Influence Mechanism of Fulvic Acid on the Strength of Cement-Solidified Dredged Sludge

Abstract

Cement solidification was a widely used method to broaden the resource utilization of dredged sludge. However, the organic matter in sludge limit the application of cement solidification. The fulic acid (FA) was used to simulate the natural organic matter in sludge. With the increase in FA content, the sludge gradually changed from low-liquid-limit clay to high-liquid-limit clay. The unconfined compression test showed that the strength of cement-solidified dredged sludge (CDS) decreased with FA content. The influence mechanism of the FA on cement solidification was revealed by a water content test, a hydration heat test, scanning electron microscopy, and thermogravimetric analysis. FA hindered the conversion of pore water to combined water and reduced the hydration heat inside CDS. The FA in sludge weakened the internal bond within CDS by hindering the development of C-S-H gel from spheroidal to flake. At the same time, the final amount of hydrates such as C-S-H, C-A-H and AFt also decreased in the CDS containing FA. The weakening mechanism of FA on the strength of CDS can be attributed to three aspects: (1) FA adsorbed on the surface of cement minerals and hindered the contact between cement minerals and pore water; (2) acidic FA reduced the pH of the pore liquid in CDS; (3) the carboxyl and hydroxyl functional groups of FA adsorbed calcium ions in pore liquid through ion coordination.

1. Introduction

A large amount of dredged sludge is produced annually in port, shipping and water environment restoration projects [1,2,3]. The geotechnical properties of these high-water-content sludge are very poor and it is difficult to use them effectively without any treatment [4,5,6]. As a widely used method, cement solidification can improve the strength of sludge and broaden the resource utilization of sludge. However, sludge with high organic matter content is often involved in engineering projects, and the organic matter composition can obviously change the physical and mechanical properties of sludge. Venda Oliveira et al. [7] studied the influence of organic matter on the preloading effect of soft foundation, and the results showed that the increase in organic matter content would lead to the increase in porosity and compression index. Zentar and abriak [8] confirmed that when the organic matter content of marine sludge was reduced by oxidation, both the liquid limit and plastic limit were significantly reduced. Hamouche et al. [9] found that the specific surface area and specific gravity of sludge tended to decrease with organic matter content, while the plastic index of sludge remained almost unchanged.

Humic acid refers to the black colloidal substances formed by the long-term decomposition of fresh organic matter by microorganisms, accounting for approximately 80% of the total organic matter content in sludge, which has the most significant impact on the properties of sludge. Kang et al. [10] studied the influence of humic acid on the strength development and microstructure of cement-solidified marine sludge at different curing ages, and the results showed that when the cement content exceeded a threshold, the negative effect of humic acid on the strength would be overcome. Zhu et al. [11] found that the relationship between unconfined compressive strength and humic acid of cement-solidified sludge was similar to that between bound water content and humic acid. The analysis showed that humic acid tended to react with Ca(OH)₂ to form a complex, so that only a small amount of calcium ions were left to participate in pozzolanic reaction. Tremblay et al. [12] found that organic matter resulted in a pH of less than 9 in the solidified soil, where no cement hydration products were produced even at high sulfate concentrations. Du et al. [13] prepared artificial organic soil by using sodium humate and confirmed that the strength of solidified soil decreased significantly with the increase in organic content.

It can be seen from the above that the organic matter in sludge can seriously weaken the reinforcement effect and limit the application of cement solidification technology. To investigate the influence mechanism of organic matter on the geotechnical properties of cement-solidified dredged sludge (CDS), fulvic acid (FA), the main component of humic acid, was used to simulate the organic matter in sludge in this study. The effects of FA on the consistency limit and grain fabric of sludge were first investigated. Based on the results of the unconfined compression tests, the effects of FA content and cement content on the strength development were evaluated. By analyzing the evolution of water content and hydration heat during the curing process of CDS, the influence law of FA on cement hydration process was proved. The micromorphology and hydration products were analyzed in detail by using scanning electron microscopy and thermogravimetric analysis, which further promoted to reveal the influence mechanism of FA on the solidification effect.

2. Materials and Methods

2.1. Materials

2.1.1. Sludge

The sludge used in this study was taken from the bottom of a river in Wuhan, China. The basic physical properties of sludge were determined according to the relevant standards [14,15,16,17], and the results are shown in

Table 1. Physical characteristics of dredged sludge.

Property	Value	Standard
Specific gravity	2.72	ASTM D854-10 [14]
Liquid limit (wL), %	42.3	ASTM D4318-10 [15]
Plastic limit (wP), %	17.8
Plastic index (IP), %	24.5
Clay fraction (d < 0.005 mm), %	20.3	ASTM D422-63 [16]
Silt fraction (0.005 mm < d < 0.075 mm), %	75.8
Sand fraction (d > 0.075 mm), %	3.9
Organic matter content, %	2.1	ASTM D2974-14 [17]

. According to the Unified Soil Classification System (USCS), the sludge was classified as low-liquid-limit clay (CL). According to the X-ray diffraction (XRD) test (Figure 1), the sludge was mainly composed of quartz, kaolinite, illite and montmorillonite. The sludge was pretreated with hydrogen peroxide to avoid the interference of natural organic matter inside the sludge. Then, the sludge was thoroughly dried at 60 °C and finally mechanically broken into powder for reserve.

2.1.2. Curing Agent

The curing agent used in this study was ordinary Portland cement (PC, ASTM Type I), and the physical properties and chemical compositions are listed in

Table 2. Properties of cement utilized for this experiment.

Physical Property	Value	Chemical Composition (%)	Value	Mineral Composition	Value
Ignition loss, %	3.76	Silica, SiO2	21.3	C3S, %	56.54
Specific gravity	3.13	Calcium oxide, CaO	64.8	C2S, %	22.56
Fineness, m2/kg	354	Alumina, Al2O3	5.2	C3A, %	8.32
Initial setting time, min	208	Ferric oxide, Fe2O3	3.3	C4AF, %	10.32
Final setting time, min	258	Magnesium oxide, MgO	2.47
UCS a in 3d, MPa	30.3	Chloride, Cl−	0.021
UCS in 28d, MPa	43.2	Sulfur oxide, SO3	2.83
		Sodium oxide, Na2O	0.08

Note: ^a means unconfined compression strength.

.

2.1.3. Organic Matter

In this experiment, the powdered fulic acid (FA) reagent was used to simulate natural organic matter in sludge, and its molecular formula was C₁₄H₁₂O₈. According to the mass ratio of 1:100, the FA was added into deionized water, dilute HNO₃ solution with pH less than 2 and NaOH solution with pH greater than 12, respectively. The results showed that the FA was easily soluble in pure water, acid or base solutions. Additionally, the pH of pure FA aqueous solution is 4.2, indicating that FA itself was an acidic substance. To investigate the influence of FA on the properties of sludge, based on the dry weight of sludge, five types of sludge with varying FA content were prepared by adding 1.5%, 3%, 4.5%, 6% and 9% FA into sludge. All types of sludge were mixed with a certain amount of deionized water to an initial water content of 60% (1.42 fold the liquid limit).

2.2. Testing Methods

In order to observe the particle size characteristics and agglomeration structure of sludge visually, the sludge was diluted 10 fold and then placed on a glass slide for microscopic observation. The unconfined compression test (UCT) was carried out by a hydraulic servo testing machine at a vertical loading rate of 1 mm/min. Since the traditional drying test method will accelerate the hydration reaction of cement in the drying process, vacuum freeze-drying method was adopted in this study to test the water content of CDS at different curing ages. The hydration heat test (HHT) of CDS was determined by direct test method according to the Chinese standard [18]. The micromorphology of CDS was observed by using scanning electron microscopy (SEM). The weight change between 30 and 940 °C was assessed by performing thermogravimetric analysis (TGA) at the heating rate of 10 °C/min with argon as stripping gas. The crushed sample pieces after the UCT were immediately dried by the vacuum freeze-drying method and ground into powder for TGA testing.

2.3. Preparation of CDS Specimens

Table 3. Mix design and test items.

Symbol	FA Content (%)	Cement Content (%)	Curing Age (d)
			UCT	HHT	SEM	TGA
F0C20	0	20	14, 28, 60	0–14	28	7, 28
F1.5C20	1.5
F3C20	3
F4.5C20	4.5
F6C20	6	20	14, 28, 60	—	—	—
F6C30		30
F6C40		40

shows the mixed design and corresponding test items. The contents of FA, water and cement were all defined as their respective mass ratios to dry sludge. The prepared sludge was homogeneously mixed with cement in a mechanical mixer. Then, the freshly CDS slurry was subsequently poured into cylindrical PVC molds (50 mm in diameter and 50 mm in height) [19]. Afterward, the prepared specimens together with the molds were then transferred into a standard curing room (20 ± 2 °C, relative humidity ≥ 95%). Finally, the demolded specimens were sealed with a plastic membrane and cured until the predetermined period (i.e., 7, 14, 28, 60 days). It can be seen from Table 3 that the cement content of sludge with FA content of 0%, 1.5%, 3% and 4.5% is 20%, while the cement content of sludge with 6% FA is 20%, 30% and 40%, respectively. Three parallel samples were prepared for each mixture to assure reproducibility, and the average value was used as the final UCS.

3. Results and Discussion

3.1. Effect of FA on the Consistency Limit and Grain Fabric of Sludge

The influence of FA on the consistency limit of sludge is shown in Figure 2. It can be seen from Figure 2a that with the increase in FA content, the liquid limit of sludge decreased almost linearly after storing for 1 day. However, the liquid limit increased significantly with FA content after storing for 7 days. The plastic limit of sludge increased slightly with FA content. In addition, the plastic limit of sludge stored for 7 days was less than that stored for 1 day. Figure 2b showed the change of plastic path of sludge with FA content after storing for 7 days. The size of graphic symbols is proportional to the corresponding FA content (1.5%, 3%, 4.5%, 6%, 9%), indicating that with the increase in FA content, the sludge gradually changed from low-liquid-limit clay (CL) to high-liquid-limit clay (CH).

Figure 3 showed the microscopic morphology of the sludge at 500 fold magnification. As shown in Figure 3a, the soil particles in sludge with 0% FA were evenly distributed and the interface between them was clear. For the sludge containing 3% FA, a large number of flocs appeared and coated soil particles, resulting in fuzzy interface of soil particles, as shown in Figure 3b. There were many flocculent aggregates in the sludge containing 6% FA, as shown in Figure 3c. Therefore, it can be concluded that the dissolved FA can agglomerate soil particles to increase the particle size of the sludge.

For the sludge stored for 7 days, FA adsorbed on the surface of clay particles combined with pore water molecules through hydrogen bonding, resulting in the increase in water film thickness of clay particles. At the same time, soil particles were connected to each other by hydroxyl and carboxyl groups in FA. Therefore, the liquid limit and particle size of sludge increased with FA content. For the sludge stored for 1 day, the sugar in FA reduced the thickness of the water film of the soil particles, leading to the decrease in the liquid limit. The sugar in the pore solution gradually decomposed with storage time, and the properties of sludge tend to be stable. Therefore, it is suggested that the sludge containing FA should be stored for at least 7 days before the experimental study.

3.2. Role of FA on the Unconfined Compression Strength (UCS) of CDS

3.2.1. Effect of FA Content on the Strength of CDS

When the cement content is fixed at 20%, the influence of FA content on the strength of CDS is shown in Figure 4. The strength of F0C20 was 1410, 2630 and 3230 kPa for 14, 28, and 60 days, respectively, while the 28-day strength of F1.5C20 decreased to 1620 kPa. When the FA content increased to 4.5% (F4.5C20), the CDS showed almost no strength performance after curing for 14 days, and its 28-day strength was still less than 1000 kPa. Therefore, the strength of CDS decreased significantly with the FA content, indicating that FA has a significant weakening effect on the strength development of CDS.

To quantify the degradation effect of FA on the strength development of CDS more clearly, the strength loss rate (η_T, %) is defined as:

$${\eta }_T=\frac{S_0-S_x}{S_0}$$

(1)

where η_T is the strength loss rate; S₀ is the strength of CDS with 0% FA; S_x is the strength of CDS containing x% FA. The larger is the η_T, the greater is the negative effect of FA on the strength. As can be seen in Figure 5, the η_T decreased with the increase in FA content, and the η_T of CDS cured for 14, 28 and 60 days were all greater than 95% when the FA content reached 4.5%. Therefore, the detrimental effect of FA on the strength is very significant, especially in the early curing stage. For instance, the η_T of F3C20 cured for 14, 28 and 60 days were 70.3, 46.9 and 39.2%, respectively. It can be inferred that the deterioration effect of FA on the strength gradually decreased with curing age.

3.2.2. Effect of Cement Content on the Strength of CDS Containing 6% FA

The effect of cement content on the strength of CDS with high organic content (6% FA) is shown in Figure 6. When the cement content increased from 20% to 30%, the corresponding 28-day strength of CDS increased by less than 30%. Additionally, the 28-day strength of CDS increased from 81.5 to 173.2 kPa even when the cement content increased to 40%, which is still not conducive to the safety of geotechnical engineering projects. It is worth noting that the 56-day strength of CDS with 40% cement content increased from 127.4 to 937.6 kPa, indicating that increasing cement content can only improve the late curing strength of CDS with high FA content to a certain extent.

3.3. The Influence of FA on the Cement Solidification Effect

3.3.1. Cement Hydration Process

Cement react with liquid pore water in the sludge to form hydrates such as ettringite (AFt), calcium aluminate hydrate (CAH) and calcium silicate hydrate (CSH). Additionally, part of pore water can be converted into combined water as a component of the hydrates with curing age, resulting in the decrease in water content of CDS, as shown in

Table 4. The hydration reaction involved in cement solidification of sludge [20].

Reaction	Chemical Formulas
The hydration reaction between cement and pore water in the sludge	2(3CaO·SiO2) + 6H2O→3CaO·SiO2·3H2O + 3Ca(OH)2 C-S-H
	2(2CaO·SiO2) + 4H2O→3CaO·SiO2·3H2O + Ca(OH)2 C-S-H
	3CaO·Al2O3 + 6H2O + Ca(OH)2→3CaO·Al2O3·6H2O C-A-H
	4CaO·Al2O3·Fe2O3 + 7H2O→3CaO·Al2O3·6H2O + CaO·Fe2O3·H2O C-F-H
	4CaO·Al2O3·13H2O + 3(CaSO4·2H2O) + 14H2O→3CaO·Al2O3·3CaSO4·32H2O + Ca(OH)2 AFt

. Therefore, the water content evolution of CDS can reflect the hydration progress of cement and further evaluate the solidification effect. Figure 7 showed the water content of CDS with curing age when the cement content is 20%. The water content of CDS (the initial value was 50%) with different FA content gradually decreased with the curing age. For instance, the water content of F0C20 decreased almost linearly within 0.5 to 250 h after cement is added. However, with the increase in FA content, the decline rate of water content in CDS decreased significantly. Furthermore, the water content of F1.5C20, F3C20 and F6C20 began to decrease significantly after curing for 0.5, 2.2 and 4.3 h, respectively, indicating that FA can delay the hydration reaction of cement. In particular, the water content of F6C20 only decreased to 48.98% after curing for 28 days, which proved that the hydration reaction of cement was severely inhibited.

The heat released during the hydration reaction of cement can lead to the rise of the temperature in CDS. In this test, the temperature change of CDS with different FA content is shown in Figure 8. Since this experiment was conducted at room temperature, the initial temperature of CDS samples fluctuated slightly in the range of 23.3~25 °C. The temperature of CDS with different FA content increased first and then decreased with curing age. For example, the initial temperature of F0C20 is 24.53 °C, and its internal temperature increased to the maximum value of 28.11 °C after curing for 3.6 h. Then, the temperature gradually decreased to nearly 20 °C after curing for 100 h. Compared with F0C20, the temperature peak of CDS containing FA decreased significantly, indicating that the FA significantly delayed cement hydration.

3.3.2. SEM Analysis

The micromorphology of CDS with different FA content cured for 28 days is shown in Figure 9a–d. Two major hydrates, acicular ettringite (AFt) and amorphous calcium silicate hydrate (C-S-H), were observed in all four samples. Existing studies have confirmed that the microstructure of C-S-H was controlled by curing conditions (age, temperature, etc.) or pore fluid environment, in which the irregular spherical structure is the intermediate of transition to flake (final state) [21]. As shown in Figure 10a, a large number of acicular ettringite and spherical C-S-H were produced in F0C20. Among them, the ettringite with well-developed crystal structure interleaved between soil particles, forming a dense and solid structure with highest strength. It can be seen from Figure 9b that the hydrates in F6C20 were mainly small-size spherical C-S-H with poor cementation effect, leading to a loose structure with lower strength. Compared with F6C20, there were more irregular spherical C-S-H with larger size in F3C20, indicating that the development degree of C-S-H in F3C20 was better, as shown in Figure 9c. As shown in Figure 9d, a large number of C-S-H in F1.5C20 was firmly attached to the surface of clay particles in flake form, forming a dense network structure with higher strength. According to the above analysis, the FA in sludge can change the morphology of hydrates (especially C-S-H gel) by hindering the development of C-S-H from spheroidal to flake, thus weakening the internal bond and strength of CDS.

3.3.3. TGA Analysis

The TGA results of F0C20 and F3C20 were characterized by the mass loss curve or the first derivative curve of mass loss with temperature, as shown in Figure 10a. The mass loss in TG curves (or the peaks in DTG curves) correspond to the thermal decomposition of hydration products. It can be seen that the weight of F0C20 and F3C20 decreased continuously with temperature in the range of 0 to 940 °C. Additionally, the total weight loss rates of F0C20 at 7 and 28 days of curing were 10.29% and 14.3%, respectively, and that of F3C20 were 9.25% and 11.15%, indicating that the total amount of hydration products of CDS increased gradually with the curing age. It should be noted in particular that FA also decomposed at high temperature, but the total weight loss rate of CDS without FA (F0C20) was still higher than that of CDS with 3% FA (F3C20) at the same curing age, which further confirmed the inhibiting effect of FA on the formation of hydration products.

As shown in Figure 10a, the substances corresponding to the four endothermic peaks in the heating range of 40–940 °C are C-S-H or AFt (50–200 °C), C-A-H or C-A-S-H (210–310 °C), CH (320–470 °C) and CaCO₃ (570–730 °C) in sequence [22,23,24,25,26,27]. Additionally, the similar evolution characteristics of the four thermogravimetric curves also indicated that there was no significant difference in mineral types between F0C20 and F3C20. However, the amount of different hydrates produced in CDS can be calculated according to the thermogravimetric curve, as shown in Figure 10b. After curing for 28 days, the total weight loss rates of F0C20 and F3C20 at 50–310 °C were 5.56% and 4.11%, respectively, indicating that more C-S-H or C-A-H gels and AFt skeletons were produced in F0C20 (consistent with the higher strength shown in Figure 4). Therefore, the addition of 3% FA reduced the amount of cement hydration products and weakened the CDS strength.

4. Discussion

Through testing and analyzing the water content, hydration heat, microscopic morphology and mineral components of CDS in the curing process, it can be inferred that the weakening mechanism of FA on the strength of CDS is as follows: (1) FA adsorbed on the surface of cement minerals and formed a thin film, which hindered the contact between cement minerals and pore water, and further slowed down the dissolution of cement minerals; (2) acidic FA reduced the pH of the pore liquid in CDS, which is not conducive to cement hydration and pozzolanic reaction [28]; (3) the carboxyl and hydroxyl functional groups of FA adsorbed calcium ions in pore liquid through ion coordination, which hindered the formation of C-S-H, C-A-H and other hydrates.

5. Conclusions

(1)

For the sludge mixed with FA and stored for 7 days, the liquid limit and particle size of sludge increased with FA content, and the sludge also gradually changed from low-liquid-limit clay to high-liquid-limit clay.

(2)

The strength of CDS decreased significantly with the FA content, and the deterioration effect of FA on the strength decreased with curing age. Furthermore, increasing cement content can only improve the late curing strength of CDS with high FA content.

(3)

FA hindered the conversion of pore water to combined water and reduced the hydration heat inside CDS. At the same time, the addition of FA deteriorated the development degree of C-S-H gel and reduced the final amount of cement hydration products. In a word, FA weakened the strength performance of CDS by hindering the polymerization process of cement hydration reaction.

(4)

For the sludge rich in organic matter, the improvement effect of increasing cement content on strength is limited. The special curing agents with oxidizing effects should be developed to effectively regulate the solidification.