Cementing efficiency factors of ceramic polishing residue in compressive strength and chloride resistance of mortar

Powder Technology 367 (2020) 163–171

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Cementing efficiency factors of ceramic polishing residue in compressive strength and chloride resistance of mortar L.G. Li a,c,⁎, Z.Y. Zhuo b, A.K.H. Kwan c, T.S. Zhang d, D.G. Lu e a

Guangdong University of Technology, Guangzhou, China Agile Property Holdings Ltd, China University of Hong Kong, Hong Kong, China d South China University of Technology, Guangzhou, China e Sun Yat-sen University, Guangzhou, China b c

a r t i c l e

i n f o

Article history: Received 4 January 2020 Received in revised form 18 March 2020 Accepted 24 March 2020 Available online 26 March 2020 Keywords: Carbon footprint Cementing efficiency Ceramic polishing residue Chloride resistance Compressive strength Waste reutilization

a b s t r a c t During the polishing process of ceramic tiles, plenty of ceramic polishing residue (CPR) is generated. In order to evaluate the reutilization of CPR as a supplementary cementitious material (SCM) in mortar/concrete, and investigate its effects on the strength and durability, a series of mortar mixes containing different CPR contents were made for conducting compressive strength test and rapid chloride permeability test. It was found that adding CPR as a SCM up to 20% could still markedly improve the compressive strength and chloride resistance, while at the same time reduce the waste disposal, cement consumption and carbon footprint for sustainable development. Moreover, the cementing efficiency factor of the CPR in 28-day compressive strength was generally higher than 1.5 whereas that in chloride resistance was generally higher than 4.0, indicating that the CPR is a highly effective SCM for replacing part of the carbon-intensive cement and improving the strength and durability performance. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Ensuring the sustainability of concrete production for construction has become an important concern of the construction industry because the manufacturing process of cement consumes 13% of the global industrial energy and discharges 7% of the carbon dioxide all over the world [1]. For the purpose of reducing the usage of cement so as to reduce the embodied energy and carbon footprint of concrete, several different methods have been employed through the reutilization of postconsumer wastes [2–5], construction wastes [6–9] and industrial wastes [10–14]. On possible reutilization of post-consumer wastes, Dong et al. [15] found that concrete containing rubber particles treated by silane coupling agent has similar mechanical performance as normal concrete. Alipour et al. [16] reported that adding recycled glass powder as cement replacement can improve the flowability of mortar. On possible reutilization of construction wastes, Florea et al. [17] noted that thermally treated recycle concrete fines exhibit reactivity similar to fly ash, whereas Ge et al. [18] showed that concrete with clay brick powder added as cement substitute still has good chloride and freezingthawing resistances. Regarding possible application of industrial wastes, de Matos et al. [19] noted that adding fly ash as cement substitute up to ⁎ Corresponding author at: Guangdong University of Technology, Guangzhou, China. E-mail address: [email protected] (L.G. Li).

https://doi.org/10.1016/j.powtec.2020.03.050 0032-5910/© 2020 Elsevier B.V. All rights reserved.

60% would enhance the long-term strength of self-compacting concrete, whereas Ting et al. [20] revealed that concrete with ultra-fine slag added could have high early strength as for the concrete with silica fume added. Unlike other solid wastes, ceramic wastes are both construction wastes and industrial wastes because some ceramic wastes are derived from old ceramic tiles or sanitary ware generated by the demolition of old buildings [21–23] and some are generated from the manufacture of ceramic products [24–27]. China is the largest producer of ceramics in the world and for this reason is generating a huge quantity of ceramic wastes every year [28,29]. In 2018, the annual production of ceramic tiles in China was 9.01 billion m2, about 60% of the total world production [30]. How to deal with the large quantity of ceramic wastes has become a critical environmental issue in China. If it could be reutilized in concrete to reduce the cement content, then both the waste disposal and cement consumption problems could be mitigated. For ceramic waste with large particle size, such as ceramic fragments obtained by crushing waste tiles or sanitary ware, it is preferred to be reutilized in concrete production as aggregate substitute [31–33]. De Brito et al. [34] pointed out that recycled ceramic waste can be used as aggregates in non-structural concrete, such as concrete pavement slab. Gonzalez-Corominas and Etxeberria [35] added ceramic aggregate to replace natural sand, and found that concrete containing up to 30% ceramic fine aggregate has similar or higher strength and durability compared to normal concrete. On the other hand, Medina et al. [36] found

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that the use of ceramic sanitary ware aggregate would improve the compressive strength, but slightly reduce the chloride resistance. Awoyera et al. [37] revealed that concrete containing 75% crushed ceramic tiles has better compressive and splitting tensile strengths than similar concrete containing normal aggregate. For ceramic waste with small particles size, such as ceramic polishing residue (CPR) which amounts to about 19 million tons generated annually from ceramic tile factories in China [28] or ground ceramic fines, some studies have been launched to explore its possible reutilization as cement replacement or as a supplementary cementitious material (SCM) [38–40]. Vejmelková et al. [41] pointed out that the optimum content of ground ceramic fines for compressive strength improvement is around 10%. Cheng et al. [42] found that the use of CPR would increase the sulphate resistance but decrease the carbonation resistance. De Matos et al. [43,44] reported that adding CPR would increase the plastic viscosity and yield stress of paste, but would reduce the cement content and enhance the passing ability of self-compacting concrete. Steiner et al. [45] showed that mortar with CPR sludge added has lower autogenous shrinkage than the conventional mortar. To assess and compare the effectiveness of different cementitious materials, the idea of quantifying the cementing efficiency of a cementitious material in terms of a cementing efficiency factor (CEF) has been developed for quite some time. The CEF is defined as the ratio of the equivalent mass of cement to the mass of the cementitious material added without influencing the performance attribute being considered [46–48] (more explanations are given in Section 5). Sellevold and Radjy [49] reported that the CEF of silica fume in terms of 28-day compressive strength was between 2 and 4. Babu and Prakash [50] found that the 28-day strength CEF of silica fume could be as high as 3.0. Domone [51] noted that the 28-day strength CEF of limestone powder was only 0.29. Cyr et al. [52] found that the 28-day strength CEF of metakaolin was dependent on the cement type and was lower at a higher metakaolin content. Lollini et al. [53] reported that the chloride resistance CEF of slag and fly ash were approximately 1.5. Li et al. [54] revealed that the 28-day strength CEF of nano-silica could be higher than 6.0. Previous studies have indicated that ceramic waste in powder form could have certain pozzolanic reactivity [39,55], but so far, the CEFs of ceramic waste have rarely been quantified. This study focused on the potential of reutilizing CPR, which, because of its high fineness close to that of cement, should have a relatively high reactivity and cementing efficiency. In order to evaluate the cementing efficiency of CPR, a series of mortar mixes with different CPR contents and various water/cementitious materials ratios were made to test their workability, compressive strength and chloride resistance, and by analyzing the test results, the compressive strength and chloride resistance CEFs of the CPR were obtained for comparison with other SCMs. 2. Raw materials and mix proportioning 2.1. Characterization of raw materials The ceramic polishing residue (CPR) used was collected from a ceramic tile factory in Foshan city, a famous ceramics industrial centre in China. The CPR was produced during the polishing and lapping of ceramic tiles. To remove the debris and moisture in the CPR, the CPR was sieved through a 1.18 mm sieve and then heated at 105 °C for 8 h. Afterwards, the CPR became a dry and light grey colour powder. A scanning electron microscope (SEM) image of the CPR particles is given in Fig. 1, from which it can be seen that the CPR particles are irregular and angular in shape and that very fine (submicron) particles are adhering to the larger particles, presumably due to electrostatic forces. X-ray fluorescence (XRF) spectrometry had been used to determine the chemical compositions of the CPR, as listed in Table 1. It is noted that the CPR is a typical ceramic material composing mainly of CaO (1.29%), SiO2 (66.34%), Al2O3 (20.11%), MgO (0.95%) and Fe2O3 (0.63%). The presence of these oxides indicates that the CPR has certain

Fig. 1. SEM image of CPR.

Table 1 Chemical compositions of CPR. Oxide

CaO

SiO2

Al2O3

MgO

Fe2O3

Na2O

K2O

Weight (%)

1.29

66.34

20.11

0.95

0.63

0.49

0.61

pozzolanic reactivity. On the other hand, the cement used was a P·O 42.5 grade ordinary Portland cement. Using XRF spectrometry, the chemical compositions of the cement had been measured, as tabulated in Table 2, which all comply with the Chinese Standard GB 175–2007 [56]. Lastly, the fine aggregate used was river sand with maximum size of 1.18 mm, water absorption of 1.10% and moisture content of 0.10%. The specific gravities of the CPR, cement and fine aggregate were tested as 2.43, 3.08 and 2.58, respectively. Moreover, the grading curves of the CPR, cement and fine aggregate, obtained by a particle size analyzer for the CPR and cement or by mechanical sieving for the fine aggregate, are presented in Fig. 2. It is noted that both the CPR and cement have continuous particle size distributions. From the grading curves, the D10, D50 and D90 of the CPR are determined as 3.3, 11.5 and 39.8 μm, respectively, whereas those of the cement are determined as 3.7, 9.4 and 21.7 μm, respectively. Overall, the CPR and cement have similar particle sizes, but the CPR has a wider particle size range. During preliminary mixing trials, it was noted that the addition of CPR as cement replacement would significantly decrease the workability of the fresh mortar mix. To attain a consistent and relatively high level of workability, a polycarboxylate based superplasticizer (SP) with specific gravity of 1.03 and solid content of 20% was dosed to each mortar mixture. The SP dosage was not fixed but was adjusted such that a consistent workability was attained.

2.2. Mortar mix design The mortar mixes to be produced for performance evaluation were each intended to be the mortar portion of a concrete mix with all

Table 2 Chemical compositions of cement. Oxide

CaO

SiO2

Al2O3

MgO

Fe2O3

SO3

Weight (%)

64.28

20.70

6.01

3.19

3.14

0.80

L.G. Li et al. / Powder Technology 367 (2020) 163–171

100

spread was within the target range, and then the SP dosage so determined was applied to the respective mortar mix during the final mortar production for testing.

CPR Cement

3. Test methods 60

40

20

0 0.1

10

1000

Particle size (µm) Fig. 2. Grading curves of CPR, cement and fine aggregate.

aggregate particles larger than 1.18 mm excluded. A total of 20 mortar mixes were made for testing. For all the mortar mixes, the paste volume (volume of water + cement + CPR) to fine aggregate volume (volume of fine aggregate up to 1.18 mm) was fixed at 3:2. Four different water/ cementitious materials (W/CM) ratios of 0.25, 0.30, 0.35 and 0.40 by mass, and five different CPR contents of 0%, 5%, 10%, 15% and 20% (by mass of total cementitious materials), were adopted. The mix compositions and proportions of the mortar mixes are tabulated in Table 3. Each mortar mix was assigned a mix number “A-B”, in which A is the W/CM ratio and B is the CPR content (%), as listed in the first column of Table 3. Since the CPR has a lower density than the cement, when CPR was added to substitute an equal mass of cement, the total volume of the powder content (cement + CPR) was increased and consequently the volume of the water was decreased to keep the same paste volume. All the mortar mixes were designed to have a target workability of flow spread within 250 ± 50 mm. To achieve this workability, SP was added to each mortar mix until the measured flow spread was as close to 250 mm as possible. The actual SP dosage (mass of liquid SP as a percentage of the mass of cementitious materials) needed was determined by carrying out a preliminary mortar mixing trial. During the trial, the SP was dosed into the mortar mix little by little until the flow

Table 3 Mix proportions of mortar mixes. Mix no.

Water Cement (kg/m3) (kg/m3)

CPR Fine (kg/m3) aggregate (kg/m3)

SP dosage (%)

Percentage reduction in cement content (%)

0.25–0 0.25–5 0.25–10 0.25–15 0.25–20 0.30–0 0.30–5 0.30–10 0.30–15 0.30–20 0.35–0 0.35–5 0.35–10 0.35–15 0.35–20 0.40–0 0.40–5 0.40–10 0.40–15 0.40–20

261 259 257 255 253 288 286 284 282 280 311 309 307 305 303 331 329 327 325 323

0 52 102 153 203 0 48 95 141 187 0 44 88 131 174 0 41 82 122 162

1.20 1.60 2.25 2.60 2.85 1.00 1.30 1.80 2.10 2.30 0.70 0.95 1.20 1.40 1.70 0.40 0.60 0.90 1.05 1.25

– 5.7 11.3 17.0 22.3 – 5.6 11.2 16.7 22.1 – 5.4 11.1 16.5 21.9 – 5.4 11.1 16.5 21.9

1044 985 926 867 811 960 906 853 800 748 889 841 791 742 694 828 783 736 691 647

1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032 1032

To measure the workability of fresh mortar, a small scale conical slump flow test [57–62] was applied. Similar to the testing procedure of the conical slump flow test for concrete [63–66], the mini cone was first filled with the fresh mortar mix and lifted vertically, and then the mean diameter (mean of two diameters in perpendicular directions) of the mortar patty formed was taken as the flow spread. The mini cone used has a height of 60 mm, a base diameter of 100 mm and a top diameter of 70 mm. More details of the testing procedures can be found in previous papers [67,68]. To measure the compressive strength of hardened mortar, three 100 mm cubes were cast and cured in a water curing tank for 28 days [69]. Then, the compressive strengths of the three cubes were measured using a compression testing machine and the average compressive strength of the three cubes tested was recorded as the cube compressive strength of the mortar. To measure the chloride resistance of hardened mortar, the rapid chloride permeability test (RCPT) stipulated in the Chinese Standard GB/T 50082–2009 [70] was carried out. The equipment used and the test procedures were actually very similar to those in the American Standard ASTM C1202–19 [71], except a slight difference in the dimension of the voltage cell. More details of the testing procedures can be found in a previous paper [72]. The test results were given in terms of the RCPT total charge passed. The higher is the total charge passed, the lower is the chloride resistance and vice versa. 4. Results obtained 4.1. Cement content The cement content is plotted against the CPR content for each W/CM ratio in Fig. 3. As expected, at a given CPR content, the cement content decreased as the W/CM ratio increased. More importantly, the cement content substantially decreased as the CPR content increased. It should be noted that as the CPR was added to substitute an equal mass of cement, the total volume of the cement and CPR was increased because the CPR has a lower density than the cement, and consequently the volume of the water was decreased to keep the same paste volume (volume of cement + CPR + water). To depict the actual decrease in cement content, the percentage reduction in cement content due to the addition of CPR has been worked out

1200

Cement content (kg/m 3

Cumulative passing (%)

Fine aggregate

80

165

1000

800

W/CM = 0.25

600

W/CM = 0.30 W/CM = 0.35 W/CM = 0.40

400 0

5

10

15

CPR content (%) Fig. 3. Cement content versus CPR content at different W/CM ratios.

20

L.G. Li et al. / Powder Technology 367 (2020) 163–171

and presented in the last column of Table 3. These results show that the addition of 5%, 10%, 15% and 20% CPR would reduce the cement content by about 5.4%, 11.1%, 16.5% and 21.9%, respectively. 4.2. Workability The SP dosage added to attain the required workability is plotted against the CPR content for each W/CM ratio in Fig. 4. It is obvious that the SP dosage increased with the decrease of W/CM ratio. This is anticipated because lowering the W/CM ratio would generally decrease the workability and increase the SP demand. Moreover, the SP dosage also increased as the CPR content increased. Nevertheless, the SP dosage still remained at lower than 3%, the maximum normal dosage recommended by the supplier of the SP. The increase in SP dosage due to the addition of CPR to replace an equal mass of cement was partly because of the slight decrease in water content (see the second column in Table 3) and partly because of the slight increase in total volume of the cement and CPR, leading to significant decrease in the water/powder ratio by volume of the paste. The irregular and angular shape of the CPR particles might have also contributed to the decrease in workability and the increase in SP dosage. During the final mortar production for testing, the flow spread of the mortar mixes varied from 214 to 293 mm, as depicted in the second column of Table A1 in the Appendix. Hence, despite unavoidable variation, the measured flow spread results were all within the target range of 250 ± 50 mm. 4.3. Cube strength The 28-day cube strength results are presented in Table A1 in the Appendix, where all details, including the individual cube strength results (the results marked as #1, #2 and #3), mean and standard deviation of each mortar mix, are given. For graphical presentation, the mean cube strength is plotted against the CPR content at different W/CM ratios in Fig. 5 with error bars added to indicate the ranges of cube strength results. It is seen that at a given CPR content, the cube strength always increased with decreasing W/CM ratio. Such observed phenomenon is just as expected [73–77]. More importantly, at all given W/CM ratios, the cube strength increased markedly when 5% CPR was added to replace cement and thereafter remained at significantly higher than that with no CPR added even when up to 20% CPR was added to replace cement. In other words, adding up to 20% CPR to replace cement always increased the cube strength. For easier reference, the percentage changes in cube strength due to addition of CPR at different W/CM ratios and CPR contents have been worked out, as tabulated in the last column of Table A1. The positive

120

28-day cube strength (MPa)

166

100

80

60 W/CM = 0.25

40

W/CM = 0.30 W/CM = 0.35 W/CM = 0.40

20 0

5

10

15

Fig. 5. 28-day cube strength versus CPR content at different W/CM ratios.

changes in cube strength due to addition of CPR indicate that the CPR is an effective SCM for replacing part of the cement and improving the cube strength of the mortar or concrete. 4.4. RCPT total charge The RCPT total charge results are presented in Table A2 in the Appendix, where all details, including the actual result of each mortar mix, are given. The RCPT total charge is correlated to the CPR content in Fig. 6. As expected, at a given CPR content, the RCPT total charge always decreased with the decrease of W/CM ratio, showing that the chloride resistance was generally better at lower W/CM ratio. This observation agrees well with those in other studies [72,78,79]. However, at the same W/CM ratio, the RCPT total charge decreased dramatically as more and more CPR was added to replace cement, or in other words, the chloride resistance increased dramatically as the CPR content increased. At a W/CM ratio of 0.30, adding 10% CPR decreased the RCPT total charge from 1814 to 517C by 71.5%, whereas adding 20% CPR decreased the RCPT total charge from 1814 to 250C by 86.2%. For easy reference, the percentage changes in RCPT total charge due to the addition of CPR at different W/CM ratios and CPR contents have been worked out, as listed in the third column of Table A2. The negative changes in RCPT total charge due to addition of CPR indicate that the CPR is an effective SCM for replacing part of the cement and increasing the chloride resistance of the mortar or concrete. Also for easy reference,

6000

4.0

W/C = 0.25

W/CM = 0.25 W/CM = 0.30

W/C = 0.30

5000

RCPT total charge (C)

W/CM = 0.35

3.0

SP dosage (%)

20

CPR content (%)

W/CM = 0.40

2.0

1.0

W/C = 0.35 W/C = 0.40

4000 3000 2000 1000 0

0.0 0

5

10

15

CPR content (%) Fig. 4. SP dosage versus CPR content at different W/CM ratios.

20

0

5

10

15

CPR content (%) Fig. 6. RCPT total charge versus CPR content at different W/CM ratios.

20

L.G. Li et al. / Powder Technology 367 (2020) 163–171

the qualitative description of the chloride ion penetrability of each mortar mix, as defined in ASTM C1202 [71], is tabulated in the last column of Table A2. It can be seen that at all W/CM ratios within the range covered in this study, the addition of at least 10% CPR to replace cement would dramatically reduce the chloride ion penetrability to “Very low”. Such dramatic reduction of RCPT total charge attained by adding CPR to replace cement may be attributed to the following factors: (i) The CPR has a wider particle size range than the cement and thus its blending with the cement would help to improve the packing density of the particle system and densify the microstructure [80,81]. (ii) The CPR has a lower density than the cement and thus its addition to replace an equal mass of cement would increase the volume of the powder content (cement + CPR) available for filling the voids between aggregate particles to improve the packing density of the particle system and densify the microstructure [80,81]. (iii) The replacement of cement by CPR decreases the formation of Ca(OH)2, which decreases the pH of the pore solution and thus increases the electrical resistivity of the mortar/concrete [82]. (iv) CPR is an aluminium-containing SCM (the Al2O3 content in the CPR is up to 20.11%) and thus can increase the capacity of the cementitious materials to bind chloride ions by forming Friedel's salt [83]. 5. Cementing efficiency The above test results reveal that the CPR used has fairly high cementing efficiencies in both the compressive strength and chloride resistance. To quantify the cementing efficiency of the CPR, the cementing efficiency factor (CEF), defined as the ratio of the equivalent mass of cement to the mass of material added [46–49], is evaluated as follows. Let the CEF of CPR be k, from which the effective water to cementitious materials ratio W/CMeff may be calculated as: W=CMeff ¼

W C þ k  CPR

ð1Þ

where W, C and CPR are respectively the water content, cement content and CPR content by mass. It is assumed that the performance attribute being considered, such as the 28-day cube strength or the RCPT total charge, may be correlated by regression analysis to the effective water/cementitious materials ratio W/CMeff. Different values of k are then tried and the k-value yielding the best correlation is the CEF. 5.1. CEF of CPR in compressive strength

relatively high. Nevertheless, up to a CPR content of 20%, the CEF is always higher than 1.5, meaning that adding CPR as cement replacement is an effective way of improving the compressive strength. Particularly, at a CPR content of 10%, the CEF is equal to 1.61 and at a CPR content of 20%, the CEF is equal to 1.53. To check the applicability and accuracy of Eq. (2), the correlation between the predicted 28-day cube strength by this equation and the measured 28-day cube strength is presented in Fig. 7. The RMS error of the prediction is only 2.84%, which should be small enough to be considered acceptable for engineering applications. 5.2. CEF of CPR in chloride resistance To evaluate the CEF of CPR in chloride resistance, a new formula based on Papadakis [85] is proposed as follows: f RCPT ¼ α eβðW=CMeff Þ

ð3Þ

in which, fRCPT is the RCPT total charge (C); α and β are numerical coefficients; and e is the Euler number. During the regression analysis, different k-values are tried until the best correlation is achieved. The values of k, α, and β so obtained are given in Table 5. It is noted that the k-value is higher at a higher CPR content and lower at a lower CPR content. Particularly, the CEF is equal to 4.14 at a CPR content of 5%, equal to 6.41 at a CPR content of 10%, and equal to 7.86 at a CPR content of 20%. Hence, within the range of CPR content from 5% to 20%, the CEF of CPR in chloride resistance keeps on increasing with the CPR content and is always larger than 4.0, indicating that the addition of CPR as cement replacement is an effective way of increasing the chloride resistance. To validate the applicability and accuracy of Eq. (3), the predicted RCPT total charge using this equation is plotted against the measured RCPT total charge in Fig. 8. A fairly small RMS error of 4.28% is obtained, which should be good enough for engineering applications. 5.3. Comparison with CEFs of other materials So far, there have been very few systematic studies on the CEFs of various powder materials in the various performance attributes of the mortar/concrete produced. Nevertheless, where there are available data in the literature, the CEFs of the CPR tested in compressive strength and chloride resistance are herein compared with those of other materials obtained by other researchers.

For the purpose of evaluating the CEF of CPR in compressive strength, a modified formula based on Hobbs [46] and Smith [84] is employed: f cu ¼ α ðW=CMeff Þn þ β

167

120

in which, fcu is the 28-day cube strength (MPa); α and β are numerical coefficients; and n is an exponent. During the regression analysis, different k-values are tried until the best correlation is obtained. The values of k, α, β and n so obtained are listed in Table 4. It is evident that the kvalue decreased as the CPR content increased, indicating that the cementing efficiency of CPR in 28-day cube strength is higher when the CPR content is relatively low and lower when the CPR content is

Predicted strength (MPa)

ð2Þ

100

RMS error = 2.84% 80

CPR = 0% CPR = 5% CPR = 10% CPR = 15% CPR = 20%

60

Table 4 CEFs in 28-day cube strength. CPR content

k-value

5% 10% 15% 20%

2.23 1.61 1.57 1.53

Note: The values of α, β and n in Eq. (2) are 21.33, 2.75 and −1, respectively.

40 40

60

80

100

Measured strength (MPa) Fig. 7. Predicted 28-day strength versus measured 28-day strength.

120

168

L.G. Li et al. / Powder Technology 367 (2020) 163–171 Table 5 CEFs in RCPT total charge.

Table 7 Comparison with CEFs in chloride resistance of other materials.

CPR content

k-value

Reference

Material

CEF in chloride resistance

5% 10% 15% 20%

4.14 6.41 7.68 7.86

Lollini et al. [53]

Fly ash Ground granulated slag Micro-silica/silica fume Nano-silica Silica fume Low-Ca fly ash High-Ca fly ash Blast-furnace slag Ceramic polishing residue

1.5 1.5 2.78–9.87 4.76–12.01 6 3 2 1.3–1.9 4.14–7.86

Li et al. [78] Papadakis [85]

Note: The values of α and β in Eq. (3) are 54.25 and 11.57, respectively.

Gruyaert et al. [86] Present study

Predicted RCPT total charge (C)

6000 5000 4000

RMS error = 4.28% 3000 2000

CPR = 0% CPR = 5% CPR = 10% CPR = 15% CPR = 20%

1000 0 0

1000

2000

3000

4000

5000

6000

Measured RCPT total charge (C) Fig. 8. Predicted RCPT total charge versus measured RCPT total charge.

The CEF of the CPR in 28-day compressive strength is compared to those obtained by Hobbs [46] for pulverized fuel ash, Wong and Abdul Razak [47] for metakaolin and silica fume, Babu and Prakash [50] for silica fume, Domone [51] for limestone powder, Cyr et al. [52] for metakaolin, Lollini et al. [53] for ground granulated slag and fly ash, Li et al. [54] for micro-silica and nano-silica, and Heidari and Tavakoli [55] for ground ceramic powder in Table 6. Overall, it is seen that the CEF of the CPR in compressive strength is generally much better than limestone powder, fly ash, ground granulated slag, and ground ceramic

Table 6 Comparison with CEFs in 28-day compressive strength of other materials. Reference

Material

Hobbs [46] Wong and Abdul Razak [47] Babu and Prakash [50] Domone [51] Cyr et al. [52]

Pulverized fuel ash Metakaolin Silica fume Silica fume Limestone powder Metakaolin

CEF in 28-day compressive strength

0.18–0.45 1.6–2.3 2.1–3.1 ≈3.0 0.29 b1.0 at metakaolin content ≥40% Lollini et al. [53] Ground granulated slag 0.69 Fly ash 0.60 Li et al. [54] Micro-silica/silica fume 2.07–2.80 Nano-silica 4.30–6.05 ≤1.0 Heidari and Tavakoli [55] Ground ceramic (not quantified) powder (particle size b75 μm) 1.53–2.23 Present study Ceramic polishing residue (particle size ≈12 μm)

powder coarser than that of the CPR used herein, as good as metakaolin at metakaolin content ≤15%, but not as good as the much more expensive micro-silica and nano-silica. The CEF of the CPR in chloride resistance is compared to those obtained by Lollini et al. [53] for fly ash and ground granulated slag, Li et al. [78] for micro-silica and nano-silica, Papadakis [85] for silica fume, low-Ca fly ash and high-Ca fly ash, and Gruyaert et al. [86] for blastfurnace slag in Table 7. Overall, it is evident that the CEF of the CPR in chloride resistance is better than fly ash and ground granulated slag, almost as good as micro-silica, but not as good as nano-silica. Finally, it is envisaged that the CEF of a material is dependent on the fineness of the material. A finer material has a larger specific surface area and thus should be more reactive. Moreover, a finer material should be able to better fill into the voids between larger size particles to increase the packing density and reduce the porosity. Hence, a higher fineness material should have higher CEFs in the strength and impermeability related properties. It is therefore recommended that in future studies, the fineness or better the particle size distribution of the material should be reported and the effects of the fineness studied. Quite possibly, grinding the material to a higher fineness could significantly increase the cementing efficiency of the material, whether cementitious like metakaolin and ceramics, or inert like limestone powder. This should also increase the amount of the material that could be added as cement replacement to lower the cement content and carbon footprint. 6. Conclusions The possible reutilization of ceramic polishing residue (CPR) in mortar and concrete production so as to minimize waste disposal and reduce cement consumption and carbon footprint has been investigated by adding CPR as cement replacement and evaluating the resulting changes in compressive strength and chloride resistance. Moreover, the cementing efficiencies of the CPR in strength and durability have been quantified in terms of cementing efficiency factors (CEFs). From the above, the following conclusions are made: (i) As expected, lowering the W/CM ratio would increase the compressive strength and chloride resistance, but this would also significantly increase the SP demand and cement consumption of the mortar production. (ii) Regardless of the W/CM ratio, adding CPR as cement replacement would increase the SP demand for a given workability, slightly improve the 28-day compressive strength and substantially enhance the chloride resistance. It allows up to 20% CPR to be added to reduce the cement consumption by about 22% while improving the compressive strength and chloride resistance of the mortar produced. (iii) By regression analysis, the CEF of CPR in 28-day compressive strength has been determined as 1.61 at 10% CPR and as 1.53 at 20% CPR. Within the range of CPR content from 5% to 20%, the CEF in 28-day compressive strength is always higher than 1.5

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materials. Since a finer material has a larger specific surface area for pozzolanic reaction and should be able to better fill into voids to increase packing density [87,88], a finer material should have higher CEFs. Hence, it would be an interesting and useful topic to study the effects of fineness on the CEFs. There is a good chance that grinding the material to a higher fineness could significantly increase the CEFs to allow larger reduction in cement content while enhancing the performance of the mortar/concrete produced simultaneously.

and therefore the CPR is a highly effective supplementary cementitious material for reducing the cement content and improving the strength of the mortar produced. (iv) By regression analysis, the CEF of CPR in chloride resistance has been determined as 4.14 at 5% CPR, as 6.41 at 10% CPR and as 7.86 at 20% CPR. Within the range of CPR content from 5% to 20%, the CEF in chloride resistance is always higher than 4.0 and therefore the CPR is a highly effective supplementary cementitious material for reducing the cement content and enhancing the durability of the mortar produced. (v) Two empirical equations for predicting the 28-day cube strength and RCPT total charge of mortar with CPR incorporated as cement replacement have been developed. These equations may be applied to the preliminary design of mortar containing up to 20% CPR.

Acknowledgements This work was supported by the financial support by the National Natural Science Foundation of China (Project Nos.: 51608131), Colleges Innovation Project of Guangdong (Project No. 2017KTSCX061) and Pearl River S&T Nova Program of Guangzhou City (Project No. 201906010064).

Before closing, it is recommended to conduct more systematic studies on the cementing efficiencies of different kinds of powder Appendix A. Detailed test results Table A1 Flow spread and cube strength results. Mix no.

Flow spread (mm)

0.25–0 0.25–5 0.25–10 0.25–15 0.25–20 0.30–0 0.30–5 0.30–10 0.30–15 0.30–20 0.35–0 0.35–5 0.35–10 0.35–15 0.35–20 0.40–0 0.40–5 0.40–10 0.40–15 0.40–20

249 270 231 214 235 234 270 291 230 284 290 236 284 274 269 231 271 292 285 293

28-day cube strength (MPa)

Percentage change in 28-day cube strength (%)

#1

#2

#3

Mean

Standard deviation

84.4 89.2 91.5 90.4 97.6 69.3 86.1 81.9 81.3 84.7 63.6 70.2 68.9 72.1 69.4 52.3 61.2 55.7 58.8 64.9

91.2 95.0 88.6 91.5 93.1 76.4 79.7 80.8 84.4 82.8 65.5 68.8 67.3 68.9 70.1 58.1 57.4 58.7 60.6 69.1

88.4 95.7 90.8 93.2 96.1 74.5 83.2 83.9 85.7 85.1 67.4 68.3 65.1 68.4 65.1 54.6 56.3 59 55.2 59.5

88.0 93.3 90.3 91.7 95.6 73.4 83.0 82.2 83.8 84.2 65.5 69.1 67.1 69.8 68.2 55.0 58.3 57.8 58.2 64.5

3.4 3.6 1.5 1.4 2.3 3.7 3.2 1.6 2.3 1.2 1.9 1.0 1.9 2.0 2.7 2.9 2.6 1.8 2.7 4.8

– +6.0 +2.6 +4.2 +8.6 – +13.1 +12.0 +14.2 +14.7 – +5.5 +2.4 +6.6 +4.1 – +6.0 +5.1 +5.8 +17.3

Table A2 RCPT total charge results. Mix no.

RCPT total charge (C)

Percentage change in RCPT total charge (%)

Level of chloride ion penetrability

0.25–0 0.25–5 0.25–10 0.25–15 0.25–20 0.30–0 0.30–5 0.30–10 0.30–15 0.30–20 0.35–0 0.35–5 0.35–10 0.35–15 0.35–20 0.40–0 0.40–5 0.40–10 0.40–15 0.40–20

943 642 346 226 177 1814 1014 517 309 250 3189 1723 692 422 284 5444 2772 997 534 378

– −31.9 −63.3 −76.0 −81.2 – −44.1 −71.5 −83.0 −86.2 – −46.0 −78.3 −86.8 −91.1 – −49.1 −81.7 −90.2 −93.1

Very low Very low Very low Very low Very low Low Low Very low Very low Very low Moderate Low Very low Very low Very low High Moderate Very low Very low Very low

170

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