Waste Marble Slurry as Partial Substitution for Cement: Effect of Water-to-Cement Ratio

Abstract

This study investigates the potential of waste marble slurry as a partial replacement for ordinary Portland cement, with particular emphases on the influence of the water-to-cement (w/c) ratio and the objectives of determining the effect of water content and the optimum marble slurry concentration. Cement pastes were prepared with three w/c ratios (0.3, 0.4, and 0.5) and five substitution levels of marble slurry (0%, 5%, 10%, 15%, and 20%). Workability was assessed through mini slump flow tests, while mechanical performance was evaluated via compressive and flexural mechanical tests. The initial and final setting times were also investigated. Electrical resistivity measurements, combined with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), were used to examine chemical composition and microstructure. Results showed that marble slurry behaves as an inert filler, rather than a reactive component. Its incorporation, up to 10%, significantly improves the fresh properties and mechanical performance of mixes with higher w/c ratios (0.4 and 0.5). At lower w/c ratios (0.3), strength was adversely affected due to insufficient hydration. Electrical resistivity measurements indicated that pastes with w/c = 0.5 and up to 10% slurry replacement became slightly more resistant to electrical current, whereas mixes with lower w/c ratios (0.3 and 0.4) showed only minor reductions at 5% and 10% cement substitution. SEM imaging demonstrated a denser microstructure when marble slurry was incorporated, consistent with a filler effect. Marble slurry was also found to accelerate the setting of cement pastes, an effect most evident at lower w/c ratios and higher substitution levels. Overall, the findings highlight that waste marble slurry can be effectively utilized at moderate replacement levels in cement-based materials, contributing to sustainable construction practices by reducing cement consumption and marble waste disposal.

1. Introduction

Waste marble slurry is an industrial by-product resulting from cutting, shaping, and polishing marble blocks. The generated slurry contains water, marble dust, and small pieces of steel from the water saw [1]. Typically, a rate of 20–25% marble waste slurry is produced, based upon the type of sawing and polishing operations [2,3,4]. Waste marble slurry is treated to remove as much water as possible. The final product, called marble sludge, is collected, but due to the large quantities produced, storage by the marble industry is not feasible. As a result, this waste is discarded in nature, with controlled or uncontrolled access, causing considerable environmental hazards and health risks [5]. In particular, the porosity and permeability of soil are reduced in the area close to the deposited waste, something that has diverse effects on its morphology, hydrology, and fertility. Also, the waste marble sludge that is left in the environment air-dries and converts into a very fine dust [6], which becomes suspended in air and causes serious health problems, such as respiratory, visual, and skin disorders [7]. For these reasons, the recycling and utilization of waste marble are critical issues, both for environmental sustainability and for financial benefits. So far, marble waste has been used in various fields, such as paper, paint, plastic, glass, and agricultural production of animal feed and lime [5].

Cement is one of the most important materials in the construction industry. Nevertheless, its production is one of the most harmful industrial processes for the environment, because large volumes of CO₂ emissions are released in the atmosphere. According to various studies, 1 tone of clinker production emits 1 tone of CO₂ [8]; as a result, the cement industry is responsible for 7% of total CO₂ emissions globally [9]. Also, the raw materials that are used for the production of cement, such as lime, are depleted continuously, because of their extended utilization.

The construction industry could play a leading role in the reuse of marble waste, due to its scale and material demands. Most related existing studies have focused on the use of marble powder, which is obtained after drying and grinding the waste. Waste marble powder has been incorporated into conventional concrete [10,11,12,13,14,15,16,17], self-compacting concrete (SCC) [18,19,20], and polymer concrete [21,22,23]. In these applications, marble powder has been used as a partial replacement for cement, sand, or both, offering environmental, economic, and technical benefits [24].

Marble slurry, however, differs from both marble powder and marble sludge in its physical form and water content. Its direct utilization as a cement replacement in cement paste has been far less studied. It is important to distinguish between marble slurry and marble sludge: slurry refers to the wet suspension of marble particles and water generated during cutting and polishing, whereas sludge denotes the semi-dry residue obtained after dewatering the slurry. In this study, the term ‘marble slurry’ is used consistently, as the experimental work employed the wet form directly. Research has shown that the addition of marble slurry in a cement mixture may lead to a decrease in workability of fresh mixtures due to the higher surface area of the slurry particles [25,26,27]. Mechanical strength improvements are often reported up to cement replacement levels of around 10–15%, whereas higher dosages, beyond 20%, lead to strength reduction [28,29,30]. Reuse of waste marble slurry as is, without any processing, in construction materials as replacement for cement could lower CO₂ emissions, lessen the usage of raw materials that are in great demand, reduce the consumption of fossil fuels and power, offer economic advantages to cement industries, and increase the consumption of a waste material which would otherwise be discarded.

Although previous studies have explored the incorporation of marble waste powder or sludge, most investigations have focused either on its use as a fine aggregate replacement or on its general influence on mechanical properties. Limited attention has been paid to the role of the water-to-cement ratio in controlling the performance of cement pastes containing marble slurry, especially in relation to fresh state properties, mechanical performance, and physicochemical properties. In addition to conventional mechanical testing, electrical resistivity was included in this study to evaluate the pore structure and matrix connectivity of cement pastes. Resistivity is widely recognized as an indicator of hydration progress, porosity, and durability in cementitious systems, and its correlation with mechanical strength provides additional insight into the microstructural effects of marble slurry incorporation.

The objective of the present study is to systematically examine the combined effects of water-to-cement (w/c) ratio and marble slurry substitution level on the performance of cement pastes. To this end, cement pastes were prepared with three w/c ratios (0.3, 0.4, and 0.5) and five substitution levels of marble slurry (0%, 5%, 10%, 15%, and 20%). This study was conducted at the paste level to provide fundamental insight into hydration behavior, filler effects, and microstructural modifications, without the additional complexity of aggregates. Paste testing enables identification of the intrinsic influences of marble slurry and w/c ratio on the binder matrix, which serves as a basis for future investigations on mortar and concrete systems. The fresh state, setting time, and mechanical, electrical, chemical, and microstructural properties were investigated through mini slump flow tests, Vicat apparatus, compressive and flexural strength testing, electrical resistivity assessments, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). By clarifying the role of marble slurry and identifying the optimum substitution levels under different w/c ratios, this study provides new insights into the effective utilization of marble slurry in sustainable cement-based materials.

2. Materials and Methods

2.1. Materials

The cementitious material used as binder was ordinary Portland cement, type CEM II 32.5R, with a relative density of 3.15 and conforming to ASTM C-150 [31] and EN-197-1/2011 [32]. Marble slurry was obtained as by-product from Democritus S.A., a marble industry located in Eastern Macedonia and Thrace (prefecture of Xanthi, Greece), which exclusively processes a single type of marble excavated in the Kavala region. Marble slurry, with a relative density of 2.75, came from the cutting, shaping, and polishing processes and was slightly air-dried. Samples were collected at different times and from different on-site disposal points of the Democritus S.A. facility, all derived from the same marble source, ensuring consistent material characteristics. The slurry was white in color and used in its wet form without further processing. Marble slurry was characterized to determine its moisture content, chemical structure, and chemical composition.

To determine the moisture content of the slurry, the samples were oven-dried at 150 °C until a constant weight was achieved, defined as the point at which two consecutive measurements yielded the same mass. Samples obtained during the same sampling event showed similar moisture contents. The by-products displayed very consistent values, ranging from 20.61% to 21.61%, with only minor fluctuations. The moisture content of marble slurry, i.e., the proportion of water it contains, is a characteristic factor of the material, as this water participates in the hydration of cement and must be considered when calculating concrete mix proportions.

FTIR spectra of marble slurry were obtained using a Perkin Elmer FT-IR/NIR spectrometer. The spectra were recorded in the wavelength range of 600–4000 cm⁻¹, with a resolution of 0.4 cm⁻¹. All measurements were performed at room temperature, and, to ensure reproducibility, at least three samples were examined. The typical transmittance spectrum of marble slurry is presented in Figure 1. As expected, the spectrum exhibits the typical features of calcium carbonate. In particular, distinct absorption peaks are observed at 1400–1405 cm⁻¹, 875–878 cm⁻¹, and 710–715 cm⁻¹, which are characteristic of the calcite structure.

The chemical composition of marble slurry was investigated with X-ray diffraction (XRD). The analysis was carried out using a D8 FOCUS diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), equipped with a nickel filter and operated at a scanning speed of 0.5 min⁻¹. The X-ray wavelength was Cu Kα radiation (λ = 1.5406 Å). The X-ray tube was operated at 40 kV and 40 mA. Prior to testing, to ensure proper sample preparation, marble by-products were dried and ground into a fine powder using a mortar, and then pressed into the sample holder. Phase identification was performed using the instrument’s XRD software (DIFFRACplus XRD Commander V6.0) and its reference library (DIFFRAC.EVA V2.0). The XRD results for marble by-products are presented in Figure 2. The material was found to consist mainly of calcite crystalline phases, with a minor presence of dolomite, identified at 31.5° 2θ in low concentration.

2.2. Mix Proportions

The experimental procedure of the present study was separated into 3 stages, with 15 cement paste mixes in total. Three water/cement proportions (0.3, 0.4, 0.5) were chosen for each set of cement paste specimens. Replacement levels of marble slurry were 0%, 5%, 10%, 15%, and 20% by weight of binder for every group of samples. The marble slurry was used for substitution of the cement powder. To ensure that the targeted w/c ratios (0.3, 0.4, 0.5) were maintained, the water present in the slurry was accounted for in the mix design. Four mixtures of cement pastes with the addition of marble slurry, as well as the control mixture without marble slurry, were prepared for every experimental stage. The mix proportions of the different samples produced are provided in

Table 1. Mix proportions of cement pastes.

Mix	Water/Cement Ratio	Substitution Ratio (%)	Cement (g)	Marble Slurry (g)	Water (g)
0.3-M0	0.3	0	600	0	180
0.3-M5	0.3	5	570	36.37	173.63
0.3-M10	0.3	10	540	72.74	167.26
0.3-M15	0.3	15	510	109.11	160.89
0.3-M20	0.3	20	480	145.48	154.52
0.4-M0	0.4	0	600	0	240
0.4-M5	0.4	5	570	36.37	233.63
0.4-M10	0.4	10	540	72.74	227.26
0.4-M15	0.4	15	510	109.11	220.89
0.4-M20	0.4	20	480	145.48	214.52
0.5-M0	0.5	0	600	0	300
0.5-M5	0.5	5	570	36.37	293.63
0.5-M10	0.5	10	540	72.74	287.26
0.5-M15	0.5	15	510	109.11	280.89
0.5-M20	0.5	20	480	145.48	274.52

. The raw materials that were used are presented in Figure 3.

2.3. Mixing, Casting, and Curing

Materials were mixed in a standard mixer to produce fresh cement paste specimens according to ASTM C305 [33]. The mixing materials used to prepare the cement paste composites are presented in Figure 3. In the case of control mixes, cement was placed in the mixer and water was added slowly. Afterwards, cement and water were left together for 30 s. Then, the mixer was turned on at low speed for another 30 s, after which the mixer was stopped to clean its internal surface, so as to achieve a better blend. Finally, the mixer was turned on again at high speed for 1 min. The same procedure was followed in the case of marble-blended cement pastes, with the only difference being that marble slurry was dissolved in the mixing water before entering the mixer. Immediately after the end of mixing, the samples were cast in well-oiled molds. The molds were circular, with dimensions of 150 × 300 mm, and prismatic, with dimensions of 20 × 20 × 80 mm (Figure 4a,b). After 24 h, specimens were removed from the molds and cured in water saturated with calcium hydroxide (lime) at room temperature for 28 days, until testing.

2.4. Characterization Methods

A series of tests was carried out in order to investigate the fresh and hardened properties, such as workability, setting time and mechanical behavior, as well as the chemical and mineralogical compositions, of cement pastes with the incorporation of waste marble slurry. The workability of the fresh mixtures was tested using the mini slump flow test (Figure 5). The mini slump flow test was conducted on three replicate samples per mixture. To conduct the test, the standard metallic truncated cone, opened at both ends, was placed on a metallic plate and filled with material (Figure 5a), after which it was removed, and vibration was applied via 25 rotations in a period of 15 s. To calculate the workability, the diameters of the mixtures were measured at 3 different points (Figure 5b), and the average value was calculated.

Initial and final setting times were determined according to EN 196-3 [34] using a Vicat apparatus (MATEST S.p.A., Arcore, Italy). Three replicate tests were performed for each mixture.

Compressive tests were conducted according to ASTM C39/C39M-20 [35] using a servo-hydraulic INSTRON 8801 Testing Device (Instron Corporation, Norwood, MA, USA). For each mixture, three cylindrical specimens (30 mm diameter × 60 mm height) were tested. A constant displacement rate of 0.25 mm/min was used. Load, displacement, and time were recorded during testing. The modulus of elasticity was calculated by setting the slope of the straight line in the stress–strain diagram. The compressive toughness was determined by calculating the area under the stress–strain curve up to a specific point (strain: 0.012 mm/mm). Results are reported as the average of three specimens.

Flexural strength was assessed by four-point bend testing on 20 × 20 × 80 mm prisms. The supporting distance was 70 mm, while the mechanical load was applied at two points, with a total distance of 35 mm, placed symmetrically with the specimen. A 10 kN MTS Insight load frame was used, and the measurements were performed by applying a constant displacement rate of 0.001 mm/sec. Three replicate specimens were tested per condition, and the average values are reported.

Electrical resistivity in cementitious materials can be measured using either non-destructive or intrusive electrical techniques. In this study, bulk resistance of hardened paste specimens was measured with a low-current, four-electrode configuration; under these conditions, the test is considered non-destructive because it does not alter the specimens. Microstructures of cement-based materials, such as pore size distribution and form of interconnections, define their durability and can be investigated through electrical resistivity tests [36,37,38]. In general, the denser the microstructure of a material is, the more resistant it is and, by extension, the better durability it has [39]. In the present study, a four-probe resistivity measurement technique was used. Immediately after mixing, four rectangular stainless steel electrodes were placed along the cross-sections of the samples, as presented in Figure 4c. After 24 h, the cement pastes were demolded and cured in water saturated with calcium hydroxide for 28 days, after which the samples were cleaned with tap water and dried in an oven at 80 °C for 72 h to remove the free water. Electrical resistance of the samples was measured using a 34450A Keysight Laboratory Digital Bench Multimeter (Keysight Technologies, Santa Rosa, CA, USA). During testing, the internal electrodes were used to record the voltage, while the external electrodes supplied the current. Measurements were recorded every 2 s over a period of 30 min to minimize deviations due to polarization effects. Resistance measurements were performed on three replicate specimens per mixture. Average values were calculated after stabilization. The average resistivity was then calculated according to Ohm’s law, based on the mean resistance values obtained during the final five minutes of the measurement period. Calculated electrical resistivity is an intrinsic, geometry-independent property of a material that quantifies how readily an electrical current can flow through its structure [40].

A D8 FOCUS X-ray diffractometer (XRD) (Bruker AXS GmbH, Karlsruhe, Germany) with Ni filter, at a scan speed of 0.5 min⁻¹, was used in order to determine the crystalline phases present, as well as the hydration reactions in the cement paste/marble slurry specimens. The wavelength of X-rays used was Cu radiation, λ = 1.5406 Å. The X-ray tube was operated at 40 kV with 40 mA. After compressive testing, the broken pieces of the cement/marble samples were grinded to a fine powder with the help of a mortar and placed into the sample holder. Then, a slice of glass was placed on the sample holder to achieve a completely smooth surface. Subsequently, the sample holder was placed in the diffractometer and the analysis began. The analysis of the results was performed through the XRD software library.

Fourier-transform infrared (FTIR) spectra of cement pastes with various dosages of marble slurry were obtained using a Perkin Elmer FT-IR/NIR Spectrometer (PerkinElmer, Waltham, MA, USA). The IR spectra were displayed from 600 to 4000 cm⁻¹, at a resolution of 0.4 cm⁻¹ at room temperature. The samples used were obtained after the compressive strength tests, in the form of crushed pieces, and grinded to a fine powder. Then, several milligrams of sample powder were placed on the diamond crystal, pressured with the swivel pressure tower, and scanned in order to produce the FTIR patterns. Microstructural analyses (XRD, FTIR) were performed on representative specimens of each mixture. Measurements were repeated on at least two samples to confirm reproducibility.

Fresh fractured samples of various concentrations of marble slurry and reference samples were coated with Chromium using a Quorum Q 150T ES Sputter Coater (Quorum Technologies Ltd., Laughton, East Sussex, UK). A Jeol 6390LV (JEOL Ltd., Tokyo, Japan) scanning electron microscope (SEM) was used. Spot size was set as 40, working distance was at 19.000, and magnification was displayed at 50 μm.

3. Results and Discussion

3.1. Workability

The workability of different mixtures is shown in Figure 6. As expected, the workability increases with increasing water-to-cement ratio (w/c). The results have shown high reputability with mostly minimal variations. In specimens with a w/c = 0.3, it is observed that, when the cement powder is replaced with up to 10% marble slurry, the workability of the material remains constant. In contrast, at higher replacement rates (15 and 20%) there is a decline, and then an increase in workability, which is accompanied by a higher standard deviation. The mixtures with w/c = 0.4 and 0.5 show similar behaviors. It is shown that, for replacement ratios up to 10%, the use of marble slurry for the replacement of cement does not affect the workability of the material. The introduction of higher contents of marble paste results in a slight reduction in workability, especially for water-to-cement ratios of 0.4 and 0.5.

To date, most of the research on the workability of cementitious composites with marble by-products have focused on the use of marble powder, rather than marble slurry, and on its effects on self-compacting concrete (SCC). Uysal and Yilmazh [41] have shown that the addition of marble powder has positive effects on the workability of SCC, especially at cement substitution levels of up to 20%. On the contrary, Shawki et al. [42] report that the addition of marble powder slightly decreases the workability of SCC.

In the present study, the results show that marble slurry, at a cement substitution level of up to 10%, does not alter workability, while higher replacement levels tend to slightly reduce flowability, particularly at conventional w/c ratios of 0.4 and 0.5. On the contrary, for w/c = 0.3 and 20% substitution, higher flow values were measured. This behavior may be explained by the balance between the filler effect and the increased water demand associated with the marble particles included in the slurry. At low replacement levels, up to 10% of cement, marble slurry acts primarily as a fine filler, improving particle packing and reducing interparticle friction. Beyond this threshold, however, possibly, the increased specific surface area and water demand of the marble slurry becomes more significant, reducing the amount of free water available for lubrication and, thereby, lowering slump flow [15]. The slight recovery at 20% replacement and w/c = 0.3 may be attributed to localized segregation or variability in the slurry–water interaction, as reflected in the higher standard deviations.

3.2. Setting Time

The effects of marble slurry addition on the setting time of cement pastes are presented in Figure 7. As expected, both the initial and final setting times increased with higher w/c ratio, reflecting the higher water content available. For example, the control mixes showed final setting times of 202, 351, and 417 min for w/c = 0.3, 0.4, and 0.5, respectively.

At w/c = 0.3, the setting time decreases progressively with increasing marble slurry content. The final setting time ranges from 165 min at 5% to 147 min at 20% cement replacement with marble slurry. A maximum reduction of approximately 28% is observed with the incorporation of marble slurry at substitution levels of 15 to 20%. For w/c = 0.4, a similar reduction trend was observed, with the final setting time decreasing from 269 min at 5% substitution to 201 min at 20% substitution. At w/c = 0.5, the setting time also decreased with slurry incorporation, although the effect was less pronounced, from 409 min at 5% to 357 min at 20% cement replacement, representing a maximum reduction of approximately 14% compared to the control.

Reductions in the initial and final setting times of cement composites with marble powder and sludge have been reported previously [43,44,45]. There are two possible mechanisms that these reductions can be attributed to. The filler effect of marble slurry provides nucleation sites that accelerate the formation of hydration products. The calcite particles present in the slurry, as confirmed by XRD analysis, can act as nucleation sites for calcium silicate hydrate formation [44]. Additionally, the use of marble sludge has been shown to raise the water demand and subsequently lower the available free water in the paste [25]. The influence is strongest at low w/c ratios, where water is already limited, leading to more significant reductions in setting time, and less pronounced at higher w/c ratios, where additional water compensates for slurry demand. Overall, the results confirm that marble slurry accelerates the setting of cement pastes, particularly at lower w/c ratios, possibly due to its filler effect and increased water demand.

3.3. Mechanical Performance

3.3.1. Compressive Strength

The typical stress–crosshead displacement curves for all the samples tested, with water-to-cement ratios of 0.3, 0.4, and 0.5 at the age of 28 days, are illustrated in Figure 8. Comparing the results of the reference samples, it is observed that, as expected, the cement paste specimens with a w/c = 0.3 display higher strength values compared to the specimens with w/c = 0.4 and w/c = 0.5. Control cement pastes with w/c = 0.3 demonstrate a compressive strength equal to 31.7 MPa, while the ones with w/c = 0.4 and w/c = 0.5 have compressive strengths equal to 30.8 MPa and 23.3 MPa, respectively. This happens due to the effect of water-to-cement ratio on the mechanical properties of cement-based products [46,47,48]. However, when marble slurry is added in the mixes, alterations are noticed considering the mechanical attributes of specimens.

The results of the average compressive strengths for different percentages of marble slurry replacement and different water-to-cement ratios are shown in Figure 9. Cement pastes with w/c = 0.3 displayed the highest performance regarding compressive strength of control samples. However, a decrease in the compressive strength was noticed when marble slurry was included as cement replacement. A reduction of approximately 28% occurred at a 5% substitution of cement with marble slurry. At 10% replacement, similar results were observed, with a 25% decrease in strength. Greater strength reduction was observed for cement paste specimens with 15% marble slurry, which was 47% lower compared to the control sample. Generally, the cement paste/marble slurry mixtures that were produced with w/c = 0.3 displayed lower compressive strength values compared to the control mix. Possibly, this occurs due to the limited amount of water used, which potentially obstructed the composite materials from being mixed properly.

In the case of cement paste/marble slurry specimens with a w/c = 0.4, when 5% and 10% of marble slurry were added, the results are very similar to the reference samples, having a very slight reduction in strength, by 5.5% and 10%, respectively. Further decrease in compressive strength was noticed when the replacement level of marble slurry increased. Particularly, 15% and 20% additions of marble slurry caused 14% and 22% strength decreases, respectively.

On the other hand, the mechanical results indicate that, at a w/c = 0.5, the compressive strength increases for a marble slurry replacement level up to 10%, and then it begins to decrease. In particular, 5% replacement of ordinary Portland cement with marble slurry leads to an increase in compressive strength of about 6%. Similarly, strength values increase by around 3% in the case of 10% replacement. This increase can possibly be attributed to a pore-filling effect of marble slurry, providing suitable nucleus for the development of hydration products [39]. The compressive strengths of cement pastes with the incorporation of marble slurry at 15% and 20% substitution ratios are decreased by almost 14% and 31% respectively, compared to the reference cement paste specimens. This decrease possibly happens due to a potential reduction in cementitious materials (C3S, C2S), which are responsible for the strength development of cement-based products [49]. This reduction is more intense for higher replacement levels of cement and is commonly known as the dilution of the pozzolanic reactions [50].

3.3.2. Modulus of Elasticity

The modulus of elasticity offers valuable information on the abilities of cement-based materials to deform elastically. The experimental results of the modulus of elasticity for the cement paste specimens with marble slurry at the age of 28 days are shown in Figure 10. The modulus of elasticity demonstrated was normalized by the average modulus of elasticity of the reference samples for each w/c ratio.

Cement paste specimens with w/c = 0.3 exhibit a gradual drop in elasticity values for all replacement levels of cement by marble slurry. In the case of samples with w/c = 0.4, the modulus of elasticity seems to be almost unaltered, with a slight increase until 10% substitution level of marble slurry compared to the control samples, but it decreases when further amounts of marble slurry are incorporated into the mixes. Considering cement pastes with various marble concentrations and w/c ratio equal to 0.5, the modulus of elasticity appears to have an increased trend up to 10% replacement level and declines for greater dosages of marble slurry (15% and 20%). It is observed that the modulus of elasticity results have close relations to the compressive strength results for all the different samples tested, as almost all the specimens displayed similar behaviors in both attributes [8].

3.3.3. Compressive Toughness

The results of the compressive toughness obtained at different replacement levels of cement by marble slurry and w/c ratios are presented in Figure 11. Starting from the samples with w/c = 0.3, there is a gradual reduction in compressive toughness for all substitution levels against the control sample. This could be ascribed to the very low water content and, by extension, to the inadequate amount of cement for the composition of cement pastes. Considering cement paste specimens with w/c = 0.4, the compressive toughness remains almost unaltered until 10% replacement with marble slurry compared to the control samples. Above that level, a gradual decrease in the compressive toughness is noticed. In the case of samples with w/c = 0.5, compressive toughness slightly improved up to a 5% substitution level, but it decreased when further amounts of marble slurry were incorporated into the mixes. In general, the results show that there is a good correlation among the compressive toughness, modulus of elasticity, and compressive strength for the majority of the cement paste samples.

3.3.4. Flexural Strength

The results of the average flexural strength for different percentages of marble slurry replacement and different water-to-cement ratios are shown in Figure 12. The addition of marble slurry to the cement paste with w/c = 0.3 did not significantly affect the flexural strength of the specimens. All the different samples with marble slurry had flexural strengths of 1.6 MPa and above. The flexural strengths for different marble slurry replacement ratios for w/c = 0.4 were also quite stable. There is a slight deviation in the specimens for 10% replacement ratio (slightly reduced flexural strength), but the results are considered within the range of the standard deviation. In the case of a higher water-to-cement ratio (0.5) for low percentages of marble slurry replacement (e.g., 5% and 10%), a significant increase in flexural strength is shown, taking values of 2.4 and 2.2 MPa, respectively. The results of flexural strength are in absolute correspondence with those of compressive strength, where a similar relative increase in mechanical strength was observed. For higher marble slurry replacement rates, the flexural strengths are significantly reduced.

The influence of marble slurry on the mechanical properties of cement pastes becomes more pronounced under two conditions: at lower w/c ratios and at higher substitution levels. At low w/c ratio, the limited availability of water reduces the hydration potential of the system. When marble slurry is added, even in small amounts, the additional surface area further increases the water demand. As a result, the beneficial filler effect is not fully developed, and strength values decrease. This possibly explains why the reductions are sharper at lower than at higher ratios.

At higher slurry replacement levels (≥15%), the dilution effect dominates across all w/c ratios. The reduction in cement content leads to fewer hydration products and weaker matrix formation. In addition, the interfacial transition zones (ITZs) around the inert marble particles may act as stress concentration sites, reducing stiffness and toughness. This explains the simultaneous declines in compressive strength, elastic modulus, and toughness observed at higher substitution levels. By contrast, at moderate marble slurry contents (≤10%) and higher w/c ratios (0.5), the filler effect contributes to a denser microstructure, supporting the slight improvements observed in compressive and flexural strengths. Marble particles fill voids, refine pore connectivity, and possibly provide nucleation sites for hydration products, thereby supporting the modest increases observed in compressive and flexural strengths [25,44].

3.4. Electrical Resistivity

Figure 13 illustrates the electrical resistivity test results conducted on cement paste specimens with three w/c ratios (0.3, 0.4, 0.5) and various replacement levels of marble slurry (0%, 5%, 10%, 15%, 20%). For comparison purposes, relative electrical resistivity values are reported. It can be observed that the two mixes (w/c = 0.3, w/c = 0.4) present decreased tendencies in electrical resistivity with the incorporation of marble slurry, compared to control samples without marble. However, these reductions are very slight for replacement levels 5% and 10%. On the other hand, the resistivity values are slightly increased for up to 10% replacement levels in the case of cement paste mix with w/c = 0.5 (Figure 13c). The electrical resistivity results can be directly correlated with the observed mechanical performance. Slightly higher resistivity values, as measured in mixes with w/c = 0.5 and up to 10% slurry replacement, indicate possible reductions in the porosity by filling the voids and reducing the interconnections of the pore structure [39,51]. As a result, the composites become slightly denser and more resistant in the electrical current. This also corresponds to the observed improvement in the compressive and flexural strengths at these replacement levels and w/c ratio. Conversely, the reduced resistivity observed at lower w/c ratios (0.3 and 0.4) suggests that slurry addition possibly increases the porosity, which is consistent with the slight reduction in strength under these conditions. This correlation confirms that resistivity is a reliable indicator of microstructural refinement and mechanical performance [3,7].

3.5. X-Ray Diffraction Analysis

X-ray diffraction tests were conducted on control samples with 0% marble slurry and samples with various marble concentrations (5%, 10%, 15%, and 20%). This process was carried out for every water-to-cement ratio (0.3, 0.4, and 0.5) tested. The purpose of XRD analysis was to investigate whether or not marble slurry participates in the chemical reactions that take place during the hydration process. XRD spectra of the mixtures of cement pastes with marble slurry for w/c = 0.3, 0.4, and 0.5 are presented in Figure 14. The major peaks that are recognized are of calcite, calcium hydroxide (portlandite), ettringite, and calcium silicate hydrate. XRD analysis of cement pastes after 28 days of curing demonstrated that there is no noticeable difference among the investigated samples. This is evident for all the w/c ratios studied. Some minor alterations in the intensity of the hydration products (C2S), (C3S), and Ca(OH)₂ can be observed as the replacement ratio of marble slurry increases, something that could possibly be attributed to the higher necessity of water for higher substitution levels of marble [3]. On the other hand, there is a slight increase in the proportion of calcite as the substitution percentage of marble increases, due to the calcium content of the marble slurry [52]. All in all, it is clear that marble slurry is an inert material and does not contribute to the chemical processes that occur throughout the hydration of the cement paste [7,51,52,53].

3.6. Fourier-Transform Infrared Spectroscopy Analysis

Infrared spectroscopy (IR) was used in order to investigate the molecular structures and characterize the chemical classes of materials qualitatively and quantitatively. The frequencies of waves measured by IR can offer information about the silicate, sulphate, and carbonate phases [54]. IR spectra of cement pastes without marble slurry (control), as well as cement pastes with different marble concentrations (5%, 10%, 15%, and 20%), for the three water-to-cement ratios (0.3, 0.4, and 0.5) are presented in Figure 15. Seven principal bands have been identified for cement paste samples, which were identical across all spectra, showing only minor variations in peak intensity. Starting from the left, the peak at around 3642 cm⁻¹ corresponds to the internal hydroxyl group in Portlandite (Ca(OH)₂), which is formed while silicate phases of cement disintegrate to form calcium silicate hydrate (C-S-H) phases during hydration. The broad peak close to 1650 cm⁻¹ is associated with O-H stretch of forcefully polarized H-bonded water, related to cementitious minerals. The peaks at approximately 1415 cm⁻¹, 874 cm⁻¹, and 712 cm⁻¹ represent CO₃²⁻ bands which occur due to C–O stretch of CaCO₃. Characteristic sulphate absorption bands are visible near to 1113 cm⁻¹, due to the vibration of the SO₄²⁻ group in sulphates. A last peak, at 960 cm⁻¹, is referred to as the SiO₄⁴⁻ band, caused by Si-O stretch, basically signifying C–S–H gel and other silicate phases [54]. In general, supplementary powder materials that participate in hydration reactions can cause shifts in the characteristic absorption peaks of cement pastes. However, in the present study, no such shifts were observed. The spectra of all mixtures remained essentially unchanged, confirming that marble slurry acts as an inert filler and does not chemically participate in the hydration process.

3.7. Microscopy Analysis

Scanning Electron Microscopy (SEM) micrographs of cement pastes including marble slurry as a replacement of cement at 10% and 20% substitution levels, and reference cement paste with 0% marble slurry, are shown in Figure 16. Fresh fractured surfaces of cement paste samples were examined, considering their microstructure at the age of 28 days. It should be noted that the SEM images presented in this study were obtained at 450× magnification. While this level of magnification does not allow for detailed visualization of hydration products, it provides useful information on the general morphology of the cement matrix. SEM images of cement pastes with the incorporation of marble slurry indicate that their structures are less porous compared to control cement paste. The capillary pores, visible in specimens without marble slurry, are decreased significantly as the replacement of cement by waste marble slurry increases up to the 10% replacement level. Pore volume reduction likely happens due to the filling property of marble, which fills the voids and results in a denser microstructure [51]. This leads to enhanced durability and strength in cement pastes.

4. Conclusions

This study investigated the partial replacement of cement with waste marble slurry in cement pastes at different w/c ratios (0.3, 0.4, 0.5) and substitution levels (0–20%). The following conclusions can be drawn:

• Workability results showed that marble slurry, at substitution levels up to 10%, has minimal influence on the flowability of cement pastes, while higher replacement levels slightly reduce workability, especially at w/c ratios of 0.4 and 0.5.
• Initial and final setting times of cement pastes are reduced with the incorporation of marble slurry, with the effect being most pronounced at lower w/c ratios and higher substitution levels. This acceleration could be attributed to the combined actions of the marble slurry particle filler effect and increased water demand.
• At w/c = 0.5, compressive and flexural strengths, modulus of elasticity, and toughness improve at moderate marble slurry substitution levels (≤10%). At w/c = 0.4, changes are minor up to 10% substitution, with reductions at higher cement substitution levels. At w/c = 0.3, all three properties decline with slurry addition, reflecting the limited water available for hydration.
• Electrical resistivity trends were consistent with the mechanical results, confirming the link between microstructural refinement at moderate slurry levels and improved performance.
• Microstructural analysis (XRD, FTIR, SEM) confirmed that marble slurry is chemically inert, acting as a filler that enhances particle packing at moderate contents.

Overall, marble slurry is not recommended for low w/c ratios (<0.4), but up to 10% substitution at higher w/c ratios improves both fresh and hardened properties. As an inert material, its effects are mainly physical, enhancing packing and pore refinement, rather than chemical reactivity. Thus, marble slurry can be effectively utilized in cementitious systems at moderate levels, supporting sustainability by reducing cement consumption, lowering CO₂ emissions, and utilizing marble industry waste. Future work should extend these findings to mortars and concretes to evaluate combined effects with aggregates.