Characteristics of fresh concretes
Setting time
Fig. 3 shows a comparison of the setting times (initial and final) for three mixtures of mortars which have the same formulation and differ only by the mixing water. The results presented in Fig. 3 allows to deduce a slight increase in the initial setting time of cement pastes mixed with purified waste water (about 3%) in comparison with those spoiled with drinking water while respecting the different standards (EN 1008 1990 and ASTM C94 1996) that limit the gap in less than one hour to 25%.
The type of mixing water influences the setting time. Water from the source reduces the start time to about 2.7%, which is acceptable according to the same standards (EN 1008 1990 and ASTM C94 1996). These results could be explained by the presence of nitrates that could react with sodium at the pH of the medium and slightly accelerate the hydration reaction; Sodium and nitrate has a promotion effect on the hydration of cement matrices at the pre-induction period. The same behavior has been reported in earlier work (Zheng et al. 2020). In addition, the slowing of the hydration of concrete mixed with the treated waste water appears to be due to the presence of orthophosphate ions which can be adsorbed on the surface of the cement particles while slowing down their dissolution (Dorozhkin 2009; Laniesse et al. 2020, Naamane et al. 2016). The setting delay obtained by the purified water is very useful for concreting, especially in hot weather in the city of Er-Rachidia, for the transport of long-distance concrete and concreting in large volume. We note that the beginnings of catch were reached between 7h18 mn and 7h 36 mn whereas those of the ends were between 10h 24 mn and 11h 02 mn.
Measurement of slump
The slump test of the low-fluid fresh concrete makes it possible to determine its consistency according to the type of water used. Substitution of drinking water by treated wastewater and groundwater reduces subsidence. The workability of concrete mixed with these waters was firm and marked a slump between 2.8 and 3 cm, due to the presence of solids. The subsidence decreased in purified wastewater due to the spongy surface of the sludge particles having a high absorption capacity. A similar observation has been made in the previous literature (Gholamreza et al. 2017).
Density of fresh concrete
The different types of mixing water had a slight impact on the density of the fresh concrete, as indicated by the density variation of less than 1% between the concretes manufactured with GW (2.00) and the TW (1.99 ) and that made with the DW (1.98) as control concrete which are all slightly lower than that of the traditional concrete B25 which must have a density around 2,35, this slight difference of density for the concrete tempered with the water from the tablecloth could be due to the presence of iron. It can act as a filler of the microstructure of cement pastes by refining the pore structure and reducing the total porosity, thus increasing the density of the composite; due to the filling, up of pores by secondary C–S–H formation (Sikora et al. 2016).
Characteristics of concretes in the hardened state
Compressive and tensile strength of hardened concretes
Fig. 5 shows the influence of the mixing water (DW, GW and TW) on the splitting resistance of concretes at the age of 7, 14, 28 and 90 days of curing. The results show a regular increase in the splitting tensile strength with the increase of immersion time for all concretes whatever the type of mixing water used. The replacement of drinking water by wastewater treated in ordinary concrete increases its resistance compared to other concretes mixed with drinking water by 3.5%, 12% and 16% at 7 days, 28 days and 90 days of cure, respectively. On the other hand, the replacement of drinking water by groundwater increased the tensile strength by 3.5% at 7 days and by 7% at 90 days.
Fig. 6 and 7 shows the evolution of the compressive strength of the ordinary B25 concretes including the three types of mixing water used (DW, GW and TW) according to the age of preservation in drinking water. The results show that:
The compressive strength of all concretes used increases regularly with age and shows no drop. At the age of 7 days, the test pieces prepared by the treated waste and the drinking water have comparable compressive strengths and are slightly higher than that of the concrete mixed with the water of the water. At the age of 1 month (+ 28 days), the development of the mechanical resistance of concretes mixed with drinking water and groundwater was similar. The compressive strength of B25 concrete mixed with treated waste water is 16.36% higher than that of concrete mixed with drinking water.
- The comparative analysis of the concrete results mixed with GW and DW shows that the compressive resistances with GW are improved at the age of 28 days and 90 days. This could be explained by the concentration of iron in the GW which greatly increases the mechanical strength. Due to the filling effect of iron, with low permeability and a significant increase in strength after 28 days of hardening. The possible reason for this increase is that iron consumes and reduces the amount and size of Ca (OH)2 crystals and fills voids in the CSH gel structure and ultimately the structure of hydrated products is denser and compact (Zhang et al. 2020; Umra Shettima et al. 2018; Vilazdeh Kiamahalleh et al. 2020). At the age of 90 days, the compressive strength is maximum in the specimens prepared with the treated wastewater. According to the literature, these results could be explained by the increase in the pH of the cementitious matrix due to the strong basicity of the TW and the high concentration of phosphate ions in this type of concrete, which precipitates calcium and calcium crystallize rapidly as hydroxyapatite preferentially to HSCs, according to the following equation (Valizadeh Kiamahalleh et al. 2020; FROHARD 2014):
As noted by Waddell (1974), a good measure of concrete quality could be obtained by using the ratio of 28-day concrete strengths (fTW28) to that of a similar mixture cast with potable water (fd). The ratio of fTW28 / fd is 1.16, which indicates that the 28-day compressive strength of concretes prepared with treated wastewater is 16% higher than that of concretes prepared with drinking water. The water of the water, the ratio of fGW28 / fd is 1.036, thus indicating a 3.6 % increase in the resistance to 28 days than that of DW. The results thus obtained favor the use of water treated wastewater from the Er-Rachidia wastewater treatment plant for the production of ordinary concrete as they significantly improved the development of B25 concrete resistance in the long term (28 days and more). Table 7 illustrates the compressive strengths and porosity at 60 days of concretes made with different types of mixing water.
As shown in Table 7, the porosity decreases with the increase of the compressive strength. Thus, concretes made with groundwater or treated wastewaters are less porous in comparison with those prepared by drinking water. This result might be explained by the formation of gypsum and ettringite that is more favored in the first case and that fill the voids left by the mixing water trapped in the pores and the capillaries of the concrete, which evaporates slowly with time. Thus, the effect of the type of mixing water used does not stop in the mechanical properties, it could also influence the porosity of the concretes and therefore its durability, since the rise of the porosity reduce the resistant of concrete.
Table 7 Resistance of mixed concrete with different types of water at 60 days and their porosity
|
DW
|
GW
|
TW
|
Rc60j[MPa] ± 1 MPa
|
25
|
32.5
|
30
|
Porosity [%] ± 0,5 %
|
7.92
|
5.59
|
6.68
|
A one-way analysis of variance (ANOVA) test
The Table. 8 presented the results of a Unidirectional Variance Analysis (ANOVA) test that was performed at a significant 5% compression resistance level at the ages of 7, 14, 28 and 90 days. There are three groups of concrete mixtures (DW, GW, and TW) and four samples for each group and age for analysis. To present the significance of the experimental results concisely, all concrete mixtures were compared together with the control mixture (DW). In the ANOVA test, if the P-value is greater than 5% in the 95% confidence interval, the difference between the results of various concrete mixtures is not significant. The results of the ANOVA statistical test at a 5% significance level showed no significant difference between the compressive strength of the concrete of various concrete mixtures at test ages with a probability value p being below the threshold of significance of 0.05 and an F value lower than the critical F value.
It shows that the development of concrete with wastewater types studied in this research does not significantly affect. Thus, these types of treated wastewater can be used for the development of concrete offering better performance in terms of resistance to fractional traction.
Table 8 A one-way ANOVA at the significant 5% compressive strength of B25 concrete mixes using various types of water at 7, 14, 28 and 90 days of age.
Descriptive statistics
|
|
|
7 Days
|
14 Days
|
28 Days
|
90 Days
|
N valide (listwise)
|
N
|
statistics
|
12
|
12
|
12
|
12
|
12
|
Interval
|
statistics
|
3.5
|
3.00
|
5.00
|
8.50
|
|
Minimum
|
statistics
|
18.00
|
24.00
|
27.00
|
32.00
|
|
Maximum
|
statistics
|
21.50
|
27.00
|
32.00
|
40.00
|
|
Medium
|
Statistics
|
19.8658
|
25.3900
|
29.2142
|
36.0000
|
|
Std Error
|
0.33389
|
0.32666
|
0.57012
|
0.90336
|
|
Standard deviation
|
statistics
|
1.15664
|
1.13158
|
1.97497
|
3.12933
|
|
Variance
|
statistics
|
1.338
|
1.280
|
3.900
|
9.793
|
|
ONEWAY 7 Days,14 Days, 28 Days, 90 Days BY VAR00005
/POLYNOMIAL=1
/MISSING ANALYSIS.
One-way ANOVA
|
Sum of squares
|
ddl
|
Standard Error
|
F
|
Fcrit
|
Signification
(p-value)
|
One-way ANOVA
|
14.716
|
11
|
1.338
|
10.874
|
13.753
|
.000
|
One-way ANOVA
|
14.085
|
11
|
2.12
|
10.604
|
17.33
|
.000
|
One-way ANOVA
|
42.905
|
11
|
1.77
|
34.634
|
59.23
|
.000
|
One-way ANOVA
|
107.720
|
11
|
1.985
|
95.891
|
70.45
|
.000
|