The Influence of Waste Cherry Pits as Coarse Aggregate and Waste Ceramics Powder on Rheological Properties and Strength of Self-Compacting Concrete ^†

Abstract

Recently, interest has grown in using alternatives to cement and aggregates to improve concrete and reduce its environmental impact. This study explores the use of cherry pit waste (CPW) as a partial substitute for coarse aggregates in self-compacting concrete (SCC) at varying rates (0–25%). Rheological, compressive strength, and ultrasonic pulse velocity tests were conducted. The results showed that CPWs reduce flowability but increase cohesion. The 5% CPW mix achieved the highest compressive strength. All the mixes remained acceptable, with classifications from SF3 to SF1. Due to CPWs’ lower density, both wet and dry weights decreased, making this a viable lightweight concrete option.

1. Introduction

Self-compacting concrete (SCC), developed in Japan in 1988 by Okamura, addresses the challenges of poor compaction due to insufficient skilled labor. It is considered highly efficient in complex construction scenarios [1]. In parallel, sustainable construction practices promote replacing natural materials with waste to reduce environmental impact and enhance resource efficiency [2,3,4]. Agricultural and industrial wastes are increasingly integrated into concrete to support sustainable development and reduce pollution [5,6]. Mohamed et al. [7] and Okechukwu et al. [8] evaluated lightweight concrete using oil palm kernel shell (OPKS/PKS), noting reductions in density and compressive strength. Walnut shells (WSs) were also studied as fine aggregate in SCC, showing reduced workability and strength with increasing WS content, though still meeting structural requirements [9]. Al-Hadithi et al. [10] used plastic fibers (PFs) and expanded polystyrene beads (EPS) to produce lightweight SCC with significantly reduced density and improved flexural strength. Other studies explored the replacement of coarse aggregate with lightweight alternatives like walnut shell-derived porcelainite and cement with waste glass powder. These substitutions reduced density and strength but remained within acceptable limits [11]. Coconut shell-based SCC showed strength losses at elevated temperatures [12,13]. The high cement and admixture costs, along with environmental concerns—mainly CO₂ emissions—are critical challenges in SCC production [14,15]. Researchers have experimented with replacing cement using various pozzolanic wastes, including glass [16], silica fume [17], fly ash [18], eggshells [19], bricks [20], and ceramics [21,22]. Ceramic tile production reached 11.9 billion m² in 2012 and continues to grow, yet only a fraction is recycled [23]. Reusing ceramic waste in concrete reduces cement usage and greenhouse gas emissions while improving thermal resistance [22]. Waste ceramic-enhanced concrete often performs better than conventional mixes [24]. Despite extensive research on agricultural and ceramic waste in concrete, no study has examined the use of cherry pits as a coarse aggregate in SCC. Prior research shows that SCC’s behavior varies depending on the type of waste added. However, the combination of ceramic powder and cherry pits remains unexplored. This study investigates the effect of partially substituting coarse aggregate with cherry pits (0–25%) in SCC. The cement content remains partially replaced with recycled ceramic waste powder across all mixes. Cherry pits, collected and processed from agricultural waste, are used as a lightweight coarse aggregate. The SCC mixes are assessed for workability (slump flow, L-box, and segregation resistance) and mechanical properties (wet/dry density, compressive strength, and ultrasonic pulse velocity).

2. Materials and Methods

2.1. Materials

Ordinary Portland Cement (OPC) with a fineness of 280 (Blaine method) was used. Its initial and final setting times were 198 min and 4.5 h, respectively, with a specific gravity of 3.15. Analysis followed IQS 5/2019 [25]. Chemical and physical properties of OPC are shown in

Table 1. Chemical properties of used powders.

Oxides (%)	Cement	Ceramic Waste
CaO	62.3	5.72
SiO2	20.28	57.4
Al2O3	5.55	17.98
Fe2O3	4.2	6.2
MgO	2.6	3.16
K2O	0.75	4.09
Na2O	0.4	2.37
SO3	2.27	-

and

Table 2. Physical properties of used powders.

Physical Properties	Test Results	Limits of IQS 5/2019
Initial setting time (Minutes)	198	≥45
Final setting times (Hours)	4.5	≤10
Fineness by Plaine method (m2/Kg)	280	≥280
Compressive strength at 2 days (MPa)	20.45	≥10
Compressive strength at 28 days (MPa)	45	≥42.5

. The ceramic powder was made from recycled ceramic waste sourced from Iraqi construction suppliers. After grinding, it was sieved through a 75 µm mesh, yielding powder with a specific gravity of 2.004. EDS analysis (Figure 1) revealed that the powder mainly contains aluminum, silicon, and oxygen. Aggregate testing followed Iraqi Standard No. 45/1984 [26].

The fine aggregate had a specific gravity of 2.65 and a water absorption of 0.82%. Its particle size distribution is shown in Figure 2. Crushed pebbles (max size 12.5 mm) were used as coarse aggregate, with a specific gravity of 2.5, SO₃ content of 0.038%, and absorption of 0.28%, as per Iraqi Standard No. 45/1984 [26]. Sieve analyses of both aggregates are shown in Figure 3. Cherry pits, cleaned, soaked, and dried, had a specific gravity of 0.815 and water absorption of 20%. A DCP Flocrete PC200 superplasticizer (SG 1.1), compliant with ASTM C494/C494M [27], was used. Tap water was used for all the mixes. The materials are shown in Figure 4.

2.1.1. Mixture Design

The SCC mix design followed the EFNARC-2005 guidelines [28], with details shown in

Table 3. Mixture proportion ratios.

Mix ID	Cement	Ceramic Powder	Sand	Gravel	Cherry Pits	Water	SP
CPW0%	400	100	790	965	0	165	8
CPW5%	400	100	790	917	48	165	8
CPW10%	400	100	790	869	96	165	8
CPW15%	400	100	790	821	144	165	8
CPW20%	400	100	790	773	192	165	8
CPW25%	400	100	790	725	240	165	8

. The water-to-binder ratio was 0.34, and the total binder content was 500 kg/m³, including 25% waste ceramic powder. A constant 3% superplasticizer (SP) dosage, based on cement weight, was used. The coarse aggregate was partially replaced with cherry pit waste (CPW) at 0%, 5%, 10%, 15%, 20%, and 25% by weight. Six SCC mixes were prepared accordingly and labeled CPW 0% to CPW 25%, with the number indicating the CPW replacement percentage. Table 3 summarizes the mixture proportions.

2.1.2. Concrete Casting and Mixing Procedure

To ensure successful SCC production, the mixing process was carefully managed following the EFNARC guidelines and the direct method by Khayat et al. [29]. In the reference mix (CPW0), half of the fine and coarse aggregates were added first, followed by water and partial CPW, and then binder and remaining aggregates, with staged additions of water and superplasticizer. Mixing continued until a uniform, homogeneous blend was achieved. For the CPW mixes, the fine aggregate was partially replaced with CPW and mixed dry before following the same procedure. Fresh SCC was tested for slump flow, T500, V-funnel, and L-box. Three 100 mm cubes per mix were used to evaluate fresh/dry density, weight loss, compressive strength, and UPV. The samples were covered for 24 h, and then cured in water at 28 °C as per ASTM C192/C192M [30].

2.2. Test Methods

To evaluate the performance of self-compacting concrete (SCC) incorporating cherry pit waste (CPW) as partial replacement of coarse aggregates and ceramic waste powder as a partial cement substitute, a comprehensive experimental program was conducted. The tests were designed to assess both the fresh and hardened properties of the developed SCC mixtures. Fresh-state tests were carried out to determine the flowability, passing ability, segregation resistance, and density of the mixes, which are critical parameters for ensuring proper self-compaction without vibration. In addition, several mechanical and non-destructive tests were performed on hardened specimens to evaluate the structural performance and internal quality of the concrete.

2.2.1. Fresh State Properties

Fresh state properties were evaluated following the EFNARC 2005 guidelines [28], including slump flow, T500 time, L-box passing ratio, and segregation resistance (Figure 5). The slump flow test assessed flowability by averaging two perpendicular diameters after lifting the slump cone. For the L-box test, the sample was poured into the container, and the sliding gate was lifted to observe flow. Passing ability was measured as the ratio of the concrete depth at the end to the start of the horizontal section. Sieve stability tests evaluated segregation resistance. A concrete rheometer measured plastic viscosity and yielded stress, while a hydrometer assessed air content. The test setups are shown in Figure 5.

2.2.2. Mechanical Properties

Compressive strength was tested at 28 days using three properly cured cubes following ASTM C109/C109M [31], as shown in Figure 6. Each specimen was placed between metal bearing plates for testing. Before compression testing, ultrasonic pulse velocity (UPV) tests were conducted on three 150 mm cubes using PUNDIT equipment, in accordance with ASTM C597 [32], to assess concrete quality. Additionally, a Schmidt rebound hammer test was performed on three 100 mm cubes as a non-destructive evaluation. UPV was measured using a 50 kHz pulse device with 0.1 precision. Wave velocity was calculated using Equation (1):

Impulse velocity (m/s) = 0.150 m/wave time (s)

(1)

3. Results and Discussion

3.1. Rheological Properties

3.1.1. Slump Flow

Figure 7 shows slump flow results for SCC with 10% ceramic waste (cement replacement) and varying CPW replacements (0–25%) of coarse aggregate by volume. All the mixes had slump flow values between 550 and 800 mm, meeting the EFNARC (2005) limits [28]. The mixes with 0% CPW reached SF3 class (high workability), while 5% CPW met SF2, and higher CPW levels fell to SF1 (lower workability but still acceptable). Increased CPW content reduced fluidity and flowability due to its high absorption and irregular shape, requiring more superplasticizer. This mirrors results from [24], where higher walnut shell content also reduced slump due to greater absorption.

3.1.2. L-Box (H2/H1)

Figure 8 presents the L-box test results for SCC with varying CPW content. According to EFNARC, the passing ratio (H2/H1) should be between 0.8 and 1 to prevent blockage. The results ranged from 0.85 to 0.92, indicating adequate passing ability for most mixes. As the CPW replacement increased, the H2/H1 ratio slightly declined but remained within the EFNARC limits [28]. The L-box test evaluates SCC’s ability to flow through congested spaces, such as reinforcement, without segregation. The three-bar version simulates denser reinforcement layouts, measuring concrete height after flowing between vertical bars to assess passing ability.

3.1.3. Segregation Ratio

According to EFNARC (2005) [28], segregation resistance is essential for ensuring the quality and uniformity of SCC, especially in tall structures where delayed segregation can cause surface defects. It becomes critical in low-viscosity or high-slump flow mixes. Workability is evaluated through segregation ratio and passing ability, as shown in Figure 9. The sieve stability test, which measures the amount of fresh SCC passing a 5 mm sieve, was used to assess segregation resistance. The results indicate that adding CPWs increased the segregation ratio and reduced flowability due to their high absorption and irregular shape. However, all the results remained within the EFNARC limits. Interestingly, segregation resistance improved at higher CPW content, likely due to better particle packing and reduced internal friction—findings supported by [33].

3.1.4. Fresh Density

Wet density in SCC refers to the fresh concrete mass per unit volume, influenced by the mix composition and specific gravities of its components. Figure 10 shows that wet density decreased as CPW content increased due to the lower specific gravity of CPWs compared to conventional gravel—consistent with previous findings [32]. This decrease is also linked to water loss in the mix, reducing specimen weight. Figure 11 and Figure 12 illustrate strong correlations between wet density and both slump flow diameter (R² = 0.90) and L-box height ratio (R² = 0.95), confirming the influence of CPW on mix behavior.

3.2. Hardened Concrete Properties

3.2.1. Compressive Strength

Compressive strength is a vital property reflecting concrete’s performance in its hardened state. Figure 13 shows the compressive strength of the SCC mixes with various CPW replacement levels. The control mix reached 48 MPa, while the 25% CPW mix had 28 MPa—still above the 17 MPa minimum for structural use. The strength values ranged from 25 to 45 MPa after 28 days, decreasing by 5–25% as the CPW content increased. This reduction aligns with prior studies [33,34], attributing the drop to the lower quality of CPW compared to natural aggregates. Additionally, the partial use of ceramic waste in place of cement and sand may enhance strength through C-S-H formation and void filling.

3.2.2. Ultrasonic Impulse Velocity (UPV)

The ultrasonic pulse velocity (UPV) test evaluates concrete quality by measuring pulse speed through the material. Conducted at 28 days, the results are shown in Figure 14. Increasing the CPW content led to a reduction in UPV values compared to the control mix. As CPW replaced coarser aggregate (0–25%), UPV declined from 4.6 km/s to 3.6 km/s, indicating increased porosity and lower concrete quality. Since UPV depends on density and elasticity, the presence of CPWs, with their lightweight and porous nature, reduced wave velocity, suggesting a decrease in structural integrity at higher replacement levels.

3.2.3. Dry Density

Figure 15 illustrates the effect of the CPW content on the dry density of SCC mixtures. At 28 days, values ranged from 1900 to 2300 kg/m³ depending on the CPW content. The decrease in density corresponds to the lower specific gravity of CPW compared to conventional aggregate. Despite the reduction, all the SCC mixes with CPW met the ACI 213R (2003) minimum of 1950 kg/m³ for air-dried lightweight concrete. As CPW replacement increased, the dry density decreased relative to the reference mix. Similar findings were reported by Hilal et al. [33].

3.2.4. Relationship Between Hardened Properties

Figure 16 and Figure 17 show the correlation between compressive strength, UPV, and dry density of the SCC mixtures at 28 days. Strong relationships were found, with R² values of 0.99 for compressive strength vs. dry density and 0.90 for UPV vs. dry density. Similar findings were reported in [35,36]. These correlations are useful when direct strength testing (e.g., cube tests) is not feasible. Figure 18 illustrates the established relationship between compressive strength and UPV for SCC containing CPW. Additional strong correlations were also found between void ratio, compressive strength, and dry density.

4. Conclusions

This study aimed to reduce environmental impact by partially replacing coarse aggregate with cherry pit waste (CPW) in SCC production. Using CPWs offers ecological benefits by repurposing agricultural waste and enhancing certain concrete properties. The main findings are as follows:

• Flowability decreased as the CPW content increased, while cohesiveness improved. The reference mix met SF3, 5% CPW mix met SF2, and others fell into SF1, yet remained acceptable per EFNARC.
• Passing ability declined and segregation risk increased with higher CPW content due to its high absorption, requiring more water.
• Wet and dry density dropped with CPW addition, as its specific gravity is lower than that of conventional aggregates.
• Both compressive strength and UPV values decreased with increasing CPW, but all the mixes remained within acceptable limits for lightweight structural concrete.
• The practical applications of SCC with CPW were highlighted more explicitly (e.g., lightweight structural elements, non-load-bearing walls, and precast elements).
• The trade-off between sustainability and performance was discussed, emphasizing that although strength and UPV decrease with higher CPW content, the values remain within acceptable limits for lightweight structural concrete.
• Possible future research directions were suggested, such as durability studies (e.g., freeze–thaw, chloride penetration, and carbonation resistance), long-term mechanical behavior, or optimization of superplasticizer dosage to mitigate the reduction in workability.
• The environmental impact and potential contribution of using agricultural and ceramic waste to circular economy practices in construction were reflected upon.