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Reutilizing ceramic polishing waste as powder filler in mortar to reduce cement content by 33% and increase strength by 85%
Powder Technology 355 (2019) 119–126
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Reutilizing ceramic polishing waste as powder filler in mortar to reduce cement content by 33% and increase strength by 85% L.G. Li a,⁎, Z.Y. Zhuo a, J. Zhu a, J.J. Chen b, A.K.H. Kwan c a b c
Guangdong University of Technology, Guangzhou, China Foshan University, Foshan, China The University of Hong Kong, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 1 November 2018 Received in revised form 1 June 2019 Accepted 12 July 2019 Available online 15 July 2019 Keywords: Ceramic waste Green mortar/concrete Microstructure Strength Waste reutilization
a b s t r a c t Usually, the ceramic wastes are reutilized as cement or aggregate replacement in concrete. Among the ceramic wastes, the ceramic polishing waste (CPW) generated during polishing of ceramic products is a powder. Herein, the CPW is added as paste replacement. This would reduce the amount of ceramic waste to be dumped, and the cement consumption and carbon footprint of the concrete production. To study the feasibility of such powder filler technology, a number of mortar mixes containing different water, cement and CPW contents were produced for workability, strength and SEM tests. It was found that with up to 20% CPW by volume added as paste replacement, the cement content could be reduced by 33%, the 7-day and 28-day cube strengths could both be increased by at least 85%, and the microstructure could be densified. Lastly, two design charts for adding CPW as paste replacement were developed. © 2019 Elsevier B.V. All rights reserved.
1. Introduction In the reutilization of a solid waste for producing mortar/concrete, depending on the source, the solid waste may be classified into industrial solid waste and construction solid waste. The industrial solid wastes are industrial by-products. Some of these, such as slag, fly ash and silica fume, have cementitious properties and thus are commonly used as supplementary cementitious materials [1–8]. On the other hand, the construction solid wastes, such as old concrete, waste glass, clay bricks, ceramic tiles and rubbers, have also been explored for possible use either as a supplementary cementitious material or as an aggregate [9–15]. Ceramics have been used for a long time all over the world [16,17]. However, a large quantity of ceramic waste is produced during manufacture of ceramic products and demolition of buildings [18]. The ceramic waste brings serious environmental problems and occupies a large landfill area [19–21]. China is the largest producer of ceramics in the world [22] and is thus also the largest producer of ceramic waste [23,24]. It is now urgent to deal with the ceramic waste problem. Currently, most of the ceramic waste is just dumped. Only a small part of the ceramic waste is crushed and reutilized to replace cement or aggregate in the production of mortar or concrete. The use to replace cement is named as cement replacement strategy whereas the use to replace aggregate is named as aggregate replacement strategy. ⁎ Corresponding author. E-mail address: [email protected] (L.G. Li).
https://doi.org/10.1016/j.powtec.2019.07.043 0032-5910/© 2019 Elsevier B.V. All rights reserved.
In the cement replacement strategy, the ceramic waste is added as partial replacement of the cementitious materials, as illustrated in Fig. 1(a). Vejmelková et al. [25] found that the frost resistance of concrete containing up to 40% fine-ground ceramics as cement replacement was as good as reference concrete. Cheng et al. [26] showed that using ceramic polishing waste as cement replacement would lower the compressive strength and carbonation resistance of concrete. Steiner et al. [27] revealed that adding ceramic tile polishing residue to replace cement up to 25% has little negative effect on the strength of mortar. Mas et al. [28] demonstrated that the addition of ceramic tile waste as cement substitution would reduce the strength of mortar, but addition up to 35% still meets the strength activity index requirements for fly ash. De Matos et al. [29] showed that replacing cement by no more than 20% porcelain polishing residue would result in similar rheological properties but better passing ability in the case of self-consolidating concrete. In the aggregate replacement strategy, the ceramic waste is added as partial replacement of the aggregate, as illustrated in Fig. 1(b). Guerra et al. [30] revealed that the addition of sanitary porcelain waste as aggregate replacement up to 9% would not impair the compressive strength of concrete. Gonzalez-Corominas and Etxeberria [31] tested that concrete with 30% ceramic fine aggregate achieved similar or better mechanical and durability properties compared to reference concrete. Medina et al. [32] showed that concrete with up to 25% ceramic sanitary ware aggregate was as durable as normal concrete. Awoyera et al. [33] reported that concrete with 75% ceramic tile waste aggregate has a higher 28-day strength than reference concrete. Elçi [34] found that
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Fig. 1. Ceramic waste addition methods.
the mechanical properties of concrete using crushed floor tile aggregate were similar to reference concrete, but those of concrete using crushed wall tile aggregate were lower than reference concrete. Anderson et al. [35] observed that the effects of ceramic tile waste were marginal and thus the use of ceramic tile waste as partial replacement of coarse aggregate is feasible. It has been noted, however, that both the two strategies of reutilizing ceramic waste have certain negative effects, albeit such reutilizations of ceramic waste would benefit waste recycling and environmental protection. In the cement replacement strategy, a relatively high cement replacement rate could cause serious strength reduction [25–28]. In the aggregate replacement strategy, the overall performance of the concrete produced could sometimes be impaired [32–34] and the cement content would not be reduced to lower the carbon footprint of the concrete production. Hence, the current strategies are not entirely satisfactory. Recently, an alternative strategy, called the paste replacement strategy, has been developed by the authors' research team. By this strategy, the solid waste is treated as a filler and added to substitute part of the cementitious paste in such a way that the total volume of the cementitious paste and the solid waste for aggregate voids filling remains unchanged. The mix proportions of the cementitious paste is also kept unchanged, as illustrated in Fig. 1(c). In previous studies, it has been demonstrated that adding limestone fines by this strategy could on one hand reduce the cement content and on other hand increase the dimensional stability, strength and durability of concrete [36–39]. This strategy has also been adopted in the reutilization of rock dust (marble dust and granite dust) and clay brick dust and in mortar/concrete. So far, the results proved that, depending on the fineness of the solid waste to be used as a filler, the addition of rock dust and clay brick dust would also effectively reduce the cement
content and improve the dimensional stability, strength and durability [40–44]. In this study, the paste replacement strategy was extended to ceramic polishing waste (CPW), a waste generated during polishing of ceramic tiles. On average, the production of 1.0 m2 of polished ceramic tiles generates about 1.9 to 2.1 kg of CPW, and in 2014, about 10 million tons of CPW was generated in China, but basically none was reutilized [45]. This waste is a powder and therefore does not require crushing or grinding. To determine the effects of adding CPW as partial paste replacement on cement content and performance of mortar so as to study the feasibility and potential of reutilizing CPW as a powder filler in mortar/concrete, trial mortar mixes at various water/cement ratios and CPW volumes were produced for testing of their workability, strength and microstructure. 2. Materials and mortar mixes tested 2.1. Raw materials The cementitious material for the mortar production was 42.5 strength class ordinary Portland cement. Its specific gravity was tested to be 3.08. The fine aggregate used was river sand. Its maximum size, specific gravity, moisture content and water absorption were tested to be 1.18 mm, 2.58, 0.10% and 1.10%, respectively. These materials were bought from the local market. The ceramic polishing waste (CPW) was obtained from a ceramics factory in Foshan city, a famous powerhouse of ceramics production in China. The CPW was generated during polishing of ceramic tiles. The original CPW collected was wet with some debris inside. In order to dry and minimize variation in quality, the CPW was treated as follows: first, the CPW was heated with an oven at 105 °C for 8 h to remove the
L.G. Li et al. / Powder Technology 355 (2019) 119–126
water and then mechanically sieved using a 1.18 mm sieve to remove the debris. After the treatment, the CPW was turned to a light grey dry powder (shown in Fig. 2) and the specific gravity of the CPW was measured as 2.43. Generally, after adding CPW, the mortar became more cohesive and stable but less flowable. To compensate for the reduction in workability and maintain the desired workability, a superplasticizer (SP) was dosed to each mortar mix. The SP used was a 3rd generation polycarboxylatebased admixture. Its solid content and specific gravity were 20% and 1.03, respectively. 2.2. Mortar mixes tested In total, 20 mortar mixes, each constituting the mortar portion of a normal concrete mix, were made for performance evaluation. These mortar mixes were designed by referring to similar mortar mixes designed and tested by previous researchers [46]. For all mortar mixes, the cementitious paste volume (cement volume plus water volume, as a percentage of the mortar volume) plus the CPW volume (as a percentage of the mortar volume) was set constant as 60%. When the CPW was added as paste replacement, it was added to reduce the cementitious paste volume by the CPW volume. The CPW volumes were set equal to 0% to 20% with an interval of 5%, and the corresponding cementitious paste volume were set as 60% to 40% with an interval of 5%. For the water/cement (W/C) ratio, it was set equal to 0.40, 0.45, 0.50 and 0.55 by mass. It is important to note that as the CPW was added to lower the volume of cementitious paste, the W/C ratio of the cementitious paste was not changed. Lastly, since the cementitious paste volume plus CPW volume was set constant at 60%, the aggregate volume (as a percentage of the total mortar volume) was set constant at 40%. The mix proportions for the mortar mixes are shown in Table 1. For easy reference, each mortar mix so produced was given an identification code of A-B, in which A represents the W/C ratio and B represents the CPW volume. Since the SP was added to maintain the desired workability, the SP dosage (as a percentage by mass of the cement plus CPW content) could not be pre-determined but was obtained by trial mortar mixing. During the trial mortar mixing, the SP was dosed into the mortar mix bit by bit until a desired flow spread of 200 to 300 mm was achieved, and then the SP dosage so obtained was applied to the respective mortar mix during the formal mortar production for testing.
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Table 1 Mix proportions of mortar mixes. Mix no.
Water (kg/m3)
Cement (kg/m3)
CPW (kg/m3)
Fine aggregate (kg/m3)
Percentage reduction in cement content (%)
0.40–0 0.40–5 0.40–10 0.40–15 0.40–20
331 304 276 248 221
828 759 690 621 552
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.45–0 0.45–5 0.45–10 0.45–15 0.45–20
349 319 290 261 232
775 710 645 581 516
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.50–0 0.50–5 0.50–10 0.50–15 0.50–20
364 333 303 273 243
728 667 606 546 485
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.55–0 0.55–5 0.55–10 0.55–15 0.55–20
377 346 314 283 252
686 629 572 514 457
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
3. Methods of testing 3.1. PSD, SEM and XRD tests The particle size distributions (PSDs) of the CPW and cement were measured by a laser diffraction particle size analyzer model Malvern Mastersizer 2000 while the PSD of the fine aggregate was measured by mechanical sieving test. The micrographs of the CPW particles and some selected hardened mortar specimens were obtained by means of a scanning electron microscope (SEM) model Hitachi S-3400 N-II, whereas the chemical compositions of the CPW were tested by an X-ray diffractometer (XRD) model Rigaku D/ MAX-Ultima IV.
3.2. Mini slump cone test Similar to the slump cone test for concrete [47], a small scale slump cone test for mortar, called mini slump cone test, proposed by Okamura and Ouchi [48], was applied in this study to determine the workability of fresh mortar mix. The mini slump cone has a base diameter of 100 mm, a top diameter of 70 mm and a height of 60 mm. Before the test, the steel plate was cleaned and pre-wetted. During the test, the mini slump cone was filled up by the fresh mortar and lifted vertically, and then the mean diameter, i.e. the mean of two diameters in perpendicular directions, of the patty formed was taken as the flow spread [49–52].
3.3. Cube strength test For each mortar mix, six 100 mm cube specimens were produced and moist cured until the age of carrying out cube strength test. By using a Matest 4000 kN compression testing machine, the compressive strengths of three specimens were measured at the age of 7 days, and the compressive strengths of the other three specimens were measured at the age of 28 days. Then, the mean value of the cube strengths of the three specimens tested at the same age was computed as the compressive strength of the mortar at the age of testing. Fig. 2. Ceramic polishing waste (CPW).
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4. Test results 4.1. PSD, SEM image and XRD pattern The PSDs of the cement, CPW and fine aggregate are shown in Fig. 3. It is observed that both the cement and CPW have continuous grading, but the mean particle size of the CPW is bigger than that of the cement and smaller than that of the fine aggregate. The SEM image and XRD results of the CPW are shown in Fig. 4 (a) and (b), respectively. As can be seen from the SEM image, it is clear that the CPW particles are angular in shape. From the XRD pattern, it is evident that the CPW is a typical ceramic material composed mainly of SiO2, Al2O3, Fe2O3 and CaO. The presence of SiO2, Al2O3 and Fe2O3 indicates that the CPW should have certain pozzolanic properties. 4.2. Cement content
(a) SEM image
20000 SiO2 Al2O3 Fe2O3 CaO
15000
I (CPS)
The cement content of each mortar mix is given in the third column of Table 1 and plotted against the CPW volume for different W/C ratios in Fig. 5. These results showed that as expected, the cement content was generally lower when the W/C ratio was higher. More importantly, as the CPW volume increased, the cement content gradually decreased. To evaluate the effectiveness of adding CPW as paste replacement in reducing the cement content, the percentage reduction in cement content attributed to the addition of CPW has been calculated, as listed in the last column of Table 1. From the table, it is evident that increasing the CPW volume would always increase the percentage reduction in cement content. For instance, a CPW volume of 5% would decrease the cement content by 8.3%, whereas a CPW volume of 20% would decrease the cement content by up to 33.3%.
10000
5000
4.3. SP dosage and flow spread
0 10
The SP dosages needed to achieve the target flow spread for the different mortar mixes were not the same. In this regard, the SP dosage results are shown in the second column of Table 2 whereas the measured flow spread results are shown in the third column of Table 2. On the whole, the flow spread results ranged from 226 to 288 mm, all within the target range. To visualize how the SP dosage varied with the W/C ratio and CPW volume, the SP dosage is plotted against the CPW volume for W/C ratios of 0.40, 0.45, 0.50 and 0.55 in Fig. 6. From the figure, it is evident that at a given CPW volume, the SP dosage was higher when the W/C ratio was lower. This is reasonable because at a lower W/C ratio, the water content was lower and thus the SP dosage necessary to achieve the target workability became higher.
30
40
50 2θ (°)
60
70
80
90
(b) XRD pattern Fig. 4. SEM image and XRD pattern of CPW.
Furthermore, regardless of W/C ratio, the SP dosage necessary to keep the desired workability increased as the CPW volume increased. For example, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% increased the SP dosage from 0.40% to 3.35%, whereas when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% increased the SP dosage from 0.14% to 2.30%. Such increases in
100
900 W/C = 0.40
. 80 Cement
CPW
Fine aggregate
60
40
20
Cement content (kg/m3 )
Cumulative percentage passing (%) .
20
W/C = 0.45
800
W/C = 0.50 W/C = 0.55
700
600
500
0
400
0.1
10
1000
Particle size (μm) Fig. 3. Particle size distributions of cement, CPW and fine aggregate.
0
5
10
15
CPW volume (%) Fig. 5. Cement content versus CPW volume at different W/C ratios.
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Table 2 Test results of mortar mixes. SP dosage (%)
W/C = 0.40
Flow spread (mm)
7-day cube strength (MPa)
28-day cube strength (MPa)
Percentage increase in 7-day cube strength (%)
Percentage increase in 28-day cube strength (%)
0.40–0 0.40–5 0.40–10 0.40–15 0.40–20
0.40 1.10 1.65 2.45 3.35
228 288 271 282 230
37.1 48.2 61.2 66.9 72.4
63.2 76.4 87.6 108.2 116.8
– 29.9 65.0 80.3 95.1
– 20.9 38.6 71.2 84.8
0.45–0 0.45–5 0.45–10 0.45–15 0.45–20
0.37 0.90 1.40 2.24 2.80
227 279 228 231 260
34.1 41.8 56.9 58.8 63.1
51.4 71.6 77.5 99.2 102.3
– 22.6 66.9 72.4 85.0
– 39.3 50.8 93.0 99.0
0.50–0 0.50–5 0.50–10 0.50–15 0.50–20
0.23 0.65 1.20 1.90 2.50
240 252 278 256 229
32.1 38.7 46.5 53.1 59.5
44.0 56.5 73.7 86.8 92.9
– 20.6 44.9 65.4 85.4
– 28.4 67.5 97.3 111.1
0.55–0 0.55–5 0.55–10 0.55–15 0.55–20
0.14 0.58 1.05 1.53 2.30
226 248 287 241 236
23.9 29.8 32.7 39.8 48.6
38.2 53.1 64.1 69.9 75.0
– 24.7 36.8 66.5 103.3
– 39.0 67.8 83.0 96.3
7-day cube strength (MPa) .
Mix no.
123
W/C = 0.45
100
W/C = 0.50 W/C = 0.55
80
60
40
20 0
5
10
15
20
CPW volume (%) Fig. 7. 7-day cube strength versus CPW volume at different W/C ratios.
necessary SP dosage caused by the addition of CPW may be explained by the consequential increase in powder content (CPW plus cement content) and decrease in water content, which together lowered the water/powder ratio and thus significantly decreased the workability of the mortar mix. 4.4. Cube strength The fourth and fifth columns of Table 2 depict the 7-day and 28-day cube strength results. To visualize how the cube strength varied with the W/C ratio and CPW volume, the 7-day and 28-day cube strength results are plotted against the CPW volume for W/C ratios of 0.40, 0.45, 0.50 and 0.55 in Figs. 7 and 8, respectively. Comparing the 7-day and 28-day cube strengths, it is obvious that at given W/C ratio and CPW volume, the 28-day cube strength was always higher than the 7-day cube strength by at least 40%. Such results are expected because with continuous moist curing, the cube strength generally increases with age. Furthermore, the curves plotted in the figures reveal that regardless
of the CPW volume and age, decreasing the W/C ratio from 0.55 to 0.40 gradually increased the cube strength. Such variation of cube strength with the W/C ratio is also expected because the cube strength is generally higher when the W/C ratio is lower. More importantly, regardless of the W/C ratio and age, the cube strength significantly improved as the CPW volume increased. At the age of 7 days, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% improved the cube strength from 37.1 to 72.4 MPa, and when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% improved the cube strength from 23.9 to 48.6 MPa. At the age of 28 days, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% improved the cube strength from 63.2 to 116.8 MPa, and when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% improved the cube strength from 38.2 to 75.0 MPa. To quantify the effects of CPW volume on cube strength, the percentage increases in 7-day and 28-day cube strengths due to the addition of CPW are listed in the sixth and seventh columns of Table 2. It is noted that the percentage increases in 7-day and 28-day cube strengths both improved as the CPW volume increased. For example, at a W/C ratio of 0.40, adding 5%, 10%, 15% and 20% CPW led to 29.9%, 65.0%, 80.3% and 95.1% increases in 7-day cube strength and 20.9%, 38.6%, 71.2% and 84.8% increases in 28-day cube strength, respectively. At a W/C ratio of 0.55, adding 5%, 10%, 15% and 20% CPW led to 24.7%, 36.8%, 66.5% and 103.3% increases in 7-day cube strength and 39.0%, 67.8%, 83.0% and 96.3% increases in 28-day cube strength, respectively. Overall,
120
4.0
28-day cube strength (MPa)__
W/C = 0.40
SP dosage (%) .
W/C = 0.45
3.0
W/C = 0.50 W/C = 0.55
2.0
1.0
100
80
60 W/C = 0.40 W/C = 0.45
40
W/C = 0.50 W/C = 0.55
20
0.0 0
5
10
15
CPW volume (%) Fig. 6. SP dosage versus CPW volume at different W/C ratios.
20
0
5
10
15
CPW volume (%) Fig. 8. 28-day cube strength versus CPW volume at different W/C ratios.
20
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the addition of up to 20% CPW would increase the 7-day and 28-day cube strengths by at least 84.8%. 4.5. Micrograph of hardened mortar The micrographs of two mortar specimens, 0.55–0 (with no CPW added) and 0.55–20 (with 20% CPW added), are shown in Fig. 9 (a) and (b), respectively. From Fig. 9(a), it can be seen that in the mortar with no CPW added, some large crystals were generated and the microstructure was rather loose with many voids embedded inside. Conversely, from Fig. 9(b), it is evident that in the mortar with CPW added, fewer large crystals were generated and the microstructure was more dense and compact. The differences in microstructure between the two mortar specimens indicate that the addition of CPW as partial paste replacement had densified the microstructure of the hardened mortar. 5. Discussions Overall, the above test results revealed that the addition of CPW as partial paste replacement would substantially lower the cement content. Moreover, such addition of CPW could at the same time significantly enhance the compressive strength and microstructure.
(a) Mortar mix 0.55-0
Therefore, this powder filler technology of adding CPW to fill into the voids between aggregate particles so that the cementitious paste volume needed to fill the voids can be reduced by the CPW volume added is a promising way of reutilizing the ceramic waste and producing low cement content mortar/concrete. The reduction in waste generation due to reutilization of ceramic waste and the reduction in carbon footprint due to production of low cement content mortar/concrete would together improve the eco-friendliness and sustainability of the ceramic manufacturing and concrete production industries. Regarding the increase in strength resulting from the addition of CPW as paste replacement, some further explanations on the possible causes and general discussions are provided as follows: (1) Adding CPW as paste replacement would not change the W/C ratio of the paste and thus, unlike adding CPW as cement replacement, would not cause any adverse effect on the strength. Hence, even if the particular CPW added has little cementing property, there at least should be no reduction in strength, despite reduction in cement content. (2) In general, the CPW has certain pozzolanic reactivity and thus could act as a supplementary cementitious material to increase the strength [25,27,28]. Moreover, the fine CPW particles may also act as nucleation sites for the precipitation of cement hydrates and thus further improve the strength. (3) The fine CPW particles are much finer than the aggregate particles and thus would fill into the aggregate voids for packing density improvement and pore structure refinement [53–60]. Such packing density improvement and pore structure refinement should have positive effects on the strength. (4) The adoption of a higher SP dosage to achieve the desired workability after adding CPW as paste replacement would help to more thoroughly disperse the solid particles in the mortar mix to attain more uniform mixing and better compaction during casting [43,44,61,62]. Both uniform mixing and good compaction should have positive effects on the strength. In theory, this strategy of adding a ceramic powder filler as paste replacement should be applicable also to crushed and fine-ground solid wastes derived from ceramic tiles, burnt clay bricks, porcelain ware and expanded clay aggregate etc. However, the effects of adding these other powder fillers are dependent on the pozzolanic reactivity and particle size of the particular powder filler added. In this regard, it is recommended that more research on the application of this strategy to other fine-ground ceramic wastes should be carried out. In the longer term, after trying different fine-ground ceramic wastes, systematic studies on the pozzolanic reactivity and filling effect of the various fineground ceramic wastes should be conducted to develop a more general theory. 6. Mortar mix design method
(b) Mortar mix 0.55-20 Fig. 9. SEM micrographs of mortar mixes at 28 days.
For the mortar mix design, there are two major mix parameters to be determined, namely, the W/C ratio and CPW volume. From the test results, the W/C ratio and CPW volume would together govern the SP dosage, cement content and strength. To enable systematic and optimum mortar mix design, two design charts are developed, as presented in Fig. 10(a) and (b). In Fig. 10(a), the 28-day cube strength is plotted against the SP dosage for various W/C ratios and CPW volumes, whereas in Fig. 10(b), the 28-day cube strength is plotted against the cement content for various W/C ratios and CPW volumes. To design a mortar mix, a target mean 28-day cube strength should first be set. Having set the target mean cube strength, the corresponding combinations of W/C ratio and CPW volume yielding the target mean cube strength may be obtained from the design charts. For a given target mean strength, several combinations of W/C ratio and CPW volume may
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7. Conclusions To study the feasibility and potential of reutilizing ceramic polishing waste (CPW) as partial paste replacement in mortar (or mortar portion of concrete), the PSD, SEM image and XRD results of a typical CPW were analyzed and a series of mortar mixes with different W/C ratios and CPW volumes were produced for testing of their fresh and hardened properties. The conclusions made are as follows:
Cube strength (MPa)
100
80
60 W/C = 0.40 W/C = 0.50 CPW = 0% CPW = 10% CPW = 20%
40
W/C = 0.45 W/C = 0.55 CPW = 5% CPW = 15%
20 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
SP dosage (%) (a) 28-day cube strength versus SP dosage
120 W/C = 0.40 W/C = 0.45 W/C = 0.50
100
Cube strength (MPa)
125
W/C = 0.55 CPW = 0% CPW = 5%
80
CPW = 10% CPW = 15% CPW = 20%
60
40
20 200
400
600
800
1000
1200
Cement content (kg/m3) (b) 28-day cube strength versus cement content Fig. 10. Mortar design chart.
(1) The CPW has a continuously graded particle size distribution as well as an angular shape. Also, it is composed mainly of SiO2, Al2O3, Fe2O3 and CaO. The mean particle size of the CPW is bigger than that of the cement but smaller than that of the fine aggregate, and thus the CPW could be used for filling the voids between the aggregate particles. (2) The addition of CPW as paste replacement in mortar would decrease the cement content but increase the SP demand. With up to 20% CPW added, the cement content could be reduced by 33% to lower the carbon footprint. (3) The addition of CPW as paste replacement in mortar would improve the cube strength and densify the microstructure of the hardened mortar. With up to 20% CPW added, the 7-day and 28-day cube strengths could both be increased by at least 85%. (4) Two mortar design charts correlating the cube strength to the SP dosage and cement content for various W/C ratios and CPW volumes have been plotted. From these charts, the combinations of W/C ratio and CPW volume satisfying the cube strength requirement and the corresponding SP dosage and cement content can be obtained directly. (5) With up to 20% CPW added, the 28-day cube strength could be increased to 116.8 MPa. Hence, unlike most other solid waste, CPW can be used for the production of high-strength concrete. Overall, it is verified that the reutilization of CPW as paste replacement in mortar is not only feasible but would also decrease the cement content by 33% and increase the 7-day and 28-day strengths by 85%. However, the results obtained herein are strictly applicable only to the particular CPW used. For other ceramic powders derived from different sources, separate tests are needed. Acknowledgements
be selected. In general, selecting a higher W/C ratio would require a higher CPW volume to achieve the strength, but simultaneously require a higher SP dosage and lead to a lower cement content. The SP dosage and cement content may be obtained directly from Fig. 10(a) and (b), respectively. By selecting a combination of W/C ratio and CPW volume that requires a lower SP dosage, the cement content would be higher. On the other hand, by selecting a combination of W/C ratio and CPW volume that leads to a lower cement content, the SP dosage would be higher. A balanced decision has to be made between low SP dosage and low cement content. The design charts can also serve to find out for various W/C ratios and CPW volumes, the ranges of cube strength and cement content that can be achieved and the SP dosage that will be needed. For examples, it can be read from the design charts that for a CPW volume of 20% and within the range of W/C ratio between 0.40 and 0.55, the cube strength would range from 75.0 to 116.8 MPa, the cement content would range from 457 to 552 kg/m3 and the SP dosage would range from 2.30% to 3.35%. Hence, although the SP dosage would be substantially increased, the addition of 20% CPW could be used for the production of high-strength concrete. Lastly, it should be noted that the actual performance of the concrete produced is dependent on the pozzolanic reactivity and particle size of the particular ceramic powder added. For ceramic powders having different characteristics, separate tests are required to generate data for modifying the design charts to suit.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Reutilizing ceramic polishing waste as powder filler in mortar to reduce cement content by 33% and increase strength by 85% L.G. Li a,⁎, Z.Y. Zhuo a, J. Zhu a, J.J. Chen b, A.K.H. Kwan c a b c
Guangdong University of Technology, Guangzhou, China Foshan University, Foshan, China The University of Hong Kong, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 1 November 2018 Received in revised form 1 June 2019 Accepted 12 July 2019 Available online 15 July 2019 Keywords: Ceramic waste Green mortar/concrete Microstructure Strength Waste reutilization
a b s t r a c t Usually, the ceramic wastes are reutilized as cement or aggregate replacement in concrete. Among the ceramic wastes, the ceramic polishing waste (CPW) generated during polishing of ceramic products is a powder. Herein, the CPW is added as paste replacement. This would reduce the amount of ceramic waste to be dumped, and the cement consumption and carbon footprint of the concrete production. To study the feasibility of such powder filler technology, a number of mortar mixes containing different water, cement and CPW contents were produced for workability, strength and SEM tests. It was found that with up to 20% CPW by volume added as paste replacement, the cement content could be reduced by 33%, the 7-day and 28-day cube strengths could both be increased by at least 85%, and the microstructure could be densified. Lastly, two design charts for adding CPW as paste replacement were developed. © 2019 Elsevier B.V. All rights reserved.
1. Introduction In the reutilization of a solid waste for producing mortar/concrete, depending on the source, the solid waste may be classified into industrial solid waste and construction solid waste. The industrial solid wastes are industrial by-products. Some of these, such as slag, fly ash and silica fume, have cementitious properties and thus are commonly used as supplementary cementitious materials [1–8]. On the other hand, the construction solid wastes, such as old concrete, waste glass, clay bricks, ceramic tiles and rubbers, have also been explored for possible use either as a supplementary cementitious material or as an aggregate [9–15]. Ceramics have been used for a long time all over the world [16,17]. However, a large quantity of ceramic waste is produced during manufacture of ceramic products and demolition of buildings [18]. The ceramic waste brings serious environmental problems and occupies a large landfill area [19–21]. China is the largest producer of ceramics in the world [22] and is thus also the largest producer of ceramic waste [23,24]. It is now urgent to deal with the ceramic waste problem. Currently, most of the ceramic waste is just dumped. Only a small part of the ceramic waste is crushed and reutilized to replace cement or aggregate in the production of mortar or concrete. The use to replace cement is named as cement replacement strategy whereas the use to replace aggregate is named as aggregate replacement strategy. ⁎ Corresponding author. E-mail address: [email protected] (L.G. Li).
https://doi.org/10.1016/j.powtec.2019.07.043 0032-5910/© 2019 Elsevier B.V. All rights reserved.
In the cement replacement strategy, the ceramic waste is added as partial replacement of the cementitious materials, as illustrated in Fig. 1(a). Vejmelková et al. [25] found that the frost resistance of concrete containing up to 40% fine-ground ceramics as cement replacement was as good as reference concrete. Cheng et al. [26] showed that using ceramic polishing waste as cement replacement would lower the compressive strength and carbonation resistance of concrete. Steiner et al. [27] revealed that adding ceramic tile polishing residue to replace cement up to 25% has little negative effect on the strength of mortar. Mas et al. [28] demonstrated that the addition of ceramic tile waste as cement substitution would reduce the strength of mortar, but addition up to 35% still meets the strength activity index requirements for fly ash. De Matos et al. [29] showed that replacing cement by no more than 20% porcelain polishing residue would result in similar rheological properties but better passing ability in the case of self-consolidating concrete. In the aggregate replacement strategy, the ceramic waste is added as partial replacement of the aggregate, as illustrated in Fig. 1(b). Guerra et al. [30] revealed that the addition of sanitary porcelain waste as aggregate replacement up to 9% would not impair the compressive strength of concrete. Gonzalez-Corominas and Etxeberria [31] tested that concrete with 30% ceramic fine aggregate achieved similar or better mechanical and durability properties compared to reference concrete. Medina et al. [32] showed that concrete with up to 25% ceramic sanitary ware aggregate was as durable as normal concrete. Awoyera et al. [33] reported that concrete with 75% ceramic tile waste aggregate has a higher 28-day strength than reference concrete. Elçi [34] found that
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Fig. 1. Ceramic waste addition methods.
the mechanical properties of concrete using crushed floor tile aggregate were similar to reference concrete, but those of concrete using crushed wall tile aggregate were lower than reference concrete. Anderson et al. [35] observed that the effects of ceramic tile waste were marginal and thus the use of ceramic tile waste as partial replacement of coarse aggregate is feasible. It has been noted, however, that both the two strategies of reutilizing ceramic waste have certain negative effects, albeit such reutilizations of ceramic waste would benefit waste recycling and environmental protection. In the cement replacement strategy, a relatively high cement replacement rate could cause serious strength reduction [25–28]. In the aggregate replacement strategy, the overall performance of the concrete produced could sometimes be impaired [32–34] and the cement content would not be reduced to lower the carbon footprint of the concrete production. Hence, the current strategies are not entirely satisfactory. Recently, an alternative strategy, called the paste replacement strategy, has been developed by the authors' research team. By this strategy, the solid waste is treated as a filler and added to substitute part of the cementitious paste in such a way that the total volume of the cementitious paste and the solid waste for aggregate voids filling remains unchanged. The mix proportions of the cementitious paste is also kept unchanged, as illustrated in Fig. 1(c). In previous studies, it has been demonstrated that adding limestone fines by this strategy could on one hand reduce the cement content and on other hand increase the dimensional stability, strength and durability of concrete [36–39]. This strategy has also been adopted in the reutilization of rock dust (marble dust and granite dust) and clay brick dust and in mortar/concrete. So far, the results proved that, depending on the fineness of the solid waste to be used as a filler, the addition of rock dust and clay brick dust would also effectively reduce the cement
content and improve the dimensional stability, strength and durability [40–44]. In this study, the paste replacement strategy was extended to ceramic polishing waste (CPW), a waste generated during polishing of ceramic tiles. On average, the production of 1.0 m2 of polished ceramic tiles generates about 1.9 to 2.1 kg of CPW, and in 2014, about 10 million tons of CPW was generated in China, but basically none was reutilized [45]. This waste is a powder and therefore does not require crushing or grinding. To determine the effects of adding CPW as partial paste replacement on cement content and performance of mortar so as to study the feasibility and potential of reutilizing CPW as a powder filler in mortar/concrete, trial mortar mixes at various water/cement ratios and CPW volumes were produced for testing of their workability, strength and microstructure. 2. Materials and mortar mixes tested 2.1. Raw materials The cementitious material for the mortar production was 42.5 strength class ordinary Portland cement. Its specific gravity was tested to be 3.08. The fine aggregate used was river sand. Its maximum size, specific gravity, moisture content and water absorption were tested to be 1.18 mm, 2.58, 0.10% and 1.10%, respectively. These materials were bought from the local market. The ceramic polishing waste (CPW) was obtained from a ceramics factory in Foshan city, a famous powerhouse of ceramics production in China. The CPW was generated during polishing of ceramic tiles. The original CPW collected was wet with some debris inside. In order to dry and minimize variation in quality, the CPW was treated as follows: first, the CPW was heated with an oven at 105 °C for 8 h to remove the
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water and then mechanically sieved using a 1.18 mm sieve to remove the debris. After the treatment, the CPW was turned to a light grey dry powder (shown in Fig. 2) and the specific gravity of the CPW was measured as 2.43. Generally, after adding CPW, the mortar became more cohesive and stable but less flowable. To compensate for the reduction in workability and maintain the desired workability, a superplasticizer (SP) was dosed to each mortar mix. The SP used was a 3rd generation polycarboxylatebased admixture. Its solid content and specific gravity were 20% and 1.03, respectively. 2.2. Mortar mixes tested In total, 20 mortar mixes, each constituting the mortar portion of a normal concrete mix, were made for performance evaluation. These mortar mixes were designed by referring to similar mortar mixes designed and tested by previous researchers [46]. For all mortar mixes, the cementitious paste volume (cement volume plus water volume, as a percentage of the mortar volume) plus the CPW volume (as a percentage of the mortar volume) was set constant as 60%. When the CPW was added as paste replacement, it was added to reduce the cementitious paste volume by the CPW volume. The CPW volumes were set equal to 0% to 20% with an interval of 5%, and the corresponding cementitious paste volume were set as 60% to 40% with an interval of 5%. For the water/cement (W/C) ratio, it was set equal to 0.40, 0.45, 0.50 and 0.55 by mass. It is important to note that as the CPW was added to lower the volume of cementitious paste, the W/C ratio of the cementitious paste was not changed. Lastly, since the cementitious paste volume plus CPW volume was set constant at 60%, the aggregate volume (as a percentage of the total mortar volume) was set constant at 40%. The mix proportions for the mortar mixes are shown in Table 1. For easy reference, each mortar mix so produced was given an identification code of A-B, in which A represents the W/C ratio and B represents the CPW volume. Since the SP was added to maintain the desired workability, the SP dosage (as a percentage by mass of the cement plus CPW content) could not be pre-determined but was obtained by trial mortar mixing. During the trial mortar mixing, the SP was dosed into the mortar mix bit by bit until a desired flow spread of 200 to 300 mm was achieved, and then the SP dosage so obtained was applied to the respective mortar mix during the formal mortar production for testing.
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Table 1 Mix proportions of mortar mixes. Mix no.
Water (kg/m3)
Cement (kg/m3)
CPW (kg/m3)
Fine aggregate (kg/m3)
Percentage reduction in cement content (%)
0.40–0 0.40–5 0.40–10 0.40–15 0.40–20
331 304 276 248 221
828 759 690 621 552
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.45–0 0.45–5 0.45–10 0.45–15 0.45–20
349 319 290 261 232
775 710 645 581 516
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.50–0 0.50–5 0.50–10 0.50–15 0.50–20
364 333 303 273 243
728 667 606 546 485
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
0.55–0 0.55–5 0.55–10 0.55–15 0.55–20
377 346 314 283 252
686 629 572 514 457
0 121 243 364 485
1032 1032 1032 1032 1032
– 8.3 16.7 25.0 33.3
3. Methods of testing 3.1. PSD, SEM and XRD tests The particle size distributions (PSDs) of the CPW and cement were measured by a laser diffraction particle size analyzer model Malvern Mastersizer 2000 while the PSD of the fine aggregate was measured by mechanical sieving test. The micrographs of the CPW particles and some selected hardened mortar specimens were obtained by means of a scanning electron microscope (SEM) model Hitachi S-3400 N-II, whereas the chemical compositions of the CPW were tested by an X-ray diffractometer (XRD) model Rigaku D/ MAX-Ultima IV.
3.2. Mini slump cone test Similar to the slump cone test for concrete [47], a small scale slump cone test for mortar, called mini slump cone test, proposed by Okamura and Ouchi [48], was applied in this study to determine the workability of fresh mortar mix. The mini slump cone has a base diameter of 100 mm, a top diameter of 70 mm and a height of 60 mm. Before the test, the steel plate was cleaned and pre-wetted. During the test, the mini slump cone was filled up by the fresh mortar and lifted vertically, and then the mean diameter, i.e. the mean of two diameters in perpendicular directions, of the patty formed was taken as the flow spread [49–52].
3.3. Cube strength test For each mortar mix, six 100 mm cube specimens were produced and moist cured until the age of carrying out cube strength test. By using a Matest 4000 kN compression testing machine, the compressive strengths of three specimens were measured at the age of 7 days, and the compressive strengths of the other three specimens were measured at the age of 28 days. Then, the mean value of the cube strengths of the three specimens tested at the same age was computed as the compressive strength of the mortar at the age of testing. Fig. 2. Ceramic polishing waste (CPW).
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4. Test results 4.1. PSD, SEM image and XRD pattern The PSDs of the cement, CPW and fine aggregate are shown in Fig. 3. It is observed that both the cement and CPW have continuous grading, but the mean particle size of the CPW is bigger than that of the cement and smaller than that of the fine aggregate. The SEM image and XRD results of the CPW are shown in Fig. 4 (a) and (b), respectively. As can be seen from the SEM image, it is clear that the CPW particles are angular in shape. From the XRD pattern, it is evident that the CPW is a typical ceramic material composed mainly of SiO2, Al2O3, Fe2O3 and CaO. The presence of SiO2, Al2O3 and Fe2O3 indicates that the CPW should have certain pozzolanic properties. 4.2. Cement content
(a) SEM image
20000 SiO2 Al2O3 Fe2O3 CaO
15000
I (CPS)
The cement content of each mortar mix is given in the third column of Table 1 and plotted against the CPW volume for different W/C ratios in Fig. 5. These results showed that as expected, the cement content was generally lower when the W/C ratio was higher. More importantly, as the CPW volume increased, the cement content gradually decreased. To evaluate the effectiveness of adding CPW as paste replacement in reducing the cement content, the percentage reduction in cement content attributed to the addition of CPW has been calculated, as listed in the last column of Table 1. From the table, it is evident that increasing the CPW volume would always increase the percentage reduction in cement content. For instance, a CPW volume of 5% would decrease the cement content by 8.3%, whereas a CPW volume of 20% would decrease the cement content by up to 33.3%.
10000
5000
4.3. SP dosage and flow spread
0 10
The SP dosages needed to achieve the target flow spread for the different mortar mixes were not the same. In this regard, the SP dosage results are shown in the second column of Table 2 whereas the measured flow spread results are shown in the third column of Table 2. On the whole, the flow spread results ranged from 226 to 288 mm, all within the target range. To visualize how the SP dosage varied with the W/C ratio and CPW volume, the SP dosage is plotted against the CPW volume for W/C ratios of 0.40, 0.45, 0.50 and 0.55 in Fig. 6. From the figure, it is evident that at a given CPW volume, the SP dosage was higher when the W/C ratio was lower. This is reasonable because at a lower W/C ratio, the water content was lower and thus the SP dosage necessary to achieve the target workability became higher.
30
40
50 2θ (°)
60
70
80
90
(b) XRD pattern Fig. 4. SEM image and XRD pattern of CPW.
Furthermore, regardless of W/C ratio, the SP dosage necessary to keep the desired workability increased as the CPW volume increased. For example, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% increased the SP dosage from 0.40% to 3.35%, whereas when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% increased the SP dosage from 0.14% to 2.30%. Such increases in
100
900 W/C = 0.40
. 80 Cement
CPW
Fine aggregate
60
40
20
Cement content (kg/m3 )
Cumulative percentage passing (%) .
20
W/C = 0.45
800
W/C = 0.50 W/C = 0.55
700
600
500
0
400
0.1
10
1000
Particle size (μm) Fig. 3. Particle size distributions of cement, CPW and fine aggregate.
0
5
10
15
CPW volume (%) Fig. 5. Cement content versus CPW volume at different W/C ratios.
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120
Table 2 Test results of mortar mixes. SP dosage (%)
W/C = 0.40
Flow spread (mm)
7-day cube strength (MPa)
28-day cube strength (MPa)
Percentage increase in 7-day cube strength (%)
Percentage increase in 28-day cube strength (%)
0.40–0 0.40–5 0.40–10 0.40–15 0.40–20
0.40 1.10 1.65 2.45 3.35
228 288 271 282 230
37.1 48.2 61.2 66.9 72.4
63.2 76.4 87.6 108.2 116.8
– 29.9 65.0 80.3 95.1
– 20.9 38.6 71.2 84.8
0.45–0 0.45–5 0.45–10 0.45–15 0.45–20
0.37 0.90 1.40 2.24 2.80
227 279 228 231 260
34.1 41.8 56.9 58.8 63.1
51.4 71.6 77.5 99.2 102.3
– 22.6 66.9 72.4 85.0
– 39.3 50.8 93.0 99.0
0.50–0 0.50–5 0.50–10 0.50–15 0.50–20
0.23 0.65 1.20 1.90 2.50
240 252 278 256 229
32.1 38.7 46.5 53.1 59.5
44.0 56.5 73.7 86.8 92.9
– 20.6 44.9 65.4 85.4
– 28.4 67.5 97.3 111.1
0.55–0 0.55–5 0.55–10 0.55–15 0.55–20
0.14 0.58 1.05 1.53 2.30
226 248 287 241 236
23.9 29.8 32.7 39.8 48.6
38.2 53.1 64.1 69.9 75.0
– 24.7 36.8 66.5 103.3
– 39.0 67.8 83.0 96.3
7-day cube strength (MPa) .
Mix no.
123
W/C = 0.45
100
W/C = 0.50 W/C = 0.55
80
60
40
20 0
5
10
15
20
CPW volume (%) Fig. 7. 7-day cube strength versus CPW volume at different W/C ratios.
necessary SP dosage caused by the addition of CPW may be explained by the consequential increase in powder content (CPW plus cement content) and decrease in water content, which together lowered the water/powder ratio and thus significantly decreased the workability of the mortar mix. 4.4. Cube strength The fourth and fifth columns of Table 2 depict the 7-day and 28-day cube strength results. To visualize how the cube strength varied with the W/C ratio and CPW volume, the 7-day and 28-day cube strength results are plotted against the CPW volume for W/C ratios of 0.40, 0.45, 0.50 and 0.55 in Figs. 7 and 8, respectively. Comparing the 7-day and 28-day cube strengths, it is obvious that at given W/C ratio and CPW volume, the 28-day cube strength was always higher than the 7-day cube strength by at least 40%. Such results are expected because with continuous moist curing, the cube strength generally increases with age. Furthermore, the curves plotted in the figures reveal that regardless
of the CPW volume and age, decreasing the W/C ratio from 0.55 to 0.40 gradually increased the cube strength. Such variation of cube strength with the W/C ratio is also expected because the cube strength is generally higher when the W/C ratio is lower. More importantly, regardless of the W/C ratio and age, the cube strength significantly improved as the CPW volume increased. At the age of 7 days, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% improved the cube strength from 37.1 to 72.4 MPa, and when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% improved the cube strength from 23.9 to 48.6 MPa. At the age of 28 days, when the W/C ratio was 0.40, increasing the CPW volume from 0% to 20% improved the cube strength from 63.2 to 116.8 MPa, and when the W/C ratio was 0.55, increasing the CPW volume from 0% to 20% improved the cube strength from 38.2 to 75.0 MPa. To quantify the effects of CPW volume on cube strength, the percentage increases in 7-day and 28-day cube strengths due to the addition of CPW are listed in the sixth and seventh columns of Table 2. It is noted that the percentage increases in 7-day and 28-day cube strengths both improved as the CPW volume increased. For example, at a W/C ratio of 0.40, adding 5%, 10%, 15% and 20% CPW led to 29.9%, 65.0%, 80.3% and 95.1% increases in 7-day cube strength and 20.9%, 38.6%, 71.2% and 84.8% increases in 28-day cube strength, respectively. At a W/C ratio of 0.55, adding 5%, 10%, 15% and 20% CPW led to 24.7%, 36.8%, 66.5% and 103.3% increases in 7-day cube strength and 39.0%, 67.8%, 83.0% and 96.3% increases in 28-day cube strength, respectively. Overall,
120
4.0
28-day cube strength (MPa)__
W/C = 0.40
SP dosage (%) .
W/C = 0.45
3.0
W/C = 0.50 W/C = 0.55
2.0
1.0
100
80
60 W/C = 0.40 W/C = 0.45
40
W/C = 0.50 W/C = 0.55
20
0.0 0
5
10
15
CPW volume (%) Fig. 6. SP dosage versus CPW volume at different W/C ratios.
20
0
5
10
15
CPW volume (%) Fig. 8. 28-day cube strength versus CPW volume at different W/C ratios.
20
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the addition of up to 20% CPW would increase the 7-day and 28-day cube strengths by at least 84.8%. 4.5. Micrograph of hardened mortar The micrographs of two mortar specimens, 0.55–0 (with no CPW added) and 0.55–20 (with 20% CPW added), are shown in Fig. 9 (a) and (b), respectively. From Fig. 9(a), it can be seen that in the mortar with no CPW added, some large crystals were generated and the microstructure was rather loose with many voids embedded inside. Conversely, from Fig. 9(b), it is evident that in the mortar with CPW added, fewer large crystals were generated and the microstructure was more dense and compact. The differences in microstructure between the two mortar specimens indicate that the addition of CPW as partial paste replacement had densified the microstructure of the hardened mortar. 5. Discussions Overall, the above test results revealed that the addition of CPW as partial paste replacement would substantially lower the cement content. Moreover, such addition of CPW could at the same time significantly enhance the compressive strength and microstructure.
(a) Mortar mix 0.55-0
Therefore, this powder filler technology of adding CPW to fill into the voids between aggregate particles so that the cementitious paste volume needed to fill the voids can be reduced by the CPW volume added is a promising way of reutilizing the ceramic waste and producing low cement content mortar/concrete. The reduction in waste generation due to reutilization of ceramic waste and the reduction in carbon footprint due to production of low cement content mortar/concrete would together improve the eco-friendliness and sustainability of the ceramic manufacturing and concrete production industries. Regarding the increase in strength resulting from the addition of CPW as paste replacement, some further explanations on the possible causes and general discussions are provided as follows: (1) Adding CPW as paste replacement would not change the W/C ratio of the paste and thus, unlike adding CPW as cement replacement, would not cause any adverse effect on the strength. Hence, even if the particular CPW added has little cementing property, there at least should be no reduction in strength, despite reduction in cement content. (2) In general, the CPW has certain pozzolanic reactivity and thus could act as a supplementary cementitious material to increase the strength [25,27,28]. Moreover, the fine CPW particles may also act as nucleation sites for the precipitation of cement hydrates and thus further improve the strength. (3) The fine CPW particles are much finer than the aggregate particles and thus would fill into the aggregate voids for packing density improvement and pore structure refinement [53–60]. Such packing density improvement and pore structure refinement should have positive effects on the strength. (4) The adoption of a higher SP dosage to achieve the desired workability after adding CPW as paste replacement would help to more thoroughly disperse the solid particles in the mortar mix to attain more uniform mixing and better compaction during casting [43,44,61,62]. Both uniform mixing and good compaction should have positive effects on the strength. In theory, this strategy of adding a ceramic powder filler as paste replacement should be applicable also to crushed and fine-ground solid wastes derived from ceramic tiles, burnt clay bricks, porcelain ware and expanded clay aggregate etc. However, the effects of adding these other powder fillers are dependent on the pozzolanic reactivity and particle size of the particular powder filler added. In this regard, it is recommended that more research on the application of this strategy to other fine-ground ceramic wastes should be carried out. In the longer term, after trying different fine-ground ceramic wastes, systematic studies on the pozzolanic reactivity and filling effect of the various fineground ceramic wastes should be conducted to develop a more general theory. 6. Mortar mix design method
(b) Mortar mix 0.55-20 Fig. 9. SEM micrographs of mortar mixes at 28 days.
For the mortar mix design, there are two major mix parameters to be determined, namely, the W/C ratio and CPW volume. From the test results, the W/C ratio and CPW volume would together govern the SP dosage, cement content and strength. To enable systematic and optimum mortar mix design, two design charts are developed, as presented in Fig. 10(a) and (b). In Fig. 10(a), the 28-day cube strength is plotted against the SP dosage for various W/C ratios and CPW volumes, whereas in Fig. 10(b), the 28-day cube strength is plotted against the cement content for various W/C ratios and CPW volumes. To design a mortar mix, a target mean 28-day cube strength should first be set. Having set the target mean cube strength, the corresponding combinations of W/C ratio and CPW volume yielding the target mean cube strength may be obtained from the design charts. For a given target mean strength, several combinations of W/C ratio and CPW volume may
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7. Conclusions To study the feasibility and potential of reutilizing ceramic polishing waste (CPW) as partial paste replacement in mortar (or mortar portion of concrete), the PSD, SEM image and XRD results of a typical CPW were analyzed and a series of mortar mixes with different W/C ratios and CPW volumes were produced for testing of their fresh and hardened properties. The conclusions made are as follows:
Cube strength (MPa)
100
80
60 W/C = 0.40 W/C = 0.50 CPW = 0% CPW = 10% CPW = 20%
40
W/C = 0.45 W/C = 0.55 CPW = 5% CPW = 15%
20 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
SP dosage (%) (a) 28-day cube strength versus SP dosage
120 W/C = 0.40 W/C = 0.45 W/C = 0.50
100
Cube strength (MPa)
125
W/C = 0.55 CPW = 0% CPW = 5%
80
CPW = 10% CPW = 15% CPW = 20%
60
40
20 200
400
600
800
1000
1200
Cement content (kg/m3) (b) 28-day cube strength versus cement content Fig. 10. Mortar design chart.
(1) The CPW has a continuously graded particle size distribution as well as an angular shape. Also, it is composed mainly of SiO2, Al2O3, Fe2O3 and CaO. The mean particle size of the CPW is bigger than that of the cement but smaller than that of the fine aggregate, and thus the CPW could be used for filling the voids between the aggregate particles. (2) The addition of CPW as paste replacement in mortar would decrease the cement content but increase the SP demand. With up to 20% CPW added, the cement content could be reduced by 33% to lower the carbon footprint. (3) The addition of CPW as paste replacement in mortar would improve the cube strength and densify the microstructure of the hardened mortar. With up to 20% CPW added, the 7-day and 28-day cube strengths could both be increased by at least 85%. (4) Two mortar design charts correlating the cube strength to the SP dosage and cement content for various W/C ratios and CPW volumes have been plotted. From these charts, the combinations of W/C ratio and CPW volume satisfying the cube strength requirement and the corresponding SP dosage and cement content can be obtained directly. (5) With up to 20% CPW added, the 28-day cube strength could be increased to 116.8 MPa. Hence, unlike most other solid waste, CPW can be used for the production of high-strength concrete. Overall, it is verified that the reutilization of CPW as paste replacement in mortar is not only feasible but would also decrease the cement content by 33% and increase the 7-day and 28-day strengths by 85%. However, the results obtained herein are strictly applicable only to the particular CPW used. For other ceramic powders derived from different sources, separate tests are needed. Acknowledgements
be selected. In general, selecting a higher W/C ratio would require a higher CPW volume to achieve the strength, but simultaneously require a higher SP dosage and lead to a lower cement content. The SP dosage and cement content may be obtained directly from Fig. 10(a) and (b), respectively. By selecting a combination of W/C ratio and CPW volume that requires a lower SP dosage, the cement content would be higher. On the other hand, by selecting a combination of W/C ratio and CPW volume that leads to a lower cement content, the SP dosage would be higher. A balanced decision has to be made between low SP dosage and low cement content. The design charts can also serve to find out for various W/C ratios and CPW volumes, the ranges of cube strength and cement content that can be achieved and the SP dosage that will be needed. For examples, it can be read from the design charts that for a CPW volume of 20% and within the range of W/C ratio between 0.40 and 0.55, the cube strength would range from 75.0 to 116.8 MPa, the cement content would range from 457 to 552 kg/m3 and the SP dosage would range from 2.30% to 3.35%. Hence, although the SP dosage would be substantially increased, the addition of 20% CPW could be used for the production of high-strength concrete. Lastly, it should be noted that the actual performance of the concrete produced is dependent on the pozzolanic reactivity and particle size of the particular ceramic powder added. For ceramic powders having different characteristics, separate tests are required to generate data for modifying the design charts to suit.
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