Global Eggshell Properties: Characterizing Variability for Sustainable Partial Cement Replacement in Hong Kong’s Concrete

1. Introduction

The modern urban areas have become a concrete jungle, and concrete has become the second most consuming material after water on the planet. Cement is no doubt the major ingredient of concrete, and its production is responsible for huge carbon footprints, which makes the cement industry the third most CO₂ emitter in the world [1]. The cement industry contributes a maximum of 8% of the global emission [1]. There are two ways to reduce the CO₂ emission from cement manufacturing plants; the first is to optimize process technology by using alternative fuel or raw material, and the second is the use of Supplementary Cementitious Materials (SCM) as a replacement for cement [2]. The supplementary cementitious materials are inorganic waste materials that possess hydraulic activity, pozzolanic activity, or sometimes dual nature [3]. Among them, limestone is one of the most commonly used SCM, and it has been used up to 10% replacement without sacrificing the compressive strength [4]. As a partial replacement for cement, limestone fillers can improve hydration in a low water-to-cement ratio [5]. Therefore, it is recommended by the ASTM to use limestone effectively up to 5% [6], whereas the BS recommends a 6% - 35% replacement level [7]. Generally, limestone can be used both as a binder replacement and as an aggregate to improve the properties of concrete [8,9]. Overall, using limestone as a replacement for cement for up to 20% can enhance the strength, workability, and durability of concrete and mortars [10,11].

The use of Eggshells (ES) has been recommended through several research studies as a potential bio-filler in cementitious materials during the past decade. The eggshells fulfill the requirements of a standard limestone for calcium silicate products as per ASTM standard specifications for limestone [12]. Incorporating eggshells as a partial replacement of cement improves the strength and other properties of cementitious materials, e.g., reduction in the setting time [13,14,15], good radiation shielding properties [16,17], and can be used up to 20% replacement level under the elevated temperature condition [18]. Several studies have been carried out on the optimal replacement of eggshells with cement considering strength as a deciding parameter, as shown in Table 1. It can be seen that 9 out of 25 studies recommend 5% replacement, 7 studies recommend 10% replacement, and the rest recommend different other replacements. Most of the studies recommends 5% replacement because limestone has complete reactivity up to 5% replacement level [2]. Likewise, eggshells have also been effectively blended in cement along with silica fume (SF) [19,20,21], fly ash (FA) [22,23,24], rice husk ash (RHA) [25], rice straw ash (RSA) [26], glass powder [27], palm oil fuel ash (POFA) [28,29,30,31], and bagasse ash [32]. In addition, eggshells have been used in special concrete as well, both in uncalcined and calcined forms like Foamed concrete [33,34], self-compacting concrete [35,36,37,38,39], and geopolymer concrete [40]. In general, using eggshells either in an uncalcined or calcined state has a positive impact on the hydration kinetics of cementitious materials [41,42,43]. Apart from using eggshells in the cementitious matrix, they can also be used as fine aggregates up to 40% without affecting the compressive strength [44].

Several SCMs and fillers have been proposed in the past few decades, while the search for new SCMs is still in progress. The chemical composition of SCMs may vary depending upon several factors like production processes and regions. For example, the composition of steel slag may vary depending on the type of furnace being used in the process of conversion from iron to steel [67]. Similarly, the Australian Fly Ash (FA) contains more SiO₂ content than the Indonesian FA and accordingly imparts more strength due to its more pozzolanic activity [68]. Therefore, it is essential to analyze the variation in the chemical compositions of different hydraulic and pozzolanic materials before proposing them for large-scale commercial application. Limestone is a less reactive hydraulic material [69], and biological limestone like eggshells have similar properties containing an overwhelmingly high content of CaCO₃. A study reports 94% - 97% CaCO₃ as an average value depending on mineral nutrition, housing system for hens, age, and animal genotype [70]. A study also reports higher content of 98.2% CaCO₃ [71], whereas another study reports low as 86.75% in eggshells of white silky chicken [72]. The quality of eggshells is defined as its resistance against breakage during handling of eggs [73]. This resistance varies from case to case and depends upon the breed and age of eggs [74], weight grade [75], color [76], and housing system [77,78]. However, this resistance majorly depends on the weight of an eggshell [79,80,81,82,83]. Therefore, a good quality egg must have a heavier eggshell, while the weight of the eggshell also varies with its size, but an average value is about 10% of the total weight of the eggshell [82]. Local weather is a significant factor affecting not only egg production but eggshell quality as well. For example, the high air temperature and the relative humidity cause heat stress, and that affects the egg's production and eggshell quality [84,85,86]. Hens in hot and humid environments cannot consume sufficient calcium and produce softer eggshells [86,87]. Likewise, it is quite possible that eggs and eggshells from different countries could have different properties depending on their mineral contents. However, a diversity of weather conditions is possible across big countries like the USA. A brief review on the specific gravity as shown in Table 2 depicts that specific gravity is different in different regions probably due to the local weather conditions and some other factors. Hence, it is important to analyze the eggshells from different countries before proposing them for large-scale industrial applications in cementitious materials.

Hong Kong is the biggest consumer of eggs in the world. Notably, the consumption per capita per year has been increasing drastically during the last decade. The domestic supply of eggs in Hong Kong was 196,000 tons in 2021. At the same time, the average consumption per capita was 26.09 kg/capita/year in 2020, as reported by the Food and Agriculture Organization (FAO) [100]. According to the Observatory of Economic Complexity (OEC), eggs were the 175^th most imported product in Hong Kong in 2021, having a total import value of 284 million USD, which ranks Hong Kong fourth in the list of most egg-importing countries [101]. Hong Kong’s market has diverse kinds of eggs from different countries of origin. Also, the import quantity of eggs was 200,000 tons, whereas the export quantity was only 4000 tons in the year 2021 [100]. Therefore, Hong Kong is an ideal place to study the characteristics of eggshells from different countries of origin and for their sustainable utilization in cementitious materials. Not only this, but the public, and specifically the restaurants, have a positive attitude towards the recycling of waste eggshells for meaningful purposes [102].

Many studies have been carried out on the viability of eggshells in cementitious materials as a cement replacement but the effect of different eggshells from different regions on the mechanical properties of cementitious material considering the quality has not been studied yet. The proposed study aims to assess the feasibility of using eggshells from different countries of origin as a cement replacement in cementitious material. For this purpose, the extent of variation in the basic properties (e.g., specific gravity and mineral content) of different eggshells and their effect on the end cementitious products were analyzed. This study will facilitate the stakeholders to develop environmentally friendly concrete containing eggshells as a cement replacement for commercial applications.

2. Materials and Methods

2.1. Market Survey and Collection of Samples

A market survey was conducted in five different supermarkets to select the eggshells from different countries of origin for detailed investigation. Out of them, three supermarkets were selected based on having the highest number of stores, one was selected based on having a variety of egg brands, and one was selling through online mode. It was found that a total of 50 different egg brands from eleven different countries are imported into these five supermarkets. Japan has the highest number of brands, followed by Thailand, the USA, and China, as shown in Figure 1(a). Whereas import share of different countries in Hong Kong as per data from OEC for 2021 is shown in Figure 1(b). For detailed characterization, eggshells from sixteen different egg brands or countries of origin were collected. These eggshells were selected based on their popularity. Among them, twelve were collected from the supermarket, whereas the rest four were collected from restaurants. The details of these samples are listed below in Table 3. These sampled eggshells were later prepared for the measurement of specific gravity, thermogravimetric analysis (TGA), and for cement replacement. For this purpose, each sampled eggshell was cleaned with tap water and dried in air. It must be noted here that no commercial method is available right now to separate the eggshell membrane on a large-scale [103]. Therefore, it was not tried to remove the eggshell membrane through any dedicated method so that to meet the practical requirements. However, the process of removal of the membrane is based on cracking and grinding [104]. Therefore, it can be presumed that a part of the organic membrane drained off due to crushing during the cleaning of eggshells.

2.2. Measurement of Specific Gravity

The specific gravity of the waste eggshell is necessary to determine because it is an important factor in the mix design of concrete. It was done using the specific gravity bottle and the ethanol as a solvent on ground sampled eggshells passed through ASTM No. 200 sieve. The density bottle was half-filled with a dried powder sample while the rest was filled with solvent and was kept in for three days to let the trapped air escape.

2.3. Thermogravimetric Analysis TGA

Thermogravimetric Analysis (TGA) was performed on each sampled eggshell to find its phase composition and compare it with the extra pure limestone (LS), which contains more than 99% CaCO₃. For this purpose, the dried eggshells were ground and passed through ASTM No. 200 sieve. The prepared eggshells having a mass of less than 10mg were put in a 20mg sample holder, and Al₂O₃ powder was used as reference material. Each sample was heated at the rate of 10^oC/min up to 1000^oC in an argon environment. Later, the results were presented in the form of the Thermogravimetric (TG) curve and Differential Thermogravimetric (DTG) curve to detect the components and their quantity. The tangential method was used to determine the mass loss corresponding to each phase change. Furthermore, the results from TGA analysis were also verified by stoichiometric analysis.

2.4. Use of Eggshells as a Cement Replacement

2.4.1. Selection of Eggshells

To replace the eggshells with cement, three different eggshells from the three different countries of origin were selected based on their popularity and mineral contents. To make the selection based on mineral contents, eggshells with low calcium carbonate, medium calcium carbonate, and high calcium carbonate were selected.

2.4.2. Properties of OPC, LS, and Selected ES

Before incorporating the eggshells in the concrete mix, each type of eggshell was ground in a ball mill under a constant weight of 12kg for 6 hours and later was sieved through ASTM No. 200 sieve. The Particle Size Distribution (PSD) for each type of eggshell, LS, and CEM I 52.5N Portland cement was carried out by laser diffraction method using ethanol as a solvent. Furthermore, the oxide composition of cement by X-Ray Fluorescence (XRF) was analyzed, and its Bogue’s components were found as shown below in Table 4. Each type of eggshell was also calcined at 800^oC for three hours. It was done so because calcination is the effective method to remove the organic matrix [105], and the resulting ash at this calcination condition consists of a blend of CaCO₃, Ca(OH)₂, and CaO [42,106]. Furthermore, a powder X-Ray Diffraction (XRD) was also performed to validate this phase composition after calcination.

2.4.3. Detail of Concrete Mixes and Strength Measurement

The maximum size of fine aggregates was 1.18mm, whereas coarse aggregates ranged between 4.75mm to 10mm. Eight batches of concrete were prepared with a general-purpose mix composition of 1:1:2 to assess the feasibility of ES and CES in concrete. Among them were the control mix, the mix with 5% limestone (LS), the mix with 5% ES, and the mixes with 5% CES, as listed below in Table 5. The replacement level was taken as 5% because most of the studies report this replacement level as mentioned before in Table 1. Finally, each of these specimens was tested to determine the 7th-day and the 28th-day compressive strength, and their Relative Strength (RS) was measured for each mix.

3. Results

3.1. Specific Gravity of Sample Eggshells

The measured specific gravity of each sample eggshell is shown below in Figure 2. The specific gravity of eggshells varies between 2.02 – 2.39 having an average value of 2.20 ± 0.01. These values of specific gravity are close to the past values as reported in the literature in Table 2. The specific gravity of eggshells is typically lower as compared to the extra pure limestone (i.e., 2.71), it is due to the porous structure of the eggshells [107]. Moreover, the specific gravity of Supplementary Cementitious Materials (SCM) is normally lower as compared to cement [108]. The presence of residual eggshell membrane also affects the specific gravity of the eggshell powder [109]. It can also be seen that eggshell with a specific gravity higher than average (i.e., 2.20) are either light brown or dark brown. In contrast, all the white eggshells (i.e., ES4, ES6, ES9, and ES16) have a specific gravity less than the average. Therefore, brown pigmentation does affect the quality of eggshells, but it needs to be investigated further in the given case. Since the specific gravity of eggshells is lower as compared to both limestone and OPC, therefore, it can also be used in the production of lightweight concrete [110,111].

3.2. Quantification of Minerals

Eggshell consists of volatile components usually water, organic components which are proteins, and the mineral part, which is CaCO₃ [106,112,113], whereas a typical thermogram is shown below in Figure 3. The average composition of eggshells comprised of volatile components 1.17 ± 0.16%, organic components 2.5 ± 0.63%, and the CaCO₃ 96.33 ± 0.67%. After complete decomposition of CaCO₃ during calcination, there was on average CaO 54.24 ± 1.12%, CO₂ emission 42.09 ± 1.05% whereas the calcium content was 38.76 ± 0.80% as tabulated below in Table 6. Furthermore, the measured mineral part by TGA was correlated with the calculation by stoichiometric analysis, as shown below in Figure 4. The correlation for CO₂ is slightly lower due to the possibility of having uncertainty. CO₂ is measured by deducting the weight of volatiles and organics from the total weight loss while it is possible that some part of the organic membrane washes away with water during the cleaning of eggshells, or sometimes there is a possibility of its concentration in the given specimen. So, such a partially true quantity of organics and volatiles directly makes the calculated CO₂ uncertain. The CaCO₃ in all eggshells ranges from 94.65% - 97.23%, which is slightly less than the extra pure limestone, whereas the color of the eggshell does not have any effect on the composition. As far as pigmentation is concerned, pigmentation has no clear effect on the mineral content. In general, the average composition of the mineral part of eggshells from different countries is very similar to that of extra pure limestone, as tabulated below in Table 7. Therefore, it is recommendable not to discard this type of biomineral in landfills; instead, it is worthwhile to extract the pure limestone from eggshells by doing either calcination or some other treatment for further use in cementitious materials.

3.3. Application of Eggshells as a Cement Replacement

3.3.1. Selection of Eggshells and their Properties

To replace the cement with eggshells, three types of eggshells from the sample eggshells were selected based on their popularity and mineral contents, i.e., low, medium, and high. Among them were American eggshells (ES7) with CaCO₃ 96.71%, Chinese eggshells (ES13) with CaCO₃ 95.35%, and Japanese eggshells (ES16) with CaCO₃ 94.65%. All these selected eggshells were also calcined under the above-mentioned calcination condition to get the calcined eggshells. The corresponding calcined American eggshells were designated as CES7, calcined Chinese eggshells as CES13, and calcined Japanese eggshells as CES16.

Since the fineness of cement additives plays an essential role in hydration kinetics and strength development, it is prime important to investigate the PSD of OPC, LS, ES, and CES. For this purpose, the PSD curve for each type of material is shown below in Figure 5, along with D[4,3], D(50), and D(90) particle sizes in Table 8.

The calcium carbonate in ES, when calcined at a certain temperature, converts into CaO along with the liberation of CO₂. Later, the CaO, because of its high reactivity with water vapors in the atmosphere, converts into Ca(OH)₂ [114]. Therefore, the partially decomposed calcium carbonate may likely consist of a mixture of CaCO₃, Ca(OH)₂, and CaO. In the given case, all the calcined eggshells consisted of a mixture of all these three components, as shown below in an XRD spectrum of LS, ES, and CES in Figure 6. Whereas the quantities of this phase composition by Rietveld refinement in Match software and are shown below in Table 9. It can be seen that Ca(OH)₂ is the major phase due to the reaction of CaO with the water vapors in the atmosphere. Therefore, the formation of Ca(OH)₂ may vary depending upon the relative humidity of the environment.

3.3.2. Compressive Strength and Relative Strength (RS) of Concrete Specimens

It has been found that replacing cement either with uncalcined eggshells or calcined eggshells is viable based on the compressive strength, as shown below in Figure 7(a). Both provide adequate strength, while the Relative Strength (RS) is shown in Figure 7(b). It can be seen that calcined eggshells are showing higher strength as compared to uncalcined eggshells. The average 7th-day compressive strength for mixes containing eggshells is 36.33Mpa, while for mixes containing calcined eggshells is 40.00Mpa. Similarly, the average 28th-day compressive strength is 49.33MPa and 54.33Mpa for mixes containing eggshells and calcined eggshells, respectively. Furthermore, the eggshells with the highest mineral content (i.e., ES7) show higher compressive strength both in calcined and uncalcined states and vice versa.

4. Discussions

4.1. Calcium Carbonate vs Specific Gravity in Uncalcined Eggshells

As explained earlier, measuring specific gravity is a direct way to measure the quality of eggshells. Similarly, the quantity of minerals can also affect the quality of the eggshell and make it harder or softer [72]. However, the quality of eggshells also varies with some other factors as well. A correlation is shown below in Figure 8 between both parameters of quality assessment i.e., CaCO₃ and the specific gravity of sampled eggshells. The specific gravity varies linearly with the CaCO₃ content in the given eggshells whereas this correlation is a bit lower. There could be many reasons behind this low correlation, the most straightforward reason is the presence of residual shell membrane [109]. It is a bit hard to remove the complete organic membrane from the eggshells during physical washing and cleaning. There is also an internal shell membrane in addition to the external shell membrane and that can only be removed by rubbing the internal surface of the shell membrane. To completely remove the organic part, it is necessary to do heat treatment like calcination [105] or a chemical treatment like a reaction with bleach solution [115].

In addition, the low correlation depicts that both specific gravity and CaCO₃ contents are not enough to describe the quality of eggshells from different regions of the world. The brown pigmentation imparts strength and the eggshell quality while it does not correlate with the egg’s internal quality [116]. Therefore, the brown eggshells are linked to the higher specific gravity [117]. This also justifies the given case like ES2, ES3, ES7, ES11, ES13, and ES14 are heavier eggshells and are either dark brown or light brown. Unlike specific gravity, brown pigmentation has no clear link with the mineral content in the given case. This is controversial to some of the previous studies because brown eggshells have more mineral content as compared to white eggshells [50,72,82]. For example, a study reports that brown eggshells have 96%-97% CaCO₃ while this quantity is around 94% in white eggshells [118]. However, there is also a conflicting opinion in some previous studies as well which indicates that brown pigmentation is not a reliable tool for assessing the quality of eggshells [117,119]. Therefore, the micro-structure is another factor that can affect both specific gravity and CaCO₃, and ultimately the quality of eggshells. Bain [120] suggested the orientation of palisade columns in the palisade layer affects the shell quality in addition to the crystal size. Since the palisade layer is the biggest layer that defines the major structural part, therefore, it is highly likely that a change in the palisade layer may affect the whole structure of an eggshell. Moreover, the housing system of egg-laying hens is an important factor that can affect the microstructure of a shell and ultimately the shell thickness and strength. It has been observed that higher numbers of pores are present in cage housing systems than in litter housing systems [121]. Hence, the cage housing system accounts for more cracked and broken eggs [122]. Although the housing system for the sampled eggshells in the present study is not known, an inference can be made that there is a diversity in the structure or pores of the eggshells which is the cause of the low correlation between specific gravity and the mineral content in addition to the presence of residual membrane. Moreover, it must be noted here that this correlation also includes the LS which has higher specific gravity due to its higher mineral content and non-pores micro-structure as compared to eggshells. In general, the quality of eggshells from different regions should be defined by their structure in addition to their mineral content and specific gravity.

4.2. Role of Calcium Carbonate

4.2.1. Calcium Carbonate vs Strength Development

The use of eggshells in uncalcined form from different regions in the given case is viable to use as a cement replacement. There is some variation as compared to the control mix and the mix with LS depending upon the mineral content, but this variation is within the acceptable limit. It is necessary to understand the determinants involved in strength development. Eggshells in an uncalcined state are an impure form of limestone and thus produce inferior strength [50]. A good correlation has been established between the strength development and the mineral content as shown below in Figure 9. The CaCO₃ is both inert and reactive, having complete reactivity up to 5% replacement [2]. The CaCO₃ reacts with the C₃A and C₄AF and forms additional hydrates like carboaluminates which imparts strength [2,123]. The quantity of CaCO₃ is the main strength contributing factor in addition to the clinker; therefore, those mixes containing the highest CaCO₃ quantity (i.e., M-ES7 and M-CES7) showing the highest strength as compared to the mixes with lowest CaCO₃ (i.e., M-ES16 and M-CES16). It can also be seen that the correlation for 28^th day strength is a bit lower as compared to the correlation for 7^th-day strength and the LS significantly improves the 7^th-day strength in contrast to 28^th-day strength by providing additional sites for nucleation and growth of hydrates [8,124,125,126,127]. The more variation or uncertainty in 28^th-day strength is due to the dilution effect [5,8,128,129]. This is because the mixes containing the LS and eggshells require less water due to the decrease of cementing part and causes the increase of w/c ratio [8,118]. Incorporation of CaCO₃ improves the compressive strength due to improvement in the degree of hydration at a low w/c ratio but the w/c ratio is high in the given case which is causing the dilution effect and the impairment of compressive strength at later ages [130].

4.2.2. Filler Effect and Heterogenous Nucleation

The strength development due to the addition of CaCO₃ is due to the filler effect in which the finer particles of CaCO₃ fill up the voids in cement. The filler effect produces more denser microstructure and increases the packing density. However, this filler effect cannot be seen if the particle size of CaCO₃ is comparable to or bigger than cement’s particle size [131]. In the given case, the D[4,3] particle size of LS, ES, and CES ranges between 13.94μm – 31.27μm which is comparable to or bigger than cement (i.e., 17.53μm). Similarly, the D(50) and D(90) particle sizes are also bigger than cement (i.e., D(50)=11.56 & D(90)=42.86) in most of the specimens. Therefore, a reduction in strength can be seen due to the dilution effect, particularly in the specimens with uncalcined eggshells. However, strength development in the specimens with LS is comparable with the control mix because D(90) particle size is lower than cement and also there is the absence of an organic matrix. In contrast, the strength development in specimens with CES is comparatively more while the quantity of CaCO₃ is relatively less due to its decomposition. There is a need to justify this phenomenon which will be justified in the next section.

In addition to the filler effect, heterogeneous nucleation is another phenomenon that can improve hydration due to the addition of CaCO₃. Unlike homogenous nucleation, the CaCO₃ particles behave as a nucleation site for C-S-H and improve the degree of hydration [128]. This is because the planar configuration of Ca and O atoms in the CaCO₃ particles is very similar to the Ca and O atoms in the C-S-H [132]. The factors influencing the heterogenous nucleation are the particle sizes [133], surface structure [134], and the quantity of CaCO₃[135]. The surface energy and absorption capacity of CaCO₃ particles increases with the decrease of particle sizes for the formation of heterogeneous nucleation. Likewise, the potential for the formation of heterogeneous nucleation also increases with the increase of CaCO₃ content. However, the part contribution of different factors has not been understood yet [8]. Since the quantity of CaCO₃ is constant in the given case, therefore, it can be assumed that both the filler effect and the heterogenous nucleation depend on the particle sizes. While the particle sizes in all non-control mixes are either comparable or greater than the cement particle sizes so dilution effect is quite explicit. Therefore, only part of the particles which are smaller than cement is taking part in the strength development due to the filler effect and heterogenous nucleation in addition to the CaCO₃ content. Despite the dilution effect, the variation in the strength of both mixes with uncalcined and calcined eggshells is acceptable.

4.3. Role of Calcium Oxide in Strength Development in Mixes with CES

Specimens containing calcined eggshells are showing better strength despite having a low quantity of CaCO₃. Therefore, it needs to investigate the strength contributing factor for the mixes with calcined eggshells. The major reason is the absence of an organic matrix, which decomposed during the calcination process. In this case, the strength contributing factor is the CaO in addition to the CaCO₃, which contributes to a slight increase in the strength development up to a certain limit [136,137] while the Ca(OH)₂ does not affect the strength [138]. This additional CaO accounts for more heat of hydration at an early stage due to its exothermic reaction with water [14,139]. Also, adding a given CES consisting of free CaO in the binder matrix can increase the strength of the concrete by improving the Hydraulic Modulus (HM) and Lime Saturation Factor (LSF) [140]. Given below are the mathematical equations for the estimation of HM and LSF.

$$\mathrm{H}\mathrm{M}=\frac{\mathrm{C}\mathrm{a}\mathrm{O}}{{\mathrm{S}\mathrm{i}\mathrm{O}}_2+{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3}$$

$$\mathrm{L}\mathrm{S}\mathrm{F}=\frac{\mathrm{C}\mathrm{a}\mathrm{O}}{{2.8\mathrm{S}\mathrm{i}\mathrm{O}}_2+{1.2\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{0.65\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3}$$

It must be noted here that CaO content for binder mix with CES consists of both CaO from XRF of OPC and from Rietveld analysis of CES. The details of HM and LSF for OPC and the binders with 5% CES replacement are shown below in Table 10. Since most of the CaO was converted into Ca(OH)₂ and the replacement level is only 5%, therefore, an extremely slight increase can be observed in HM and LSF of binders with CES. It means that the quantity of free CaO from the CES contributing to the strength is negligible while CaCO₃ is still present and contributing the strength. It is hard to find which phase is contributing more to strength in addition to the absence of the organic part but both CaO and CaCO₃ are the strength contributors. Furthermore, it is recommended to incorporate pozzolanic materials along with CES with decomposed calcium carbonate to consume the additional Ca(OH)₂ produced during the calcination for achieving high durability.

5. Conclusions

This study was carried out to assess the suitability of waste eggshells from different countries of origin for application in cementitious materials as a cement replacement. The focus was on the extent of variation in eggshells quality and its effect on the cement concrete when replaced with cement. The following conclusions can be drawn based on a detailed investigation of sixteen different eggshells from different countries of origin.

• The specific gravity of eggshells from across the world is lower as compared to the extra pure limestone due to the presence of an organic matrix. Thus, the eggshells are impure biological limestone having less mineral content as compared to extra pure limestone. The brown pigmentation in eggshells causes higher specific gravity but it does not affect the mineral content. Furthermore, the quality of eggshells from different regions across the world can be defined by their micro-structure in addition to their specific gravity and mineral contents.
• The eggshells from different regions across the globe both in uncalcined state and calcined state with decomposed CaCO₃ are viable to use as a replacement of cement. The variation in the strength due to the variation in mineral content is acceptable. However, the strength of mixes with calcined eggshells is closer to the control mix and mix with limestone. The CaCO₃ content is the major contributor towards strength development by producing filler/dilution effect and heterogenous nucleation depending upon the size of particles in addition to the CaCO₃ content whereas CaO is another factor towards strength development by increasing the quantity of free CaO in the cementitious matrix containing calcined eggshells.