Combined Potential of Quarry Waste Fines and Eggshells for the Hydrothermal Synthesis of Tobermorite at Varying Cement Content

By inergency On Mar 14, 2024

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1. Introduction

Huge amounts of quarry by-products are generated globally mostly from the production of crushed stone or coarse aggregate. These quarry by-products, also known as ‘quarry wastes’, contain considerable amounts of fine particles that exhibit variable compositions of minerals. In general, quarry waste consists of different material types invariably known as “quarry fines”, “quarry dust”, “stone by-products”, “recycled aggregates”, “quarry powder wastes”, and so forth [1,2,3]. Quarry dust is considered a residue that forms after rock crushing and screening, with particles less than 75 μm, consisting of silt, clay, and non-quartz particles. This makes the quarry industry unsustainable since large amounts of these fine materials are produced, which is about 20% to 25% of the total output of rock processing, which is considered unmarketable and is generally disposed of in landfills [4,5,6].

Aside from the sustainability issues within the extractive industries, quarry waste fines pose environmental and social impacts, especially at high proportions of extremely fine particles as these are prone to mobilization under the action of gravity and wind. Consequently, these dust emissions pose health risks to workers and surrounding communities. If not properly managed, these dusts may also change native soil properties and possibly destroy vegetation [2,7]. In addition, quarry dust can cause water contamination and further affects communities when uncontrolled dust finds its way into water sources, making water unpleasant for consumption [8,9,10].

As a response, many researchers from different countries have been exploring the potential uses of quarry by-products. Most of the proposed recycling methods are for structural purposes such as building materials, road development, aggregates, bricks, and tiles. Specifically, quarry fines are applied as a partial replacement for sand in the production of various types of concrete. The amount of substitution varies according to the chemical and mineralogical properties of different types and sources of quarry wastes. However, based on the majority of these studies, only 40% to 50% of the replacement of sand with quarry waste is the optimum dosage since incorporating a higher amount of this material reduces the concrete’s strength and durability [3,11,12]. In addition, the high proportion of fine particles and the presence of other elements in quarry waste have adverse impacts on other properties (i.e., cohesive property, workability, density, and permeability) of concrete. It was reported that the workability of the concrete mix is compromised due to the higher water absorption of quarry fines than sand. As a result, the replacement level of this material for this application is limited [11,13,14].

In addition to quarry waste fines, eggshell is another type of waste that has not been adequately explored as a potential raw material for tobermorite synthesis. When eggshells are improperly disposed of in the environment, they become pollutants, posing health hazards due to fungal growth. The scale of this problem is substantial. By 2030, the global egg production is projected to yield around 90 million tons, translating to approximately 7.2 million tons of eggshell waste annually [15,16]. Hence, strategies for recycling discarded eggshells are also constantly being investigated. According to the studies, eggshell powder (ESP) can be employed as a replacement for cement which can be incorporated into concrete, cement mortar, and brick. Based on these findings, ESP is suitable for structural systems since it contains high amounts of calcium, which can be combined with pozzolanic materials. However, some studies reported a reduction in strength when cement is replaced with high percentages of ESP, especially above 10%. Furthermore, with high ESP dosage, the modulus of elasticity is also decreased [17,18,19]. Thus, the optimum percentage of ESP recommended is only 10% to 15%, whether in concrete, cement mortar, reinforced concrete, or clay brick applications [20].

To sum up, quarry waste fines (QWF) and eggshell powder (ESP) have been found to have potential by many researchers. However, the recyclability of these materials still has some constraints. The aforementioned published studies indicate that QWF and ESP can only partially replace traditional building raw materials to a limited extent. At this point, most studies do not recommend using these wastes as the primary raw materials for a product. Aside from that, no research has yet been reported that eggshells can be paired with quarry waste fines found in Mandulog, Iligan City, Philippines, specifically to produce blended mixes for the synthesis of tobermorite.

Currently, the QWF from Mandulog, Iligan City, has not been thoroughly explored in terms of its recycling potential. The Mandulog QWF or microfines are undesirable for aggregate businesses since they affect the concrete properties. The contributing factors of the occurrence of these fine materials such as silt and clay together with the fines coming from the sand and gravel may be due to sedimentation and siltation [21]. To address these problems, this study seeks to evaluate the characteristics of Mandulog QWF, along with ESP, and investigate their reaction products after autoclaving or hydrothermal treatment.

Hydrothermal synthesis is generally defined as a method of crystallizing substances in high-temperature and -pressure environments. During this hydrothermal treatment, products are strengthened due to the complex reactions affecting the calcium–silicate-hydrate (CSH) phase leading to the formation of tobermorite. Furthermore, the hydrothermal process contributes to the high utilization of solid wastes because of the stimulation of mineral activity. For this reason, various industrial solid wastes can be used for the production of tobermorite-bearing products like AAC [22,23,24,25]. Likewise, this study examined the presence of tobermorite mineral since it is considered the main reaction product during the hydrothermal curing process which plays a major role in the structural integrity and strength of the product [26,27,28,29].

This study aims to investigate the suitability of QWF and ESP as raw materials for the hydrothermal synthesis of tobermorite. Specifically, this study intends to (1) evaluate the chemical and mineralogical properties of the raw materials; (2) determine the physical and mechanical properties of the autoclaved samples; and (3) determine the presence of tobermorite in the autoclaved samples (at 10%, 15%, and 20% OPC, respectively). The use of additives and pore formers such as gypsum and aluminum powder is not included since this study concerns only the reaction between QWF and ESP, and this serves as preliminary research toward the development of AAC using the aforementioned waste materials. Additionally, only OPC was added which acts as the binder in the sample forming process and was limited only to three variations—10%, 15%, and 20%, respectively. This was carried out to evaluate the dependability of QWF-ESP mix with cement. Furthermore, only a Ca/Si ratio of 0.80 was followed in the mixture preparation of QWF-ESP samples [30,31,32]. Meanwhile, the hydrothermal conditions were fixed at 180 °C for 6 h [23,24,33,34,35].

The research presents new possible recycling routes for QWF and ESP which will contribute to the reduction in environmental load and waste management costs in quarry industries, agriculture, and commercial sectors. Furthermore, this study provides additional insights into the potential use of QWF and ESP as possible starting materials for the synthesis of tobermorite for the future development of autoclave concrete products. In effect, such wastes may be converted into valuable resources and may reduce the demand for raw materials, especially river sand and limestone, in the future.

2. Materials and Methods

This study explores the potential of quarry waste fines (QWF) and eggshell powder (ESP) for the synthesis of tobermorite, a significant component in autoclaved aerated concrete (AAC). The chosen methodologies are designed to evaluate the chemical and mineralogical properties of QWF and ESP and to investigate the mixture of these materials to form tobermorite under specific hydrothermal conditions. Each step of the experiment, from the initial preparation of QWF and ESP to the detailed analysis of the autoclaved samples, has been carefully selected to provide insight into the feasibility of using these waste materials in new and sustainable ways.

The procedure used to conduct the experiments generally includes (1) raw materials preparation, (2) characterization of the waste materials QWF and ESP, (3) sample preparation, (4) measuring and evaluation of the physical and mechanical properties of autoclaved samples, and (5) evaluation of the hydrothermal reaction products between QWF –ESP and confirmation of the tobermorite phase.

2.1. Raw Materials Preparation

This study utilized QWF sourced from a river sand quarry site in Mandulog, Iligan City, Philippines. In the preparation of the QWF, foreign materials were removed and discarded by sieving QWF on a size 20-mesh sieve (840 microns). The screened QWF was then wet-milled until a uniform particle size distribution with a fineness of ≤75 microns (using a 200-mesh sieve) was achieved [36]. Lastly, the wet-milled QWF was oven-dried for at least 4 h at 110 °C.

For the ESP preparation, eggshells from raw chicken eggs were used. The collected eggshells were washed thoroughly with water and were then oven-dried for 4 h at 110 °C. The dried eggshells were milled for at least 5 h using a porcelain ball mill to produce a fineness of ≤75 microns. Afterward, the ESP was subjected to pre-treatment which entailed a calcination process to remove volatile substances and purify the material. This procedure was performed by subjecting ESP to 1000 °C in a firing furnace [37]. The prepared QWF and ESP are displayed in Figure 1.

2.2. Raw Material Characterization

X-ray fluorescence analysis (XRF) and X-ray diffraction analysis (XRD) were carried out to determine the chemical composition and mineralogical characteristics of the raw materials used for the formulation of the QWF-ESP mix. Approximately 15 g of the powdered sample was prepared for each of the aforementioned analyses. The analyses were carried out via X-ray fluorescence spectroscopy (XRF, EDXL300, Rigaku Corporation, Tokyo, Japan) and X-ray diffraction spectroscopy (XRD, MultiFlex, Rigaku Corporation, Tokyo, Japan). For the XRD, the sample was placed in a platinum sample holder and analyzed at a heating rate of 2 °C/min. In addition, the thermogravimetric analysis and differential thermal analysis (TGA–DTA) were performed on the raw materials to determine the mass loss and microstructural changes, as well as to identify minerals and hydrates, complementing the XRD results [38,39,40]. For TGA–DTA, approximately 70 mg of sample was placed on an aluminum crucible and subjected to a heating rate of 10 °C/min for up to 1000 °C in oxygen atmosphere.

2.3. Sample Preparation

The samples in Figure 2a–c were produced using QWF, ESP, and OPC, following a Ca/Si ratio of 0.8 in the formulation [32,33,34]. The mixture proportions used to prepare the samples are displayed in Table 1. The QWF-ESP formulations were designed with increasing OPC concentrations (10, 15, and 20 percent, respectively). Based on the chemical composition results of the raw materials shown in Table 2, the precise proportions were determined by taking into account the total CaO and SiO₂ and equated it to 0.8 Ca/Si. The loss on ignition (LOI) of individual materials was not yet accounted in the presented percent proportions. It was during the preparation and weighing of the batches that adjustments were made as the LOI of each raw material was considered. After weighing, water was added to the proportioned solids in a 0.70 water–solid ratio and was thoroughly mixed to form a slurry. The slurry was then poured and cast into a cubic mold (2.4 × 2.4 × 2.4 cm). After the cast mixture was hardened, it was de-molded and autoclaved at 180 °C for 6 h [25,26,35,36,37]. The control sample (Figure 2d) was formulated with the traditional raw materials, lime, and silica with only 10% OPC, while the Ca/Si ratio, water–solid ratio, and autoclaving conditions were consistent with those used for the QWF-ESP samples.

2.4. Determination of Physical and Mechanical Properties

The physical and mechanical properties which include the bulk density, percent water absorption, percent volume of permeable voids, and compressive strength tests were performed on the cured samples. Before testing, the samples were prepared via oven drying at 100–110 °C. The bulk density and water absorption tests were conducted adhering to the ASTM C642-06 guidelines [41]. On the other hand, the compressive strength was determined in accordance with ASTM C1386-98, 2017 [42], using a universal testing machine (Zhejiang Tugong Instrument Co., Ltd., Shaoxing, China).

2.5. Determination of Phase Composition of the Autoclaved Samples

X-ray diffraction analysis (XRD, MultiFlex, Rigaku Corporation, Tokyo, Japan) was carried out on the autoclaved samples to determine the presence of the tobermorite and other mineral phases. This further compared the resulting products of the hydrothermal reaction between QWF and ESP to the control sample (lime and silica). This procedure required the autoclaved sample to be crushed and pulverized using a mortar.

4. Implications

As previously stated, the investigation of blended QWF and ESP to form tobermorite and the effects of varied OPC amounts on the properties of the autoclaved samples were the main emphasis of this study. Tobermorite is responsible for the strength of autoclaved aerated concrete (AAC). According to the literature, AAC is one of the confirmed green structures that permits the use of many raw material types in its manufacture. Some of these substitute materials are effective in reducing cement consumption in the production of AAC, thus leading to a reduction in greenhouse gases [76,77]. The characterization of QWF and ESP, in this study, revealed that these waste materials contain the key components for tobermorite synthesis such as SiO₂ and CaO. Aside from construction applications, tobermorite has been gaining more attention in recent years due to its high utilization value in chemical and mechanical industries, its economy of materials, as well as its potential for environmental cleanup purposes [75,78,79,80].

In the present work, the tobermorite phase was formed despite the differences in mineralogical characteristics of QWF and ESP from the traditional raw materials (i.e., lime, sand, and chemical-grade silica). However, the QWF-ESP samples have lower compressive strengths than the reference sample. Nevertheless, these findings will serve as a starting point for future innovation that will encompass technical challenges associated with exploring the vast potentials of tobermorite and the hydrothermal process to utilize waste by-products for the development of an environmentally sustainable building material.

Furthermore, using QWF as the main source of silica and calcined ESP as the source of lime could potentially conserve sand or silica resources and limestone. QWF is also composed of finer materials than sand. Moreover, QWF is much cheaper compared to commercial chemical-grade silica. Thus, recycling waste by-products like QWF and ESP not only reduces environmental loads but also promotes resource efficiency in the building sector. Finally, it should be noted that this is a preliminary study that has been conducted to determine the potential of the mixture of QWF and ESP as possible alternatives to the traditional AAC raw materials (i.e., lime and silica sand) in the synthesis of tobermorite. Understanding the properties of the raw materials to determine the appropriate pretreatment method and to optimize their proportions in the mix, as well as considering the concentration of OPC, and hydrothermal curing conditions becomes necessary, and we must come up with a high-quantity of crystalline tobermorite. More importantly, this study provides the individual chemical and mineralogical characteristics of QWF and ESP which could be used not only for tobermorite synthesis but for other recycling or waste valorization and solidification strategies (i.e., heavy metal immobilization).

5. Conclusions and Recommendations

In this study, the chemical, mineralogical, and physical characteristics of QWF and ESP were evaluated through XRF, XRD, and TGA-DTA techniques. In addition, their suitability to form tobermorite-bearing materials with different amounts of OPC binders was also investigated via physical and mechanical property tests and XRD analysis. After examining the results, the following conclusions can be drawn:

QWF and ESP can be used as a starting material for tobermorite synthesis in terms of their CaO and SiO₂ content. The QWF was found to have a considerable amount of silica (SiO₂) (53.77%), which is comparable with the silica content range of fly ash. On the other hand, ESP makes a rich source of calcium oxide (CaO) of 97.80%.
The sample with only 10% OPC exhibited the highest strength and best physical properties compared to QWF-ESP samples with 15% and 20% OPC. This is advantageous in terms of saving raw materials (OPC) and waste proportion optimization.
Tobermorite was produced using QWF and ESP at a 0.80 Ca/Si ratio through hydrothermal treatment at 180 °C for 6 h, as confirmed by the XRD results. The tobermorite peaks were visible in the QWF-ESP samples, and the peak intensities were closely similar to the lime–silica formulation. Regardless of the OPC dosage, the tobermorite phase was formed using the QWF-ESP mix.

Furthermore, to obtain products with ideal properties, crystalline tobermorite should be the main phase formed after hydrothermal treatment [23,24]. Hence, the present work needs further improvement since the QWF-ESP formulations were not sufficient in terms of achieving a comparable strength to the reference sample, suggesting that the amount of tobermorite formed was also insufficient. Nevertheless, since lower OPC had positive effects on the compressive strength, it is highly recommended to conduct a follow-up experiment using the same or lower range of OPC at varying hydrothermal temperatures or curing times to further validate the findings in this study. If this is not possible for the casting method, it is also suggested to explore other forming methods such as semi-dry pressing. Additionally, it is recommended to vary the mix design and incorporate additives (i.e., gypsum or anhydrite) as possible methods to enhance the properties of the cured product. Furthermore, investigating the QWF, ESP, and OPC reactivity to determine the major source of tobermorite formation needs to be explored. This may involve measuring the solubility of quartz present in the raw materials, especially in QWF, in comparison with other silica or quartz sources. Employing this approach will help determine whether the pre-treatment of QWF is necessary to yield more crystalline tobermorite.

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