Preliminary Synthesis of Calcium Silicates using Oil Palm Leaves and Eggshells

A new synthetic procedure is described for the synthesis of calcium silicate derivatives, using natural resources such as eggshell (ES) for calcium and oil palm leaves (OPL) for silica, which do not require prepurification. The reaction is performed by directly converting two weight ratio of the precursors, ES:3OPL and ES:6OPL, to dried-powder form by heat treatment at 900 °C for two hours. The results demonstrate that the concentration of the precursors has an effect on the morphology and crystallinity of the calcium silicate derivatives, mainly Ca2SiO4 and CaSiO3. X-ray diffraction results reveal that the reaction product obtained using a 1:3 ratio is quite pure, and mainly consisted of calcium silicate in the form of Ca2SiO4. The CaSiO3 was also identified in ES:6OPL, together with a small amount of excess non-reacted crystalline silica. Furthermore, a scanning electron microscopy analysis shows that both reaction products have a coarse surface. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Numerous literatures have paid specific attention to the green synthesis of calcium silicates. For example, wollastonite (CaSiO3) was successfully prepared using a hydrothermal method from CaO obtained from eggshell and SiO2 extracted from diatomite. This method generates a wollastonite mixture of -CaSiO3 and -CaSiO3 [1]. Another approach was devel-carbon at 1320 °C [6]. Another important derivative is Ca2SiO4, known as calsil or larnite depending on its crystallite structure, where the basic arrangement of larnite or others (, , and ) has a heteropolyhedral glaserite framework [7]. Extensive research has been conducted on the synthesis of Ca2SiO4, including common synthetic and green methods. For instance, Ca2SiO4 was prepared from the combination of eggshell and tetraethyl orthosilicate (TEOS) as precursors, using a sol-gel combustion treatment [8]. In addition, other reports established the use of natural silica, rice husk, CaO, and eggshell to form calcium silicate derivatives or silica-impregnated CaO. This technique is dependent on the reaction conditions, and the product derived from the use of silicaimpregnated CaO obtained from rice husk and eggshell is effective as a catalyst in biodiesel preparation [9].
Calcium silicate in the form of CaSiO3 has some industrial applications due to its inherent properties including thermal stability, chemical inertness, and low thermal conductivity and expansion [10]. These materials also serve as important fertilizers that ensure the productivity of some plants, and the silicon component plays a major role in increasing resistance against diseases, abiotic stresses, insect attack, and the effect of climate change [11]. Thus, as a silicate fertilizer, the CaSiO3 can be added to urea as a nitrogen source for paddy growth and to enhance tropical soil productivity, a process that is claimed to reduce N2O emission [12]. Furthermore, a commercially available CaCO3 has been used as a precursor of porous -Ca2SiO4 by its reaction with SiO2 in the presence of tripholyphospate and polyurethane foam, which serve as stabilizer and template, respectively [13].
From an environmental perspective, the use of sustainable natural resources as precursors in the preparation of advanced materials is preferred over materials of synthetic origin. Some recent researches have reported oil palm leaves (OPL) as a potential source for natural SiO2, which is used in several purposes [14][15]. In addition, oil palm ash (OPA), which is obtained from an incinerator at the palm oil mill, form nanostructural materials by high-energy ball milling. The resulting product consists of silica or silicon dioxide (SiO2), which is perhaps the most essential substance in OPA [16]. To the best of our knowledge, no studies describing the use of OPL as a source of silica for the synthesis of materials such as wollastonite, larnite, or calsil have been reported. In contrast with reported procedures, the reactions con-ducted in this investigation do not require stabilizers, such as: tricalcium phosphate and the surfactant sodium dodecyl sulfate [13,17]. In the context of sustainable technology, this report describes research results pertaining to the synthesis of silica-based materials using local bioresources and the characterization of the obtained products.

Materials and Methods
Oil palm leaves (OPL) and eggshell (ES) were washed with aquadest, wiped with tissue, and dried on air. Each material was grounded to powder using a juice mixer and passed through 325 mesh sieves, before repeating the drying process. The synthesis was carried out using two weight ratios of ES and OPL powders, and the corresponding samples were called ES:3OPL and ES:6OPL. For ES:3OPL, 1 g of ES powder was weighed into a furnace crucible, followed by addition of 3 g OPL powder.
The mixture was homogenized using a glass spatula, and a small amount of demineralized water was added to form a sticky reaction mixture. The crucible was then placed in a furnace and subjected to a heating process. A similar procedure was applied for the sample ES:6OPL, in which 1 g ES and 6 g OPL powders were subjected to the heating treatment. The furnace temperature was increased from room temperature to 900 °C within 4 h, and was kept at that temperature for 2 h. After cooling down the furnace for 12 h, the sample was ready for further analysis.
X-ray diffraction (XRD) analysis was performed to determine the crystallinity phase and the diffraction pattern of the materials, using an X'PERT Powder-PANalytical PW 30/60 instrument. Surface morphology analysis was acquired on a JEOL JSM 6510 LA instrument equipped with an energy dispersive X-ray

Results and Discussion
A solid-state reaction between OPL and ES powders was performed as shown in Figure 1. The amount of the products obtained using the samples ES:3OPL and ES:6OPL was 0.6301 g and 1.561 g, respectively. By mixing the powders of yellow-white ES (Figure 1(a)) and green-brown OPL (Figure 1(b)), the mixture shown in Figure 1(c) was obtained. Subsequent addition of a small amount of water to ensure homogeneity afforded the material depicted in Figure 1(d). This mixture was heated at 900 °C for 2 h, leading to the formation of a gray-white solid material (Figure 1(e)), which was characterized as shown in Figure 2.
Since the XRD analysis of ES has been largely investigated [18][19][20], it was not addressed in the present study. Figure 2(a) shows the amorphous characteristic of the OPL powder. The wide peaks at 21.6° and 29.9° and other small peaks can be attributed to the corresponding crystal plane of disordered carbon [21], and are possibly overlapped with those of the silica precursors. After heat treatment at 900 °C for 2 h, the resulting white-gray solid material was subjected to XRD analysis ( Figure   2(b)). A mixture of compounds was identified in the material, being crystalline silica the major component. This result is congruent with previous reports revealing the presence of silica in OPL [14][15]. Figure 3 shows the results of further XRD characterization of the reaction products. Figure 3(a) shows major diffraction peaks at 2θ of 32.1°, 32.4°, and 41.1°, which can be attributed to Ca2SiO4 (dicalcium silicate) (JCPDS 00-031-0299), and the XRD pattern was compared with that of other reported approaches [19,[22][23][24][25]. It should be noted that the present XRD profile reveals that the reaction products mainly consist of the Ca2SiO4 phase. This XRD profile was in line with that obtained from the reaction of commercial TEOS as a source of sil-    ica and calcium nitrate as CaO source and other reports [7,26]. This suggests that the reactions using the ES-to-OPL ratio of 1:3 tend to produce calcium silicates with Ca2SiO4 as the major product and CaSiO3 as the minor product. In addition, some impurities were detected, including excess of mineral CaO, which was identified by the presence of peaks at 17.9°, 34.1°, and other overlapped peaks, and was converted to Ca(OH)2 by hydration under air condition. This XRD data was compared with reported results [27][28] and supported by JSPDS 01-073-5492 and ICDD 00-044-1481.
Other peaks at 26.8°, 29.92°, 39.3°, 44.7°, and various small peaks were also observed, more clearly in Figure 3(b), indicating the presence of reasonable amounts of CaSiO3. This was compared to previous XRD results on calcium silicates synthesized using pure silica and pure CaO precursors [29], and referred to JCPDS #00-043-1460 [30,23]. It was found that higher amounts of CaSiO3 were obtained in the present study, most likely due to the fact that our method preserves the high content of silica precursor in OPL. Despite the higher content of silica and other products such as CaSiO3 in the reaction mixture having a weight ratio of 1:6, similar products were obtained. The XRD patterns displayed in Figure 3a show that the silica was consumed completely, as indicated by the absence of a silica peak as discussed for Figure 1b, whereas an excess of calcium hydroxide was observed. On the other hand, upon increasing the amount of OPL, a silica peak was observed at 2θ = 21°, and no CaO/Ca(OH)2 was found. These evidences indicate that both precursors were converted to the corresponding calcium silicates such as CaSiO3 and Ca2SiO4. Despite the lack of experimental evidence, the reaction mechanism for the formation of the calcium silicates derivatives CaSiO3 and Ca2SiO4 can be predicted from other studies [24,31]. The interaction between CaCO3 in ES and silicon-based complex compounds in OPL at 900 °C may proceed as follows: The calcium silicate product was subjected to scanning electron microscopy (SEM) analysis, and the corresponding results are depicted in Figure 4. Micrometer-sized particles with various shapes are present in both powders, and an irregular surface morphology can be observed. As can be seen in Figure 4(a)-(b), the product of ES:3OPL exhibits an irregular multi-layered surface, whereas more uniform calcium silicate particles are found in Figure 4(c)-(d) for the product of ES:6OPL. These morphologies are similar to those obtained using other methods [32][33]. From these SEM results, the surface porosity of the materials cannot be predicted.
Furthermore, an EDX analysis was performed to investigate the major elements present in the products, and the results are shown in Figure 5. Figure 5(a) and 5(b) show the sample composition, which mainly consisted of oxygen, silicon, and calcium, with a minor quantity of copper, magnesium, and zirconium. The EDX data shows that the silicon element comes from OPL, which is in agreement with previous reports [14][15]. The Ca/Si ratio could not be determined because the products were not purified. The present method provides new information on the green synthesis of calcium silicates, mainly Ca2SiO4 and CaSiO3, which has been reported using other silica sources [3,5,7,34].

Conclusions
Powder materials containing calcium silicate derivatives, mainly Ca2SiO4 and a small amount of CaSiO3, were successfully synthesized via the solid-state reaction of ES and OPL powders. This approach was executed by simply mixing both materials in the presence of a small amount of water, followed by heating at 900 °C for 2 h. Further investigation for the application of the reaction products and the use of OPL as a source of silica for preparing other products are on-going in our laboratory.