Feasibility of Using Recycled Construction and Demolition Materials for Deep Soil Mixing

Abstract

Reusing construction and demolition wastes for geotechnical and geo-environmental purposes has already become a research hotspot. This study was carried out to evaluate the feasibility of using recycled construction and demolition wastes in a partial substitution of cement to enhance the mechanical properties of soft soil. The strength and stiffness development of two types of recycled material (RM1 and RM2), incorporated with peat and clayey soil under 7, 14, and 28 days’ curing time, was investigated based on unconfined compressive strength and free–free resonance frequency test methods. The findings demonstrated that clayey soil showed an average of 2.5 times higher strength than peat with the addition of recycled materials, regardless of the type. However, after 14 days of curing, the strength remained constant for peat soil. Moreover, it is concluded that the studied granular recycled materials could be used to replace a part of the cement content to improve the strength and stiffness properties.

1. Introduction

Due to their unsatisfying hydraulic and mechanical properties, soft soils have always been a major concern in geotechnical engineering. Scientists are searching for environmentally friendly and cost-effective solutions that can help to improve these properties because using traditional materials, such as cement and lime, are energy-consuming and relatively expensive. As a result, waste management has received more attention over the last decade in order to achieve the sustainable development goals (SDGs) suggested by the United Nations at the international level [1,2].

Using recycled materials, such as waste tires, plastic waste, recycled crushed concrete, and recycled glass powder, as an alternative to traditional materials has become increasingly popular [3]. Among these, wastes from the construction, reconstruction, extension, maintenance, and destruction of buildings and other infrastructures are typically referred to as construction and demolition wastes (C&DW). These wastes are heterogeneous residues comprised of several materials used to construct a building or infrastructure [4]. Moreover, the European Commission has classified C&DW as a priority category due to the large amounts of waste they produce and their great potential for recycling and reuse. Therefore, researchers suggested the use of C&DW as a partial substitution for traditional soil stabilizers, such as cement, lime, etc., might reduce greenhouse gas (GHG) emissions [5,6]. Furthermore, slag from blast furnaces is a type of industrial solid waste that may be used for high-value purposes. The blast furnace slag is commonly used in the formulation of various kinds of cement [7]. Zhang et al. [8] investigated the effect of basicity on blast furnace slag. This study found that increased alkalinity improves the denitration performance of blast furnace slag. The curing temperature has a significant impact on the hydration of blast furnace slag [9]. Zeghichi et al. [10] investigated the effect of the alkaline activation of slag cement with clinker. The results showed that alkaline activators accelerate hydration and stimulate the hardening process.

Road base and sub-base layers have been the main focus of utilizing C&DW in geotechnical engineering. Arulrajah A. et al. [11] investigated blending various types of recycled plastic wastes with crushed brick and reclaimed asphalt material in terms of strength, stiffness, and resilient moduli. It has been found that a polyethylene plastic/demolition waste blend is suitable for road construction material. Tavakoli Mehrjardi G. et al. [5] evaluated the physical and mechanical properties of C&DW, employing sieve analysis, Atterberg limits, modified Proctor test, California Bearing Ratio, and direct shear test to use in road construction as a sub-base material. The findings demonstrated that the physical and mechanical properties of the investigated C&DW materials mostly complied with the requirements to be used as sub-base materials in road construction.

Soil reinforcement with waste carpet fibres is a well-established technique for soils having low tensile and shear strength. Murray J. et al. [12] evaluated the possible reuse of carpet waste fibres in soil that is classified as sandy silt. It was concluded that the peak stress increases with an increasing amount of carpet fibres. Another worldwide concern is the recycling of plastic water bottles, which challenges the reuse of these materials in soil applications. Luwalaga J. G. [13] studied soil strength’s improvement by the addition of uniformly graded polyethylene terephthalate (PET) plastic waste flakes (ranging between 10 and 1.18 mm) to uniformly graded sand. Test results indicated that the optimum PET plastic dosage is 22.5%, which suggests a limit for the dosage of added waste material.

Dredging works are carried out to keep waterways navigable or even to create new canals, which results in significant amounts of dredged sediments. Regarding the application of landfills, Di Emidio G. et al. [14] evaluated the possible reuse of dredged sediments for hydraulic barriers in the field of geo-environmental engineering. Long-term polymer treatment preserved the soil’s low hydraulic conductivity to electrolyte solutions. Compared to untreated soil, the polymer treatment increased the amount of heavy metals that the soil could adsorb, such as Cu²⁺ and Pb^2+.

Cement-treated soils are widely used worldwide for soft ground improvement [15]. Furthermore, using C&DW as aggregates in cement-stabilized soils for foundations has been reported by researchers as a green geotechnical solution [16,17,18]. Zhang G. et al. [19] studied the strength enhancement of the silty clay incorporated with Portland cement and C&DW at several percentages by performing an unconfined compressive strength (UCS) test. The results indicated that adding Portland cement increased the strength, while the excessive addition of C&DW caused the improvement to drop. Sharma A. and Sharma R.k. [20] conducted a laboratory study on clayey soil to study the stress–strain and volumetric behaviour with the addition of C&DW. The findings showed that adding C&DW increased the stress–strain characteristics, and the volumetric strain decreased.

Weak mechanical characteristics in the soil can cause severe settlements, eventually surpassing a structure’s serviceability limit and causing failure. The soil is typically in situ mechanically deep-mixed with binders, such as cement, to enhance the strength properties. Deep soil mixing (DSM) technology has been developed for several structural environmental purposes, including liquefaction barriers, soil reinforcement, land and slope stabilisation, and retaining structures [21]. Previous decades have also witnessed the use of recycled wastes, such as C&DW, to stabilize soft soils using the DSM method. [22].

The compressive strength of cement-treated soils is typically considered a critical factor in characterizing soil behaviour, and it is significantly influenced by the curing time and temperature. Bagriacik B. [23] studied the feasibility of using alkali-activated C&DW as a stabilizer in soil improvement at different curing times and temperatures. This study suggested 21 days of curing, after which significant enhancement was not observed. Additionally, the results demonstrated that the bearing capacity of soil samples increased as the curing temperature increased. The UCS of cement-treated soils is also remarkably influenced by the water content. Several studies focused on the effect of water content used for mixing and the water/cement ratio of cement-treated soils [24,25]. According to Consoli et. al. [26], the moulding water content is a primary factor in determining the UCS of cement-treated soils. A study also reported that UCS values change with the changing water content [27].

The stress–strain behaviour of soil is complex and non-linear, yet an essential parameter for various geotechnical design applications. Young’s modulus (E) and the shear modulus (G) of the soil are not constant. Therefore, the small-strain moduli E₀ and G₀ are defined because, under 0.001% strain level, E and G are constant and linear. Bender/extender elements testing is commonly used to determine the small-strain modulus. However, the free–free resonant frequency (FFR) test can be a promising alternative due to its simplicity for measuring the small-strain modulus of cement-treated soils [28]. Verástegui Flores D. et al. [29] studied to validate the free–free resonant frequency for cement-treated soil and confirmed the reliability of the results obtained from the FFR test.

The main purpose of this research was to study the possibility of using recycled construction and demolition waste materials as an alternative to cement in DSM technology by improving the mechanical properties of soft soil. Therefore, this study investigated the impact of the addition of two types of recycled materials, with the partial substitution of cement, to peat and clayey soil under 7, 14, and 28 days of curing time. The strength and stiffness properties of cemented soft soils were investigated based on unconfined compressive strength (UCS), small-strain Young’s modulus (E₀), and small-strain shear modulus (G₀). This study contributes to sustainable development resulting from the recycling and reuse of C&DWs, which will lessen the demand for non-renewable resources. Recycled C&DWs will have a lower overall environmental effect and take up less room in landfills when used for soil stabilization. Moreover, this study also used the FFR test as a novel approach to determine the mechanical characteristics of cement-treated soils as a simplified approach.

2. Materials and Methods

2.1. Materials

Soft peat and clayey soil that is collected from Ostend on the Belgian coast were chosen to study in this research. The soft peat and clayey soil were sampled using undisturbed continuous drillings, and the organic content was determined as ±81.0% and ±19.0%, respectively. Peat is distinguished from other organic soil materials by an organic content higher than 75% (ASTM D 2974). Based on the test results of the organic content, it could be concluded that the samples were classified as full peat. The average wet density was determined as 1.09 g/cm³. The clay mineral composition on the Belgian coast was examined in the MOCHA project [30]. The results are as follows: Illite between 15 and 83%, Smectite between 1 and 80%, kaolinite between 1 and 36, and minimal differences for chlorite. The soil is classified as heavy historical polder clay from Ostend, considered in the specific Fluvic Gleyic Cambisol soil group. Due to the limited age or rejuvenation of the soil material, cambisols are soils with a moderately developed profile that may be found in various environments. Therefore, it is not easy to summarize all cambisols’ mineralogical, physical, and chemical characteristics. However, most cambisols have a neutral to weakly acidic soil reaction, satisfactory chemical fertility and active soil fauna, high porosity, good structural stability, good water-holding capacity, and internal drainage [31]. The average water content for peat and clayey soil was determined by drying at 105 °C, 475.1%, and 62.3%. The overview of the soil used in this research is shown in

Table 1. The overview of soil (peat and clay).

Mean Depth (m)	1.88	1.68	4.98	6.28
W0	38.7	36.1	478.4	76.1
S0	100.0	100.0	99.0	100.0
e0	1.07	0.94	7.66	1.91
Wf	34.5	31.3	228.3	41.7
Sf	100.0	100.0	83.1	72.9
ef	0.75	0.70	4.12	0.93
Liquid limit (%)	44.7	57.3	62.1	48.8
Plastic limit (%)	26.0	28.9	46.9	29.6
Plasticity index (%)	18.7	28.4	15.2	19.2
Average organic content (%)	3.49	5.18	11.8	3.38
Compression index (Cc)	0.28	0.26	4.81	0.81
Pre-consolidation stress (kPa)	95.0	100.0	100.0	80.0
Soil classification	Lean Clay	Elastic Silt	Elastic Silt	Lean Clay

. Figure 1 represents the sampled soil of the drillings. In the top left corner of the figure, the term “GL” denotes the ground level. The initial and final properties of the regular oedometer-tested specimens are presented in Table 1. The water content (w₀ and w_f), the saturation degree (S₀ and S_f), and the void ratio (e₀ and e_f) before and after the test are provided. An overview of the Atterberg limits (ASTM D 4318) and organic content (ASTM D 2974) of the clayey soil at various reference depths can also be found in Table 1. Based on these properties and the results of executed aerometer tests (ASTM D 422–63), the clayey soil was also classified according to ASTM D 2487. For clayey soil, the compression index was determined to be related to each other as Cc = PI/74.

Kazemian S. et al. [32] studied peat in literature and concluded that the compression index of peat soil ranges from 2 to 15. The compression index is high since water is expelled simultaneously from within and among the peat particles during primary and secondary compression. The calculated compression index at 4.98 m depth was located within the range reported by [32]. Morever, the samples collected at 6.28 m depth were evaluated as clayey soil containing peat.

The SLW Foundation Group supplied two different crushed granular recycled materials used in this study as a partial substitution for cement in soft soils. The first recycled material (RM1) was composed of a mixture of crushed mortar, crushed concrete, and sand, while the other recycled material (RM2) was a mixture of sand, stones, and crushed bricks. The water content of RM1 and RM2 was calculated from the mean of three measurements as 14.8% and 13.6%, respectively. The soil classification of RM1 and RM2 is given in

Table 2. The soil classification of RM1 and RM2.

	RM1 (%)	RM2 (%)
Gravel (>2.00 mm)	39.9	24.8
Sand	49.9	55.6
Silt	7.1	15.2
Clay	3.1	4.4

.

Blast-furnace slag cement, type CEM III/A 42.5 N LA (Holcim-Belgium), was used in this study as a binding agent. The chemical composition of the cement used in this study is shown in

Table 3. Chemical composition of CEM III/A 42.5 N LA.

CaO	52.2
SiO2	25.8
Al2O3	8.1
Fe2O3	2.3
MgO	4.4
Na2O	0.36
K2O	0.6
Na2O-eq	0.75
SO3	3.1
Other	1.7

. CEM III/A 42.5 N LA is a blast furnace cement according to EN 197-1, containing Portland clinker and granulated blast furnace slag as main ingredients. The Portland clinker content ranges between 35% and 64%, while the blast furnace slag content ranges between 36% and 65%. This cement has a limited alkali content (LA) (see also Table 3, Na₂O-eq < 0.90%). This cement has a slower hardening rate compared to that of Portland cement (CEM I). Due to its composition, this cement is suitable for applications below the ground’s surface and where durability and better chemical resistance are important. De-ionized water was utilized to simulate the addition of wet recycled materials to peat and clayey soil and cement based on the wet deep mixing method, due to the need for additional water to keep the recycled materials pumpable. Therefore, the water-cement ratio of 0.7 was used in this study for “wet mixing” samples.

2.2. Experimental Program

A study was conducted on the impact of various factors on the stiffness and strength of the stabilized soft soil specimens:

• Recycled material type and amount;
• Curing time;
• Cement content.

Table 4. The testing program.

	Mixture	RM Type and Amount (kg/m³)	Water Content	Cement Amount (kg/m³)	Curing (Days)
Clay	DM	-	Wn	300	7-14-28
	WM, RM1	RM1: 300	Wn+210 L/m³	300	7-14-28
	WM, RM2	RM2: 300	Wn+210 L/m³	300	7-14-28
	WM	-	Wn+210 L/m³	300	14
	DM, RM1	RM1: 300	Wn	300	14
	WM, RM1+	RM1: 500	Wn+210 L/m³	300	14
	WM, RM1+ C−	RM1: 500	Wn+70 L/m³	100	14
Peat	DM	-	Wn	300	7-14-28
	WM, RM1	RM1: 500	Wn+210 L/m³	300	7-14-28
	WM, RM2	RM2: 500	Wn+210 L/m³	300	7-14-28
	WM	-	Wn+210 L/m³	300	14
	DM, RM1	RM1: 500	Wn	300	14
	WM, RM1+	RM1: 700	Wn+210 L/m³	300	14
	WM, RM1+ C−	RM1: 700	Wn+70 L/m³	100	14

briefly shows the testing program of this research. The experimental program was divided into two parts; first, the type of RM and curing time were investigated based on UCS and FFR test methods. Second, the selected optimum RM type was used to examine the increase of RM content and decrease in cement content for both peat and clayey soil. The samples prepared using the dry DSM technique in this study were named “DM” as they were produced by mixing cement and natural soil without additional water. The water content for DM samples resulted from the initial water content of the native soft soil. Samples prepared according to the wet DSM method were named “WM” in this study. These samples contained cement and recycled material and had a water/cement ratio of 0.7. The mixtures “WM, RM1+” were prepared to study the effect of increasing recycled material content. Moreover, the mixtures “WM, RM1+ C−” were prepared to examine the increase of recycled material while decreasing the cement content, and these mixtures were compared with the “WM, RM1” mixtures. Curing times of 7, 14, and 28 days were adopted in this study.

2.3. Sample Preparation

Sample preparation was carried out according to EuroSoilStab [33]. The mixtures presented in Table 4 were prepared using a Domo DO9070KR mixer at the speed of 350 rpm until a visually homogeneous material was obtained.

The base soil was mixed for 30 s, cement was added, the mixing continued for two minutes, and the final three minutes were devoted to adding water and/or recyclable materials. In order to prevent excessive stiffening of the mixture during material preparation as a result of cement hydration, the mixing time was limited to a minimum. The EuroSoilStab also reports limiting the mixing time to protect the peat fibres. The mixing procedure was carried out for each test specimen individually. The samples were manufactured with a suggested D/L ratio of 0.5, measuring 100 mm in height and 50 mm in diameter [29], following the proposed compaction method in EuroSoilStab. The only difference was that the specimens were compacted at ten layers instead of the proposed four layers on order to develop a more uniform and reliable compaction method. A 98 kPa load was applied to the specimen for 3 s after filling it with an exact amount of material, and then the specimen was immediately unloaded for 5 s. For each compacted layer, this loading and unloading cycle was repeated five times. After straightening the compacted sample, the specimen was left to settle for 60 min at a temperature of 20 ± 2 °C. After that, a Hydraulic Extruder HS 16.06 from HEICO was used to extrude the specimens. Upon sample preparation, each sample was securely wrapped in plastic wrap and placed in a humid box that was kept at a temperature of 10 ± 1 °C for 7, 14, and 28 days of curing time.

2.4. Testing Methods

The mechanical performance of the mixtures was evaluated based on the unconfined compressive strength (UCS) test and the free–free resonance (FFR) test methods.

The UCS tests were executed with a Wykeham Farrance type Tritech 50 compression machine according to ASTM D 2166–00 [34]. For all samples, the loading was applied by producing a constant axial strain of 0.5 mm/min. It was assumed that all samples had a height of exactly 10 mm. Therefore, an axial strain rate of 0.5%/min was applied. The axial strain and axial normal compressive stress were found using the following relationships from ASTM D2166–00:

σ = P/A,

(1)

ε = ΔL/L₀

(2)

A = A₀/(1 − ε)

(3)

where σ is the compressive stress [kPa], P is the corresponding force [kN], ε is the axial strain for the given load, A₀ is the initial cross-sectional area of the specimen [mm²], A is the corresponding cross-sectional area [mm²], L₀ is the initial length of the test specimen [mm], and ΔL is the length change of specimen [mm].

A key parameter for many geotechnical design applications, small-strain stiffness can also be utilized to predict other soil properties indirectly. Using the free–free resonant frequency approach, the small-strain stiffness modulus in the longitudinal (E₀) and transversal direction (G₀) was calculated in this study [29]. This method places the test specimen on top of a soft foam layer to approach fully free boundary conditions. After that, one end of the sample is put against an accelerometer or an acoustic meter to measure vibrations, while the other end is hit lightly with a hammer. Figure 2 shows the FFR testing setup that was employed in this research.

The calculation of the small-strain modulus was as follows:

E0 = ρ·v_p² = ρ·(2·L·f_L)²,

(4)

G0 = ρ·vs² = ρ·(2·L·f_T)²,

(5)

where v_p and vs are the compressive and shear wave velocity, respectively. ρ is the bulk density, L is the height, and f_L and f_T are the longitudinal and transversal resonant frequency. It should be noted that both formulas assume that the wavelength (λ) of the vibrating sample is equal to twice its length (λ = 2 L). Additionally, these formulas are applicable for free–free specimens having D/L ≤ 0.5, according to ASTM C 215 [35].

3. Results and Discussion

3.1. The Effect of RM Type and Curing Times

Recycled materials can be used in soil stabilization with additives such as cement and lime to improve the mechanical properties of soft soil and contribute to a sustainable environment.

Figure 3 demonstrates the effects of recycled material type and curing time on clayey soil based on UCS and small-strain moduli. The cement and recycled material content for the mixtures shown in Figure 3 were kept constant at 300 kg/m³. The UCS values of DM mixtures after 7, 14, and 28 days of curing were an average of 2.7 times higher than that of the “WM, RM1” and “WM, RM2” mixtures. The difficult compaction of the WM samples can explain this. Conversely, the addition of RM materials to WM mixtures, regardless of the type, increased strength and stiffness. This indicates the partial substitution of cement with RM could improve the mechanical properties of clayey soil. The UCS of mixtures containing RM1 type was higher than those with RM2 at each curing time, while the small-strain moduli of samples with RM1 and RM2 showed no significant difference for both longitudinal and transversal directions. The results of UCS and FFR tests of samples with 7, 14, and 28 days of curing time are also presented in Figure 3. Increasing the curing time also increased the strength and stiffness of all mixtures. However, the increase rate between 14 and 28 days was not as high as that of the first 7 to 14 days. Higher UCS and G₀ values were obtained with RM1 compared with those of RM2 mixtures. However, it was also found that E₀ values did not show a trend over RM type (Figure 3b). It should also be noted that E₀ and G₀ values were determined to be close for RM1 and RM2. Therefore, UCS can be a good indication for selecting RM type.

The UCS and small-strain moduli for peat in the function of curing time are shown in Figure 4. The cement content for the samples shown in Figure 4 was 300 kg/m³, while the recycled material content was 500 kg/m³. The UCS values of peat samples, after 28 days of curing, were increased from 371 kPa to 487 kPA with the addition of RM1 and to 423 kPa with the addition of RM2. The same trend was also observed for stiffness. Therefore, it can be concluded that strength and stiffness are increased with the addition of RM, regardless of the type. The results also show that WM mixtures showed higher values than those of DM mixtures at each curing time. Although RM2 samples had higher results (UCS: 443 kPa, E₀: 0.36 GPa, G₀: 0.13 GPa) after 14 days than those of RM1 samples (UCS: 381 kPa, E₀: 0.31 GPa, G₀: 0.11 GPa), after 28 days of curing, it was observed that RM1 mixtures (UCS: 487 kPa, E₀: 0.39 GPa, G₀: 0.15 GPa) had higher strength and stiffness properties than those of RM2 mixtures (UCS: 423 kPa, E₀: 0.36 GPa, G₀: 0.13 GPa). Moreover, the strength and stiffness were increased by increasing the curing time for the mixtures prepared with RM1. Conversely, the UCS of RM2 mixtures dropped after 14 days of curing. The E₀ and G₀ values remained constant and/or did not clearly change after 14 days of curing. The minor variation in the results could be attributable to variations in the RM compositions, since RM is a construction and demolition waste that contains several types of materials.

Although the in situ strength characteristics of clayey soil and peat were considered comparable, significantly greater strengths were obtained for clayey mixtures compared with those of peat in this study. After 14 days of curing, the UCS value of DM clayey soil sample was 3031 kPa, while those of the DM peat samples were 400 kPa. The same trend was also observed for 14 and 28 days of curing and with the addition of RM. This can be related to the higher RM content used in peat mixtures (500 kg/m³) than that of the clayey soil mixtures (300 kg/m³). Another possibility is that peat and blast furnace cement interacted less favorably than clayey soil at very early strength stages. The water content of the cemented peat ranges from 80 to 160%, which is still relatively substantial despite the lower RM content in soft peat. During 7 and 28 days of cure, the water content remained constant. This indicates a significant amount of unbounded water in the samples, which is harmful to the soil’s mechanical behavior. Therefore, this phenomenon can also explain the lower strength results of peat compared to clayey soil. However, for both clayey soil and peat, the addition of RM to the native soil increased the mechanical properties. Since RM1 showed good performance for peat and clayey soil, it was determined to continue the second phase of the study by using RM1. Additionally, the target value of 540 kPa for clayey soil, as proposed by the SLW Foundations group, was for part of the WM-RM1 and WM-RM2 samples already reached after 7 days. Additionally, the UCS values of peat soil after 14 days was not changed. Therefore, in the study’s second phase, it was decided to continue with 14 days of curing for both the clay and peat soil.

3.2. The Effect of RM and Cement Content

The increase in RM content and decrease in cement content were evaluated at 14 days of curing in this phase, and the results are demonstrated in Figure 5. Considering the previous results, RM1 showed significantly higher mechanical properties for both peat and clayey soil than RM2; therefore, it was decided to continue this part of the study using RM1. The increase of RM content for clayey soil was evaluated by increasing the RM content to 500 kg/m³ while the cement content remained at 300 kg/m³ (WM, RM1+), and the results were compared with those of the mixtures containing 300 kg/m³ RM and cement (WM, RM1). Conversely, the effect of the RM content was evaluated for peat by increasing the RM content to 700 kg/m³ while cement content remained at 300 kg/m³ (WM, RM1+). The results indicated that increased RM content reduced the strength and stiffness properties of clayey soil, while it increased these properties for peat. As it can also be concluded from Figure 3 and Figure 4, increased RM content can be related to the difficult compaction of clayey soil mixtures. Furthermore, the cement content was reduced to 100 kg/ m³ while the RM content was increased for peat to 700 kg/m³ and for clayey soil to 500 kg/m³ (WM, RM1+ C−) in order to evaluate the partial substitution of cement with RM. The results showed that the decrease in cement content negatively affected the strength of soft soil. This can indicate inadequate bonding in these mixtures due to the lack of the amount of binding agent, cement. This also indicates that the water/cement ratio is a determining parameter with regard to the mechanical properties.

3.3. Relation between Secant Young’s Modulus E₅₀ and UCS

The Secant Young’s modulus E₅₀ at 50% of deviatoric stress (UCS) was determined according to the EuroSoilStab [33]:

E₅₀ = σ_c/(ε_c − ε_be)

(6)

In which σ_c is 50% of the deviatoric stress (UCS), ε_c the strain at σ_c, and ε_be the bedding error correction.

Figure 6a,b present E₅₀ in the function of UCS for the peat and clayey cemented soils, respectively. The results show that similar relations between E₅₀ and UCS were found for the peat and clayey mixtures. It should be noted that the range (comparing the lower and upper bouncaries) of the studied correlation varies quite significantly compared with those of correlations in the literature [36]. However, it should be emphasized that different mixtures were used for the correlation, such that obtaining a wide range is plausible. Geotesting Express reported 50·UCS < E₅₀ < 150·UCS [37], and Futaki et al. [38] calculated 50·UCS < E₅₀ < 150·UCS, which are both the boundary in this research.

4. Conclusions and Recommendations

This study evaluated the effects of RM type, curing time, and RM and cement content for peat and clayey soil. Based on the analysis of the laboratory test results obtained, the following conclusions can be drawn:

• Adding RM increased the unconfined compressive strength of peat and clayey soil for WM mixtures, regardless of the type. The maximum strength with 1490.7 kPa was observed at 28 days of curing for the clayey soil by adding RM1.
• In terms of UCS testing, significantly greater strengths were seen for the clayey materials compared with those of the peat samples. The small-strain stiffness moduli in both longitudinal and transverse directions followed the same pattern. It was also found that the water/cement ratio is an important factor in the mix design for both peat and clayey samples. Adding water to the clayey soil made a very sticky mixture, which was hard to compact. Consequently, the samples were of lower quality, which influenced the investigation of the effects of using recycled materials as an alternative to cement. Future research should take into account the impact of the water-to-cement ratio.
• Increasing the curing time also increased the strength of the clayey soil; however, the strength remained constant after 14 days of curing for peat samples. Therefore, it is recommended to study the long-term strength development of different mixtures. Because of the difference in nature between clayey soil and peat, a difference in strength development over time could also occur.
• Increasing RM content increased the strength of peat samples while reducing the strength of clayey soil samples.
• The results obtained from this study comply with the results of other research studies, which proved the use of C&DW materials in soil stabilization by improving the mechanical properties. The findings of this study might assist the soil improvement industry by demonstrating how these waste products can be successfully used in construction when secondary additives are used.