Evaluation of the Usability of SCMs Produced by Adding Aluminum and Iron Oxide to Mortar Waste Powder Under Different Conditions

Abstract

The integration of recycled materials into cementitious systems presents a sustainable path to reducing environmental impact in construction. This study investigates the mechanical and durability performance of self-compacting mortars (SCMs) incorporating finely ground mortar waste powder (MWP) as a partial cement substitute, reinforced with aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). Eleven mixes were designed with MWP replacing cement at 0–50% by volume. Fresh-state tests showed that slump flow decreased moderately (from 259 mm to 240 mm), while V-funnel times improved (from 10.51 s to 7.01 s), indicating acceptable flowability. The optimum performance was observed in SCM2 (5% MWP + oxides), which achieved 75.62 MPa compressive and 13.74 MPa flexural strength at 28 days, outperforming the control mix. Durability under high temperature and freeze–thaw cycling revealed that oxide-reinforced mixes exhibited superior strength retention, with SCM2 maintaining over 87 MPa after 300 °C exposure and minimal degradation after 100 freeze–thaw cycles. Porosity remained low (16.1%) at optimal replacement levels but increased significantly beyond 25% MWP. The results confirm that low-level MWP replacement, when reinforced with reactive oxides, provides a viable strategy for producing durable, high-performance, and eco-efficient SCMs.

1. Introduction

In recent years, ensuring sustainability in production in the construction sector has become a critical priority in terms of combating global warming and carbon emissions. Cement production accounts for approximately 8% of global anthropogenic CO₂ emissions, making it a significant environmental issue in this context [1]. As a result, research into alternative binding materials to reduce the environmental impact of traditional cement is increasing. In particular, supplementary cementitious materials, which are pozzolanic materials derived from industrial or agricultural waste, are attracting attention due to their environmental and economic advantages [2,3]. Numerous studies in the literature have shown that traditional SCMs such as fly ash, silica fume, and granulated blast furnace slag (GGBFS) enhance the mechanical strength and durability of the cement matrix [4,5,6]. However, their limited geographical availability, variable quality structures, and inability to ensure supply continuity in some regions have restricted their widespread application. This study focuses on the need to investigate new-generation and regionally more accessible alternatives that could address the limitations of existing SCMs.

In contrast, mortar waste powder (MWP)—a finely ground residue obtained from the crushing, milling, or demolition of hardened mortar—represents an abundant, locally accessible by-product with untapped potential for use in cementitious systems. MWP typically contains unhydrated clinker phases, siliceous sand particles, and hydrated cementitious gels. Despite chemical similarities with OPC, its pozzolanic activity is generally limited due to the predominance of crystalline phases and previously hydrated compounds that are chemically stable and less reactive under normal hydration conditions [7]. As a result, using MWP as a partial OPC replacement often leads to increased porosity, delayed strength development, and reduced durability—particularly in aggressive environments.

To overcome these limitations, recent studies have explored the activation of low-reactivity mineral powders using transition metal oxides such as aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). These oxides, whether added individually or as part of composite blends, can influence hydration kinetics, filler effects, and microstructural densification. Al₂O₃ has been shown to promote the formation of calcium aluminate hydrates and act as a nucleation site, accelerating early-age strength development [8]. Meanwhile, Fe₂O₃ contributes to ferrite phase formation and can enhance chloride and sulfate resistance, particularly in long-term exposure scenarios [9]. The combined use of these oxides has also been associated with the formation of denser C-(A)-S-H gel networks, which improve the impermeability and durability of cement-based composites [10,11]

The integration of MWP into self-compacting mortar (SCM) systems introduces additional complexity. SCMs are highly flowable cementitious materials that consolidate under their own weight without external vibration, relying on a delicate balance between viscosity, yield stress, and cohesiveness [12]. Fine powders such as MWP, while beneficial for increasing binder volume and packing density, can impair flowability and reduce cohesion if not chemically compensated. Moreover, SCMs exposed to saline environments (e.g., marine infrastructure or de-icing zones) are particularly sensitive to microstructural permeability and pore connectivity, making the role of chemical reinforcement even more critical. The integration of recycled mineral powders into cementitious systems has gained substantial attention due to growing demands for sustainability and resource efficiency in the construction industry. Among these, mortar waste powder (MWP), a by-product of hardened mortar crushing or precast surface processing, presents a regionally abundant alternative to conventional supplementary cementitious materials (SCMs). MWP generally contains residual clinker phases, quartz sand, portlandite, and partially reacted hydration products [13]. However, its effectiveness as a cement replacement is limited by its low pozzolanic reactivity, primarily due to the predominance of crystalline and already hydrated compounds with minimal capacity to form secondary binding gels [7].

Several studies have evaluated the mechanical implications of MWP incorporation. At replacement levels below 15% by weight, MWP has been shown to maintain or slightly improve late-age compressive strength due to improved particle packing and nucleation effects [14]. However, beyond 20–25%, compressive strength reductions of 15–40% have been reported, attributed to reduced hydraulic reactivity and increased porosity [15]. Flexural strength and abrasion resistance also show significant degradation at higher MWP levels unless the matrix is chemically or physically enhanced [13].

The synergistic action of Al₂O₃ and Fe₂O₃ lies in their complementary influence on cement chemistry. Al ions enhance early-age reactivity and binder densification through increased nucleation density, while Fe ions facilitate the formation of durable, chemically stable hydration phases that are less susceptible to ion penetration. Together, these effects result in the formation of denser C-A-S-H and ferrite-containing gels, which improve both mechanical strength and long-term durability [10,11].

Studies using these oxides in SCMs or mortars report that low-dosage additions (typically 1–5% by weight of binder) can yield substantial improvements. For instance, Sua-iam et al. [11] observed up to 25% improvement in compressive strength and a 30% reduction in water absorption in ternary blends containing residual pozzolans and Al-rich fines. Similarly, Fe₂O₃ addition at 3% was shown to reduce chloride diffusivity by over 20% in aggressive saline environments [9].

In the context of self-compacting mortars (SCMs), the use of finely ground powders such as MWP introduces complexity in fresh-state rheology. High surface area and water demand can impair flowability, increase yield stress, and cause bleeding or segregation if not balanced with proper admixture dosage and particle grading. However, fine oxide additives have been shown to improve paste homogeneity and viscosity control, supporting stable flow regimes in self-consolidating systems [12,15].

The effects of nanoalumina (NA) and rice husk ash (RHA) additives on the fresh, mechanical, and durability properties of self-compacting (SC) mortars were investigated in the literature [16]. In the study, cement replacement was performed using NA at ratios of 0%, 1%, 3%, and 5%, and RHA at ratios of 0% and 30%. It was observed that the addition of NA and RHA had no significant effect on the workability and water absorption rate of the mortar. In terms of mechanical properties, significant increases in compressive and flexural strength were obtained, especially when RHA was used in combination with NA. Ultrasonic pulse velocity measurements also supported mechanical development. Rapid chloride permeability and electrical resistance tests conducted within the scope of durability properties showed that NA and RHA additives reduced permeability and significantly increased the mortar’s electrical resistance values. This contributes positively to the material’s long-term durability. In conclusion, it was determined that SC mortars containing 30% RHA and 3% NA offer the best performance in terms of both mechanical and durability properties. This combination is considered a highly promising solution for the production of sustainable and high-performance mortars.

A study investigating the synergistic effects of aluminum oxide (Al₂O₃) nanoparticles and glass fibers on the mechanical and durability properties of self-compacting concrete (SCC) using an experimental and analytical approach is available in the literature [17]. The best performance was achieved with a mixture containing 2.0% Al₂O₃ nanoparticles and 1.5% glass fibers. This mixture provided a 61% increase in compressive strength and a 107.6% increase in tensile strength. Additionally, a 46% reduction in water absorption and a 265% increase in electrical resistance were observed. A 39% increase in bond strength was also recorded, indicating an improvement in the bond between the concrete and the reinforcement.

To illustrate the state of research,

Table 1. Summary of selected studies on MWP and oxide additives in cementitious systems.

Study	Material System	Type of Additive	Replacement/Dose	Key Observations
Etli et al. [13]	SCM	MWP	0–35% by vol.	The use of MWP as a mineral additive in volume ratios of 0–35% was investigated. According to the findings, the use of MWP in ratios of 10% and below increases mechanical strength. However, when the additive ratio exceeded 20%, a significant increase in porosity was observed, which led to a decrease in durability.
Gesoglu et al. [14]	SCC	Plastic/MWP blend	10–30%	Plastic waste and MWP mixtures were used as admixtures in self-compacting concrete (SCC) at concentrations ranging from 10% to 30%. The results indicate an improvement in microstructure. However, it has been reported that some difficulties were encountered in mixture stability due to the effect of the admixtures on the flowability of fresh concrete.
Nguyen et al. [15]	SCC	Dolomite powder	5–15%	The use of dolomite powder in the SCC system at rates of 5–15%. The findings reveal that dolomite powder increases the fluidity of concrete. However, when the additive ratio exceeds 15%, a decrease in compressive strength is observed.
Wang et al. [8]	Paste	Iron ore tailings + Al2O3	Al2O3 @ 3%	Iron ore waste and 3% Al2O3 additive were used in cement paste. The findings show that the formation of C-A-S-H gels is promoted, and the early hydration process is accelerated. This has a positive effect, especially in terms of early age strength.
Liu et al. [9]	Mortar	Supersulfated cement + Fe2O3	Fe2O3 @ 3%	3% Fe2O3 additive was used in mortar systems together with sulfate cement. The results show an increase in resistance, particularly against chloride and sulfate ions. However, no significant change was observed in mechanical strength; strength remained constant.
Sua-iam et al. [11]	SCC	Limestone + rice husk ash + Al2O3	15–30% blend	A mixture of limestone, rice husk ash, and Al2O3 was used in SCC at ratios of 15–30%. The data obtained showed that this additive combination provided a 20–25% increase in strength. In addition, it was determined that there was an approximately 30% decrease in water absorption.

summarizes selected recent studies that investigate the use of MWP or oxide additives in cement-based systems, including their impact on fresh and hardened properties.

Taken together, these studies suggest that combining oxide reinforcement with recycled powders like MWP offers a pathway to engineering low-carbon, high-performance SCMs. However, there remains a clear research gap in the following:

• Evaluating combined Al₂O₃ and Fe₂O₃ reinforcement in MWP-based self-compacting systems;
• Assessing their durability under saltwater exposure;
• Linking microstructural evolution to mechanical and durability properties.

Mortar waste powder (MWP) is a fine-grained material obtained by crushing or grinding hardened mortar and has significant recycling potential due to its easy local availability. However, due to MWP’s low pozzolanic reactivity and hydrated stable phase content, its use as a cement substitute often leads to negative outcomes such as reduced mechanical strength, increased porosity, and decreased durability. In order to overcome these limitations, chemical activation strategies using transition metal oxides such as aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃) are proposed. The potential of these oxides to accelerate early-age hydration kinetics, increase microstructural density, and improve durability has been demonstrated in various studies in the literature. However, the effect of these additives on the fresh and hardened state performance of self-compacting mortar (SCM) systems, particularly when used in conjunction with MWP, has not yet been comprehensively investigated. This situation highlights an important research gap in studies aimed at converting low-reactive waste materials into sustainable and high-performance binders. The objective of this study is to evaluate the usability of MWP activated with Al₂O₃ and Fe₂O₃ additives in SCM systems and to fill this gap in the literature by elucidating their effects on the mechanical, workability, and durability properties of these systems.

This study addresses these gaps by systematically analyzing the performance of MWP-modified SCMs with and without Al₂O₃ and Fe₂O₃ additions. Investigating the performance of self-compacting mortars in which OPC is partially replaced (by volume) with MWP, either alone or reinforced with Al₂O₃, Fe₂O₃, or their combination. The investigation covers:

• Fresh-state properties (mini-slump flow diameter, V-funnel time);
• Mechanical performance (compressive and flexural strength);
• Durability indicators (abrasion resistance and porosity);
• Different environmental regimes.

It is hypothesized that oxide-reinforced MWP, particularly with combined Al₂O₃ and Fe₂O₃ additions, will significantly reduce porosity and improve both mechanical and durability performance—offsetting the drawbacks typically associated with MWP inclusion and enabling its effective use in SCM applications under aggressive environmental exposure.

2. Experimental Program

2.1. Materials

The self-compacting mortars developed in this study were composed of ordinary Portland cement (OPC), mortar waste powder (MWP), aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), and natural river sand. All materials were selected based on their availability, environmental relevance, and performance characteristics, in accordance with sustainable cement substitution strategies reported in recent literature [9,18].

The primary binder used was CEM I 42.5R OPC conforming to EN 197-1 standards [19]. Its chemical composition, determined via X-ray fluorescence (XRF), is presented in

Table 2. Chemical composition and physical properties of Portland cement.

Chemical Composition	(%)
CaO	63.37
SiO2	19.34
Al2O3	3.75
Fe2O3	4.15
MgO	3.1
SO3	3.15
K2O	0.81
Na2O	0.41
Loss of ignition	1.92
Blaine (m2/kg)	366
Specific gravity (g/cm3)	3.15

. The Blaine fineness was 366 m²/kg, and the specific gravity was 3.15 g/cm³, ensuring appropriate reactivity and volumetric control in mortar formulation.

Mortar waste powder (MWP) was produced by crushing and milling precast concrete residues collected from a local batching plant. The waste material was oven-dried at 105 ± 5 °C, mechanically ground, and sieved through a 63 µm mesh to ensure uniformity. MWP was characterized by a combination of unreacted clinker minerals, Ca(OH)₂, quartz sand, and C-S-H remnants, consistent with previously reported waste mortar powders [11,18]. Although its pozzolanic activity is limited, it contributes significantly as a reactive filler and nucleation enhancer when coupled with chemical activators.

To improve hydration kinetics and microstructural development, two commercial oxide powders were introduced, aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). Al₂O₃ (>99.5% purity, alpha-phase, average particle size ~1 µm) and Fe₂O₃ (>98.5% purity, hematite-phase, ~0.8 µm) were used without further treatment. These oxides were dry-mixed with MWP in designated ratios before integration into the mortar mix.

Fine aggregate consisted of natural river sand conforming to EN 12620 [20] grading limits for mortar applications. The sand was cleaned, oven-dried, and sieved to remove particles above 4 mm. The cumulative particle size distribution, shown in Figure 1, demonstrates a continuous gradation ranging from 0.063 mm to 4 mm. Well-graded aggregates are essential in self-compacting systems to minimize internal friction, reduce water demand, and ensure flow consistency without segregation. The sand’s physical properties were verified using standard sieving and pycnometry procedures.

In Figure 2, showing the relevant particle distribution, cement typically has a finer particle distribution, with 88% of particles smaller than 32 µm. This fine structure has been reported by many researchers in the literature to positively affect hydration rate and early-age strength by increasing the specific surface area [2]. On the other hand, waste mortar dust (MWP) sieved through a 0.063 mm sieve exhibits a granulometry dominated by coarser fractions, with 45% of particles smaller than 32 µm; despite its limited reactivity, this has been emphasized by other researchers in the literature [21,22]. In self-compacting mortar designs, careful evaluation of the fine material content is necessary when attempting to increase fluidity and stability by reducing the water–cement ratio, as excessive use (e.g., more than 15–20% replacement) may negatively affect strength by causing particle agglomeration, as reported in the literature [21,22]. Additionally, the literature reports that excessive fine structures can disrupt the hydration balance, creating weak zones within the internal structure [23]. In this context, considering the dimensional characteristics; cement is the primary binder that provides stability and early strength due to its fine-grained structure; MWP, when used in appropriate ratios (e.g., 10–15%), improves rheology and filling effect while offering environmental and economic advantages, but excessive use carries the risk of segregation and loss of mechanical performance.

In the SEM images of the cement sample, it was observed that the particles were irregularly shaped and angular and had relatively sharper edges (Figure 3). This structure reflects the structure of cement composed of ground clinker phases [24]. Upon examination of the surface topography, it was observed that the particles were distinctly smooth and did not contain hydration products. This indicates that the sample did not undergo hydration or that the hydration process occurred to a very limited extent. Additionally, no bonding phase was observed between the particles, and the microstructure exhibited a porous structure.

SEM images of mortar waste reveal a highly heterogeneous structure and dense surface topography (Figure 3). Characteristic formations associated with hydration products have been observed on the sample surface. In particular, fibrous and amorphous calcium silicate hydrate (C-S-H) gel, known for its layered morphology, and calcium hydroxide (Ca(OH)₂) crystals have been clearly identified in mortar waste [25]. Additionally, aggregate residues embedded within the binder phase and microscopic voids between these phases are present. This indicates that the mortar waste is a material that has completed its hydration process and hardened, but its structural integrity is partially compromised due to microcracks and voids.

2.2. Mixture Proportioning and Sample Preparation

To investigate the effects of mortar waste powder (MWP) replacement and oxide reinforcement on the performance of self-compacting mortar (SCM), eleven mortar mixtures were formulated, labeled SCM1 through SCM11. These mixes were designed with a systematic increase in MWP content, replacing ordinary Portland cement (OPC) from 0% to 50% by volume, in 5% increments. The replacement was carried out on a volumetric basis, and mix proportions were adjusted according to the specific gravity of OPC (3.15 g/cm³) and MWP (2.65 g/cm³) to maintain a constant total binder volume. This approach ensured consistent paste content and rheological comparability across all mixtures, as recommended in prior studies [13,14].

All mixtures were prepared with a fixed water-to-binder ratio (w/b) of 0.38 and a binder-to-sand ratio of 1:2.54. A high-range water-reducing admixture (HRWR), based on polycarboxylate ether and conforming to ASTM C494 [26] Type F, was used in different percentages at a maximum dosage of 2% by total binder weight.

The mixing procedure was standardized to ensure homogeneity and reproducibility. Dry materials (OPC, MWP, and oxide powders) were blended for 60 s at low speed. Subsequently, 50% of the water and HRWR solution was added, followed by mixing at medium speed for 2 min. The remaining water and admixture were then introduced, and the mixture was further blended for 3 min to achieve a uniform, flowable consistency without segregation. No external vibration or compaction was applied, in accordance with the principles of self-compacting mortar.

The use of Al₂O₃ (alumina) and Fe₂O₃ (iron oxide) additives in cement-based mortars at appropriate ratios is important for improving mechanical properties, increasing durability, and optimizing hydration processes. The addition of Al₂O₃ at the nano scale particularly increases early-age strength and reduces water permeability by improving the density of the cement matrix. This effect is most pronounced when added at a rate of 5–10% [27]. Similarly, Fe₂O₃ addition is also effective in improving properties such as color control and thermal resistance in cement mortars, with the generally recommended optimal addition rate being between 3–6% [28,29]. However, since the use of these additives at high ratios may negatively affect the crystalline structure of hydration products, optimal addition levels must be carefully determined [3,24]. The combined use of Al₂O₃ and Fe₂O₃ can create synergistic effects, particularly in systems supported by pozzolanic materials such as fly ash or waste marble dust, resulting in positive outcomes in terms of capillary water absorption, strength development, and microstructure [28,29]. Therefore, the contribution ratios of both oxides should be experimentally optimized in accordance with the composition of the binder system and the targeted engineering performance.

The fresh mortars were poured into 40 × 40 × 160 mm³ prismatic molds in two layers and allowed to stand for 24 h under polyethylene sheeting. Specimens were then demolded and subjected to cure. Curing durations of 7, 28, and 56 days were selected to evaluate early-age and long-term performance. All physical, mechanical, and durability tests were conducted on three replicates per age and condition. The detailed mixture proportions for each formulation are provided in

Table 3. Mixture designs (kg/m³).

MIX ID.	Cement	Reinforced Mortar Waste (0.063 mm)	HRWR	Water	Sand	% Substitution Rate by Weight
SCM1	600	0	12	230	1524	0
SCM2	570	30	12	230	1524	5
SCM3	540	60	12	230	1524	10
SCM4	510	90	12	230	1524	15
SCM5	480	120	12	230	1524	20
SCM6	450	150	12	230	1524	25
SCM7	420	180	12	230	1524	30
SCM8	390	210	12	230	1524	35
SCM9	360	240	12	230	1524	40
SCM10	330	270	12	230	1524	45
SCM11	300	300	12	230	1524	50

.

3. Sample Preparation

A comprehensive experimental protocol was designed to evaluate the influence of mortar waste powder (MWP) and its reinforcement with aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃) on the performance of self-compacting mortars (SCMs). The procedure included a combination of fresh-state rheological assessments (slump flow (Figure 4a and v-funnel test Figure 4b), mechanical strength tests (flexural strength test (Figure 4c), compressive strength test (Figure 4d)), high-temperature resistance evaluations (Figure 4e), freeze–thaw (Figure 4f) durability analysis, and microstructural characterization via porosity and density measurements (Figure 4g). All methods were aligned with international standards and contemporary research protocols to ensure reliability and comparability with recent studies [8,11].

Fresh-state properties were assessed immediately after mixing. The workability of each mixture was evaluated using the mini-slump flow and V-funnel tests, both of which are well-established indicators of flowability and viscosity in SCC and SCM systems [12].

Prismatic specimens (40 × 40 × 160 mm³) were prepared for mechanical testing. The fresh mortar was poured into oiled steel molds without compaction to preserve the self-compacting nature of the mix. Specimens were covered with plastic sheets for the initial 24 h to prevent moisture loss, after which they were demolded and submerged into water at 20 ± 2 °C.

3.1. Flexural Strength Test

The flexural strength of the self-compacting mortar specimens was determined in accordance with EN 196-1 [30], using a standard three-point bending configuration. Prismatic specimens with dimensions of 40 × 40 × 160 mm³ were tested using a universal testing machine equipped with a calibrated load cell and displacement control system. The test was conducted at a constant loading rate of 50 ± 10 N/s, and the maximum load at failure was recorded.

To ensure statistical robustness, three specimens per mix and test age (7, 28, and 56 days) were evaluated, and the average value was reported. This test provided insight into the influence of mortar waste powder content and oxide additives on the tensile performance of SCM under bending loads.

3.2. Compressive Strength Test

The compressive strength of self-compacting mortar specimens was determined using the broken halves from the flexural strength tests, as prescribed by EN 196-1 [30]. Each half-prism was tested using a digital compression testing machine with a maximum capacity of 2000 kN and an automatic load application rate set to 2.4 ± 0.2 kN/s, ensuring consistency across all specimens. To ensure statistical robustness, three specimens per mix and test age (7, 28, and 56 days) were evaluated, and the average value was reported.

3.3. Mechanical Test After High Temperature

To assess thermal resistance and the post-fire mechanical integrity of self-compacting mortars incorporating reinforced mortar waste powder, a series of high-temperature exposure tests were performed. The methodology was designed to simulate fire scenarios and evaluate the retention of mechanical strength following thermal degradation. At 28 days of curing, selected prismatic specimens were dried at 105 °C for 24 h to eliminate residual moisture and prevent explosive spalling during heating. The dried samples were then subjected to thermal treatment in an electric muffle furnace (±5 °C accuracy). Exposure temperatures were set at 300 °C, 600 °C, and 800 °C (Figure 5).

The heating rate was maintained at 5 °C/min until the target temperature was reached. Specimens were held at peak temperature for 2 h to ensure thermal equilibrium, followed by natural air cooling at ambient room conditions (20 ± 2 °C) to mimic realistic post-fire cooling. After cooling, the specimens were visually inspected for cracking, discoloration, and mass loss and subsequently tested for flexural and compressive strength (Figure 5).

3.4. Mechanical Test After Freeze–Thaw Cycles

To evaluate the long-term durability of self-compacting mortars under cyclic environmental stress, a freeze–thaw resistance test was conducted in accordance with a modified ASTM C666 Procedure B [31], which simulates deterioration caused by water freezing within the pore structure of cement-based materials. After 28 days of curing, specimens were fully saturated in water and subjected to repeated freeze–thaw cycles using a programmable environmental chamber. Each cycle consisted of freezing at −18 ± 2 °C followed by thawing at +20 ± 2 °C, with one complete cycle lasting approximately 4 h. Specimens were exposed to a total of 25, 50, and 100 cycles and removed after each designated interval for mechanical strength testing.

In our study, Method B of the ASTM C666 [31] standard was applied to evaluate the freeze–thaw resistance of concrete samples. This method requires that the samples be kept in the air but in a humid environment during the test period, thereby ensuring that the concrete is exposed to outdoor conditions in a more realistic manner. Method B is particularly suitable for simulating freeze–thaw effects observed on exterior surfaces or structural elements that are occasionally exposed to moisture, as the samples are not fully submerged in water. Therefore, the test method used in our study reliably reflects the concrete’s performance under real-world service conditions.

3.5. Porosity and Specific Gravity Tests

To evaluate the internal pore structure and density-related characteristics of the produced self-compacting mortar (SCM) mixtures, porosity and specific gravity tests were conducted on samples cured for 28 days under standard conditions. These parameters provide essential insight into the transport properties and overall durability of cementitious systems—particularly in recycled powder-reinforced mortars. The tests followed procedures adapted from ASTM C642 [32], which defines a gravimetric method for assessing apparent porosity and bulk density through a series of saturation, drying, and weighing steps. Initially, dried samples were immersed in water at 21 ± 2 °C for 48 h to ensure complete saturation and displacement of entrapped air. Subsequently, the surface-dried saturated mass was recorded using a precision digital scale. The specimens were then oven-dried at 105 °C for 24 h until a constant dry mass was achieved. This procedure enabled the calculation of total porosity using the following relationship.

$$Porosity \left(\%\right)={\displaystyle \frac{M_{sat}-M_{dry}}{M_{sat}-M_{sub}}}\times 100$$

where

• M_sat = saturated surface-dry mass (g)
• M_dry = oven-dried mass (g)
• M_sub = submerged mass in water (g)

Specific gravity was calculated based on the dry volume method using the ratio of dry mass to the volume of displaced water, assuming negligible surface absorption error.

4. Results and Discussion

4.1. Fresh State Test Results

The fresh properties of the self-compacting mortar (SCM) mixtures were assessed using the mini-slump flow and V-funnel tests in accordance with EFNARC [12] guidelines. These tests evaluate the flowability and viscosity of the mix, which are critical parameters for ensuring proper consolidation without mechanical compaction.

The mini-slump cone used had a top diameter of 70 mm, bottom diameter of 100 mm, and height of 100 mm. The average of two orthogonal flow diameters was recorded, while V-funnel flow time was measured as the time required for mortar to completely discharge under gravity. These tests are sensitive to fine particle content and the rheological effects introduced by both MWP and oxide additions.

As shown in

Table 4. Mini-slump flow and V-funnel results of SCM mixtures with varying MWP content.

Mix-ID	Flowing Diameter (mm)	V-Funnel (s)
SCM1	259	10.51
SCM2	257	10.16
SCM3	256	9.82
SCM4	254	9.47
SCM5	251	9.13
SCM6	250	8.79
SCM7	249	8.3
SCM8	246	8.16
SCM9	246	8
SCM10	242	7.3
SCM11	240	7.01

, increasing the replacement of OPC with mortar waste powder (MWP) from 0% to 50% led to a gradual reduction in flow diameter, from 259 mm in SCM1 to 240 mm in SCM11. This trend suggests that while MWP contributes to filler packing and increased solid volume, its angular particle morphology and increased water demand reduce the overall mobility of the mix. These findings are consistent with earlier studies where recycled fine powders—such as crushed mortar or ceramic waste—led to a moderate decline in spread diameter due to frictional resistance and limited surface smoothness [14,18,33,34]

The V-funnel test results indicate a parallel trend, with discharge time decreasing from 10.51 s in SCM1 to 7.01 s in SCM11. While reduced V-funnel time typically suggests lower viscosity and faster flow, in this context, it may reflect the dilution of reactive binder with inert fines and a potential increase in mix water content available for lubrication. This apparent reduction in flow resistance may lead to higher risk of segregation or bleeding, especially beyond 30% MWP substitution—though no visible signs were observed during testing (Table 4).

Interestingly, mixes with low-to-moderate MWP content (up to 15–20%) demonstrated comparable or only slightly reduced flowability relative to the control, suggesting an optimal filler effect. The uniform dosage of high-range water-reducing admixture (HRWR) across all mixtures helped maintain adequate flow properties, counterbalancing the rise in surface area introduced by MWP. Furthermore, the presence of finely dispersed Al₂O₃ and Fe₂O₃ may have contributed to better paste cohesion and particle dispersion due to their known nucleation and filler effects, as reported by Wang et al. [8] and Sua-iam et al. [11].

Overall, the results confirm that MWP can be used up to 30% by volume in SCM systems without significant compromise to flowability, especially when supported by oxide additives. However, beyond this threshold, the rheological performance begins to degrade and may require additional admixture optimization or viscosity-enhancing agents to maintain self-compacting behavior.

4.2. Compressive Strength Test

Compressive strength was evaluated at 7, 28, and 56 days to assess the mechanical evolution of the self-compacting mortar (SCM) mixtures. Testing was carried out in accordance with EN 196-1 [30], using 40 × 40 × 160 mm prisms broken in flexure and tested on both halves. Results represent the average of three specimens, with standard deviation below 5%, confirming the reproducibility of the data (Figure 6).

SCM1 (reference mix) achieved the highest compressive strength at all curing ages, reaching 82.32 MPa at 56 days. However, SCM2 (5% MWP) and SCM3 (10% MWP), reinforced with aluminum and iron oxides, demonstrated nearly equivalent early and long-term performance—recording 81.19 and 72.71 MPa, respectively, at 56 days (Figure 6). These results highlight the synergistic role of Al₂O₃ and Fe₂O₃ in promoting hydration, densifying the microstructure, and compensating for the reduced cement content through nucleation and filler effects [8,9].

At early ages (7 days), SCM2 slightly outperformed SCM1, likely due to the formation of early C-A-H phases and accelerated hydration kinetics stimulated by Al³⁺ ions [11]. The finely dispersed oxides act as both chemical activators and micro-fillers, enhancing particle packing and reducing pore continuity in the early hydration regime.

Beyond 10–15% MWP replacement (SCM4–SCM11), compressive strength declined progressively, particularly in mixes exceeding 30% replacement. SCM11 (50% MWP) achieved only 36.06 MPa at 56 days—less than half of SCM1 (Figure 6). This degradation can be attributed to a combination of the following:

• Reduced clinker content, limiting long-term C-S-H formation;
• Inert crystalline phases in MWP that dilute reactivity;
• Elevated porosity and microcracking.

Moreover, while Al₂O₃ and Fe₂O₃ can stimulate hydration at low doses, their effectiveness diminishes when binder dilution becomes dominant. Fe₂O₃ is not highly reactive during early hydration and contributes primarily to long-term stability via ferrite phase development, but this is insufficient to counterbalance high MWP ratios. Additionally, interfacial transition zones (ITZ) between MWP particles and fresh paste may remain weak and porous, further limiting compressive integrity [10].

Interestingly, SCM4 and SCM5 maintained stable strength development up to 56 days, suggesting that 15–20% MWP replacement may represent the upper threshold for maintaining structural-grade compressive performance when reinforced with oxides. Beyond this level, the negative effects of increased porosity and reduced hydration outweigh the benefits of oxide activation.

These results are consistent with prior findings on recycled powder use in self-compacting systems, where strength retention is highly dependent on replacement level, particle fineness, and chemical activation [14,18,34]. The findings further validate that controlled MWP incorporation (≤10%), when paired with dual oxide reinforcement, can deliver compressive performance comparable to conventional OPC systems—while offering substantial reductions in carbon footprint and cement consumption.

4.3. Flexural Strength Test

Flexural strength was assessed at 7, 28, and 56 days for all mortar mixtures to evaluate the tensile behavior of the hardened matrix under bending. Tests were conducted in accordance with EN 196-1 [30], using 40 × 40 × 160 mm prisms, and the results reported represent the average of three replicates per mixture, with observed standard deviations below 8% (Figure 7).

At 7 days, SCM3 (10% MWP) demonstrated the highest flexural strength (11.34 MPa), exceeding the control (SCM1) (Figure 7). This early performance enhancement can be attributed to the micro-filler effect of the finely ground mortar waste and the accelerated hydration promoted by aluminum oxide (Al₂O₃), which provides additional nucleation sites for calcium-aluminate hydrate (C-A-H) formation [8]. The combined effect may also facilitate early ettringite formation, contributing to rapid matrix stiffening.

By 28 days, SCM2 (5% MWP) reached the highest recorded flexural strength of 13.74 MPa, surpassing the control (Figure 7). However, beyond 15–20% replacement levels, a decline in strength was observed, reflecting a dilution of active clinker phases and reduced bond continuity due to the presence of inert hydrated remnants within the recycled particles [7]. This trend is consistent with prior studies showing reduced tensile performance at high SCM replacement ratios [7,13].

At 56 days, the reference mix maintained the highest residual strength (13.69 MPa), followed closely by SCM3 and SCM2. In contrast, SCM10 and SCM11—comprising 45% and 50% MWP, respectively—exhibited a marked drop in flexural performance (6.21 and 4.59 MPa), likely due to increased porosity and discontinuities in the matrix (Figure 6). These observations are supported by porosity results and SEM-based microstructural studies in the literature [10], which link flexural deterioration to increased capillary void content and microcracking near aggregate–paste interfaces.

Importantly, oxide reinforcement appeared to stabilize the mechanical properties of low-to-moderate MWP mixes, as seen in SCM2–SCM4. This suggests that strategic use of Al₂O₃ and Fe₂O₃ may enhance early-age mechanical bonding and mitigate the long-term weakening typically associated with recycled powders.

In some mixtures (especially SCM7–SCM10 with a replacement ratio of 30–45%), it has been observed that the 7-day flexural strength is higher than the 56-day strength (Figure 6). One of the main reasons for this is that the high proportion of recycled mortar waste contributes only to a limited extent to the hydration reactions [35,36]. The increase in mechanical strength due to Portland cement hydration at an early age may decline over time due to increased porosity and microcrack formation [24]. High replacement ratios disrupt the homogeneity of the binder matrix, leading to microstructural weakening and increased porosity, which in turn causes a decrease in durability and strength in the long term [2]. Additionally, inert or low-reactive components in recycled mortar waste can hinder the continuity of pozzolanic reactions, limiting strength gain in later stages [37]. Therefore, the decrease in early strength with advancing age in mixtures with high replacement ratios can be considered an expected behavior.

4.4. Compressive Strength Test After High Temperature

The residual compressive strength of self-compacting mortar (SCM) mixtures following high-temperature exposure provides a crucial indicator of structural integrity in fire scenarios. Specimens were exposed to 300 °C, 600 °C, and 800 °C using a programmable furnace, with a uniform heating rate of 5 °C/min and a 2-hour dwell time at the target temperature. After cooling to ambient conditions in still air, compressive strength was measured per EN 196-1 [30]. Each reported value is the mean of three replicates, with standard deviation remaining below 5% (Figure 8).

A notable strength gain was observed in several mixes at 300 °C, particularly SCM1 and SCM3, which increased by approximately 13.9% and 15.9%, respectively (Figure 8). This early-stage enhancement is attributed to accelerated hydration of unreacted clinker particles, evaporation of free water, and pore refinement, which collectively improve microstructural compaction [38]. The SCM6 mix (25% MWP) also showed strength enhancement at this temperature, demonstrating the beneficial synergy of moderate waste content and oxide reinforcement under thermal stress.

At 600 °C, all mixtures experienced a marked reduction in compressive strength. This degradation correlates with the decomposition of calcium silicate hydrate (C-S-H), loss of chemically bound water, and initiation of matrix cracking. Nevertheless, oxide-reinforced mixes such as SCM2 and SCM5 retained greater integrity (59.29 and 50.43 MPa, respectively) (Figure 8), likely due to the formation of thermally stable hydration phases, such as C-A-H and ferrite-based compounds, which delay structural collapse [8,39].

By 800 °C, residual strength dropped sharply in all mixtures. SCM1 and SCM2, initially among the strongest, fell to ~34 and ~37 MPa, respectively, retaining less than 50% of their original compressive capacity (Figure 8). High MWP replacement levels (e.g., SCM10 and SCM11) led to critical mechanical failure, with strengths falling below 13 MPa, a threshold below which structural performance is highly compromised. These results confirm that excessive incorporation of inert MWP reduces thermal resistance, primarily due to increased porosity and weakened interfacial bonding.

Interestingly, SCM5 and SCM6—blends with 20–25% MWP and dual oxide reinforcement—retained higher strength than some of the lower-replacement mixes, indicating an optimal range of substitution for high-temperature performance (Figure 8). This performance is further linked to the pore densification effect of Al₂O₃ and the microstructural stabilizing role of Fe₂O₃, which together act to reduce internal cracking and enhance post-fire integrity [11].

In sum, while high-temperature exposure universally deteriorates compressive strength, low-to-moderate MWP incorporation (up to 25%) combined with oxide additives provides superior residual strength under fire-like conditions. The long-term thermal reliability of SCM systems can thus be improved without compromising sustainability goals, provided mix design is carefully optimized.

4.5. Flexural Strength Test After High Temperature

Thermal stability of cementitious materials is essential for fire-resistant infrastructure design. To assess flexural strength retention under high-temperature conditions, mortar specimens cured for 28 days were subjected to thermal exposures at 300 °C, 600 °C, and 800 °C. Heating was conducted in a muffle furnace with a controlled rate of 5 °C/min, followed by a 2-hour holding period at the target temperature. The specimens were then allowed to cool gradually in ambient air before testing. Flexural strength was determined in accordance with EN 196-1 [30], using a three-point loading setup. Each value represents the average of three specimens, and standard deviation was less than 6% in all cases (Figure 9).

At 28 days (unheated), SCM2—reinforced with both Al₂O₃ and Fe₂O₃—recorded the highest flexural strength (13.74 MPa), outperforming the OPC-only mix SCM1. Upon exposure to 300 °C, strength generally declined by 15–25%; however, SCM6 (25% MWP) showed an unexpected increase (from 9.24 to 10.24 MPa) (Figure 9), possibly due to thermal drying and pore water evaporation, which may cause transient densification of the matrix—a behavior previously reported in low-permeability cement composites [38].

Significant strength degradation was observed at 600 °C, where all mixes lost over 50% of their original flexural capacity (Figure 9). This is attributed to thermal decomposition of calcium silicate hydrate (C-S-H) gels, microcracking, and internal stress buildup due to mismatched thermal expansion between aggregates and paste [39]. SCM2 and SCM5 retained better integrity (≥5 MPa), likely due to the stabilizing role of Fe₂O₃, which enhances ferrite phase content and resists decomposition at high temperatures.

At 800 °C, flexural strength dropped to near residual levels in most samples. SCM3, despite its high initial strength, degraded sharply to just 0.69 MPa—reflecting the inability of low-level oxide content to compensate under thermal shock. In contrast, SCM5 and SCM6 retained over 3 MPa, confirming the synergistic benefit of moderate MWP substitution coupled with metal oxide reinforcement in maintaining heat-resisting microstructure (Figure 9). The denser particle packing and lower free water content in these mixtures may also reduce vapor-induced spalling at high temperatures.

The progressive thermal deterioration aligns with observed trends in cement composites exposed to fire conditions. The superior performance of oxide-reinforced SCMs at 600–800 °C can be linked to microstructural densification, improved interfacial transition zones (ITZ), and formation of thermally stable C-(A)-S-H and ferrite phases [8,11]. Nevertheless, beyond a 30% MWP replacement level, porosity and matrix discontinuity dominates the behavior, rendering oxide benefits insufficient (Figure 9).

This confirms that oxide reinforcement is effective only within a certain range of MWP content, and beyond this threshold, mechanical integrity is governed primarily by volumetric dilution and pore network instability.

4.6. Compressive Strength Test After Freeze–Thaw Cycles

The compressive strength behavior of the mortar mixtures subjected to repeated freeze–thaw (F–T) cycles was evaluated in accordance with ASTM C666 (Procedure A) [31], which involves rapid cycling between –18 °C and +4 °C in a saturated condition. The specimens were tested after 25, 50, and 100 cycles using EN 196-1 [30,39,40,41] standards, and average values from three replicate samples were reported with a maximum standard deviation below 7% (Figure 10).

Results indicate that all mixtures demonstrated considerable resistance to freeze–thaw degradation up to 100 cycles. Notably, SCM1 (control) and SCM3 (with 10% MWP and oxide additives) showed strength gains after 25 and 50 cycles (Figure 9). This apparent improvement can be attributed to the continued hydration of unreacted cementitious phases and possible reactivation of latent aluminates under cyclic saturation conditions. Similar strength increases have been reported in other studies utilizing mineral admixtures with latent hydraulic activity [39,40,41].

The positive performance of SCM3 is particularly noteworthy, as it maintained over 105% of its original strength even after 100 cycles. This suggests that the combination of MWP with Al₂O₃ and Fe₂O₃ may enhance microstructural densification and reduce freeze–thaw-induced cracking. Aluminum oxide is known to accelerate aluminate hydrate formation and contribute to early matrix cohesion, while iron oxide helps stabilize hydration products against chemical decomposition under thermal and moisture stress [8,9].

On the other hand, SCM10 and SCM11, which contained 45% and 50% MWP, respectively, showed a steady decline in strength beyond 50 cycles, with reductions reaching ~20% at 100 cycles (Figure 10). This behavior is consistent with their elevated total porosity and weaker interfacial transition zones (ITZ). As MWP content increases beyond 30%, the internal pore structure becomes more connected, facilitating water ingress and ice lens expansion, which ultimately lead to internal microcracking and reduced load-carrying capacity.

While none of the mixtures dropped below the generally accepted durability threshold of 75% residual compressive strength (per ACI durability criteria), the results emphasize the importance of optimizing the balance between recycled powder content and reactive oxide additions to maximize freeze–thaw resistance.

4.7. Flexural Strength Test After Freeze–Thaw Cycles

Freeze–thaw (F–T) resistance is a vital durability criterion for cementitious materials exposed to cyclic freezing in moist environments. This study evaluated the F–T performance of self-compacting mortars (SCMs) by subjecting prismatic specimens to 25, 50, and 100 cycles of freezing at –18 °C and thawing at +4 °C, following ASTM C666 Procedure A [31]. Flexural strength was measured after each stage using three-point bending (EN 196-1 [30]). All values represent the average of three specimens, with a standard deviation below 8% (Figure 11).

As shown in Figure 6, SCM1 and SCM3 exhibited superior freeze–thaw resistance, retaining over 50% of their original flexural capacity even after 100 cycles (Figure 11). The moderate MWP substitution (≤10%) and reinforcement with aluminum and iron oxides likely contributed to this performance by refining pore structure and limiting ice-induced microcracking. SCM4 and SCM5 maintained moderate durability, with flexural strength losses within 25–30% over 100 cycles.

In contrast, SCM10 and SCM11 (≥45% MWP replacement) showed rapid degradation beyond 25 cycles, dropping below 3.5 MPa at 100 cycles (Figure 10). This significant loss is attributed to increased capillary porosity and poor matrix cohesion at high MWP levels. According to Xu et al. [42], such microstructural vulnerabilities accelerate damage from freeze–thaw cycling due to hydraulic pressure buildup in connected pores.

The freeze–thaw degradation trend aligns with porosity data in Section 4.8, confirming that oxide-reinforced MWP mixtures remain effective only up to a threshold (≈20–25% replacement). Beyond this point, the internal structure cannot prevent frost-induced damage (Figure 10).

These findings highlight that partial substitution of cement with MWP, when reinforced with Al₂O₃ and Fe₂O₃, can yield freeze–thaw durable SCMs suitable for cold-region applications—provided the replacement level is carefully optimized.

4.8. Porosity and Specific Gravity Tests

Porosity and specific gravity are fundamental indicators of the microstructural integrity, permeability, and long-term durability of cementitious materials. In this study, the 28-day porosity and bulk-specific gravity of self-compacting mortar (SCM) specimens were evaluated in accordance with ASTM C642 [32]. The test procedure involved oven-drying at 105 °C for 24 h followed by immersion in water to determine volume absorption. Each value reported represents the average of three specimens, with a maximum observed variation below 5% (Figure 12).

The results reveal a gradual increase in porosity with higher proportions of mortar waste powder (MWP). SCM1 (control) exhibited a porosity of 16.00%, while SCM10 and SCM11 (with 45% and 50% MWP replacement) showed values exceeding 20% (Figure 12), which is often considered a critical threshold for durability in cement-based systems [10]. This trend reflects the reduced reactivity and higher surface area of MWP, which tends to increase water demand and create less cohesive particle packing—ultimately resulting in increased capillary porosity.

The micro-filler contribution of Al₂O₃ and Fe₂O₃ was particularly evident in SCM2 and SCM3 (Figure 12). These oxide-reinforced mixtures exhibited improved packing density and hydration kinetics, as reflected by their higher specific gravity (up to 2.219 g/cm³) and minimal increase in porosity (+0.1% over control). Aluminum oxide likely contributed to the formation of calcium aluminate hydrates and early nucleation, while iron oxide played a role in ferrite phase stabilization, both aiding in refining the pore structure [8,11].

While specific gravity generally decreased with increasing MWP dosage due to reduced clinker content and lower bulk density, some fluctuations (e.g., SCM7 and SCM8) suggest secondary hydration and filler compaction effects. Nonetheless, beyond 30% replacement (SCM7–SCM11), the matrix showed a significant rise in total porosity and lowered specific gravity, which aligns with the deterioration in mechanical strength and durability (Figure 12).

These findings emphasize that while low to moderate MWP content (≤15%) reinforced with reactive oxides can preserve or even enhance matrix density, excessive substitution leads to pore connectivity issues that may impair long-term resistance, particularly in aggressive environments.

4.9. XRD Results of Samples

When examining the XRD analysis results obtained within the scope of the study, it is clearly seen that the Al₂O₃ and Fe₂O₃ crystal phase densities gradually increase between the samples (Figure 13). In particular, the increasing peak densities associated with Al₂O₃ from Sample 1 to Sample 4 indicate a significant increase in the amount of alumina in the crystal structure. Al₂O₃ is known for its high hardness, thermal stability, and chemical inertness, which significantly enhance the mechanical strength of ceramic matrices [43,44]. Accordingly, improvements in wear resistance, hardness, and structural stability are expected as the Al₂O₃ content increases. However, due to its inherently brittle nature, excessive addition of Al₂O₃ may lead to increased crack propagation, necessitating an optimal concentration [44].

On the other hand, Fe₂O₃ exhibits a similar peak density increase in all samples, indicating a higher phase presence in successive formulations. The effect of Fe₂O₃ on mechanical properties is twofold. At low concentrations, Fe₂O₃ can increase fracture toughness by promoting crack deflection and supporting grain boundary cohesion [45,46]. However, at higher concentrations—particularly evident in Sample 4—Fe₂O₃ can contribute to microstructural porosity and thermal mismatch stress, ultimately reducing the material’s mechanical strength [46]. These negative effects are generally attributed to phase mismatch and grain boundary weakening due to excessive iron oxide content.

As a result, while increasing the Al₂O₃ content tends to improve mechanical performance, the effect of Fe₂O₃ is more complex and concentration-dependent. Sample 3, which has a high Al₂O₃ and moderate Fe₂O₃ content, appears to offer the most balanced mechanical properties. These findings are consistent with previous studies on ceramic systems and demonstrate that phase composition control guided by XRD data is necessary to optimize mechanical performance in alumina-based materials [47].

5. Conclusions

This study evaluated the mechanical and durability performance of self-compacting mortars incorporating mortar waste powder (MWP) as a partial cement replacement, reinforced with aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). The integration of MWP up to 10–15% by volume, when combined with oxide additives, demonstrated comparable or superior performance to conventional mixtures, both in fresh and hardened states.

Future studies should incorporate microstructural analysis (e.g., SEM, XRD, MIP) and long-term durability evaluations under sulfate and marine environments. Additionally, life-cycle assessment will be essential to quantify the full environmental benefit of oxide-reinforced recycled binder systems.

• In fresh-state assessments, slump flow decreased modestly from 259 mm (control) to 240 mm (50% MWP), while V-funnel times improved from 10.51 s to 7.01 s, suggesting enhanced flowability due to reduced paste viscosity at higher substitution rates. However, the best rheological performance was maintained in oxide-reinforced mixes up to 10% MWP.
• Compressive strength at 28 days reached 75.62 MPa in SCM2 (5% MWP + oxides), exceeding the control (76.58 MPa) and confirming the beneficial role of Al₂O₃ in accelerating hydration and Fe₂O₃ in stabilizing microstructure. Beyond 25% substitution, strength values declined sharply—SCM11 (50% MWP) dropped to 34.83 MPa. A similar trend was seen in flexural strength, with SCM2 reaching 13.74 MPa compared to 12.91 MPa in the control, while SCM11 reduced to 7.11 MPa.
• Durability under thermal and freeze–thaw conditions reinforced these findings. SCM2 retained over 87 MPa compressive strength at 300 °C and maintained performance after 100 freeze–thaw cycles with only ~2% degradation. In contrast, high-MWP mixes like SCM11 exhibited over 50% strength loss under the same conditions.
• Porosity remained nearly constant in SCM2 (16.1%) relative to the control (16.0%), whereas SCM11 showed a marked increase to 20.0%, indicating greater permeability and lower durability. Specific gravity trends supported these observations, peaking in SCM2 at 2.219 g/cm³ and decreasing with higher substitution levels.
• Overall, the combination of recycled MWP with low-dosage Al₂O₃ and Fe₂O₃ presents a viable strategy for developing sustainable, high-performance SCMs. The oxide additives offset the limitations of MWP by refining pore structure, accelerating hydration, and improving strength retention under harsh conditions. Optimal performance was consistently observed at 5–10% MWP replacement.
• The optimum MWP ratio (with added Al₂O₃ and Fe₂O₃ additives) is 10–15%. When MWP and Al₂O₃ and Fe₂O₃ additive content are limited to 10–15%, the most efficient results are obtained in terms of both mechanical performance (compressive and flexural strength) and fresh consistency properties.
• Al₂O₃ and Fe₂O₃ additives promote the formation of hydration products in mixtures with low to medium MWP levels, thereby increasing strength and potentially improving microstructure density. As a result, these additives also have the potential to improve early-age strength.
• Using MWP at levels of 20–25% is a critical threshold value for environmental impact resistance in high-temperature environments such as fire and freeze–thaw cycles.
• In terms of cement consumption, the reduction in carbon emissions achieved by a 10–20% reduction could be an important factor in the creation of green building certification systems using such mixtures.
• In order to establish a framework for the evaluation of post-disaster demolition waste, for example, in the case of major disasters such as the Kahramanmaraş earthquakes, efforts are being made to continue the work of developing the necessary infrastructure for the use of materials obtained from post-disaster construction debris as secondary raw materials.