Comparative Analysis of Industrial Waste as Supplementary Cementitious Materials—A Preliminary Study

Abstract

This preliminary study investigates the viability of substituting high-performance Aalborg white Portland cement (CEM I 52.5 R) with five diverse industrial byproducts: wood ash, silica waste, clay brick, glass fibre, and calcined sewage sludge ash. Sewage sludge ash was produced in a laboratory from two different sludges from wastewater treatment plants in the Latvian cities of Jelgava and Liepaja. The research evaluates the influence of substitution levels ranging from 5% to 20% on the rheology of fresh material and its early-age mechanical performance (day 7). Results indicate that particle morphology largely dictates workability; porous and angular materials, such as wood ash, clay brick, and sewage sludge ash, reduce flowability, whereas non-absorbent milled glass fibres unexpectedly improve spread diameter. Regarding mechanical performance, glass fibre and clay brick waste demonstrated the highest potential, exceeding the 48–62 MPa reference compressive strengths by achieving up to 69 MPa at a 10% substitution level. Conversely, wood ash and silica waste exhibited significant strength degradation at higher substitution levels, due to agglomeration and high water demand. This approach not only identifies viable waste streams for cement substitution but also diverts significant industrial waste from landfills, thereby reducing CO₂e emissions and advancing more sustainable construction practices.

1. Introduction

The construction industry faces increasing pressure to mitigate its environmental footprint, particularly from the production of Ordinary Portland Cement (OPC). In 2023, cement production accounted for approximately 7–8% of global CO₂e emissions, around 2.4 GtCO₂e, making it one of the top individual contributors to anthropogenic greenhouse gas emissions [1]. The calcination of limestone to produce clinker releases significant amounts of greenhouse gases, estimated at roughly 900 kg of CO₂e per ton of cement [2,3,4,5]. To address this challenge, researchers and engineers are increasingly exploring supplementary cementitious materials (SCMs) that can partially replace clinkers without compromising the mechanical integrity of the composite. Reducing the cement content by incorporating industrial and municipal byproducts not only diverts waste from landfills but also lowers the embodied energy of the final construction material [6]. Consequently, the development of eco-efficient cementitious composites has become a critical area of study, aiming to strike a balance between sustainability and necessary engineering performance.

Wood ash, specifically the light fraction collected from the electrostatic precipitators of central heating systems fuelled by chipped wood, represents a promising biomass-derived alternative to traditional coal fly ash. Unlike the spherical particles of coal ash that enhance flow, wood ash particles from electrostatic filters are typically irregular, angular, and highly porous, which significantly influences the rheology of the fresh mix. Research indicates that the high specific surface area and hygroscopic nature of wood ash increase water demand, leading to a reduction in spread diameter and cone height as substitution levels increase [7]. Studies have shown that a 15% replacement can reduce the slump flow by roughly 10–15% if the water content is not adjusted [8,9]. However, regarding mechanical performance, the high calcium and silica content of this light fraction ash promotes hydraulic and pozzolanic activity. The literature suggests that at moderate substitution levels of 10–15%, compressive strengths on day 7 can reach approximately 85% of those of control samples, with absolute values often remaining above 40 MPa, rendering it a viable partial binder for structural applications [8].

Silica waste behaves similarly to wood ash in terms of water demand but offers better reactivity at early ages. Due to its extremely high specific surface area, silica waste significantly increases water demand, which can drastically reduce the cone height and spread diameter if not compensated with superplasticizers. Literature suggests that without flow agents, a 15% replacement can reduce workability by over 30% [10,11]. However, regarding mechanical performance, silica fume is highly effective at early ages, as it accelerates hydration through its nucleation effect. Research demonstrates that mixes with 10% silica fume can achieve compressive strengths on day 7 exceeding 60 MPa, often surpassing control samples by 10–15% due to the rapid formation of calcium silicate hydrate (C-S-H) gel and the densification of the matrix [10].

Clay brick waste presents unique challenges for achieving a consistent fresh mix due to the angular nature and high porosity of the ground particles. When used as a cement replacement, the high-water absorption of brick powder, often reported to be around 10% to 15% by mass, absorbs mixing water, leading to a noticeable reduction in spread diameter and stiffer consistency [12,13]. Despite this, the pozzolanic reactivity of fine brick dust contributes to increased strength after initial setting. While 7-day strength is generally lower than that of pure cement, optimized mixtures with up to 10% substitution have shown compressive strengths of approximately 35–40 MPa on day 7, which is sufficient for many non-structural or semi-structural applications, provided the particle size is milled sufficiently fine to promote early reactivity [12].

Glass fibre waste impacts the fresh and hardened states through physical rather than chemical mechanisms during the first week of curing. The inclusion of fibres increases the viscosity of the mix, creating a resistance to flow that results in reduced spread diameters and cone heights, particularly at dosages above 1–2% [14]. However, even at substitution levels of 5% to 10% of the binder mass (in powder or fine fibre form), the primary benefit observed on day 7 is the preservation of structural integrity. While the compressive strength on day 7 may remain similar to or slightly lower than the control values, around 45 MPa has been observed; the post-peak behaviour and resistance to early-age shrinkage cracking are significantly improved, which is vital for the material’s durability [15,16].

Sewage sludge ash (SSA) introduces complex chemical interactions that influence both rheology and hydration kinetics. For this study, SSA is sourced from wastewater treatment plants in the Latvian cities of Jelgava and Liepaja. It is prepared in the laboratory by drying sewage sludge at 105 °C, calcining it in a rotary kiln at 25 RPM with a 5° inclination at 750 °C, and milling it in a planetary ball mill for 10 min at 300 RPM. This preparation is crucial, as the irregular particle shape and high porosity of SSA typically increase water demand, reducing the flow spread compared to OPC pastes. Furthermore, SSA often contains phosphates, which can retard cement hydration and potentially affect early strength. However, recent studies indicate that with proper calcination, the retarding effect is minimized, allowing 10% replacement mixes to achieve 7-day strengths of 38 MPa, which is comparable to fly ash blends [17].

While the substitution of OPC with established SCMs such as fly ash and ground granulated blast furnace slag is well documented, the use of emerging industrial by-products remains fragmented in the literature [18]. Recent studies have largely focused on the isolated effects of single waste streams, such as wood ash or sewage sludge ash (SSA), often in the context of standard-strength grey cement [18,19]. However, a comprehensive framework that evaluates the relative performance of diverse industrial residues—ranging from biomass and chemical waste to construction and municipal by-products—within a single experimental matrix is largely missing.

Utilizing a high-performance binder, such as Aalborg white cement 52.5 R, is a strategic choice to counteract the potential early-strength loss associated with these waste substitutions. The “R” classification denotes rapid hardening, ensuring a high intrinsic 7-day compressive strength and providing a safety margin when diluting the cement content with 5%, 10%, 15%, and 20% waste. By leveraging the rapid hydration of this specific cement, it becomes possible to incorporate lower-reactivity waste, such as clay brick or SSA, while still satisfying the rigorous mechanical demands of early-stage construction. This approach enables a direct assessment of how each waste type affects the rheological balance and hydration speed, without the risk of producing non-viable, weak composites. By reducing the amount of cement needed for the material, total emissions can be lowered. Using a small lab-scale planetary ball mill as an example of a pretreatment method, a tonne of waste can be processed while emitting up to 20 kg of CO₂e. With an industrial mill, this value can be lower. As processing is not emission-intensive, pretreatment of waste material would still yield a lower-emission final product.

The novelty of this research lies in its systematic comparative evaluation of five distinct industrial waste streams (wood ash, silica waste, clay brick, glass fibre, and calcined sewage sludge ash) specifically integrated into a high-performance white Portland cement (CEM I 52.5 R) matrix. This choice of binder is significant: white cement’s unique mineralogical composition, characterized by high tricalcium silicate (C₃S) and low iron content, reacts differently with the chemical constituents of SCMs than traditional OPC. Furthermore, this study bridges a critical gap by linking the fresh-state rheological behavior (critical for workability) with early-age mechanical development (7-day strength) [19,20]. By providing a direct performance hierarchy across these diverse materials, this study establishes a baseline for selecting SCMs in high-strength, rapid-construction applications where early property development is paramount, thereby offering a more robust scientific contribution than regional case studies.

This study aims to compare the fresh-state properties and early-age mechanical performance of cement pastes incorporating various industrial wastes available in Latvia and the Baltic states. These materials were selected as representative samples of diverse industrial by-product streams, allowing for a comparative analysis of different waste chemistries (aluminous, siliceous, and biomass-derived). Workability was assessed through cone height and spread diameter measurements, while day 7 compressive tests evaluated early strength. Substitution levels of 5–20% were applied using wood ash, silica waste, clay brick powder, glass fibre waste, and calcined sewage sludge ash. The goal is to identify mix designs that maintain adequate flowability and achieve sufficient early strength, supporting sustainable construction by reducing clinker use and diverting waste from landfills. The experimental workflow is illustrated in Figure 1.

2. Results and Discussion

2.1. Particle Morphology

The SEM micrographs of WA reveal a highly irregular and angular particle morphology with a significant presence of porous structures (Figure 2). Unlike coal fly ash, which typically contains smooth, spherical particles that enhance flowability through a “ball-bearing” effect, the wood ash particles observed here are jagged and inconsistent in size [21]. This rough and pitted surface texture explains the increased water demand and reduced spread diameter observed in the fresh state; the porous nature of these particles absorbs mixing water, reducing the free water available for lubrication. This observation aligns with Siddique’s [22] findings, who noted that wood ash from biomass combustion lacks the spherical geometry of coal ash, resulting in higher specific surface areas and irregular shapes that significantly increase inter-particle friction. The absence of spherical particles in the WA samples confirms that the physical mechanism of workability improvement observed in conventional fly ash concrete is absent here; instead, angularity contributes to a stiffer mix, necessitating careful water management to maintain consistency.

The microstructure of the SW displays a distinct morphology characterized by extreme fineness and a strong tendency toward agglomeration. The particles (Figure 3) appear as dense clusters of sub-micron-sized grains, a typical feature of silica-based byproducts with high amorphous silicon dioxide content [23]. These agglomerates create a rough, textured surface area that increases the water demand of the mixture, correlating directly with the reduction in flowability observed in fresh state tests. This morphology is consistent with descriptions by Mukharjee and Barai [10], who reported that the high surface energy of nano- and micro-silica particles leads to flocculation, trapping water within the agglomerated structures, and reducing the workability of the paste. However, this same fineness suggests a high potential for reactivity; the surface area provides abundant nucleation sites for the formation of C-S-H gel, which densifies the interfacial transition zone (ITZ) and contributes to rapid early-age strength development despite the rheological challenges.

The SEM analysis of CB waste reveals particles that are predominantly angular, blocky, and sharp-edged, resulting from mechanical crushing and milling. The images (Figure 4) show a lack of smooth surfaces, with particles exhibiting a fractured, crystalline-like appearance typical of ceramic materials [24]. This angular morphology enhances the internal friction of the cementitious mix, effectively “locking” the particles together and resisting flow, which parallels the reduced slump spread observed in the fresh property tests. Furthermore, the surface of the brick particles appears coarse and micro-porous. This agrees with Bideci et al. [12], who identified that the inherent porosity of fired clay brick waste acts as an internal water reservoir. At the same time, this absorbs water during mixing, reducing initial workability, and may also support internal curing mechanisms later in the hardening process. The irregular geometry observed here contrasts with the smooth texture of the cement grains, indicating that the brick powder acts more as a friction-inducing filler in its fresh state rather than as a lubricant.

The morphology of the GF waste is distinct from the other wastes, characterized by high-aspect-ratio fragments and shattered fibrous bundles. Even after milling, the material retains a rod-like or needle-like structure, with smooth surfaces typical of vitrified glass. The SEM images (Figure 5) indicate that while milling reduced the fibre length, it did not produce a globular powder; instead, a network of short fibres and shards remains. This structure creates a “bird-nesting” effect in the fresh mix, where fibres entangle and form a lattice that resists flow and slump, significantly increasing viscosity. This matches the behavior described by Tibebu et al., who noted that milled glass fibers often act as micro-reinforcement, bridging micro-cracks even at early ages [25]. The smooth surface of the fibres initially suggests a purely physical bond, which explains why early strength is maintained through crack arrest rather than an immediate chemical reaction. However, the high silica content implies potential pozzolanic reactivity over longer curing periods.

The SEM analysis of the SSA highlights the critical role of the laboratory milling process in transforming its morphology. Prior to milling, the calcined SSA from Jelgava and Liepaja (Figure 6a) appears as large, highly porous, and irregularly fused agglomerates with a sponge-like, open surface texture. This is a direct result of the devolatilization and gas release during the 750 °C incineration process, creating a structure that is extremely detrimental to workability due to its high-water absorption capacity [26]. However, after 10 min of planetary ball milling (Figure 6b), the morphology transforms into finer, more discrete angular particles, although they retain a significant degree of surface roughness compared to cement. Milling effectively breaks down the large, porous clusters, densifying the material and exposing new, reactive surfaces. This observation is consistent with Rutkowska et al. [27], who found that milling calcined SSA is essential for breaking down the porous skeleton, improving particle packing, and mitigating the high water demand of the raw ash. The irregular, angular nature of the milled SSA confirms why the material reduces flowability, as the particles still interlock and frictionally resist flow. However, the drastic size reduction is crucial for enhancing its filler effect and pozzolanic reactivity in the hardened state.

2.2. Fresh Material Properties

The analysis of fresh material properties, specifically cone height and spread diameter, reveals a divergence in rheological behaviour depending on the type of waste used. The cone height results (Figure 7) demonstrate that incorporating wood ash (WA) and sewage sludge ash (SSA) increases the mix’s stiffness relative to the reference interval of 25–41 mm. The WA series exhibited the highest cone retention, ranging from 45 mm at 5% substitution to 53 mm at 20% substitution. This behaviour correlates directly with the SEM analysis, which revealed that WA particles are highly angular and porous, leading to increased inter-particle friction and water absorption. This aligns with findings by Teker Ercan et al. [7], who reported that the high specific surface area and hygroscopicity of biomass ash drastically increase water demand. Similarly, the SSA (L) series exhibited a linear increase in cone height up to 50 mm at a 20% replacement rate. This reduced slump retention is attributed to the “sponge-like” open surface texture of the SSA observed in the microscopic analysis, which absorbs mixing water and reduces the available free water, a phenomenon consistent with the high-water demand of SSA reported by Rutkowska et al. [27].

In terms of spread diameter (Figure 8, left), the majority of waste streams (WA, SW, CB, and SSA) show a consistent reduction in flowability compared to the reference. In the static test (0 jolts), the WA, SW, and CB series generally fell below the reference interval of 129–145 mm. The SSA (L) samples exhibited the lowest workability, dropping to approximately 96 mm at 20% substitution. This reduction is supported by Siddique [22], who noted that ash’s irregular shape increases inter-particle friction, resisting flow. Likewise, the clay brick (CB) samples showed a decrease in spread, attributable to the intra-particle porosity of the crushed brick acting as a water reservoir, a mechanism detailed by Bideci et al. [12]. Even silica waste (SW), despite being non-porous, showed reduced spread at a high substitution level (20%), likely due to flocculation of its extremely fine particles, which trap water, as described by Mukharjee and Barai [10].

However, the glass fibre (GF) series presented a distinct anomaly, exhibiting a significant increase in spread diameter compared to the reference. At 20% substitution, the GF spread diameter reached 230 mm before jolting, exceeding the control. This result contradicts the general trend observed by Tibebu et al. [25], who typically found that fibrous materials reduce workability due to entanglement. The enhanced fluidity observed here can be analysed by examining the particle morphology in the SEM images. Unlike the porous cement or ash particles, the milled glass fibres possess smooth, vitrified surfaces that are completely non-absorbent. By replacing 20% of the water-absorbing cement with non-absorbent glass fibres, the effective water-to-binder ratio of the paste is likely to have increased, providing more free water to the mix. Additionally, the smooth, rod-like glass particles may have reduced the paste’s internal friction coefficient compared to angular cement grains, effectively acting as a lubricant rather than a reinforcement in the fresh state.

While the initial cone height and static spread indicated a general stiffening effect for most waste materials, the application of mechanical energy increased the spread diameter across all series (Figure 8, right). This response is characteristic of cementitious systems, in which vibration temporarily overcomes the yield stress, allowing the material to flow.

For the wood ash (WA), sewage sludge ash (SSA), and clay brick (CB) series, the increase in spread after 5 jolts was modest compared to the reference. These materials maintained a more cohesive, viscous consistency even under dynamic loading. This behaviour is largely dictated by particle morphology; as seen in the SEM analysis, these particles exhibit high angularity and intra-particle porosity. This porous structure traps a portion of the mixing water, effectively increasing the solid volume fraction and inter-particle friction. According to research by Teker Ercan et al. [7], the high water absorption of biomass ashes leads to rapid loss of free water, significantly limiting the material’s ability to deform, even when subjected to the mechanical energy of the impact table. Similarly, Rutkowska et al. [27] study on sewage sludge ash emphasizes that the irregular geometry of these particles creates a “locking” effect, requiring higher energy to initiate flow than standard Portland cement.

In contrast, the silica waste (SW) and glass fibre (GF) series displayed a more pronounced response to the jolting process. In the case of SW, although fine particles increase viscosity through flocculation, mechanical energy likely breaks these weak agglomerates, thereby increasing the spread diameter. However, the most significant change was observed in the GF series, which transitioned from a stable paste to a highly fluid state. This divergence suggests that the lack of surface porosity and the smooth, vitrified texture of the glass, as identified in the morphology section, prevents the water-binding effect seen in the ashes. Consequently, once the 5 jolts disrupt the initial yield stress, the excess free water in the GF mixes leads to spread diameters that exceeded the reference interval. This phenomenon highlights how the physical surface characteristics of waste, specifically whether it is absorbent or non-absorbent, are the primary drivers of workability under dynamic conditions, rather than just the replacement level itself.

2.3. Density and Mechanical Compressive Strength

The density results across all mixes reveal modest but consistent reductions compared to the reference cement paste (1925 kg/m³) (

Table 1. Density compilation of all the samples.

Sample	Material Density, kg/m3	Sample	Material Density, kg/m3	Sample	Material Density, kg/m3
REF	1925 ± 37
WA5	1932 ± 11	CB5	1928 ± 20	J5	1880 ± 18
WA10	1898 ± 15	CB10	1909 ± 21	J10	1878 ± 16
WA15	1893 ± 9	CB15	1908 ± 23	J15	1877 ± 13
WA20	1871 ± 12	CB20	1884 ± 30	J20	1891 ± 20
SW5	1869 ± 20	GF5	1941 ± 15	L5	1911 ± 19
SW10	1808 ± 23	GF10	1893 ± 19	L10	1885 ± 26
SW15	1763 ± 30	GF15	1900 ± 23	L15	1883 ± 16
SW20	1685 ± 13	GF20	1893 ± 28	L20	1856 ± 20

). Wood ash (WA) and glass fibre (GF) samples maintained densities closest to the reference, with WA ranging from 1932 kg/m³ at 5% substitution to 1871 kg/m³ at 20%, and GF ranging from 1941 to 1893 kg/m³. This relative stability reflects their particle morphology: WA’s particle angular porosity absorbs water but still packs efficiently, while GF’s smooth, non-absorbent surfaces prevent significant density loss.

SW exhibited sharper declines, from 1869 kg/m³ at 5% to 1685 kg/m³ at 20%, due to its extremely fine particles and tendency to agglomerate, which trap water and reduce effective packing. CB waste samples maintained similar densities to the reference, dropping from 1928 to 1884 kg/m³ when substitution is increased, reflecting its porous ceramic nature. Sewage sludge ash (SSA) samples exhibited modest densities, dropping from 1911 kg/m³ at 5% substitution to 1856 kg/m³ at 20% substitution. Because the density of waste materials is lower than that of cement, the overall density of the samples can be reduced while maintaining the reference interval compressive strength.

Overall, density reductions correlate with particle porosity and water absorption capacity. However, density alone is not a reliable predictor of mechanical performance, as highly porous materials, such as silica waste, can still enhance strength through pozzolanic reactivity. This aligns with recent findings by Rutkowska et al. [27] and Verma et al. [28], who emphasized that microstructural reactivity often outweighs density in determining composite performance.

The compressive strength results reveal a clear hierarchy of performance at an early age (Figure 9). At a 5% substitution, all samples fall within the marginal error range of the reference interval of 48–62 MPa. With the 10% substitution, only the CB10 and GF10 samples show an increase in compressive strength, with the lower marginal error placing them at the top of the reference interval. The performance of GF at low dosage is consistent with micro-reinforcement and crack-bridging mechanisms that preserve load-carrying capacity and delay microcrack coalescence, even though GF’s contribution is primarily physical rather than chemical. Almost all sample series show a decrease in compressive strength with increasing waste substitution for cement. The WA sample series exhibits a gradual decrease from 48 to 29 MPa, as in the SW samples, with a drop in strength from 50 MPa to 25 MPa at a 5–20% substitution rate. SW’s early-age reputation is not realized here at 20%, likely due to agglomeration and insufficient dispersion without superplasticizers, which can suppress its nucleation benefits. The CB and GF sample series show the most promising results, with the 20% substitution still within the reference interval. CB’s stability suggests sufficient fineness and reactive aluminosilicate phases that drive pozzolanic contributions and a beneficial internal curing effect from micro-porosity, which support strength despite clinker dilution. Comparing the SSA sample series, a decrease is noticeable, indicating limited early reactivity under the chosen curing and mixing conditions. In the SSA (L) sample series, an increase in compressive strength is noticeable with increasing cement substitution from 15% to 20%, reaching 43 MPa on day 7. All ash-type replacements, WA (29 MPa), SW (25 MPa), SSA(J) (39 MPa), and SSA(L) (43 MPa), drop below the reference interval at 20% substitution of cement, highlighting early-age sensitivity to water demand, porosity, and potential hydration retarders, like phosphates in SSA, unless admixture strategies or processing, like finer grinding, and optimized calcination, are intensified.

Wood ash samples exhibited trends similar to those reported by Popławski & Lelusz [25], who observed pressures of 40–45 MPa at 10–15% WA with mechanical activation. The results gathered in this study are within the expected bounds, although further strength gains may be possible with finer milling or blended activation strategies.

Silica waste samples reached 50 MPa at 5%, but fell to 25 MPa at 20%, which is lower than expected for silica fume. Antoni [29] found that silica fume mixes often exceed 60 MPa on day 7 due to nucleation effects. The discrepancy likely stems from agglomeration or a lack of superplasticizers in the mix, which suppress dispersion and reactivity.

The clay brick waste in the mixes reached 65 MPa at 10% and 48 MPa at 20%, which is notably higher than the typical values reported in the literature. Saravanan & Kabeer [30] modeled ceramic waste powder mixes and found day 7 strengths ranging from 45 to 55 MPa at 10–20% replacement, depending on fineness and curing conditions. The study’s results suggest better grinding or reactivity, possibly due to optimized particle size or internal curing from micro-porosity.

Recent studies confirm that glass fibre waste can significantly enhance early compressive strength when finely milled. In this study, GF achieved 69 MPa at 10% and 59 MPa at 20%, both within or above the reference interval. This aligns with findings by Zaid et al. [31], who demonstrated that a 20% substitution of waste glass as a binder material serves as an optimal dosage for enhancing mechanical properties. They attributed these gains to the high pozzolanic reactivity of finely ground glass particles, which facilitates the formation of additional calcium silicate hydrate (C-S-H) gel, thereby densifying the microstructure and improving the interfacial transition zone between the fibers and the cement matrix.

Sewage sludge ash (SSA) samples showed 59 MPa at 5% (Jelgava) and 48 MPa at 20% (Liepaja), but dropped to 39 MPa and 43 MPa at 20% substitution, respectively. These values are consistent with those reported by de Azevedo Basto et al. [32], who found 41 MPa at 10% SSA when the material was properly calcined and milled. The results confirm that SSA can be viable at low dosages but suffers at higher levels without admixture support.

A comparative analysis of the various waste streams reveals that particle morphology is the fundamental driver of both rheological and mechanical performance. Individually, WA and SSA exhibit high water demand due to their angular, porous particles, which reduces flowability as substitution levels increase. While CB shares this angularity, its micro-porosity acts as an internal water reservoir, providing a beneficial internal curing effect that helps maintain 7-day strength. SW poses a different challenge—its extreme fineness leads to agglomeration, resulting in the greatest strength degradation at 20% substitution due to poor dispersion. Conversely, GF is unique because its non-absorbent, smooth surfaces increase spread diameter by raising the effective water-to-binder ratio while providing micro-reinforcement that preserves high compressive strength. The varying degree of inter-particle friction, as established by the SEM analysis, suggests that the physical interaction between waste fragments and cement grains is as critical as the chemical hydration process. Specifically, the high specific surface area of the porous wood ash and sewage sludge ash particles creates significant viscous drag, effectively immobilizing the paste, a phenomenon corroborated by Siddique [8]. In contrast, the smooth surfaces of glass fiber minimize this drag, acting more as a lubricant than a filler. Furthermore, the modest density reductions observed across the series confirm that these materials’ impact on the fresh state is primarily governed by their surface characteristics and absorption capacity rather than simple volumetric displacement [28].

When comparing all streams collectively, a clear hierarchy of viability emerges. CB is the most effective supplementary cementitious material, and GF acts as micro-reinforcement for high-performance applications, consistently maintaining or exceeding reference strengths at up to 20% replacement. In contrast, WA, SW, and SSA are more sensitive to the “dilution effect,” showing substantial strength loss unless paired with optimized processing or chemical admixtures, such as superplasticizers, to manage their high interparticle friction and water absorption. Despite these modest density reductions, the mechanical integrity on day 7 demonstrates that density is not a reliable proxy for strength. The superior performance of the clay brick and glass fiber series-achieving up to 69 MPa highlights that localized micro-reinforcement and internal curing mechanisms can successfully offset the clinker dilution effect [14,16]. Conversely, the failure of silica waste at 20% substitution reveals a critical threshold where physical agglomeration overrides the material’s pozzolanic potential [10,11]. This suggests that the high-performance Aalborg cement provides a robust hydration matrix capable of masking the lower reactivity of certain wastes, but only until the inter-particle packing is severely disrupted by excessive dosages or poor dispersion of ultra-fine particles, as highlighted by Antoni and Verma [29,30].

3. Materials and Methods

3.1. Raw Materials

3.1.1. CEM I 52.5 R Aalborg White Cement

CEM I 52.5 R is a high-performance hydraulic binder that conforms to the EN 197-1 standard [33]. This cement type (Sakret, Brocēni, Latvia) is a pure Portland cement. Unlike blended cements, such as CEM II, a CEM I classification means it consists almost entirely of Portland cement clinker (typically 95–100%) and a small amount of gypsum to control the setting time. It contains no secondary constituents or fillers, such as limestone.

The distinct white color of Aalborg White cement is achieved by using raw materials with exceptionally low iron and manganese oxide content, such as ultra-pure limestone and fine-grained sand. This high purity and whiteness make it a preferred choice for architectural concrete, precast panels, and decorative mortars where aesthetics and bright color are essential. Its high reflectivity also provides benefits for traffic safety elements, such as curbs and median barriers.

This cement is designated as high-strength class 52.5 and rapid hardening classification R. This indicates a very high early compressive strength, specified as ≥21 MPa at 1 day, ≥39 MPa after 2 days, and a standard compressive strength of ≥52.5 MPa after 7 days. Technical data often shows typical performance exceeding these minimums, with 28-day strengths ranging from 66 to 76 MPa [34]. Other key properties include a low alkali content (≤0.3%) and high sulphate resistance.

3.1.2. Wood Ash

Wood ash (WA) was collected from an electrostatic filter from a “Bauskas novada komunālserviss” Ltd. central heating system in the city of Iecava, Latvia. The system has a capacity of 3 MW and uses chipped wood as a fuel source. Figure 10 shows the macroscopic nature of the WA.

3.1.3. Silica Waste

Silica byproduct waste (SW) was collected from a Lifosa fertilizer production plant (Kėdainiai, Lithuania). The material consists mainly of amorphous silica (≈78 wt%) with fluoride compounds (≈8 wt%) and minor oxides [35]. This industrial byproduct, generated during the neutralization of hexafluorosilicic acid, was previously stockpiled in landfills due to fluorine contamination. For experimental purposes, the silica waste powder was dried at 105 °C for 2 h. The powdered silica waste (Figure 11) was evaluated as a potential source of reactive silica and alumina for supplementary cementitious materials. Particle morphology was analysed using digital microscopy and SEM to assess its structure and reactivity.

3.1.4. Clay Brick

Clay brick waste was collected from construction and demolition sites and consisted primarily of illite-based clay bricks. The recovered material was manually sorted to remove impurities, and then crushed and ground in the laboratory using a planetary ball mill PM 400 (Retsch, Haan, Germany) for 10 min at 300 RPM to obtain a fine powder (Figure 12). After pulverization, the clay brick waste powder was used as a raw material for this study, designated CB. This powdered clay brick waste was assessed as an industrial byproduct with potential pozzolanic properties for use as a supplementary cementitious material in cement paste. Particle morphology and texture were analyzed using digital microscopy and SEM to evaluate their structure and reactivity.

3.1.5. Glass Fiber

Glass fibre waste was collected from Valmiera Glass Group JSC (Valmiera, Latvia). The waste material consisted of crushed glass fibres approximately 50 mm in length, which were then cut in the manufacturing plant and ground in the laboratory using a Retsch PM 400 planetary ball mill for 10 min at 300 RPM to produce a powdered material. This powdered glass fibre waste (GF) was evaluated as an industrial byproduct with potential pozzolanic properties for application as SCM in cement paste. GF particle morphology was analysed by digital microscope and SEM. Figure 13 shows the macroscopic particles of the GF.

3.1.6. Sewage Sludge

For the study, sewage sludge was obtained from the wastewater treatment plants in Jelgava and Liepaja. The samples were not specially treated or dried before being processed in the laboratory to analyze the product created during wastewater treatment at wastewater treatment plants.

The wastewater sludge consisted mainly of water, accounting for >76% of its total mass, and this water must be removed during further processing. The sludge consisted of a large proportion of biological mass, which could contain various disease-causing pathogens that could endanger the health or lives of people in contact with it. For safety reasons, additional personal protective equipment was used.

3.2. Processing of the Sewage Sludge

The mass of the sample is determined by comparing its weight before and after drying. The sewage sludge (SS) was dried in a drying oven at 105 °C for 72 h. After drying, the sewage sludge lost 76.3% of its mass. Its consistency became dry, brittle, and lumpy (see Figure 14a,b). The dried sludge lumps could be easily broken apart by hand. After drying, the sewage sludge was brown, and large hair and other fiber particles were visible. After drying, the samples were stored in a sealed container.

After the drying process, the SS was incinerated in a rotary kiln (Keramserviss, Ādaži, Latvia) at 25 RPM and 5° inclination, heated to 750 °C, to remove any remaining organic matter. The dried sludge was weighed before and after incineration, and the mass loss was calculated. The SS lost 77.8% of its mass. The samples changed color from dark brown to light brown on the surface and became much more brittle; the unpleasant odor also disappeared. After incineration, the sewage sludge is less hazardous to health, as it has been sterilized at a temperature above 121 °C. After incineration, the sewage sludge ash (SSA) was stored in a sealed container.

After incineration, the SSA had to be ground to obtain powder for further use in the study (Figure 15a). Milling was performed in a laboratory planetary mill PM 400 (Retsch, Haan, Germany) for 10 min at a speed of 300 RPM. The set time and revolutions per minute were chosen based on previous experience working with SS and were sufficient to grind the SSA into a fine powder (Figure 15b). The obtained SSA powder was stored in a sealed container until the next stage of the study.

3.3. Mix Design and Sample Preparation

The compositions include CEM I 52.5 R and various waste materials, such as wood ash (WA), silica (SW), clay brick (CB), glass fiber waste, and two different milled sewage sludge ashes (SSA), to assess the effects of substituting cement with these waste materials. A total of 24 sample series were prepared. The REF sample series was produced using only CEM I 52.5 R and served as a reference series for the other sample series. For each subsequent sample series, a portion of the cement was replaced with different waste materials at varying percentages (5%, 10%, 15%, and 20%, respectively). The compositions of the prepared sample series are shown in

Table 2. Mix design of the sample.

Sample	Composition, Mass Parts							Water
	СЕM I 52.5 R	FA	SW	CB	GF	SSA (J)	SSA (L)
REF	100							40
WA5	95	5						40
WA10	90	10
WA15	85	15
WA20	80	20
SW5	95		5					40
SW10	90		10
SW15	85		15
SW20	80		20
CB5	95			5				40
CB10	90			10
CB15	85			15
CB20	80			20
GF5	95				5			40
GF10	90				10
GF15	85				15
GF20	80				20
J5	95					5		40
J10	90					10
J15	85					15
J20	80					20
L5	95						5	40
L10	90						10
L15	85						15
L20	80						20

. No superplasticizers were used in the making of the samples.

The dry ingredients were mixed in a bowl with a spoon for 1 min. Then water was added, and the mixture was mixed for 2 min with an electric hand mixer on the lowest setting (600 RPM). Then, mixing was carried out for 2 min at the mixer’s fastest speed (1200 RPM). This sequence of steps and the selected time ensured a homogeneous mixture for all compositions. The amount of water added to the mixture was the same in all cases: 8 mass parts, or a W/B ratio of 0.4. The W/B ratio was set to the same for each waste series to allow a direct comparison of the materials.

After mixing, the homogenous mass was tested for its cone slump and spread diameter. Afterwards, the mass was poured into oiled 2 cm × 2 cm × 2 cm silicone molds. The moulds were covered with plastic film and inserted in a sealable plastic bag to prevent moisture loss. The samples were left in the molds for 24 h.

After demoulding, the samples were cured in water till the testing day. Before testing, samples were removed, dried according to BS EN 196-1 [36], and tested for their mechanical compressive strength.

3.4. Characterisation Techniques

The macrostructure of the samples was captured using a Bresser DST-1028 digital microscope (Bresser GmbH, Rhede, Germany) at 15× magnification.

The samples’ microstructure and morphology were examined using the Hitachi TM3000 tabletop scanning electron microscope (SEM) (Hitachi High-Technologies Corporation, Tokyo, Japan). SEM provides high-resolution imaging of surface structures and particle morphology, enabling imaging of non-conductive samples without a conductive coating. SEM analysis provided valuable insights into the microstructural properties of the samples. SEM images were taken at 5 kV with magnifications of 100×, 500×, and 1200×.

The material’s viscosity was measured in accordance with EN 206-1 [37]. Cone height and spread diameter were assessed, and 5 jolts on the impact table from ASTM C143 [38] were performed to induce changes in the material. After the jolts, the diameter was remeasured, and conclusions about the viscosity were made.

Compressive strength of the samples was tested on day 7. The cubic samples were measured and weighed for their volume and density in accordance with EN 12390-7 [39]. Using a Controls 50-C56G2 compressive strength testing system (Controls Group, Milan, Italy), the mechanical compressive strength of cubic samples was determined at a strain rate of 0.8–1.0 MPa/s. For each test, 6 parallel samples were tested to obtain more reliable results.

4. Conclusions

This study indicates that while high-performance cement can effectively accommodate lower-reactivity waste streams, the physical morphology of the byproducts, specifically their angularity and porosity, is the dominant factor under the investigated factors of fresh-state behaviour and early structural integrity. Given the preliminary nature of this study, the conclusions primarily reflect early-age performance trends rather than long-term durability behaviour. Key conclusions revolve around morphology, early compressive strength, and dosage limitations: the angular and porous nature of wood ash, clay brick, and sewage sludge ash increased water demand and reduced workability, while the smooth, non-absorbent surface of glass fibre waste improved flowability under dynamic conditions.

Glass fibre waste and clay brick waste demonstrated the highest potential for structural applications, achieving compressive strengths of up to 69 MPa and 65 MPa at a 10% substitution, respectively, which is consistent with their favourable particle morphology and suggests a contribution from crack-bridging effects and pozzolanic potential rather than definitive confirmation of these mechanisms. Sewage sludge ash, although limited in early strength at high dosages, achieves 39–59 MPa with substitution, showing promise for non-structural applications and eco-friendly products, especially when combined with optimized processing or chemical activation. In contrast, wood ash and silica waste are also more suitable for non-structural applications, as their high water demand, particle agglomeration, and limited early reactivity limit strength development at higher dosages. These findings highlight the importance of selecting waste types based on performance requirements to ensure both sustainability and durability in construction.

Future research should focus on long-term mechanical performance and durability (freeze–thaw, chloride penetration, carbonation), optimize silica waste mixes with superplasticizers to overcome agglomeration, explore chemical activation for wood ash and sewage sludge ash to enhance pozzolanic reactivity, perform life-cycle assessments to quantify CO₂e reduction and landfill diversion, and evaluate scalability for industrial implementation, mix design optimization to improve the workability and reactivity of selected wastes, and environmental and scalability assessments to support industrial implementation.