Calcination Analysis of CaCO₃ from Waste Oyster Shells for Partial Cement Replacement

1  Introduction

Aceh Province in Indonesia is rich in marine resources and abundant fishery products such as oysters. Many types of shellfish grow in Aceh’s waters, including oysters, with a continuous increase in annual production [1]. Generally, communities living in coastal areas work as oyster fishermen who collect catches near river estuaries, and the shells are typically discarded without further use. Waste Oyster Shells (WOS) that accumulate along the shoreline can cause environmental pollution in surrounding communities [2]. When WOS is left for a long time, there is a tendency for prolonged environmental pollution. Therefore, in the future, a large cost will be required to restore the beauty of the environment [3].

Oyster is a type of shellfish that grows in seawater or river estuaries, characterized by hard shells with chalky texture [4,5]. The hard shells function as a protective covering for the oyster body. WOS contains a very high amount of calcium carbonate (CaCO3), at approximately 98.00% [6]. Other types of shellfish also contain high levels of CaCO3, including mollusks [7], cockles [8], blood cockles [9], periwinkle [10], snails [11], and green mussels [12]. Several studies have widely used seashells as a substitute for cement in concrete [13,14]. Others stated that waste materials used to replace cement are coal bottom ash [15], rubber powder [16], silica fume [17], and fly ash [18].

The CaCO content in WOS has been proven effective as a binder material in paste and concrete, specifically for partial cement replacement. Previous studies have also shown that WOS powder can be used to replace materials in concrete, including partial cement, and fine or coarse aggregate [19]. This waste serves as cement substitute materials that have water-absorbing properties, with a low absorption rate. Based on previous studies, the water absorption level of WOS ranged from 1.00% to 7.66% [3,20]. When WOS is substituted into cement, it can accelerate the binding process with other materials in paste and concrete, leading to a more homogeneous mixture and improved early-age strength [21,22].

Generally, cement paste is a component of concrete, composed of a mixture of cement and water experiencing chemical reactions to form hydration products and water-filled pores [23]. It is dimensionally unstable and is not used in construction without the addition of stabilizing agents such as aggregates [24]. Cement paste forms when water is added to the concrete mixture, functioning as binding, coating, and lubricating [25]. To assess the homogeneity of cement bonding, a setting time test is conducted according to ASTM standards [26]. Setting time is divided into two categories, namely initial and final. The initial setting time refers to the time from mixing cement and water until paste loses plasticity. Meanwhile, the final setting time is the time it takes for paste to harden completely. The initial setting time is determined when penetration reaches 25 mm and should not occur earlier than 45 min according to ASTM C150 [26].

One of the major oxides in cement is calcium oxide (CaO), which functions as a binding agent. To achieve chemical properties similar to cement, CaCO3 must first be calcined. The calcination process in WOS converts CaCO3 into CaO [27], which accounts for approximately 63% of cement composition [22]. The calcination process typically uses a temperature of 800°C [28].

Calcination is a chemical reaction process aimed at altering the chemical properties of a material through heating at specific temperature. During calcination, other chemical elements such as CO2 are removed [5]. This process is carried out by heating the material at the designated temperature using a microwave or electric furnace, with the duration adjusted according to the material [29]. High-temperature calcination can produce a large amount of lime [30]. Moreover, calcining at 1200°C leads to complete calcination but requires a long duration often referred to as “dead burnt” [31].

To examine the result of calcination, a laboratory test known as X-Ray Fluorescence (XRF) is required. XRF testing is a chemical analysis method used to determine calcination outcome, which consists of several elements expressed as percentages. This test material can be in powder or liquid form [32].

Currently, CaO content in WOS has been widely used as a partial replacement for cement, fine aggregate, or coarse aggregate in concrete. The optimal calcination temperature for producing high CaO content has not yet been clearly established. Therefore, this study aimed to determine the optimum calcination temperature to produce high CaO content, the initial setting time of WOS paste, and the best compressive strength.

Previous studies have performed calcination of WOS at a single level for 2 h using WOS as a substitute material in cement, with a particle size of 38 µm in diameter like Ordinary Portland Cement (OPC) (sieve No. 400). In this study, WOS was calcined at temperatures of 700°C, 800°C, 900°C, and 1000°C, serving as a 5% partial replacement for cement in the paste. WOS had a particle size of 75 µm (sieve No. 200), with a constant calcination duration of 2 h. This method was in line with previous studies, which reported CaO content in WOS calcined at temperatures of 700°C [33], 800°C [34], 900°C [27], and 1000°C [35]. However, there was no analysis of variation in MgO content from different WOS calcination temperature. The MgO content in WOS is crucial, due to the significant impact on the porosity and compressive strength of the paste.

For the experiment, WOS was collected from Krueng Neng, Gampong Surien, Meuraxa District, Banda Aceh City, Indonesia. XRF tests were conducted to determine the CaO content, which was used to analyze the compressive strength test of the paste. Subsequently, normal consistency and initial setting time tests were performed using a Vicat apparatus to obtain the optimum water content and setting time.

The optimum compressive strength of the paste was analyzed using ANOVA and Response Surface Methodology (RSM) through Design Expert software. RSM allows the development of a robust predictive model considering various effects, including linear, quadratic, and interaction effects between independent variables. One of the advantages of RSM is the requirement for fewer experiments compared to other methods to achieve the same level of accuracy. This allows for the determination of the ideal mixture to achieve the optimum compressive strength at a lower cost.

2  Material and Methods

This study was carried out based on ASTM (American Society for Testing and Materials) standards. The conditions stated in ASTM standards included chemical characterization of cement using XRF testing [36], normal consistency testing [37], setting time testing [26], and compressive strength testing of paste [38]. The procedures conducted in this study were as follows:

2.1 Material and Equipment Preparation

Before performing chemical characterization and setting time testing of paste, the initial procedure carried out was the preparation of materials, which included,

1.    Cement used is Ordinary Portland Cement (OPC) Type 1, produced by Lafarge Cement Indonesia Company.

2.    The water used in this study is clean, odorless tap water obtained from the laboratory.

3.    WOS was obtained from Krueng Neng, Gampong Surien, Meuraxa Subdistrict, Banda Aceh City, Indonesia. The sample was collected and brought to the laboratory for cleaning of any attached dirt using tap water. This was followed by soaking in tap water for 24 h to ensure thorough cleaning. Subsequently, the sample was placed in an oven for 24 h at a temperature of 105°C–110°C and crushed using a Los Angeles machine after drying [39]. The WOS was ground using a No. 200 sieve (0.075 mm) to produce a powder, which was used as a partial cement replacement [40]. The crushing process is shown in Fig. 1.

Figure 1: The WOS crushing process

The equipment used in this study complies with ASTM standards. This includes ASTM No. 200 standard sieve, containers or pans, drying oven, Los Angeles machine, a 5000-g capacity scale with 0.01-g accuracy, mixer, porcelain bowl, ebonite rings, vicat apparatus with a conical mold (70 mm bottom diameter, 60 mm top diameter, 40 mm height), glass plate, stopwatch, and other supporting tools (ruler, measuring cup, dipper, and bucket).

2.2 Calcination

Calcination was conducted in a chemical laboratory to remove moisture, carbon dioxide, and other volatile components by applying controlled heating. The test was performed on waste oyster shell (WOS) material that had been previously ground and passed through an ASTM No. 200 sieve (75 µm). The calcination process was carried out in a controlled environment, where the WOS samples were placed in a heat-resistant crucible and inserted into an electric furnace. The specimens were subjected to temperatures of 700°C, 800°C, 900°C, and 1000°C, with each calcination maintained for 2 h. The furnace temperature was increased gradually until the target temperature was reached, after which the heating was maintained for 2 h and then turned off. This procedure is consistent with previous studies that also utilized a 2-h calcination period at similar temperature levels: 700°C [41], 800°C [42], 900°C [43], and 1000°C [44]. After the heating process, the WOS samples were allowed to cool naturally at room temperature for 6 h. Post-calcination, the WOS exhibited distinct physical and chemical characteristics compared to the uncalcined control. The detailed calcination process used in this study is illustrated in Fig. 2.

Figure 2: WOS calcination process

2.3 Test Specimen Design

The specimen in this study was paste shaped into ebonite rings with the following dimensions, namely 70 mm bottom diameter, 60 mm top diameter, and 40 mm height. The tests conducted on specimen included normal consistency and setting time. XRF testing was performed to determine chemical content in the laboratory. Meanwhile, compressive strength testing used cube molds measuring 5 cm × 5 cm × 5 cm, as shown in Table 1.

2.4 Chemical Properties Testing

Chemical properties of OPC Type 1 and WOS were examined through XRF testing based on ASTM [36]. The test was performed on OPC Type 1 and calcined WOS samples. Solid powder samples weighing 3 g were used.

2.5 Determination of Water Content

The water content in paste was determined using the normal consistency test according to ASTM [37]. The initial step was to weigh cement, WOS, and water. After measurement, the total weight of cement and WOS was set to 400 g using a digital scale with 0.01 g precision. WOS was used to replace 5% of cement by weight and initial water content was assumed to be 25% of the combined weight of cement and WOS. After mixing the materials to form a paste, the mixture was placed in an ebonite ring mold and tested with a Vicat apparatus. When the Vicat needle penetration was below 9 mm, additional water was added until the penetration reached between 9–11 mm.

2.6 Setting Time Test

Setting time test was carried out on cement paste and WOS mixed paste, based on ASTM [26]. The procedure for testing the setting time of cement paste mixed with WOS is as follows:

a.   Weigh cement, water, and WOS that is used to replace 5% of cement. Therefore, the weight of WOS is 5% of cement weight.

b.   Put the weighed cement, WOS, and water into a mixer.

c.   Mix the ingredients in the mixer until a homogeneous paste is formed, for approximately 3 min.

d.   Shape paste into a ball using both hands.

e.   Toss paste ball back and forth between hands at a distance of 15 cm, six times.

f.   Place paste ball into the ebonite ring and place it on the Vicat apparatus.

g.   Release the Vicat needle at the first 15-min interval and record the penetration depth.

h.   Release the Vicat needle again after another 15 min and record the penetration depth, ensuring the measurement point is +5 mm and +10 mm from the edge of the ebonite ring.

i.   Observe the initial setting time of cement paste, which occurs when the penetration depth reaches 25 mm. A graph of the initial setting should be made. The data recorded include the penetration time in minutes and the depth in millimeters [45].

2.7 Paste Mix Design

In this study, paste mix design was determined based on the water content obtained from the normal consistency test. The variables used to produce paste were cement replacement and calcination temperature, with the responses being compressive strength and density. Furthermore, paste mixing was conducted in the laboratory through four trial combinations, with details shown in Table 2.

2.8 Compressive Strength Test of Paste

The compressive strength test of paste was conducted in accordance with ASTM [38]. The specimens used were cubes measuring 50 mm × 50 mm × 50 mm. This test consisted of five variations as shown in Table 1, each tested with three specimens, totaling 15 for the compressive strength test.

The homogeneous cement paste mixed with WOS was placed into cube molds and compacted according to ASTM standards. After one day of curing, specimens were submerged in clean water and cured for 28 days, followed by weighing and measurement. Subsequently, paste specimens were placed into a Universal Testing Machine for compressive testing. The compressive strength of paste was calculated using the following formula.

fm=PA.(1)

Legend: fm = Compressive strength of paste (MPa); P = Maximum failure load (Newton); A = Cross-sectional area of the specimen (mm2).

2.9 Statistical Analysis

In this study, the compressive strength test results of paste were analyzed using the ANOVA (Analysis of Variance) statistical test, applying RSM through the Design Expert software. The variables used were cement replacement (denoted as A) and calcination (denoted as B). The responses obtained were compressive strength and bulk density of paste. A quadratic model was designed and presented in the form of contour and 3D graphs. Based on the results, a model that produces a p-value < 0.05 is considered acceptable, showing a significant effect on the increase in compressive strength.

3  Results and Discussion

Calcination test and chemical property analysis of WOS served as preliminary data before conducting the normal consistency, setting time, and compressive strength tests of paste. Based on the test results, the following data were obtained and analyzed:

3.1 Calcination Test Results

Calcination test was carried out cleaned and oven-dried WOS samples. Heating or calcination at 700°C, 800°C, 900°C, and 1000°C led to changes in the physical properties of WOS, namely in terms of texture and color. At 900°C and 1000°C, the texture became rougher, and the color turned whitish, as shown in Fig. 3. Therefore, calcined WOS had to be ground again using a No. 200 sieve. According to previous studies, WOS used as cement substitute has varying particle sizes, namely 150 µm (sieve No. 100) [46], 75 µm (sieve No. 200) [47–49], and 38 µm (sieve No. 400) [50].

Figure 3: OPC cement and calcined WOS

Based on Fig. 3, a significant color difference can be observed between Type 1 OPC and the calcined WOS. The color of Type 1 OPC cement is dark gray, while the calcined WOS becomes more whitish. Clumping of WOS was observed after calcination at 900°C and 1000°C. This was due to the calcination temperature being too high and the duration being only 2 h. Based on previous studies, higher calcination temperature (900°C–1100°C) could lead to a faster process, which required a longer time for CaO to form [31]. Therefore, to allow proper mixing into a paste, the clumped WOS was crushed and sieved using No. 200. Higher calcination temperature also corresponds to harder WOS texture and more pronounced color change. These results correlate with previous studies, where high-temperature calcination of WOS leads to changes in both color and particle size [35]. Another study also reported that calcining WOS at high temperatures led to a rapid change in color [51].

3.2 Chemical Composition Analysis

The Rigaku instrument is used only to detect chemical composition, while the LOI (Loss of Ignition) value is obtained from TGA (Thermogravimetric Analysis) testing, which is not measured in this study. The physical properties examined were specific gravity and SSA (Specific Surface Area). The specific gravity test was conducted using the Le Chatelier flask method, in accordance with ASTM C188 standard [52]. Meanwhile, SSA testing was performed using the BET (Brunauer-Emmett-Teller) [53]. Based on the BET method, the SSA of WOS with a particle size of 75 µm was found to be 0.276 m2/g. In comparison, previous studies reported that the SSA of OPC cement was 0.352 m2/g [54], showing a lower value of WOS. Therefore, water absorption is also lower and the workability of WOS paste improves [55,56]. Table 3 shows the percentage of chemical composition and the specific gravity of OPC and WOS produced from calcination at 700°C to 1000°C.

Based on Table 3, a significant difference is observed in the amount of CaO produced. The XRF test results show that the CaO content for OPC Type 1, WOS1 (700°C), WOS2 (800°C), WOS3 (900°C), and WOS4 (1000°C) were 55.8%, 45.3%, 49.2%, 54.3%, and 57.4%, respectively. This showed that higher calcination temperature led to greater production of CaO. In comparison, WOS2 produced more CaO than WOS1 and had the lowest MgO content among all samples. Previous studies also stated that calcining at 800°C could perfectly transform CaCO3 into CaO [34]. Based on the results, the CaCO3 content of uncalcined WOS was 87.56 [57], 95.32 [58], 95.99 [59], 95.99 [60], 97.11 [21], 97.20 [61], and 98.00 [6].

The specific gravity of OPC was found to be higher compared to WOS. This was because OPC particles are smaller in size compared to WOS. With smaller OPC particles, the surface area is larger and requires more water. Therefore, the volume of cement becomes smaller, which can increase the specific gravity value. Previous studies reported similar results, with OPC having a specific gravity of 3.14 g/cm3 and WOS of 2.67 g/cm3 [21].

The optimum temperature for producing CaO was 1000°C and the amount of CaO achieved was greater than OPC Type 1. Other studies have also confirmed that calcination at 1000°C yields a high CaO content [31]. XRF results show that all calcined samples had high CaO and low MgO content. According to a report, the quality of cement can be evaluated based on CaO and MgO content [62]. Another study also suggests that low MgO content leads to better homogeneity in cement paste when mixed with water [63].

3.3 Normal Consistency Test Results

The normal consistency test was carried out on Type 1 OPC and WOS-mixed paste. The results provided the percentage of water used in making paste, which was expressed as Water (W) and Derivation (D). Detailed data are presented in Table 4 below.

Based on Table 4, each sample experienced multiple trials to determine the appropriate water percentage for paste preparation. The amount of water used was determined based on the vicat needle penetration, with a target of 9–11 mm. Furthermore, water content obtained for each paste sample, namely OPC-NC, WOS1-NC, WOS2-NC, WOS3-NC, and WOS4-NC, was 29%, 31%, 29%, 30%, and 31%, respectively. The WOS2-NC paste showed similar physical properties to the OPC-NC control, requiring the same water percentage (29%) to initiate cement hydration. However, the vicat needle penetration of WOS2-NC was lower than OPC-NC due to reduced MgO content, which accelerated the hydration process (paste hardening). Previous studies reported that lower MgO content accelerated paste hardening, thereby reducing vicat needle penetration [64,65]. Another study found that WOS with 0.20% MgO and OPC Type 1 with 1.40% MgO caused a lower slump in WOS, accelerating paste and concrete setting [66].

3.4 Setting Time Test Results

The setting time test was performed immediately after determining the water content from the normal consistency. This test was conducted at 15-min intervals and measured vicat needle penetration, with results shown in Table 5.

Table 5 shows that vicat penetration for all pastes is 40 mm during the first three 15-min intervals. The test was stopped once the penetration dropped below 25 mm. For the control paste (OPC-ST), the condition occurred at 90 min, while WOS was achieved at 75 min. The final results of paste setting time test are shown in Fig. 4.

Figure 4: Setting time graph of WOS-mixed paste

Based on Fig. 4, a higher calcination temperature of WOS leads to lower vicat needle penetration in paste. The high CaO content in the 5% WOS-mixed paste reduced plasticity, leading to faster hardening compared to the OPC control paste. The results also showed that the initial setting time of the WOS-mixed paste was shorter compared to OPC cement. This was because WOS required more water than OPC cement. As a result, it has better workability and takes longer to form a rigid structure, showing that the initial setting time is extended [67]. Previous studies found that CaO addition could shorten setting time and accelerate hydration [68,69]. Other reports suggested that higher calcination temperature of WOS increased CaO content, thereby enhancing the workability performance of paste and concrete [46–48].

3.5 Paste Mix Design Results

Paste mix design was based on water percentage obtained from the normal consistency test. Manual mixing was performed in the laboratory for cube specimens measuring 5 cm × 5 cm × 5 cm, with details shown in Table 6.

Based on Table 6, workability for each variation was significantly influenced by the amount of water added. Higher water content led to a greater W/C ratio, showing better workability and ease of mixing. The OPC-fm paste had lower workability compared to WOS paste due to high MgO content in OPC, as shown in Table 3. Similarly, previous studies stated that replacing 5% to 10% WOS with cement could dilute paste and increase the W/C ratio [70,71]. Other reports confirmed that adding WOS to cement enhances paste workability [21]. WOS substituted into cement could also dilute paste mixture, thereby increasing the water-to-cement (W/C) ratio and improving workability. As a result, paste hydration increases to reduce the intensity of C3S and C2S diffraction peaks.

3.6 RSM Optimization Analysis

3.6.1 Results of Compressive Strength and Density Tests of Paste

Compressive strength and bulk density tests were conducted after the specimens reached 28 days of age. The specimens were immersed in a curing tank with a humidity temperature of 28°C. Previous studies reported that the curing temperature for immersed paste typically ranged around 27°C ± 2°C [5]. Table 7 shows results for hardened paste, which include compressive strength and density from 5 variations. Based on the results, the variables used in this study consisted of cement replacement and calcination, with the outputs being response 1 (compressive strength) and response 2 (density). The data were also statistically analyzed using ANOVA with the RSM through the Design Expert application, applying a quadratic model represented in contour and 3D plots.

The quadratic model was used in the ANOVA, with cement replacement variable denoted by A and calcination temperature by B. The ANOVA results for compressive strength and density are presented in Table 8.

Both compressive strength and density tests produced statistically significant data (p < 0.05). The variables used in this study significantly influenced the increase in compressive strength of paste. The F-value for compressive strength was 315.23 with a p-value of 0.0414, showing a significant model. For density, although the model was significant overall, individual terms showed no substantial impact. Furthermore, the standard deviation obtained was ideal, namely <3.00. This showed that all values in the dataset were nearly identical to the mean. Previous studies have stated that a standard deviation smaller than mean showed a low level of data deviation, indicating good data quality [72].

From responses 1 and 2 in Table 7, contour and 3D surface plots were generated using Design Expert. This showed the relationships between cement replacement and calcination temperature, as presented in Figs. 5 and 6.

Figure 5: Contour and 3D response surface plot for compressive strength

Figure 6: Contour and 3D response surface plot for density

Figs. 5 and 6 show that cement replacement and calcination temperature significantly influence the increase in compressive strength and density. When WOS replacement is increased to 15% and calcined at 700°C to 900°C, both strength and density improve. The most significant improvement occurs near 800°C with higher WOS content.

The compressive strength results in Fig. 5 show that the maximum strength for 5% WOS replacement is found in WOS2-fm at 71.37 MPa, due to low MgO content produced from calcining WOS at 800°C. Liu et al. (2012) observed that low MgO enhanced paste hardening and strength [64]. Similarly, Liao et al. (2022) found that WOS calcination at 800°C significantly increased mortar strength [27]. In line with previous studies, MgO content of 0.33 in calcined WOS could significantly enhance the compressive strength of mortar. This was due to the CaO content in WOS, which provided space for Ca(OH)2 to participate in the pozzolanic reaction. The resulting C-S-H gel fills the voids in the mortar, thereby increasing the density of the mortar structure [73]. Other studies on the use of fly ash as a substitute material in paste have shown the production of different chemical compositions compared to WOS. It was also reported that calcining fly ash at 800°C for use in cement could significantly enhance the compressive strength of paste. The reaction was attributed to the high content of SiO2, a primary chemical compound in cement production, amounting to 48.70%. Specifically, SiO2 plays a crucial role in increasing density and strength, reducing permeability, and improving resistance to corrosion [74].

In this study, the minimum compressive strength occurred in WOS4-fm at 47.45 MPa, which was lower than OPC-fm. High calcination temperature may lead to excessive CaO, which can slow hardening and reduce strength [75]. According to previous studies, excessive addition of CaO into mortar increases porosity, leading to a significant reduction in the compressive strength [76]. Another study found that when CaO was added to cement in high proportion, cement particles could not disperse properly, thereby reducing the fluidity of cement paste mixed with WOS. This suggested that paste ability to flow decreased, causing the formation of voids and a reduction in compressive strength [77]. Another study reported that when cement hydration occurred with a high CaO content, there was a decrease in the formation of C-S-H gel. Therefore, porosity increased, water absorption became higher, and the compressive strength of cement declined [46].

Fig. 6 shows a similar trend for density, where the maximum was 2.14 g/cm3 (WOS2-fm), and the minimum was 2.04 g/cm3 (WOS4-fm). All WOS-mixed paste had higher density than OPC-fm, except WOS4-fm. The results of chemical testing showed that MgO content in WOS calcined at 800°C was the lowest. This showed that hydration did not occur rapidly, preventing cracking in paste and leading to a denser paste. According to previous studies, when only a small amount of WOS was replaced with cement, the required water amount was low, leading to a lower production of Ca(OH)2. Therefore, the performance of paste improved, and the bulk density also increased [78]. High calcination causes CaCO3 to turn into CaO, resulting in weight reduction [79], while lower CaO content helps fill voids and increases density [80].

3.6.2 Mathematical Model

Table 8 shows the analysis of compressive strength and paste density test results using ANOVA. Based on this analysis, a mathematical model can be determined using the Design Expert application by referring to the coefficients in terms of coded factors, with details shown in Table 9.

The equation in terms of coded factors can be used to make predictions about the response for given levels of each factor. By default, the high levels of factors are coded as +1 and the low levels are −1. The coded equation is useful for identifying the relative impact of factors by comparing the coefficients. Moreover, the mathematical models between the factors and responses in this study are given by Eqs. (2) and (3).

Compressive Strength=78.06+27.41A−9.92B−11.70B2(2)

Density=2.16+0.075A−0.03B−0.045B2(3)

Legend: A = Cement Replacement (%); B = Calcined (°C).

3.6.3 Prediction of the Optimum Level

The optimum level was predicted by determining the criteria for the variables affecting the response. In this study, the variables used were cement replacement and calcination temperature. Cement replacement variable had two levels of 0% and 5%, while calcination variable had four levels, namely 700°C, 800°C, 900°C, and 1000°C. In RSM, the objective for cement replacement and calcination was set to in range, while the compressive strength was set to maximize and density was place at none. By using the Design Expert software and the RSM method, the optimum combination of levels was obtained as shown in Fig. 7, namely cement replacement = 5%, calcined at 786.438°C.

Figure 7: Effect of the optimum level on study variables

Based on Fig. 7, the optimum level of each variable was shown by a peak point with a desirability value of 1.000. It was also observed that paste strength decreased as desirability reached 0.000. The strength significantly decreases when cement replacement is below 5% and calcination exceeds 800°C. As shown in Fig. 6, the optimum response for maximum compressive strength can be determined. The optimum response can be visualized in the form of a contour plot, as shown in Fig. 8.

Figure 8: Contour plot for desirability and all responses

Based on Fig. 8, the desirability value reaches 0.986 when cement replacement is increased to 4% and 5%, and calcination temperature is set between 700°C and 870°C. The compressive strength of paste reaches the peak when cement replacement is increased and calcination is carried out at temperature between 750°C and 820°C. Meanwhile, the optimum paste density is achieved if cement replacement is increased to between 3% and 5% and calcination temperature ranges from 700°C to 870°C. The predicted optimum compressive strength and paste density are shown in Table 10. Therefore, with variable levels of cement replacement at 5% and calcination temperature at 786.44°C, the responses obtained are a compressive strength of 71.028 MPa and a density of 2.140 g/cm3.

4  Conclusion

In conclusion, the results showed that the optimum temperature for producing CaO was 800°C. Therefore, replacing WOS with cement at 5% and a temperature of 800°C could yield ideal chemical properties, namely high CaO and low MgO content. These chemical components showed the potential to accelerate the paste hydration process.

This study showed that replacing WOS with cement at 5% and calcination temperature of 800°C could significantly increase the compressive strength of the paste. This was due to the moderate rate of cement hydration and the formation of a perfect bond between cement, WOS, and water. The optimum level for achieving maximum compressive strength and density responses was found to be at 5% cement replacement and calcination temperature of 786.44°C.

Using WOS in paste production added value to the material, as the high CaO content made it a viable substitute for one of the main raw materials in cement manufacturing. However, the limitation of this study was the use of a relatively large WOS particle size of 75 µm. This limited the results regarding the ideal WOS particle size in terms of CaO content and compressive strength improvement of paste. Therefore, varying WOS particle sizes in future studies could help overcome the limitation. Varying calcination duration for WOS could also be considered as a subject. Further analyses such as TGA, XRD, FTIR, and SEM-EDS, including LCA and TEA assessments were also recommended to strengthen the evaluation of CaO reactivity and the impact on mechanical performance and material sustainability.

Acknowledgement: The authors are grateful to the University of Syiah Kuala and the University of Iskandar Muda for providing tools for conducting the study. Furthermore, the authors are grateful for the contributions of Rudiansyah and Tarmizi who provided assistance with the experimental work.

Funding Statement: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Author Contributions: Bunyamin Bunyamin carried out the teoretical analysis and experimental studies, participated in the sequence alignment and drafted the manuscript. Taufiq Saidi guided the whole study and edited the writing of the article. Muttaqin Hasan participated in the experimental studies, participated in the design of the specimens and perfomed the experimental data analysis. Sugiarto Sugiarto helped to draft manuscript, participated in data processing and article editing. All authors reviewed the results and approved the final version of the manuscript.

Availability of Data and Materials: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest to report regarding the present study.

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