Experimental Study on the Strength and Stress–Strain Properties of Waste Concrete Fine Aggregate and Cement-Solidiﬁed Sludge

: In this study, waste concrete ﬁne aggregates and cement are applied to sludge solidiﬁcation and resource recycling. The unconﬁned compressive strength (UCS) test is performed to investigate the variation in the strength and stress–strain properties of the solidiﬁed sludge with the content and particle size of waste concrete ﬁne aggregate, cement content, and curing time. The results show that incorporating waste concrete ﬁne aggregates can improve the UCS of cement-solidiﬁed sludge, which can achieve the optimum effect when the ﬁne aggregate content ranges from 12% to 15%. However, compared with the ﬁne aggregate content, the cement content and curing time are the main factors in improving the strength of waste concrete ﬁne aggregate and cement-solidiﬁed sludge (WCSS). The stress–strain curves of WCSS comprise four stages. The failure strain ε f of WCSS with four ﬁne aggregate contents decreases in a power function with the increase in q u , and ε f is mostly distributed when the content is 1.1–2.1%. A linear relationship is observed between E 50 and q u . This study attempts to promote the recycling of waste concrete ﬁne aggregates and obtain solidiﬁed sludge with excellent mechanical properties, providing some reference for practical engineering applications.


Introduction
With the rapid economic boom and urbanization, China has accelerated the seaport extension and large-scale land reclamation in coastal areas to address increasing land resources in short supply. Therefore, massive amounts of dredged silt has inevitably been produced in the governance of ports, harbors, inland rivers and lakes, and channels [1,2], with its end disposal bringing a heavy economic burden and environmental problems to society. To achieve a virtuous circle of economy and the environment, reasonable use of idle dredged sludge has become a research hotspot in recent years, represented by the current solidification treatment of sludge with curing agents [3][4][5][6]. Cement can generate cementation products, namely CAH and CSH, through a series of chemical reactions, which can bond soil particles and form clusters. Thus, cement has been extensively used as a curing agent to solidify sludge [7].
At the end of the last century, concerning the solidification mechanism of cement soil [8,9], the hydration products generated by the early hydrolysis and hydration of cement ensured the rapid improvement of the early strength of cement soil, while the pozzolanic reaction could promote the bonding strength between cement soil particles, allowing for long-term enhanced strength of the cement soil. With the wide application of cement soil in engineering, Portland cement production inevitably emits a large amount of carbon dioxide, accounting for 5-8% of the total anthropogenic emissions and causing a great burden on the environment [10,11]. Dredging sludge has a high content of organic matter, which is usually accompanied by organic and inorganic pollutants [12]. Moreover, the high moisture content in the sludge, the large proportion of fine-grained soil, and the poor gradation are not conducive to forming hydration products. The loose and porous structure of solidified products makes it difficult to construct the skeleton effectively [13], and the ideal curing effect is hardly achieved by increasing the cement content alone. Therefore, many scholars focus on reducing the amount of cement in sludge solidification to lessen the cement production-induced carbon dioxide emission and realize environmental protection. Other curing materials are applied to partially replace and interact with cement to form a composite-curing agent with an enhanced solidification effect, which has become the current research hotspot.
Liu [14] used steel slag, cement, and metakaolin as composite-curing agents to solidify soft clay. The results indicated that the composite-curing agent could effectively stabilize soft clay. In response to the difficulty in treating sludge with a high organic matter content using a cement-curing agent, Gu [15] developed a novel solidifier with cement, fly ash, slag, phosphogypsum as a curing agent, and the strong oxidants KMnO 4 and GH as additives. The results showed that soft clay with a high organic content could obtain a better curing effect after using alkali-activated cementitious material and a strong oxidant. Zhang et al. [16] added the cement and ethylene-vinyl acetate copolymer to loess as a soilcuring agent. They observed that with an increase in the ethylene-vinyl acetate copolymer, the cement represented increased hydration products, decreased pores, and dense structure, obviously improving the performance of stabilized loess. In addition, Estabragh [17] found that an appropriate fiber content could improve the unconfined compressive strength (UCS) and axial strain at failure of cement soil, which transformed the brittle properties of the cement soil's change in ductility. The research and application of cement-based compositecuring agents verified the feasibility of curing sludge with a composite-curing agent.
The production of construction waste in China generally covers about 30% of municipal garbage production, with a total annual output of up to two billion tons [18] and increasing at a rate of 10% each year. Among the construction waste, waste concrete accounts for 50-60% of it [19], occupying limited land resources and severely threatening the surrounding environment every year. Currently, one method of recycling waste concrete is to crush, clean, and grade waste concrete to obtain recycled aggregate particles. The particles with a size less than 4.75 mm after crushing are called recycled fine aggregate, an artificial aggregate with a complex composition and high permeability generally used to prepare recycled mortar [20]. The various properties of waste concrete aggregates have also been explored [21][22][23]. Still, the research on the recycling of waste concrete fine aggregates in engineering is lacking, especially for waste concrete fine aggregate-solidified sludge. This is mainly because waste concrete fine aggregate does not react with sludge directly, and the effect of using waste concrete fine aggregate alone to solidify sludge is not obvious. However, waste concrete fine aggregate has a certain strength. When it interacts with other curing agents capable of generating cementation products, the generated cementation products can make soil particles attach to the skeleton formed by the aggregate. Moreover, fine aggregate can also absorb excess water and improve the strength of the sample to a certain extent.
Therefore, this study uses fine aggregate with a particle size of 0-4.75 mm obtained from waste concrete treatment as an additive to solidify the dredged sludge together with cement. While reusing construction waste resources, the dredged sludge can also achieve an excellent solidification effect. Based on the UCS test data, the effects of the fine aggregate content, particle size, cement content, and curing time on the strength of the waste concrete fine aggregate and cement-solidified sludge (WCSS) were investigated. The stress-strain characteristics of the WCSS were analyzed, and the feasibility of WCSS was proven.

Materials
The sludge used in this study was obtained from a coastal dredged sludge dump in the Wenzhou area (referred to as Wenzhou sludge). A laser particle size analyzer was employed to analyze the particle size of the sludge sample, with the physical property indexes and particle size composition shown in Table 1. The Wenzhou sludge samples were classified as high liquid limit clay CH [24]. Figure 1 shows the X-ray diffraction test results. As can be observed, in the Wenzhou sludge, quartz was the main mineral component. The contents of illite and kaolinite were also high, while the contents of montmorillonite and mica were low. The Wenzhou sludge had a relative density of 2.69 and a natural moisture content of 90-110%, about twice the liquid limit. The moisture content of the undisturbed sludge was adjusted to 125% to meet the test requirements. The cement used in the test was P. O42. 5 ordinary Portland cement produced by Huaxin Cement Co., Ltd. in Ezhou, China, with the chemical composition shown in Table 2. The waste concrete fine aggregate came from the waste concrete in the road reconstruction project. The concrete was crushed by a jaw crusher to obtain waste concrete fine aggregate with particle sizes of 0-2.36 mm and 2.36-4.75 mm. Then, the waste concrete fine aggregate was dried. Table 3 lists the performance specifications of the waste concrete fine aggregate.

Experimental Methods
To study the influence of the size and content of the waste concrete fine aggregate, cement content, and curing time on the strength and stress-strain properties of solidified sludge, in this experiment, a sludge sample with a water content of 125% was utilized, and waste concrete fine aggregate particles with sizes of B1 (0-2.36 mm) and B2 (2.36-4.75 mm) were selected. To investigate the effect of the content of fine aggregate particles and cement on solidified sludge, samples with four contents Ci of waste concrete particles (defined as the mass ratio of waste concrete fine aggregate to dry silt soil) were selected, which were 9%, 12%, 15%, and 18%. The four cement contents Di (defined as the mass ratio of cement to dry silt soil) were 10%, 12%, 14%, and 16%. In the experiment, a control

Experimental Methods
To study the influence of the size and content of the waste concrete fine aggregate, cement content, and curing time on the strength and stress-strain properties of solidified sludge, in this experiment, a sludge sample with a water content of 125% was utilized, and waste concrete fine aggregate particles with sizes of B 1 (0-2.36 mm) and B 2 (2.36-4.75 mm) were selected. To investigate the effect of the content of fine aggregate particles and cement on solidified sludge, samples with four contents C i of waste concrete particles (defined as the mass ratio of waste concrete fine aggregate to dry silt soil) were selected, which were 9%, 12%, 15%, and 18%. The four cement contents D i (defined as the mass ratio of cement to dry silt soil) were 10%, 12%, 14%, and 16%. In the experiment, a control group with a fine aggregate content of 0% was set for comparison. The detailed information of 120 kinds of samples is displayed in Table 4. Moreover, three groups of parallel samples were prepared for each kind of sample. The curing temperature was set at 30 • C, higher than the normal curing temperature of 25 • C. This could accelerate the hydration and pozzolanic reaction between the cement, water, and sludge soil particles after the samples were kept in closed plastic bags for curing. The UCS parameters of the samples were measured at 7, 14, and 28 d with a TSZ-1A automatic triaxial apparatus following standard practices [25]. Figure 2 shows the triaxial apparatus used in this test and the partially prepared WCSS specimens.

Experimental Methods
To study the influence of the size and content of the waste concrete fine aggregate, cement content, and curing time on the strength and stress-strain properties of solidified sludge, in this experiment, a sludge sample with a water content of 125% was utilized, and waste concrete fine aggregate particles with sizes of B1 (0-2.36 mm) and B2 (2.36-4.75 mm) were selected. To investigate the effect of the content of fine aggregate particles and cement on solidified sludge, samples with four contents Ci of waste concrete particles (defined as the mass ratio of waste concrete fine aggregate to dry silt soil) were selected, which were 9%, 12%, 15%, and 18%. The four cement contents Di (defined as the mass ratio of cement to dry silt soil) were 10%, 12%, 14%, and 16%. In the experiment, a control group with a fine aggregate content of 0% was set for comparison. The detailed information of 120 kinds of samples is displayed in Table 4. Moreover, three groups of parallel samples were prepared for each kind of sample. The curing temperature was set at 30 °C, higher than the normal curing temperature of 25 °C. This could accelerate the hydration and pozzolanic reaction between the cement, water, and sludge soil particles after the samples were kept in closed plastic bags for curing. The UCS parameters of the samples were measured at 7, 14, and 28 d with a TSZ-1A automatic triaxial apparatus following standard practices [25]. Figure 2 shows the triaxial apparatus used in this test and the partially prepared WCSS specimens.   Two groups of waste concrete fine aggregate with particle sizes of 0-2.36 mm and 2.36-4.75 mm were selected for the experiment. The influence of the particle size of the waste concrete fine aggregate on the test strength was analyzed by the unconfined compression test. The variations in strength of the samples of two kinds of particle sizes with an increasing cement content under four curing periods are shown in Figure 3. Although the UCS value varied in the two groups, the maximum difference in the UCS value between the two groups was only 7.78 kPa, 7.05 kPa, 9.2 kPa, and 10.92 kPa, which was not significant. This indicated that when the particle size of the waste concrete was within 0-4.75 mm, it had little effect on the UCS of the WCSS.

Content of Waste Concrete Fine Aggregate
The particle size of the waste concrete fine aggregate had little effect on the UCS when it was 0-4.75 mm (as illustrated in Section 3.1.1). On this basis, in this section, the samples with a particle size of 0-2.36 mm were selected to analyze the effect of the content of the waste concrete fine aggregate on the strength of the sample. Figure 4 shows the UCS variations of specimens of four cement contents with increased fine aggregate contents. Compared with the cement-solidified sludge samples, the WCSS represented an effectively improved UCS at various ages. With the elevated fine aggregate content, the UCS of the WCSS increased first and then decreased. The maximum UCS mostly occurred when the fine aggregate content was 12% or 15%.

Content of Waste Concrete Fine Aggregate
The particle size of the waste concrete fine aggregate had little effect on the UCS when it was 0-4.75 mm (as illustrated in Section 3.1.1). On this basis, in this section, the samples with a particle size of 0-2.36 mm were selected to analyze the effect of the content of the waste concrete fine aggregate on the strength of the sample. Figure 4 shows the UCS variations of specimens of four cement contents with increased fine aggregate contents. Compared with the cement-solidified sludge samples, the WCSS represented an effectively  • The UCS of the sample was macroscopically determined by the synergistic action of the cement-solidified sludge and the support skeleton formed by the waste concrete fine aggregate. When a small number of fine aggregates was added, the waste concrete fine aggregates were surrounded and separated by soil, leading to the failure to form a complete support skeleton, or the support skeleton was discontinuous in the soil. The integrity of the support skeleton could be improved with an enhanced UCS by increasing the fine aggregate content of the waste concrete. The mixed cement could also react to generate cementation products that filled the pores in the soil and the skeleton, bonding the soil particles to the skeleton and further improving the UCS of the solidified sludge. However, excess waste concrete fine aggregates failed to strengthen the formed support skeleton and even harmed the skeleton. When stressed, the original skeleton structure in the soil was destroyed, and excess fine aggregates also hindered the effective connection of cementation products generated by cement hydrolysis and hydration reactions. As a result, the UCS of the sample decreased. The curing period of 28 d was set for analysis. With the growing content of the waste concrete fine aggregate, the UCS of the WCSS with four cement contents was continuously improved. An inflection point appeared when the content was enhanced from 12% to 15%, where the UCS reached the maximum.
Compared with those of the cement-solidified sludge samples (the fine aggregate content was 0%), the maximum UCS values of the four WCSS samples rose by 10.83%, 8.52%, 11.6%, and 8.58%. The UCS declined after the inflection point, and the curing effect decreased with a fine aggregate content of 18%. Compared with the maximum UCS, the UCS decreased by 6.8%, 3.06%, 5.23%, and 2.55%. However, a fine aggregate content of 18% could still improve the effect of cement-solidified soil. Two main factors affected the UCS variation of the samples.

•
The UCS of the sample was macroscopically determined by the synergistic action of the cement-solidified sludge and the support skeleton formed by the waste concrete fine aggregate. When a small number of fine aggregates was added, the waste concrete fine aggregates were surrounded and separated by soil, leading to the failure to form a complete support skeleton, or the support skeleton was discontinuous in the soil. The integrity of the support skeleton could be improved with an enhanced UCS by increasing the fine aggregate content of the waste concrete. The mixed cement could also react to generate cementation products that filled the pores in the soil and the skeleton, bonding the soil particles to the skeleton and further improving the UCS of the solidified sludge. However, excess waste concrete fine aggregates failed to strengthen the formed support skeleton and even harmed the skeleton. When stressed, the original skeleton structure in the soil was destroyed, and excess fine aggregates also hindered the effective connection of cementation products generated by cement hydrolysis and hydration reactions. As a result, the UCS of the sample decreased.

•
With the water content elevated, the distance between the soil particles and the clusters was enlarged, resulting in weaker soil fabric and cementation. Therefore, a high water content also greatly impacted the UCS [13]. Due to the high porosity and coarse surface of the waste concrete fine aggregate, an appropriate content of waste concrete fine aggregate could absorb water in the sludge, thus causing a water content reduction. Finally, the UCS of the dredging sludge was improved. However, after adding too many waste concrete fine aggregates, redundant fine aggregates absorbed the moisture that should have undergone hydrolysis and hydration reactions with the cement. Therefore, the water content of the sample was further reduced, resulting in insufficient solidification, fewer cementation products, and resultant failure to form an effective link between the soil particle clusters and the waste concrete fine aggregates. Based on the above test results, the UCS improvement of the sample was most significant when the content of the waste concrete fine aggregate was 12-15%. Figure 5 shows the variation in UCS with the cement content under four contents of waste concrete fine aggregate and different curing times. Under constant temperature water bath curing, when the cement content increased from 10% to 16%, the UCS of the solidified sludge sample was significantly improved, being positively correlated with the cement content. When the curing time was 28 d, compared with that of the specimens with 10% cement content, the maximum UCS of the specimens with 16% cement content increased by 38.15%, 37.02%, 45.14%, and 45.95% under four particle contents. During curing, the cement continued to undergo hydrolysis and hydration reactions. The cementation products gradually filled the pores between the silt particles and the waste concrete fine aggregates. Then, effective connections were formed between the silt particles and between the silt particles and the fine aggregates. Increasing the cement content could result in more generated cementation products and an increased UCS for the sample. The waste concrete fine aggregates were not involved in the series of hydrolysis and hydration reactions. Compared with the content of waste concrete fine aggregate, the cement content exerted a more significant impact on the UCS of the WCSS.

Multiple Linear Regression Analysis of the UCS of the WCSS
Based on the test data of the WCSS, the UCS of the sample group with a particle size of 0-2.36 mm was analyzed by multiple linear regression using SPSS software [26]. The dependent variable y was set as the UCS of the solidified sludge, x 1 was the fine aggregate content of the waste concrete, x 2 was the cement content, and x 3 was the curing time.
As shown in Table 5, the adjusted R 2 was 0.959, indicating a high fitting degree for the linear regression equation. The Durbin-Watson index was 1.712, which was less than 4, identifying that the error in the analysis was independent. The multivariate linear regression equation obtained showed excellent accuracy. The significance level was less  (Table 6), indicating the impact of the content of the waste concrete fine aggregate, cement content, and curing age on the UCS.

Multiple Linear Regression Analysis of the UCS of the WCSS
Based on the test data of the WCSS, the UCS of the sample group with a particle size of 0-2.36 mm was analyzed by multiple linear regression using SPSS software [26]. The dependent variable y was set as the UCS of the solidified sludge, x1 was the fine aggregate content of the waste concrete, x2 was the cement content, and x3 was the curing time.
As shown in Table 5, the adjusted R 2 was 0.959, indicating a high fitting degree for the linear regression equation. The Durbin-Watson index was 1.712, which was less than 4, identifying that the error in the analysis was independent. The multivariate linear regression equation obtained showed excellent accuracy. The significance level was less than 0.05 (Table 6), indicating the impact of the content of the waste concrete fine aggregate, cement content, and curing age on the UCS.    As shown in Table 7, the unstandardized coefficients of the model were obtained. The coefficient of the fine aggregate content was 0.379, that of the cement content was 6.545, and that of the curing time was 5.377. Thus, a multivariate linear regression equation was established, as shown in Equation (1), where the significance levels of x 2 and x 3 (cement content and curing time, respectively) were less than 0.05, indicating that the cement content and curing time impacted the UCS significantly. In contrast, the significant level of x 1 (fine aggregate content of the waste concrete) was greater than 0.05, indicating that compared with the cement content and curing time, the fine aggregate content of the waste concrete was not the main factor affecting the strength, consistent with the conclusion in Section 3.1.3. The maximum standard residual was 1.745 (Table 8), which was less than 3, indicating no abnormal phenomenon in the model: y = −54.654 + 0.379x 1 + 6.545x 2 + 5.377x 3 (1)

Stress-Strain Curve
The optimal fine aggregate content in the waste concrete was 12-15% (Section 3.1.2). Therefore, the representative sample group with a fine aggregate dosage of 12%, a particle size of 0-2.36 mm, and a cement content of 16% was selected to analyze the influence of the curing age on the stress-strain curve. Figure 6 displays the stress-strain curve of this sample group obtained from the UCS test of the WCSS sample. As can be seen, the stress-strain curves under different curing periods showed similar distributions, being roughly divided into four stages.
The first stage was the compaction stage, which featured a nonlinear stress-strain relationship and concave curve. The main reason for this was the large fine aggregate particle size of the waste concrete, the rough and uneven surface, and the high porosity of the WCSS sample. When the fine aggregates were added to the cement-solidified sludge, they could not be uniformly distributed to the sample. Therefore, the sample porosity increased. Under pressure, the pores were closed, and the solidified sludge deformation was intensified.
The second stage was the elastic deformation stage, characterized by an approximately linear stress-strain curve. With the increasing uniaxial pressure, the porosity of the solidified sludge decreased, and the sample presented a dense structure. The sample deformation was mainly elastic. The first stage was the compaction stage, which featured a nonlinear stress-strain relationship and concave curve. The main reason for this was the large fine aggregate particle size of the waste concrete, the rough and uneven surface, and the high porosity of the WCSS sample. When the fine aggregates were added to the cement-solidified sludge, they could not be uniformly distributed to the sample. Therefore, the sample porosity increased. Under pressure, the pores were closed, and the solidified sludge deformation was intensified.
The second stage was the elastic deformation stage, characterized by an approximately linear stress-strain curve. With the increasing uniaxial pressure, the porosity of the solidified sludge decreased, and the sample presented a dense structure. The sample deformation was mainly elastic.
The third stage was the plastic yield stage. When the stress of the WCSS sample was close to the peak value, the stress-strain curve was nonlinearly correlated, and the curve was bent. Additionally, the sample deformation was mainly plastic. The stress increased slowly with the elevated strain, and the sample deformation entered the yield stage. In this process, the sample elasticity declined, and the plasticity rose. After reaching the peak value, with the continuous enhancement of strain, the yield stress of the sample began to decrease. The stress-strain curve showed a linear downward trend, and obvious cracks began to appear in the sample.
The fourth stage was the destruction stage. As the strain continued to increase, the stress lessened rapidly, indicating that the WCSS sample gradually lost its bearing capacity, and the failure form was mainly a brittle failure.
The comparison of the stress-strain curves under different curing periods indicated that with the prolonged curing time, the descending section was steeper, and the inflection point of the plastic yield stage was more obvious; that is, the WCSS represented an increased brittleness and decreased plasticity.  The third stage was the plastic yield stage. When the stress of the WCSS sample was close to the peak value, the stress-strain curve was nonlinearly correlated, and the curve was bent. Additionally, the sample deformation was mainly plastic. The stress increased slowly with the elevated strain, and the sample deformation entered the yield stage. In this process, the sample elasticity declined, and the plasticity rose. After reaching the peak value, with the continuous enhancement of strain, the yield stress of the sample began to decrease. The stress-strain curve showed a linear downward trend, and obvious cracks began to appear in the sample.

Effect of the Curing Time
The fourth stage was the destruction stage. As the strain continued to increase, the stress lessened rapidly, indicating that the WCSS sample gradually lost its bearing capacity, and the failure form was mainly a brittle failure.
The comparison of the stress-strain curves under different curing periods indicated that with the prolonged curing time, the descending section was steeper, and the inflection point of the plastic yield stage was more obvious; that is, the WCSS represented an increased brittleness and decreased plasticity. Figure 7 shows the variation trend of the stress and strain of the WCSS specimens with the curing time increased when the fine aggregate content of the waste concrete was 12%. The UCS q u grew linearly with the prolonged curing age. The UCS values of the samples were significantly improved during 28 d curing. During the curing period from 7 d to 28 d, the q u of the four groups of samples rose by 99.61 kPa, 108.5 kPa, 130.17 kPa, and 139.2 kPa. Under the same cement content, ε f decreased linearly with the increasing curing time, indicating that the brittleness of the WCSS samples increased with the growth of the curing age. At the same curing time, a higher cement content implied a greater brittleness of the WCSS, consistent with the variation in the strength and strain of the cement soil with the curing time in Li's research [27].

Effect of the Curing Time
Due to the rapid hydrolysis and hydration of the cement at the early stage of curing, tricalcium silicate could react to form hydration products which have cementation action. Hydration products made rapid connections between adjacent soil particles and between the soil particles and the waste concrete fine aggregates. Cementation products could also be attached to the skeleton formed by the waste concrete fine aggregates, thus further improving the strength of the sample. At this time, the solidified sludge began to show the characteristics of hardening, and the failure morphology of the sample under external force was gradually dominated by brittle failure. Therefore, ε f decreased rapidly, while q u quickly increased. The UCS of the cement soil could be enhanced rapidly with its curing time due to its early hydrolysis and hydration reaction. The WCSS also showed this property, whose hydrolysis and hydration reaction enabled the UCS to increase rapidly with the curing time. In addition, the WCSS could show strain hardening and strain softening properties similar to cement soil with the curing time. The curing time was an important factor for strength improvement and hardening of the WCSS. This also reflected that the incorporation of waste concrete fine aggregate did not change the properties of the cement soil. Due to the rapid hydrolysis and hydration of the cement at the early stage of curing, tricalcium silicate could react to form hydration products which have cementation action. Hydration products made rapid connections between adjacent soil particles and between the soil particles and the waste concrete fine aggregates. Cementation products could also be attached to the skeleton formed by the waste concrete fine aggregates, thus further improving the strength of the sample. At this time, the solidified sludge began to show the characteristics of hardening, and the failure morphology of the sample under external force was gradually dominated by brittle failure. Therefore, εf decreased rapidly, while qu quickly increased. The UCS of the cement soil could be enhanced rapidly with its curing time due to its early hydrolysis and hydration reaction. The WCSS also showed this property, whose hydrolysis and hydration reaction enabled the UCS to increase rapidly with the curing time. In addition, the WCSS could show strain hardening and strain softening properties similar to cement soil with the curing time. The curing time was an important factor for strength improvement and hardening of the WCSS. This also reflected that the incorporation of waste concrete fine aggregate did not change the properties of the cement soil.   Figure 8 illustrates the relationship between the UCS q u and the failure strain ε f of the specimens with four fine aggregate contents of waste concrete of 9%, 12%, 15%, and 18%. The q u increased with the decreasing failure strain, indicating that the WCSS showed the characteristics of hardening. This indicates that the deformation of the WCSS was small when the bearing capacity was not lost. This was because the cementation products generated by the cement continued to make soil particles form clusters and strengthen the supporting effect of the skeleton formed by the fine aggregate. Moreover, the cementation products could be continuously generated with the increasing curing time and fill the pores in the solidified sludge, as well as the pores between the soil particles and the fine aggregates. As a result, the solidified sludge structure became increasingly dense. The failure strain was mostly between 1.1% and 2.1%. Therefore, the WCSS showed excellent engineering properties. when the bearing capacity was not lost. This was because the cementation products generated by the cement continued to make soil particles form clusters and strengthen the supporting effect of the skeleton formed by the fine aggregate. Moreover, the cementation products could be continuously generated with the increasing curing time and fill the pores in the solidified sludge, as well as the pores between the soil particles and the fine aggregates. As a result, the solidified sludge structure became increasingly dense. The failure strain was mostly between 1.1% and 2.1%. Therefore, the WCSS showed excellent engineering properties. According to the study of ordinary cement solidified soil by Tang et al. [28], the relationship between the εf and qu of cement soil is a power function. This relation could also be used to fit the UCS and failure strain of the WCSS in this experiment. The fitting correlation coefficient increased with the rising fine aggregate content. The power function of each fine aggregate content can be expressed as in Equations (2) According to the study of ordinary cement solidified soil by Tang et al. [28], the relationship between the ε f and q u of cement soil is a power function. This relation could also be used to fit the UCS and failure strain of the WCSS in this experiment. The fitting correlation coefficient increased with the rising fine aggregate content. The power function of each fine aggregate content can be expressed as in Equations (2)- (5).

Relationship between the UCS and Failure Strain
In the resource utilization of WCSS, the strain corresponding to a specific strength was estimated according to this empirical equation, which had a certain engineering application value.

Relationship between E 50 and q u
The deformation modulus refers to the ratio of compressive stress to corresponding compressive strain under unconfined conditions, which can reflect the ability of a material to resist elastic-plastic deformation. The deformation modulus is an essential parameter in calculating ground settlement. Therefore, it is significant to investigate the deformation modulus of WCSS. However, since the WCSS sample had nonlinear deformation, the deformation modulus was not constant. For this reason, this paper used the deformation modulus E 50 to characterize the deformation characteristics of the sample. E 50 is the slope of the origin corresponding to 50% of peak stress on the stress-strain curve, as expressed in Equation (6): where ε 0 . 5 is the strain value corresponding to the stress of 0.5 q u in the stress-strain curve. Figure 9 shows the linear fitting relationship based on the data points of E 50 and q u when the fine aggregate particle size was 0-2.36 mm. When q u increased, E 50 was enhanced linearly. Considering the influence of the cement content and fine aggregate content of waste concrete, the points of E 50 and q u were all concentrated in the range of E 50 = (38.87-83.33)q u : E 50 = 70.77q u , R 2 = 0.98 (7) to resist elastic-plastic deformation. The deformation modulus is an essential parameter in calculating ground settlement. Therefore, it is significant to investigate the deformation modulus of WCSS. However, since the WCSS sample had nonlinear deformation, the deformation modulus was not constant. For this reason, this paper used the deformation modulus E50 to characterize the deformation characteristics of the sample. E50 is the slope of the origin corresponding to 50% of peak stress on the stress-strain curve, as expressed in Equation (6): where ε0.5 is the strain value corresponding to the stress of 0.5 qu in the stress-strain curve. Figure 9 shows the linear fitting relationship based on the data points of E50 and qu when the fine aggregate particle size was 0-2.36 mm. When qu increased, E50 was enhanced linearly. Considering the influence of the cement content and fine aggregate content of waste concrete, the points of E50 and qu were all concentrated in the range of E50 = (38.87-83.33)qu: Equation (7) can be used to fit the relationship between E50 and qu and the correlation coefficient. Equation (7) can be used to fit the relationship between E 50 and q u and the correlation coefficient.
By calculation, R 2 = 0.97. According to this equation and the above range relationship, the deformation modulus of the solidified sludge under a certain strength could be estimated. The equation and range could be used as a parameter guide for analyzing the elastic-plastic deformation resistance and resource utilization of WCSS.

Effect of the Curing Time and Fine Aggregate Content on the Deformation
Modulus E 50 Figure 10 shows the variation in E 50 of the content samples of four fine aggregate contents with an increasing curing time and the fine aggregate content when the cement content was 16% and the fine aggregate particle size was 0-2.36 mm. With an increased curing time, the E 50 values of the four kinds of solidified sludge were significantly improved. With a curing time from 7 d to 14 d, the maximum E 50 value of the solidified sludge was 8.95 MPa, and the maximum increase was 4.47 MPa. During the curing period of 7 d, despite the rapid hydrolysis and hydration reaction of the cement, fewer cementation products were generated, and the ability of the WCSS to resist deformation was insufficient. When the specimen underwent damage from an external force and large strain during deformation, the E 50 value was low. With the prolonged curing time, the cement fully reacted, and the generated cementation products could establish effective connections in the sample. The cohesion between soil particles gradually increased, and the solidification effect was obvious. The stress-strain curve of the solidified sludge became steeper with the increasing curing time, showing that the brittleness of the solidified sludge increased and the plasticity decreased. Therefore, when the specimen was damaged, the failure strain was small, and ε 0 . 5 was also lessened. Therefore, E 50 continued to grow in the late stage of curing. of 7 d, despite the rapid hydrolysis and hydration reaction of the cement, fewer cementation products were generated, and the ability of the WCSS to resist deformation was insufficient. When the specimen underwent damage from an external force and large strain during deformation, the E50 value was low. With the prolonged curing time, the cement fully reacted, and the generated cementation products could establish effective connections in the sample. The cohesion between soil particles gradually increased, and the solidification effect was obvious. The stress-strain curve of the solidified sludge became steeper with the increasing curing time, showing that the brittleness of the solidified sludge increased and the plasticity decreased. Therefore, when the specimen was damaged, the failure strain was small, and ε0.5 was also lessened. Therefore, E50 continued to grow in the late stage of curing.  Figure 10 also reflects the variation in E50 for the WCSS with different fine aggregate contents. When the fine aggregate content was less than 15%, E50 increased with the enhanced fine aggregate content. When the dosage exceeded 15%, E50 began to decrease. The reason for this was that when the fine aggregate content was more than 15%, excess fine aggregates would absorb too much water, and the hydrolysis and hydration reaction of the cement were not complete. As a result, insufficient cementation products filled the pores in the WCSS. The synergistic effect between the cementation products generated by the cement and the support skeleton formed by the waste concrete fine aggregate was also affected, and the WCSS structure was not dense enough. Excess fine aggregate particles also negatively impacted the skeleton of the WCSS sample. Moreover, it also caused friction and collision with the formed skeleton when the sample was under stress and destroyed the support skeleton. Finally, the ability of the WCSS to resist deformation would decrease to some extent. Therefore, an ideal curing effect could be achieved when the fine aggregate content was moderate, proving that the optimal dosage of waste concrete fine aggregate was 12-15%.  Figure 10 also reflects the variation in E 50 for the WCSS with different fine aggregate contents. When the fine aggregate content was less than 15%, E 50 increased with the enhanced fine aggregate content. When the dosage exceeded 15%, E 50 began to decrease. The reason for this was that when the fine aggregate content was more than 15%, excess fine aggregates would absorb too much water, and the hydrolysis and hydration reaction of the cement were not complete. As a result, insufficient cementation products filled the pores in the WCSS. The synergistic effect between the cementation products generated by the cement and the support skeleton formed by the waste concrete fine aggregate was also affected, and the WCSS structure was not dense enough. Excess fine aggregate particles also negatively impacted the skeleton of the WCSS sample. Moreover, it also caused friction and collision with the formed skeleton when the sample was under stress and destroyed the support skeleton. Finally, the ability of the WCSS to resist deformation would decrease to some extent. Therefore, an ideal curing effect could be achieved when the fine aggregate content was moderate, proving that the optimal dosage of waste concrete fine aggregate was 12-15%.

Conclusions
In this study, the strength and stress-strain properties of the WCSS were analyzed by a UCS test. The main conclusions are as follows: (1) When the fine aggregate content of the waste concrete was 12-15%, the improvement effect of the UCS of the WCSS was the most obvious. This indicates the feasibility of improving the strength of cement-solidified sludge by adding waste concrete fine aggregates. Nevertheless, compared with the fine aggregate content, the cement content and curing time were the main factors affecting the UCS. (2) Multiple linear regression analysis between the UCS, fine aggregate content, cement content, and curing time satisfied the equation y = −54.654 + 0.379x 1 + 6.545x 2 + 5.377x 3 , where y is q u , x 1 is the fine aggregate content, x 2 is the cement content, and x 3 is the curing time. This equation can be used to estimate the UCS of WCSS samples when the fine aggregate content, cement content, and curing age are known.
(3) The stress-strain curves of the solidified sludge changed with different waste concrete particle contents, cement contents, and curing ages, which comprised four stages: the compaction stage, elastic deformation stage, plastic yield stage, and destruction stage. The peak strength of the WCSS increased with the decrease in the failure strain. The failure strain in this study was mostly distributed between 1.1% and 2.1%. Under four fine aggregate contents, the relationship between the strength and failure strains of solidified sludge can be expressed by a power function. (4) There was a linear relationship between the deformation modulus E 50 and the UCS of the WCSS, and the E 50 corresponding to a certain strength value could be fitted by Equation (7). When the fine aggregate content was 9-15%, E 50 increased with the rising fine aggregate content, but when the content was 15-18%, E 50 began to decrease.
The above research shows that adding waste concrete and cement can solidify sludge. Compared with cement-solidified soil, WCSS presents a higher strength and favorable engineering characteristics. However, this study only tested WCSS within a 28-d curing time. Whether a synergistic effect between the cementation products and support skeleton can be further promoted under long-term curing requires further study.