2.1.1. Collection of Raw Materials

The fired clay brick waste (grog) (RB) was obtained from the Local Materials Promotion Authority (MIPROMALO), Cameroon. The collected grog was oven dried at 105˚C ± 5˚C for 24 h to remove all unwanted substances. It was later ground and sieve at 2 mm sieve using a standard sieve and package in a plastic bag for further use.

The egg shell was gotten from a bakery in the Etoug-Ebe neighborhood Yaounde, Cameroon. They were washed under a tap flowing water to get rid of contaminants and unwanted particles found on the surface membrane. The washed eggshell was later sun dry at room temperature (23˚C ± 3˚C) for 5 days then later oven-dried at 105˚C ± 5˚C for 24 h to eliminate the remaining water moisture from the eggshell. The raw materials were crushed and sieved at 75 µm to obtain fine particles and eliminates the coarse particles. Samples were packaged in hermetically sealed bags for further use. The resulting white eggshell powder was calcined at 900˚C (CO) for 2 h in an electric oven.

Coconut shells were obtained from farmers dealing with coconut farming in Yaounde. The fiber was extracted from the coconut using a hammer. The fiber was oven dry at 105˚C ± 5˚C for 24 h and later crushed and sieved through a 250 µm sieve, packaged and labeled (CN) for further analysis. In order to have water that is free from impurities and contaminants, the water was obtained from the Water Utilities Corporation (Camwater) NC 102-115 [49] which is the Cameroonian standard applied for compressed earth blocks. Water facilitates rapid hydration reaction and paste-bind the matrix’s constituent parts together.

2.1.2. Preparation of Mortar Samples

Samples formulated and cured (Figure 1) involved combining a 10:2 ratio of earth to water to create a recycled brick dust mortar with dimensions of 2 × 2 × 2 cm. A total mass of 360 g was used to prepare the samples per formulation as shown by RB_1-x-yCO_xCN_y, where x represents the weight percentage of calcined eggshells (CO) in total powders, which started between 0 and 0.4 (wt.%), y represents the weight percentage of coconut shells (CN) in total powders, which started between 0 and 0.4 (wt.%), and 1-x-y represents the weight of grog (RB). Three (03) samples per formulation were used and the averages taken to calculate mean values and standard deviation.

Figure 1. Samples formulated and cured.

2.2.1. X-Ray Fluorescence (XRF)

The X-ray fluorescence is an analysis that allows the chemical composition of the sample to be determined. The chemical composition of the raw materials (grog, coconut shell and calcined eggshell) was determined using a Zetium PANATICAL apparatus with a power of 1 KW. Before carrying out the analysis, each sample powder was mixed with lithium borate salt and vitrified in a Pt crucible at 1600˚C at the Materials Analysis Laboratory of the Artisan Mining Support and Promotion Framework (CAPAM), now SONAMINES, in order to determine the lead content. At each lead content, a peak is determined which indicates the presence energy position at the same intervals as indicated by the markers.

2.2.2. X-Ray Diffraction Analysis (XRD)

In order to qualitatively identify several crystalline compounds and the crystallographic forms of materials, the XRD quantitative method was applied. Ex-situ XRD data were collected on a STOE Stadi-p X-ray powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany) using Cu Kα1 radiation (λ = 1.54056 Å; Gemonochromator; flat samples) in transmission geometry with a DECTRIS® MYTHEN 1K detector (DECTRIS, Baden-Daettwil, Switzerland).

2.2.3. Fourier Transformed Infrared Spectroscopy (FT-IR)

To measure the impurity and assess the nature of molecules in a material, the FTIR analysis is used which is based on the absorption reaction between infrared radiation that passes through matter. The excitation vibrations of samples are selectively observed and each molecule or group of molecules make up vibrational levels which corresponds to precise energies. The measurements of infrared spectroscopy are measured in transmittance using a spectrometer. A Bruker Vertex 80v with KBr was used by mixing 1 g of each sample with 200 mg of KBr and pressing at 100 kN using a hydraulic press (ENERPAC P392, USA) to obtain each pellet. The infrared spectrum of each pellet was recorded in the range of 400 - 4000 cm⁻¹ with a resolution of 2 cm⁻¹ and 32 scans. The samples were recorded using OPUS Spectroscopy software.

2.2.4. Characterization Methods of Physico-Mechanical Properties

The prepared samples were tested on 28, 56 and 90 days of ambient temperature (23˚C ± 3˚C). curing. These properties include: water absorption, apparent porosity, bulk density, moisture content, compressive strength and elastic modulus.

1) Bulk density and Moisture content. The weight of each sample was registered.

Three samples each of 2 × 2 × 2 cm clay mortar were used for each formulation to determine the average density according to BS EN 14617-1 [50]. This calculation was done using the formula in Equation (1) below.

With BD: bulk density in g·cm⁻³; M₁ = initial mass of sample after 28 and 56 days of curing in g; V: volume of sample in cm³. Moisture content was measured at 28 and 56 days. Samples were introduced into the electric oven at 105˚C ± 5˚C for 24 h to obtain the dry mass. Three (03) samples from each formulation were used to obtain their mean values and later calculated their standard deviations. The formula for moisture content is shown in Equation (2):

Whereby M₁ = initial mass of sample after 28 and 56 days of curing in g;

M₂ = dry mass of sample after 24 h in an oven in g.

2) Water absorption and Apparent porosity

According to ASTM C373-88 standard [51] water absorption determines the quantity of water retained by the material for 24 h, and also it gives information on the stability of the material in water. It is shown in Equation (3) below and was determined using an average of 3 samples: Whereby M₂ = dry mass of sample after 24 h in an oven in g; M₃ = wet mass of sample after 24 h in water in g. Apparent porosity reveals the porous nature of the material and gives information on voids within the material. The averages of three (03) samples were used following Equation (4) such that MC, WA, AP (%); ρ_e = density of water (1 g·cm⁻³); V = volume of sample in cm³; M₁ = initial mass of sample after 28 and 56 days of curing in g; M₂ = dry mass of sample after 24 h in an oven in g; M₃ = wet mass of sample after 24 h in water in g.

3) Compressive strength and elastic modulus

This corresponds to the maximum load per unit area, subjected to stress under the specific conditions at ambient temperature which can resist before breaking. It was measured at 28 and 90 days. While the Elastic modulus gives information on the elasticity and degree of deformation of a material. As concerns compression strength, this was determined at 28 and 90 days using an average of 3 samples. It is linked to compressive strength by Hooke’s law, and is shown on Equation (5)

With CS = compressive strength in MPa; EM = elastic modulus in MPa; ε = stretching or displacement.

The chemical composition of grog, coconut shell and calcined eggshell is shown in Table 1. The results from grog revealed important contents such as silica (SiO₂), alumina (Al₂O₃) and iron oxide (Fe₂O₃) as follows: 51.47%, 21.16% and 10.71%, respectively, with a silica ratio of ( SiO 2 Al 2 O 3 + Fe 2 O 3 ) = 1.61. The coconut shell powder contained 58.44% of SiO₂, 24.54% of Al₂O₃ and 13.39% of Fe₂O₃, with silica ratio ( SiO 2 Al 2 O 3 + Fe 2 O 3 ) = 1.54. The calcined eggshell contained 4.24% of (SiO₂), 0.76% of Al₂O₃, 0.08% of Fe₂O₃ with silica ratio ( SiO 2 Al 2 O 3 + Fe 2 O 3 ) = 5.04, and the high content of calcium oxide (CaO) of 53.75% was the result of decomposition of calcium carbonate (CaCO₃) at 900˚C. The results of grog and coconut shell show the presence of three major oxides: silica, alumina and iron oxide. The sums of the major oxides (SiO₂ + Al₂O₃ + Fe₂O₃ = 83.34%) for grog and (SiO₂ + Al₂O₃ + Fe₂O₃ = 96.37%) for coconut shell, respectively, which more than 70% revealed the pozzolanic nature of raw materials by ASTM C618 standard [52].

Table 1. XRF of raw materials.

Figure 2 indicates the X-Ray Diffraction of grog (waste brick), eggshell powder calcined at 900˚C, coconut shell, and mortar samples. However, Figure 2(a) indicates the raw materials while Figure 2(b) indicates the mortar samples. From Figure 2 below, the following abbreviations are represented on the diagram as: D = RB_0.74CO_0.26CN₀; E = RB_0.74CO_0.13CN_0.13; I = RB_0.6CO_0.13CN_0.27; G = RB_0.6CO_0.4CN₀; F = RB_0.74CO₀CN_0.26.

Figure 2. XRD pattern of: (a) raw materials, (b) mortar samples.

As indicated in Figure 2(a), the peak characteristic of grog is Quartz (SiO₂) and minor peaks consist of Hematite (Fe₂O₃), Corundum (Al₂O₃) (crystalline pure nature) [14], and unreactive phase nature Halite (NaCl) [53]. This is due to the presence of chloride element contained in the others fraction in XRF analysis, and the presence of sodium in SEM-EDX analysis [54]. Coconut shell shows no peak and presents an amorphous texture [55] which is almost completely amorphous with a broad peak or hump shown between 20˚ and 30˚ (2θ) [56]. The calcined eggshell powder is chemically composed of Calcite (CaCO₃) indicates that during decomposition, calcination was not yet complete and that organic matter was still present in the final product. One more phase is observed as Calcite in the form of CaO [57] (Equation (6)), which is important for a pozzolanic reaction to affect the properties of clay bricks [58]. While coconut shell and calcined eggshell can be good candidates as additives to create pores [59]. From Figure 2(b) the samples D, E, I, G and F are identical and present all phases into raw materials, that presented the crystalline phases of calcite and quartz adding to calcium silicate hydrate (C-S-H) gel [5]. In this study, no significant differences were observed due to the different formulations [60], while no formation of intermediates in any of the five samples was observed. The decomposition of calcium carbonate is shown in Equation (6) below.

Figure 3 shows the FT-IR spectra of the raw materials (grog, calcined eggshell powder and coconut shell powder) and mortar samples (D, E, I, G and F) mentioned in Figure 3 below.

Figure 3. Infrared spectrum of: (a) raw materials, (b) mortar samples.

The Fourier Transformed Infrared Spectrum (FT-IR) of the RB (grog) (Figure 3(a)) is measured within the range limit of 3837 - 3643 cm⁻¹ attributing to the stretching vibration of O-H and H-O-H bonds [61], indicating that the partial hydration after calcination, is as a results of the moisture present in the environment. While the peak at 2192 cm⁻¹ reveals the stretching vibration of C = C. The peak at 1416 cm⁻¹ correspond to the stretching vibration of C-O bond [62], indicating the carbonation of CO₂ from ambient air. The peaks at 1031 cm⁻¹ and 533 cm⁻¹ correspond to the deformation vibration of Si-O and Si-O-Si bonds respectively [61], which indicating the presence of amorphous quartz in the material [61]. The peaks at 913 cm⁻¹ and 747 cm⁻¹ indicate the deformation vibration of Al-O-H and Si-O-Al bonds respectively [59][61].

The FT-IR spectrum of CO (calcined eggshell) (Figure 3(a)) shows the range of 3748 - 3641 cm⁻¹ which attributes the stretching vibration of O-H and H-O-H bonds [63], and indicating the presence of moisture in the environment [58]. The range of 2183 - 1792 cm⁻¹ indicates the stretching vibration of C = O carbonyl bonds, originating from calcination eggshells at 900˚C [58]. The peaks at 1394 cm⁻¹, 871 cm⁻¹ and 711 cm⁻¹ revealing the presence of C-O bond (this is due to partial carbonation of CaO) [64], and the organic matter [58].

Concerning the FT-IR spectrum of CN (coconut shell) (Figure 3(a)), within the range of 3734 - 3326 cm⁻¹ attributes the stretching vibration of O-H and H-O-H bonds [59][65]. The peaks at 2919 cm⁻¹ and 1507 cm⁻¹ revealing the presence of stretching vibration of C-H and C=C bonds [15][65]. The different peaks at 1456 cm⁻¹, 1422 cm⁻¹, 1373 cm⁻¹, 1329 cm⁻¹, 1239 cm⁻¹ and 770 cm⁻¹ correspond the stretching vibration of C-O bond [65], while the peaks at 1033 cm⁻¹ and 896 cm⁻¹ correspond to Si-O bonds indicating the presence of amorphous silica present in the material [15][55]. The peak at 1732 cm⁻¹ reveals unconjugated carbonyl C = O [55][65]. The FT-IR spectra of samples D, E, I, G and F (Figure 3(b)) show similar identity for peaks characteristics from obtained results. Results from XRD and FT-IR analysis show no compositional and chemical differences between the mortar samples [66], and show the formation of identically phases of samples after curing time [67]. Due to the addition of raw materials, the possibility of chemical properties of the crystallinity is revealed [68]. Results from XRD and FT-IR analysis revealed that after curing time, the crystalline phases initially present in the raw materials were also found in the chemical properties of mortar samples, which justify their partial formation into the process mixture [69].

The physico-mechanical properties of samples are represented on Table 2 below. The physico-mechanical properties of earth mortars (moisture content, water absorption, apparent porosity, bulk density, compressive strength and elastic modulus) were determined and shown in Figure 4 below.

Results obtained for physico-mechanical properties at 28 and 56 days (bulk density, moisture content, water absorption and apparent porosity), and at 28 and 90 days for (compressive strength and elastic modulus) are presented in Figure 4 below.

Figure 4(a) shows bulk density. The bulk density of samples shows a slight decrease in strength values. For RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀, their densities between 28 and 56 days of curing decreases with an increase in the percentage of calcined eggshell from: 2.37 to 2.33, 2.31 to 2.29 and 2.22 to 2.04 g·cm⁻³, respectively. The bulk density values decrease from 28 to 56 days in ambient curing condition as 1.68%, 0.86%, and 8.82%, respectively. These results are similar to those found by [70], who obtained less density concretes based on calcined eggshell. While for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6CO₀CN_0.4, the bulk density between 28 and 56 days of curing decreases with an increase in the percentage of coconut shell from: 2.39 to 2.35, 2.22 to 2.10 and 2.22 to 2.20 g·cm⁻³, respectively. The decrease in bulk density value at ambient curing temperature was 1.27%, 5.71%, and 0.90% at 28 to 56 days, respectively. These results were compared with that obtained by [71], indicating that the density of types of concretes decreases with an increase in the percentage replacement of granite with coconut shell powder. Since the coconut shell powder are lightweight and occupy substantial amount of space during formulation, the particles were not closely bonded to each other as a result of the mould casting method. However, it can be attributed to the decrease in values of bulk density [72].

Table 2. Physical and mechanical properties of mortar samples at 28, 56 and 90 days.

Whereby A = RB₁CO₀CN₀; B = RB_0.87CO_0.13CN₀; C = RB_0.87CO₀CN_0.13; D = RB_0.74CO_0.26CN₀; E = RB_0.74CO_0.13CN_0.13; F = RB_0.74CO₀CN_0.26; G = RB_0.6CO_0.4CN₀; H = RB_0.6CO_0.27CN_0.13; I = RB_0.6CO_0.13CN_0.27; J = RB_0.6CO₀CN_0.4; K = RB_0.87CO_0.065CN_0.065; L = RB_0.67CO_0.26CN_0.07; M = CH_0.67CO_0.07CN_0.26.

Figure 4. Average curves at 28 and 56 days of: (a) bulk density, (b) moisture content, (c) water absorption, (d) apparent porosity, at 28 and 90 days of: (e) compressive strength, (f) elastic modulus.

However, for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07, RB_0.67CO_0.07CN_0.26 and RB₁CO₀CN₀, their bulk density at 28 and 56 days of curing increases and decreases from: 2.22 to 2.25, 2.22 to 2.16, 2.14 to 2.22, 2.18 to 1.97, 2.22 to 2.16, 2.22 to 2.16 and 2.31 to 2.35 g·cm⁻³, respectively. The increase in the bulk density value in ambient curing temperature at 28 and 56 days of curing was 1.35, 3.73, and 1.73% for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.13CN_0.27 and RB₁CO₀CN₀, respectively, On the other hand, the decrease in the bulk density value in ambient curing condition at 28 and 56 days was 2.70%, 9.63%, 2.70%, 2.70% for RB_0.6CO_0.27CN_0.13, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07 and RB_0.67CO_0.07CN_0.26, respectively. However, these values are greater than 1 g·cm⁻³ and comply with EN 771-1 standard for protected constructions [73].

Figure 4(b) shows moisture content variation. The moisture content of samples shows a great decrease in values. For RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀, their moisture content between 28 and 56 days of curing decreases from: 18.75% to 13.39%, 15.63% to 14.53% and 13.84% to 7.13%, respectively. The moisture content values decrease from 28 to 56 days in ambient curing temperature as 28.58%, 7.03%, and 48.48%, respectively. These results were compared with that obtained of [74], indicating that moisture content decreases when the percentage of eggshell powder increases. While for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6CO₀CN_0.4 the moisture between 28 and 56 days of curing decreases from: 21.06 to 15.93%, 20.22% to 10.88% and 18.93% to 16.98%, respectively. The decrease in the moisture content value in ambient curing condition was 25.78%, 46.19%, and 10.30% at 28 to 56 days, respectively. According to [75], the moisture content decreases with an increase in percentage of crushed aggregates. As for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07, RB_0.67CO_0.07CN_0.26 and RB₁CO₀CN₀, their moisture content at 28 to 56 days of curing decreases from: 17.59% to 14.79%, 16.34% to 12.46%, 15.70% to 14.92%, 15.41% to 5.27%, 15.09% to 12.49%, 20.19% to 14.39% and 19.35% to 15.03%, respectively. The decrease in the moisture content value at 28 to 56 days of curing in ambient condition was 15.91%, 23.74%, 4.96%, 65.80%, 17.22%, 28.72%, 22.32%, respectively.

Figure 4(c) indicates that when there is an increase in the percentage of calcined eggshell at 900˚C, and a decrease in the percentages of grog, the water absorption test slightly decreases then later and increases. For RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀ the water absorption test decreases as 29.16% to 28.88%, and increases of 29.21% to 29.80% and 26.57% to 28.60% up to 28 and 56 days of curing, respectively. However, there is a decrease in water absorption with an increase in the percentage of coconut shell which follows an increase as mentioned in the formulations for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6CO₀CN_0.4 with values as 44.22% to 40.02%, 49.38% to 47.81% and 55.55% to 51.18% up to 28 and 56 days of curing, respectively. The decrease in the water absorption value in ambient curing temperature was 9.49%, 3.17%, and 7.86% at 28 to 56 days, respectively. Results are in line with [76], who demonstrated how water absorption decreases when curing time increases.

The mix design of both calcined eggshell and coconut shell with the different formulations are given below with their corresponding water absorption for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07 and RB_0.67CO_0.07CN_0.26. The water absorption at 28 and 56 days of curing decreases and increases from: 40.68% to 39.13%, 34.80% to 35.16%, 49.38% to 48.31%, 40.68% to 41.17%, 34.39% to 34.08% and 47.27% to 60.72%, respectively. The decrease in the water absorption content value at 28 to 56 days of curing in ambient condition was 3.81%, 2.16%, and 0.90% for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.67CO_0.26CN_0.07, respectively. While the increase in the water absorption value at 28 to 56 days in ambient temperature was 1.03%, 1.20%, 28.45% for RB_0.6CO_0.27CN_0.13, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.07CN_0.26, respectively. These results were compared with that of [77], indicating the type of bricks which become more porous when water absorption ranges between 20 and 55%. However, the results of [53] showed the capacity of bricks to absorb water and also increased as the porosity of bricks increased.

Generally, mortars based on calcined eggshells absorb less water. These results are similar to those found by [78], who obtained less absorbent concretes based on eggshells.

Figure 4(d) shows apparent porosity. The apparent porosity of samples shows a constant increase in values. So for RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀, their apparent porosity remains constant and increases from: 58.33% to 58.33%, 58.33% to 58.33% and 52.08% to 54.16% up to 28 and 56 days of curing, respectively. While for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6CO₀CN_0.4 the apparent porosity at 28 and 56 days of curing decreases from: 87.50% to 79.16%, 91.66% to 89.58% and 104.11% to 93.75%, respectively. These results were compared with that of [76], indicating which apparent porosity decreases when curing time increase. For RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07 and RB_0.67CO_0.07CN_0.26, their apparent porosity at 28 and 56 days of curing decreases, remains constant and increases from: 77.08% to 75.00%, 66.66% to 66.66%, 91.66% to 91.66%, 77.08% to 77.08%, 66.66% to 64.58% and 87.50% to 112.50%, respectively. These values of apparent porosity are greatly improved due to increase in pores size of mortars and the cohesion mixture decreases [79]. However, the type of bricks with higher porosity are recommended for its thermal insulation properties [80].

Figure 4(e) revealed an increase in the percentage of calcined eggshell sample at 900˚C, while there is a decrease in the percentages of grog, as the compressive strength increases. For RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀ the compressive strength test at 28 and 90 days of curing increases from: 1.38 to 1.87, 2.70 to 4.61 and 3.88 to 4.14 MPa, respectively. These results are in agreement with [41], indicating the increase of mechanical properties. The compressive strength values increase from 28 to 90 days in ambient curing condition as 35.50%, 70.74%, and 6.70%, respectively. While on the other hand, there is an increase in compressive strength with an increase in the percentage of coconut shell, then a decrease from above mentioned formulations for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6CO₀CN_0.4 with values as 1.52 to 3.01, 2.01 to 4.26 and 1.34 to 3.42 MPa up to 28 at 90 days of curing, respectively. The increase in the compressive strength value in ambient curing temperature was 98.02%, 111.94%, and 155.22% at 28 to 90 days, respectively. These samples have revealed a gain in compressive strength from 28 to 90 days, according to [81]. The different values such as 2.01, 2.70, 3.01, 3.42, 3.88, 4.14, 4.26 and 4.61 MPa respectively are higher than the Cameroonian [49] standards (which sets compressive strength at 2 MPa). Thus, seven values 2.70 MPa, 3.01 MPa, 3.42 MPa, 3.88 MPa, 4.14 MPa, 4.26 MPa and 4.61 MPa are highest than the Netherland NEN 3835 standard [82] (which sets compressive strength and falls within the range limit of 2.5 - 5 MPa), while these results are similarly with [83], who demonstrated that compressed earth bricks stabilized by 8% of cement achieved the compressive strength between 2.5 and 3.5 MPa. The compressive strength is profoundly affected by firing temperature, method of production and physical, chemical and mineralogical properties of the raw materials [59]. The better improved values of compressive strengths contribute to the creation of an environmental friendly building materials, and promote a circular economy as stated by [84], and also to achieve sustainable development of building materials [85]. However, these results are applied in building and construction materials and decoration as stipulated by [86]. Indeed, a high percentage of silica (SiO₂) has reacted with calcium oxide (CaO) forming calcium-silicate-hydrate (C-S-H), which increases compressive strength [87]. While the curing time increased, the hydration or lime-pozzolan reaction increased, which accelerated the pozzolanic reactivity [87].

The formulations of the mix design of both calcined eggshell and coconut shell, are given below with their corresponding compressive strength such as RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.67CO_0.07CN_0.26 and RB₁CO₀CN₀. Their strength between 28 and 90 days of curing increases from: 1.30 to 1.89, 0.73 to 1.40, 1.34 to 2.22, 0.29 to 0.58, and 1.54 to 2.39 MPa, respectively. For RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07 the strength between 28 and 90 days of curing decreases from: 1.16 to 0.85, 0.74 to 0.57 MPa, respectively has demonstrated on how compressive strength decreases when the percentage of coconut shell increases [88]. The increase in the compressive strength value in ambient curing condition was 45.38%, 91.78%, 65.67%, 100%, 55.19% at 28 to 90 days, respectively. While the decrease in the compressive strength value was 26.72%, 22.97% at 28 to 90 days, respectively. These results have been compared with that of [60], indicating the significant increase of the compressive strength at ambient curing, and have been presented a gain of the compressive strength values from 28 to 90 days [1][5], this is due to pozzolanic effect of calcined eggshell and coconut shell [79].

Figure 4(f) shows elastic modulus. The elastic modulus of samples shows a great increase in values. So for RB_0.87CO_0.13CN₀, RB_0.74CO_0.26CN₀ and RB_0.6CO_0.4CN₀, their elastic modulus between 28 and 90 days of curing increases and decreases from: 8.41 to 10.79, 9.74 to 9.51 and 5.85 to 12.00 MPa, respectively. In general terms, the use of highly porous materials (water-absorbent materials) can reduce elastic modulus of concrete [89]. While for RB_0.87CO₀CN_0.13, RB_0.74CO₀CN_0.26 and RB_0.6 CO₀CN_0.4 the elastic modulus between 28 and 90 days of curing increases from: 9.46 to 10.36, 8.46 to 13.25 and 5.72 to 12.53 MPa, respectively. The increase in the elastic modulus value in ambient curing temperature was 9.51%, 56.61%, and 119.05% at 28 to 90 days, respectively. These results were compared with that of [90], indicating the use of adequate content of lightweight sand results in an increase in elastic modulus. And finally for RB_0.74CO_0.13CN_0.13, RB_0.6CO_0.27CN_0.13, RB_0.6CO_0.13CN_0.27, RB_0.87CO_0.065CN_0.065, RB_0.67CO_0.26CN_0.07, RB_0.67CO_0.07CN_0.26 and RB₁CO₀CN₀, their elastic modulus at 28 and 90 days of curing increases and decreases from: 9.74 to 10.32, 4.95 to 10.06, 9.05 to 12.60, 7.11 to 9.92, 4.39 to 8.20, 3.25 to 8.21 and 10.89 to 8.78 MPa, respectively. The elastic modulus values increase from 28 to 90 days in ambient curing condition as 5.95%, 103.23%, 39.22%, 39.52%, 86.78%, 152.61%, respectively, and decrease as 19.37% for sample (RB₁CO₀CN₀) at 28 to 90 days in ambient curing condition. However, the use of water-absorbent materials can reduce elastic modulus of concrete [89]. Moreso, in this study, it was established that addition of coconut shell particle greatly improved elastic modulus [91].

The correlation coefficient denoted as R² is a statistical measure representing the proportion of the variance in the dependent variable that is predictable from independent variables in a regression model. R² indicates the strength of the relationship between independent and dependent variables. The different limits range of correlation coefficient are: 0 - 0.2 (weak correlation) means a very little variation in the dependent variable, 0.2 - 0.4 (moderate correlation) means and explains some of the variations in the dependent variable, 0.4 - 0.6 (strong correlation) means a strong part of the variation in the dependent variable, 0.6 - 0.8 (very strong correlation) means and explains a huge portion of the variation in the dependent variable, and 0.8 - 1 (perfect correlation) means and explains all of the variations in the dependent variable.

Correlations between physical properties are shown in Figure 5 and Figure 6 at 28 and 56 days. Figure 7 shows correlations between mechanical properties at 28 and 90 days. However, the correlation coefficients and range limits between properties are shown on Table 3 below.

Table 3. Correlation coefficients and range limits between properties at 28, 56 and 90 days.

Figure 5. Linear correlation curves of physical properties at 28 days.

Figure 6. Linear correlation curves of physical properties at 56 days.

Figure 7. Linear correlation curves at 28 and 90 days of mechanical properties.

The linear correlation curves Figure 5 shows variation between different physical properties at 28 days of curing such as bulk density, water absorption, and moisture content. Figure 6 indicates the linear correlation curves of variation between different physical properties at 56 days of curing such as bulk density, water absorption, and moisture content.

Figures 5(a)-(c) and Figures 6(a)-(c) show the correlations between bulk density of mortars under investigation and their water absorption, moisture content, and apparent porosity, respectively. Figures 5(a)-(c) and Figures 6(a)-(c) present a linear increase in their water absorption, moisture content, and apparent porosity with an increase in bulk density, and the correlation coefficients are as follow R² = 0.67, R² = 0.73, R² = 0.62 and R² = 0.86, R² = 0.89, R² = 0.85, respectively. This all shows a very strong correlation and fall within the range limit of 0.6 - 0.8 and 0.8 - 1. Result of correlation between bulk density and moisture content explains that the increase of bulk density with moisture content increases [92]. However, the results of correlation coefficients between bulk density, water absorption and apparent porosity have explained that the bulk density increases with an increase in the water absorption and apparent porosity [93].

Figure 5(d) & Figure 5(e) and Figure 6(d) & Figure 6(e) show the correlations between water absorption of mortars under investigation, their moisture content, and apparent porosity, respectively. Figure 5(d) & Figure 5(e) and Figure 6(d) & Figure 6(e) present a linear increase in their moisture content, and apparent porosity with an increase in water absorption, and the correlation coefficients are R² = 0.81, R² = 0.99, R² = 0.82 and R² = 0.99, respectively. The above shows perfect correlation and fall between 0.8 - 1. Result of correlation between water absorption and apparent porosity also explains why more porous materials with open and interconnected pores have greater water absorption [94]. Moreover, this explains why water absorption depends on the apparent porosity of mortars [95]. However, the result of correlation between water absorption and moisture content shows that the water absorption increases with an increase in moisture content [96].

Figure 5(f) and Figure 6(f) show the correlation between moisture content of mortars under investigation, and their apparent porosity. Figure 5(f) and Figure 6(f) present a linear increase in their apparent porosity with an increase in moisture content, and the correlation coefficients are R² = 0.77 and R² = 0.83, respectively. This shows two types of very strong correlation and perfect correlation and fall within the range limits of 0.6 - 0.8 and 0.8 - 1, respectively. Result of correlation between apparent porosity and moisture content explains that the apparent porosity increases with an increase in moisture content [97].

Rasool et al., 2023 who worked on “Experimental study on strength and endurance performance of burnt clay bricks incorporating marble waste”, and they established a linear correlation between water absorption and apparent porosity, and their results showed a correlation coefficient of 0.92 indicating a perfect correlation. Another researcher such [98] determined a linear correlation between water absorption, and apparent porosity. They also obtained a correlation coefficient of 0.98 for relationship, which is a perfect correlation.

However, [99] determined the linear correlations between apparent porosity, and water absorption, and between bulk density and apparent porosity, respectively. They obtained the correlation coefficients of 0.96 and 0.98 for relationships, respectively, which is a perfect correlation.

Figure 7 above shows the linear correlation curves between elastic modulus and compressive strength at 28 days and 90 days of curing.

Figure 7(a) shows a linear increase in elastic modulus with an increase in compressive strength. The correlation coefficient is as follows R² = 0.66, it shows very strong correlation and falls within the range limit of 0.6 - 0.8. Figure 7(b) shows a linear increase in elastic modulus with an increase in compressive strength. The correlation coefficient is R² = 0.96, it shows perfect correlation and falls between 0.8 - 1. These results were in agreement with that of [100], indicating that the elastic modulus increases with an increase in the compressive strength.

The relationship between water absorption (WA), and bulk density (BD) at 28 and 56 days of curing increases from R² = 0.67 to R² = 0.86 and it is observed that there is a significant effect on the curing condition. On the other hand, at 28 and 56 days of curing, the correlation between moisture content (MC), and bulk density (BD) increases from R² = 0.73 to R² = 0.89, and it is observed that there is a significant effect on the curing condition. While the correlation between apparent porosity (AP), and bulk density (BD) at 28 and 56 days, increases from R² = 0.62 to R² = 0.85, it shows a significant effect on the curing condition. Thus, the relationship between MC and WA at 28 and 56 days of curing increases from R² = 0.81 to R² = 0.82, showing a non-significant effect on curing condition. While for the relationship between AP and WA at 28 and 56 days of curing, the correlation coefficient value as R² = 0.99 for all, showing a non-significant effect. And then the correlation between AP and MC at 28 and 56 days of curing, increases from R² = 0.77 to R² = 0.83 and it demonstrates a significant effect of curing condition. And finally for the relationship between elastic modulus (EM) and compressive strength (CS), the correlation at 28 and 90 days of curing increases from R² = 0.66 to R² = 0.96, indicating a significant effect on curing condition.