Substitution of Sand in Concrete Blocks with Coconut Fiber and Cattle Manure: Effects on Compressive Strength and Thermal Conductivity

Abstract

Improving the energy performance of buildings is critical in the construction sector. This study investigates the effects of incorporating coconut mesocarp fibers (F = Fiber) and bovine manure (M = Manure) on the thermal conductivity and compressive strength of concrete blocks. Bovine manure and coconut fiber replaced the block sand at maximum concentrations of 10 and 1.5%, respectively. Thermal conductivities were measured according to the ASTM C177 (2013) standard, compression tests were performed using the ASTM C140 standard, and characterization assays such as Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were performed to determine the morphological properties of the final material and its constituents. The results showed a 50% reduction in the thermal conductivity coefficient of the blocks when 10 and 1.5% of the sand was replaced with manure and coconut fiber, respectively. Similarly, incorporating coconut fiber at percentages of 0.5, 1, and 1.5% improved compressive strength results. Blocks comprising 0.5, 1, and 1.5% fiber or a mix of 3% manure and 1.5% fiber attained the compressive strength requirements established by the standard. This study demonstrated the feasibility of using coconut fiber mixed with cattle manure as a substitute for up to 2.5% of the sand in non-structural wall elements manufacturing, attaining a decrease in thermal conductibility of around 10%.

1. Introduction

Mitigating climate change is currently one of the biggest challenges faced by the construction industry [1,2]. Because buildings consume approximately 40% of the global energy [3], improving insulation materials within building envelopes to reduce the thermal load in living spaces is crucial [4,5]; improving insulation materials reduces the thermal load in living spaces and the energy required for comfort [6]. Incorporating an insulation material into the material composition of buildings is one of the most commonly used alternatives for improving it. The green building approach links the use of environmentally friendly materials for construction buildings with reduced energy consumption [7,8].

Synthetic materials from the petrochemical industry, such as polystyrene, or natural sources that require high energy consumption during processing, such as glass and rock wool, are mainly used for building materials insulation improvement. These materials have significant adverse environmental impacts [8]. Hence, the use of recycled waste has drawn a lot of attention, including the application of several agricultural wastes such as maize husk after separation of the corn, maize cob after separation of the corn, coconut pith with groundnut shell, sheep’s waste wool, sunflower stalks, corn peel, wheat straw board composed of coconut coir and durian peel, kenaf binderless board, expanded vermiculite and perlite, stubble fiber after harvesting of wheat and barley, and bagasse [9].

Several studies have assessed the mechanical properties of natural fibers when used in minimal percentages (1–2%) to reinforce building materials, aiming to strike a balance between improving thermal insulation and maintaining compressive strength in concrete blocks by replacing some of the conventional constituents used in block manufacturing [4]. Adding coconut fiber (10, 15, and 20% of reference cement volume) to uncooked blocks reduced the final material density, thermal conductivity, and compressive strength; the optimum proportion of coconut fiber was established to be 20% [10]. Using bagasse ash at 2, 4, and 8% of the weight of compressed earth blocks in non-structural masonry elements resulted in mechanical property improvement. The blocks with 8% bagasse ash and 12% Portland cement presented a compressive strength of 2.89 MPa.

Manure as an additive to improve construction materials has been much less studied. The use of a few manures as building material additives is reported in the specialized literature, mainly focused on the influence on strength: horse [11], cow [12] and goat [13]. Nevertheless, to the authors’ best knowledge, there have not been any reported studies about the influence of coconut fiber and cattle manure, used alone or mixed, on building materials’ thermal conductivity, which is the main contribution of this research.

Colombia has a strong dependency between per capita electricity consumption and the climatic zone [14]. Since electricity consumption in tropical zones such as Cordoba is driven by building comfort needs, avoiding larger thermal loads is essential. Therefore, the availability of cheap and abundant materials capable of reducing the transconductance of external wall elements would reduce electricity demand.

The successful use of natural and synthetic fibers to improve the ductility, toughness, and flexibility of building materials has been reported in the literature [15,16]. Although synthetic fibers exhibit better mechanical behavior, the use of natural fibers presents several advantages owing of their abundance, cost-effectiveness, low-density, biodegradability, and potential to enhance the energy performance of buildings by enhancing the thermal insulation of envelopes [17]. Agricultural waste, often produced in substantial quantities and commonly incinerated after harvest at planting sites, resulting in significant environmental repercussions, can serve as a viable fiber source for manufacturing construction materials [18].

Vibro-compacted concrete blocks are one of the most widely used building materials for residential construction in countries such as Colombia, because they are easy to produce and have adequate compressive strength. However, in recent years, the scarcity of river sand in Colombia has posed challenges, necessitating the exploration of alternative materials for block manufacturing [19]. The revaluation of waste as a substitute for sand in block manufacturing represents a sustainable approach to address this issue [20]. Natural fibers emerge as a particularly promising option for this purpose because of their abundance and minimal environmental impact [21].

The Department of Córdoba in Colombia produces 29,000 tons of coconut per year, generating 2000 tons of waste with the potential to be used as fiber reinforcement for making composite materials for buildings. This approach not only offers significant environmental benefits but also addresses the pressing need for alternative resources to conventional materials, thereby reducing carbon emissions and the ecological impact of buildings. Additionally, emissions from their free decomposition on land, where they are usually disposed of, can also be avoided [22,23]. Also, there are in the department around 2.6 million heads of cattle [24], which generate approximately 26 million tons of manure yearly, constituting a significant part of the department’s greenhouse gas emissions [25].

The use of cattle manure as a partial replacement for sand in manufacturing vibro-compacted cement blocks has not yet been reported on. Manure incorporation should be carefully evaluated due to the chemical and biological processes involved. According to Li et al. [26], the high content of Proteobacteria and Actinobacteria in manure poses potential health risks, as these organisms can cause diseases in humans. However, using low levels of manure, about 10%, is harmless [27]. The use of binders, such as Portland cement and hydrated lime, aids in binding the manure, thereby mitigating health risks and increasing the lifespan and durability of adobe blocks [17,28].

Standard benchmarking is required to improve the suitability and applicability of these materials. Water absorption remains a concern for almost all products developed from agricultural waste; therefore, a combination of more than one of these types should be investigated to determine their technical and economic viability. This study assessed the effects of incorporating coconut mesocarp fibers and bovine manure on the thermal conductivity and compressive strength of vibro-compacted concrete blocks when used as a sand substitute. These wastes are typically disposed of through natural decomposition in soil or incineration, leading to CO₂ emission, particulate matter generation in air, and water contamination. However, they can be used as an alternative to replace sand in the production of concrete blocks under the criteria of the standards established for the final use of this material.

2. Materials and Methods

Fibers from the mesocarp of coconuts, from the Cocos nucifera variety “Alto Caribe”, and dry cattle manure were used to produce vibro-compacted blocks and test specimens for thermal conductivity tests. Before production, the raw materials were subjected to drying and storage processes under controlled conditions to ensure the reproducibility of the results. Coconut mesocarp was extracted using a cutting tool (Figure 1a). The samples were packed in plastic bags to maintain their moisture content. The lengths of the extracted fibers ranged from 150 to 250 mm. Manure was collected manually (Figure 1b).

2.1. Physicochemical Characterization of Added Materials

The proximate analysis of the materials followed ASTM D3172-13 [29] and included the determination of moisture content, volatile matter, ash content, and fixed carbon. Moisture content was measured by drying the sample at 105 °C until a constant weight was achieved. Volatile matter was assessed by heating the dried sample in a covered crucible at 950 °C for 7 min in an inert atmosphere. Ash content was obtained by combusting the sample at 750 °C in an open crucible until only mineral residue remained. Fixed carbon was calculated by subtracting the moisture, volatile matter, and ash content from the original sample weight. Each measurement was performed in triplicate to ensure accuracy. The higher heating value was determined using an adiabatic calorimeter according to the standard ASTM D5865 2013 [30].

2.1.1. Elemental Analysis

The elemental analysis of the coconut fiber was carried out using Leco Truspec model elemental analyzer equipment (LECO Corporation, St Joseph, MI, USA). The elements present were reported to be nitrogen, carbon, hydrogen, sulfur, and oxygen. The reported values represent the averages of three test results.

2.1.2. Fourier-Transform Infrared (FTIR) Spectroscopy

FTIR characterization was carried out on sieved powder samples of bovine manure and coconut fibers to detail their structural chemical characteristics. FTIR spectra were obtained using an Itracer-100 FTIR spectrophotometer (Perkin Elmer, Waltham, MA, USA) with an attenuated total reflection (ATR) function. Each spectrum was obtained by consecutive scans in the range of 4000–500 cm⁻¹, at a resolution of 1 cm⁻¹.

2.1.3. Density of Coconut Fiber and Bovine Manure Determination

The fibers were dried by exposure to the sun for 7 h per day over a span of five days. The Archimedes principle was used to determine the density of the coconut fiber. A portion of the coconut fiber was weighed using an Ohaus CP413^® (OHAUS CORPORATION, Parsippany, NJ, USA) reference balance with a resolution of 0.0001 g. This weighted portion of coconut fiber was then immersed in a 500 mL graduated reservoir with water, and the volume of water displaced was used to calculate the fiber density. The manure was sun-dried for 7 h per day over a span of ten days. It was crushed in a hammer mill with a 2 mm screen to achieve sufficient compaction and good finish in the blocks. The density of the manure was measured using the absolute density method. An Ohaus CP413^® reference balance with a resolution of 0.0001 g and a 50 mL test tube were used. Initially, an empty test tube was weighed, following which the test tube was filled with manure. Subsequently, the test tube was weighed again.

2.1.4. Scanning Electron Microscopy (SEM)

SEM was used to observe the surface morphology of the concrete matrix and to determine the compounds or functional groups of the organic residues. The concrete samples prepared for this test had volumes of 2 cm³. This analysis was performed using SEM (Thermionic) (JEOL JMS-6490LV) (JEOL USA, Inc., Peabody, MA, USA) at an accelerating voltage of 20 kV. Samples previously immersed in liquid nitrogen were sputter-coated with gold (approximately 50 nm thick).

2.2. Manufacture of Concrete Blocks and Specimens for Thermal Conductivity Tests

Table 1. Coconut fiber and manure addition.

Designation	Manure %	Fiber %
CB	0	0
B M0-F0.5	0	0.5
B M0-F1.0	0	1
B M0-F1.5	0	1.5
B M3-F0	3	0
B M6-F0	6	0
B M10-F0	10	0
B M3-F0.5	3	0.5
B M3-F1.0	3	1
B M3-F1.5	4	1.5
B M6-F0.5	6	0.5
B M6-F1.0	6	1
B M6-F1.5	6	1.5
B M10-F0.5	10	0.5
B M10-F1.0	10	1
B M10-F1.5	10	1.5

details the combinations of coconut fiber and manure incorporated into the studied mixtures. In the Supplementary Materials section, details are provided on how sand was replaced with fiber and manure. Additionally, the mixing and molding method, the molds used, their sizes, the curing method, and the period before testing are explained. A factorial analysis involving a series of experimental tests was conducted to establish the optimal proportions of these components. These tests assessed the effects of varying coconut fiber levels (0%, 0.5%, 1%, 1.5%) and manure content (0%, 3%, 6%, 10%) on the blocks’ physical and thermal properties. Three blocks were produced for each composition to conduct comparative tests on strength and water absorption. Additionally, two specimens from each composition underwent thermal conductivity testing. The table’s notation is designed to provide clear information about the composition of the mixtures. In this notation, ‘M’ represents the percentage of manure, while ‘F’ represents the fiber percentage. For example, ‘B M3-F1.0’ indicates a mixture that contains 3% manure and 1% fiber.

The blocks’ thermal conductivity was determined as per ASTM C177 [31]. Measurements were conducted using a heating box designed for room temperature conditions, as detailed in Annex 1 of the standard. A wooden mold with dimensions of 300 mm × 300 mm × 25 mm served as the base for the specimens. The apparatus shown in Figure 2 controls the supplied heat flow, records the surface temperatures over time, and determines the thermal conductivity based on the range of temperatures that exhibit steady-state behavior.

The thermal conductivity was calculated using Equation (1). The heat flow was divided into two equal parts for each of the test specimens. Hence, the average between the two faces of the evaluated specimen was determined. Subsequently, Equation (1) was applied.

$$\mathrm{k}={\displaystyle \frac{\mathrm{V}\mathrm{I}∆\mathrm{X}}{\mathrm{A}∆\mathrm{T}}}$$

(1)

• A = Heat transfer area (m²)
• k = Thermal conductivity (Wm⁻¹K⁻¹)
• ∆T = Temperature differential (K)
• ∆x = Material thickness (m)
• V = Voltage (V)
• I = Intensity of the electric current (A)

2.3. Compressive Strength of Studied Concrete Blocks

A compression test was performed using an MTS Criterion Model 45 (MTS Systems, Eden Prairie, MN, USA) universal testing machine according to the ASTM C140 standard [32]. The speeds used in the tests were 0.03 mm s⁻¹. Three tests were performed for each percentage of mixes in the 48 blocks. Each block was randomly tested and removed when the load fluctuated.

2.4. Statistical Data Analysis

A factorial experimental design with multiple levels was implemented to evaluate the properties of blocks with fiber and manure. The sample size was adjusted to preserve the degrees of freedom with the inherent variability of the operational conditions of block manufacturing. Fifteen experimental runs of blocks with various percentages of fiber and manure were conducted, along with a control block made only of concrete for comparison. Thermal conductivity, compressive strength, and water absorption were measured as response variables, and an analysis of variance was performed to determine the statistical significance of the percentages of fiber and manure on these responses, including interactions between factors.

Subsequently, a thorough response surface methodology was used to determine the optimal percentage of coconut fiber and manure that would provide the block with the best properties, utilizing multi-objective optimization through a desirability function, with response variables of equal importance.

Table 2. Objective variables in multi-response optimization.

Response Variable	Target	Importance
Thermal conductivity	Minimize	3
Compression strength	Maximize	3
Water absorption	Minimize	3

shows the configuration of the response variables for this thorough analysis.

3. Results and Discussion

3.1. Proximate Analysis of Materials

The results of the proximal analysis are listed in

Table 3. Results of proximate analysis of materials.

Samples	M	Ash	VM	FC	HHV
	(%)	(%)	(%)	(%)	kJ/kg
Manure	11.78	31.71	44.76	11.75	11,914
Coconut Fiber	5.62	1.48	72.05	20.85	18,966

. The residual moisture (M) of the manure was twice that of the coconut fiber, which was less than 10%; hence, the calorific value (HHV) of coconut fiber was 59% higher than that of manure. Coconut fiber contains much less ash than manure, making coconut fiber combustion more efficient because the ash content represents the non-combustible residue left after combustion. Manure volatile material (VM) content was 61% lower than that of coconut fiber, which ignites better. In other words, the coconut fibers can quickly release more energy during combustion due to their high volatile matter content. Additionally, the presence of more fixed carbon (FC) in coconut fiber contributes to a more prolonged and stable combustion, making it an efficient fuel source.

3.2. Materials Density

Table 4. Material density.

Materials	$\mathbf{Density}\mathbf{\left(}{\mathbf{g}\mathbf{/}\mathbf{c}\mathbf{m}}^{\mathbf{3}}\mathbf{\right)}$
Coconut Fiber	0.9
Manure	0.24

lists the densities of the studied materials, concurrent with the results reported in other studies [33,34]. However, the density of the coconut fibers was higher than that of the manure. Therefore, a greater volume of manure than coconut fiber is required in the mortar preparation process.

The sand used for the vibro-compacted blocks and thermal conductivity specimens was coarse, with a fineness modulus of 2.9 according to the ASTM C33-16 standards [35]. Grey ultracem-type ART cement (for structural use) with a density of 2300 kg m⁻³ was used. In general, increasing the amounts of coconut fiber and manure in the mixture decreased the weight of the specimens. Replacing sand (dense material) with coconut fiber and manure (light materials) resulted in an increase in the total mix volume, even after compaction. This increase in the compacted mix volume resulted in a decrease in specimen weight and density (

Table 5. Materials used to 1 m³ of mortar.

Material	Dry Weight (Kg)	Specific Weight (g/cm3)	Volume (L)
Cement	195.9	3.1	63.19
Sand	1093.122	1.43	764.42
Water	172.392	1	172.39
Total	1461.414		1000

).

3.3. Fourier-Transform Infrared (FTIR) Spectroscopy

Figure 3 shows the FTIR results for coconut fiber and bovine manure. The FTIR spectra of the coconut fiber (Figure 3a) exhibits a series of peaks at a particular wavelength (λ), indicating the bonding vibrations of the functional groups present in the material. The initial disturbances observed were located at a wave number of 3314 cm⁻¹. This vibration was attributed to the axial stretching of O-H hydroxyl groups [36,37]. Further vibrations were observed at 2891 cm⁻¹, corresponding to the symmetric and asymmetric stretching vibrations of the C-H bonds of the methylene (CH₂) and methyl (CH₃) functional groups. The perturbation generated at 1717 cm⁻¹ corresponded to the carbonyl group that is characteristically found in the range of 1725–1700 cm⁻¹ [38]. A following valley, owing to the vibrations generated by the bending of H-O-H groups (belonging to water) and humidity absorbed by the fibers, was observed at 1604 cm⁻¹. Further vibrations were observed at 1013 and 514 cm⁻¹. This set of vibrations, typical of the sample fingerprint, are characteristic of the functional groups C-C, C=C, O-H, CO, C-O-C, CH, and aromatic bonds contained in the glucose present in cellulose [37,38].

The FTIR spectrum of the manure (Figure 3b) revealed the presence of a peak in the region between 1680 and 1640 cm⁻¹, related to the stretching vibration of CO double bonds. In addition, the peak at 1532 cm⁻¹, assigned to the stretching vibration of the C=C aromatic double bond of lignin compounds, indicates a high content of straw and fibers in the livestock feed. Likewise, a weak curve was observed in the region around 1545 cm⁻¹, attributable to the vibration captured in the N-H plane. In addition, a region close to 1365 cm⁻¹, which was assigned to the vibration of the C-N group of the primary and secondary aromatic amides, and another region located between 1265 and 1240 cm⁻¹ were determined to be the stretching vibrations of the C-N and C-O groups, respectively. The extensive peak between 1200 and 900 cm⁻¹, centered at 1031 cm⁻¹, formed a region of overlapping peaks. This included the C-O stretching vibration around 1160 and 1080 cm⁻¹, as well as the P–O and Si– stretching vibrations of phosphates and silicates, at 1100 and 1000 cm⁻¹, respectively. Furthermore, some mineral and lignin components were observed in the regions below 1000 and 899 cm⁻¹, where the stretching vibrations of the C-O-C bonds of cellulose occur [39].

3.4. Analysis of Scanning Electron Microscopy (SEM) and Optical Microscopy Results

The optical microscopy results provide insights into the adhesion properties of the materials when mixed with block mortar (cement plus sand). The analysis involved selecting a mixed sample and examining the resulting adhesion, as depicted in Figure 4a,b. A comparison was made between the appearance of the coconut fiber before (Figure 4c) and after mixing. This comparison revealed that adhesion occurred on the fiber surface, indicating that the fibers effectively bond with the mortar (Figure 4d). This adhesion is crucial for ensuring the structural integrity and durability of the composite material. The detailed optical microscopy results highlight the potential of using these fibers as a reinforcing agent in building materials, improving adhesion and overall material performance.

The surface characterization of the blocks B M3-F0 and B M3-F1.5 was performed to evaluate the morphologies of the coconut fiber and bovine manure using SEM. Figure 5a,b shows the SEM images of bovine manure in the blocks B M3-F0 (at 50 μm) and B M3-F0 (at 10 μm), respectively. Figure 5c shows the SEM images of the bovine manure mixed with coconut fiber in B M3-F1.5 at 50 μm, and Figure 5d shows the SEM image of block B M3-F1.5 at 10 μm. Organic fibers with diameters of ~50 μm, covered by several layers, were observed in M3-F0 (Figure 5a). This result is similar to that reported by Ormaechea et al. [40]. The vegetal particles in the bovine manure were composed of tissues and organs that have different proportions of lignin (2.7–5.7%), hemicellulose (1.4–3.3%), and cellulose (1.6–4.7%). Figure 5b shows that the bovine manure particles have a small space of less than 10 μm between them and the concrete matrix. Furthermore, the spaces were due to air particles that were trapped between the bovine manure particles and concrete matrix. Some of these bovine manure particles adhered to the surface of the fibrous material, and different minerals were present in the concrete matrix.

In Figure 5c,d, coconut fibers with diameters of ~50 μm, covered by several minerals from the concrete matrix and a few particles of bovine manure, can be observed in the sample from block B M3-F1.5. In addition, cracks were observed around the fibers, producing pores or cavities in the concrete matrix. These results are similar to those reported by Asim et al. [4]. In the microstructure of concrete with natural fibers, there are pores that improve the material thermal insulation (0.026 W m⁻¹ K⁻¹). As the fiber content increased in the concrete matrix, the number of pores increased; simultaneously, the volume of trapped air increased, resulting in better thermal insulation. In contrast, coconut fibers are composed of biopolymers, such as cellulose, lignin, and hemicellulose. Lignin creates a thermal barrier that reduces the heat flow passing through a concrete matrix.

Table 6. Results of elemental analysis.

Samples	C (%)	H (%)	N (%)
Manure	28.2	6	2
Coconut Fiber	52.7	4.3	0.22

shows the results of the elemental analyses of both materials. The fibers have a higher carbon content and lower percentages of hydrogen and nitrogen compared to the manure. Other components include lignin and cellulose [41]. The carbon and hydrogen contents were similar to those of rice husk, providing adequate biofuel conditions. The low nitrogen contents of these two materials result in lower NOx emissions when used as biofuels in furnaces and boilers [42].

3.5. Specimens Thermal Conductivity

Figure 6 illustrates the decrease in thermal conductivity upon the addition of coir fiber and bovine manure, with the samples designated as F0, F0.5, F1, and F1.5 representing fiber percentages of 0%, 0.5%, 1.0%, and 1.5%, respectively. Despite the limited variation in sample density and blends, the newly manufactured blocks exhibit relatively low thermal conductivity, ranging from 0.88 W/mK (B M10-F1.5) to 1.66 W/mK (control sample). This decrease represents approximately 50% compared to the reference sample. In the figure, the thermal conductivity of B M0-F1.5 is 1.34 W/mK, reflecting a 20% reduction compared to the heat conduction capacity of the control block and is significantly close to the results reported by Zhang et al. [43].

It is important to note that the technical building standards do not specify thermal conductivity values for individual blocks but for the complete wall system, which includes the block, finishes, and thermal insulation. The standards only state that the block’s thermal conductivity should be as low as possible. However, the literature indicates that blocks’ thermal conductivity can vary between 0.3 and 1.9 W/mK, so all the blocks analyzed in this study are within this acceptable range [44].

3.6. Compressive Strength of the Studied Vibro-Compacted Blocks

Figure 7 presents the compressive strengths of the different mixes. Notably, the blocks with added coir fiber, specifically B M0-F1.0 and B M0-F1.5, demonstrated a significant 5% and 15% increase in compressive strength, respectively, surpassing that of the control block. The vibro-compacted blocks B M0-F0.5, B M0-F1.0, B M0-F1.5, and B M3-F1.5 all met the compressive strength requirements outlined in ASTM C129-17 [45], validating the effectiveness of the mixing process. However, a reduction in compressive strength was observed for various fiber/mulch/concrete mixes. This reduction can be attributed to the development of strength properties in the fiber/manure/concrete mix, which primarily depends on the formation of reinforcement–matrix, matrix–matrix, and reinforcement–reinforcement bonds. Bonding is affected by the dimensions, surface conditions, and percentages of fiber and manure present in each volume of the composite material. Therefore, for the mixtures used in the experiment, the variation in the fraction of the constituents resulted in a decrease in the bond strengths of the specimens, leading to lower compressive strengths. The improvement of compressive strength is not always assured when a fiber is used as a reinforcement material, as highlighted by M. Benzerara et al. [46]. Their study reveals that the incorporation of fibers can sometimes decrease the compressive strength, with the values dropping from 3.3 MPa for extruded flat blocks to 1.2 MPa for fiber-reinforced blocks. In a similar study, the use of palm fibers for reinforcement improved the tensile strength of earth-based composites, where a 12% increase in tensile strength was reported for reinforcement with 0.75% palm fibers. In the case of jute and coconut fibers with 2.5% reinforcement in concrete, the improvements in ultimate compressive strength were 6.7 and 3.7%, respectively, and the corresponding improvements in thermal insulation were 2.6 and 6.5%, respectively [4]. The inclusion of fibrous material improves mechanical strength; however, values higher than 10% impair the performance of adobes owing to the high presence of organic matter in the material. High shrinkage was observed in these compositions owing to the absence of sand in the blocks [47].

Water absorption in the blocks with different proportions of coconut fiber or manure was higher compared to the percentage of absorption in the control block. Figure 8 shows that this increase is due to the replacement of the volume percentage of a mineral, such as sand, by organic compounds sensitive to the effects of humidity, such as cattle manure and coconut fiber. When the prepared mix was wetted, the amount of water remaining was limited because the coconut fiber and bovine manure absorbed a large percentage. The vibro-compacted blocks had a dry mix to achieve better compaction and mitigate pore generation, thereby reducing water retention and increasing the percentage of water absorption.

3.7. Analysis of Variance (ANOVA)

The ANOVA analysis revealed that the percentage of bovine manure has the most significant impact on the thermal conductivity of the blocks, being statistically substantial, along with the quadratic interaction of bovine manure–bovine manure and the percentage of fiber. These results indicate that the manure content and its quadratic effect significantly influence thermal conductivity, while the fiber also plays an important role. Although the fiber–fiber and fiber–manure interactions showed positive effects, they were not statistically significant, suggesting their influence was low. However, this is not decisive for the thermal behavior of the material.

For compressive strength, the percentage of bovine manure was again the most influential variable, being statistically significant along with the quadratic interaction of bovine manure–bovine manure and the percentage of fiber. The significance of these variables underscores that both the manure content and its quadratic interaction, along with the amount of fiber, are crucial for enhancing the load-bearing capacity of the blocks. The bovine manure–bovine manure interaction and the percentage of fiber presented positive effects, suggesting that increases in these components improve the compressive strength of the blocks, confirming their importance in the design of construction materials.

Regarding water absorption, the percentage of bovine manure again stood out as the variable with the greatest effect and was statistically significant, along with the percentage of fiber. These factors are crucial in determining the blocks’ capacity to absorb water, essential for evaluating the material’s durability in humid environments. Additionally, the percentage of bovine manure and the interactions between manure–fiber and fiber–fiber showed positive effects on water absorption, indicating that these components increase absorption capacity. However, it is essential to consider the potential negative impact on structural integrity and compressive strength. This analysis highlights the need to balance physical and mechanical properties when optimizing block formulations properly. Figure 9 presents the statistical effect of the percentage of manure, the rate of fiber, and their interactions on thermal conductivity, compressive strength, and water absorption.

3.8. Response Surface Optimization

The multi-objective optimization using the response surface methodology revealed that the optimal combination of components to maximize the desired properties of the blocks is achieved with a high fiber content and a low manure content. This finding aligns with the need to balance the material’s thermal, mechanical, and water absorption properties. Specifically, the analysis determined that the optimal point is achieved with a mixture containing 1.5% fiber and 0% manure, reaching a desirability of 0.64.

The high fiber content in the optimal mixture suggests that this component significantly enhances the material’s mechanical and thermal properties. The fiber reinforces the block matrix, increasing compressive strength and possibly reducing thermal conductivity by acting as an insulator. The absence of manure in the optimal mixture indicates that while this component may positively affect certain aspects, such as water absorption, it is not favorable when seeking an optimal balance of all the considered properties. Manure could increase the block’s porosity and water absorption capacity due to its organic nature, which could deteriorate its mechanical strength and dimensional stability. Figure 10 shows the contour plot of the response surface of the desirability function constructed in the analysis.

A comparative analysis of these results, particularly for block B M0-F1.5, which exhibited the specific thermal conductivity and compressive strength, also demonstrated favorable water absorption characteristics. Block B M0-F1.5 meets the requirements established in the ASTM C90-16 standard [48] and is classified as a light block with a density of 1600 kg m⁻³, water absorption percentage of 7.83%.

Table 7. Blocks parameters.

Block	Control Block	B M0-F1.5
Water absorption (%)	6.99	7.83
Compressive strength (MPa)	5.66	5.93
Thermal conductivity (W K−1 m−1)	5.17	1.33

lists the descriptions of the control blocks and B M0-F1.5 blocks used in this study.

4. Conclusions

Vibro-compacted blocks and plates with varying percentages of coconut fiber (0.5, 1, and 1.5%) and bovine manure (3, 6, and 10%) were prepared to measure the thermal conductivity. A proximate analysis evidenced that coconut fiber had a residual moisture content, ash content, volatile matter content, fixed carbon content, sulfur content, and calorific value of 5.62%, 1.48%, 72.05%, 20.85%, 0.08%, and 18,966 kJ/kg, respectively. Manure analysis results showed a residual moisture content, ash content, volatile matter content, fixed carbon content, sulfur content, and calorific value of 11.78%, 31.71%, 44.76%, 11.75%, 0.26%, and 11,914 KJ/kg, respectively. In the elemental analysis, coconut fibers showed 52.7, 4.3, and 0.22% of carbon, hydrogen, and oxygen, respectively, whereas manure had 28.2, 6, and 2% of carbon, hydrogen, and oxygen, respectively. In thermal conductivity tests, a 50% reduction was observed for B M10-F1.5 compared with the control block, indicating a significant reduction. Furthermore, compression tests revealed that blocks containing only manure, 6% manure with coconut fiber, and 10% manure with coconut fiber experienced an almost 90% reduction in strength, thereby failing to meet any standard. In contrast, blocks containing 1 and 1.5% coconut fiber exceeded the compressive strength of the control block by 5 and 15%, respectively, complying with the ASTM C129-17 standards. The B M0-F1.5 mixture met the ASTM C129-17 standards for strength, with a thermal conductivity ~20% lower than the control. During block manufacturing, blocks with over 10% manure revealed poor compaction, leading to crack formation. Extraction of more sand from the mixture resulted in a remarkable decrease in strength when the percentage of manure was increased.

The results suggest that the use of coconut fiber mixed with cattle manure as a substitute for sand would be possible in the range of 1.5 manure and 1% fiber because the reduction of compressive strength to the 5 MPa limit established for non-structural wall elements also attained a decrease in thermal conductibility of around 10%, enabling its application in the exterior walls of multi-story buildings and arrangements, which most determine the thermal load due to the sun’s action and heat gain in habitable spaces.

This research demonstrates the viability of substituting around 2.5% of the sand in manufacturing vibro-compacted blocks for non-structural applications in the specific conditions of the Córdoba Department in Colombia; through use of coconut fiber and cattle manure as additives, a 10% reduction in wall transmittance was achieved, which would contribute to building energy performance. More research is needed to standardize the mixed and manufacturing procedure for actual application to ensure more precise mix proportions. Innocuity and other practical issues related to logistics, manipulation, and the market should also be addressed.