3.1. Treated Domestic Wastewater Quality
Mixing water quality plays an essential role in the preparation of concrete [
26]. Drinkable water can be used as mixing water for concrete production. Furthermore, some water types that are not suitable for drinking may be fit for concrete making [
27,
28].
Table 4 shows that the TSS concentration of PTW (148 mg/L) and STW (10.5 mg/L) were below the tolerated limit of suspended fine particles in mixing water (2000 mg/L) according to the Portland Cement Association (PCA) [
27]. However, a suspended solid of 50,000 mg/L is still tolerated in case of reusing of wastewater from mixer washout operations. The results also showed that the TDS concentrations of PTW and STW (900 and 974 mg/L) are lower than the maximum permissible concentration (2000 mg/L). The total organic content of the STW (46.9 mg/L) is lower than the maximum permissible level while the organic concentration of PTW (538 mg/L) exceeded the maximum permissible level. The values of chloride and pH are under the maximum permissible limits.
E. coli is still considered as the most important indicator of public health [
29].
Based on the acceptable criteria for water to be used in concrete (
Table 4), the STW is suitable for concrete production whereas the PTW should be tested and compared with specimens made with FW or DW to ensure that the impurities in PTW do not adversely affect the mechanical properties of the concrete [
30]. The PTW should be pretreated to reduce the microorganism content before the water can be in direct contact with humans [
31]. Regarding the Jordanian standard specifications, there are no available limits for dissolved impurities in mixing water and their possible negative impact on concrete properties.
3.2. Effects of Treated Wastewater on Cement Paste
The results presented in
Table 5 show that the initial setting time of cement paste mixed with PTW and STW were up to +5 and +30 min compared to control. Furthermore, the water/cement ratio shows a slight increase of up to 0.4% and 0.6% for PTW and STW, respectively. Moreover, the results of the soundness test of cement by the Le-Chateliers method show no significant differences between using PTW, STW, and control. According to the requirement of the Jordanian standard (No. 30-1/2007) [
35], the maximum expansion must be ≤10mm.
These results are in harmony with ASTM C94 [
24] requirements on initial setting time where the initial setting time of cement paste made with the questionable water must not be more than 60 min earlier than that made with the same cement using DW. Shekarchi et al. [
7] and Al-Ghusain et al. [
14] reported that the dissolved salts can increase the initial setting time up to +70 min in case of using treated domestic wastewater on cement paste.
3.3. Effects of Treated Wastewater on Mortar Properties
The results depicted in
Figure 3 reveal that mortar made with STW at curing time (7, 28, and 200 days) has no significant negative effect on the mortar’s compressive strength. However, at 120 days there is a slight reduction in compressive strength. Using PTW as mixing water led to a reduction in the compressive strength at curing time (7, 28, 120, and 200 days). This could be due to the effect of organic contents, which may have contributed to mortar strength reduction.
Mortar relative strength index (R
s) can be defined as the ratio of the compressive strength of mortar or concrete to that of the control [
36].
Table 6 represents the mortar R
s of mixes using PTW and STW at the wet curing ages of 7, 28, and 120 days. The relative index in
Table 6 indicates that using PTW as mixing water for mortar production led to a significant reduction in the compressive strength between 13.6% and 16.2%, while addition of the STW led to a slight increase in R
s at curing age (28 days) and a slight decrease in R
s at long term curing time (120 days). According to the ASTM C109 [
20], water is suitable for production concrete if the mortar which was made with it has strength at 7 days curing time that is equal or less than 10% reduction compared to control samples made with drinkable water.
A stereo microscope is employed to produce three-dimensional photos and low magnification observation.
Figure 4 apparently shows that there are no significant differences in air void size and distribution between mortar specimens made with PTW, STW, and DW. There is no micro-crack effect on the surface of mortar specimens. The mortar specimen structure and the matrix between aggregates are structurally intact.
3.4. Effects on Concrete Properties
The best indicator for concrete workability is slump value. The results of slump test on concrete using PTW, STW, and Control (FW for full-scale or DW for bench-scale) as mixing water are presented in
Figure 5. In the case of the bench-scale mixtures, the results reveal that using PTW and STW led to a decrease in slump by 2.5 and 1.5 cm, respectively. In the case of the full-scale mixtures, using PTW and STW led to a decrease in slump by 4 and 1 cm, respectively. The difference between bench-scale and full-scale the slump value is attributed to apply superplasticizer by 0 and 5 kg/m
3, respectively. Application of PTW led to a decrease in the concrete workability and increase in the concrete viscosity. The total dissolved solids and SO
42− in the PTW and STW are higher than the control mixing water (
Table 4). The achieved observations are consistent with previous studies [
7,
10]. One possible explanation is that the dissolved solids in the PTW reduce the concrete slump value. Tang and Gartner [
37] and Lawrence [
38] reported that the major dissolved solids in the solution phase (Ca
2+, K
+, Na
+, SO
42−, and OH
−) were very important factors in governing the characteristics of early cement paste hydration. They found that also presence of relatively soluble sulfates retarded the initial C
3A hydration.
In the case of the lab mixtures, the compressive strength of concrete utilizing PTW, STW, and DW at curing ages 7, 28, 120, and 200 days are shown in
Figure 6. In comparison with concrete made by DW, the compressive strength of concrete made by PTW and STW at 7 days shows a slight decrease with no significant differences. The compressive strength of STW at 28 days curing age shows no significant differences. However, at curing ages of 120 and 200 days, it shows a significant reduction up to 10%. The concrete made with PTW shows a significant reduction in the compressive strength of up to 19.6% at 120 days (
Table 6). In the case of scale-up mixtures to ready-mix concrete (
Figure 7): In comparison with concrete made by FW, the compressive strength of concrete made by PTW and STW at 7 days shows with no significant differences. The compressive strength of STW at 28 days curing age shows a slight decrease with no significant differences. In addition, at curing age of 120 days, it shows a significant reduction of up to 7.2%. The concrete made with PTW shows a significant reduction in the compressive strength of up to 19.6% at 120 days. This reduction in the compressive strength is placed on the concrete which is made by PTW outside the allowable threshold.
The present results are not in disagreement with AbdolChini and Mbwambo [
39] who reported that the properties of concrete are not affected by the use of recycled water (washing wastewater from ready–mixed concrete plant). However, these study results show that the type of recycled water may influence concrete properties.
Domestic wastewater has different pollutant contents. Therefore, its impact on concrete properties should be different. According to the IS 456-2000 [
40] and Kucche et al. [
26], the reduction of compressive strength of concrete should not exceed 15% of the mean compressive strength of concrete specimen made with drinkable water.
The effect of dilution of PTW, using dilution ratio of (PTW:DW) 1:0, 3:1 and 1:1, on the development of strength is presented in
Figure 8. It is clear that there are no significant effects of dilution of PTW on compressive strength at various curing ages. Based on these results, dilution of PTW is not the solution to enhance the PTW properties as mixing water.
The water absorption parameter is one of the most important indicators of the concrete’s potential durability and quality [
28]. The average water absorption of concrete performed with PTW, STW, and DW is 1.70%, 1.74% and 1.74%, respectively. The result shows that the effect of using domestic wastewater as mixing water on water absorption is not significant. This result is in agreement with Shekarchi et al. [
7] who reported that utilizing treated wastewater as mixing water does not affect the concrete’s durability and water absorption.