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The use of water treatment plant sludge ash as a mineral addition
Construction and Building Materials 94 (2015) 513–520
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The use of water treatment plant sludge ash as a mineral addition A.L.G. Gastaldini ⇑, M.F. Hengen, M.C.C. Gastaldini, F.D. do Amaral, M.B. Antolini, T. Coletto Federal University of Santa Maria, 97105-900 Rio Grande do Sul, Brazil
h i g h l i g h t s It is possible to obtain environmentally sound concrete by using WTP sludge ash. The use of WTP sludge ash helps reduce greenhouse gases emissions. The use of WTP sludge ash reduces the discharge of this material in watercourses. 3
It is possible to reduce the consumption of cement in concrete by 37–200 kg m
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 19 May 2015 Accepted 12 July 2015
Keywords: Pozzolanic activity Cement substitution Characterization Compressive strength Water treatment plant sludge Chapelle test
a b s t r a c t For decades, the sludge produced in water treatment plants (WTP) was dissolved in water and then discharged in watercourses. WTP sludge is rich in pathogens and metals and when discharged in watercourses, it increases the amount of suspended solids, eventually causing the water body to silt up. Existing legislation in Brazil prohibits the discharge of WTP sludge in watercourses, but the practice persists. This study investigated the possibility of using WTP sludge as a mineral addition. First, the pozzolanicity of WTP sludge with Portland cement and the concentration of fixed calcium hydroxide using the Chapelle test were determined after exposing the material to different calcination temperatures and residence times. The samples with the best results were used to investigate the performance of concrete mixes where WTP sludge was substituted for Portland cement in concentrations ranging from 5% to 30% in three different water/binder (w/b) ratios (0.35, 0.50 and 0.65). Results indicate that the use of WTP sludge ash improves the strength of concrete mixes when compared with concrete with rice husk ash or silica fume. By using WTP sludge ash, it is possible to obtain the same strength of a concrete mix with 100% Portland cement and reduce the consumption of cement by 37–200 kg m3 of concrete, depending on the concentration of substitution and the desired strength level. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The growing demand for products and services associated with population growth is responsible for the expansion of industrial activity, which in turn results in an increase in the consumption of materials. Civil construction has considerable environmental impacts because of the consumption of natural resources and the generation of waste associated with this industry. The concrete industry is one of the major users of natural resources and its growth has resulted in significant environmental impacts associated with the use of raw materials, including water, as well as the release of greenhouse gases during the production of Portland cement. The industry has been trying to mitigate such effects by reducing emissions during the manufacture of Portland cement. Modern ⇑ Corresponding author. E-mail address: [email protected] (A.L.G. Gastaldini). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.038 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
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plants release 0.7 metric ton of CO2 for each metric ton of clinker produced. In addition, rice husk ash, silica fume and blast furnace slag are now used as substitutions for clinker. Recent studies have attested the viability of using other materials as mineral additions [1–4]. The use of mineral additions as partial substitutions for Portland cement has become a pressing need, not only because of the need to reduce the environmental impact associated with the manufacture of cement but also to improve the durability of concrete structures exposed to harsh environments, such as sulfates, chlorides, alkali-aggregate reactions, etc. [5–7]. This is due to the fact that mineral additions change the composition of the pore solution [8]. This way, electric conductivity is changed [9]. The substitutions refine the pore structure [10], which reduces permeability and enhances strength, even though in some cases strength increases more slowly. The optimal concentration of a given substitution depends on the type of cementitious material used, the intended modifications
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to the concrete and the characteristics of the environment to which the concrete will be exposed. The use of calcined clay as a partial substitution for Portland cement has been studied in detail because of its technical, economic and environmental benefits [11–15]. On the other hand, population growth also translates as an increase in water consumption, which results in more sludge being produced by water treatment plants (WTP). The amount and composition of the sludge depends on the volume of water treated in the plant, the process used and the characteristics of untreated water. It is often the case that water suppliers criticize the quality of untreated water. However, some WTPs end up discharging their waste in water courses, which runs contrary to their own interests. WTP sludge is a type of solid waste and must be processed and disposed of accordingly to prevent environmental damage. WTP sludge is rich in pathogens and metals. When discharged in watercourses, it increases the amount of suspended solids, eventually causing the water body to silt up [16]. Even though existing legislation in Brazil prohibits the discharge of WTP sludge in watercourses, the practice persists. Many water suppliers in Brazil have signed agreements with the government that give them up to 30 years before they fully comply with existing legislation. As a result, they will continue discharging sludge and harming the environment for years to come. Several researchers have attested the viability of using untreated sludge as a partial substitution for fine aggregate or cement [17,18] or as a partial substitution for the siliceous material in the manufacture of cement [19]. WTP sludge has also been used as lightweight coarse aggregate (water treatment sludge and softwood sawdust composite) [20] and in the production of heavy clay [21]. However, there are few studies on the possibility of using WTP sludge ash as a pozzolanic agent in concrete and Portland cement mortars. The present study thus aims to determine the optimal temperature and residence time to yield a material with pozzolanic activity that can be used as a partial substitution for cement without compromising mechanical strength and production cycles and that can improve the sustainability of construction sites and reduce concrete costs. 2. Experimental program
Table 1 Physical and chemical characteristics of the cementitious materials. Physical properties Characteristic
2.2. Mixture proportions A total of seven binder mixes were prepared. The reference mixes were labeled REF and the mixtures with 5%, 10%, 15%, 20%, 25% and 30% of WTP sludge ash were labeled 5SA, 10SA, 15SA, 20SA, 25SA and 30SA, respectively.
Sludge ash (600 °C)
3.13 1.14
2.56 27.7
Particle size distribution (lm) D(v,0.1)* D(v,0.5)* D(v,0.9)*
3.67 14.1 38.81
– 20.07 65.06
Chemical composition (weight%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O MnO TiO2 P2O5 LOI
20.4 4.37 2.64 62.9 2.7 2.2 0.13 0.95 <0.10 0.29 <0.10 3.16
66.2 17.7 8.76 0.57 0.96 – 0.32 1.16 0.13 0.86 0.33 3.37
* equivalent spherical diameter (of the same volume) 10%, 50% and 90% of the particle distribution are below.
Table 2 Composition of the concrete mixtures (kg m3). Mixture
w/b
CM
PC
SA
Fine agg.
Coarse agg.
water
P
SP
(kg m3) REF
0.35 0.50 0.65
487 359 284
487 359 284
– – –
633 740 804
1076 1055 1045
170 180 185
1.46 – –
– – –
5SA
0.35 0.50 0.65
487 359 284
463 341 270
24 18 14
626 736 798
1076 1055 1045
170 180 185
1.94 – –
– – –
10SA
0.35 0.50 0.65
487 359 284
438 323 256
49 36 28
623 732 798
1076 1055 1045
170 180 185
4.84 – –
1.45 – –
15SA
0.35 0.50 0.65
487 359 284
414 305 241
73 54 43
619 731 797
1076 1055 1045
170 180 185
4.84 0.36 –
1.93 – –
20SA
0.35 0.50 0.65
487 359 284
390 287 227
97 72 57
614 729 795
1076 1055 1045
170 180 185
4.84 1.07 –
2.17 – –
25SA
0.35 0.50 0.65
487 359 284
365 269 213
122 90 71
609 725 792
1076 1055 1045
170 180 185
4.84 1.43 –
2.41 – –
30SA
0.35 0.50 0.65
487 359 284
341 251 199
146 108 85
606 722 790
1076 1055 1045
170 180 185
4.84 3.21 0.71
3.13 – –
2.1. Materials High-early strength Portland cement and WTP sludge ash were used as binders in this experiment. The WTP sludge was first dried in an oven at 110 °C for 24 h. It was then homogenized and calcined in a muffle kiln at temperatures of 400, 500, 600 and 700 °C, with residence times of 1 and 2 h, except for the temperature of 700 °C, in which case a residence time of 30 min was used. After calcination, the ashes were ground in a ball mill for 1 h. The physical and chemical characteristics of the different cementitious materials are shown in Table 1. The fine aggregate used in the experiment consisted of natural quartz sand, with specific weight of 2.66 g/cm3, unit mass of 1.62 g/cm3, fineness modulus of 1.85 and maximum particle size of 1.2 mm. The coarse aggregate consisted of crushed stone with specific weight of 2.48 g/cm3, unit mass of 1.38 g/cm3 and maximum particle size of 19.0 mm. Concrete samples were prepared with a plasticizer additive, except for those with a w/b ratio of 0.35 and those with 30% sludge ash, which required a superplasticizer additive (modified carboxylic ether), even when w/b of 0.50 was used. The casting temperature was set at 18 ± 1 °C and the mix water was heated or cooled to adjust for the temperature of the other materials [22]. The quantities of materials used per cubic meter of concrete are shown in Table 2.
PC
Specific gravity (g/cm3) Specific surface area BET (m2/g)
w/b – water/binder ratio; CM – cementitious materials; PC – Portland cement; SA – sludge ash; P – plasticizer; SP – superplasticizer.
For each mixture, three w/b ratios (0.35, 0.50 and 0.65) were used with a mortar content of 51% by weight of dry materials. In the mixtures with WTP sludge ash, the amount of sand was adjusted to ensure that the mortar content was the same as that in the reference mixture. This was necessary because of the lower specific gravity of sludge ash when compared with Portland cement.
2.3. Testing procedures 2.3.1. Assessment of pozzolanic activity with Portland cement Samples were tested according to Brazilian standard ABNT NBR 5752:2014 [23]. This procedure is similar to the one defined in ASTM C 311. The difference is the use of a plasticizer to achieve the same flow for samples with the same w/b ratio. The method consists of preparing mortar test specimens with mix proportion = 1:3 and w/b ratio = 0.48. Mortar ‘‘A’’ was prepared with Portland cement CP II F, while in
A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520 Mortar ‘‘B’’ 25% of pozzolanic material is used as a substitution for the cement. Cement type CP II F contains, in addition to clinker and gypsum, 6–10% of ground limestone. Both mortars should display the same consistency and a plasticizer should be used, as required. The strength activity index of cement was calculated as the ratio between the compressive strength of mortar B at 28 days divided by the compressive strength of mortar A (reference) at the same age, multiplied by 100. 2.3.2. Determination of fixed calcium hydroxide – modified Chapelle method Samples were tested according to Brazilian standard ABNT NBR 15895-10 [24]. The method consists of stirring a mix of 1 g of pozzolanic material and 2 g of calcium oxide at 90 °C for 16 h in 250 mL water. The result is expressed as the amount of fixed calcium hydroxide per gram of pozzolanic material. 2.3.3. XRD and thermogravimetric analysis XRD measurements were carried out in a Rigaku DMAX 1100 diffractometer using CuKa radiation with a wavelength of 1.54 Å operating at 40 kV and 40 mA. Step scanning was used with a scan speed of 2°/min and the data were collected in the 5–50° interval (2h). Thermogravimetric analysis (TG-DTG) was carried out in an ATD-TG LabSys Evo Setaram. The samples were heated from 20 to 1000 °C at a constant rate of 10 °C/min in a nitrogen atmosphere. 2.3.4. Compressive strength Eight cylindrical test specimens ø 10 20 cm were cast for each mix, four of which were wet-cured for 7 days and four for 28 days. The test specimens were removed from the moulds after 24 h and stored in a moist cabinet at 23 ± 2 °C and RH > 95% until ready for testing.
3. Results and discussion 3.1. Determination of pozzolanic activity with Portland cement Table 3 presents the results of compressive strength and the strength activity index of cement for the different calcination temperatures and residence times in the test. Results show that even when the lowest temperature and shortest residence times are used (400 °C for 1 h) in a mix with only 75% of cement, compressive strength results at 28 days (44 MPa) are very similar to the values found for the reference mix – 100% Portland cement (45 MPa). Compressive strength for the mix calcined at 600 °C for 1 h was 54.5 MPa, which is higher than the values found for the mix calcined at 700 °C for 30 min and for 2 h (50.2 and 50.7 MPa, respectively). Fig. 1 presents the particle size distribution for calcination temperatures of 600 °C and above, which yielded the highest compressive strength. Similar particle size distributions were observed in the samples with WTP sludge ash calcined at different temperatures and residence times. In order to determine the optimal calcination temperature, it is important to ensure that the particle size distribution of the calcined clays are similar. This way, the physical effect associated with the substitution of WTP sludge ash for cement will be the same. It should be remembered that the particles of WTP sludge ash are slightly larger than those of Portland cement. Table 3 Compressive strength of mortars at 28 days and strength activity index. Mixture
Compressive strength 28 days (MPa)
S.A.I⁄ (%)
REF 400-1 h 400-2 h 500-1 h 500-2 h 600-1 h 600-2 h 700-1/2 h 700-1 h 700-2 h
45.0 44.0 45.4 47.3 48.6 54.2 51.4 50.2 56.3 50.7
100 98 101 105 108 120 114 112 125 113
S.A.I⁄: strength activity index.
515
Even though the best strength activity index was that of in the sample with WTP sludge ash calcined at 700 °C for 1 h (56.3 MPa), this value was very close to that of the sample calcined at 600 °C for 1 h (54.2 MPa – just 3.5% lower). An increase in strength of only 3.5% does not offset the energy required to reach a temperature of 700 °C. Therefore, a temperature of 600 °C for 1 h was adopted for concrete testing. 3.2. Determination of fixed calcium hydroxide – modified Chapelle method Table 4 presents the results of the fixed calcium hydroxide test, according to Brazilian Standard NBR 15.895/10. The results show that the values are higher than those reported by Raverdy et al. [25]. These authors classifiy low reactivity materials as those with a value of 330 mg of CaO per gram of material. The use of longer residence times at 600 °C resulted in slightly lower pozzolanic activity. However, at 700 °C, this reduction amounted to almost 10%. It was observed that longer residence times caused particles to agglomerate as they start to fuse together. Brazilian Standard ABNT NBR 15894-1:(2010) [26] establishes that the pozzolanic reactivity of a metakaolin sample must not be lower than 750 mg Ca(OH)2/g of metakaolin while French standard AFNOR NF P 18 – 513 [27] sets a value of 700 mg Ca(OH)2/g of metakaolin. The ash, after calcination at 700 °C for 30 min and at 600 °C for 1 h, shows a concentration of fixed calcium hydroxide of 680 and 673 mg Ca(OH)2, respectively. This value corresponds to 90% of the value prescribed by the Standard. However, this WTP sludge ash does not meet the requirements of the Standard concerning the concentration of Al2O3, which must fall within the 32–46% range, nor the concentration of SiO2 which must not exceed 65%. It does, however, meet all remaining requirement of the Standard in question. 3.3. Results of XRD and thermogravimetric analysis The diffractogram in Fig. 2 shows that the WTP sludge ash is made up mainly of quartz and kaolinite. There are also small amounts of feldspar and hematite. Thermogravimetric graphs were used to determine mass losses. The theoretical mass loss of kaolinite (13.9%) was used to estimate the concentration of this mineral in the samples. Results are shown in Table 5. Thermal analysis also indicates that the sample shows a mass loss between 200 and 400 °C associated with an exothermal peak corresponding to the decomposition of organic matter. In general terms, it can be said that the pozzolanic activity of a clay increases as the amount of amorphous or random structures in the material also increases. These include materials such as diatomites of clay minerals. The best clays for thermal activation are rich in clay minerals (concentrations above 50%) and have low associated quartz content (high hardness phase, no pozzolanic activity). Kaolin clays are best for the production of thermally activated pozzolans, which display optimal activity after calcination between 600 and 800 °C [28–30]. Calcined WTP sludge ash samples in this study show a concentration of kaolinite of 37%, which is lower than the recommended minimum. However, compressive strength levels can be maintained and even increased from 28 days by substituting calcined clays containing a large proportion of impurities (more than 40% of quartz) for 30% of the cement in mortars [31]. Similarly, it has been reported that ‘‘a material with pozzolanic activity similar to commercial grade MKs can be produced with clays that have a kaolinite content <40%, as long as it undergoes
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Fig. 1. Particle size distribution (laser) of cement and WTP sludge ash calcined at different temperatures and different residence times.
Table 4 Results of fixed calcium hydroxide – modified Chapelle method. Temperature
Calcium hydroxide fixed (mg Ca(OH)2/g sample)
600 °C 600 °C 700 °C 700 °C 700 °C
673 666 680 643 620
1h 2h 1/2 h 1h 2h
thermal treatment between 600 and 800 °C. For optimal performance, the material should be sedimented, in order to enrich the clay minerals [32]. 3.4. Compressive strength Table 6 shows the values of compressive strength and the strength activity index (in percentage terms) in relation to the reference sample after wet curing for 7 and 28 days, respectively.
The partial substitution of WTP sludge ash for cement resulted in an increase in compressive strength when compared with the reference concrete (100% Portland cement), both at 7 and 28 days. The exception was for the mix with 5% WTP sludge ash and w/b ratio of 0.35 at 7 days. For the remaining mixes, the increase in strength ranged from 3% to 30%, depending on the w/b ratio used and the concentration of the substitution of WTP sludge ash for cement. The fact that improved strength can be observed from an early age in the mixes with WTP sludge ash when compared with the reference concrete indicates that this substitution does not require longer casting times and therefore the workflow in construction sites is not affected. All mineral additions, be they cementitious or pozzolanic, contribute to improving mechanical strength. Some display this effect early on, such as silica fume or rice husk ash produced under controlled burning. Other materials, such as fly ash or slag, require longer periods unless they display high fineness. The improved mechanical strength is a result of the pozzolanic reactions and also
Fig. 2. XDR – WTP sludge.
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A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520 Table 5 Thermogravimetric analysis results – WTP sludge. Mass losses (%) Sample
20–200 °C
200–400 °C
400–800 °C
WTP sludge ash
5.3
5.4
4.9
Kaolinite content (%)
Table 6 Compressive strength of concrete mixes with WTP sludge ash and strength activity index of mixes in relation to reference mix. Mixture
w/b
Compressive strength (MPa)
Mixture
w/b
Compressive strength (MPa)
37
The kaolinite content was determined in relation to the theoretical mass loss of this material = 13.9%.
1
Table 7 Compressive strength of concrete mixes with 10%, 20% and 30% of rice husk ash (RHA) – curing 28 days.
Cs mixture/Cs reference
7
28
7
28
0.35 0.50 0.65
49.5 34.8 25.4
54.7 40.7 28.8
100 100 100
100 100 100
5SA
0.35 0.50 0.65
46.3 34.7 28.4
56.5 43.5 34.5
1.03 1.07 1.20
1.03 1.07 1.20
10SA
0.35 0.50 0.65
54.8 35.0 29.4
66.5 44.0 36.0
1.21 1.08 1.25
1.22 1.08 1.25
15SA
0.35 0.50 0.65
55.8 35.3 28.0
67.0 48.5 35.5
1.22 1.19 1.23
1.22 1.19 1.23
20SA
0.35 0.50 0.65
55.8 37.5 27.8
69.0 45.6 32.3
1.26 1.12 1.12
1.26 1.12 1.12
25SA
0.35 0.50 0.65
54.5 41.3 29.0
71.0 47.8 33.3
1.30 1.17 1.16
1.30 1.17 1.16
30SA
0.35 0.50 0.65
53.5 38.5 27.2
65.3 46.3 33.0
1.19 1.14 1.14
1.19 1.14 1.14
w/b1 – water/binder ratio; SA – sludge ash WTP.
the filler effect, which increases compactness. In the case of WTP sludge ash, the occurrence of pozzolanic reactions is confirmed by the result of the Chapelle test, discussed in Section 3.2. A comparison of the compressive strength of concrete mixes with WTP sludge ash (10%, 20% and 30%) with mixes with rice husk ash (RHA) burned under controlled conditions [33] yielded similar results (Table 7). In this case, the mixes were prepared with the same type and brand of cement, similar aggregates and the same w/b ratios. The only difference was that the mixes with rice husk ash required the use of a superplasticizer additive, thus making them more expensive than the mixes with WTP sludge ash. In Table 8, the values for concrete mixes with WTP sludge ash are compared with those prepared with RHA burned under controlled conditions and low concentrations of carbon graphite. Again, mixes were prepared with identical w/b ratios, the same type and brand of Portland cement and similar aggregates and were cured in a moist cabinet for 3 and 7 days and tested at 28 days. For the sake of comparison, composite concrete mixes with 5% and 10% silica fume (5SF and 10SF) were also prepared [34]. For the concrete mixes cured in a moist cabinet for 3 days, those with WTP sludge ash usually showed higher compressive strength when compared to the samples with rice husk ash and silica fume substitutions in the same concentrations.
SA⁄
REF
0.35 0.50 0.65
53.7 47.3 27.6
54.7 40.7 28.8
10RHA
0.35 0.50 0.65
68.1 46.9 31.7
66.5 44.0 36.0
20RHA
0.35 0.50 0.65
72.0 52.3 33.2
69.0 45.6 32.3
30RHA
0.35 0.50 0.65
67.4 50.1 29.9
65.3 46.3 33.0
Age (days)
REF
RHA
SA⁄ – mixes with WTP sludge ash and the same concentration of substitution.
Table 8 Compressive strength of concrete with rice husk ash with low graphite carbon content or with silica fume. Mixture
w/b
Compressive strength (MPa) Curing 3 days
Curing 7 days
Age (days) 28 REF
0.35 0.50 0.65
54.0 37.3 29.0
54.7 40.7 28.8
58.0 40.0 33.2
54.7 40.7 28.8
5RHA
0.35 0.50 0.65
56.0 43.6 33.6
56.5 43.5 34.5
60.2 47.6 36.0
56.5 43.5 34.5
10RHA
0.35 0.50 0.65
59.3 40.5 29.0
66.5 44.0 36.0
65.0 42.0 35.6
66.5 44.0 36.0
20RHA
0.35 0.50 0.65
61.0 40.6 30.0
69.0 45.6 32.3
72.0 43.0 32.0
69.0 45.6 32.3
30RHA
0.35 0.50 0.65
65.6 47.0 31.0
65.3 46.3 33.0
72.6 49.2 31.6
65.3 46.3 33.0
5SF
0.35 0.50 0.65
54.0 44.0 36.6
56.5 43.5 34.5
59.7 50.0 38.0
56.5 43.5 34.5
10SF
0.35 0.50 0.65
53.5 43.5 37.6
66.5 44.0 36.0
55.5 48.2 39.5
66.5 44.0 36.0
NB: results in bold – mixes with WTP sludge ash and the same concentration of substitution.
The results of Chapelle method tests carried out in the samples with RHA and silica fume yielded results of 1365 and 1513 mg Ca(OH)2/g sample, respectively. These are much higher than the values recorded for WTP sludge ash (673 mg Ca(OH)2/g sample). Therefore, the results of compressive strength corroborate the finding that even clays that would be considered as ‘low grade’ for the production of pozzolans cannot be disregarded in developing countries [32]. 3.5. Economic feasibility of using WTP sludge ash From the pairs of values of compressive strength at 28 days and the w/b ratios, the coefficients of equation Cs = A/Bw/b were
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Table 9 Coefficients equations C = D/ECs and Cs = A/Bw/b. Mixture
w/b
Cementitious materials – C (kg m3)
Compressive strength 28 days (MPa)
REF
0.35 0.50 0.65
487 359 284
54.7 40.7 28.8
5SA
10SA
15SA
20SA
25SA
30SA
–
2
R2 = 1.0 Cs ¼ 116:6864 w=b 8:513456
2
0.35 0.50 0.65
487 359 284
R = 0.98 C ¼ 122:87906 Cs
56.5 43.5 34.5
R = 0.99 Cs ¼ 126:2019 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.97 C ¼ 161:9419 0:9833Cs
66.5 44.0 36.0
R2 = 0.96 Cs ¼ 131:3381 w=b
0:97586
2
5:1772
7:7339
2
0.35 0.50 0.65
487 359 284
R = 0.98 C ¼ 155:6019 Cs
67.0 48.5 35.5
R = 0.99 Cs ¼ 140:3112 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.96 C ¼ 180:2122 0:9856Cs
69.0 45.6 32.3
R2 = 0.98 Cs ¼ 165:6965 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.97 C ¼ 178:9445 Cs
71.0 47.8 33.3
R2 = 0.99 Cs ¼ 167:7014 w=b
0.35 0.50 0.65
0:9831
0:9859
2
R = 0.99 C ¼ 164:7168 0:9834Cs
487 359 284
65.3 46.3 33.0
8:3078
12:6201
11:9293
2
R = 0.99 Cs ¼ 144:7362 w=b 9:7423
calculated for all mixes in the study (Table 9). This way, the w/b ratios for the mixes with WTP sludge ash required to obtain the same compressive strength value as the reference mix at 28 days were calculated – 54.7, 40.7 and 28.8 MPa. These were labeled (w/b⁄). From the pairs of values ‘binder consumption’ and ‘compressive strength’, the coefficients of equation C = D/ECs were calculated for all mixes in this study (Table 9). This way, the binder consumption of the mixes with WTP sludge ash required to obtain the same
compressive strength value as the reference mix at 28 days – 54.7, 40.7 and 28.8 MPa were calculated. These were identified as ‘Cementitious Materials Consumption’. From this value, Portland cement (‘‘PC⁄⁄’’) and sludge ash consumptions were calculated with reference to the cement substitution content. In the mix with 5% WTP sludge ash, 468 kg of binder are required to obtain a compressive strength value of 54.7 MPa. This weight is broken down as follows: 445 kg of cement (0.95 * 468) and 23 kg of sludge ash (0.05 * 468). To obtain the same strength in the reference sample, 485 kg of cement are needed. Therefore, the 23 kg of WTP sludge ash are equivalent to 42 kg of cement (487–445 kg). Thus, from the data in Table 9, it was possible to establish the reduction in Portland cement consumption in the concrete mixes prepared with different concentrations of WTP sludge ash to obtain the same strength as the reference concrete (Table 10). In one city in the state of Rio Grande do Sul, Brazil, the local WTP pays a contractor U$19.15 m3 to have the sludge collected plus U$0.79 to have the material transported to a site located 150 m from the plant. In this case, this is possible because the plant has a permit to use a nearby plot as a landfill site. Dumping costs in the landfill run at U$36.10 m3, so the grand total is U$56.04 m3. This works out as a cost of approximately U$0.0467 per kg of sludge dumped in the landfill, which is more than twice the cost of 19-mm crushed rock (U$ 0.018) and 5.6 times the cost of washed and sieved sand (U$0.0083). In another city in the same state, the sludge the material, after centrifugation, is transported to a landfill site located approximately 300 km from the plant at an approximate cost of U$57.76 m3 of sludge. The several steps in the manufacture of cement (i.e. grinding limestone to a specific size, mixing it with clay after drying, burning, cooling, milling) mean that a huge amount of energy is required to obtain the final product. The current cost of 1 kg of high early strength Portland cement runs at U$0.2238. To produce WTP sludge ash, the sludge is first dried at 105 °C, then calcined at 600 °C for 1 h and finally milled for 1 h. This means
Table 10 Reduction in Portland cement consumption to obtain the same strength as the reference concrete. Mixture
w/b
Materials (kg/m3) C.M.
PC
Ludge Ash
Cs28
w/b⁄
C.M.
PC⁄⁄
Ludge Ash
Reduction PC***
468 332
445 315
23 17
42 37
***
***
***
⁄⁄⁄
406 321
365 289
41 32
81 70
***
***
***
⁄⁄⁄
396 312
337 265
59 47
150 94
***
***
***
⁄⁄⁄
REF
0.35 0.50 0.65
487 359 284
487 359 284
– – –
54.7 40.7 28.8
5SA
0.35 0.50 0.65
487 359 284
463 341 270
24 18 14
56.5 43.5 34.5
0.35 0.50 0.65
487 359 284
438 323 256
49 36 28
66.5 44.0 36.0
0.35 0.50 0.65
487 359 284
414 305 241
73 54 43
67.0 48.5 35.5
20SA
0.35 0.50 0.65
487 359 284
390 287 227
97 72 57
69.0 45.6 32.3
0.44 0.55 0.69
399 326 274
319 261 219
80 65 55
168 94 65
25SA
0.35 0.50 0.65
487 359 284
365 269 213
122 90 71
71.0 47.8 33.3
0.45 0.57 0.70
389 319 269
292 239 202
97 79 67
195 120 82
30SA
0.35 0.50 0.65
487 359 284
341 251 199
146 108 85
65.3 46.3 33.0
0.43 0.56 0.71
410 324 266
287 227 186
123 97 80
200 132 98
10SA
15SA
0.37 0.54 ***
0.43 0.57 ***
0.44 0.59 ***
w/b = water/binder ratio; w/b⁄ = water/binder to obtain the same strength as the reference concrete; C.M. = cimentitious materials; PC = Portland cement; SA = sludge ash water treatment plants; Cs = compressive strength. *** Not established because it falls outside the confidence interval of the equations.
A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520
that far less energy is required when compared with Portland cement. There are additional benefits such as the strategic advantage of preserving limestone and clay quarries, the environmental benefit of reducing greenhouse gases emissions as well as providing a better destination for this product. Finally, there is an increase in concrete strength provided by the use of this material. Therefore, the use of WTP sludge ash in concrete is an economically sound alternative. 4. Conclusions The following conclusions can be drawn from the experimental results: 1. The analysis of the results of the determination of pozzolanic activity with Portland cement shows that even at the lowest calcination temperature tested (400 °C), the values obtained are similar to those of the reference mixture. The optimal pozzolanic activity index was obtained with a calcination temperature of 700 °C for 30 min. However, this value is only 3.5% higher than the value obtained with a calcination temperature of 600 °C for 1 h, which requires less energy. As a result, using a temperature of 700 °C is not economical. 2. The analysis of the results of determination of the concentration of fixed calcium hydroxide – Chapelle method – showed that the best results are obtained with a calcination temperature of 700 °C for 30 min. However, this value is very close to the one obtained with a calcination temperature of 600 °C for 1 h, which requires less energy. The resulting value is more than twice the limit required to classify a material as having low pozzolanic activity. 3. The concentration of kaolinite (% – dry weight) was 37%, which is below the minimum desired level for calcined clays (50%). The amount of associated quartz should be low, which is not the case with this material. 4. For identical w/b ratios, the concrete mixes prepared with WTP sludge ash showed increases in strength ranging from 3% to 30%, depending on the w/b ratio and the concentration of substitution used. The fact that improved strength is already present after 7 days means that the flow of operations in construction sites is not affected. 5. A comparison of the performance of mixes prepared using the same concentration of WTP sludge ash, low graphite carbon rice husk ash burned under controlled conditions or silica fume as a substitution for cement shows that the values obtained with the use of WTP sludge ash are higher, which attests the viability of using the material. Rice husk ash and silica fume showed far higher results of fixed calcium hydroxide in the Chapelle test when compared with WTP sludge ash, which indicates that this test cannot be used to rule out the possibility of using a mineral addition. 6. The same level of compressive strength observed in the reference concrete can be obtained with reductions in cement consumption ranging from 37 to 200 kg of cement per m3 of concrete. The amount depends on the required strength level and the concentration of WTP sludge ash used. Acknowledgements The authors would like to acknowledge the following for their support: FAPERGS and CNPq for their financial support and initial research scholarships; BASF for the materials supplied; and CORSAN (Rio Grande do Sul State Waterworks) for providing samples of WTP sludge and technical data.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The use of water treatment plant sludge ash as a mineral addition A.L.G. Gastaldini ⇑, M.F. Hengen, M.C.C. Gastaldini, F.D. do Amaral, M.B. Antolini, T. Coletto Federal University of Santa Maria, 97105-900 Rio Grande do Sul, Brazil
h i g h l i g h t s It is possible to obtain environmentally sound concrete by using WTP sludge ash. The use of WTP sludge ash helps reduce greenhouse gases emissions. The use of WTP sludge ash reduces the discharge of this material in watercourses. 3
It is possible to reduce the consumption of cement in concrete by 37–200 kg m
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 19 May 2015 Accepted 12 July 2015
Keywords: Pozzolanic activity Cement substitution Characterization Compressive strength Water treatment plant sludge Chapelle test
a b s t r a c t For decades, the sludge produced in water treatment plants (WTP) was dissolved in water and then discharged in watercourses. WTP sludge is rich in pathogens and metals and when discharged in watercourses, it increases the amount of suspended solids, eventually causing the water body to silt up. Existing legislation in Brazil prohibits the discharge of WTP sludge in watercourses, but the practice persists. This study investigated the possibility of using WTP sludge as a mineral addition. First, the pozzolanicity of WTP sludge with Portland cement and the concentration of fixed calcium hydroxide using the Chapelle test were determined after exposing the material to different calcination temperatures and residence times. The samples with the best results were used to investigate the performance of concrete mixes where WTP sludge was substituted for Portland cement in concentrations ranging from 5% to 30% in three different water/binder (w/b) ratios (0.35, 0.50 and 0.65). Results indicate that the use of WTP sludge ash improves the strength of concrete mixes when compared with concrete with rice husk ash or silica fume. By using WTP sludge ash, it is possible to obtain the same strength of a concrete mix with 100% Portland cement and reduce the consumption of cement by 37–200 kg m3 of concrete, depending on the concentration of substitution and the desired strength level. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The growing demand for products and services associated with population growth is responsible for the expansion of industrial activity, which in turn results in an increase in the consumption of materials. Civil construction has considerable environmental impacts because of the consumption of natural resources and the generation of waste associated with this industry. The concrete industry is one of the major users of natural resources and its growth has resulted in significant environmental impacts associated with the use of raw materials, including water, as well as the release of greenhouse gases during the production of Portland cement. The industry has been trying to mitigate such effects by reducing emissions during the manufacture of Portland cement. Modern ⇑ Corresponding author. E-mail address: [email protected] (A.L.G. Gastaldini). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.038 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
.
plants release 0.7 metric ton of CO2 for each metric ton of clinker produced. In addition, rice husk ash, silica fume and blast furnace slag are now used as substitutions for clinker. Recent studies have attested the viability of using other materials as mineral additions [1–4]. The use of mineral additions as partial substitutions for Portland cement has become a pressing need, not only because of the need to reduce the environmental impact associated with the manufacture of cement but also to improve the durability of concrete structures exposed to harsh environments, such as sulfates, chlorides, alkali-aggregate reactions, etc. [5–7]. This is due to the fact that mineral additions change the composition of the pore solution [8]. This way, electric conductivity is changed [9]. The substitutions refine the pore structure [10], which reduces permeability and enhances strength, even though in some cases strength increases more slowly. The optimal concentration of a given substitution depends on the type of cementitious material used, the intended modifications
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to the concrete and the characteristics of the environment to which the concrete will be exposed. The use of calcined clay as a partial substitution for Portland cement has been studied in detail because of its technical, economic and environmental benefits [11–15]. On the other hand, population growth also translates as an increase in water consumption, which results in more sludge being produced by water treatment plants (WTP). The amount and composition of the sludge depends on the volume of water treated in the plant, the process used and the characteristics of untreated water. It is often the case that water suppliers criticize the quality of untreated water. However, some WTPs end up discharging their waste in water courses, which runs contrary to their own interests. WTP sludge is a type of solid waste and must be processed and disposed of accordingly to prevent environmental damage. WTP sludge is rich in pathogens and metals. When discharged in watercourses, it increases the amount of suspended solids, eventually causing the water body to silt up [16]. Even though existing legislation in Brazil prohibits the discharge of WTP sludge in watercourses, the practice persists. Many water suppliers in Brazil have signed agreements with the government that give them up to 30 years before they fully comply with existing legislation. As a result, they will continue discharging sludge and harming the environment for years to come. Several researchers have attested the viability of using untreated sludge as a partial substitution for fine aggregate or cement [17,18] or as a partial substitution for the siliceous material in the manufacture of cement [19]. WTP sludge has also been used as lightweight coarse aggregate (water treatment sludge and softwood sawdust composite) [20] and in the production of heavy clay [21]. However, there are few studies on the possibility of using WTP sludge ash as a pozzolanic agent in concrete and Portland cement mortars. The present study thus aims to determine the optimal temperature and residence time to yield a material with pozzolanic activity that can be used as a partial substitution for cement without compromising mechanical strength and production cycles and that can improve the sustainability of construction sites and reduce concrete costs. 2. Experimental program
Table 1 Physical and chemical characteristics of the cementitious materials. Physical properties Characteristic
2.2. Mixture proportions A total of seven binder mixes were prepared. The reference mixes were labeled REF and the mixtures with 5%, 10%, 15%, 20%, 25% and 30% of WTP sludge ash were labeled 5SA, 10SA, 15SA, 20SA, 25SA and 30SA, respectively.
Sludge ash (600 °C)
3.13 1.14
2.56 27.7
Particle size distribution (lm) D(v,0.1)* D(v,0.5)* D(v,0.9)*
3.67 14.1 38.81
– 20.07 65.06
Chemical composition (weight%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O MnO TiO2 P2O5 LOI
20.4 4.37 2.64 62.9 2.7 2.2 0.13 0.95 <0.10 0.29 <0.10 3.16
66.2 17.7 8.76 0.57 0.96 – 0.32 1.16 0.13 0.86 0.33 3.37
* equivalent spherical diameter (of the same volume) 10%, 50% and 90% of the particle distribution are below.
Table 2 Composition of the concrete mixtures (kg m3). Mixture
w/b
CM
PC
SA
Fine agg.
Coarse agg.
water
P
SP
(kg m3) REF
0.35 0.50 0.65
487 359 284
487 359 284
– – –
633 740 804
1076 1055 1045
170 180 185
1.46 – –
– – –
5SA
0.35 0.50 0.65
487 359 284
463 341 270
24 18 14
626 736 798
1076 1055 1045
170 180 185
1.94 – –
– – –
10SA
0.35 0.50 0.65
487 359 284
438 323 256
49 36 28
623 732 798
1076 1055 1045
170 180 185
4.84 – –
1.45 – –
15SA
0.35 0.50 0.65
487 359 284
414 305 241
73 54 43
619 731 797
1076 1055 1045
170 180 185
4.84 0.36 –
1.93 – –
20SA
0.35 0.50 0.65
487 359 284
390 287 227
97 72 57
614 729 795
1076 1055 1045
170 180 185
4.84 1.07 –
2.17 – –
25SA
0.35 0.50 0.65
487 359 284
365 269 213
122 90 71
609 725 792
1076 1055 1045
170 180 185
4.84 1.43 –
2.41 – –
30SA
0.35 0.50 0.65
487 359 284
341 251 199
146 108 85
606 722 790
1076 1055 1045
170 180 185
4.84 3.21 0.71
3.13 – –
2.1. Materials High-early strength Portland cement and WTP sludge ash were used as binders in this experiment. The WTP sludge was first dried in an oven at 110 °C for 24 h. It was then homogenized and calcined in a muffle kiln at temperatures of 400, 500, 600 and 700 °C, with residence times of 1 and 2 h, except for the temperature of 700 °C, in which case a residence time of 30 min was used. After calcination, the ashes were ground in a ball mill for 1 h. The physical and chemical characteristics of the different cementitious materials are shown in Table 1. The fine aggregate used in the experiment consisted of natural quartz sand, with specific weight of 2.66 g/cm3, unit mass of 1.62 g/cm3, fineness modulus of 1.85 and maximum particle size of 1.2 mm. The coarse aggregate consisted of crushed stone with specific weight of 2.48 g/cm3, unit mass of 1.38 g/cm3 and maximum particle size of 19.0 mm. Concrete samples were prepared with a plasticizer additive, except for those with a w/b ratio of 0.35 and those with 30% sludge ash, which required a superplasticizer additive (modified carboxylic ether), even when w/b of 0.50 was used. The casting temperature was set at 18 ± 1 °C and the mix water was heated or cooled to adjust for the temperature of the other materials [22]. The quantities of materials used per cubic meter of concrete are shown in Table 2.
PC
Specific gravity (g/cm3) Specific surface area BET (m2/g)
w/b – water/binder ratio; CM – cementitious materials; PC – Portland cement; SA – sludge ash; P – plasticizer; SP – superplasticizer.
For each mixture, three w/b ratios (0.35, 0.50 and 0.65) were used with a mortar content of 51% by weight of dry materials. In the mixtures with WTP sludge ash, the amount of sand was adjusted to ensure that the mortar content was the same as that in the reference mixture. This was necessary because of the lower specific gravity of sludge ash when compared with Portland cement.
2.3. Testing procedures 2.3.1. Assessment of pozzolanic activity with Portland cement Samples were tested according to Brazilian standard ABNT NBR 5752:2014 [23]. This procedure is similar to the one defined in ASTM C 311. The difference is the use of a plasticizer to achieve the same flow for samples with the same w/b ratio. The method consists of preparing mortar test specimens with mix proportion = 1:3 and w/b ratio = 0.48. Mortar ‘‘A’’ was prepared with Portland cement CP II F, while in
A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520 Mortar ‘‘B’’ 25% of pozzolanic material is used as a substitution for the cement. Cement type CP II F contains, in addition to clinker and gypsum, 6–10% of ground limestone. Both mortars should display the same consistency and a plasticizer should be used, as required. The strength activity index of cement was calculated as the ratio between the compressive strength of mortar B at 28 days divided by the compressive strength of mortar A (reference) at the same age, multiplied by 100. 2.3.2. Determination of fixed calcium hydroxide – modified Chapelle method Samples were tested according to Brazilian standard ABNT NBR 15895-10 [24]. The method consists of stirring a mix of 1 g of pozzolanic material and 2 g of calcium oxide at 90 °C for 16 h in 250 mL water. The result is expressed as the amount of fixed calcium hydroxide per gram of pozzolanic material. 2.3.3. XRD and thermogravimetric analysis XRD measurements were carried out in a Rigaku DMAX 1100 diffractometer using CuKa radiation with a wavelength of 1.54 Å operating at 40 kV and 40 mA. Step scanning was used with a scan speed of 2°/min and the data were collected in the 5–50° interval (2h). Thermogravimetric analysis (TG-DTG) was carried out in an ATD-TG LabSys Evo Setaram. The samples were heated from 20 to 1000 °C at a constant rate of 10 °C/min in a nitrogen atmosphere. 2.3.4. Compressive strength Eight cylindrical test specimens ø 10 20 cm were cast for each mix, four of which were wet-cured for 7 days and four for 28 days. The test specimens were removed from the moulds after 24 h and stored in a moist cabinet at 23 ± 2 °C and RH > 95% until ready for testing.
3. Results and discussion 3.1. Determination of pozzolanic activity with Portland cement Table 3 presents the results of compressive strength and the strength activity index of cement for the different calcination temperatures and residence times in the test. Results show that even when the lowest temperature and shortest residence times are used (400 °C for 1 h) in a mix with only 75% of cement, compressive strength results at 28 days (44 MPa) are very similar to the values found for the reference mix – 100% Portland cement (45 MPa). Compressive strength for the mix calcined at 600 °C for 1 h was 54.5 MPa, which is higher than the values found for the mix calcined at 700 °C for 30 min and for 2 h (50.2 and 50.7 MPa, respectively). Fig. 1 presents the particle size distribution for calcination temperatures of 600 °C and above, which yielded the highest compressive strength. Similar particle size distributions were observed in the samples with WTP sludge ash calcined at different temperatures and residence times. In order to determine the optimal calcination temperature, it is important to ensure that the particle size distribution of the calcined clays are similar. This way, the physical effect associated with the substitution of WTP sludge ash for cement will be the same. It should be remembered that the particles of WTP sludge ash are slightly larger than those of Portland cement. Table 3 Compressive strength of mortars at 28 days and strength activity index. Mixture
Compressive strength 28 days (MPa)
S.A.I⁄ (%)
REF 400-1 h 400-2 h 500-1 h 500-2 h 600-1 h 600-2 h 700-1/2 h 700-1 h 700-2 h
45.0 44.0 45.4 47.3 48.6 54.2 51.4 50.2 56.3 50.7
100 98 101 105 108 120 114 112 125 113
S.A.I⁄: strength activity index.
515
Even though the best strength activity index was that of in the sample with WTP sludge ash calcined at 700 °C for 1 h (56.3 MPa), this value was very close to that of the sample calcined at 600 °C for 1 h (54.2 MPa – just 3.5% lower). An increase in strength of only 3.5% does not offset the energy required to reach a temperature of 700 °C. Therefore, a temperature of 600 °C for 1 h was adopted for concrete testing. 3.2. Determination of fixed calcium hydroxide – modified Chapelle method Table 4 presents the results of the fixed calcium hydroxide test, according to Brazilian Standard NBR 15.895/10. The results show that the values are higher than those reported by Raverdy et al. [25]. These authors classifiy low reactivity materials as those with a value of 330 mg of CaO per gram of material. The use of longer residence times at 600 °C resulted in slightly lower pozzolanic activity. However, at 700 °C, this reduction amounted to almost 10%. It was observed that longer residence times caused particles to agglomerate as they start to fuse together. Brazilian Standard ABNT NBR 15894-1:(2010) [26] establishes that the pozzolanic reactivity of a metakaolin sample must not be lower than 750 mg Ca(OH)2/g of metakaolin while French standard AFNOR NF P 18 – 513 [27] sets a value of 700 mg Ca(OH)2/g of metakaolin. The ash, after calcination at 700 °C for 30 min and at 600 °C for 1 h, shows a concentration of fixed calcium hydroxide of 680 and 673 mg Ca(OH)2, respectively. This value corresponds to 90% of the value prescribed by the Standard. However, this WTP sludge ash does not meet the requirements of the Standard concerning the concentration of Al2O3, which must fall within the 32–46% range, nor the concentration of SiO2 which must not exceed 65%. It does, however, meet all remaining requirement of the Standard in question. 3.3. Results of XRD and thermogravimetric analysis The diffractogram in Fig. 2 shows that the WTP sludge ash is made up mainly of quartz and kaolinite. There are also small amounts of feldspar and hematite. Thermogravimetric graphs were used to determine mass losses. The theoretical mass loss of kaolinite (13.9%) was used to estimate the concentration of this mineral in the samples. Results are shown in Table 5. Thermal analysis also indicates that the sample shows a mass loss between 200 and 400 °C associated with an exothermal peak corresponding to the decomposition of organic matter. In general terms, it can be said that the pozzolanic activity of a clay increases as the amount of amorphous or random structures in the material also increases. These include materials such as diatomites of clay minerals. The best clays for thermal activation are rich in clay minerals (concentrations above 50%) and have low associated quartz content (high hardness phase, no pozzolanic activity). Kaolin clays are best for the production of thermally activated pozzolans, which display optimal activity after calcination between 600 and 800 °C [28–30]. Calcined WTP sludge ash samples in this study show a concentration of kaolinite of 37%, which is lower than the recommended minimum. However, compressive strength levels can be maintained and even increased from 28 days by substituting calcined clays containing a large proportion of impurities (more than 40% of quartz) for 30% of the cement in mortars [31]. Similarly, it has been reported that ‘‘a material with pozzolanic activity similar to commercial grade MKs can be produced with clays that have a kaolinite content <40%, as long as it undergoes
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Fig. 1. Particle size distribution (laser) of cement and WTP sludge ash calcined at different temperatures and different residence times.
Table 4 Results of fixed calcium hydroxide – modified Chapelle method. Temperature
Calcium hydroxide fixed (mg Ca(OH)2/g sample)
600 °C 600 °C 700 °C 700 °C 700 °C
673 666 680 643 620
1h 2h 1/2 h 1h 2h
thermal treatment between 600 and 800 °C. For optimal performance, the material should be sedimented, in order to enrich the clay minerals [32]. 3.4. Compressive strength Table 6 shows the values of compressive strength and the strength activity index (in percentage terms) in relation to the reference sample after wet curing for 7 and 28 days, respectively.
The partial substitution of WTP sludge ash for cement resulted in an increase in compressive strength when compared with the reference concrete (100% Portland cement), both at 7 and 28 days. The exception was for the mix with 5% WTP sludge ash and w/b ratio of 0.35 at 7 days. For the remaining mixes, the increase in strength ranged from 3% to 30%, depending on the w/b ratio used and the concentration of the substitution of WTP sludge ash for cement. The fact that improved strength can be observed from an early age in the mixes with WTP sludge ash when compared with the reference concrete indicates that this substitution does not require longer casting times and therefore the workflow in construction sites is not affected. All mineral additions, be they cementitious or pozzolanic, contribute to improving mechanical strength. Some display this effect early on, such as silica fume or rice husk ash produced under controlled burning. Other materials, such as fly ash or slag, require longer periods unless they display high fineness. The improved mechanical strength is a result of the pozzolanic reactions and also
Fig. 2. XDR – WTP sludge.
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A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520 Table 5 Thermogravimetric analysis results – WTP sludge. Mass losses (%) Sample
20–200 °C
200–400 °C
400–800 °C
WTP sludge ash
5.3
5.4
4.9
Kaolinite content (%)
Table 6 Compressive strength of concrete mixes with WTP sludge ash and strength activity index of mixes in relation to reference mix. Mixture
w/b
Compressive strength (MPa)
Mixture
w/b
Compressive strength (MPa)
37
The kaolinite content was determined in relation to the theoretical mass loss of this material = 13.9%.
1
Table 7 Compressive strength of concrete mixes with 10%, 20% and 30% of rice husk ash (RHA) – curing 28 days.
Cs mixture/Cs reference
7
28
7
28
0.35 0.50 0.65
49.5 34.8 25.4
54.7 40.7 28.8
100 100 100
100 100 100
5SA
0.35 0.50 0.65
46.3 34.7 28.4
56.5 43.5 34.5
1.03 1.07 1.20
1.03 1.07 1.20
10SA
0.35 0.50 0.65
54.8 35.0 29.4
66.5 44.0 36.0
1.21 1.08 1.25
1.22 1.08 1.25
15SA
0.35 0.50 0.65
55.8 35.3 28.0
67.0 48.5 35.5
1.22 1.19 1.23
1.22 1.19 1.23
20SA
0.35 0.50 0.65
55.8 37.5 27.8
69.0 45.6 32.3
1.26 1.12 1.12
1.26 1.12 1.12
25SA
0.35 0.50 0.65
54.5 41.3 29.0
71.0 47.8 33.3
1.30 1.17 1.16
1.30 1.17 1.16
30SA
0.35 0.50 0.65
53.5 38.5 27.2
65.3 46.3 33.0
1.19 1.14 1.14
1.19 1.14 1.14
w/b1 – water/binder ratio; SA – sludge ash WTP.
the filler effect, which increases compactness. In the case of WTP sludge ash, the occurrence of pozzolanic reactions is confirmed by the result of the Chapelle test, discussed in Section 3.2. A comparison of the compressive strength of concrete mixes with WTP sludge ash (10%, 20% and 30%) with mixes with rice husk ash (RHA) burned under controlled conditions [33] yielded similar results (Table 7). In this case, the mixes were prepared with the same type and brand of cement, similar aggregates and the same w/b ratios. The only difference was that the mixes with rice husk ash required the use of a superplasticizer additive, thus making them more expensive than the mixes with WTP sludge ash. In Table 8, the values for concrete mixes with WTP sludge ash are compared with those prepared with RHA burned under controlled conditions and low concentrations of carbon graphite. Again, mixes were prepared with identical w/b ratios, the same type and brand of Portland cement and similar aggregates and were cured in a moist cabinet for 3 and 7 days and tested at 28 days. For the sake of comparison, composite concrete mixes with 5% and 10% silica fume (5SF and 10SF) were also prepared [34]. For the concrete mixes cured in a moist cabinet for 3 days, those with WTP sludge ash usually showed higher compressive strength when compared to the samples with rice husk ash and silica fume substitutions in the same concentrations.
SA⁄
REF
0.35 0.50 0.65
53.7 47.3 27.6
54.7 40.7 28.8
10RHA
0.35 0.50 0.65
68.1 46.9 31.7
66.5 44.0 36.0
20RHA
0.35 0.50 0.65
72.0 52.3 33.2
69.0 45.6 32.3
30RHA
0.35 0.50 0.65
67.4 50.1 29.9
65.3 46.3 33.0
Age (days)
REF
RHA
SA⁄ – mixes with WTP sludge ash and the same concentration of substitution.
Table 8 Compressive strength of concrete with rice husk ash with low graphite carbon content or with silica fume. Mixture
w/b
Compressive strength (MPa) Curing 3 days
Curing 7 days
Age (days) 28 REF
0.35 0.50 0.65
54.0 37.3 29.0
54.7 40.7 28.8
58.0 40.0 33.2
54.7 40.7 28.8
5RHA
0.35 0.50 0.65
56.0 43.6 33.6
56.5 43.5 34.5
60.2 47.6 36.0
56.5 43.5 34.5
10RHA
0.35 0.50 0.65
59.3 40.5 29.0
66.5 44.0 36.0
65.0 42.0 35.6
66.5 44.0 36.0
20RHA
0.35 0.50 0.65
61.0 40.6 30.0
69.0 45.6 32.3
72.0 43.0 32.0
69.0 45.6 32.3
30RHA
0.35 0.50 0.65
65.6 47.0 31.0
65.3 46.3 33.0
72.6 49.2 31.6
65.3 46.3 33.0
5SF
0.35 0.50 0.65
54.0 44.0 36.6
56.5 43.5 34.5
59.7 50.0 38.0
56.5 43.5 34.5
10SF
0.35 0.50 0.65
53.5 43.5 37.6
66.5 44.0 36.0
55.5 48.2 39.5
66.5 44.0 36.0
NB: results in bold – mixes with WTP sludge ash and the same concentration of substitution.
The results of Chapelle method tests carried out in the samples with RHA and silica fume yielded results of 1365 and 1513 mg Ca(OH)2/g sample, respectively. These are much higher than the values recorded for WTP sludge ash (673 mg Ca(OH)2/g sample). Therefore, the results of compressive strength corroborate the finding that even clays that would be considered as ‘low grade’ for the production of pozzolans cannot be disregarded in developing countries [32]. 3.5. Economic feasibility of using WTP sludge ash From the pairs of values of compressive strength at 28 days and the w/b ratios, the coefficients of equation Cs = A/Bw/b were
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Table 9 Coefficients equations C = D/ECs and Cs = A/Bw/b. Mixture
w/b
Cementitious materials – C (kg m3)
Compressive strength 28 days (MPa)
REF
0.35 0.50 0.65
487 359 284
54.7 40.7 28.8
5SA
10SA
15SA
20SA
25SA
30SA
–
2
R2 = 1.0 Cs ¼ 116:6864 w=b 8:513456
2
0.35 0.50 0.65
487 359 284
R = 0.98 C ¼ 122:87906 Cs
56.5 43.5 34.5
R = 0.99 Cs ¼ 126:2019 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.97 C ¼ 161:9419 0:9833Cs
66.5 44.0 36.0
R2 = 0.96 Cs ¼ 131:3381 w=b
0:97586
2
5:1772
7:7339
2
0.35 0.50 0.65
487 359 284
R = 0.98 C ¼ 155:6019 Cs
67.0 48.5 35.5
R = 0.99 Cs ¼ 140:3112 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.96 C ¼ 180:2122 0:9856Cs
69.0 45.6 32.3
R2 = 0.98 Cs ¼ 165:6965 w=b
0.35 0.50 0.65
487 359 284
R2 = 0.97 C ¼ 178:9445 Cs
71.0 47.8 33.3
R2 = 0.99 Cs ¼ 167:7014 w=b
0.35 0.50 0.65
0:9831
0:9859
2
R = 0.99 C ¼ 164:7168 0:9834Cs
487 359 284
65.3 46.3 33.0
8:3078
12:6201
11:9293
2
R = 0.99 Cs ¼ 144:7362 w=b 9:7423
calculated for all mixes in the study (Table 9). This way, the w/b ratios for the mixes with WTP sludge ash required to obtain the same compressive strength value as the reference mix at 28 days were calculated – 54.7, 40.7 and 28.8 MPa. These were labeled (w/b⁄). From the pairs of values ‘binder consumption’ and ‘compressive strength’, the coefficients of equation C = D/ECs were calculated for all mixes in this study (Table 9). This way, the binder consumption of the mixes with WTP sludge ash required to obtain the same
compressive strength value as the reference mix at 28 days – 54.7, 40.7 and 28.8 MPa were calculated. These were identified as ‘Cementitious Materials Consumption’. From this value, Portland cement (‘‘PC⁄⁄’’) and sludge ash consumptions were calculated with reference to the cement substitution content. In the mix with 5% WTP sludge ash, 468 kg of binder are required to obtain a compressive strength value of 54.7 MPa. This weight is broken down as follows: 445 kg of cement (0.95 * 468) and 23 kg of sludge ash (0.05 * 468). To obtain the same strength in the reference sample, 485 kg of cement are needed. Therefore, the 23 kg of WTP sludge ash are equivalent to 42 kg of cement (487–445 kg). Thus, from the data in Table 9, it was possible to establish the reduction in Portland cement consumption in the concrete mixes prepared with different concentrations of WTP sludge ash to obtain the same strength as the reference concrete (Table 10). In one city in the state of Rio Grande do Sul, Brazil, the local WTP pays a contractor U$19.15 m3 to have the sludge collected plus U$0.79 to have the material transported to a site located 150 m from the plant. In this case, this is possible because the plant has a permit to use a nearby plot as a landfill site. Dumping costs in the landfill run at U$36.10 m3, so the grand total is U$56.04 m3. This works out as a cost of approximately U$0.0467 per kg of sludge dumped in the landfill, which is more than twice the cost of 19-mm crushed rock (U$ 0.018) and 5.6 times the cost of washed and sieved sand (U$0.0083). In another city in the same state, the sludge the material, after centrifugation, is transported to a landfill site located approximately 300 km from the plant at an approximate cost of U$57.76 m3 of sludge. The several steps in the manufacture of cement (i.e. grinding limestone to a specific size, mixing it with clay after drying, burning, cooling, milling) mean that a huge amount of energy is required to obtain the final product. The current cost of 1 kg of high early strength Portland cement runs at U$0.2238. To produce WTP sludge ash, the sludge is first dried at 105 °C, then calcined at 600 °C for 1 h and finally milled for 1 h. This means
Table 10 Reduction in Portland cement consumption to obtain the same strength as the reference concrete. Mixture
w/b
Materials (kg/m3) C.M.
PC
Ludge Ash
Cs28
w/b⁄
C.M.
PC⁄⁄
Ludge Ash
Reduction PC***
468 332
445 315
23 17
42 37
***
***
***
⁄⁄⁄
406 321
365 289
41 32
81 70
***
***
***
⁄⁄⁄
396 312
337 265
59 47
150 94
***
***
***
⁄⁄⁄
REF
0.35 0.50 0.65
487 359 284
487 359 284
– – –
54.7 40.7 28.8
5SA
0.35 0.50 0.65
487 359 284
463 341 270
24 18 14
56.5 43.5 34.5
0.35 0.50 0.65
487 359 284
438 323 256
49 36 28
66.5 44.0 36.0
0.35 0.50 0.65
487 359 284
414 305 241
73 54 43
67.0 48.5 35.5
20SA
0.35 0.50 0.65
487 359 284
390 287 227
97 72 57
69.0 45.6 32.3
0.44 0.55 0.69
399 326 274
319 261 219
80 65 55
168 94 65
25SA
0.35 0.50 0.65
487 359 284
365 269 213
122 90 71
71.0 47.8 33.3
0.45 0.57 0.70
389 319 269
292 239 202
97 79 67
195 120 82
30SA
0.35 0.50 0.65
487 359 284
341 251 199
146 108 85
65.3 46.3 33.0
0.43 0.56 0.71
410 324 266
287 227 186
123 97 80
200 132 98
10SA
15SA
0.37 0.54 ***
0.43 0.57 ***
0.44 0.59 ***
w/b = water/binder ratio; w/b⁄ = water/binder to obtain the same strength as the reference concrete; C.M. = cimentitious materials; PC = Portland cement; SA = sludge ash water treatment plants; Cs = compressive strength. *** Not established because it falls outside the confidence interval of the equations.
A.L.G. Gastaldini et al. / Construction and Building Materials 94 (2015) 513–520
that far less energy is required when compared with Portland cement. There are additional benefits such as the strategic advantage of preserving limestone and clay quarries, the environmental benefit of reducing greenhouse gases emissions as well as providing a better destination for this product. Finally, there is an increase in concrete strength provided by the use of this material. Therefore, the use of WTP sludge ash in concrete is an economically sound alternative. 4. Conclusions The following conclusions can be drawn from the experimental results: 1. The analysis of the results of the determination of pozzolanic activity with Portland cement shows that even at the lowest calcination temperature tested (400 °C), the values obtained are similar to those of the reference mixture. The optimal pozzolanic activity index was obtained with a calcination temperature of 700 °C for 30 min. However, this value is only 3.5% higher than the value obtained with a calcination temperature of 600 °C for 1 h, which requires less energy. As a result, using a temperature of 700 °C is not economical. 2. The analysis of the results of determination of the concentration of fixed calcium hydroxide – Chapelle method – showed that the best results are obtained with a calcination temperature of 700 °C for 30 min. However, this value is very close to the one obtained with a calcination temperature of 600 °C for 1 h, which requires less energy. The resulting value is more than twice the limit required to classify a material as having low pozzolanic activity. 3. The concentration of kaolinite (% – dry weight) was 37%, which is below the minimum desired level for calcined clays (50%). The amount of associated quartz should be low, which is not the case with this material. 4. For identical w/b ratios, the concrete mixes prepared with WTP sludge ash showed increases in strength ranging from 3% to 30%, depending on the w/b ratio and the concentration of substitution used. The fact that improved strength is already present after 7 days means that the flow of operations in construction sites is not affected. 5. A comparison of the performance of mixes prepared using the same concentration of WTP sludge ash, low graphite carbon rice husk ash burned under controlled conditions or silica fume as a substitution for cement shows that the values obtained with the use of WTP sludge ash are higher, which attests the viability of using the material. Rice husk ash and silica fume showed far higher results of fixed calcium hydroxide in the Chapelle test when compared with WTP sludge ash, which indicates that this test cannot be used to rule out the possibility of using a mineral addition. 6. The same level of compressive strength observed in the reference concrete can be obtained with reductions in cement consumption ranging from 37 to 200 kg of cement per m3 of concrete. The amount depends on the required strength level and the concentration of WTP sludge ash used. Acknowledgements The authors would like to acknowledge the following for their support: FAPERGS and CNPq for their financial support and initial research scholarships; BASF for the materials supplied; and CORSAN (Rio Grande do Sul State Waterworks) for providing samples of WTP sludge and technical data.
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