Mechanical Properties of Sand Concrete Containing Recycled Marble Powder and Steel Fibres at Ambient and Elevated Temperatures

Rapid urbanization has led to an unprecedented demand for sand, which is extensively extracted from rivers and mountainous regions. This excessive exploitation has caused significant resource depletion and severe environmental degradation [1], adversely impacting ecosystems and prompting global research efforts to explore sustainable alternatives to natural sand in concrete, particularly using waste materials [2-4]. Among these alternatives, glass has been identified as a high-quality and sustainable substitute for natural sand in concrete production, as demonstrated by the findings of Dadouch et al. [5]. Previous studies have examined various aspects of concrete modifications. Ravina and Mehta [6] investigated the impact of fly ash on compressive strength and reported substantial improvements in concrete performance, particularly in lean mixtures. Furthermore, according to Wang et al. [7], the incorporation of biochar as a 100 vol% replacement for sand reduced concrete shrinkage by 65% at 28 days while ensuring improved long-time strength development. Marble powder has also been extensively studied as a potential replacement for natural sand in concrete, with numerous investigations reporting its effectiveness [8-14].

Although the effects of cement substitution and steel fibre addition have been studied independently, limited research has addressed their combined impact on the performance of sand concrete. In accordance with the relevant standard [15], sand concrete is a cement-based material composed exclusively of fine aggregates (≤ 5 mm), without coarse aggregates. Owing to this composition, it exhibits a denser and more homogeneous structure than conventional concrete and demonstrates superior mechanical properties compared to mortar [16]. These characteristics make sand concrete an effective material for structural applications requiring high compactness, good workability, and reliable mechanical performance. When reinforced with fibres, sand concrete becomes a high-performance composite material in which the fibres play a crucial role in enhancing mechanical properties [17]. Fibre reinforcement improves tensile/flexural strength, ductility, and toughness through crack-bridging mechanisms, while also reducing shrinkage cracking and enhancing durability. The homogeneous, aggregate-free microstructure ensures uniform fibre dispersion [18]. In addition, fibres contribute to improved thermal resistance at high temperatures by mitigating spalling and helping to preserve structural integrity, making fibre-reinforced sand concrete particularly suitable for demanding applications, including structures exposed to fire.

Regarding the effect of marble waste, Bayraktar et al. [19] reported that substituting silica sand with waste marble powder (WMP) in foamed concrete significantly enhances mechanical properties, including compressive, tensile, and flexural strengths, while reducing drying shrinkage by up to 25.55%. Similarly, Shukla et al. [20] found that replacing cement with marble dust yields optimal strength at approximately 15%; however, beyond this level, strength decreases and brittleness increases. In a related study, Venkataramana and Babu [21] investigated marble waste as a coarse aggregate replacement and observed a gradual decline in strength with increasing substitution levels up to 50%. To address these limitations, several studies have highlighted the effectiveness of fibre reinforcement. Bayraktar et al. [19] demonstrated that incorporating basalt fibres improves mechanical performance and crack control, with an optimal fibre content of around 1%. Likewise, Venkataramana and Babu [21] showed that adding 1-2% steel fibres effectively compensates for strength loss caused by high marble waste content while enhancing ductility. Karimipour et al. [22] further showed that marble slurry powder (MSP), due to its high fineness and filler effect, contributes more significantly to compressive strength than other fillers. However, its effect on tensile strength is limited unless combined with fibres. The incorporation of steel fibres (0.1–0.2%) significantly improves both compressive and especially splitting tensile strength through crack-bridging mechanisms and enhanced ductility. Overall, the combined use of MSP and steel fibres results in a denser microstructure and superior mechanical performance.

The present work investigates a novel sand concrete composite incorporating recycled marble powder as filler and steel fibres as reinforcement, evaluated under both ambient conditions and elevated temperature exposure up to 600 ℃. A temperature of 600 ℃ was selected as a representative high temperature exposure because it lies just above the critical threshold at which concrete typically experiences a 50–60% reduction in strength and the onset of pronounced visible cracking. In a previous study by the authors (currently under publication), sand-based concrete without steel fibres was exposed to 20 ℃, 150 ℃, 300 ℃, 450 ℃, 600 ℃, and 800 ℃. The results showed that at 600 ℃, the concrete retained only minimal residual strength, while at 800 ℃ it had deteriorated to a condition unsuitable for mechanical testing. These observations are consistent with the findings of Phan and Carino [23], who reported that both compressive strength and tensile strength decrease markedly beyond 400–600 ℃, with damage particularly severe above 600 ℃. The use of waste marble, either as an aggregate or as an admixture, has been shown to enhance concrete durability, confirming its suitability for both conventional and self-compacting concrete (SCC) mixes [23, 24]. Vardhan et al. [25] further demonstrated that waste marble improves mechanical strength, permeability resistance, and microstructural characteristics of concrete, with optimal performance achieved at a 40% replacement. The addition of steel fibres into manufactured sand concrete (MSC) has attracted significant research interest. Sand concrete, as a quasi-brittle material, inherently exhibits low tensile strength and poor crack resistance, which limit its structural applications.

The inclusion of steel fibres effectively inhibits and reduces the propagation of macroscopic cracks [26], thereby improving the material's overall performance and durability. Cai et al. [27] investigated the fracture behaviour of steel fibre-reinforced manufactured sand concrete (SFRMSC) and demonstrated improvements in residual strength, fracture toughness, and fracture energy with increasing fibre volume fraction. Murali and Vinodha [28] assessed the impact of the failure strength of steel hybrid fibre reinforced concrete (SHFRC) under freeze-thaw cycles, highlighting the critical role of long fibres in enhancing strength. Ammari et al. [29] examined the durability of sand concrete with lignocellulosic and steel hybrid fibres, showing enhanced performance under severe conditions with increasing steel fibre content. Jiao et al. [30] also reported superior mechanical and fracture properties in SFRMSC compared to conventional mixes. The addition of fibres was found to reduce fluctuations in the acoustic parameter, with a steel fibre content of 1% yielding the highest flexural strength. Durgun and Sevinc [31] investigated the effects of replacing conventional limestone aggregate with basalt, barite, marble, and pumice on the engineering properties and durability of concrete. The produced specimens were subjected to comprehensive testing, including unit weight, compressive strength, splitting tensile strength, ultrasonic pulse velocity (UPV), water absorption, and apparent porosity. Furthermore, high-temperature performance was assessed by exposing the specimens to temperatures of 600 ℃ and 800 ℃. SCC is increasingly used in high-rise and industrial buildings, which are particularly vulnerable to fire hazards due to dense electrical and gas systems. Ahmad et al. [32] evaluated the performance of SCC after exposure to 500 ℃.

Post-exposure evaluation included visual inspection for cracking and spalling, along with measurements of mass loss, compressive strength, and tensile strength. Their results indicated that the addition of steel fibres improved the performance of SCC at elevated temperatures. In contrast, the control mix without steel fibres exhibited considerable degradation when exposed to 500 ℃. Although the effect of high temperatures on concrete has been extensively studied, sand concrete containing marble powder and steel fibres remains largely unexplored. In Algeria, the marble manufacturing industry generates substantial quantities of waste, yet recycling rates remain very low. From environmental, economic and safety perspectives, this study aims to evaluate sand concrete formulated by replacing 40% of cement with WMP as filler and incorporating steel fibres at different volume fractions (0%, 0.25%, 0.50%, 0.75%). Accordingly, the key and novel objective of the present work is to investigate the behaviour of a new sand concrete composite incorporating recycled marble powder filler and steel fibre reinforcement, under two conditions: ambient temperature and exposure to 600 ℃.

3.1 Slump test and density of sand concrete

Figure 7 illustrates the variation in slump values for the reference sand concrete and fibre-reinforced sand concrete containing 0.25%, 0.50%, and 0.75% fibres. The reference sand concrete exhibits the highest slump value at 8.5 cm, indicating good workability. Incorporating 0.25% fibres slightly reduces the slump to 8 cm, reflecting a minor decrease in workability while remaining within acceptable handling limits. As the fibre content increases to 0.50%, the slump further decreases to 7.3 cm, indicating a more noticeable reduction in workability. At 0.75% fibre content, the slump reaches 7 cm, representing the lowest workability among all mixtures tested. These results show a clear inverse relationship between fibre content and workability. The progressive reduction in slump with increasing fibre content can be attributed to increased friction and entanglement of the fibres within the matrix. Adequate fibre dispersion was ensured despite the limitations of the slump test, which reflects yield stress but not viscosity, the latter being critical for fibre dispersion. Accordingly, for the sand-based concrete containing 0.75% steel fibres, the amount of superplasticizer was increased to achieve an acceptable slump of approximately 7 cm.

Figure 8 shows the variation in the 28-day density of fresh cured concrete for the reference sand concrete and the fibre-reinforced mixtures samples containing 0.25%, 0.50%, and 0.75% fibres. The reference sand concrete exhibits the lowest density, measured at 2.235 g/m³. Addition of 0.25% fibres leads to a notable increase in density to 2.319 g/m³, indicating improved compactness. This upward trend continues at 0.50% fibre content, where the density reaches 2.329 g/m³, though the rate of increase becomes less pronounced. At 0.75% fibre content, the density further rises slightly to 2.337 g/m³, suggesting diminishing gains with higher fibre content. Overall, the results demonstrate that fibre addition contributes to an increase in the density of sand concrete as previously confirmed by Wang and Niu [40].

Figure 7. Variation of slump sand concrete

However, the effects become less pronounced as fibre content increases, indicating a potential saturation threshold beyond which further fibre addition yields limited benefits in terms of density enhancement.

Figure 8. Density variation of fresh cured concrete at 28 days

3.2 Mechanical characterization of unheated specimens

Table 8 shows the mechanical properties of the developed sand concrete composite incorporating recycled marble powder as filler and steel fibre reinforcement at ambient temperature. The statistical analysis of the compressive strength at 20 ℃ as function of fibre content for the various curing ages are tabulated in Tables A1-A4. The results clearly demonstrate the effectiveness of this incorporation strategy in enhancing the overall performance of sand concrete. Previous studies support these findings. For instance, Kabbo et al. [41] reported that implementing different proportions of marble powder with a fixed amount of steel fibres significantly enhanced the mix's flowability, passing ability, and viscosity. Furthermore, the same study indicated that a combination of 10% WMP and 5% silica fume (SF) produced a linear increase in both compressive strength and splitting tensile strength. In addition, Ahmad and Zhou [42] demonstrated that the incorporation of steel fibres and marble waste considerably enhanced the performance of SCC under aggressive environmental conditions.

Table 8. Mechanical properties of unheated specimens

3.2.1 Compression strength

Figure 9 presents steel fibre-reinforced concrete samples containing different fibre contents (0.25%, 0.5% and 0.75%) following compression testing at 7 aging days. Figure 10 shows the development of compressive strength in sand concrete samples incorporating different steel fibre contents (0%, 0.25%, 0.50% and 0.75%), measured at curing ages of 7, 14 and 28 days. For all mixtures, compressive strength increases with curing time, reflecting the hardening process of concrete. The incorporation of steel fibres enhances compressive strength compared with the reference mix, with the most pronounced improvement observed at a fibre content of 0.50%.

Beyond 0.50% fibre content, strength gains tend to plateau, indicating that higher fibre contents do not lead to further significant improvements in compressive strength. This trend is consistent with findings reported in the literature [43], where an optimal combination of 1% steel fibres and 15% marble dust as a cement replacement produced the highest mechanical strength, while higher contents resulted in significant strength reduction.

Figure 9. Test specimen after compression test, (a) BS25, (b) BS50, (c) BS75

In the present study, sand concrete incorporating marble waste as partial aggregate replacement and 0.50% steel fibres provides favourable balance between mechanical performance and material efficiency. The results indicate that a moderate steel fibre content of approximately 0.50% is optimal for maximising compression strength.

Figure 10. Compressive strength of sand concrete incorporating marble filler, with and without steel fibre reinforcement

Figure 11. Tensile strength of sand concrete incorporated with marble filler, with and without steel fibre reinforcement

3.2.2 Tensile strength

Figure 11 illustrates the evolution of tensile strength for sand concrete mixes with and without steel fibres, measured at curing ages of 7, 14 and 28 days. The fibre-free sand concrete exhibits a gradual increase in bending strength increases with time, reaching about 8.5 MPa at 28 days, which confirms continued strength development after the initial hardening period, albeit at a lower level than fibre-reinforced mixtures. The incorporation of 0.25% steel fibres always leads to a slight but consistent improvement in bending strength, achieving about 9 MPa at 28 days. Increasing the fibre content to 0.50% does not result in a proportional gain, as the flexural strength at 28 days remains comparable to, or slightly lower than, that of the 0.25% fibre mixture, although a steady improvement is observed during early hardening. A further increase to 0.75% fibre content yields similar results with a strength around 9 MPa at 28 days, indicating that beyond a certain threshold the contribution of additional fibres becomes marginal and may even stagnate. This behaviour is in line with findings reported in the literature [27], which suggests that higher fibre contents enhance aggregate-matrix but can also promote the formation of microcracks, limiting further strength gains. Overall, while steel fibre addition improves the mechanical properties of sand concrete, these results highlight the importance of optimising fibre dosage to maximize performance while controlling material costs. The statistical analysis of the tensile strength at 20 ℃ as function of fibre content for the various curing ages are shown in Tables A5-A8.

3.3 Residual characteristics of heated specimens

Table 9 summarises the results of the residual properties of steel fibre-reinforced sand concrete with fibre contents of 0%, 0.25%, 0.50%, and 0.75%. According to the findings reported in this study [27], although the incorporation of steel fibres enhances the post-peak behaviour of sand concrete, it may also increase the presence of initial defects such as pores and bond interfaces. The statistical analyses of compressive strength and mass loss as functions of fibre content at 600 °C are reported in Tables A9 and A10, respectively, whereas Tables A11 and A12 report the corresponding analyses for tensile strength and ultrasonic velocity at 600 °C.

Table 9. Residual mechanical characteristics of specimens heated at 600 ℃

3.3.1 Mass loss of fibred sand concrete

As shown in Figure 12, a significant mass loss is observed at 600 ℃, primarily attributed to the evaporation of free and bound water as well as the progressive decomposition of cement hydration products. In fibre-free sand concrete, uncontrolled thermal cracking promotes rapid vapor migration, resulting in severe matrix damage that is often accompanied by localized spalling and marked surface degradation. The addition of fibres contributes to controlling the initiation and propagation of thermal microcracks, promoting a more gradual release of water vapor and reducing the buildup of internal pressure. This mechanism decreases the effective permeability of the material during heating and mitigates spalling, resulting in a systematic reduction in mass loss with increasing fibre content. Visual observations of the specimens after fire exposure support this trend: fibre-free sand concrete displays extensive surface damage with open cracks and spalled zones, whereas fibre-reinforced sand concrete exhibits finer, more uniformly distributed microcracks and a comparatively stable surface. These macroscopic observations are consistent with the measured mass loss results and confirm the beneficial role of fibres in improving the overall thermal stability of sand concrete, in agreement with findings reported in this study [43], which demonstrated that polypropylene and/or steel fibres reduce spalling and physical degradation in high-performance concrete exposed to elevated temperatures.

Figure 12. Mass loss of fibre-reinforced sand concrete after exposure to 600 ℃

3.3.2 Flexural strength

In contrast to the behaviour observed in compression, the flexural strength (Figure 13) of sand concrete is significantly enhanced by the addition of fibres, even after exposure to 600 ℃. Fibre-free sand concrete exhibits very low flexural strength (1.95 N/mm²), reflecting a brittle behaviour governed by rapid and uncontrolled crack propagation. At a low fibre content of 0.25%, the flexural strength increases markedly to 3.50 N/mm², corresponding to an improvement of approximately 80% compared with the reference sand concrete. This pronounced gain highlights the effectiveness of the crack-bridging mechanism widely reported in the literature, whereby fibres carry tensile stresses after matrix cracking, delay the initiation and propagation of thermally induced cracks, and enhance the residual ductility of the material.

Figure 13. Effect of fibre content on flexural strength after exposure to 600 ℃

For higher fibre contents (0.50% and 0.75%), the flexural strength remains higher than that of the fibre-free concrete, although the measured values are lower than the peak observed at 0.25%. This non-monotonic evolution reflects a balance between the beneficial effects of crack bridging and the adverse consequences associated with higher fibre content, such as partial thermal degradation of fibres, degradation of the fibre–matrix bond, and less homogeneous fibre dispersion or local fibre agglomeration. The slight recovery observed at 0.75% (2.77 N/mm²) further suggests the influence of random fibre dispersion, a phenomenon frequently reported in experimental studies on fibre-reinforced concrete and known to cause fluctuations in residual flexural strength.

Overall, these results confirm that fibres retain a significant residual effectiveness in flexure at elevated temperatures, with improved post-cracking behaviour and toughness remaining the dominant contribution. These findings are consistent with those reported in this study [44], who showed that even after exposure to temperatures as high as 600–800 ℃, fibres significantly enhance post-cracking behaviour by limiting crack opening and propagation and maintaining higher residual flexural strength than fibre-free sand concrete. The observed reduction in residual flexural strength at higher fibre content is also in agreement with previous studies [31], which attribute the non-monotonic evolution of residual flexural strength to fibre clumping, random dispersion, and partial thermal degradation.

3.3.3 Compressive strength

At an exposure temperature of 600 ℃, the residual compressive strength (Figure 14) of sand concrete shows an overall decreasing trend with increasing fibre content, although this evolution is not strictly monotonic. The fibre-free sand concrete exhibits the highest residual compressive strength (25.20 N/mm²), which can be attributed to an initially more compact and homogeneous cementitious matrix, free from additional discontinuities related to fibre–matrix interfaces. The incorporation of 0.25% fibres results in a significant decrease in compressive strength, primarily due to thermal degradation of the cement matrix in the 400–600 ℃ range, including the dehydroxylation of portlandite (Ca(OH)₂), partial decomposition of cement hydrates, and increased porosity. Under these conditions, the fibre-matrix interfaces act as preferential zones for crack initiation and propagation, thereby limiting the effectiveness of fibres under compressive loading despite their beneficial role in tension. At a fibre content of 0.50%, a slight recovery in compressive strength is observed (26.57 N/mm²), suggesting the presence of an optimal intermediate content at which fibres contribute to partial microcrack bridging and improved local stress redistribution without inducing excessive porosity. Similar observations have been reported in the literature for fibre-reinforced concretes exposed to 600 ℃, where moderate fibre contents help restrain the coalescence of thermally induced microcracks.

Figure 14. Effect of fibre content on compressive strength after exposure to 600 ℃

In contrast, increasing the fibre content to 0.75% results in a more pronounced decrease in compressive strength (22.00 N/mm²), which is generally associated with aggravated thermal damage, arising from the accumulation of voids left by fibre degradation, non-uniform fibre distribution, and an overall weakening of the load-bearing matrix.

Thus, despite the localized crack-bridging effects at intermediate fibre contents observed at 0.50%, the overall influence of fibres on residual compressive strength at elevated temperatures remain unfavourable. These findings are consistent with the conclusions reported in this study [45], which showed that fibre addition, particularly polypropylene fibres, reduces residual compressive strength after exposure to 600 ℃ due to thermal degradation of the fibres and deterioration of the fibre–matrix interfaces.

3.3.4 Ultrasonic velocity

The UPV measured after exposure to 600 ℃ in Figure 15 shows a reduction compared with values typically recorded at ambient temperature, indicating significant internal microstructural damage. All measured velocities fall within the range of 1923 to 2177 m/s, confirming that the concrete has been severely affected by high temperature exposure, since sound, undamaged concrete generally exhibits ultrasonic velocities exceeding 3500 m/s) [37]. The control sand concrete displays the highest pulse velocity of 2177 m/s, whereas fibre-reinforced concretes show slightly lower values, though without evidence of abrupt degradation despite the high-temperature exposure. This overall reduction in pulse velocity is mainly attributed to the formation of thermal microcracks and increased porosity. The drop observed at a fibre content of 0.25% fibre content (1923 m/s) correlates well with the corresponding decrease in compressive strength, suggesting an increase in porosity or internal microcracking, likely caused by fibre degradation at elevated temperatures.

The variations observed with different fibre contents suggest a dual role of fibres: while they introduce local discontinuities that tend to slow the propagation of ultrasonic waves, they also contribute to controlling the formation of major cracks. As a result, pulse velocity values for fibre-reinforced concrete remain within a relatively narrow range, indicating that, despite thermal exposure, the fibres provide a degree of microstructural stabilization by limiting the development of severe cracking.

Figure 15. Effect of fibre content on ultrasonic velocity

Future research will focus on investigating the evolution of porosity in fibre-reinforced concrete after fire exposure and characterising thermally induced microcracking within the material. Attention will be given to correlating these microstructural changes with residual mechanical properties. This approach will provide deeper insight into the post-fire behaviour of fibre-reinforced concrete and support the optimization of its performance under elevated-temperature conditions.