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Article

Effect of Incorporating Cement and Olive Waste Ash on the Mechanical Properties of Rammed Earth Block

by
Hassan Ghanem
1,*,
Chouk El Bouz
1,
Rawan Ramadan
1,
Adrien Trad
2,
Jamal Khatib
1 and
Adel Elkordi
1,3
1
Faculty of Engineering, Beirut Arab University, Beirut 12-5020, Lebanon
2
Masonry and Structure Department, French Building Federation, 75016 Paris, France
3
Department of Civil and Environmental Engineering, Faculty of Engineering, Alexandria University, Alexandria 5424041, Egypt
*
Author to whom correspondence should be addressed.
Infrastructures 2024, 9(8), 122; https://doi.org/10.3390/infrastructures9080122
Submission received: 14 June 2024 / Revised: 17 July 2024 / Accepted: 23 July 2024 / Published: 25 July 2024
(This article belongs to the Section Infrastructures Materials and Constructions)
Browse Figures
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Abstract

:
Rammed earth blocks have recently gained substantial popularity in construction materials due to their environmental benefits, energy saving, and financial effectiveness. These benefits are even more pronounced if waste materials such as olive waste ash (OWA) are incorporated in rammed earth blocks. There is limited information on the use of OWA in rammed earth blocks. This paper investigates the use of OWA and cement in improving rammed earth block characteristics. OWA was incorporated to partially replace the soil by 10, 20, 30 and 40% of its weight and cement was added in percentages of 2, 4, 6 and 8% by the dry weight of the composite soil. Proctor, unconfined compressive strength (UCS), and California Bearing Ratio (CBR) tests were performed at 7, 28, and 56 days. Results indicated that OWA inclusion decreased the maximum dry density while it increased the optimum moisture content. However, cement addition improved the maximum dry density of soil. The UCS results revealed that OWA possessed cementitious and pozzolanic behavior, and soil mechanical properties improved by up to 30% due to OWA inclusion, after which there was a significant drop of 40%. The trend in the CBR results was similar to those of UCS. To further clarify the experimental results, a mathematical model was proposed to determine the variation in strength as a function of time. Furthermore, correlations between soil mechanical properties were conducted. Predicted equations were developed to determine the properties of rammed earth block. All in all, the inclusion of OWA in cement stabilized earth block suggests the potential to improve the properties of rammed earth blocks.

1. Introduction

Rammed earth block construction is an ancient yet enduring architectural method that spans across many cultures for centuries. In recent years, with rising concern for the environment and the environmental impact of materials typically utilized in modern building construction, rammed earth has become a good candidate for future infrastructure materials [1,2,3]. Grounded in simplicity and resource efficiency, rammed earth construction involves the compression of a mixture of earth materials into solid blocks or walls. The materials used are commonly available but locally sourced, including sand, gravel, clay, and sometimes a bit of stabilizers like cement [4,5,6,7,8,9]. Its structures are exceptionally durable and exhibit thermal mass properties and an aesthetic appeal which the natural environment perfectly complements. The renewed interest in rammed earth construction is due to several factors (sustainability, energy efficiency, cost-effectiveness and resilience) [10,11,12,13,14]. Since its typical making, unlike concrete which has a carbon footprint, the low carbon footprint of rammed earth is due to the abundant locally available materials and low manufacturing energy [15,16,17,18,19]. Furthermore, the thermal mass in rammed earth also reduces the need for mechanical heating and cooling systems, making it energy efficient [20,21,22,23,24].
The different rammed earth block stabilization methods are presented in Figure 1. As shown, the major stabilization approaches are mechanical and chemical, based on the construction process. Mechanical stabilization focuses on reducing soil voids, while chemical stabilization enhances strength and decreases permeability [25]. To achieve this, several stabilizing additives and reinforcements have been used to improve the rammed earth mechanical properties, such as lime, fly ash, granitic residual soils, textile reinforcement, and fabric strips [26,27,28,29,30,31,32,33,34]. For example, Liu et al. [32] studied the performance of external canvas, bamboo, and tarpaulin fibers with three different adhesives. The report conclusion showed that the use of tarpaulin fibers, sodium silicate, and hardener could increase the load capacity by 38% and maximum horizontal displacement by 75% in the retrofitting of rammed earth walls. Also, fly ash and calcium carbide residue were used to stabilize rammed earth [33]. The results of the chemical stabilization study were remarkably effective in changing the properties of the substance. In addition to that, Araldi et al. [34] studied recycled concrete aggregates as additives for rammed earth, and it was found that they could decrease cement need.
Furthermore, several studies focused on enhancing seismic resilience and sustainability in rammed earth construction by various techniques of retrofitting or reinforcement. For example, Sen and Saha., [35] investigated the seismic performance of rammed earth block using typical Tripura soil and Bamboo Fiber. The results showed an increase in structural strength and ductility by many folds. The findings indicate a reduction by up to 46.67% in the lateral periods and an increment of 4.50 to 6.31 times in seismic strength when compared with the control mix. In addition, a study on retrofitting using natural and artificial fibers demonstrated significant improvements in mechanical strength through empirical equations [36]. The research showed notable enhancements in shear, compressive, and flexural strengths achieved by adding treated fiber strips [36]. Also, Koutous and Hilali., [37] investigated the mechanical performance of rammed earth reinforced with plant fibers. It was shown that such plant fibers, such as barley straw and date palm, greatly increased the tensile strength and changed the stiffness of rammed earth. Ramezannia et al. [38] carried out a life cycle assessment (LCA) in Australia and observed great savings on embodied energy and greenhouse gas emissions by replacing the cement in stabilized rammed earth construction with natural fibers. Results indicated that the potential savings would be from 37% to 49.7% of the highest sensitivity of fossil fuel used. Another study reported the behavior of seismic fiber-reinforced rammed earth walls [39]. It was shown that, during seismic loading, the jute and straw fiber reinforcements provided the structure with significant strengthening. In addition to the seismic resilience studies, a study was conducted on a comparative analysis of geopolymer and cement solidification of organic clay [40]. The study found that geopolymer significantly enhanced compressive strength and durability, as tested through microscopic analysis. Geopolymer-treated clay exhibited up to 1.92 times higher unconfined compressive strength (UCS) compared to cement-treated clay, highlighting its effectiveness in sustainable soil stabilization.
Olive waste ash (OWA) is obtained from burning a wide range of olive by-products. These include olive seeds, pulp, pruning residues, and other residuals from olive oil production. It has shown potential as a stabilizing agent in various construction applications due to its cost-effectiveness, abundance, and eco-friendliness. For example, Al-Akhras and Abdulwahid. [41] used OWA as a substitute for both Portland cement and mortar sand. In their study, they found an increase in the mechanical strength of the mixture with the substitution of sand by OWA. For the replacement of sand by limestone sand, compressive strength increased by 9%, 22%, 35%, and 37% for 0%, 5%, 10%, and 15% in the material featuring replacement OWA content, and the flexural strength rose by 5, 4, 13 and 13% for the same replacements, respectively. In another study conducted by Al-Akhras et al. [42], the effects of OWA on the behavior of concrete under high temperatures were examined. The obtained results stated that the performance of OWA concrete at high temperatures was better than the performance of the control concrete. The effect of adding OWA in high-strength geopolymer concrete (HSGC) through partial replacement of fly ash (FA) and/or granulated blast furnace slag (GBFS) in the presence of rice husk ash (RHA) was examined [43]. In that study, it was observed that the utilization of 20% OWA with FA brought about an increase of 8.9% in compressive strength after 28 days. The same applied to the substitution of 30% OWA with GBFS and 30% OWA with a combination of GBFS and FA, which resulted in an increase in compressive strength of 20% and 17.8%, respectively. OWA also showed an impact on the microstructure density of the HSGC samples. The optimal substitution ratio ranged from 20% to 30% by weight [43]. Another study investigated the replacement of cement in high-strength concrete (HSC) at percentages of rice husk ash (RHA) and olive waste ash (OWA). The optimal dosages were 20% RHA and 5% OWA. The result showed that this application increased the compressive strength by around 58.7% [44]. In addition, the effects of using OWA on the physical and mechanical characteristics of the concrete mixture were investigated [45]. Results indicated that the use of 15% OWA gave low compressive strength and workability but increased the durability against various weather conditions and high temperatures (~170 °C).
Furthermore, the impact of OWA on asphalt cement and asphalt concrete mixtures has been studied [46]. Limestone and valley gravel were mixed with asphalt cement (80/100) and OWA. The physical aspects of the asphalt binder regarding OWA were studied through five phases of the asphalt binder mixed with OWA at different volumetric proportions of OWA (0%, 5%, 10%, 15%, and 20%). Results revealed that the Marshall stability test was similar to that of the control asphalt at 10% OWA in terms of the impact of OWA on asphalt binder, but the retained Marshall stability increased with the increased amount of OWA. However, the dynamic modulus decreased with the increased OWA proportion, temperature, and loading frequency for both aggregates [46]. In another study, the effect of adding Olive Husk Ash (OHA) on the properties of asphalt concrete mixtures was investigated [47]. The major findings show that the incorporation of OHA as filler in the asphalt binder enhances Marshall stability and reduces the voids in the mineral aggregate, flow, retained stability, stiffness, and retained stiffness when 10% to 15% of the asphalt binder is substituted with OHA [48].
To summarize, there are many studies on the use of OWA in mortar, concrete, and asphalt concrete [49,50]. However, to the best of the authors’ knowledge, the incorporation of OWA in rammed earth block has not been investigated. This paper aims to address this gap in this area. Therefore, in this study, the mechanical properties of rammed earth blocks stabilized using cement and OWA were investigated. Initially, the soil was replaced by 10, 20, 30 and 40% of its weight by OWA, and cement was then added in percentages of 2, 4, 6 and 8% by the dry weight of the composite soil. Accordingly, the Proctor test and the UCS and CBR tests were conducted on the stabilized soil. As a result, the optimal binder content for the stabilized rammed earth block was determined.

2. Experimental Program

2.1. Materials

2.1.1. Soil

The soil utilized in this study was obtained from the Menyeh Area within the north district of Lebanon. It is classified under the unified soil classification system (ASTM D 2487) [51] as poorly graded sand (SP). The physical properties of this soil are depicted in Table 1, and the particle size distribution of soil is shown in Figure 2.

2.1.2. Cement

The cement in this study is used as a stabilizer to improve the soil. It was obtained from Alsabeh, Lebanon, and has been recognized as PA-L 42.5. This cement meets both European requirements (CEM II/A-L), established under EN 197, and Lebanese regulations, as specified by LIBNOR. The chemical properties of cement are illustrated in Table 2.

2.1.3. Olive Waste Ash (OWA)

OWA was obtained from Koura region in North Lebanon. The production of OWA follows several steps. First, olive pomace wastes, a residue from the extraction of olive oil, were collected from olive presses. These collected olive pomaces were boiled for 8 h. After cooling, the ashes were grounded by a Los Angeles machine and then sieved. Materials collected are those passing through sieve No.4 and retained at No. 200. OWA has a density and specific gravity of 670 kg/m3 and 2.16, respectively. The particle size distribution and chemical properties of the cement and OWA used are presented in Figure 2 and Table 2, respectively.

2.2. Mix Proportions

The aim of this research is to maximize the use of OWA in construction application, since it is a waste material. Some researchers have used up to 50% waste material as a soil replacement [52,53]. Generally, cement is added up to 10% by weight of dry soil in order to improve the properties of the soil [54,55]. In this investigation, twenty-four different soil mixes were prepared to assess the suitability of cement and OWA in rammed earth block application. Different combinations of OWA and cement were employed. The soil was partially replaced by weight by different OWA percentages (10, 20, 30 and 40%), and cement was added in percentages of 2, 4, 6, and 8% by weight of the composite dry soil (soil + OWA). The abbreviations S for soil, C for cement, and OWA for olive waste ash were used to label the samples, followed by a number labeling the content of the material included. For example, S-C2-OWA10 denotes that 10% of soil was replaced with OWA and 2% of cement was added to the composite dry weight of soil. The experimental plan is presented in Table 3. A flow chart illustrating the procedure is displayed in Figure 3.

2.3. Sample Preparation and Testing Methods

2.3.1. Sample Preparation

The soil and OWA were dried in the oven at 100 °C for 24 hrs until a constant weight was achieved. After that, the dry materials (i.e., soil + OWA) and the cement were mixed until a homogeneous mix was achieved. Water was then added slowly to the dry mixtures until homogeneity was obtained.

2.3.2. Compaction Test

Standard Proctor compaction tests were performed on all mixes to determine their compaction characteristics (Maximum Dry Unit Weight (MDUW) and Optimum Moisture Content (OMC)) in accordance with ASTM D 698–12 [56]. The process involved placing the soil with a chosen water content into a cylindrical metal mold in three layers of approximately equal height. Each layer was compacted using an automated proctor rammer, dropped from 30.5 cm, with 25 blows per layer (see Figure 4a). The mold, with dimensions of 4 inches (10.2 cm) diameter, 4.58 inches (11.63 cm) height, was used. After compaction, the blocks were extracted from the molds using a manual extruder (see Figure 4b). A representative sample of 500 g was placed in the oven at 110 °C for 24 hrs to determine the water content.

2.3.3. Unconfined Compressive Strength (UCS)

Testing method for the UCS was completed in accordance with ASTM D2166 [57]. In order to meet the 2:1 ratio required for this test, sample specimens measuring 8 inches (20.3 cm) in height and 4 inches (10.1 cm) in diameter were prepared at the OMC. The samples were compacted in five layers, with each layer being compacted by a rammer dropped from a distance of 18 inches (45.7 cm). Then, the samples were removed from the molds with the help of an extruder (Figure 4b) and were then wrapped in plastic cling-wrap and stored in an environmentally controlled storage room until their scheduled date of testing (7, 28, and 56 days). (Figure 5a). It should be noted that the base plates were aligned before testing and the surfaces of the specimen were horizontal. The UCS (Figure 5b) is obtained by dividing the maximum axial load by the cross-sectional area of the rammed earth block, as follows:
σ U C S = P max A 0
where σ U C S is the unconfined compressive strength, Pmax is the maximum load, and A 0 is the initial cross section area. Three replicate specimens were prepared for each mix, and the average value was taken.

2.3.4. California Bearing Ratio (CBR)

The California Bearing Ratio test for rammed earth blocks was performed according to the ASTM D1883 standard [58] and was very crucial in assessing the strength and load-bearing capacity of the material. The data collection was conducted at 7, 28, and 56 days. Cylindrical molds with a diameter of 6 inches (15.24 cm) and a height of 7 inches (17.8 cm) were used. During the CBR test, a specified plunger or probe was employed to penetrate the rammed earth block samples, while a mechanical loading device applied gradual and continuous load to measure resistance encountered during penetration, as shown in Figure 6. The CBR is calculated as follows:
C B R = P P s × 100
where:
  • P = measured pressure for site soils [N/mm2]
  • PS = pressure to achieve equal penetration on standard crushed stone [N/mm2]
It should be noted that, during the CBR testing, it was observed that the inclusion of 6% and 8% cement as a stabilizer achieved CBR values greater than 100%. Therefore, the authors have decided to exclude the CBR results for mixes containing 6% and 8% cement.
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Figure 6. CBR test: (a): CBR testing machine; (b) sample before CBR test; (c) sample after CBR test.
Figure 6. CBR test: (a): CBR testing machine; (b) sample before CBR test; (c) sample after CBR test.
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2.3.5. UCS Data Analysis for Rammed Earth Block

Several models for predicting UCS values using artificial intelligence (e.g., Gaussian Process Regression, Adaptive Neuro Fuzzy Inference System, Artificial Neural Network and Gene Expression Programming) exist [59,60,61,62,63,64]. However, there are hardly any models to describe the changes in the UCS over time. Construction and geotechnical engineers are interested in many parameters that can characterize the UCS of rammed earth blocks. Those parameters are the initial UCS rate (I-UCS) and the ultimate UCS (U-UCS). The I-UCS helps the engineer to have a quick idea about the strength of rammed earth block in the first couple of days after compaction. The U-UCS is also a crucial parameter. It indicates the maximum expected value of UCS after reaching a stable cementitious reaction level. To analyze those essential parameters, and to determine the UCS with time, a hyperbolic model (Figure 7) is used, as follows [65,66,67,68]:
U C S = t 1 a + t b
where:
  • UCS = unconfined compressive strength (kPa);
  • T = time (days);
  • a = I-UCS (kPa/day);
  • b = U-UCS (kPa).
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Figure 7. UCS parameters.
Figure 7. UCS parameters.
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To find out the effect of each component in the mix and its percentage on the initial strength gain (I-UCSi) and the ultimate strength (U-UCSi), a regression analysis was performed. The regression parameters were determined using Microsoft Excel V2021 Software. The results were then normalized to determine the contribution of each component. Multiple linear regression equations for that analysis are shown below:
U-UCSi = a0 + a1. (%Ci) + a2. (%OWAi) + a3. (%C × %OWAi)
I-UCSi = b0 + b1. (%Ci) + b2. (%OWAi) + b3. (%C × OWAi)
where
  • U-UCSi = Ultimate Strength for mix i (i = 1, 2, … 15);
  • I-UCSi = Initial Strength Gain for mix i (i = 1, 2, … 15);
  • %Ci = % cement in mix i (i = 1, 2, … 15);
  • %OWAi = % of olive waste ash in mix i (i = 1, 2, … 15);
  • a0, a1, a2, a3 = regression parameters for the ultimate strength;
  • b0, b1, b2, b3 = regression parameters for the initial strength gain.

3. Results

3.1. Compaction Characteristics

The Proctor test results are depicted in Figure 8. The variation in MDUW and OMC of soil with cement (S-C) is shown in Figure 8a. As inferred, MDUW slightly increases with the addition of cement while reducing the OMC for all specimens. Specifically, using 8% of cement caused the MDUW to increase from 18.1 kN/m3 for compacted pure soil to 18.9 kN/m3. However, for the same cement content, the OMC of pure soil decreased from 16.4% to 14.8%. Previous studies confirm this deduction [69,70]. This is attributed to the fact that water used in hydration reduces the free water needed for reaching the maximum compaction. However, the MDUW increases due to the cement particles filling the voids between the soil particles, thus making the packing of the mixture denser [70]. These densification effects become more significant at a high cement content (8%) since the additional cement provides better binding and reduces overall voids, thus increasing the MDUW [71,72,73].
Furthermore, the compaction characteristics of soil with olive waste ash (OWA) are illustrated in Figure 8b. The observed increase in OMC and decrease in MDUW when soil is partially replaced by OWA is well documented. For example, the MDUW of pure soil is 18.1 kN/m3. This value gradually drops with the addition of 10%, 20%, 30%, and 40% OWA, where it achieves the lowest MDUW at 40% OWA (14.4 kN/m3). In terms of OMC, it increases from 16.4% in pure compacted soil to 20% with the incorporation of 40% OWA (S-OWA40). These findings can be attributed to the physical and chemical properties of OWA. In fact, OWA has a high absorption capacity (~30%), being lighter and more porous compared to soil. This leads to an increase in the amount of water required to reach optimal compaction. Furthermore, the lower density of OWA compared to soil results in a reduction in the overall dry density of the mix [42,43]. As the proportion of OWA increases, the mix becomes less dense. This is due to the additional void spaces in OWA within the compacted structure, which are not fully occupied by soil particles [45]. This increase in void spaces and the overall lighter nature of OWA contribute to the observed decrease in MDUW. Additionally, the different gradation of OWA can disrupt the soil matrix, leading to a less efficient packing of particles, which further reduces the MDUW while increasing the OMC due to the higher water demand to fill the voids and achieve lubrication during compaction [47].
The compaction test results for mixes (S-C-OWA) are illustrated in Figure 8c–f. The combination of cement with OWA tends to decrease the MDUW while increasing the OMC for all mixes. For example, the MDUW for soil with 4%C (S-C4-OWA0) is 18.6 kN/m3. This value goes down to 17.51, 16.6, 15.7 and 14.8 kN/m3 in S-C4-OWA10, S-C4-OWA20, S-C4-OWA30 and S-C4-OWA40 mixes with an OMC of 16.5, 17.5, 18.5%, and 19.5%, respectively. Among all (S-COWA) mixes, the lowest MDUW is achieved for a mix with 2% cement and 40% OWA (S-C2-OWA40), where it records 14.72 kN/m3. As elucidated, the combination of soil, cement, and OWA changes the parameters in the OMC and MDUW. This is because of the interaction of the properties and the synergistic effects of the materials [42,43,44]. OWA, with its high absorption capacity (~30%) and lower specific gravity, increases the OMC because it absorbs a substantial amount of water and requires more moisture to achieve optimal compaction [66,69]. Additionally, cement consumes water and forms cementitious bonds that improve the density and stability of the mixture. In the case of the mixture with soil and cement, the OMC further increases with the addition of OWA because the absorption characteristics of OWA dominate the requirements of moisture [71,72]. On the other hand, the MDUW decreases in comparison with the mix of soil and cement. This is due to the addition of OWA, which introduces more voids and reduces the overall density. Therefore, with the increase in the proportion of OWA, the mixture becomes lighter and demands more water, leading to a higher OMC and lower MDUW compared to the soil–cement mixes [73,74].

3.2. Unconfined Compressive Strength (UCS)

The UCS development over time for all rammed earth block mixtures is plotted in Figure 9. Also presented in the plots are the calculated UCS values using the mathematical model presented earlier. As seen from the plots, the model fits to the measured data are very encouraging for all mixes, with minimal differences between the measured and predicted UCS values. To demonstrate the accuracy of the proposed model, the coefficient of determination R2 was determined. Results are presented in Table 4. As shown, R2 values exceed 0.99 for all mix combinations, ensuing the model accuracy in predicting UCS values.
As expected, the addition of cement significantly enhances the UCS at all times of curing, as shown in Figure 9a. For instance, the UCS for pure compacted soil is 79 kPa after 56 days, which is in agreement with results reported elsewhere [75,76]. This value increases to 530, 1186, 1315 and 1553 kPa with the integration of 2, 4, 6 and 8% cement, respectively. This is equivalent to a percentage increase of 570.9, 1401.27, 1564.56 and 1865.82%, respectively. The addition of cement increases the UCS of the soil essentially by the process of soil stabilization. In fact, cement, acting as a stabilizer, reacts with water in the soil through hydration, forming cementitious bonds to hold soil particles together. This further improves cohesion, decreases voids, and changes soil properties such as strength and stability [34,41,69,77].
The effect of OWA on mixes containing pure soil is interesting to notice. Results are presented in Figure 9b. As shown, the UCS values go up as OWA is added to the mix. For example, the UCS values increase from 79 to 382, 535, 764 and 866 kPa for mixes containing 10, 20, 30 and 40% OWA, respectively. This indicates that OWA possesses some cementitious behavior. In fact, the chemical composition of OWA contains 36.1% CaO, which can react with water to form hydrated gels, such as calcium aluminate silicate (C-A-S-H), portlandite (C-H), and calcium silicate hydrate (C-S-H), which appear at later ages of curing, resulting in the high strength of the matrix [78,79,80].
UCS results for mixes containing cement and OWA shown in Figure 9c–f display similar characteristic patterns. It can be noticed that that the UCS values increase when OWA is incorporated in the mixes containing cement by up to 30%. This was followed by a significant drop at 40% OWA. For instance, at 56 days, the UCS value for the S-C2-OWA0 mix is 530 kPa. This value goes up to 1155, 1651 and 2655 kPa with the incorporation of 10%, 20% and 30% OWA. This increase is equivalent to 118%, 211.51%, and 400.94%, respectively. Then, the UCS value drops to 2019 kPa at 40% OWA. This trend was similar at all levels of cement additions (4, 6 and 8%). This may be attributed to OWA having a pozzolanic behavior in addition to the cementitious behavior mentioned previously. This can be deducted by comparing mix S-C2% with S-C2-OWA10, S-C2-OWA20 and S-C2-OWA30, where the UCS keeps going up to 30% OWA replacement. Therefore, it appears that, at 30% OWA replacement, the pozzolanic reaction is optimal, as the calcium hydroxide (Ca(OH)2) resulting from cement hydration reacts with OWA in the production of further calcium silicate hydrate (C-S-H) [45,69,78], thus leading to strength enhancement. However, at 40% OWA replacement, the excessive ash, with its much lower density, may play the role of filler, bringing about insufficient cementitious material and a less cohesive microstructure that can cause a drop in strength [74,77,78]. The set of reaction between the mix ingredients is illustrated in Figure 10.

3.3. UCS Parameters

To further illustrate the experimental findings, initial strength rate (I-UCS) and ultimate strength (U-UCS) values were determined using the proposed hyperbolic model. Results for all the mixes are shown in Figure 11. All data presented are consistent with the experimental UCS results. As expected, the presence of cement increases the initial and ultimate strength of rammed earth block, as cement binds soil particles together to form a solid matrix. As seen in Figure 11a, the I-UCS and U-UCS are 47, 200, 394, 467, and 587 kPa/day and 83, 530, 1223, 1522, and 1584 kPa for mixes S, S-C2, S-C4, S-C6 and S-C8, respectively.
The I-UCS and U-UCS of soil with various percentages of OWA are displayed in Figure 11b. It can be seen that both I-UCS and U-UCS increase as OWA percentages goes up. The maximum value of both parameters is observed for 40% OWA replacement (465 kPa/day and 891 kPa, respectively), associated with an increase of 889.4% and 973.5%, respectively, compared to mix (S-OWA0). This can signal to the fact that OWA contains some cementitious behavior. By comparing mixes containing OWA with mixes containing cement (S-C versus S-OWA), it can be noticed that the I-UCS and U-UCS are lower for mixes containing OWA. Thus, cement is the main contribution to the initial and ultimate strength.
For mixes containing cement and OWA (Figure 11c–f), it can be seen that the I-UCS and U-UCS increase as OWA is added to the mix by up to 30%, after which there is a significant drop at 40%. For example, for a mix with 6% cement (Figure 11e), the I-UCS records 467, 964, 1089, and 1752 kPa/day, and U-UCS achieves 1522, 2113, 2521, and 2672 kPa for S-C6-OWA0, S-C6-OWA10, S-C6-OWA20, and S-C6-OWA30, respectively. These increases represent 107.1, 133.2, and 275.2% and 38.8, 65.6, and 163.5, respectively. However, at 40% OWA, the I-UCS and U-UCS decreases to 1154 kPa/day and 2672 kPa. This pattern is observed at other cement levels (2, 4 and 8%). This behavior suggests that OWA possesses pozzolanic properties in addition to its cementitious characteristics, and, at 30% OWA replacement, the pozzolanic reaction is optimal. OWA was found to be a pozzzolanic material when used as a partial cement replacement in concrete [72,78,79].

3.4. UCS Regression Analysis

A statistical analysis on the 24 mixes was conducted to shed light on the study’s findings. Figure 12 shows the contribution of each component on the initial strength gain and ultimate strength. As depicted, the combined effect of cement and OWA contributes 90% of the initial strength rate, whereas the contribution of cement and OWA represents 9% and 1%, respectively. This can be related to the synergistic effect of C&OWA that accelerates the initial strength gain, possibly due to the cementitious reaction that takes place between cement, OWA, and water. From the dominance of cement in contributing to the early strength, it follows that it acts as the primary binder in the early stages of hydration, while OWA supplements this action through the enhancement of the cementitious reaction, which leads to quick strength development [71,79]. The same trend is observed for the ultimate strength, whereby the combination of C&OWA contributes 57% of the ultimate strength and the remaining 38% and 5% are attributed to cement and OWA, respectively. This implies that the combination of cement and OWA continues to contribute to the reaction until the ultimate strength is gained. The above result indicates the importance of the combined C&OWA in enhancing both the initial and ultimate strength of the mixtures. The significant contribution of cement to the ultimate strength emphasizes its significance in providing long-term structural integrity, and the persistent contribution of OWA through the cementitious and pozzolanic behavior underlines its effectiveness as a supplementary material throughout the hydration process [41,44,71].

3.5. California Bearing Ratio (CBR)

CBR values at 7, 28, and 56 days are presented in Figure 13. For initial observations (Figure 13a), as cement content increases over time, there is a gradual improvement in CBR as well. Particularly, there is a significant elevation in CBR values at 56 days. For example, the CBR for soil is 25%; this soil is classified as SW since the CBR range is 20–40 according to the unified soil classification system (USCS) [51]. This value increases and reaches 31% and 59% for C2 and C4, respectively. This means that, with the addition of 4% cement, the soil classification has changed from SW to GW, illustrating the importance of using cement, which acts as a binder, filling the voids between soil particles and creating stronger bonds. This process leads to densification and aggregation of the soil particles, resulting in a coarser overall texture. Furthermore, the positive influence of extended self curing in reinforcing the CBR properties of rammed earth block emphasizes the importance of cement incorporation for structural enhancement.
As displayed in Figure 13b, the CBR values for S-OWA mixes show a consistent increase over time, with an increase in OWA content suggesting ongoing improvements in block strength. For example, at 56 days, the CBR for the control mix (soil) is 25%; this value rises to 41, 42, 46 and 47% for 10, 20, 30, and 40% OWA substitution. This highlights the significance of having OWA in the mix for improving soil structural integrity and enhancing rammed earth block performance [33,34].
Mixes comprising cement and OWA yield distinct patterns and yield higher CBR values compared to mixes with OWA only, as presented in Figure 13c,d. For example, at 56 days, the CBR values are 41, 42. 46 and 47% for S-OWA10, S-OWA20, S-OWA30 and S-OWA40, whereas they are 52, 59, 78, and 70% for S-C2-OWA10, S-C2-OWA20, S-C2-OWA30, and S-C2-OWA40, respectively. This rise in the CBR can be attributed to the collaborative effects of the pozzolanic properties of OWA, which enhance binding and cohesion within the composite. The pozzolanic reaction generates additional cementitious compounds as OWA content increases, positively influencing the soil–OWA matrix’s resistance to deformation. Similar to UCS, it was noticed that, beyond the optimum threshold of 30% OWA, a drop in the CBR may occur. This may be due to an excess of OWA compromising effective cement–soil particle interaction and inducing a dilution effect that diminishes overall cementitious properties.

3.6. Correlations between CBR and UCS

To measure and describe the strength between the CBR and UCS, a Pearson correlation analysis is conducted. The correlations for all mixes (S-C, S-OWA, S-C2-OWA and S-C4-OWA) are presented in Figure 14. As shown in Figure 14a, the Pearson correlation coefficient (PCC) for S-OWA is 0.5, signaling a moderate positive linear relationship, but it is not strong. The p-value is 0.0105, indicating that the relationship between the UCS and CBR is not statistically significant. On the other side, the PCC is 0.86, 0.94, and 0.85 for S-C, S-C2-OWA and S-C4-OWA mixes, respectively, indicating a solid correlation when cement and OWA are present together in the mixture (Figure 14b–d). The calculated p-values for the S-C, S-C2-OWA, and S-C4-OWA mixes are 0.028, 1.028 × 10−7, and 8.579 × 10−5, indicating that the correlations between the CBR and UCS are statically significant.
Overall, these correlations can be attributed to several reasons. The first one is the fact that both UCS and CBR are tests to determine the ability of the material to withstand different types of loads [81,82]. Since the fundamental load-bearing mechanism is the same, this automatically results in a strong correlation. Secondly, OWA, with its physical and chemical nature, possesses some pozzolanic behavior, which improves the cementitious properties of mixes. Therefore, the properties of stability, cohesion, UCS, and, subsequently, the CBR values are improved too [83].

4. Conclusions

The objective of this study is to investigate the effect of cement and OWA on the mechanical properties of rammed earth blocks. Tests conducted on the mixes are the standard Proctor test, UCS test, and CBR test. Based on this current investigation, several points can be mentioned:
  • The presence of cement and OWA altered the compaction characteristics of the mixtures. The Proctor test results showed that replacing soil with OWA reduced the MDUW but raised the OMC moisture. However, the cement addition improved the compaction characteristics of the soil. The highest values of MDUW and OMC were recorded in the S-C8 and S-OWA40 mixes, respectively.
  • Replacing soil with up to 40% OWA caused an increase in the UCS and CBR. This indicates that OWA possesses cementitious properties. Adding cement to the soil—OWA system increased the UCS and CBR by up to 30% OWA inclusion, after which there was a significant drop at 40% OWA. The rise in UCS and CBR of up to 30% OWA content may be attributed to its pozzolanic nature, facilitating binding and cohesion within the composite. However, beyond this threshold, UCS and CBR declined due to potential overfilling of voids and dilution effects.
  • The proposed hyperbolic model aligned well with the UCS experimental data for all mixes showing a correlation coefficient above 98%. The predicted initial strength rate (I-UCS) and ultimate strength (U-UCS) had maximum values at 30% OWA inclusion. The UCS statistical analysis showed that the combined effect of OWA and cement contributed to 90% of I-UCS and 57% of U-UCS.
  • A positive correlation was determined between the UCS and CBR, especially when cement and OWA were incorporated in the mixture (S-C-OWA). The coefficient of determination R2 ranged from 0.66 to 0.99.
  • All in all, this study showed that the soil, which is a primary resource, can be replaced with OWA, which is considered a waste material. If cement is added to the soil–OWA mixtures, the properties of the rammed earth blocks can be significantly improved.
  • The results of this investigation are applicable to poorly graded soil with OWA acting as an up to 40% replacement. Further studies could be carried out on other types of soil (e.g., clay) and other OWA replacement percentages. Also, using fibers can improve the soil stabilization properties.

Author Contributions

Conceptualization, H.G.; methodology, C.E.B., R.R. and H.G.; formal analysis, R.R. and H.G.; writing—original draft preparation, H.G. and R.R.; writing—review and editing, J.K., A.T. and R.R.; supervision, J.K. and A.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors dedicate this work to the memory of the late Diala Tabbal. She will be deeply missed.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Rammed earth block stabilization methods [25].
Figure 1. Rammed earth block stabilization methods [25].
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Figure 2. Particle size distribution of soil and OWA.
Figure 2. Particle size distribution of soil and OWA.
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Figure 3. Flow chart of the experimental program.
Figure 3. Flow chart of the experimental program.
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Figure 4. Compaction test: (a) standard proctor; (b) manual extruder; (c) representative sample.
Figure 4. Compaction test: (a) standard proctor; (b) manual extruder; (c) representative sample.
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Figure 5. (a) UCS wrapped sample; (b) UCS testing machine.
Figure 5. (a) UCS wrapped sample; (b) UCS testing machine.
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Figure 8. Compaction curves for soil: (a) S-C; (b) S-OWA; (c) S-C2-OWA; (d) S-C4-OWA; (e) S-C6-OWA; (f) S-C8-OWA.
Figure 8. Compaction curves for soil: (a) S-C; (b) S-OWA; (c) S-C2-OWA; (d) S-C4-OWA; (e) S-C6-OWA; (f) S-C8-OWA.
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Figure 9. UCS test results: (a) S-C; (b): S-OWA; (c) S-C2-OWA; (d) S-C4-OWA; (e) S-C6-OWA; (f) S-C8-OWA, Note: E = experimental data, C = calculated.
Figure 9. UCS test results: (a) S-C; (b): S-OWA; (c) S-C2-OWA; (d) S-C4-OWA; (e) S-C6-OWA; (f) S-C8-OWA, Note: E = experimental data, C = calculated.
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Figure 10. A mechanistic diagram displaying the set of reactions between mix ingredients.
Figure 10. A mechanistic diagram displaying the set of reactions between mix ingredients.
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Figure 11. Initial strength rate (I-UCS) and ultimate strength (U-UCS) for soil mixes: (a) S-C; (b) S-OWA; (c) S-C2-OWA; (d) S-C4-OWA (e) S-C6-OWA and (f) S-C8-OWA.
Figure 11. Initial strength rate (I-UCS) and ultimate strength (U-UCS) for soil mixes: (a) S-C; (b) S-OWA; (c) S-C2-OWA; (d) S-C4-OWA (e) S-C6-OWA and (f) S-C8-OWA.
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Figure 12. Contribution of mix components to (a) initial strength gain and (b) ultimate strength.
Figure 12. Contribution of mix components to (a) initial strength gain and (b) ultimate strength.
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Figure 13. Variation in CBR with time for all mixes: (a) S-C; (b) S-OWA; (c) S-C2-OWA and (d) S-C4-OWA.
Figure 13. Variation in CBR with time for all mixes: (a) S-C; (b) S-OWA; (c) S-C2-OWA and (d) S-C4-OWA.
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Figure 14. Correlation between UCS and CBR for all mixes: (a) S-C; (b) S-OWA; (c) S-CE-OWA and (d) S-C4-OWA.
Figure 14. Correlation between UCS and CBR for all mixes: (a) S-C; (b) S-OWA; (c) S-CE-OWA and (d) S-C4-OWA.
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Table 1. Physical properties of the soil.
Table 1. Physical properties of the soil.
PropertiesSymbolValue
Optimum moisture content (%)OMC16
Maximum dry unit weight (kN/m3)MDUW18.2
Mean grain size: D60 (mm)MGS0.55
Coefficient of uniformityCU4.58
Coefficient of curvature: CcCC0.54
Plasticity index (%)PI0
Table 2. Chemical composition of cement and OWA.
Table 2. Chemical composition of cement and OWA.
OxideSiO2Al2O3Fe2O3CaOMgOSO3K2ONa2OLOI
Cement (%)18.533.933.0661.781.742.920.470.186.3
OWA (%)24.733.413.8336.132.810.039.561.4214.7
Table 3. Experimental design table.
Table 3. Experimental design table.
Parameters
Test CombinationsCement content (%)OWA content (%)
Undisturbed/Compacted soil--
Soil + cement2, 4, 6, 8-
Soil + OWA-10, 20, 30, 40
Soil + cement + OWA2, 4, 6, 810, 20, 30, 40
Table 4. Coefficient of determination R2 for UCS results.
Table 4. Coefficient of determination R2 for UCS results.
MixesR2MixesR2MixesR2MixesR2MixesR2MixesR2
S-C20.999S-OWA100.998S-C2-OWA100.999S-C4-OWA100.994S-C6-OWA100.999S-C8-OWA100.994
S-C40.999S-OWA200.996S-C2-OWA200.999S-C4-OWA200.997S-C6-OWA200.999S-C8-OWA200.999
S-C60.991S-OWA300.988S-C2-OWA300.996S-C4-OWA300.999S-C6-OWA300.998S-C8-OWA300.998
S-C80.996S-OWA400.999S-C2-OWA400.999S-C4-OWA400.998S-C6-OWA400.999S-C8-OWA400.994
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MDPI and ACS Style

Ghanem, H.; El Bouz, C.; Ramadan, R.; Trad, A.; Khatib, J.; Elkordi, A. Effect of Incorporating Cement and Olive Waste Ash on the Mechanical Properties of Rammed Earth Block. Infrastructures 2024, 9, 122. https://doi.org/10.3390/infrastructures9080122

AMA Style

Ghanem H, El Bouz C, Ramadan R, Trad A, Khatib J, Elkordi A. Effect of Incorporating Cement and Olive Waste Ash on the Mechanical Properties of Rammed Earth Block. Infrastructures. 2024; 9(8):122. https://doi.org/10.3390/infrastructures9080122

Chicago/Turabian Style

Ghanem, Hassan, Chouk El Bouz, Rawan Ramadan, Adrien Trad, Jamal Khatib, and Adel Elkordi. 2024. "Effect of Incorporating Cement and Olive Waste Ash on the Mechanical Properties of Rammed Earth Block" Infrastructures 9, no. 8: 122. https://doi.org/10.3390/infrastructures9080122

APA Style

Ghanem, H., El Bouz, C., Ramadan, R., Trad, A., Khatib, J., & Elkordi, A. (2024). Effect of Incorporating Cement and Olive Waste Ash on the Mechanical Properties of Rammed Earth Block. Infrastructures, 9(8), 122. https://doi.org/10.3390/infrastructures9080122

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