Effect of Super Retarder on Recycled Water and Concrete Properties of Waste Slurry in Mixing Plant

Abstract

A large amount of waste slurry water will be generated in the production process of concrete mixing plant, due to the complex composition of waste slurry water, if it is not handled in time when stored, serious coagulation will occur, which will accelerate the loss of equipment and reduce the utilization rate. In this paper, a super retarder suitable for waste slurry recycled water from concrete mixing plant was prepared using composite technology. The waste slurry recycled water mixed with super retarding agent was characterized by using microscopic testing means XRD, TG and DTG. The waste slurry recycled water mixed with super retarding agent was used to replace tap water in the production of concrete, and its effect on the workability and mechanical properties of concrete was investigated. It was found that the compounding of Butane 2-phospho-1,2,4-tricarboxylic acid (PBTCA) with Reclaimed water treatment agent (ACS) resulted in a setting time of 64 h for 10% concentration of recycled water, with optimal retarding effect. When PBTCA:ACS was 1:20, mixed at 1.5% of the mass of recycled water, the 1 h slump of concrete had no loss, the loss of extension was 15 mm, the 7 days compressive strength was increased by 3.5 MPa, and the 28 days compressive strength was increased by 3.0 MPa. The microscopic results showed that the use of ACS and PBTCA does not affect the type of cement hydration products, but only affects the the rate of hydration product generation.

1. Introduction

Concrete, the most widely utilized construction material globally, produces substantial waste slurry water during its manufacturing process [1]. A concrete plant with an annual output of approximately 1.2 million cubic meters generates 60 tons of wastewater annually from cleaning equipment. This waste slurry water primarily originates from gravel separators, transportation trucks, production site rinsing water, and some rainwater. It contains a significant amount of cement, mineral powder, and fly ash, along with their hydration products, and its direct discharge leads to severe pollution of water resources and the environment [2,3]. As modern industrialized production technology advances and societal demands for environmental protection escalate, people’s awareness of sustainable development continues to grow. China faces a significant challenge due to its scarcity of water resources, making the judicious utilization of recycled water crucial for conserving water and protecting the environment. The treatment and application of recycled water in concrete mixing plants by ready-mixed concrete producers have garnered societal attention and are now considered a key metric for assessing the green credentials of mixing plants [4]. Consequently, it is imperative for enterprises to ensure that wastewater is treated to meet the established discharge standards before it is released from the mixing plant.

After identifying the primary physical and chemical characteristics of waste slurry water, Shan et al. [5] investigated the impact of each critical factor on the treatment of concrete effluent using single-factor experiments, orthogonal tests, and response surface methodology. They analyzed the effect and mechanism of influence of each factor at a micro level. Liu et al. [6] investigated the effect of placing time of recycled slurry water with a concentration of 6.4% in mixing plant on the performance of concrete, and analyzed the changes of hydration products and microscopic morphology of recycled slurry water in cement slurry with different placing time by XRD and SEM. Hu. [7] investigated the impact of varying dosages of plant wastewater (25%, 35%, and 45%) on the mass loss, appearance, dynamic elastic modulus, and compressive strength of concrete subjected to freeze-thaw cycles. Furthermore, a freeze-thaw damage model specific to concrete containing wastewater was developed.

Arunvivek et al. [8] experimentally validated the mechanical and durability properties of concrete containing waste cement sludge and silica fume, and identified the optimal cement replacement rate. Gazzar et al. [9] investigated the mechanical and durability properties of concrete prepared with wastewater and showed that the mechanical properties of concrete prepared with wastewater do not deteriorate, and even improve in terms of homogeneity, with no increase in the risk of corrosion. Bouaich et al. [10] investigated the use of wastewater in mortar preparation and found that wastewater, when combined with CEM III/A 42.5 cement, was able to produce mortars with high-strength properties for a wide range of engineering environments. Babu et al. [11] investigated the ideal pH range for concrete mixing water and found that a pH range of 7.2 to 7.6 slightly alkaline helps to ensure concrete stability. Chen et al. [12] used different proportions of wastewater instead of tap water for concrete mixing, and found that wastewater concrete has better compressive properties than ordinary concrete, with 75% wastewater mixing increasing the compressive strength of concrete by about 20%, and the advantage is more obvious in the early age.

In summary, domestic and foreign research on waste slurry recycled water is mostly focused on its use in the reproduction of concrete, and the impact of mixing waste slurry recycled water on the working performance, mechanical properties, durability performance of concrete, etc., and there are certain constraints. Most concrete mixing plants choose to use the establishment of cisterns to store these waste slurry water, the use of physical and chemical means of treatment, so that it can eventually reach the degree of recycling. However, due to the complex composition of waste slurry water, condensation will occur at the bottom of the cistern plate in the storage process, which will reduce the utilization rate of waste slurry recycled water, accelerate the mixing plant equipment loss, affecting the further recycling. Domestic and foreign research on the condensation problem of waste slurry recycled water is relatively small, and there is basically no research on the mixing of super retarder waste slurry recycled water concrete. Therefore, in this paper, we developed a super retarding agent that can effectively retard the condensation of recycled water through synthesizing and compounding technology, and used the waste slurry recycled water with super retarding agent in the production of concrete. The results of the research can make the waste slurry recycled water of mixing station realize more efficient utilization, reduce the loss of mixing station, and have guiding significance for the reasonable utilization of waste slurry recycled water of concrete mixing station.

2. Materials and Composition

2.1. Raw Materials

Gelling material: In this study, P-I 42.5 ordinary silicate cement was used, which has a fineness of 0.8%, density of 3.12 g/cm³, specific surface area of 356 m²/kg, satisfactory stability, initial setting time of 128 min, final setting time of 189 min, and a Dx(50) of 17.8 μm. The chemical composition is listed in

Table 1. Chemical composition of cement (%).

CaO	MgO	SiO2	SO3	Al2O3	Fe2O3	R2O	f-CaO	Loss	Cl−
62.49	4.04	20.16	2.30	4.68	3.51	0.53	0.91	1.98	0.046

. Class F Class II fly ash, S95 grade mineral powder.

Super Retarder: Tetrasodium hydroxyethylidene diphosphate (HEDP-4 Na); Butane 2-phospho-1,2,4-tricarboxylic acid (PBTCA); Tetrasodium iminodisuccinate (IDS); Acrylic acid-2-acrylamide-2-methylpropanesulfonic acid (AA/AMPS); Reclaimed water treatment agent (ACS); Polyamino polyether methylidene phosphonic acid (PAPEMP); Sediment Dispersant (SDP-90).

Waste slurry recycling water: from a mixing station in Beijing, concentration: 7–20%.

Sand: Zone II mechanism medium sand with a fineness modulus between 2.3 and 3.0 and Zone III fine sand with a fineness modulus between 1.3 and 1.6 were used.

Stone: Use 5–25 continuously graded gravel with no more than 10% needled particles.

Water reducing agent: Polycarboxylate super- plasticizer was employed as a superplasticizer, and its water reduction rate and solid content were 40% and 50%, respectively.

Water: Clean tap water.

2.2. Concrete Mix Proportions

The concrete mix proportions were listed in

Table 2. Experimental mix ratios (kg/m³).

Cement	Fly Ash	Mineral Powder	Sand		Stone	Water	Superplasticizer
			2.3–3.0	1.3–1.6
230	66	88	650	199	1000	158	6.9

with a W/B ratio of 0.43.

2.3. Sample Preparation

Concrete test block: mold size of 160 mm long × 40 mm wide × 40 mm high, after vibration is completed maintenance 24 h demolding, the temperature of 20 ± 2 °C, humidity > 50%. and then moved to the standard curing room for curing, the temperature of 20 ± 2 °C, humidity > 95%. The same age under the preparation of three test blocks, test results take the average value.

Microscopic specimen preparation: The solid components in the waste slurry recycled water were selected, washed with anhydrous ethanol, powder was ground and sieved through 400 mesh using an agate mortar, dried at 45 °C for 24 h, and placed in a vacuum-sealed bag for sealing and preservation.

3. Test Methods

3.1. Setting Time

Reference group preparation: weigh 500 g of waste slurry recycled water in a 10 × 12 cm cylindrical plastic container and place it in the maintenance box of the YH-40B for maintenance, with 99% humidity and 20 °C temperature. Weigh three identical ones under the same component. The super retarder mixed group has one more step than the reference group, after weighing the waste slurry recycling water, it is necessary to weigh the corresponding mass of super retarder. This type of addition is the external mixing method. The Setting time is calculated from the time the weighing is completed and recorded as h1. During the test, the container is removed from the curing box and operated with an air compressor by adjusting the air pressure to 6, connecting a hose at the blowing port so that the hose is tightly pressed against the wall of the container and operated at a distance of 1 cm from the wall of the container. If no small fragments are blown up during the operation, the solution is judged to be in an unsolidified state at this time, as shown in Figure 1. The same operation is performed on the remaining two sets of solutions at 30-min intervals until small intensity-free fragments appear. The time at which the intensity-free fragments appeared was recorded as h2. Finally, the time at which the recovered water began to solidify was obtained by calculating h2–h1.

3.2. Slump and Extension Test

Determine the slump and extension of concrete in accordance with GB/T50080-2002 [13] Test Methods for Properties of Ordinary Concrete Mixes.

The slump is tested by slump cylinder, and the mixed concrete is evenly loaded in three layers, and each layer is inserted and pounded 25 times with a pounding rod, and scraped flat with a scraper after pounding. Lift the slump cylinder smoothly, measure the height difference between the highest point of the mixture and the cylinder after slumping, in mm, accurate to 5 mm. Direct recording of measured values.

When the concrete mix is no longer spreading, use a steel ruler to measure the maximum diameter of the spreading extension surface of the concrete mix, and the diameter in the direction perpendicular to it, with an accuracy of 1 mm, and take the arithmetic average as the result of the extension test.

3.3. Compressive Strength

Determine the compressive strength of concrete in accordance with GB/T50081-2002 [14] Test Method for Mechanical Properties of Ordinary Concrete; There are three test blocks under each age and the results are taken as arithmetic mean.

3.4. Microtesting

X-ray diffraction (XRD) instrument model: Japan Rigaku SmartLab SE, Cu target, scanning range 5~70°, scanning speed of 2°/min. Thermogravimetric analyzer model: Setline STA, Setaram, temperature range of 60–1000 °C, the amount of sample of about 20 mg, the rate of temperature increase of 10 °C/min, argon atmosphere for the Test.

4. Results

4.1. Effect of Using a Single Super Retarder on the Setting Time of Waste Slurry Recycled Water

Seven kinds of super retarders were selected, and the effect of different dosage of super retarders on the setting time of waste slurry recycled water was investigated by external doping method. Figure 2 shows the effect of 0.5%, 1.0%, 1.5%, 2.0% super retarding agent on the setting time of recycled water, the dosage of super retarding agent is the ratio of the mass of recycled water.

As can be seen in Figure 2, after using 0.5% of super retarding agents, the four super retarding agents, SDP-90, ACS, PBTCA and HEDP-4Na, can make the slurry recycled water setting time of more than 10 h, compared with the reference group of 7 h, it can be seen that the four super retarding agents 0.5% dosage of the retarding effect of the better. Among them, SDP-90 has the best effect on retardation of slurry recycling water, and the final measurement of the beginning of the setting time is 13 h. However, when using the two super retarders AA/AMPS or IDS, the setting time of the waste slurry recycled water was 7 h and 8 h, respectively, which was not much different from that of the reference group of 7 h. It can be seen that the retardation effect is not good when these two super retarders are used individually at a dosage of 0.5%.

As can be seen in Figure 2, after using 1.0% of super retarding agents, SDP-90, ACS and PBTCA three super retarding agents make the slurry recycled water setting time more than 14 h, it can be seen that these three super retarding agents 1.0% dosage retarding effect is better. Among them, PBTCA super retarding agent has the best effect on retardation of slurry recycling water, and the final measurement of the beginning of setting time is 18 h. However, when using AA/AMPS or PAPEMP these two super retarding agents, the setting time of waste slurry recycled water is 7.5 h and 9.5 h respectively, which is not much different from the reference group of 7 h; when using HEDP-4Na or IDS these two super retarding agents, the setting time of waste slurry recycled water is 10.5 h and 11 h respectively, which is insignificant compared to the reference group of retarding setting time. It can be seen that 1.0% dosage of AA/AMPS, PAPEMP retardation effect is not good, HEDP-4Na, IDS retardation effect is general.

As can be seen in Figure 2, after using 1.5% of super retarding agents, the three super retarders SDP-90, ACS and PBTCA make the slurry recycled water setting time more than 18 h, which can be seen that these three super retarders are more effective in the dosage of 1.5% to slow down the sloughing effect. Among them, PBTCA super retarding agent has the best effect on retardation of slurry recycling water, and the final measured start of setting time is 22.5 h. However, when using HEDP-4Na or IDS, the setting time of waste slurry recycled water is 12 h, which is not obvious compared with the reference group; when using AA/AMPS or PAPEMP, the setting time of waste slurry recycled water is 8 h and 10 h, which is not much different from the reference group of 7 h; It can be seen that at 1.5% dosage of super retarding agent, the retarding effect of HEDP-4Na and IDS is general, and the retarding effect of AA/AMPS and PAPEMP is not good.

As can be seen in Figure 2, after using 2.0% of super retarding agents, the two super retarders ACS and PBTCA made the slurry recycled water setting time more than 27 h, which can be seen that the two super retarders in the dosage of 2.0% retardation effect is better. Among them, PBTCA super retarding agent has the best effect on retarding the condensation of waste slurry recycled water, and the final measured time of the beginning of condensation is 30 h. However, when using the two super retarding agents HEDP-4Na or IDS, the setting time of waste slurry recycled water is 14 h and 17 h respectively, which is not obvious compared with the reference group; when using the two super retarding agents AA/AMPS or PAPEMP, the setting time of waste slurry recycled water is 9 h and 11 h respectively, which is not much different from the reference group of 7 h. It can be seen that 2.0% dosage of super retarding agent HEDP-4Na, IDS retarding effect in general, AA/AMPS, PAPEMP retarding effect is not good.

4.2. The Effect of Super Retarder on the Setting Time of Waste Slurry Recycling Water Under the Condition of Compound Mixing

ACS is a special super retarder for waste slurry water from concrete mixing plant which is currently used on a small scale, therefore, ACS was selected to be compounded with other super retarders with a view to obtaining a better retardation effect.

The six super retarding agents used under single blending conditions were compounded with ACS in the ratio of 1:20, and the compounded reagents were mixed with tap water in the ratio of 1:15 to make six super retarding solutions. According to the proportion of 1.5% of the total amount of recycled water (external mixing method) was added to different concentrations of waste pulp recycled water to determine the setting time of waste pulp recycled water. Figure 3 shows the effects of different kinds of compounded super retarding agents on the setting time of waste slurry recycled water. The concentrations of the waste slurry recycled water were 7%, 10%, 15% and 20%.

As can be seen from Figure 3, compared with single blending, the retardation effect of super retardant is significantly improved after compounding with ACS. When the concentration of slurry recycled water is 7%, PBTCA or SDP-90 compounded with ACS make the setting time of waste slurry recycled water more than 70 h. Among them, the setting time of waste slurry recycled water after compounding of PBTCA and ACS reaches 73 h, with a significant retardation effect. HEDP-4Na or IDS compounded with ACS made the setting time of waste slurry recycled water reach about 50 h, and the effect of retardation was general; AA/AMPS or PAPEMP compounded with ACS made the setting time of waste slurry recycled water reach about 40 h, and the effect of retardation was poor.

As can be seen from Figure 3, when the concentration of slurry recycled water is 10%, PBTCA or SDP-90 compounded with ACS make the setting time of waste slurry recycled water more than 60 h. Among them, after the compounding of PBTCA and ACS, the setting time of waste slurry recycled water reached 64 h, and the effect of retardation was very significant; after the compounding of HEDP-4Na or IDS with ACS, the setting time of waste slurry recycled water reached about 40 h, and the effect of retardation was general; after the compounding of AA/AMPS or PAPEMP with ACS, the setting time of waste slurry recycled water reached about 60 h, and the effect of retardation was general; after the compounding of AA/AMPS or PAPEMP with ACS, the effect of retardation was very significant. After compounding AA/AMPS or PAPEMP with ACS, the setting time of waste slurry recycled water reached about 30 h, and the effect of retarding coagulation was poor.

As can be seen from Figure 3, when the concentration of slurry recycled water is 15%, SDP-90 or PBTCA compounded with ACS make the waste slurry recycled water setting time are more than 45 h. Among them, ACS and PBTCA compounding, make the waste slurry recycled water setting time reached 49 h, the best effect of retardation. After the compounding of HEDP-4Na or IDS with ACS, the setting time of waste slurry recycled water reached about 40 h, and the effect of retarding coagulation was average; after the compounding of AA/AMPS or PAPEMP with ACS, the setting time of waste slurry recycled water reached about 30 h, and the effect of retarding coagulation was poor.

As can be seen from Figure 3, when the concentration of slurry recycled water is 20%, The super retarders such as HEDP-4Na, PBTCA, etc., compounded with ACS have similar effects in delaying coagulation. Among them, PBTCA, IDS, SDP-90 compounded with ACS made the setting time of waste slurry recovery water reach about 35 h; HEDP-4Na, AA/AMPS, PAPEMP compounded with ACS made the setting time of waste slurry recovery water of reach about 30 h. It can be seen that when the concentration of waste slurry recycled water is higher, the compounding super retarder can delay the setting time to a certain extent, but the delaying effect is not obvious, and the delaying effect of each compounding super retarder is not much different.

4.3. Effect of Compounding Super Retarder on Concrete Properties

ACS+PBTCA and ACS+SDP-90, which have good retarding effect under compound mixing conditions, were selected, and the compounding ratios of the two groups of super retarding agents were changed respectively to study their effects on the working performance and compressive strength of concrete. The results of working performance test are shown in

Table 3. Effect of PBTCA and ACS at different ratios on the workability of concrete.

PBTCA:ACS	Slump/mm	Extensibility/mm	1 h Slump/mm	1 h Extensibility/mm
blank	235	590	210	435
1:10	245	615	210	450
1:20	240	630	245	615
1:30	235	605	215	510
1:40	235	595	240	545
1:50	240	590	220	430

and

Table 4. Effect of SDP-90 and ACS at different ratios on the workability of concrete.

PBTCA:ACS	Slump/mm	Extensibility/mm	1 h Slump/mm	1 h Extensibility/mm
blank	235	590	210	435
1:10	260	640	235	470
1:20	255	635	240	550
1:30	250	630	240	525
1:40	245	620	240	560
1:50	240	610	230	465

, and the results of compressive strength test are shown in Figure 4 and Figure 5.

As shown in Table 3, compared to the reference group, the slump and extensibility of concrete exhibited a pattern of initial increase followed by decrease as the ratio of PBTCA to ACS increased. When PBTCA:ACS reached 1:20, there was no loss of 1 h slump and the loss of 1 h extensibility was 15 mm. Concrete has good workability in this condition.

Table 4 shows the effect of SDP-90 compounded with ACS at different ratios and added to waste slurry recycling water on the slump, extension, 1 h slump and 1 h extension of concrete at 1.5% admixture. As can be seen from Table 4, with the increase of the ratio of PBTCA to ACS, the slump and extensibility of concrete showed a decreasing trend. When SDP-90:ACS was 1:20, the 1 h slump loss was the smallest, only 5 mm; at this time, the extension loss was 85 mm. Compared with the reference group, when using the compound super retarder of SDP-90 and ACS, the 1 h slump loss of concrete were all significantly reduced, but the 1 h extension loss were all larger.

As can be seen from Figure 4, after mixing the compound super retarder of PBTCA and ACS, the waste slurry recycled water did not adversely affect the compressive strength of concrete in both low and high ratios, but improved the 7 days and 28 days compressive strengths. When PBTCA:ACS was 1:20, the 7 days compressive strength was increased by 3.5 MPa and the 28 days compressive strength was increased by 3.0 MPa; when PBTCA:ACS was 1:30, the 7 days compressive strength was increased by 2.2 MPa and the 28 days compressive strength was increased by 5.5 MPa. As can be seen from Figure 5, after admixing the compounded super retarding agent of SDP-90 and ACS, the waste slurry recycled water has little effect on the compressive strength of concrete, but the overall trend is decreasing, and when SDP-90:ACS is 1:10 and 1:20, the phenomenon of water secretion occurs.

In summary, although either PBTCA or SDP-90 compounded with ACS can effectively retard the setting time of waste slurry recycled water, the effect of waste slurry recycled water on the working and mechanical properties of concrete is minimized when using the super retarding additive compounded with PBTCA and ACS, and it can increase the 7 days and 28 days compressive strengths of concrete when PBTCA:ACS is 1:20.

4.4. Microanalysis

The super retarder PBTCA and ACS were selected with a fixed ratio of 1:20, and XRD, DTG, TG and other microscopic tests were used to study their effects on the hydration of cement in waste slurry recycled water at different ages. The dosages were 0.5%, 1.5% and 2.0% (external mixing method), and the concentration of waste slurry recycled water was 10%.

Study of the types and amounts of hydration products at different ages using XRD:

Figure 6 shows the effect of different dosages of ACS+PBTCA on the type and amount of cement hydration products in 10% concentration of waste slurry recycled water as characterized by XRD.

As can be seen from Figure 6, the types of crystalline phases in the hydration products did not change at all ages after doping with ACS+PBTCA. The calcium alumina peak intensities of blank and ACS+PBTCA doped specimens with different amounts of ACS+PBTCA were similar for the specimens with ages of 1 day, 3 days and 7 days, and the calcium hydroxide peak intensities of ACS+PBTCA doped specimens with ages of 1 day and 3 days were lower than those of the blank specimens; The calcium hydroxide peak intensity of the 0.5% ACS+PBTCA doped specimen at the age of 7 days is higher than that of the blank specimen, and the calcium hydroxide peak intensity of the remaining two doped ACS+PBTCA specimens is similar, and both are lower than that of the blank group and the 0.5% ACS+PBTCA doped specimen group.

After adding ACS and PBTCA compound super retarder, it reacts with Ca(OH)₂ on the surface of the generated clinker phase to form “insoluble” calcium phosphate, which hinders normal hydration and slows down the setting time of waste slurry recycled water. At the same time, after adding the compound super retarder, the content of C₃S is obviously increased, so it is favorable to the compressive strength.

Study on the types and amounts of hydration products at different ages using DTG:

Four components were selected for the test, one group was a blank group and the remaining three groups were control groups with ACS+PBTCA. The concentration of recycled water was 10%. Thermogravimetric tests were performed on the four cement pastes separately. The changes in hydration products of the four pastes were analyzed at different hydration times. Figure 7 shows the effect of different dosages of ACS+PBTCA on cement paste in recycled water as characterized by DTG.

As shown in Figure 7, ACS+PBTCA does not cause new hydration products to appear in the cement paste. The more obvious characteristic peaks in the figure are the dehydration and heat absorption peak of free water or water of crystallization inside the decomposition of calcite near 100 °C, the CH decomposition peak near 430 °C and the CaCO₃ decomposition peak near 700 °C, in which CaCO₃ mainly comes from the raw materials in the cement, and a small amount of carbonation will occur in the cement during the storage process.

It can be seen that, with the advance of hydration time, the DTG curves of cement slurry and blank cement slurry in recycled water with 10% concentration of waste slurry doped with different amounts of ACS+PBTCA gradually converge, indicating that doping of ACS+PBTCA affects the rate of generation of hydration products, but does not have an effect on the type of cement hydration products.

Study of the types and amounts of hydration products at different ages using TG:

Figure 8 shows the effect of different dosages of ACS+PBTCA on cement paste in 10% concentration of waste slurry recycled water as characterized by TG.

As can be seen from Figure 8, the hydration product has three distinct absorption peaks in the range of 45~900 °C, which are TG1: 45~400 °C, TG2: 400~540 °C, and TG3: 540~900 °C.

The combined water and CH volumes were calculated by the following equations, respectively:

$$\mathrm{P}1=\mathrm{T}\mathrm{G}1+{\displaystyle \frac{\mathrm{T}\mathrm{G}3}{3}}$$

(1)

$$\mathrm{P}2=({\displaystyle \frac{\mathrm{T}\mathrm{G}2}{18}}+{\displaystyle \frac{2}{3}}\times {\displaystyle \frac{\mathrm{T}\mathrm{G}3}{44}})\times 74$$

(2)

$$\mathrm{P}3=\mathrm{T}\mathrm{G}1+\mathrm{T}\mathrm{G}2+{\displaystyle \frac{\mathrm{T}\mathrm{G}3}{3}}+{\displaystyle \frac{2}{3}}\times {\displaystyle \frac{\mathrm{T}\mathrm{G}3}{44}}\times 18$$

(3)

where P1 is the percentage of bound water in CSH gel and AFt, P2 is the percentage of calcium hydroxide, P3 is the percentage of total bound water, and TG1 to TG3 are the three significant heat losses in the TG curve. The results of the analysis are shown in

Table 5. Amount of chemically bound water and calcium hydroxide (%) in the hydration products of each specimen.

Serial Number	45~400 °C TG1	400~540 °C TG2	540~900 °C TG3	CSH and AFt Dehydration	CH	Total Combined Water
blank—1 day	5.22	2.56	2.94	6.20	13.82	9.56
blank—3 days	7.65	3.64	3.51	8.82	18.90	13.42
blank—7 days	17.12	3.14	2.16	17.84	15.33	21.57
0.5%—1 day	1.75	0.30	1.50	2.25	2.92	2.96
0.5%—3 days	3.22	0.65	1.95	3.87	4.86	5.05
0.5%—7 days	8.20	3.76	3.25	9.28	19.10	13.93
1.5%—1 day	2.37	0.24	1.65	2.92	2.84	3.61
1.5%—3 days	3.33	1.02	2.18	4.06	6.64	5.67
1.5%—7 days	2.64	0.30	2.65	3.52	4.20	4.55
2%—1 day	1.90	0.15	1.65	2.45	2.47	3.05
2%—3 days	1.79	0	1.65	2.34	1.85	2.79
2%—7 days	1.94	0.13	2.19	2.67	2.99	3.40

.

As can be seen from Table 5, the total bound water quantity of the blank and 0.5% ACS+PBTCA doped specimens gradually increased with the extension of age, the total bound water quantity of the 1.5% ACS+PBTCA doped specimens firstly increased and then decreased, and the total bound water quantity of the 2.0% ACS+PBTCA doped specimens firstly decreased and then increased. At early age (1~3 days), the CH and total bound water of ACS+PBTCA specimens doped with different dosages were lower than those of blank specimens. At the age of 7 days, the CH of the 0.5% ACS+PBTCA doped samples was higher than that of the blank samples, and the CH of the rest of the doped samples was lower than that of the blank samples.

5. Conclusions

The paper discusses a method for managing waste slurry water generated in concrete mixing plants by developing a super retarder using composite technology. The waste slurry water was mixed with a super retarding agent, characterized through X-ray diffraction (XRD), thermogravimetric (TG), and differential thermogravimetric (DTG) analyses. The treated waste slurry water was used in place of tap water for concrete production. The study evaluated the impact on concrete workability and mechanical properties. Based on the presented results of the experimental research, we can draw the following conclusions:

The super retarder can retard the setting time of waste slurry recycling water under both single mixing and compounding conditions. Under single mixing conditions, SDP-90, ACS, PBTCA retardation effect is better, HEDP-4Na and IDS retardation effect is general, AA/AMPS and PAPEMP retardation effect is worse. Compared with single blending, when PBTCA and ACS were blended, the setting time of waste slurry recycled water was more significantly retarded.

There are good and bad types of compounding super retarder, among which, when PBTCA is compounded with ACS, it is favorable to the working and mechanical properties of concrete, the optimal ratio is 1:20, and the optimal mixing amount is 1.5% of the mass of recycled water of waste slurry.

The microscopic study found that the types of crystal phases in the hydration products of cement at all ages in the waste slurry recycled water did not change after the incorporation of the blended super retarder of PBTCA and ACS. Combined with the thermogravimetric curve analysis, the use of super retarder blended with PBTCA and ACS only affects the rate of generation of hydration products, and has no effect on the types of cement hydration products.

After mixing the super retarder compounded with PBTCA and ACS, the waste slurry recycled water replaces the mixing water for concrete production, which can effectively improve the workability and mechanical properties. When PBTCA:ACS reached 1:20, there was no loss of 1 h slump and the loss of 1 h extensibility was 15 mm. The 7 days compressive strength increased by 3.5 MPa and the 28 days compressive strength increased by 3.0 MPa relative to the reference group.