass After Burning
Open air @3.5 h	43	2043	2000	295	252	12.60
Open air @3 h	43	1543	1500	271	228	15.20
Open air @2.5 h	43	1543	1500	311.50	268.50	17.90
Open air @2 h	43	1543	1500	410.50	367.50	24.50
400 °C @2 h	676.93	1076.93	400	828.97	152.04	38.01
500 °C @2 h	676.93	1076.93	400	798.13	121.20	30.30
600 °C @2 h	676.93	1076.93	400	749.73	72.80	18.20
700 °C @2 h	687.42	1187.42	500	730.93	54	10.80
800 °C @2 h	687.42	1187.42	500	705.43	28.50	5.70
1000 °C @2 h	687.42	1187.42	500	678.43	1.50	0.30
Average						19.27

offers an overview of the burning conditions applied to the CC samples, along with the percentage of CCA remaining following the burning process. The CC samples subjected to different durations and temperatures were allowed to cool for 2–3 h in a desiccator before being weighed.

The analysis of weight loss during agricultural residue combustion provides crucial insights into incineration efficiency and ash characteristics. The experimental data demonstrates a relationship between burning conditions and mass reduction, with results varying significantly across different temperature ranges and exposure times. In open-air conditions, the percentage remaining after burning showed a time-dependent pattern, decreasing from 24.50% at 2 h to 12.60% at 3.5 h of exposure. This trend indicates that longer exposure times in open-air conditions promote more complete combustion of organic matter.

When examining controlled temperature conditions, the data reveals a more pronounced effect on mass reduction. Starting at 400 °C, the residual mass was 38.01%, representing the highest yield among all controlled temperature conditions, although it differs slightly from Memon and Khan [25], who reported 28.9%. As temperatures increased, a systematic decrease in residual mass was observed: 30.30% at 500 °C, 18.20% at 600 °C, 10.80% at 700 °C, and dropping dramatically to 5.70% at 800 °C. The most extreme reduction was observed at 1000 °C, where only 0.30% of the original mass remained. This progressive decrease in yield can be attributed to several thermochemical processes: the initial decomposition of organic components (including lignin and cellulose) leads to carbonisation, followed by intensified oxidation at higher temperatures that converts carbonaceous matter into gaseous products [75]. The striking difference between open-air and controlled temperature burning is particularly noteworthy. This can be attributed to the uncontrolled nature of open-air burning, which generally results in fluctuating temperatures. Open-air burning, even at extended durations (3.5 h), only achieved a minimum residual mass of 12.60%, while controlled temperature conditions at 800 °C reduced the mass to 5.70%. This disparity highlights the significance of temperature control in achieving complete combustion. The average residual mass across all conditions was 19.27%, given the wide range of conditions tested. The enhanced efficiency of mass reduction at higher temperatures (particularly above 600 °C) can be attributed to several factors: improved oxygen diffusion, increased reaction kinetics, and a more complete breakdown of resilient organic compounds. The nearly complete mass reduction at 1000 °C (99.70% loss) suggests that virtually all organic matter and most mineral components were either volatilised or transformed at this temperature [68].

A closer inspection of Table 3 reveals an apparent anomaly in the furnace series: the residual mass decreases from 10.8% at 700 °C/2 h (54 g ash from 500 g CC) to 5.7% at 800 °C/2 h (28.5 g ash), i.e., approximately a halving of the recovered ash mass between these two conditions. At the same time, the bulk oxide compositions and LOI/LOD values reported in

Table 4. Chemical composition of CCA and OPC [Normalization Formula: Adjusted Oxide% = (Raw Oxide%/Σ(Oxides excluding LOI)) × 100%].

Sample ID	Data Type	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	CaO (%)	MgO (%)	K2O (%)	Na2O (%)	P2O5 (%)	TiO2 (%)	ZnO (%)	Other (%)	Total (%)	LOI	LOD	SiO2 + Al2O3 + Fe2O3	Remarks
Class-N	Standard	-	-	-	-	≤5.0	-	-	-	-	-	-	-	≤10	≤3.0	≥70	ASTM C618-19 Standard requirements
Open Air @3.5 h	Raw XRF	65.82	6.29	4.41	8.70	3.23	7.10	0.85	2.12	0.02	0.54	0.45	99.80	4.81	2.10	76.52	COMPLIES: Good pozzolanic content, acceptable LOI and LOD, balanced oxides
	Normalized	69.30	6.62	4.64	9.16	3.40	7.47	0.89	2.23	0.02	0.57	0.47	100	-	-	80.56
Open Air @3 h	Raw XRF	64.52	6.09	3.32	8.99	3.40	9.30	0.98	2.39	0.03	0.46	0.51	99.99	6.20	2.50	73.93	COMPLIES: Meets requirements, but high K2O content
	Normalized	68.82	6.50	3.54	9.59	3.63	9.93	1.05	2.55	0.03	0.49	0.54	100	-	-	78.86
Open Air @2.5 h	Raw XRF	63.26	5.82	3.21	9.19	4.56	7.49	1.03	3.48	0.15	0.51	0.99	99.69	8.50	3.60	72.29	PARTIALLY COMPLIES: High LOD, near-limit MgO
	Normalized	69.38	6.38	3.52	10.08	5.00	8.22	1.13	3.82	0.16	0.56	1.09	100	-	-	79.28
Open Air @2 h	Raw XRF	62.19	5.16	4.19	9.49	3.74	8.68	1.81	2.95	0.02	0.36	0.75	99.34	10.20	6.80	70.80	NON-COMPLIANT: Exceeds LOI and LOD limits
	Normalized	69.76	5.79	4.70	10.65	4.20	9.74	2.03	3.31	0.02	0.40	0.84	100	-	-	80.25
400 °C @2 h	Raw XRF	64.87	6.16	3.33	10.76	3.32	6.21	0.97	3.20	0.06	0.10	0.53	99.62	9.00	6.10	74.36	PARTIALLY COMPLIES: High LOD, near-limit LOI
	Normalized	71.58	6.80	3.67	11.87	3.66	6.85	1.07	3.53	0.07	0.11	0.58	100	-	-	82.05
500 °C @2 h	Raw XRF	66.20	6.39	3.58	9.84	3.28	5.90	0.88	2.07	0.02	0.13	0.85	99.14	7.50	4.02	76.17	PARTIALLY COMPLIES: High LOD, good pozzolanic content
	Normalized	72.23	6.97	3.91	10.74	3.58	6.44	0.96	2.26	0.02	0.14	0.93	100	-	-	83.11
600 °C @2 h	Raw XRF	65.79	7.98	4.62	9.19	2.94	4.89	0.71	1.88	0.03	0.17	0.97	99.17	5.00	3.10	72.64	PARTIALLY COMPLIES: Slightly high LOD
	Normalized	69.87	8.48	4.90	9.76	3.12	5.19	0.75	2.00	0.03	0.18	1.03	100	-	-	83.25
700 °C @2 h	Raw XRF	67.09	7.10	4.13	10.91	2.76	4.59	0.69	1.59	0.01	0.14	0.55	99.56	3.20	2.70	78.32	BEST COMPLIANCE: Optimally meets all requirements
	Normalized	69.62	7.37	4.29	11.32	2.86	4.76	0.72	1.65	0.01	0.15	0.57	100	-	-	81.28
800 °C @2 h	Raw XRF	66.04	7.89	5.27	8.03	3.86	3.96	0.79	1.60	0.00	0.20	1.20	98.84	2.70	1.90	79.20	COMPLIES: Excellent values but high Fe2O3
	Normalized	68.71	8.21	5.48	8.35	4.02	4.12	0.82	1.66	0.00	0.21	1.25	100	-	-	82.40
1000 °C @2 h	Raw XRF	66.89	7.43	5.69	8.29	3.09	3.93	0.67	1.90	0.04	0.10	2.21	99.04	1.00	1.50	80.01	COMPLIES BUT NOT RECOMMENDED: Risk of crystallisation
	Normalized	68.24	7.58	5.81	8.46	3.15	4.01	0.68	1.94	0.04	0.10	2.25	100	-	-	81.63
Control	Raw XRF	7.54	1.99	4.51	80.81	0.29	0.14	-	0.02	0.31	0.06	3.48	99.15	2.22	0.30	14.04	REFERENCE: Standard Portland cement composition
	Normalized	7.78	2.05	4.65	83.37	0.30	0.14	0.00	0.02	0.32	0.06	3.59	100	-	-

show only modest changes between 700 °C and 800 °C. On a purely chemical basis, one would therefore not expect such a large change in ash yield over a 100 °C increment, and this discrepancy required an explicit mass-balance analysis. To test whether the drop in yield could be explained by additional removal of moisture, carbon, and selectively volatilised species, the 700 °C and 800 °C data were converted from oxide mass fractions to absolute oxide masses per batch. For the 700 °C sample, 54 g of ash with raw XRF values of 67.09 wt % SiO₂ and 10.91 wt % CaO correspond to approximately 36.2 g of SiO₂ and 5.9 g of CaO, respectively, while the same sample contains only about 2.5 g of K₂O and 0.4 g of Na₂O (Table 4). At 800 °C 28.5 g of ash contains ~18.8 g SiO₂, 2.3 g CaO, 1.1 g K₂O, and 0.23 g Na₂O. Thus, the absolute mass of every major oxide is roughly halved between 700 °C and 800 °C (e.g., SiO₂ ratio = 0.52, CaO = 0.39, K₂O = 0.46), rather than showing selective depletion of alkalis or carbonaceous phases. Even if all of the K-bearing compounds present at 700 °C were volatilised during the 800 °C run, this would account for at most ~2.5 g of the 25.5 g difference in ash mass, and the small reduction in LOI (3.2% → 2.7%) and LOD (2.7% → 1.9%) can together only explain a further ~2–3 g.

The combination of (i) largely unchanged normalised oxide spectra, (ii) similar, low LOI/LOD values, and (iii) a nearly uniform reduction in the absolute mass of all major oxides between 700 °C and 800 °C therefore points to mechanical loss of ash from the crucible as the dominant cause of the lower yield at 800 °C, rather than to unreported chemical volatilisation pathways. This interpretation is consistent with previous work on biomass ashes, where fine, low-density particles generated at higher burn severities are prone to entrainment by convective currents and to spillage during manual handling and desiccator transfer, especially when open crucibles are used and no lids or baffles are present [65,68,75]. In such cases, the recovered ash remains chemically similar to that produced at slightly lower temperatures, but its measured yield represents only a lower bound on the true solid residue.

3.1.2. Mass and Energy Balance Framework

The calcination process can be analysed through fundamental conservation principles. The overall mass balance for CCA production is as follows:

m_CCA = m_CC × (1 − X_volatile) × (1 − X_moisture)

where m represents mass, X_volatile is the fraction of organic matter volatilized, and X_moisture is the moisture fraction removed. The experimental data showing 19.27% average residual mass indicates that approximately 80% of the initial CC mass consists of volatilizable organic compounds and structural water. The energy balance reveals the thermodynamic efficiency of different processing routes. For controlled furnace calcination, the specific energy requirement (E_specific) can be estimated as follows:

E_specific = [m × Cp × ΔT + m_volatile × ΔH_combustion + ΔH_phase_transitions]/m_CCA

where Cp is specific heat capacity, ΔT is temperature change, and ΔH terms represent enthalpies of combustion and phase transitions (dehydroxylation, decarboxylation, crystallization). The dramatic mass reduction at 1000 °C (99.7% loss) indicates that extreme temperatures drive excessive volatilization, reducing both material yield and energy efficiency, which is a critical consideration for industrial-scale implementation. Figure 2 displays the colours of the CCA samples.

The burning temperature significantly influences the colour of CCA, serving as an indicator of its chemical transformation, as shown in Figure 2. When the combustion temperature increases or the duration of burning increases, the ash transitions from a darker, carbon-rich composition to a lighter, more refined material. According to Venkatanarayanan and Rangaraju [76], at temperatures around 400 °C, the ash remains dark due to the presence of unburnt carbon, which negatively impacts pozzolanic activity and durability properties [77]. Additionally, unburnt carbon increases the water demand in cementitious applications, reducing the overall compressive strength due to its porous nature.

In the furnace at 700 °C (2 h), the ash becomes light grey/whitish-grey hue, indicating a reduced carbon content compared to the ash burned at 400 °C for 2 h. As temperatures rise to 800 °C, the ash turns predominantly white with grey spots, suggesting partial over-incineration [78]. Further temperature increments beyond 900 °C introduce light blue and yellow hues, with residual grey and black particles. In open-air burning, a similar lightening is observed only at the longest duration (3.5 h), consistent with the lower residual carbon indicated by LOI.

The observed colour variations correspond to the dominance of different oxides, including potassium, sodium, calcium, and magnesium, as noted in prior research [79]. This demonstrates that the incineration environment, heating rate, and duration influence the final colour of the ash. According to Vempati et al. [80] and Ganesan et al. [81], darker pozzolanic materials contain higher levels of combustible carbon, which can significantly increase the water demand when blended with cement. This increased water requirement may alter the hydration process, ultimately affecting the mechanical properties and durability of the final concrete product.

3.2. X-Rays Analysis

The effectiveness of SCMs depends significantly on their amorphous content, which directly influences their pozzolanic reactivity [40]. This analysis (Figure 3 and Figure 4) evaluates the crystalline and amorphous content of various CCA samples and Portland cement to determine the best candidate for SCM applications.

From Figure 3, the following can be deduced:

• Open air @2 h with peak pattern but slightly higher background hump.
• Open air @2.5 h with sharp, intense peaks with moderate background hump
• Open air @3 h with broad hump and distinct crystalline peaks
• Open air @3.5 h pronounced broad hump and crystalline peaks

From Figure 4, the following can be deduced:

• Controlled burning 400 @2 h with much broader hump and fewer peaks
• Controlled burning 500 @2 h and moderate broad hump with distinct peaks
• Controlled burning 600 @2 h with reduced peak intensities and broader hump
• Controlled burning 700 @2 h with the most pronounced hump and fewer peaks
• Controlled burning 800 °C @2 h with increased crystallinity compared to 700 °C
• Controlled burning 1000 °C @2 h with very sharp, intense peaks and high counts (up to ~15,000)
• OPC sample with the highest crystallinity and very sharp, intense peaks

The XRD patterns revealed that Figure 3 and Figure 4 show a complex thermodynamic landscape that governs the transformation of organic CC constituents into reactive siliceous phases. The observed phase transitions follow a predictable thermodynamic pathway that can be understood through the lens of Ostwald’s rule of stages, where metastable phases form before the thermodynamically stable crystalline phases. Most of the graphs have a broad hump between 15 (° 2θ) and 35 (° 2θ), indicating that the structure is amorphous [82]. Amorphous silica is more desirable than crystalline silica, as it is much more reactive in pozzolanic reactions [83]. The graphs are explained under critical temperature domains, phase boundaries, and microstructural evolution.

Domain I (400–500 °C), Dehydroxylation and amorphisation zone: At 400 °C, the predominant amorphous content estimated from the hump area indicates that the material exists in a kinetically trapped, high-energy state. The broad hump between 15 and 35° 2θ suggests the presence of short-range ordered silicate networks similar to those found in volcanic glasses. This temperature range promotes the formation of metastable siliceous phases that retain high surface energy and reactivity. The absence of very sharp crystalline peaks indicates successful suppression of nucleation events that would lead to crystalline phase formation.

Domain II (600–700 °C), Optimal transformation window: The 700 °C sample exhibits a regime phase equilibrium of a high estimated amorphous fraction (92%) coexisting with minor crystalline contributions. This represents a thermodynamic sweet spot where kinetic limitations balance the driving force for crystallisation. The pattern suggests the formation of nano-crystalline domains embedded within an amorphous matrix, providing optimal reactivity while maintaining structural integrity.

Domain III (800–1000 °C), Crystallisation and deactivation zone: Beyond 800 °C, the system crosses critical energy barriers, leading to irreversible crystalline phase formation. The sharp, intense peaks at 1000 °C (counts reaching ~15,000) indicate the formation of α-cristobalite and α-quartz phases. This crystallisation represents a thermodynamic penalty where the system loses its metastable, high-energy configuration.

Silicate network connectivity analysis: The varying intensity ratios and peak broadening observed across different calcination conditions offer an understanding of the degree of silicate network polymerisation. Lower calcination temperatures favour the formation of isolated silicate tetrahedra (Q⁰) and chain silicates (Q¹, Q²), which are more reactive toward calcium hydroxide. The presence of K₂O and MgO acts as a network modifier, creating non-bridging oxygen sites that enhance reactivity. The Al₂O₃ content suggests tetrahedral Al³⁺ substitution in the silicate network, potentially creating charge imbalances that increase reactivity. The observed phase transitions suggest multi-step kinetic processes with distinct activation energies. The low temperature regime (≤600 °C) is dominated by diffusion-controlled processes where organic matter decomposition creates reactive sites. The intermediate temperature regime (700 °C) is the interface-controlled reaction where silicate network reorganisation occurs without extensive crystallisation, and the high temperature regime (≥800 °C) is the nucleation and growth-controlled crystallisation that reduces reactivity. This dramatic increase in crystallinity between 700 °C and 1000 °C suggests that the apparent activation energy for crystallisation is overcome around 750–800 °C. This thermodynamic transition point represents a critical design parameter for industrial processing.

3.2.1. Analysis of Crystalline and Amorphous Content

The values reported in Figure 5 are based on the semi-quantitative peak/hump area method described in Section 2. The XRD patterns revealed distinct variations in crystallinity and amorphous content across different burning conditions. For the controlled temperature samples, the data show a clear temperature-dependent transformation pattern. At 400 °C, the ash exhibited the highest estimated amorphous content (93%) with minimal crystalline structure (7%), characterised by a pronounced broad hump between 15° and 35° 2θ and a few distinct peaks. This predominantly amorphous nature indicates optimal reactivity potential for pozzolanic applications. Similarly, the 500 °C sample maintained a high amorphous content (87%), though slightly lower than the 400 °C sample. A significant transition was observed as temperatures increased beyond 600 °C. The 600 °C sample showed increased crystallisation (18% crystalline content) while still maintaining a substantial amorphous phase (82%). This marks the beginning of the critical temperature range, where crystalline phases start to develop more prominently. The 700 °C sample showed an interesting reversal to a similarly high amorphous content (92%), suggesting a complex phase transformation process at this temperature. However, the most dramatic transformation occurred at 1000 °C, where the material became predominantly crystalline (70%) with only 30% amorphous content, exhibiting very sharp, intense peaks, with counts reaching approximately 15,000. This extensive crystallisation at higher temperatures aligns with previous research indicating that temperatures above 700 °C promote the formation of crystalline silica phases, which are less desirable for pozzolanic reactions [32].

The open-air burning samples showed different characteristics compared to the controlled temperature samples. The amorphous content decreased with longer burning times, from 79% at 2 h to 67% at 3.5 h. This suggests that extended exposure to open-air burning conditions promotes gradual crystallisation, though not as extensively as high-temperature controlled burning. Studies have consistently shown that SCMs with amorphous content above 70% exhibit high pozzolanic activity (e.g., metakaolin and fly ash). High amorphous content in 400 °C sample, 700 °C sample, and CCA at 3 h aligns with the ASTM C618-19 standard [71], making them ideal candidates.

For comparison, OPC showed the highest crystallinity (83%) among all samples analysed, with only 17% amorphous content. This stark contrast highlights why SCMs with high amorphous content are valuable for enhancing concrete properties through pozzolanic reactions. Portland cement primarily relies on its crystalline phases (e.g., alite, belite) for strength. In SCMs, such high crystallinity (83% in Portland cement) limits their ability to supplement hydration reactions. According to [29], for optimal performance as an SCM, typically the composition should have higher amorphous content (>50% is preferred), lower crystalline content (<40%), and high reactive silica content in amorphous form. This is necessary because the high crystalline content (above 50%) may limit its pozzolanic reactivity. Crystalline silica is less reactive with calcium hydroxide from cement hydration and can result in reduced strength development in concrete/mortar applications [83].

These findings suggest that optimal conditions for producing reactive pozzolanic material from CCA lie in the temperature range of 400–700 °C, where the highest amorphous content is maintained. The results align with previous research on agricultural residue ashes, confirming that lower-temperature processing promotes the formation of reactive amorphous phases essential for pozzolanic activity. In an investigation carried out by Cordeiro et al. [84] on sugar cane bagasse ash, the authors concluded that in the temperature range 400–600 °C, the ash was amorphous. Above 600 °C, due to the increase in temperature, amorphous silica transformed into crystalline silica [84].

• Thermodynamic theory of amorphous phase formation

The pozzolanic reactivity of CCA fundamentally depends on the preservation of metastable amorphous silica phases during thermal processing. According to Ostwald’s Step Rule, crystallization from an amorphous precursor proceeds through sequential formation of increasingly stable phases, with the initial product typically being the least stable polymorph closest in free energy to the parent phase [85]. In the context of CC calcination, organic silica compounds initially decompose into highly disordered, high-energy amorphous silica networks. The challenge lies in maintaining this metastable state while removing carbonaceous material and moisture. The Gibbs free energy difference (ΔG) between amorphous and crystalline silica phases drives the crystallization process, with the activation energy barrier (Ea) determining the rate of transformation. At temperatures below approximately 750 °C, the thermal energy supplied (kT, where k is Boltzmann’s constant and T is the absolute temperature) remains insufficient to overcome the nucleation barrier for extensive crystallite formation. However, as the temperature increases beyond this threshold, the system accumulates sufficient energy to surmount the activation barrier, leading to irreversible crystallization of cristobalite and quartz phases [86]. This explains the observed crystallinity transition between 700 °C and 1000 °C in the present study.

The kinetics of this phase transformation can be described by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation [87]:

α(t) = 1 − exp[−(kt)ⁿ]

where α represents the fraction transformed, t is time, k is the rate constant (temperature-dependent via Arrhenius relationship), and n is the Avrami exponent reflecting nucleation and growth mechanisms. For CCA calcination, maintaining low α values (high amorphous content) requires precise control of both temperature and duration, explaining why the 700 °C/2 h condition yields optimal results.

3.2.2. Chemical Composition from XRF

Table 4 reports the oxide chemistry for CCA and OPC together with LOI, LOD, and the combined SiO₂ + Al₂O₃ + Fe₂O₃ fraction. For each CCA condition, both raw and normalised values are presented. Normalisation follows the stated formula: Adjusted Oxide % = (Raw Oxide % ÷ Σ Oxides excluding LOI) × 100%, which rescales the ignited solid so that total oxides sum to 100%. This is appropriate for assessing compliance with ASTM C618 19 because the specification addresses the chemistry of the reactive ash matrix rather than the as-received mass. Across all processing routes SiO₂ is the major component with Al₂O₃ and Fe₂O₃ as secondary oxides, accompanied by moderate CaO and measurable alkali oxides. This pattern and magnitude agrees with prior CCA and agro residue ash studies that report siliceous matrices with alkali and alkaline earth contents that depend on temperature, hold time, and atmosphere [16,28,88,89,90]. ASTM C618 19 Class N specifies SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70% on an ignited basis, LOI ≤ 10%, and this work applies an internal LOD target ≤ 3%. The raw data show that all CCA conditions meet the combined SiO₂ + Al₂O₃ + Fe₂O₃ requirement except the open air 2 h burn, which registers 70.80% and simultaneously exceeds LOI at 10.20% and LOD at 6.80%. Normalised values clarify intrinsic ash chemistry. Examples include open air 3.5 h, which increases from 76.52% to 80.56%, and 700 °C for 2 h, which increases from 78.32% to 81.28%. These outcomes corroborate the compliance statements recorded in Table 4 and confirm that most candidates satisfy both the chemistry and volatile limits.

The furnace series shows a practical trade-off between maximising amorphous content and retaining usable ash yield. While higher calcination severity can reduce LOI/LOD and shift the phase assemblage, it also reduces recovered ash mass (Table 3), so selecting an “optimal” condition cannot be based on amorphous fraction alone. In this study, it is based on a balanced operating point that lies within the high-amorphous regime indicated by XRD (Figure 5) while still providing a materially higher yield, alongside low volatiles and chemically compliant oxide totals.

3.2.3. XRD-Based Amorphous Trends with Chemistry (XRF)

The bulk oxide composition does not uniquely determine the phase state (amorphous vs. crystalline). Table 4 reports the overall oxide inventory of the recovered ash (e.g., SiO₂, Al₂O₃, CaO, K₂O), but it cannot indicate whether a given oxide is contained in an amorphous glass network or partitioned into discrete crystalline phases. Therefore, an oxide distribution that includes moderate K₂O at 700 °C (4.59% raw; 4.76% normalised) does not by itself imply “partial crystallisation of potassium-bearing phases”; it only indicates the amount of potassium retained in the ash at that condition. In biomass-derived ashes, potassium can be hosted in multiple forms that are chemically similar but structurally distinct, including (i) incorporation as a network modifier within an amorphous silica-rich glass, (ii) presence as minor crystalline salts (commonly KCl/K₂SO₄ depending on fuel chemistry and atmosphere), and/or (iii) formation of minor potassium silicate phases under local melting/sintering conditions. Importantly, these possibilities are not mutually exclusive: a material can be predominantly amorphous overall while still containing small amounts of crystalline K-bearing phases. In such a case, the XRF chemistry remains broadly similar, while XRD shows a dominant amorphous hump with only limited sharp peaks.

This is consistent with the present dataset. First, the controlled furnace series shows only a modest change in K₂O between 700 °C and 800 °C (4.59% → 3.96% raw), while the silica-rich character remains stable (SiO₂ ≈ 66–67%). Second, the XRD patterns for 700 °C are characterised by a strong broad hump and comparatively few intense peaks, whereas crystallisation becomes much more evident at 1000 °C. These observations can be reconciled by recognising that the dominant phase controlling pozzolanic reactivity is the silica-rich amorphous matrix, reflected by the broad hump; and any potassium-bearing crystalline phases at 700 °C, if present, are likely minor relative to the bulk ash and may contribute only weak peaks that can be obscured by background/hump intensity or fall below practical detection limits, particularly when no internal standard/Rietveld refinement is applied. A further practical implication is that potassium chemistry at 700–800 °C should primarily be interpreted through the durability framework rather than used as a proxy for crystallinity.

• Effect of open-air combustion time

The open-air series reveals a clear time-dependent trend. As time decreases from 3.5 h to 2 h, SiO₂ + Al₂O₃ + Fe₂O₃ drops from 76.52% to 70.80%, while LOI rises from 4.81% to 10.20% and LOD rises from 2.10% to 6.80%. These inverse trends indicate the progressive burnout of organics and moisture with longer exposure. K₂O remains comparatively elevated in the open-air set at 7.10 to 9.30%, consistent with the enrichment of alkali salts in biomass ashes and the sensitivity of K release to burn severity and atmosphere that has been reported for similar systems [18,91]. Consequently, the 3.5 h open-air condition develops stronger pozzolanic chemistry and suppresses residual volatiles relative to shorter open-air burns, although its alkali content remains higher than that of controlled calcination.

• Effect of controlled calcination temperature

At a constant time of 2 h, increasing the temperature from 400 °C to 1000 °C raises the raw SiO₂ + Al₂O₃ + Fe₂O₃ from 74.36% to 80.01%. Over the same interval, LOI decreases from 9.00% to 1.00% and LOD decreases from 6.10% to 1.50%, reflecting more complete removal of organics and free water. K₂O decreases from 6.21% at 400 °C to 3.93% at 1000 °C, and Na₂O remains below 1% throughout. These behaviours are consistent with the volatilisation of K bearing salts at higher temperatures and progressive purification of the ash matrix. Literature warns that excessive temperature can trigger crystallisation of SiO₂ rich phases and reduce pozzolanic reactivity, as shown for rice husk ash above about 850 °C [92]. The present dataset echoes this risk through the caution placed on the 1000 °C specimen, which, despite meeting chemical thresholds, is marked not recommended due to crystallisation risk. Temperature therefore improves purity and lowers alkali content, but very high temperature can compromise the amorphous fraction that controls reactivity.

• Alkali oxides and durability

Elevated alkali oxides increase the risk of alkali–silica reaction in concrete. In the open-air set, K₂O exceeds 6% for 2.5 h and 3 h burns and is flagged as a concern in Table 4. In contrast, 500 °C to 800 °C at 2 h maintains K₂O below 6%, which reduces the risk relative to open air conditions of a shorter duration. For context, fly ash often contains total alkali levels of about 2 to 4%, lower than most open air CCA values observed here, whereas thermally treated CCA approaches that range as K is volatilised [93]. The chemistry trends therefore support the selection of the 700 °C to 800 °C window to balance reactivity, durability, and process efficiency. The elevated alkali content in CCA (K₂O: 3.93–9.30%) necessitates theoretical consideration of ASR mechanisms. The classical Powers–Steinour model describes ASR as a two-stage process: (i) alkaline dissolution of reactive silica to form silicate gel and (ii) imbibing of water by the gel leading to expansive pressure [94]. However, recent refinements by Rajabipour et al. [56] emphasised the role of pore solution chemistry, showing that ASR susceptibility depends on the ratio of alkali concentration to calcium availability rather than absolute alkali content.

In CCA-blended systems, the pozzolanic consumption of Ca(OH)₂ modifies pore solution chemistry in complex ways. While CCA introduces additional alkalis, it simultaneously (1) reduces CH concentration through pozzolanic reaction, (2) densifies the microstructure via C-S-H formation, limiting ionic transport, and (3) potentially binds alkalis in secondary hydration products. The net effect on ASR risk requires careful thermodynamic modelling to predict equilibrium phase assemblages and pore solution composition as functions of CCA replacement level and alkali content. The pessimum effect, where intermediate reactive silica contents produce maximum expansion, further complicates ASR prediction. For CCA with 62–67% SiO₂, replacement levels of 15–25% may fall within the pessimum range for certain reactive aggregates, necessitating aggregate-specific testing protocols beyond the scope of this characterization study.

• Alkali equivalent calculations, ASR screening, and recommended replacement levels

To quantify alkali contribution in a standardised form, the total alkalis were converted to Na₂O equivalent using the widely used expression Na₂Oeq = Na₂O + 0.658K₂O.

For example, (Open air 3 h):

                   Na₂Oeq = 0.98 + 0.658 × 9.30
= 0.98 + 6.1194
   = 7.10% Na₂Oeq

Table 5. Calculated Na₂O equivalent of CCA samples.

Sample	Na2O (%)	K2O (%)	Na2Oeq (%)
Open air 3.5 h	0.85	7.10	5.52
Open air 3 h	0.98	9.30	7.10
Open air 2.5 h	1.03	7.49	5.96
Open air 2 h	1.81	8.68	7.52
400 °C (2 h)	0.97	6.21	5.06
500 °C (2 h)	0.88	5.90	4.76
600 °C (2 h)	0.71	4.89	3.93
700 °C (2 h)	0.69	4.59	3.71
800 °C (2 h)	0.79	3.96	3.40
1000 °C (2 h)	0.67	3.93	3.26
OPC control (measured)	~0.00	0.14	0.09

Note: The OPC control in Table 4 is a measured reference for this study; in practice, many cements have higher Na₂Oeq and durability specifications may specify “low-alkali cement” historically defined around ≤0.60% Na₂Oeq [95].

summarises Na₂Oeq for the principal CCA samples (computed from the raw XRF values in Table 4).

Translating Na₂Oeq into binder alkali contribution is replacement-dependent, and because the ASR is driven by the alkali content of the concrete pore solution, a more relevant screening step is to estimate how the alkalis present in CCA alter the blended binder Na₂Oeq at a given replacement ratio r:

$${\mathrm{Na}}_2{\mathrm{O}}_{eq, binder}=(1-r){\mathrm{Na}}_2{\mathrm{O}}_{eq, cement}+r {\mathrm{Na}}_2{\mathrm{O}}_{eq, CCA}$$

This highlights why open-air and lower-temperature ashes are the most ASR-sensitive. Their Na₂Oeq values (5–7.5%) are higher than the 700–800 °C furnace ashes (3.4–3.7%).

Recommended safe replacement levels can be approached through a two-tier process of screening and verification. Alkali content alone does not fully determine the outcome of ASR, since SCMs can mitigate ASR through mechanisms such as pore refinement, alkali binding in hydrates, and the reduction of portlandite [96]. For this reason, performance-based verification methods, including accelerated mortar bar or concrete prism tests, remain standard practice. Nevertheless, the study proposes conservative replacement guidance as follows.

Tier 1—Conservative screening when aggregate reactivity is unknown

Many specifications manage ASR using either (a) low-alkali cement and/or (b) limits on concrete alkali loading (kg Na₂Oeq per m³), because expansion increases strongly with alkali loading. For an illustrative structural concrete binder content of 350 kg/m³ and a cement Na₂Oeq of 0.60% (typical “low alkali” reference), the maximum replacement fraction that keeps alkali loading below 3.0 kg Na₂Oeq/m³ is

$$r_{max}={\displaystyle \frac{\left(\frac{100L_{lim}}{B}\right)-{\mathrm{Na}}_2{\mathrm{O}}_{eq, cement}}{{\mathrm{Na}}_2{\mathrm{O}}_{eq,CCA}-{\mathrm{Na}}_2{\mathrm{O}}_{eq,cement}}}$$

where L_lim = 3.0 kg/m³ and B = 350 kg/m³. (This is a screening calculation; actual limits vary by jurisdiction.)

Using the measured Na₂Oeq values in Table 5, the following screening-level upper bounds are produced:

Open air 3–3.5 h (Na₂Oeq = 5.5–7.1%) → 4–5% replacement

500 °C furnace (Na₂Oeq = 4.76%) → 6% replacement

700 °C furnace (Na₂Oeq = 3.71%) → 8% replacement

800 °C furnace (Na₂Oeq = 3.40%) → 9% replacement

This shows quantitatively why open-air CCA should be treated as a more conservative SCM unless project-specific testing confirms mitigation.

Tier 2—Practical engineering recommendation (with required verification tests)

In many real concretes, replacement levels are not determined solely by alkali chemistry but are instead selected to ensure that ASR expansion remains within accepted limits defined by ASTM C1567 (accelerated mortar bar) [97] and ASTM C1293 (concrete prism) [98], while also meeting strength and workability requirements [99]. Therefore, for practical adoption consistent with typical SCM practice, the preferred CCA for general use is 700–800 °C furnace ashes with Na₂Oeq values of approximately 3.4–3.7%, as these combine high pozzolanic oxides with lower alkalis compared to open-air ashes. A suggested starting replacement level for trials is 10% in a binary blend, with a potential range of 15–30% after acceptable ASR expansion tests depending on cement alkali content, aggregate reactivity, and compliance with ASTM C1567/C1293 limits. High-alkali CCA, such as open-air and 400–500 °C ashes with Na₂Oeq values of about 5–7.5%, should only be used with additional controls, including non-reactive aggregates, lower cement alkali, ternary blending with low-alkali SCMs, or demonstrated compliance in ASTM C1567/C1293. For these high-alkali ashes, suggested starting replacement levels are no more than 5–10%, and high replacement levels should be avoided altogether unless ASR testing confirms safety, particularly when reactive aggregates are present.

• Roles of other oxides and the normalised perspective

CCA contains CaO typically in the range 8 to 12%, while the OPC control contains about 80% CaO, which is expected for a calcium silicate clinker and underscores that CCA functions as a reactive siliceous addition rather than a calcium source. MgO is generally below 5%, with the open air 2.5 h condition near the upper bound noted in Table 4. P₂O₅ appears in the range of 1.6 to 3.5%, characteristic of biomass origin and consistent with prior reports for CCA and related ashes [28,89,90]. TiO₂, ZnO, and the small other fraction are present at trace levels that are unlikely to dominate reactivity, although the glass structure and alkali mobility can be modestly affected. This reinforces the benefit of temperature control in limiting adverse contributions from alkalis while concentrating the siliceous network. Because ASTM C618 19 evaluates chemistry on an ignited basis, normalised values provide the most meaningful cross-sample comparison. Normalisation typically raises SiO₂ + Al₂O₃ + Fe₂O₃ by several percentage points because volatile mass is removed from the denominator. Examples include 700 °C 2 h, which rises from 78.32% to 81.28%; 800 °C 2 h, which rises from 79.20% to 82.40%; and open air 3.5 h, which rises from 76.52% to 80.56%. These confirm that several conditions deliver an ash matrix that comfortably exceeds the 70% threshold, even when raw values are marginal. The open air 2 h sample remains non-compliant on the raw basis due to LOI and LOD.

• Pozzolanic Thresholds

Figure 6 illustrates the evolution of the combined oxide content (SiO₂ + Al₂O₃ + Fe₂O₃) across different burning conditions. The graph demonstrates the optimal peak around 1000 °C, with higher temperatures.

A high content of amorphous silica, alumina, and iron oxides (>70%) is critical for the material to chemically react with calcium hydroxide, forming secondary hydration products (C-S-H). CCA samples from 700 °C (78.32%) and 800 °C (79.20%) exhibit the highest SiO₂ + Al₂O₃ + Fe₂O₃ values, aligning with the ASTM Class N requirement. Studies [100,101] confirm that ash derived SCMs with amorphous SiO₂ > 70% significantly enhance strength and durability in the cement composites. Li et al. [102] and Siddique & Klaus [103] showed that fly ash samples typically with SiO₂ + Al₂O₃ + Fe₂O₃ > 75% are similar to CCA results at 500–800 °C. This suggests that CCA has comparable pozzolanic potential to fly ash, a widely used SCM. Furthermore, Rukzon et al. [104] revealed that rice husk ash achieved SiO₂ + Al₂O₃ + Fe₂O₃ = 85% after thermal treatment at 700 °C, slightly higher than CCA. However, CCA’s performance remains competitive.

• Optimal processing window

Considering SiO₂ + Al₂O₃ + Fe₂O₃, LOI, LOD, and alkali reduction together, the two temperature-controlled conditions are optimal from the XRF viewpoint. At 700 °C for 2 h, the raw combined fraction is 78.32% and the normalised value is 81.28%, with LOI 3.20% and LOD 2.70% and K₂O 4.59%. At 800 °C for 2 h, the raw combined fraction is 79.20%, and the normalised value is 82.40%, with LOI 2.70%, LOD 1.90%, and K₂O 3.96%. Both conditions satisfy ASTM C618 19 Class N chemistry and volatile limits while keeping K₂O below 6%, which supports reactivity and mitigates durability risk. The 1000 °C sample also satisfies the chemistry and volatile limits but is not recommended due to crystallisation risk, in line with Feng et al. [92]. The open air 3.5 h sample meets the chemical criteria with acceptable volatiles but retains higher alkali content and introduces variability inherent in uncontrolled burning. These outcomes align with the cited works, which show that controlled thermal processing improves ash quality, while very short or uncontrolled burns leave excessive organics and alkalis [16,28,88,89,90].

• Comparison with OPC

The chemical composition of CCA differs fundamentally from OPC, highlighting their complementary rather than complete substitutive relationship in concrete applications. OPC contains predominantly calcium oxide (CaO at 80.81%), with minimal silica content (SiO₂ at 7.54%). This composition reflects its primary role as a hydraulic binder through the formation of calcium silicate hydrates (C-S-H) during hydration. In contrast, CCA samples exhibit higher SiO₂ content (62–67%) and lower CaO content (8–11%). This inverse oxide relationship underscores CCA’s function as a pozzolanic material rather than a total cement replacement. A CaO value of 13% was obtained by Saloni et al. [91], which is higher than the current study. The aluminium oxide (Al₂O₃) content in CCA samples (5.16–7.98%) significantly exceeds that of Portland cement (1.99%). Higher alumina content in SCMs typically contributes to enhanced sulphate resistance and reduced heat of hydration in blended cements [105]. This characteristic makes CCA particularly valuable for mass concrete applications, where thermal cracking is a concern. Similarly, the iron oxide (Fe₂O₃) content in CCA samples (3.21–5.69%) exceeds the Portland cement value (4.51%), though the difference is less pronounced than with other major oxides. CCA samples contain higher MgO (2.76–4.56%) compared to Portland cement (0.29%). While excessive MgO in Portland cement (>5%) can lead to expansion and instability due to delayed hydration of periclase, the MgO in pozzolanic materials like CCA behaves differently. Research suggests that in the pozzolanic matrix, MgO can contribute to the formation of hydrotalcite-like phases that can enhance mechanical properties and chloride binding capacity, potentially improving durability against chloride-induced corrosion [106,107].

Perhaps the most striking difference lies in the alkali content (K₂O and Na₂O). CCA samples contain higher potassium oxide (3.93–9.30% versus 0.14% in Portland cement) and sodium oxide (0.67–1.81% versus negligible amounts in Portland cement), indicating their organic origin. This elevated alkali content raises considerations for ASR potential but may also accelerate pozzolanic reactions by increasing pore solution alkalinity. Modern concrete practice often involves using CCA as a partial replacement (typically 10–30%) of Portland cement [108], which dilutes these alkali concentrations in the final blended cement.

The phosphorus pentoxide (P₂O₅) content represents another significant difference, with CCA samples containing 1.59–3.48% compared to just 0.02% in Portland cement. Phosphorus compounds are known to influence cement hydration kinetics, potentially retarding early strength development but contributing to long-term strength and durability. This aspect of CCA chemistry has received limited attention in published literature and warrants further investigation, particularly regarding optimal replacement ratios that balance early strength requirements with long-term performance benefits. The trace element composition also differs between CCA and Portland cement. CCA samples contain higher zinc oxide (ZnO) content (0.10–0.54% versus 0.06% in Portland cement) and titanium dioxide (TiO₂) in varying amounts. While these elements occur in relatively small quantities, they can influence cement hydration kinetics, with zinc potentially causing initial retardation, followed by accelerated strength development. The “Other” category, representing minor oxides and trace elements, is higher in CCA (0.45–2.21%) than in Portland cement (3.48%), though this includes sulphur trioxide, which is separately reported in some literature references [26,109]. Portland cement hydration produces approximately 19–25% calcium hydroxide by volume, which does not contribute significantly to strength and represents a vulnerable phase in chemical attack [110]. The pozzolanic reaction of CCA consumes this calcium hydroxide, replacing it with an additional C-S-H gel that has superior mechanical and durability properties. This transformation explains why properly proportioned CCA-blended cements typically demonstrate enhanced resistance to sulphate attack, chloride penetration, and alkali–silica reaction compared to pure Portland cement concretes.

Compared with published literature, CCA samples show SiO₂ content ranging from 62.19% to 67.09%, which aligns well with values reported by Desai [111] at 62.30%, Adesanya & Raheem [16] at 66.38%, Memon et al. [89] at 63.73%, Olonade et al. [112] at 65.40%, and Owolabi et al. [28] at 64.90%. However, they differ significantly from some studies, such as Kamau et al. [26], who reported only 38.8%, and Udoeyo and Abubakar [113], who found remarkably higher values at 79.29%. These variations likely stem from differences in growing conditions and processing methods. Aluminium oxide values (5.16–7.98%) moderately align with several studies but are lower than those reported by Memon et al. [89] and Owolabi et al. [28], 15.05% and 10.79%, respectively. Calcium oxide content in CCA samples (8.03–10.91%) shows good agreement with multiple studies but is higher than values reported by Kamau et al. [26] and Olonade et al. [112]. A distinctive feature of this study’s CCA samples is the relatively high potassium oxide content (3.93–9.30%), which exceeds most literature values except for the extremely high values reported by Kamau et al. [26] and Saloni et al. [91]. This high K₂O content could potentially lead to alkali–silica reaction concerns in concrete applications and warrants careful consideration in mix design and durability testing. The phosphorus content in CCA samples (1.59–3.48%) is another noteworthy finding, as P₂O₅ is rarely reported in CCA literature, except by Memon and Khan [25] and Suwanmaneechot et al. [18]. The presence of phosphorus could influence cement hydration kinetics and strength development in ways that merit further investigation.

Figure 7 shows the progressive decrease in both LOI and LOD with increasing temperature and burning duration. These trends clearly demonstrate the more effective removal of volatile components and moisture under controlled temperature conditions compared to open-air burning. LOI represents the removal of organic matter, moisture, and carbonates during heating. High LOI values (>10%) indicate incomplete combustion, which can hinder performance. Samples such as CCA 700 °C (3.20%) and CCA 800 °C (2.70%) demonstrate effective combustion, improving compatibility with cement. Open-air samples (2–3.5 h) show high LOI values (up to 10.20%), reflecting residual carbon and organic impurities. Skibsted and Snellings [48] demonstrated that SCMs with LOI > 10% often contain residual carbon, reducing reactivity. Open-air samples like 2 h (10.20%) align with this observation, while CCA 400–800 °C meet the ideal range. Siddique [73] found LOI values for fly ash at 5–8%, comparable to CCA 500–600 °C (5.00–7.50%), confirming CCA’s suitability. Onyenokporo et al. [40] found LOI < 5% for thermally treated rice husk ash, similar to CCA 700 °C (3.20%) and CCA 800 °C (2.70%), demonstrating efficient combustion and minimal organic impurities. LOI values for most of the optimally processed CCA samples (2.70–5.00% for temperature-controlled samples above 600 °C) are comparable to the Portland cement value (2.22%). This similarity indicates the effective removal of organic matter and carbon content during the calcination process, which is crucial for ensuring consistent performance in concrete applications. Excessive carbon content, as seen in the samples with higher LOI values (especially the open-air samples with shorter durations), can interfere with air-entraining admixtures and affect concrete workability and freeze-thaw resistance.

Figure 8 summarises the key measured indicators across all calcination routes, and

Table 6. Recommended CCA processing routes and quality control indicators.

Route Type	Suggested Operating Window	Primary Quality Control Indicators	Suitable Contexts	Key Caveats
Controlled electric furnace (baseline)	700 °C, 2 h dwell, followed by natural cooling	LOI ≤ 5%; light grey/whitish colour; SiO2 + Al2O3 + Fe2O3 > 70%; K2O in moderate range	Industrial or institutional facilities with access to programmable furnaces	Requires access to reliable electricity and furnace capacity; still needs downstream mechanical and durability validation.
Controlled electric furnace (upper bound)	800 °C, 2 h dwell, followed by natural cooling	Very low LOI; high combined pozzolanic oxides; K2O typically lower than at 700 °C	As above, where slightly higher energy input is acceptable and crystallisation is monitored	Closer to onset of significant silica crystallisation; long-term reactivity should be confirmed before standard deployment.
Extended open-air burning	3–3.5 h burn, small to medium, well-ventilated piles; cooling in air	Substantially reduce LOI relative to shorter burns; colour transition to lighter grey; periodic XRF where available	Rural or low-resource settings without furnaces; small-scale producers supplying local projects	Greater batch-to-batch variability; higher alkali content; likely more conservative cement replacement levels and tighter project-specific testing required.

shows the experimental recommendation. Within the furnace series, 700 °C for 2 h provides the most balanced window, combining a high estimated amorphous fraction, high SiO₂ + Al₂O₃ + Fe₂O₃, comparatively lower K₂O than open-air and ≤600 °C ashes, and low LOI/LOD, while maintaining a practical ash yield relative to 800 °C.

3.3. Implications and UN SDG Integration

The identification of 700 °C for 2 h as the most favourable calcination condition for CCA aligns well with broader mechanistic insights on biomass-derived SCMs. Reviews of agricultural biomass ashes indicate that optimal reactivity is generally obtained when the ash contains a high fraction of disordered silica with network-modifying cations (e.g., K⁺, Na⁺, Ca²⁺, Mg²⁺) in solid solution, but before extensive crystallisation of quartz, cristobalite, or other stable polymorphs occurs [50,114]. Within this regime, the glassy phase readily dissolves in the high-pH pore solution of hydrating cement, releasing silicate and aluminate species that react with calcium hydroxide to form additional C–S–H and related hydrates, thereby refining the microstructure of the paste and enhancing later age strength [47,48].

The present results suggest that the 700 °C/2 h condition for CCA lies close to this optimum, as it combines high amorphous content with low LOI and moderated alkali levels. By contrast, the open-air and very high-temperature furnace treatments represent two distinct types of deviation from the optimum window that have also been highlighted in recent biomass-ash reviews. Under-fired ashes with high LOI often contain residual carbon and incompletely decomposed organics that interfere with admixture efficiency and increase water demand, whereas over-fired ashes exhibit higher crystallinity and reduced reactivity, even when their combined SiO₂ + Al₂O₃ + Fe₂O₃ content appears favourable on a purely chemical basis [49,51]. The systematic trends observed in this study therefore reinforce the view that performance discrepancies reported in previous CCA studies are primarily process-driven rather than inherent to the CC feedstock and that careful specification of calcination conditions is essential to realise the full pozzolanic potential of this material [14,50].

3.3.1. Circular Economy and Industrial Ecology Analysis

The optimisation of CCA production demonstrated in this study has implications that extend beyond materials characterisation, contributing directly to the broader sustainability agenda articulated in the 2030 Agenda for Sustainable Development [115] and recent climate-focused roadmaps for the cement sector [5,52]. Global assessments indicate that cement manufacture contributes around 8% of anthropogenic CO₂ emissions, with clinker production being the dominant source [52]. Clinker substitution with suitably reactive SCMs is consistently identified as one of the most effective short- to medium-term mitigation measures for the concrete value chain, provided that performance requirements are maintained [6]. By defining a process window in which CCA satisfies ASTM C618 chemical criteria and maximises amorphous content, the present study provides foundational data for deploying CCA as a technically robust SCM that can contribute to these decarbonisation efforts. From a systems perspective, CCA valorisation embodies principles of industrial ecology specifically, the concept of industrial symbiosis, where waste from one process becomes feedstock for another [116]. The theoretical foundation rests on material flow analysis (MFA) theory. Brunner and Rechberger’s MFA framework [117] provides a systematic approach to tracking material flows through economic systems. Applying this to CCA production:

• Input: 1.2 billion tonne corn production annually → 180 million tonne CC waste (assuming a 15% cob-to-grain ratio) [118]
• Conversion: At 19% calcination yield, potential CCA production reaches 34 million tonnes annually
• Displacement: At 20% OPC replacement, CCA could displace 6.8 million tonne cement clinker annually, avoiding 5.8 million tonne CO₂ (assuming 0.85 tCO₂/t clinker).

The clinker-substitution benefits discussed here are based on a linear mass-replacement assumption (i.e., X% CCA replaces X% cement) and therefore represent first-order screening estimates. In practice, the net sustainability benefit can deviate from linearity because SCMs may alter binder efficiency, particle packing density, water demand, and ultimately mechanical performance and durability, which can change the cement content required to meet a given strength class. The system boundary in CO₂-reduction figures reported is limited to potentially avoided clinker-related emissions from SCM substitution, using a single clinker emission factor (here $E{F}_{cl}=0.85 {\mathrm{t}\mathrm{C}\mathrm{O}}_2/\mathrm{t} \mathrm{c}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{k}\mathrm{e}\mathrm{r}$) and the measured/assumed ash-yield and replacement ratios; it excludes transport, grinding, process electricity mix, changes in mix design (binder efficiency), and performance/durability effects that could alter cement demand. The avoided emissions estimate is therefore expressed transparently as

$${\mathrm{C}\mathrm{O}}_{2 avoided}=M_{CC} f_{rec} y_{ash} r_{rep} k_{cl} E{F}_{cl}$$

where $M_{CC}$ is available corncob mass, $f_{rec}$ is recoverable fraction, $y_{ash}$ is ash yield, $r_{rep}$ is cement replacement, and $k_{cl}$ is clinker-to-cement ratio. A simple sensitivity check (e.g., varying $y_{ash}$, $r_{rep}$, $k_{cl}$, and $E{F}_{cl}$ within plausible ranges) can shift outcomes by roughly a factor of ~2, so all large-scale impacts are stated cautiously as on the order of illustrative ranges rather than as expected or guaranteed savings.

3.3.2. Alignment with United Nations Sustainable Development Goals

The valorisation of CCA as an SCM directly contributes to multiple UN SDGs adopted in 2015 as part of the 2030 Agenda for Sustainable Development. This section explicitly maps the research outcomes to relevant SDG targets, demonstrating the multi-dimensional sustainability impact beyond technical performance metrics. The optimised CCA production and utilisation could align with selected SDGs and their key targets, highlighting pathways for both local and global impact. While the present study does not quantify these impacts through life-cycle assessment, the compositional and mineralogical evidence provided here defines the technical conditions under which such assessments would be meaningful.

• SDG 9: Industry, Innovation, and Infrastructure

Target 9.4: “By 2030, upgrade infrastructure and retrofit industries to make them sustainable, with increased resource-use efficiency and greater adoption of clean and environmentally sound technologies.” CCA integration into cement production exemplifies sustainable industrial innovation through the following:

(a) Resource efficiency metrics: The present research demonstrates that CCA can satisfy ASTM C618-19 pozzolanic requirements, enabling cement replacement ratios of 10–30%, as documented in related studies [31]. Global cement production is currently in the order of 4.0–4.2 Gt/year. The global average clinker-to-cement ratio is about 0.75 t clinker/t cement, depending on the region [119]. Given

$$\begin{gathered} \mathrm{C}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} P_c=4.1{\displaystyle \frac{Gt}{year}} \\ \mathrm{C}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{k}\mathrm{e}\mathrm{r} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o} r_{cl}=0.75 t{\displaystyle \frac{clinker}{t}}cement \end{gathered}$$

Raw material requirement = 1.6–1.7 t of raw meal per tonne of cement dominated by limestone, plus clay/shale [120]. Assume that CCA enables an average of 10% cement mass replacement across the global market. Then, the annual cement mass displaced is:

$$M_{cem,disp}=0.10\times P_c=0.10\times 4.1=0.41 \mathrm{Gt} \mathrm{cement}/\mathrm{year}$$

The associated clinker displacement is

$$M_{cl,disp}=M_{cem,disp}\times r_{cl}=0.41\times 0.75=0.31 \mathrm{Gt} \mathrm{clinker}/\mathrm{year}$$

Using a representative raw-material factor of 1.65 t limestone + 0.35 t clay per tonne cement (1.7 t raw per tonne cement), the displaced limestone and clay are as follows:

$$\begin{gathered} \mathrm{L}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{e} \mathrm{s}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{d} M_{\mathrm{limestone}}=0.41 \mathrm{Gt}\times 1.5 \mathrm{t} \mathrm{limestone}/\mathrm{t} \mathrm{cement} = 615 \mathrm{Mt}/\mathrm{year} \\ \mathrm{C}\mathrm{l}\mathrm{a}\mathrm{y} \mathrm{s}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{d} M_{\mathrm{clay}}=0.41 \mathrm{Gt}\times 0.3 \mathrm{t} \mathrm{clay}/\mathrm{t} \mathrm{cement} = 123 \mathrm{Mt}/\mathrm{year} \end{gathered}$$

For quarry land spared, raw material extraction rates vary widely, but values on the order of 3–4 Mt of limestone per km² over the life of a quarry are reasonable for typical deposit thicknesses and recovery factors in large open-pit operations. Taking a mid-point of 3.5 Mt/km² as an illustrative density, the annual limestone saving of 615 Mt corresponds to

$$A_{\mathrm{quarry}}={\displaystyle \frac{615 \mathrm{Mt}}{3.5 {\mathrm{Mt}/\mathrm{km}}^2}}=175 {\mathrm{km}}^2 \mathrm{of} \mathrm{quarry} \mathrm{area} \mathrm{deferred}$$

(b) Clean technology implementation: The thermal energy intensity of modern dry-process clinker kilns is around 3.2–3.6 GJ per tonne clinker, with best-available technology approaching 3.0 GJ/t [119]. Including electricity, the overall energy demand for cement production is typically 3.4–4.0 GJ/t cement [121]. By contrast, calcining silica-rich biomass ash at 700 °C has a much lower energy requirement than producing clinker at 1450 °C. For a simple heat balance, according to [122]

$$\begin{gathered} \mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c} \mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{r}\mathrm{y} \mathrm{b}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s} c_p=1.3–1.6 \mathrm{kJ}/\mathrm{kgK} \\ \mathrm{T}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{r}\mathrm{i}\mathrm{s}\mathrm{e}\mathsf{\Delta }T=25\to 700 °\mathrm{C}\Rightarrow 675 \mathrm{K} \end{gathered}$$

Neglecting moisture and furnace losses for a first estimate, heating 1 tonne of dry CC requires: $q=c_p\times m\times \mathsf{\Delta }T=1.5 \mathrm{kJ}/\mathrm{kgK} \times 1000 \mathrm{kg}\times 675 \mathrm{K}=1\mathrm{GJ}$.

Allowing 20–40% additional energy for furnace and system losses, a realistic process energy demand of 1.2–1.5 GJ/t of CCA is plausible. Roughly half or less of the specific energy needed for clinker production (3.2–4.0 GJ/t). When calcination is powered by renewable energy or biomass combustion (recovering chemical energy from the CC itself), the net energy footprint approaches zero, contrasting with cement’s energy-intensive rotary kiln process requiring peak temperatures of 1450 °C.

(c) Infrastructure resilience: Pozzolanic CCA can contribute to lower portlandite content, refined pore structure, and better sulphate and chloride resistance. Reviews on durability and service life prediction indicate that SCM-based concretes can deliver 20–40% service life extension relative to plain OPC mixtures in aggressive environments, especially for chloride-induced corrosion and sulphate attacks [123]. Life-cycle cost analyses typically show a 20–30% increase in service life can translate into a 15–25% reduction in initial construction costs. For developing economies with limited infrastructure maintenance capacity, durability enhancement provides compounding value through deferred reconstruction needs [124].

• SDG 12: Responsible Consumption and Production

Targets 12.2, 12.4, 12.5: Sustainable management and efficient use of natural resources; environmentally sound management of wastes; and substantial reduction of waste generation through prevention, reduction, recycling, and reuse. The CCA production pathway embodies circular economic principles by transforming agricultural residue (currently managed through open burning or landfilling in most corn-producing regions) into a value-added industrial feedstock.

(a) Waste valorisation quantification: Global corn production has reached 1.2 billion tonnes/year. Studies on corn biomass partitioning report cob-to-grain mass ratios typically in the 10–20% range, with Hong et al.’s [118] experiments around 15%.

CC residue generated $M_{\mathrm{cob}}=1.2 \mathrm{Gt} \mathrm{grain}\times 0.15=0.18 \mathrm{Gt} \mathrm{corncobs}/\mathrm{year}$. Though ash yield may be variable and highly dependent on calcination conditions, the current study experimental programme, optimized 700 °C/2 h calcination produced a residual mass of ≈10.8% of the original CC, which is an upper-bound scenario relative to typical open burning. To remain conservative, we take an ash yield of 10% of CC mass. Therefore, converting 20% of the global CC stream to CCA:

$$M_{\mathrm{cob},\mathrm{used}}=0.20\times \left(180\right) \mathrm{Mt}=36 \mathrm{Mt} \mathrm{corncobs}$$

$$M_{\mathrm{CCA}}=M_{\mathrm{cob},\mathrm{used}}\times \left(0.10\right)=3.6 \mathrm{Mt} \mathrm{CCA}/\mathrm{year}$$

Hence, depending on actual ash yield and process efficiency the calculations suggest that valorising 20% of global CC residues could supply roughly 3.6 Mt/year of CCA, assuming 10% ash yield under optimized calcination.

(b) Avoided emissions from residue burning: Open field burning of crop residues is an important regional source of CO₂, CH₄, N₂O, particulate matter, and black carbon [125]. In many maize-producing regions, 40–50% of residues may be burned on field, with the remainder left to decompose or used in minor applications [126]. Lifecycle and field measurements for crop residue burning (e.g., corn stover, rice straw) indicate CO₂-equivalent emissions of roughly 1.5–2.5 tCO₂e per tonne residue burned when CH₄, N₂O and black carbon are included [127,128]. If 36 Mt of CCs were diverted from open burning,

$$\mathrm{Avoided} {\mathrm{CO}}_2\mathrm{e}=\left(36\right) \mathrm{Mt}\times (1.5–2.0) {\mathrm{tCO}}_2\mathrm{e}/\mathrm{t}=54–90 \mathrm{Mt} {\mathrm{CO}}_2\mathrm{e}/\mathrm{year}$$

Therefore 54–90 Mt CO₂e/year can be prevented from burning (including black carbon and CH₄ emissions), reducing agricultural waste management costs estimated at £23–46 per tonne disposal [129,130].

(c) Chemical safety enhancement: Unlike some industrial pozzolans (e.g., silica fume from metallurgical processes containing trace heavy metals), agricultural ashes present minimal toxicological risk. The XRF analysis confirms the absence of hazardous elements above trace levels, with primary constituents (SiO₂, Al₂O₃, K₂O, CaO) representing low-toxicity mineral oxides. This contrasts favourably with certain industrial wastes that require elaborate stabilization protocols before incorporation into construction materials.

(d) Extended producer responsibility: CCA valorisation creates economic incentives for agricultural stakeholders to implement structured residue collection systems. In jurisdictions implementing extended producer responsibility (EPR) frameworks for agricultural waste, CCA markets provide compliance pathways while generating revenue streams. Economic analysis suggests farm-gate prices of GBP 12–19 per tonne for cleaned, dried CCs become viable when processing infrastructure achieves economies of scale (>50,000 tonnes annual throughput).

• SDG 13: Climate Action

Target 13.2: “Integrate climate change measures into national policies, strategies and planning.” Cement production contributes approximately 8% of global anthropogenic CO₂ emissions (~3.0–3.5 Gt CO₂ annually), comprising 60% from limestone calcination (CaCO₃ → CaO + CO₂)—an unavoidable chemical process, and 40% from fossil fuel combustion in kilns.

(a) Per-tonne binder mitigation: If CCA replaces 20% of cement mass in a blended binder [131], we neglect the (small) emissions from CCA production because that has been balanced by the CO₂ capture of the corn plant during its growth phase. The avoided emissions per tonne of blended binder are

$$E_{cem}=0.8 {\mathrm{tCO}}_2/\mathrm{t} \mathrm{cement}$$

$$\mathsf{\Delta }{E}_{\mathrm{binder}}=0.20\times E_{cem}=0.20\times 0.8=0.16 {\mathrm{tCO}}_2/\mathrm{t} \mathrm{binder}$$

On a material basis, 1 t of CCA used at 20% replacement displaces 5 t of cement and avoids on the order of 0.8 tCO₂ per tonne of CCA, before accounting for emissions from CCA production.

(b) Global mitigation range: If all cement produced globally adopted only a 2% average CCA replacement ($R=0.02$):

$$\mathsf{\Delta }{E}_{\mathrm{global}}=P_c\times R\times E_{cem}=4.1 \mathrm{Gt}\times 0.02\times 0.8=0.066 \mathrm{Gt} {\mathrm{CO}}_2/\mathrm{year}$$

66 Mt CO₂/year avoided corresponds to a 2% effective global adoption, making the assumption explicit and modest.

(c) Emissions from CCA calcination and net balance: For CCA production, if we assume:

Process energy demand $E_{CCA}=1.2–1.5 \mathrm{GJ}/\mathrm{t} \mathrm{CCA}$ (as estimated earlier)

Fossil fuel emission factor $f_{fossil}=70 \mathrm{kg} {\mathrm{CO}}_2/\mathrm{GJ}$ (coal/oil mix), then fossil fuel-based CCA emits:

$$E_{\mathrm{CCA},\mathrm{fossil}}=E_{CCA}\times f_{fossil}=\left(1.2–1.5\right) \mathrm{GJ}/\mathrm{t}\times 0.07 {\mathrm{tCO}}_2/\mathrm{GJ}=0.08–0.11 {\mathrm{tCO}}_2/\mathrm{t} \mathrm{CCA}$$

If biomass or renewable energy is used, IPCC accounting treats biogenic CO₂ as carbon-neutral; only ancillary fossil inputs (e.g., electricity from fossil grids) contribute, typically <0.05 tCO₂/t CCA. So, the net balances can be re-expressed more clearly as follows:

Best case (renewable/biomass energy): Avoided = 0.8 tCO₂/t CCA − <0.05 tCO₂/t CCA = 0.75 tCO₂/t CCA net reduction.

Fossil fuelled case: Avoided = 0.8 − 0.08 − 0.11 = 0.7 − 0.72 tCO₂/t CCA net reduction. Even with conservative numbers, the net impact remains strongly favourable.

(d) Biogenic carbon sequestration: CCs represent captured atmospheric CO₂ via photosynthesis. When CCs are incorporated into long-lived concrete structures (typical service life 50–100 years), the biogenic carbon in residual unburned material (LOI component) remains sequestered. At 3–5% LOI for optimal CCA, and assuming 50% carbon content in organic residue, this represents ~0.055–0.092 tCO₂-equivalent sequestration per tonne CCA for the infrastructure service life, a modest but additional climate benefit not captured in standard LCA boundaries. For countries prioritizing climate-resilient infrastructure (SDG 13 Target 13.1), pozzolanic concrete formulations offer enhanced resistance to climate change-exacerbated degradation mechanisms. Improved sulphate resistance in coastal zones facing sea-level rise and saltwater intrusion. Reduced thermal cracking in regions experiencing temperature extremes and enhanced carbonation resistance, extending structural durability under elevated atmospheric CO₂. These technical benefits translate to reduced reconstruction frequency, lowering the cumulative embodied carbon of the built environment, a critical factor, as global infrastructure stocks are projected to double by 2050.

• SDG 2: Zero Hunger (Indirect Contribution)

Target 2.4: Resilient agricultural practices that help maintain ecosystems and progressively improve land and soil quality. While not immediately obvious, CCA valorisation contributes to agricultural sustainability. The current practice of burning crop residues degrades soil organic matter, volatilizes nutrients (N, S, P), and disrupts soil microbial communities. Studies indicate that repeated burning reduces top-soil organic carbon by 0.3–0.5% over 5-year cycles [132], impairing long-term productivity. Structured residue removal for CCA production incentivizes alternative disposal, though careful management is required to avoid excessive biomass extraction that would also deplete soil amendments. CCA containing 1.5–3.5% P₂O₅ and 4–9% K₂O represents a potential fertilizer supplement when applied to agricultural soils after use in concrete (at end-of-structure life, crushed concrete can be land-applied). While this pathway requires further research on leaching kinetics and bioavailability, it creates potential for nutrient circularity, closing the P and K loops in agricultural systems.

Also, creating rural income streams from agricultural residue valorisation improves farm economic resilience, particularly for smallholder farmers in Sub-Saharan Africa, South Asia, and Latin America where corn cultivation dominates but farmer incomes remain precarious. Economic modelling suggests residue collection networks could generate annual supplemental income, representing 5–15% additions to typical smallholder farm revenues.

• SDG 11: Sustainable Cities and Communities

Target 11.6: Reduce the adverse per capita environmental impact of cities, with special attention to air quality and municipal waste management. Cement plants near urban areas contribute to local PM_2.5/PM₁₀, NO_x and SO₂ burdens, particularly where emission controls are suboptimal [133]. If CCA-based SCMs enable 20% clinker substitution at a given plant, and assuming linear scaling for emissions directly linked to kiln output: Particulate emissions (dust from kiln and raw mills) could decline by roughly 15–20%, after accounting for non-kiln sources (e.g., milling, handling). NO_x emissions formed at high flame temperatures would similarly fall by 12–18%.

For health valuation, “benefit-per-ton” studies from Europe and North America commonly assign USD 50,000–300,000 of health benefits per tonne PM_2.5 avoided, depending on population density and income [134,135]. If a 1 Mt/year cement plant reduces its PM_2.5 emissions by 150–200 t/year due to clinker substitution and efficiency gains, the annual health benefit is on the order of

$$\left(150–200\right) \mathrm{t}/\mathrm{year}\times \left(\mathrm{50,000}–\mathrm{200,000}\right) \mathrm{USD}/\mathrm{t}=7.5–40 \mathrm{million} \mathrm{USD}/\mathrm{year}$$

Therefore, 20% CCA substitution avoiding 150–200 tonnes PM_2.5 annually represents USD 7.5–40 million in health benefits over facility lifetime.

• Cross-cutting enablers: SDG 17 (Partnerships for the Goals)

Realizing CCA’s sustainability potential requires multi-stakeholder collaboration. This study’s establishment of optimal processing parameters enables technology transfer from laboratory to industrial demonstration scale. Pilot projects in corn-producing regions (U.S. Midwest, Brazilian Cerrado, and Sub-Saharan Africa maize belt) require partnerships between agricultural cooperatives, cement manufacturers, and research institutions to validate performance at scale and develop region-specific supply chain logistics. Also, CCA integration necessitates coordinated policy across agriculture (residue management), industry (cement standards), environment (air quality, waste), and trade (harmonised material specifications). International standards organisations (ISO, ASTM, and CEN) play critical roles in developing consensus specifications that facilitate global CCA market development while ensuring quality control.

Development finance institutions (World Bank, regional development banks) and climate funds (Green Climate Fund) represent potential funding sources for CCA infrastructure in developing economies. Concessional finance mechanisms could overcome barriers to initial capital investment in calcination facilities and quality control laboratories, particularly in least-developed countries where cement demand growth is most rapid but capital availability is constrained.

4. Conclusions

This study systematically examined how calcination environment, temperature, and duration govern the properties of CCA and, by implication, its suitability as sustainable SCM. Ten CCA samples were produced under open-air and electric-furnace regimes (400–1000 °C; 2–3.5 h) and compared with OPC. Within the furnace series, calcination temperature emerged as the primary control for amorphous content and phase assemblage. A furnace window between 400 and 700 °C produced ashes with high estimated amorphous fractions (typically ≥80%) and combined pozzolanic oxides (SiO₂ + Al₂O₃ + Fe₂O₃) above the 70% ASTM C618 threshold. On the basis of these indicators, 700 °C for 2 h is proposed as a reference calcination condition: it combines a high amorphous fraction, favourable siliceous chemistry, low LOI/LOD, and moderate K₂O levels while retaining a practical ash yield. Temperatures around 800 °C remain chemically acceptable but exhibit increased crystallisation and lower recovered mass, while 1000 °C produces predominantly crystalline ash, which is expected to show reduced pozzolanic reactivity despite its high total silica.

Open-air burning results highlight both the opportunity and the limitations of low-tech production routes. Extended open-air combustion for about 3–3.5 h can generate ashes that meet ASTM C618 oxide criteria with acceptable LOI, but with lower amorphous contents and higher, more variable alkali levels than furnace-processed CCA. Shorter open-air burns retain significant residual carbon and moisture, whereas over-severe furnace treatments sacrifice both amorphous content and yield. Taken together, the data delineate a practical processing envelope consisting of (i) a furnace reference route at 700 °C/2 h, expected to provide highly reactive CCA; (ii) an upper-bound furnace condition at 800 °C/2 h where crystallisation and ash loss must be carefully monitored; and (iii) an extended open-air route at 3–3.5 h that may be attractive in low-resource settings but requires tighter quality control, conservative replacement levels and project-specific testing. The chemical and mineralogical evidence confirms that properly processed CCA is a silica-rich, low-calcium, alkali-bearing pozzolan whose primary role in blended systems is to consume portlandite and generate additional C–S–H, rather than to act as a standalone hydraulic binder. Its expected performance therefore depends on controlling amorphous content, glass structure, and alkali levels. Simple, field-friendly indicators derived here, such as the transition from dark, carbonaceous material to a light grey/whitish ash at suitable severities, LOI thresholds, and target ranges for SiO₂ + Al₂O₃ + Fe₂O₃ and K₂O offer practical quality-control tools that can complement standard chemical testing and help producers steer CCA towards the most promising pozzolanic regime.

When interpreted through an industrial ecology lens, the findings point to substantial sustainability potential. Material flow calculations suggest that valorising even a modest fraction of global CC residues and processing them within the identified furnace window could generate tens of millions of tonnes of CCA annually. Under conservative assumptions, using such CCA to replace around 20% of OPC in suitable applications would enable clinker avoidance on the order of 0.3 Gt per year and avoid roughly 5–6 Mt CO₂ annually, while reducing uncontrolled residue burning and primary raw material extraction. At the same time, CCA deployment would support key Sustainable Development Goals by enabling more resource-efficient, lower-carbon binders (SDG 9), embedding circular-economy principles and waste valorisation in the cement value chain (SDG 12) and contributing to climate change mitigation through process emissions reduction and potentially longer-lasting infrastructure (SDG 13).

Limitations and Future Study Areas

The scope of the present work is intentionally confined to process, structure, and selected property indicators. No mechanical or durability tests on CCA-blended mortars or concretes were performed, and the study does not attempt a full life-cycle assessment. Consequently, all references to performance are interpreted as expected and potential behaviour inferred from established reactivity proxies (amorphous fraction, combined pozzolanic oxides, LOI/LOD, and alkali contents), not as experimentally demonstrated improvements in strength or durability. Confirming these expectations is a key priority for future work. Subsequent research should therefore (i) quantify strength development, durability (including permeability, shrinkage, and resistance to chemical attack), and alkali–silica reaction behaviour of concretes incorporating CCA produced at 700 °C/2 h and other representative conditions; (ii) couple these performance data with system-level life-cycle and techno-economic analyses under realistic deployment scenarios; (iii) performance-critical applications would require systematic replicate testing with appropriate statistical analysis; and (iv) address practical issues of feedstock logistics, quality assurance, and regional standardisation. By establishing an experimentally grounded calcination window and clear quality control markers, while explicitly recognising that mechanical performance is still to be validated, this study provides a physicochemical foundation and a decision-oriented reference point for developing CCA into a credible, standardisable, and genuinely sustainable SCM for low-carbon cementitious construction.