Influence of Wastewater Content on Mechanical Properties, Microstructure, and Durability of Concrete

Abstract

In this study, high concentration wastewater from ready-mixed concrete plants was used to replace potable water as mixing water of concrete, with replacement rates of 0%, 25%, 50%, 75%, and 100%, by weight. The solid content of the wastewater was 12%. Five groups of C20 concrete mix proportions were designed. Different concrete properties, including workability, compressive strength and durability under freeze–thaw cycles, carbonization, and drying conditions, were studied, and the effect of the increase in the proportion of wastewater as a replacement for potable water was investigated. The microstructural attributes of the developed C20 concrete were studied through X-ray diffraction (XRD), thermal analysis (TG-DSC), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), and mercury intrusion porosimetry (MIP). Finally, the economic benefits of replacing potable water with wastewater were analyzed. The results indicate that using wastewater for concrete mixing reduces workability and a superplasticizer is needed to ensure adequate concrete workability. At the ages of 7, 28, and 56 days, with the increase in the proportion of wastewater as a replacement for potable water (0, 25%, 50%, 75%, 100%), the compressive strength of concrete shows a trend of first decreasing, then increasing, and then decreasing. When the proportion of wastewater replacing potable water is 75%, the concrete compressive strength is the highest. The microstructure showed that the main products of wastewater-mixed concrete are calcite (CaCO₃), portlandite (Ca(OH)₂), ettringite (Aft), and calcium silicate hydrate (C-S-H). Adding wastewater to concrete does not lead to the formation of new products in the concrete. Wastewater can fill the concrete pores well, thus optimizing the pore structure. When the proportion of wastewater replacing potable water is 75%, C20 concrete has the densest microstructure, lower porosity, and better pore structure. Durability properties further indicate that 25%, 50%, and 75% of wastewater replacing potable water can improve the concrete’s frost resistance. However, there is a negative impact on the carbonation resistance of wastewater. Wastewater replacing 75% potable water by weight can improve the drying shrinkage of concrete. The recycling of wastewater is not only green and environmentally friendly but also has good economic and environmental benefits.

1. Introduction

Concrete is the most widely used building material in the world at present. It is estimated that nearly 500 L of freshwater is consumed per cubic meter of concrete produced [1]. Generally, concrete is transported from the mixing plant to the construction site by a concrete mixer truck. After each transportation, the washing of each mixer truck produces about 570–1300 L of wastewater [2,3]. The wastewater produced by flushing contains unhydrated cement, fly ash, and additives. The direct discharge of high alkalinity and residual substances may cause significant environmental distress [4,5,6,7]. In order to avoid this situation, the wastewater from the ready-mixed concrete plant needs to be treated before it can be discharged, which incurs considerable costs for the ready-mixed concrete plant [2,8].

In recent years, many researchers have investigated the use of wastewater from the ready-mixed concrete plant to replace freshwater (potable water) for concrete mixing [9,10]. The mechanical properties, durability, and microstructure of concrete mixed with wastewater have been thoroughly analyzed. Wastewater from ready-mixed concrete plants is highly alkaline, and its PH value is generally 10–13 [8,9,10,11]. The density of wastewater can reach 1098 kg/m³ [12]. Additionally, the storage time of wastewater has a great influence on the working performance and strength of concrete. Once the storage time exceeds eight hours, the cement particles in wastewater are completely hydrated, thereby not affecting the concrete setting and hardening [2].

In terms of liquidity, researchers have given conflicting views. De Matos et al. [8], Chatveera et al. [10], and Audo et al. [13] found that wastewater addition can inhibit concrete fluidity. Asadollahfardi et al. [14] discovered that even diluted wastewater still leads to a significantly decreased concrete fluidity. Asadollahfardi et al. [15] and Tsimas et al. [16] demonstrated that the mixing of wastewater has little influence on concrete fluidity. Kadir et al. [11] found that concrete fluidity increases with wastewater content. In addition, Su et al. [17] pointed out that mixing the upper, middle, and lower layers of wastewater in the wastewater sedimentation tank has different effects on concrete fluidity. The concrete mixed with the bottom wastewater has a shorter setting time and lower fluidity, and the top and middle wastewater will not significantly affect the fluidity of fresh concrete. In terms of condensation time, Asadollahfardi et al. [15,18] conducted experimental statistics on the setting time of concrete mixed with wastewater and confirmed that the mixing of wastewater shortened the setting time; however, it was well within the acceptable range. Klus et al. [19] observed that using 20% and 50% wastewater instead of clean water to mix mortar shortened the setting time by 15 min.

In terms of mechanical properties, Tsimas et al. [16] and De Matos et al. [8] compared the concrete compressive strength before and after wastewater addition, and found that adding wastewater to concrete did not reduce the 7-day concrete strength; rather it improved the concrete to some extent. Brian et al. [20] found that the concrete made of wastewater has higher compressive strength than that made of tap water. However, some studies have shown that wastewater addition may reduce the concrete compressive strength; for example, De Matos et al. [8] and Chatveera et al. [10,21] showed that wastewater addition could reduce the 28-day concrete strength. In addition, some researchers pointed out that wastewater does not significantly affect compressive [9,18], tensile [18], and bending strength [18]. In terms of durability, Chatveera et al. [10,17] and Su et al. [17] studied the influence of wastewater on the weight loss caused by drying shrinkage and acid erosion of concrete and found that with the increase in the proportion of wastewater, drying shrinkage and acid erosion was aggravated. Chatveera et al. [21] studied the concrete permeability and sulfate resistance of wastewater-mixed concrete containing superplasticizers or fly ash. It was shown that when wastewater was added to concrete alone, it negatively impacted the concrete impermeability and sulfate resistance. However, the impermeability and sulfate resistance of concrete were improved when wastewater was combined with fly ash or superplasticizer in concrete. These differences may be caused by the difference in solid content and storage time of wastewater [22].

Microstructural characteristics and mechanisms were also investigated, besides the fresh and hardened state properties. Audo et al. [13] found by XRD and DTA that the wastewater contains silica (SiO₂), calcium carbonate (CaCO₃), calcium hydroxide (Ca(OH)₂), calcium silicate hydrate (C-S-H), and gypsum. Chatveera et al. [10] used SEM and confirmed that the wastewater sediments appeared as ettringite (Aft) and C-S-H gel. Tsimas et al. [16] evaluated the wastewater particles by XRF, XRD, and thermogravimetric analysis. The results showed that the wastewater particles were mainly CaCO₃ and SiO₂. De Matos et al. [8] reported that the reaction rate of cement could be increased by 72 h by wastewater. Additionally, it was shown through thermogravimetric analysis that there was more Ca(OH)₂, C-S-H, and Aft in the wastewater samples added for 3 days.

To sum up, at present, there are many studies on the workability and mechanical properties of concrete mixed with wastewater from ready-mixed concrete plants, whereas very few have evaluated the durability properties, such as frost resistance and carbonation resistance. Additionally, the microstructural investigations are primarily focused on the material composition and hydration products of wastewater. In addition, there are few reports on the micro-morphology and pore structure changes of concrete mixed with wastewater from ready-mixed concrete plants, and the macro-micro mechanism analysis of concrete mixed with wastewater from ready-mixed concrete plants has not been studied yet. Therefore, this paper uses 0%, 25%, 50%, 75%, and 100% proportions of wastewater from ready-mixed concrete plants to replace potable water as mixing water for C20 concrete. The effects of wastewater from ready-mixed concrete plants on the workability, compressive strength, frost resistance, carbonation resistance, and drying shrinkage of C20 concrete are studied. X-ray diffraction analysis (XRD), thermal analysis (TG-DSC), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and mercury intrusion porosimetry (MIP) were used to study the influence of mixing plant wastewater with the increase in the proportion of wastewater as a replacement for potable water on the microstructure and pore structure of C20 concrete. The action mechanism of mixing plant wastewater on compressive strength, durability, and microstructure of C20 concrete with the increase of wastewater as a replacement for potable water was studied by combined macroscopic and microscopic methods.

2. Test Materials and Methods

2.1. Materials

P.O42.5 ordinary Portland cement produced by Sanmenxia Tengyue Tongli Cement Co., Ltd.(Zhengzhou, China) was used in this study. The chemical composition is shown in

Table 1. Chemical properties of wastewater.

Chemical Properties	Wastewater
PH	12
Total solids content, ppm	121,210
TDS	391
Chloride ion (Cl−), ppm	78
Sulfate ion (SO42−), ppm	34

ppm: parts per million.

. Coarse aggregate with a 5–20 mm particle size, 0.57% water absorption, and continuous gradation was used as coarse aggregate. Fine aggregate comprised local river sand with fineness modulus of 2.6, silt content of 0.8%, bulk density of 1470 kg/m³, and apparent density of 2650 kg/m³. Grade I fly ash, produced by Henan Longquan Jinheng Electric Power Co., Ltd. (Zhengzhou, China), with a 93% water demand ratio, was used. All the materials used in the test meet the national standards, and the equivalent amount of fly ash was used to replace cement. A polycarboxylate superplasticizer with a 26% water-reducing rate produced by Luoyang Junjiang Building Materials Technology Co., Ltd. (Zhengzhou, China) was used as an admixture.

Potable water and wastewater from ready-mixed concrete plants were used as mixing water for concrete. The wastewater (solid content 12% and the PH value of 12) was obtained from the treated waste materials produced in the concrete production process of Henan Fifth Construction Ready-mixed Concrete Factory in Zhengzhou City (as shown in Figure 1). The recycling process of wastewater is shown in Figure 2. The dry wastewater particles obtained by natural air drying of wastewater were ground and sieved with a 45 μm square hole sieve, with a fineness of 17.6%. The chemical properties of wastewater are shown in Table 1. The wastewater storage time is above 48 h. The main oxides of wastewater particles are shown in

Table 2. Chemical characteristics of wastewater and cement.

Composition	Portland Cement	Wastewater
	Weight% (% by Mass)
CaO	63.22	41.95
SiO2	18.63	32.64
Al2O3	4.93	13.24
MgO	1.03	3.7
Fe2O3	3.99	3.63
SO3	3.01	1.74
Na2O	0.79	0.524 1.24
K2O	0.47

. The microstructure of wastewater particles was further analyzed by SEM and EDS, as shown in Figure 3 and

Table 3. Element analysis of wastewater particles (atomic fraction)/at %.

Energy Spectrum	Total	C	O	Ca	Si	Al	S
	100	84.4	13.1	1.9	0.3	0.2	0.1

. The phase composition was analyzed by XRD with an X-ray diffractometer, as shown in Figure 4. The results indicate that the CaO content in wastewater particles is less than that in cement; however, the SiO₂ and Al₂O₃ contents are noticeably higher than that in cement, which may be attributed to the decrease of CaO content caused by the reaction between CaO and water when washing the remaining concrete. In addition, the SiO₂ content in wastewater particles is increased because of the sand in concrete production. As shown in Figure 3, the particle size of wastewater is generally between 1 and 20 μm, and many pores are observed. From the EDS elemental analysis, the main elements of wastewater particles are found as C, O, and Ca, indicating the presence of large amounts of CaCO₃ (Table 2). Further phase analysis of wastewater particles by XRD (as shown in Figure 4) shows that the main products of wastewater particles are CaCO₃, SiO₂, and Aft; CaCO₃ and SiO₂ were contributed by fine sand and gravel [16], whereas Aft mainly came from cement hydration.

2.2. Experimental Methods

Five groups of C20 concrete mix proportions were designed based on the proportion of wastewater replacing potable water. Wastewater replaced 0%, 25%, 50%, 75%, and 100% of the potable water, by weight, respectively. The mix proportion of each concrete group is shown in

Table 4. Mix proportions of C20 concrete (kg/m³).

Code	Wastewater	Potable Water	Cement	Fine Aggregate	Coarse Aggregate (5~20 mm)	Fly Ash	Superplasticizer
C20-0	0	174	260	778	1122	66	5.5
C20-25	43.5	130.5	260	778	1122	66	5.5
C20-50	87	87	260	778	1122	66	8.2
C20-75	130.5	43.5	260	778	1122	66	11.4
C20-100	174	0	260	778	1122	66	13.0

and

Table 5. Freeze–thaw cycle mix proportions of C20 concrete (kg/m³).

Code	Wastewater	Potable Water	Cement	Fine Aggregate	Coarse Aggregate (5~20 mm)	Fly Ash	Superplasticizer
C20-0	0	168	256	810	1047	64	5.5
C20-25	42	126	256	810	1047	64	5.5
C20-50	84	84	256	810	1047	64	8.2
C20-75	126	42	256	810	1047	64	11.4
C20-100	168	0	256	810	1047	64	13.0

Note: C20-X represents a kind of concrete, and X is the mass percentage of wastewater instead of potable water, C20-0 is the control concrete.

. The slump and expansion of fresh concrete were tested. The compressive strength of concrete at 3, 7, 28, and 56 days was tested. Compressive strength concrete samples were cast in 100 × 100 × 100 mm³ molds.

The concrete with a curing age of 28 days was also tested by subjecting it to 100 freeze–thaw cycles. Compressive strength, flexural strength, relative dynamic elastic modulus, water absorption, and mass loss of concrete were determined after every 25 freeze–thaw cycles. The 40 × 40 × 160 mm³ prisms were made for testing the freeze–thaw resistance. The rules of relative dynamic elastic modulus are as follows. An ultrasonic nondestructive detector is used to measure the propagation time T of the ultrasonic wave in concrete. Calculate the relative dynamic elastic modulus of concrete according to Equations (1) and (2) [23] and Equation (3) [24].

$$V=\frac{T}{L}$$

(1)

$$E_d=\frac{\rho V_{}^2(1+\mu )(1-2\mu )}{1-\mu }$$

(2)

$$E_d=\frac{E_n}{E_0}\times 100\%$$

(3)

where T is the ultrasonic propagation time; L is the longitudinal length of concrete. E is the dynamic elastic modulus of concrete. μ is Poisson’s ratio, and μ is generally 0.15 when the material is concrete. ρ is the density of concrete. V is the longitudinal wave velocity of the ultrasonic wave propagating in concrete. E_d represents the relative dynamic elastic modulus of concrete. E_n represents the dynamic elastic modulus of the specimen after N freeze–thaw cycles. E₀ indicates the dynamic elastic modulus of the specimen without freezing and thawing.

The anti-carbonization concrete specimens were cast in 100 × 100 × 300 mm³ molds. When the concrete specimens were cured to the specified age of 26 days, they were put into an oven and dried at 60 ± 2 °C for 48 h. After cooling, the specimens were sealed with paraffin, leaving only one side. At last, the specimens were put into a concrete rapid carbonization test chamber (temperature: 20 ± 2 °C, relative humidity: 70 ± 5%, and concentration of carbon dioxide: 20 ± 3%). After carbonization for 3, 7, 14, and 28 days, the specimen was split, and the thickness was half of the width of the specimen (50 mm). The carbonation depth of cracked specimens was tested with a 1% phenolphthalein alcohol solution.

Drying shrinkage tests on 100 × 100 × 515 mm³ concrete specimens were conducted after 1, 3, 7, 14, and 28 days. Casting, molding, and curing of all concrete specimens were carried out as per SL/T 352 5150-2020 [24].

For microscopic tests, typical samples of cube compressive concrete specimens with ages of 7 and 28 days were selected for XRD, TG-DSC, SEM, EDS, and MIP.

3. Test Results and Analysis

3.1. Workability

Fresh concrete workability refers to fluidity, cohesiveness, and water retention. The slump and expansion degrees were used to express the fluidity of concrete. As shown in

Table 6. Workability of C20 fresh concrete (mm).

Code	Before Adjustment			After Adjustment
	Slump	Expansion	Superplasticizer (kg/m3)	Slump	Expansion	Superplasticizer (kg/m3)
C20-0	185	635	5.5	185	635	5.5
C20-25	164	578	5.5	164	578	5.5
C20-50	100	435	5.5	160	565	8.2
C20-75	0	200	5.5	170	598	11.4
C20-100	0	200	5.5	170	595	13.0

, with the increase in the proportion of wastewater as a replacement for potable water, the fresh concrete slump and expansion degree decreased sharply when the superplasticizer proportion was unchanged. When wastewater completely replaces potable water, fresh concrete loses its fluidity. This is because the actual water/binder ratio decreases due to the presence of wastewater particles; since the wastewater particles are porous (Figure 3) and can absorb part of the mixing water [25]. In order to obtain a reasonable slump, the dosage of C20-50 and C20-75 superplasticizer was adjusted. After adjusting the proportion of superplasticizer, C20-50 and C20-75 can have good workability. Although C20-100 can achieve better workability, it is not recommended for use due to its lower early strength.

3.2. Compressive Strength

The compressive strength test values of C20 concrete with different proportions of wastewater replacing potable water at different curing ages (3, 7, 28, and 56 days) are shown in Figure 5.

Overall, the influence of wastewater replacement rate on the cube compressive strength of C20 concrete has a different effect with the increasing age. With increasing the proportion of wastewater as a replacement for potable water, the cube compressive strength of concrete first increased and then decreased at the 3-day curing age. When the curing ages were 7, 28, and 56 days, the compressive strength of concrete first decreased, then increased, and then decreased.In addition, after 3 days, the compressive strength of concrete cubes of C20-25, C20-50, and C20-75 increased by 15.5% (1.9 MPa), 30.3% (3.7 MPa), and 68.9% (8.4 MPa), respectively, compared with C20-0. Nevertheless, the compressive strength of C20-100 concrete decreased by 22.1% (2.7 MPa). Therefore, the addition of wastewater can improve the 3-day concrete strength, which may be attributed to the filler and nucleation effect accelerating the cement hydration, resulting in a denser structure [26]. However, too much wastewater may reduce the strength of C20-100. The reason may be that adding a superplasticizer prolongs the setting time and slows down the early strength growth [25,27].

The changing trend of concrete compressive strength was similar for 7, 28, and 56-day ages. Considering the 28-day age as an example, with the increase in the wastewater potable water content, the concrete compressive strength for C20-25, C20-50, and C20-100 decreased by 14.9% (4.7 MPa), 4.8% (1.5 MPa), and 8.3% (2.6 MPa), respectively, compared with C20-0. However, the strength of C20-75 increased by 5.4% (1.7 MPa). It may be attributed to the absorption of hydration water by the wastewater particles, reducing the saturated pores for hydrate growth and precipitation. Additionally, the saturated pores filled with water are smaller than the critical pores for hydrate growth and precipitation, leading to decreased strength [28].

To sum up, increasing the proportion of wastewater as a replacement for potable water will have different effects on the cube compressive strength of C20 concrete at different ages. When the proportion of wastewater replacing potable water is 75%, the compressive strength of C20 concrete is the highest. Appropriate wastewater instead of potable water can enhance the compressive strength of C20 concrete.

3.3. XRD

The XRD test results of concrete specimens at the ages of 7 and 28 days are shown in Figure 6. The main products in concrete are SiO₂, Ca(OH)₂, CaCO₃, and Aft. Wastewater addition does not lead to the formation of new hydration products. From Figure 3 and Figure 4, it can be seen that wastewater particles contain more SiO₂, CaCO_3, and Ca(OH)₂, resulting in a certain increase in the kurtosis of SiO₂, CaCO₃, and Ca(OH)₂ after wastewater addition (Figure 6a). Additionally, it can be noticed (Figure 6b) that with the increase in the proportion of wastewater as a replacement for potable water, the peaks and kurtosis of CaCO₃ increase to some extent at 2θ = 29° (θ is the diffraction angle of the diffractometer). The kurtosis is the highest for C20-75.

Ca(OH)₂(2θ = 18°) kurtosis was abundant at 7 days, which decreased significantly at 28 days. This may be attributed to the pozzolanic reaction after adding fly ash, consuming the portlandite, and reducing Ca(OH)₂ content.

3.4. TG-DSC

The TG-DSC test results of concrete specimens at the ages of 7 and 28 days are shown in Figure 7. The first endothermic peak is between 420 °C and 550 °C, which is the dehydration of Ca(OH)₂ [29]. The second visible peak lies between 680 and 790 °C, which corresponds to the decarbonation of calcite (CaCO₃) [29,30]. From the thermogram and the area analysis of characteristic peaks of C-S-H and Ca(OH)₂, it can be seen that with the increase in the proportion of wastewater as a replacement for potable water, the concrete contains more corresponding products. Moreover, with the increase in the wastewater potable water content and age, no new substances are found. However, at the age of 28 days, the area of the first endothermic peak was obviously reduced compared with that at the age of 7 days. It shows that the content of Ca(OH)₂ is obviously reduced [31], which is consistent with the results of the XRD analysis. In addition, Ca(OH)₂ and CaCO₃ were observed in SEM analysis, which indicated that the results of TG-DSC analysis were consistent with those of SEM analysis.

3.5. SEM

SEM test results of C20 concrete specimens at the ages of 7 and 28 days are shown in Figure 8 and Figure 9. With the increase in the proportion of wastewater as a replacement for potable water, the internal change trend of concrete is gradually denser at first, then gradually more porous. With the increase in curing age, each concrete specimen becomes denser. This is consistent with the macroscopic strength change.

The internal morphology of the C20-0 concrete is a compact structure combining gelatinous shapes and crystals. As shown in Figure 8a, many C-S-H crystals and some needles can be observed. These hydrates form a morphology with a relatively compact structure but with some pores. With the addition of wastewater, tiny pores appear in the internal morphology of C20-25 specimens, which are interwoven with many needles or rods, forming a network-like structure (Figure 8b). With the increase in the proportion of wastewater as a replacement for potable water, the small pores of C20-50 and C20-75 specimens are gradually filled, and the surface is gradually denser. At the same time, wastewater particles may not be closely linked with concrete, and there are cracks between them. As shown in Figure 8e, after potable water is completely replaced by wastewater, the concrete interior is loose and porous, and many cracks are observed. As shown in Figure 9, it can be observed that the SEM images of all concrete specimens at 28 days are denser than those at 7 days. Ca(OH)₂ produced by hydration of ordinary Portland cement further reacts with the silica phase present in fly ash, forming C–S–H and other gelling compounds, such as calcium aluminate hydrate (C₄AH₁₃ and C₂AH₈) and calcium silicate aluminate hydrate (C₂ASH), filling the pores [32]. Additionally, it is seen from Figure 9b,d that there are honeycombs or clustered wastewater particles in the concrete matrix, with needle-like or rod-like substances nearby, which may lead to concrete strength reduction. Finally, as shown in Figure 9e, at C20-100, the appearance of concrete is “sandy”, and almost no complete cementitious system has been developed. The main reason for this may be that wastewater particles absorb cement hydration water [31,33,34,35].

Further analysis of the products formed in concrete was carried out by EDS. The results are shown in

Table 7. Element analysis of C20 concrete (atomic fraction)/at %.

Code	Energy Spectrum	Total	C	O	Ca	Si	Al	Mg	S	K	Fe
C20-0 7 days		100	28	49.1	1.4	18.2	2.1	/	0.3	0.2	0.7
C20-25 7 days		100	19.3	55.1	9.2	5.7	8.8	0.3	0.9	0.3	0.4
C20-50 7 days		100	20.2	47.3	9.1	15.5	4.4	2.9	0.2	/	0.4
C20-75 7 days		100	10.1	49.2	39.2	0.9	0.2	/	0.4	/	/
C20-100 7 days		100	10	51.2	21.8	8.8	5.9	0.4	0.5	0.8	0.6
C20-0 28 days		100	18	59.9	10.9	5.9	1.8	2.3	0.6	0.1	0.5
C20-25 28 days		100	14.3	61.3	10.5	5.3	2.4	4.9	0.4	0.4	0.5
C20-50 28 days		100	22.1	58	9.7	5.8	2.2	0.5	0.9	0.3	0.5
C20-75 28 days		100	20.7	55.3	4.6	12.2	5.9	0.4	0.2	0.3	0.4
C20-100 28 days		100	16.8	58.7	12.6	6.9	2.4	0.6	1.2	0.2	0.6

, which shows that the main chemical elements of concrete are C, O, Ca, Si, Al, Mg, and S. Through XRD analysis, it is found that the needle-like or rod-like substances may be Aft crystals (Figure 8b). With the increase in the proportion of wastewater as a replacement for potable water, the needle-like or rod-like Aft gradually decreases, while the honeycomb or cluster-like substances gradually increase (Figure 9c,d) [36]. This may be due to certain pores in C20 concrete that provide space for the growth of Aft (Aft generates expansion stress). When the expansion stress exceeds the tensile stress of concrete, cracks and pores will be generated, which will lead to the reduction of concrete strength [37]. The petal-shaped material (Figure 8c) may be a mixture of monosulfur calcium aluminate hydrate (Afm), calcium carboaluminate, calcium chloroaluminate, and hydrated calcium aluminate, which may be due to the low-sulfur Afm formed by hydration of aluminum-containing components in wastewater particles in the absence of enough crystalline calcium sulfate (CaSO₄·2H₂O) [38]. Lastly, CaCO₃ was found in XRD and SEM (Figure 9d), and the crystal appeared at 2θ = 29 (θ is the diffraction angle of the diffractometer). Excess CaCO₃ may come from wastewater particles (Figure 3). CaCO₃ is characterized by high strength and insolubility in water, and it can improve the concrete strength if it is filled in the gaps.

3.6. MIP

The pore structure of cementitious materials is very complex. Under different mechanisms, pores of different shapes and sizes are formed on different scales [39]. Porosity and pore structure have a key influence on the strength of concrete [40,41,42,43]. The porosity and pore structure of C20 concrete specimens at the age of 7 and 28 days are shown in Figure 10. At the 7-day age, with the addition of wastewater, wastewater particles filled the pores of the C20 concrete well. The porosity first decreased and then increased with the increase in the proportion of wastewater as a replacement for potable water. When the proportion of wastewater replacing potable water was 75%, the porosity was the lowest. Compared with C20-0, the porosity of C20-75 decreased by 15.5%. However, excessive wastewater particles (C20-100) may absorb too much cement hydration water or lead to the aggregation and agglomeration of wastewater particles, resulting in extra pores. Then, the porosity increases by 0.6%. At the 28-day age, the porosity of each group of specimens decreased to varying degrees compared with the age of 7 days. Although C20-0’s porosity decreased by 25.3% to 7.7382%, lower than that of C20-25 (9.3224%), C20-50 (7.5884%), and C20-100 (8.1355%), using wastewater instead of potable water can still reduce the content of C20 concrete (Figure 4).

According to Wu et al. [40], pores with d ≥ 100 nm are considered harmful, pores with 50 ≤ d ≤ 100 nm are considered less harmful, and pores with d ≤ 50 nm are considered harmless. The pore structure distribution of C20 concrete samples with different proportions of wastewater to replace potable water is shown in Figure 10.

At the age of 7 days, the pore structure changes as follows. When d ≥ 100 nm, compared with C20-0, the proportion of pores with d ≥ 100 nm in wastewater-mixed concrete decreased by 20.7% (C20-25), 36.1% (C20-50), 22.5% (C20-75), and 53.8%, respectively. When 50 ≤ d ≤ 100 nm, the proportion of 50 ≤ d ≤ 100 nm pores increased after adding wastewater. Compared with C20-0, the proportion of 50 ≤ d ≤ 100 nm pores in concrete increased by 129.1% (C20-25) and 67.6% (C20-50) with the increase in the proportion of wastewater as a replacement for potable water. When d ≤ 50 nm, compared with C20-0, the proportion of pores with d ≤ 50 nm in concrete with the increase in the proportion of wastewater as a replacement for potable water is 26.6% (C20-25), 20.8% (C20-50), 23.8% (C20-75), and 41.2% (C20-100), respectively. The wastewater particles can be well filled in macropores, reducing the macropore proportion. Wastewater particles play a good filling effect [27,44].

At 28-day age, the pore structure changes are as follows. When d ≥ 100 nm, compared with C20-0, the proportion of pores with d ≥ 100 nm in wastewater-mixed concrete increased by 9.6% (C20-25), 52.4% (C20-50), 7.5% (C20-75), and 0.3% (C20-100), respectively. When 50 ≤ d ≤ 100 nm, compared with C20-0, the proportion of 50 ≤ d ≤ 100 nm pores in concrete with the increase in the proportion of wastewater as a replacement for potable water decreased by 27.4% (C20-25), 9.4% (C20-50), and 48.5% (C20-75), respectively. When d ≤ 50 nm, the percentage of pores with d ≤ 50 nm in C20-25, C20-75, and C20-100 increased by 7.8%, 34.9%, and 15.3%, respectively, compared to C20-0, whereas that for C20-50 decreased by 24.5%. The pore structure of C20 concrete, aged 28 days, is different from that of 7 days. At 28 days, the porosity proportion of C20-0 d ≥ 100 nm decreased, whereas the proportion of 50 ≤ d ≤ 100 nm and d ≤ 50 nm pores increased. The change in C20 concrete after adding wastewater is that the proportion of pores with 50 ≤ d ≤ 100 nm decreases, while the proportion of pores with d ≥ 100 nm and d ≤ 50 nm increases. This change may be because the pores filled by wastewater particles are mainly the pores with d ≤ 100 nm, which makes the proportion of pores with 50 ≤ d ≤ 100 nm smaller than that with d ≥ 100 nm.

The pore size distribution curves of C20 concrete samples with different proportions of wastewater replacing potable water are shown in Figure 11. At the 7-day age, the curve of d ≥ 100 nm gradually decreases with the increase of the proportion of wastewater as a replacement for potable water, which indicates that wastewater particles fill the pores of d ≥ 100 nm well. The most probable pore diameter moves slightly to the right. This change may be that wastewater particles slightly destroy smaller pores, increasing the number of small pores. On the other hand, large pores may become smaller and smaller. This is consistent with the size of wastewater particles observed in Figure 3. The 2–20 nm particles can fill larger pores and turn them into smaller pores. At the same time, it may make C-S-H discontinuous, resulting in a slight increase in small pores. At the age of 28 days, the most probable pore diameter of C20-0 moves to the right, and the curve part of d ≥ 100 nm decreases. However, the curve part of d ≥ 100 nm of the concrete sample mixed with wastewater decreases little. This may be because the wastewater particles accelerate the cement hydration and fill the large pores. In addition, it is observed that C20-75 has the smallest possible pore size and the lowest porosity, which is consistent with the results of compressive strength and SEM.

Summarily, combined with the cube compressive strength, it can be seen that porosity has a decisive influence on the strength. Replacing 75% of potable water with wastewater can reduce the porosity of C20 concrete, optimize the pore structure of C20 concrete, and then increase the proportion of harmless pores, thus improving the compressive strength.

3.7. Durability of Wastewater-Mixed Concrete

3.7.1. Frost Resistance

• Compressive strength loss rate

The results of the compressive strength loss rate of C20 concrete with different proportions of wastewater replacing potable water under freeze–thaw cycles are shown in Figure 12. With the increase in the proportion of wastewater as a replacement for potable water, the compressive strength loss of C20 concrete first decreases, then increases. With the increase in freeze–thaw cycles, the compressive strength of C20 concrete with different proportions of wastewater replacing potable water all gradually decreased. C20-25, C20-50 C20-75, and C20-100 had better frost resistance than the control concrete (C20-0) after being subjected to 100 freeze–thaw cycles. The loss rate of compressive strength decreased by 36.5%, 43.9%, 49.9%, and 18.9%, respectively.

2.

Loss rate of flexural strength

The flexural strength loss rate of C20 concrete with different proportions of wastewater replacing potable water under freeze–thaw cycles is shown in Figure 13. With the increase in the proportion of wastewater as a replacement for potable water, the flexural strength loss of C20 concrete firstly decreases and then increases. With the increase in freeze–thaw times, the flexural strength of C20 concrete with different proportions of wastewater replacing potable water all gradually decreased. C20-25, C20-50, and C20-75 had better frost resistance than C20-0 after being subjected to 100 freeze–thaw cycles. The loss rate of flexural strength decreased by 17.7%, 31.7%, and 34.3%, respectively. However, C20-100 had poorer frost resistance, and the loss rate of flexural strength increased by 32.3%.

3.

Relative dynamic elastic modulus

The results of the relative dynamic elastic modulus of C20 concrete with different proportions of wastewater replacing potable water under freeze–thaw cycles are shown in Figure 14. With the increase in the proportion of wastewater as a replacement for potable water, the relative dynamic elastic modulus of C20 concrete first increases and then decreases. With the increase in freeze–thaw times, the relative dynamic elastic modulus of C20 concrete with different proportions of wastewater replacing potable water gradually decreased. C20-25, C20-50, and C20-75 had better frost resistance than C20-0 after 100 freeze–thaw cycles, and the corresponding relative dynamic elastic modulus increased by 39.1%, 173.9%, and 78.3%, respectively. However, C20-100 had poor frost resistance, and the relative dynamic elastic modulus decreased by 21.7%.

4.

Water absorption

The water absorption rate of C20 concrete with different proportions of wastewater replacing potable water under freeze–thaw cycles is shown in Figure 15. The water absorption rate can well reflect the change of porosity of the specimen after freeze–thaw cycles [45]. With the increase in the proportion of wastewater as a replacement for potable water, the water absorption of C20 concrete with different proportions of wastewater replacing potable water decreased, then increased, and then decreased. With the increase in freeze–thaw times, the water absorption of C20 concrete with every proportion of wastewater replacing potable water gradually increases. After 50 freeze–thaw cycles, C20-25 and C20-50 had better frost resistance than C20-0, and the water absorption decreased by 3.3% and 12.2%, respectively. C20-75 and C20-100 had poor frost resistance, and the water absorption increased by 72.3% and 54.5%, respectively. After 100 freeze–thaw cycles, C20-25 had better frost resistance than C20-0, and the water absorption decreased by 31.9%. Meanwhile, C20-50, C20-75, and C20-100 exhibited poorer frost resistance, and the water absorption increased by 10.5%, 31.2%, and 15.4%, respectively.

5.

Mass loss

The quality loss of C20 concrete with different proportions of wastewater replacing potable water under freeze–thaw cycles is shown in Figure 16. With the increase in the proportion of wastewater as a replacement for potable water, the quality loss of C20 concrete first decreases, then increases, and then decreases again. With the increase in freeze–thaw times, the quality loss of C20 concrete with different proportions of wastewater replacing potable water gradually decreased. C20-25 has better frost resistance than C20-0 after 100 freeze–thaw cycles, and the corresponding mass loss decreases by 47.3%, whereas C20-50, C20-75, and C20-100 exhibited worse performance (the mass loss increases by 29.0%, 15.1%, and 25.3%, respectively).

To sum up, replacing 25%, 50%, and 75% of potable water with wastewater can improve the frost resistance of C20 concrete and reduce the freeze–thaw failure rate. However, excessive wastewater, such as a 100% proportion of wastewater as a replacement for potable water, can accelerate the damage rate of C20 concrete. The reasons may be as follows. When the temperature is lower than 0 °C, the water in the capillary pores starts freezing, forcing the excess water to flow to larger pores [46]. At the same time, the water tends to move to the surface and freeze, causing the sample to dry [47] and forming pressure. When the pressure exceeds a certain level, the sample may be damaged. However, when the temperature rises, due to the partial pores generated by the last freezing, more water can enter the concrete, leading to the greater frost heaving force generated by the next freezing to destroy the concrete [48] (C20 concrete freeze-thaw cycle failure is shown in the Figure 17). In addition, in the beginning, the low content of wastewater particles will fill some pores of C20 concrete, which will refine some pores and improve its frost resistance. However, the wastewater particles do not have the cementing/binding effect. With the increase of wastewater particles, although the pores may be further reduced, there may be agglomeration, resulting in the wastewater particles being not tightly attached to C-S-H in concrete. Finally, because of the porous structure of wastewater particles (Figure 3), the penetration of water in the concrete will be accelerated. The frost heaving stress that leads to increased water absorption will increase, thus accelerating the damage rate of C20 concrete.

3.7.2. Carbonation Resistance

The test results of carbonation depth of C20 concrete with different proportions of wastewater replacing potable water are shown in Figure 18. The carbonation diagrams of C20 concrete are shown in

Table 8. Carbonized specimen of C20 concrete.

	C20-0	C20-25	C20-50	C20-75	C20-100
3 days
7 days
14 days
28 days

. The addition of wastewater reduces the carbonation resistance of C20 concrete. With the increase of carbonation age, the carbonation depth of C20 concrete specimens with different proportions of wastewater replacing potable water gradually increases. Compared with C20-0, the carbonization depth of C20-25, C20-50, C20-75, and C20-100 increased by 30.7 mm, 8.5 mm, 25.4 mm, and 24.8 mm, respectively, after accelerated carbonization for 28 days.

The possible reasons for such reduced carbonation resistance are as follows. With the increasing proportion of wastewater replacing potable water, wastewater particles are filled in concrete, reducing the number and volume of macropores. However, wastewater particles hinder the continuous deposition of C-S-H, simultaneously increasing the capillary pores. In addition, the wastewater particles are porous (Figure 3), which leads to the fact that, although the total volume of the filling part is reduced, there are still micropore channels. This provides a way for CO₂ to spread through concrete, which leads to a decrease in the carbonation resistance of concrete. Finally, the PH value of wastewater is 12, which is alkaline. With the increase of alkali content, the substitution rate of OH^- ions in concrete pore solution will increase, and the solubility of CaCO₃ after carbonization will decrease, i.e., the substitution rate of Ca²⁺ in pore solution will decrease, and the Ca(OH)₂ crystals supplemented with Ca²⁺ substitution rate will be easily dissolved, thus accelerating carbonization [49].

3.7.3. Drying Shrinkage

The drying shrinkage of C20 concrete with different proportions of wastewater replacing potable water is shown in Figure 19. With the increase in the proportion of wastewater as a replacement for potable water, the shrinkage of C20 concrete first increases, then decreases, and then increases again. With the increase in drying shrinkage time, the shrinkage of concrete with different proportions of wastewater replacing potable water gradually increases. Compared with C20-0, C20-25, C20-50, and C20-100 exhibited poor drying shrinkage after 28 days, in which the shrinkage increased by 30.2%, 16.1%, and 2.4%, respectively. However, C20-75 exhibited better drying shrinkage, and shrinkage decreased by 29.4%.

The reasons may be as follows. First, when the proportion of wastewater as a replacement for potable water is low (25%), the wastewater particles absorb part of the hydration water, leading to a decrease in the internal humidity of C20 concrete. Additionally, wastewater particles increase the number of capillary holes, increase the tension of capillary holes, and increase the shrinkage of concrete [50]. However, with the increase of the wastewater proportion of potable water, after the free water in the concrete is partially absorbed by wastewater particles, most of it is consumed by cementitious materials, which reduces the gelled empty water and its absorbed water, resulting in reduced the shrinkage of concrete [51]. When wastewater completely replaces potable water, the hydration water absorbed by wastewater particles will further increase, reducing gel water and increasing pores due to insufficient hydration and precipitation. After air and water are unsaturated, the evaporation loss of water absorbed by wastewater will increase the shrinkage of concrete compared with the proportion of 75% wastewater as a replacement for potable water.

4. Economic and Efficiency Analysis

In the economic analysis, the normalized cost index (NCI), i.e., the raw material cost per cubic meter of concrete, is used to estimate the economic benefits of wastewater mixing concrete when superplasticizer is used instead of the cement slurry to obtain the same fluidity. The unit price of each raw material is shown in

Table 9. Unit price list of materials.

Raw Materials	Cement	Fly Ash	Water	Fine Aggregate	Coarse Aggregate	Superplasticizer
Unit price (CNY/kg)	0.48	0.2	0.03	0.12	0.16	2

. The standardized cost index calculation formulae are given in Equations (4) and (5).

$$\mathrm{NCI}=\frac{\mathrm{C}}{{\mathrm{C}}_{\mathrm{Ref}}}$$

(4)

$$\mathrm{C}={{\displaystyle \sum }}_{i=1}^n{M}_i{P}_i$$

(5)

where C is the unit price of concrete per cubic meter (CNY/kg); C_Ref is the unit price of reference mixture (CNY/kg); M_i is the dosage of each raw material (kg); P_i is the unit price of each raw material (CNY/kg).

For simple calculation—based on the fitting of the relationship between the slump of this test and the content of the superplasticizer—it can be determined that 0.423 kg/m³ superplasticizer is required for every 10 mm increase in the slump of 1 m³ concrete. From experience, for every 10 mm increase in the concrete slump of 1 m³, it is necessary to increase water by 3 kg/m³. Every 1 m³ of wastewater produced by concrete can recycle CNY 1.75 worth of sand and gravel, and every 1 kg of wastewater consumed can save CNY 0.015 of disposal charges [52].

The cost index and compressive strength at 28 days of curing age are shown in Figure 20. With the increase in the proportion of wastewater as a replacement for potable water, the cost index of each mixture ratio first decreases and then increases. When superplasticizer is used to obtain slump, the cost indexes of C20-50, C20-75, and C20-100 increase by 0.6%, 2.3%, and 2.6%, respectively. When using cement slurry to obtain slump, C20-50, C20-75, and C20-100 cost indices increased by 1.7%, 6.8%, and 7.6%, respectively. Using a superplasticizer to increase fluidity (slump) is more economical than cement slurry.

5. Conclusions

In this paper, the influence and mechanism of different high concentration wastewater replacement ratios on the workability, mechanical properties, durability, and microstructure of C20 concrete were studied. The conclusions of the obtained results are summarized as follows.

(1)

As the proportion of potable water replaced by wastewater in ready-mixed concrete plants increases, additional superplasticizer need to be added for better fluidity. It is suggested that the wastewater replacement rate should not exceed 75%. Compared with ordinary concrete, concrete mixed with wastewater has better compressive strength. When the replacement rate of wastewater is 75%, it can be increased by 5.4%. This advantage is more obvious in the early stage.

(2)

The microstructure shows that the addition of wastewater will not lead to new substances in concrete. The addition of wastewater fills the internal pores of the concrete, optimizes the pore structure of the concrete, and reduces the porosity of the concrete.

(3)

Appropriate wastewater instead of potable water can improve the frost resistance and shrinkage resistance of concrete. However, the addition of wastewater will reduce the carbonation resistance of concrete.

(4)

The influence of wastewater content on concrete impermeability, mechanical properties and durability of different grades of concrete, and the influence of wastewater storage time on concrete performance need further clarification.