Study on mix proportions and mechanical properties of controlled low strength material (CLSM) using river sludge

Abstract

This study examines the preparation of controlled low strength material (CLSM) utilizing dredged river sludge as the primary raw material. Laboratory experiments were conducted to analyze the influence of various mix ratios—particularly the water-to-solid ratio (W/S), lime-to-soil ratio (L/S), and supplementary materials including fly ash and slag—on the flowability and unconfined compressive strength (UCS) of the solidified material. The findings indicate that flowability increases with both W/S and L/S ratios. The UCS displays an inverse relationship with the W/S ratio and a positive correlation with the L/S ratio. The addition of slag and fly ash enhances performance, with slag demonstrating superior strengthening properties. Drawing from the experimental results, the study concludes that the optimal mix proportion is highly dependent on specific engineering requirements. For higher-strength road subgrade backfill (requiring 400–800 kPa UCS), an L/S ratio of 0.20 and a W/S ratio of 0.72 was identified as optimal. Conversely, for high-flowability pipeline trench backfill (requiring 200–400 kPa UCS), an L/S ratio of 0.15 and a W/S ratio of 0.80 was found to be ideal.

Introduction

The management and utilization of dredged sludge from rivers, lakes, and reservoirs presents a significant environmental and engineering challenge. Sludge typically exhibits high water content, low strength, and high compressibility, rendering it unsuitable for direct construction applications without treatment^1,2. Nevertheless, given that sludge frequently contains mineral-rich and silty components, its stabilization through binding agents can transform it into a viable construction material³.

Controlled Low Strength Material (CLSM) represents a flowable, self-compacting cementitious material commonly employed in civil engineering applications including trench backfills, subbase fillings, and utility encasements. Its low strength and high flowability facilitate placement without mechanical compaction while meeting engineering performance requirements^4–7. The incorporation of dredged sludge into CLSM has emerged as a promising sustainable approach for waste utilization.

Research has investigated the incorporation of fly ash, cement, slag, and other industrial by-products to enhance the mechanical and rheological performance of CLSM^8–12. The mechanical properties of CLSM, particularly flowability and unconfined compressive strength (UCS), are crucial determinants of field applicability. Variables including binder content, water content, and the incorporation of supplementary cementitious materials (SCMs) significantly influence performance outcomes^13–17.

This research evaluates the engineering feasibility of utilizing river sludge as a raw material in CLSM production. Through comprehensive laboratory testing, the research analyzes the effects of water-to-solid and lime-to-soil ratios, along with the addition of fly ash and slag, on the material’s workability and strength. The study aims to establish a scientifically sound mix design framework meeting both fluidity and strength performance criteria, enabling broader CLSM application in infrastructure backfill projects.

Materials and methods

Materials

The primary raw material utilized in this study comprised dewatered sludge obtained from a river dredging project in Shishou City, Hubei Province, China. Ordinary Portland cement (P.O 42.5) served as the main binder, while fly ash and ground granulated blast furnace slag (GGBFS) functioned as SCMs. Tap water was employed for mixing.

The sludge was characterized by high moisture content, a significant proportion of fine particles, and a relatively high organic matter content (5.3%). The major chemical components of the sludge, determined by X-ray fluorescence (XRF), were SiO₂, Al₂O₃, Fe₂O₃, and CaO. Mineralogical analysis by X-ray diffraction (XRD) revealed that the primary minerals included quartz, illite, chlorite, and feldspar.

The P.O 42.5 cement had an initial setting time of 45 min and a final setting time of 600 min. Its 3-day and 28-day compressive strengths were 17.0 MPa and 42.5 MPa, respectively. The material used was Grade II fly ash, with an average particle size of 20.8 μm and a water requirement of 100%. The slag was Grade S95, featuring an average particle size of 12.5 μm, a 7-day activity index of 65%, and a 28-day activity index of 85%.

According to the chemical composition analysis by X-ray fluorescence (XRF), the content of active components (CaO, SiO₂, and Al₂O₃) in the slag was over 85%, with a high proportion of CaO. In contrast, the chemical composition of the fly ash was predominantly SiO₂, and Al₂O₃.

According to the Technical Standard for Pre-mixed Flowable Fill (T/CECS 1037-2022), the recommended dosage of the binder in the CLSM should be between 7 and 25%, and the design specifications must meet the requirements listed in Table 1.

Table 1.

Design standard for CLSM unconfined compressive strength (UCS).

Application category	Minimum strength (MPa)
Utility trenches, trenches, and voids	0.2–0.4 MPa (when not explicitly specified by engineering requirements)
Subgrade backfill	Depth below subgrade (m)	Class a	Class b
	0.0– 0.8	0.8	0.6
	0.8–1.5	0.6	0.4
	> 1.5	0.4

A total of 16 mixtures were designed with varying parameters: the W/S ratio was set to 0.72, 0.76, 0.80, or 0.84, while the L/S ratio was set to 0.10, 0.15, 0.20, or 0.25, and proportions of fly ash and slag (0–20%).

Methods

The flowability of fresh CLSM was tested using a standard flow cone in accordance with Technical Standard for Pre-mixed Flowable Fill (T/CECS 1037-2022). The average diameter of the spread was recorded after lifting the cone.

UCS tests were conducted at 7, 14, and 28 days using an electro-hydraulic universal testing machine. The loading rate was 1 mm/min. Three specimens per group were tested, and average values were recorded.

Results

Flowability

Table 2 presents the flow diameters of CLSM mixtures with varying W/S and L/S ratios. Flowability increased with both water-to-solid and lime-to-soil ratios. The maximum measured flow spread exceeded 230 mm, demonstrating excellent self-compacting capability. However, the analysis revealed a time-dependent decrease in flowability. Following 30 min of static standing, the flow spread decreased by 2% to 9%, correlating with binder content.

Table 2.

Mobility of fluid cured soils with different ratios.

No	L/S ratios	W/S ratios	material			Flowability (mm)	Flowability at 30 min (mm)
			Lime/(g)	Soil/(g)	Water/(g)
1–1	0.10	0.72	50.12	501.22	396.15	120	113
1–2	0.10	0.76	50.16	501.55	418.24	157	153
1–3	0.10	0.80	50.10	501.00	440.12	201	197
1–4	0.10	0.84	50.03	500.30	462.12	215	208
2–1	0.15	0.72	60.00	400.04	331.23	153	144
2–2	0.15	0.76	60.01	400.05	349.60	184	173
2–3	0.15	0.80	60.02	400.06	368.02	219	210
2–4	0.15	0.84	60.09	400.05	386.51	266	255
3–1	0.20	0.72	80.02	400.03	345.61	175	160
3–2	0.20	0.76	80.02	400.04	364.85	209	201
3–3	0.20	0.80	80.03	400.03	384.01	257	237
3–4	0.20	0.84	80.01	400.00	403.23	295	278
4–1	0.25	0.72	90.01	360.02	324.01	186	169
4–2	0.25	0.76	90.08	360.05	342.03	229	211
4–3	0.25	0.80	90.02	360.03	360.02	265	247
4–4	0.25	0.84	90.03	360.01	378.06	306	281

Figures 1 and 2 present the SEM micrographs of the CLSM before and after curing, respectively(20,000 × magnification). Before curing, the soil particles exhibit a loose and disordered arrangement, lacking a distinct structural framework. Inter-particle contact points are minimal, and the particles are primarily connected by the cohesive force of the binder. The soil matrix is characterized by large pores, high porosity, and a non-uniform pore distribution consisting of a mixture of macro- and micro-pores. After curing, the particles are arranged in a more orderly and compact manner, forming a relatively stable structure. The number of contact points between particles increases, and the inter-particle cohesive forces are significantly enhanced. Consequently, the pores within the soil matrix are markedly reduced in size, leading to a decrease in overall porosity.

Fig. 1.

SEM micrograph of the CLSM specimen before curing (20000x).

Fig. 2.

SEM micrograph of the CLSM specimen after curing (20000x).

During the quiescent period, the hydration of cementitious particles and physicochemical interactions between nascent hydration products (e.g., C–S–H gels) and clay minerals induce the formation of a three-dimensional flocculated network. This evolving microstructure physically entraps free water and imparts a yield stress to the system, which macroscopically manifests as a time-dependent increase in apparent viscosity and a corresponding reduction in flowability. This behavior is attributed to thixotropic characteristics, wherein weak flocculated structures develop over time.

Unconfined compressive strength (UCS)

The UCS results at 7, 14, and 28 days are summarized in Figs. 3, 4 and 5. Strength development showed a typical growth trend over time. At 7 days, UCS ranged from 0.07 to 0.63 MPa, and continued to increase to a maximum of 0.916 MPa at 28 days in optimal mixes. Based on the data analysis from Figs. 3, 4 and 5, the strength growth rate of the material exhibits distinct time-dependent characteristics; specifically, the rate during the 7–14 day curing period is significantly higher than that of the 14–28 day period. The early-age strength development is primarily dominated by the rapid hydration reaction of the Ordinary Portland cement, which serves as the main binder. During this stage, the cement clinker reacts rapidly to generate a large volume of calcium silicate hydrate (C–S–H) gel—the primary contributor to strength—thereby achieving rapid strength growth. As the curing age progresses, the readily reactive cement particles are gradually consumed, and the hydration rate decelerates as the process becomes diffusion-controlled.

Fig. 3.

7-day UCS at different mix ratios.

Fig. 4.

14-day UCS at different mix ratios.

Fig. 5.

28-day UCS at different mix ratios.

Admixture effects

Figure 6 illustrates the flowability and 7-day unconfined compressive strength characteristics following the incorporation of 0–20% fly ash and slag into the CLSM with a mix ratio of 2–1 to 3–4 mixture. In this study, the admixture dosage is defined as the mass of the admixture expressed as a percentage of the total mass of the binder (cement and admixture). As shown in the figure, the addition of fly ash and slag enhanced flowability.

Fig. 6.

Flowability and 7-day UCS diagram after adding 0–20% fly ash and slag to the CLSM.

At equivalent dosages, the strengthening effect of slag on the CLSM is significantly superior to that of fly ash. Slag particles are generally finer than fly ash particles, allowing them to more effectively fill the interstitial voids between cement and sludge particles. This micro-filling effect increases the density of the slurry and reduces its porosity. Additionally, these finer particles provide more heterogeneous nucleation sites for cement hydration products, which promotes the early hydration process to some extent.

The chemical activities of the two materials also differ. Fly ash primarily exhibits pozzolanic activity, undergoing a secondary reaction with the calcium hydroxide (CH) produced during cement hydration to form calcium silicate hydrate gel. Blast furnace slag, in contrast, possesses latent hydraulic activity. In addition to the pozzolanic reaction, its vitreous structure, activated by the high-pH environment from cement hydration, can react directly with water to form C–S–H gel similar to that from cement. Therefore, its contribution to strength is greater.

When the admixture content reaches 20%, the flow spread increases by more than 15% compared to the control group. Higher L/S ratios enhance both early-age and long-term strength, whereas higher W/S ratios have an adverse effect. The experimental results indicate that when the admixture dosage is in the 10–15% range, the 7-day UCS of the CLSM reaches its peak. If the dosage exceeds 15%, the strength begins to decrease due to a dilution effect, as the proportion of cement—the main contributor to early strength—is excessively reduced, leading to an insufficient formation of early hydration products.

Fly ash and slag exhibited distinct performance patterns: both materials initially enhanced strength up to a dosage of 10–15%, beyond which strength decreased due to dilution of the cementitious phase. Mixtures containing slag outperformed those with fly ash additions, achieving unconfined compressive strength values up to 20% higher at equivalent dosages.

Optimal mix proportions for CLSM engineering applications

The standards for the flowability and Unconfined Compressive Strength (UCS) of CLSM for road subgrade and pipeline trench backfill applications are shown in Fig. 7. For road subgrade backfill, the required flowability is 140–200 mm, and the UCS is 400–800 kPa; for pipeline trench backfill, the recommended flowability is 200–250 mm, and the UCS is 200–400 kPa. The mix proportions suitable for road subgrade backfill are primarily concentrated in the range of L/S ratios from 0.15 to 0.25 and W/S ratios from 0.72 to 0.80, while those suitable for pipeline trench backfill are mainly found in the range of L/S ratios from 0.10 to 0.20 and W/S ratios from 0.76 to 0.80.

Fig. 7.

Flowability and UCS standards for CLSM in different backfill applications.

Upon comprehensive consideration, for road subgrade backfill, specimens 3–1 (L/S = 0.20, W/S = 0.72) is the mix proportions that meet the performance requirements. For pipeline trench backfill, specimen 2–3 (L/S = 0.15, W/S = 0.80) is the mix proportion that satisfies the performance requirements.

Conclusions

This research examined the effects of mix proportion parameters on the workability and mechanical performance of CLSM manufactured using river sludge. Through systematic laboratory experiments, the following conclusions were established:

Material flowability was positively correlated with both the W/S and L/S ratios. Unconfined compressive strength (UCS) was positively correlated with lime-to-soil ratio and negatively correlated with water-to-solid ratio. Strength increased significantly over time, with the highest UCS reaching 0.916 MPa at 28 days.

The addition of slag and fly ash improves both flowability and strength. Slag provides a superior strengthening effect compared to fly ash, and the optimal admixture dosage for 7-day strength was found to be in the 10%–15% range.

The optimal mix proportion for CLSM is contingent upon the specific performance criteria of the engineering application, rather than a single universal range. For road subgrade backfill, which requires higher strength (UCS of 400–800 kPa) and moderate flowability (140–200 mm), the optimal mix proportion was identified as specimen 3–1, with an L/S ratio of 0.20 and a W/S ratio of 0.72. For pipeline trench backfill, which requires high flowability (200–250 mm) and moderate strength (UCS of 200–400 kPa), the optimal mix proportion was identified as specimen 2–3, with an L/S ratio of 0.15 and a W/S ratio of 0.80.

These results establish a practical framework for the engineering application of sludge-based CLSM, facilitating the sustainable utilization of dredged sediments while fulfilling construction performance requirements.

Author contributions

Jinyan Zhu designed the research study, performed the data analysis, and wrote the manuscript. Lei Li conducted the experiments and collected the data. Ziyi Huang and Haopeng Xu provided critical feedback and helped shape the research, analysis, and manuscript. All authors discussed the results and contributed to the final manuscript.

Funding

The authors would like to acknowledge the financial supports from National Key R&D Program of China (2022YFC3202704).

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.