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Influence of waste clay bricks as fine aggregate on the mechanical and microstructural properties of concrete
Construction and Building Materials 228 (2019) 116757
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of waste clay bricks as fine aggregate on the mechanical and microstructural properties of concrete Juntao Dang a, Jun Zhao b,⇑ a b
School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China School of Mechanics and Engineering Science, Zhengzhou University, Zhengzhou 450001, China
h i g h l i g h t s Different additional water volumes of waste clay bricks (WCBF) were added into concrete. Macroscopic and microscopic characterization of concrete with WCBF were studied. Mechanical properties of concrete deteriorated with increasing of additional water. WCBF are effective in enhancing the compactness of interfacial transition zone.
a r t i c l e
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Article history: Received 18 March 2019 Received in revised form 28 July 2019 Accepted 18 August 2019
Keywords: Waste clay brick Fine aggregate Recycled concrete Mechanical properties Microstructure
a b s t r a c t To satisfy the demand of modern concrete and environmentally friendly society, the value-added utilization of waste clay bricks could be effectively improved without sacrificing the strength of sustainable concrete. This paper investigated the influence of variable replacement ratios (0%, 25%, 50%, 75% and 100%) and additional water volumes (no extra water, partially extra water and totally extra water) of fine aggregates made of waste clay bricks (WCBF) on the properties of recycled concrete (RBC). Based on the pore structure, microscopic morphology and chemical element of microscopic test results, the mechanism of the WCBF on the properties of concrete were analyzed. The results indicated that the density of RBC was gradually reduced with the increasing of additional water volume and replacement ratio of WCBF. The compressive strength of RBC with no extra water and partially extra water within 50% replacement ratio was comparable with normal concrete (NC), but the compressive strength of RBC with fully extra water and partially extra water beyond 50% replacement ratio was degraded. The splitting tensile strength of RBC was comparatively increased or similar with the decrease of additional water volume, while the RBC yielded lower elastic modulus especially beyond 50% replacement ratio. The microscopic results revealed that the RBC porosity and total pore volume, as well as the pore content with diameter higher than 100 nm of RBC were increased. In addition, the interfacial transition zone was compacted because of the pozzolanic activity together with the penetration of cement matrix into the surface of the WCBF by about 120 mm. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The booming development of the global economy and construction industry, as well as the frequent occurrence of natural disasters have generated tremendous amounts of construction and demolition wastes (CDW), which contain concrete, brick, glass, plastic and ceramic [1–4]. Particularly in China, with the accelerated urbanization, the generation of CDW from rural region was
⇑ Corresponding author at: School of Mechanics and Engineering Science, Zhengzhou University, Science Road 100, Zhengzhou 450001, Henan, China. E-mail address: [email protected] (J. Zhao). https://doi.org/10.1016/j.conbuildmat.2019.116757 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
mainly composed of waste clay bricks and waste mortar. The traditional way of disposing CDW was physical landfill, which resulted in a secondary pollution of the environment. Furthermore, the urbanization process has facilitated the requirement for construction, and numerous building materials (such as concrete, mortar and brick) were consumed. Additionally, the considerable quantities of non-renewable resource (such as virgin aggregate and cement) were depleted which further aggravated the burden of environment [5]. Currently, the natural aggregate and cement were substituted by the recycled materials like supplementary cementitious materials [6–8], recycled fine aggregate [9–12], and recycled coarse aggregate [13–16] which were able to prepare
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sustainable concrete or mortar after processing. These methods not only resulted in the reduction of excessive excavation of natural aggregate, which contributed to alleviate the hazard of environment and reduced the CO2 emission, but also satisfied the requirement of construction and expansion of urbanization. In recent years, the different particle sizes of waste clay bricks as alternative materials were prepared by sorting, screening and crushing like recycled coarse aggregate (with particles larger than 5 mm in diameter) [17,18], recycled fine aggregate (with particles lower than 5 mm in diameter) [19,20], and supplementary cementitious material (with particles lower than 0.125 mm in diameter) [21,22]. Zong et al. [23] researched the influence of the replacement ratio (30%, 40% and 50%) of recycled coarse aggregate from waste bricks on the mechanical properties and durability. It was concluded that due to the lower hardness of waste clay bricks, a considerable reduction on RBC strength were achieved. Rashid et al. [24] studied the fresh and compressive strength of recycled concrete prepared with coarse ceramic aggregate. It was showed that the incorporation of coarse ceramic aggregate with 30% replacement ratio into concrete obtained the higher compressive strength but lower environmental impact, which satisfied the requirement of sustainable recycled concrete. Additionally, Ge et al. [25] and Qu et al. [26] investigated that the recycled waste clay powder partially replaced the cement without sacrificing the performance because of the pozzolanic activity of micro-powder from waste clay bricks. With respect to the fine ceramic recycled aggregates and fine aggregate from waste clay bricks (WCBF), Siddique et al. [27] studied the effect of different replacement ratios (0–100%) of fine ceramic aggregate on the mechanical properties of recycled concrete with various water to binder ratio. It was found that fine ceramic aggregate resulted in an increase of mechanical strength and enhanced the generation of hydration products, which indicated that fine ceramic aggregate was feasible for preparing the concrete. Alves [28] and Vieira [29] have evaluated the performance of RBC with incorporation of fine ceramic aggregate with 20%, 50% and 100% replacement ratio. The result illustrated that the performance of concrete with fine ceramic aggregate was in accordance with the NC. However, Gonzalez-Corominas and Etxeberria [30] shown that compared to the NC, the RBC with less than 30% replacement ratio of fine ceramic aggregate resulted in the similar performance or slightly improved the performance of concrete. Dang et al. [5] have studied the effect of particle size distribution (PSD) and water content of WCBF on the properties of mortar. The result revealed that the WCBF with partially water content and 0–5 mm PSD had a beneficial improvement on the strength, while the WCBF with totally water content and 0.15–5 mm PSD had an adverse effect on the strength. In general, the waste clay bricks were simply backfilled due to the substantial defect of coarse aggregate from waste clay bricks, which was not suitable for preparation of concrete. On the other hand, both the economic value and the utilization rate were reduced due to the comparatively insufficient research with respect to the WCBF. Hence, the influences of variable replacement ratios (0%, 25%, 50%, 75% and 100%) and additional water volumes (no extra water, partially extra water and totally extra water) on the properties of concrete at different ages have been investigated. Based on the mechanical characterization of concrete, the microscopic tests have been carried out to further analyze the mechanism of the WCBF on recycled concrete. 2. Experimental investigations 2.1. Materials In this study, the crushed stone as coarse aggregate mainly constituted of limestone, as well as possessed the consecutive gradation of 5–20 mm, the water absorption of 0.9% and the apparent density of 2771 kg/m3. The natural fine aggre-
gate (NF) was river sand and the physical properties are presented in Table 1. In addition, Table 2 shows the chemical composition of cement and WCBF, while Table 3 shows the mechanical properties of 42.5 Ordinary Portland cement. Both the mixing water and additional water used for casting concrete were tap water. The slump of the NC was adjusted to 180 ± 20 mm by incorporating the polycarboxylate superplasticizer about 5.14 kg/m3. The WCBF included clay bricks and waste mortar derived from the CDW of masonry structure as shown in Fig. 1. The physical properties and chemical composition of the WCBF are illustrated in Tables 1 and 2, respectively. The grading curves of WCBF and NF are presented in Fig. 2. It can be observed that the fineness modulus of WCBF after processed is comparably consistent with the NF. While after mechanical crushing, the micro-powder content of WCBF is considerably increased up to 18.2% compared with the NF. The microscopic morphology of the WCBF through the SEM image is showed in the Fig. 3. It can be concluded that WCBF formed rough surface and loose texture.
2.2. Mixing proportion and specimen preparation As shown in Table 4, the mix proportion of concrete is designed in compliance with JGJ55-2011 [31]. The water cement (w/c) ratio of concrete is 0.35. The NF is substituted by WCBF at the replacement ratio of 0%, 25%, 50%, 75% and 100% by absolute volume, which can be represented by W0, W25, W50, W75 and W100. The additional water volume is determined by the water absorption of WCBF, which is represented by D (no extra water was the dry state of WCBF), T (totally extra water was 100% saturated surface dry state of WCBF), and P (partially extra water was 75% saturated surface dry state of WCBF). For example, CW25T represents the specimen of 25% replacement ratio, and totally extra water volume of WCBF. The mixing procedure of NC and RBC is presented in Fig. 4. The total water of recycled concrete included mixing water for conventional concrete and additional water volume of WCBF. The specimen was compacted by vibrating table. After casting, the specimen was kept a day in ambient temperature. Then the demolded specimens moved to the curing room (95% relative humidity and 20 ± 2 °C temperature).
2.3. Testing methods 2.3.1. Density The density of concrete was carried out in accordance with BS EN 12390-7 [32], which measured the saturated density and oven-dry density at 28 and 90 days.
2.3.2. Investigation of the mechanical properties The mechanical properties of concrete were evaluated according to GB/T500812002 [33], including the compressive strength, splitting tensile strength and modulus of elasticity of concrete. The compressive strengths of concrete at 3 d, 7 d, 14 d, 28 d, 56 d and 90 d were determined by cubic samples of 100 mm 100 mm 100 mm. The splitting tensile strengths of concrete at 28 d and 90 d were assessed by cubic samples of 100 mm 100 mm 100 mm. The modulus of elasticity of concrete at 28 d and 90 d were conducted on prismatic samples of 150 mm 150 mm 300 mm. 2.3.3. Investigation of the microstructure After 90 days’ curing, the dimension of 3–5 mm sample without coarse aggregate was taken out from the core of concrete specimen and immediately soaked in anhydrous alcohol to stop hydration. Then the sample was dried in the oven at 50 °C for 72 h before testing. The pore structure of concrete was characterized by mercury intrusion apparatus (MIP) with the tested pore size range of 0.003–1000 mm. After spraying with carbon, the microscopic morphology and interfacial transition zone (ITZ) of concrete were observed by scanning electron microscope (SEM) at different magnification. The elemental analysis of concrete was measured by energy dispersive X-ray spectroscopy (EDS) at the accelerating voltage of 20 kV based on the SEM. The sample was grounded, polished and sprayed with carbon before quantitative line scanning.
Table 1 Physical properties of fine aggregates. Sample
NF
WCBF
Density (kg/m3) Loose bulk density (kg/m3) Compact bulk density (kg/m3) Loose porosity (%) Compact porosity (%) Crush index (%) Saturated surface dry absorption (%) Fineness modulus SO3 content (%)
2596 1474 1560 43.2 39.9 6 1.1 2.86 0.2
2548 1170 1305 54.1 48.8 39.4 8.6 2.9 0.56
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J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757 Table 2 The chemical composition of cement and WCBF.
Cement WCBF
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
K2O
TiO2
19.24 66.26
4.08 16.42
3.25 6.18
62.47 5.97
4.81 1.36
4.19 –
1.38 2.63
– 0.93
Table 3 Properties of 42.5 Ordinary Portland cement. Setting time/min Initial setting 158
Final setting 198
Flexural strength/MPa
Compressive strength/MPa
3d 6
3d 23.6
28 d 8.7
28 d 48.9
Fig. 3. The SEM image of WCBF.
Fig. 1. The WCBF with the particle size of 0–5 mm.
especially on the oven-dry density. Furthermore, the density of RBC is slightly declined with the increasing of replacement ratio. In addition, the additional water volume of WCBF plays a crucial role in the RBC density. It can be clearly suggested that there is a liner reduction in RBC density with the increasing of additional water volume. The influence of the WCBF with no extra water on the density of RBC is negligible. However, compared to the NC, the saturated density and oven-dry density of RBC comparatively decreased with the increasing of additional water volume, achieving a maximum reduction of 6% and 5% respectively, when totally extra water was 100% SSD (T). On the one hand, this is attributed to the porosity of both waste brick and mortar yielded an inferior density of WCBF than the NF, resulting in a lower density of RBC. On the other hand, the effective w/c ratio is responsible for the density of concrete. The effective w/c ratio of RBC is gradually increased with the increasing of additional water volume, thus reducing the compactness of RBC and eventually decreasing the density of RBC. 3.2. Compressive strength
Fig. 2. Grading curves of fine aggregates.
3. Results and discussion 3.1. Density Fig. 5 presents the effect of WCBF with different additional water volumes and replacement ratios on density of concrete. At 28 and 90 days, the oven-dry density and saturated density of NC are 2328–2331 kg/m3 and 2416–2419 kg/m3, respectively, while the oven-dry density and saturated density of RBC are 2180– 2299 kg/m3 and 2299–2386 kg/m3, respectively. It can be observed that NC with higher density is less influenced by curing age, but the RBC with lower density is affected by curing age to some extent,
The effect of WCBF on the compressive strength of concrete is illustrated in Fig. 6 and Table 5. As shown in the figure, the increasing tendency of compressive strength of RBC is consistent with the NC, which increases with the increase of curing age. The different additional water volumes of WCBF have a remarkable impact on the compressive strength compared to replacement ratio. In general, the compressive strength of RBC with no extra water of WCBF is the maximum, followed by RBC with partially extra water of WCBF, being the compressive strength of RBC with fully extra water of WCBF the minimum. When the WCBF with no extra water is incorporated in the concrete, the RBC with 25% replacement ratio is slightly increased by 6% and 9% respectively at 28 and 90 days compared to the NC. As the replacement ratio increases up to 50% of WCBF with no extra water, the compressive strength of RBC is comparable to the NC. Nevertheless, the compressive strength of RBC is gradually decreased with the increase of additional water volume. When the additional water volume of WCBF is partially extra water, the compressive strength of RBC within 50% replacement ratio is
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Table 4 Mix proportion of concrete (kg/m3). Sample
W/C
Water (kg)
Cement (kg)
Coarse aggregate (kg)
River sand (kg)
WCBF (kg)
Additional water (kg)
Wa/C
CW0 CW25D CW50D CW25T CW50T CW75T CW100T CW25P CW50P CW75P CW100P
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
180 180 180 180 180 180 180 180 180 180 180
514 514 514 514 514 514 514 514 514 514 514
1052 1052 1052 1052 1052 1052 1052 1052 1052 1052 1052
672 504 336 504 336 168 0 504 336 168 0
0 165 330 165 330 495 660 165 330 495 660
0 0 0 14 28 43 57 11 21 32 43
0 0 0 0.03 0.06 0.08 0.11 0.02 0.04 0.06 0.08
Note: Wa is additional water introduced to WCBF.
Fig. 4. The mixing procedure of concrete, (a) recycled concrete, (b) normal concrete.
Fig. 5. Effect of WCBF on density of concrete, (a) oven-dry density, (b) saturated density.
Fig. 6. Effect of WCBF on compressive strength of concrete.
approximately identical to the NC. For the replacement ratio beyond 50%, there is a gradual reduction of compressive strength. At 28 and 90 days, the maximum reduction of compressive strength of RBC is 7% and 6%, respectively. When the additional water volume of WCBF is fully extra water, the compressive strength of RBC is decreased by a maximum of 11% and 8% respectively at 28 and 90 days compared to NC. This is due to the WCBF with no extra water can absorb free water from the internal concrete, thus reducing the effective w/c ratio. Whilst the incorporation of WCBF may create the pozzolanic activity, generating more stable and uniform hydration products, as well as leading to the formation of a compactness ITZ and dense cement matrix. Moreover, the combination of the irregular surface of WCBF and the penetration of the cement matrix to the surface of the WCBF contribute to enhance the adhesion force of ITZ further, which is fundamental to improve the compressive strength. This is confirmed by the microscopic morphology of the ITZ from SEM test.
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J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757 Table 5 Mechanical properties of concrete at different ages. Sample
CW0 CW25D CW50D CW25T CW50T CW75T CW100T CW25P CW50P CW75P CW100P
Compressive strength (MPa)
Splitting tensile strength (MPa)
Modulus of elasticity (GPa)
3d
7d
14 d
28 d
56 d
90 d
28 d
90 d
28 d
90 d
52.8 57.6 54.4 47.3 47.5 47.8 46.2 48.9 51.9 50.3 49.0
53.2 58.0 57.7 49.6 52.2 52.8 50.1 53.9 55.8 54.2 54.9
56.6 63.8 60.9 50.3 54.4 56.6 53.7 57.3 59.9 60.9 58.0
63.4 67.5 64.3 56.4 59.1 59.4 58.8 60.7 60.8 61.6 59.0
63.8 70.4 65.5 59.5 59.8 59.7 58.8 62.1 63.4 63.6 59.4
66.9 73.1 67.9 62.5 65.0 61.9 61.7 66.8 68.5 64.5 62.9
4.65 5.15 5.08 4.75 4.80 4.87 4.77 4.77 4.69 4.80 4.85
4.80 5.20 5.10 4.93 4.90 4.91 4.87 4.87 4.90 4.87 5.00
35.4 34.7 34.5 31.5 30.5 30.3 31.6 32.0 30.6 32.4 31.5
41.2 41.8 41.3 39.4 38.5 37.2 33.0 38.4 40.1 37.7 34.6
However, with the increasing of additional water volume and replacement ratio, the effective w/c ratio of RBC is progressively increased. Therefore, the influence of the inferior properties of WCBF is gradually increased so as to offset the beneficial effect of WCBF, thus leading to a degradation of the compressive strength. In addition, as shown in Table 6 and Fig. 3, the significant increase of total pore volume and porosity are justified by the presence of WCBF, which possess both porous structure and the micro-crack because of the mechanical crush. Hence, the compressive strength of RBC is reduced. 3.3. Splitting tensile strength Fig. 7 and Table 5 display the influence of WCBF on the splitting tensile strength of concrete at different ages. As shown in figure, the increasing trend of splitting tensile strength of RBC is in accordance with NC. Moreover, the splitting tensile strength of RBC is slightly increased at 90 days. In general, it is worthwhile to noted that the splitting tensile strength of RBC is comparatively enhanced compared to NC. This is attributed to the rough irregular surface of WCBF, which improves the adhesion force and compactness of ITZ. The different additional water volumes of WCBF have slightly effect on the splitting tensile strength of RBC. As shown in the Fig. 7, the splitting tensile strength of RBC incorporating no extra water of WCBF is the maximum, followed by the RBC with partially extra water of WCBF, being the splitting tensile strength of RBC with fully extra water of WCBF the minimum. When the WCBF with no extra water is incorporated in the RBC, the splitting tensile strength of RBC is relatively higher than that of NC, but there is a negligible reduction of the splitting tensile strength with increasing of replacement ratio. At 28 and 90 days, the splitting tensile strength of RBC is maximally increased by 11% and 8%, respectively. When the additional water volume of WCBF is partially extra water, the splitting tensile strength of RBC is increased by a maximum of 4%. While the addition water volume is increased up to fully extra water, the splitting tensile strength of RBC is increased by 5% and 3% maximally at 28 and 90 days. This result
Fig. 7. Effect of WCBF on splitting tensile strength of concrete.
shows that the splitting tensile strength of RBC is marginally decreased with the increasing of additional water volume. The favorable performance of RBC with WCBF is attributed to three reasons as follows. First, the porous structure of WCBF can absorb part of the free water, which reduces the effective w/c ratio and improves the hardness of cement matrix and ITZ. Second, the pozzolanic activity of WCBF contributes to the formation of hydration product, which can enhance the mechanical interlocking of interface. Finally, the micro-powder of WCBF enables to fill the void and crack in the vicinity of WCBF, which can promote the generation of a dense structure. Hence, the multiple effects of WCBF overwhelm the detrimental effect of the porous structure of itself, which improves the splitting tensile strength. This result is further demonstrated by the close interlocking between the WCBF and the cement matrix from the SEM observation.
Table 6 Result of MIP analysis for concrete at 90 days. Samples
CW0 CW50D CW50T CW100T CW50P CW100P
Total pore volume (mL/g)
0.0577 0.062 0.077 0.0916 0.0622 0.079
Porosity (%)
12.57 13.02 15.28 17.51 13.11 15.66
Pore size distribution (mL/g) <4.5 nm
4.5–50 nm
50–100 nm
>100 nm
0.0037 0.0035 0.0041 0.0048 0.0029 0.0037
0.0215 0.0181 0.0305 0.0405 0.0223 0.0302
0.0038 0.0086 0.0025 0.0023 0.0020 0.0039
0.0287 0.0318 0.0399 0.0440 0.0350 0.0412
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3.4. Modulus of elasticity Fig. 8 and Table 5 depict the effect of WCBF on the elastic modulus of concrete at different ages. It can be observed that the increasing trend of elastic modulus of RBC is consistent with NC. At 28 days, compared to the NC, the replacement ratio of WCBF has negligible effect on the elastic modulus, while the elastic modulus of RBC is slightly affected by the WCBF with different additional water volumes. With the increasing of curing age, the elastic modulus of RBC is gradually increased. Moreover, the influence of replacement ratio and additional water volume on the elastic modulus of RBC is progressively obvious. It is noted that there is a linearly worsening tendency on the elastic modulus of RBC in compliance with the order of no extra water, partially extra water and fully extra water of WCBF. When the additional water volume of WCBF is no extra water, the elastic modulus of RBC is comparable with NC. Because of the WCBF with higher absorption capacity, there is a reduction of the effective w/c ratio of RBC, which basically compensates the degradation induced by the porous characteristic of the WCBF. However, it is clearly observed that the elastic modulus of RBC is gradually decreased with the increasing of additional water volume. When the additional water volume of WCBF is partially extra water, the elastic modulus of RBC is marginally decreased by 3% with the replacement ratio increases up to 50% at 90 days compared to the NC. However, when the replacement ratio increases up to 100%, the elastic modulus of RBC is comparatively reduced by 16% compared to the NC. When the additional water volume of WCBF is fully extra water, the elastic modulus of RBC exhibits a linearly descending trend with the increasing of replacement ratio at 90 days. And the WCBF with fully extra water has a little adverse impact on the elastic modulus of concrete with the replacement ratio no larger than 50%, while the elastic modulus of RBC with 100% replacement ratio can be achieved a maximum reduction of 20% at 90 days. Generally, the poorer properties of WCBF result in both weak stiffness and lower strength, which may be responsible for the deterioration of elastic modulus of RBC. In addition, the effective w/c ratio of RBC is gradually increased with the increasing of additional water volume, which increases the porosity of RBC, thus resulting in an abruptly decrease in the elastic modulus of RBC. Although the WCBF can enhance the compactness of ITZ and cement matrix, it cannot compensate the defect caused by its own porous structure. Particularly, when the replacement ratio is beyond 50%, the WCBF has a considerably unfavorable impact on
Fig. 8. Effect of WCBF on modulus of elasticity of concrete.
the elastic modulus of RBC. As shown in Table 6, based on the pore structure of concrete by MIP test, the incorporation of WCBF brings about a significantly increase in the pore content with diameter higher than 100 nm, which plays a predominant role in the reduction of elastic modulus of recycled concrete.
3.5. Investigation of the microstructure 3.5.1. Mercury intrusion porosimetry The effect of WCBF on the pore structure of recycled concrete at 90 days is presented in Table 6. As shown in the table, the porosity and total pore volume of NC is 12.57% and 0.0577 mL/g, respectively, while the porosity and total pore volume of RBC presents the values of 13.02–17.51% and 0.062–0.0916 mL/g, respectively. It is noted that the porosity and total pore volume of RBC increases with the addition of WCBF. The porosity and total pore volume of CW50D and CW50P increases by 8% and 4% compared to NC. Nevertheless, with the increase of additional water volume, the pore characteristic of recycled concrete is obviously worsened. The total pore volume and porosity of CW50T is maximum increased by 33% and 22%, respectively, compared to NC. When the replacement ratio of WCBF is increased up to 100%, the porosity and total pore volume of recycled concrete are increased prominently. Compared to the NC, the total pore volume and porosity of CW100P are increased by 37% and 25%. When the additional water volume of WCBF is rose up to the fully extra water, the total pore volume and porosity of CW100T are characterized by a maximum increase of 59% and 39%, respectively. It can be illustrated that the WCBF has prominent detrimental effect on the pore characteristic of RBC. Furthermore, the pore structure of RBC is gradually deteriorated with the increase of additional water content and replacement ratio. Fig. 9 shows the effect of WCBF on cumulative pore volume of concrete. The cumulative pore volume curves of CW50D and CW50P are basically consistent with the normal concrete. The WCBF is preferably embedded into the cement matrix with low w/c ratio, thus improving the compactness of ITZ and impeding the entry of mercury. However, compared to the NC, the cumulative pore volume curves of CW50T, CW100P and CW100T are overall upward shifted. The result exhibits that the deterioration of pore characteristic is proportional to both the replacement ratio and additional water volume of WCBF. According to the investigation of Metha and Monteiro [34], the pore size distribution of concrete is classified according to the following pore diameter intervals: less than 4.5 nm, 4.5–50 nm, 50–100 nm, and higher than 100 nm. As shown in Table 6, it is noted that the pore content
Fig. 9. Effect of WCBF on cumulative pore volume of concrete.
J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757
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Fig. 10. The SEM images of normal concrete at 90 days, (a) magnified 500 on CW0, (b) magnified 2000 on CW0.
with diameter higher than 100 nm is prominently increased with the addition of WCBF. When the WCBF replacement ratio is 50%, the pore content with diameter higher than 100 nm of CW50D, CW50P and CW50T is increased by 11%, 22% and 39% respectively compared to the normal concrete. When the replacement ratio of WCBF is increased up to 100%, the pore content with diameter higher than 100 nm of CW100P and CW100T is significantly increased by 44% and 53% respectively compared to the normal concrete. It can be illustrated that the pore content with diameter higher than 100 nm of RBC is substantial higher than NC due to the presence of WCBF. 3.5.2. Scanning electron microscope Figs. 10 and 11 present the ITZ microscopic morphology of normal concrete and recycled concrete at 90 days. It is noted that there is an obvious micro-crack between NF and cement matrix because the micro-bleeding of NF forms a relative higher w/c ratio around the ITZ. Nevertheless, there is no visible distinctive debonding between WCBF and cement matrix, which is substantially different to the ITZ between NF and cement matrix. This is attributed to the porous structure of WCBF as well as the mitigation of wall effect caused by the absorption of free water around the ITZ. Therefore, the ITZ between WCBF and cement matrix is well-bonded together, thus enhancing the compactness of ITZ. Through the observation with different magnification, it is clearly indicated that the WCBF is wrapped or embedded in the cement matrix, which is tightly integrated with the cement matrix. Compared to the inert NF, the pozzolanic reaction between WCBF and Ca(OH)2 from the cement matrix contributed to the formation of hydration products (C-S-H, AFt and C-A-H) which uniformly wrapped on the surface of WCBF and filled in the vicinity of cement matrix and ITZ. In addition, the powder of WCBF not only has a better micro-filling effect, but also has a larger specific surface area
which is readily penetrated by the alkaline solution, leading to adequately exert on pozzolanic reactivity. Additionally, the rough irregular surface of WCBF effectively enhances the mechanical interlocking of ITZ between WCBF and cement matrix, which is essential in improving the splitting tensile strength. However, only an amount of hydration products is detected in a certain range on the surface of WCBF. It can be deduced that the limit depth of WCBF is penetrated by cement matrix, which is not effectively penetrated in the core of WCBF. Therefore, the detrimental effect on the strength of RBC cannot be offset sufficiently by the modified boundary of WCBF, which further demonstrate that the WCBF result in the deterioration of the pore structure of recycled concrete. The most prominent representation of deterioration is given by the increase in pore content of pore diameter higher than 100 nm. 3.5.3. Energy dispersive X-ray spectroscopy Figs. 12 and 13 present chemical element analysis around the ITZ of concrete. As shown in Fig. 12, it is noted that the element composition is obviously distinguished in the adjacent NF for normal concrete. The NF mainly contains Si element, but the cement matrix includes Si, Al and Ca element. Compared to the normal concrete, there is no variation in the element types of CW50P, while the WCBF contains a large amount of Si and Al elements. This is consistent with the result of the X-ray fluorescence test analysis. The chemical composition of SiO2 and Al2O3 in the WCBF account for 82.68% as presented in Table 2. In addition, as shown in the Fig. 13, the distribution of Si and Al element components in the WCBF is gradually decreased from the interior of WCBF to the cement matrix, while the content of Ca element is progressively stabilized from the boundary of WCBF to the cement matrix. Furthermore, it is clearly observed that a relative amount of Ca element is well concentrated on the boundary of
Fig. 11. The SEM images of recycled concrete at 90 days, (a) magnified 500 on CW50P, (b) magnified 2000 on CW50P.
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Fig. 12. The SEM image and element distribution of concrete with CW0.
Fig. 13. The SEM image and element distribution of concrete with CW50P.
the WCBF compared to the NF. Therefore, it is evident that the Ca element component is penetrated from cement matrix to the WCBF about 120 mm. This is attributed to the porous structure and rough surface of WCBF generates the adhesion force to the cement slurry, which causes the cement slurry to penetrate into the WCBF. Whilst the pozzolanic reactivity between the active component of WCBF and Ca(OH)2 from cement hydration will take place near the boundary of WCBF, thus facilitating the generation of the C-A-H and C-S-H of hydration products within the reaction rim. Moreover, this result is basically consistent with SEM images from Fig. 11, which suggest that the amounts of hydration products of Ca element fill the void in a certain thickness in the vicinity of WCBF. Hence, due to the multiple effect of mechanical interlocking and the homogeneous hydration product from pozzolanic reaction, the strength of ITZ is enhanced, and the porosity and micro-crack in the thickness of the WCBF rim is filled. These beneficial effects are fundamental to obtain the acceptable mechanical properties of RBC, especially the enhancement of splitting tensile strength. 4. Conclusion This paper studied the influence of additional water volume and replacement ratio of WCBF on the density and mechanical properties. Based on the SEM, EDS and MIP test results, the morphology of
microstructure, the chemical element analysis and the pore structure of concrete have been researched. (1) The density of RBC is gradually declined with the increasing of replacement ratio. The influence of the WCBF with no extra water on the concrete density is negligible, but the density of RBC is comparatively reduced with the increasing of additional water volume. (2) The additional water volume and replacement ratio of the WCBF play a predominant role in the compressive strength. The compressive strength of RBC with no extra water of WCBF is similar or higher than the NC. When the additional water volume of the WCBF is partially extra water, the compressive strength of RBC within 50% replacement ratio is comparable with NC, while the compressive strength of RBC beyond 50% replacement ratio is slightly decreased. Additionally, the compressive strength of RBC with the fully extra water of WCBF results in a maximum reduction of 8%. (3) The incorporation of WCBF marginally strengthens the splitting tensile strength, while the WCBF with no extra water enhances the splitting tensile strength of RBC with the maximum increase of 8%. (4) The WCBF within 50% replacement ratio has minor effect on the elastic modulus of concrete. But with the increasing of additional water volume and replacement ratio, the WCBF
J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757
decrease the elastic modulus of concrete with a maximum loss of 20%. (5) The porosity and total pore volume of recycled concrete is significantly increased with the incorporation of WCBF, especially the pore content of pore diameter higher than 100 nm. Whilst the satisfactory compactness of ITZ between the WCBF and cement matrix is improved due to the rough irregular surface and pozzolanic activity of WCBF. In addition, the cement matrix can effectively penetrate into the surface of the WCBF by about 120 mm, which promotes the generation of more stable and dense hydration products and enhances the strength and compactness of the ITZ.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Research reported here was supported by the National Key R&D Program of China (2016YFE0125600) and Program for Innovative Research Team of Education Ministry of China (IRT_16R67). References [1] X.S. Shi, F.G. Collins, X.L. Zhao, Q.Y. Wang, Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete, J. Hazard. Mater. 237–238 (2012) 20–29. [2] M. Bravo, J. de Brito, L. Evangelista, J. Pacheco, Durability and shrinkage of concrete with CDW as recycled aggregates: benefits from superplasticizer’s incorporation and influence of CDW composition, Constr. Build. Mater. 168 (2018) 818–830. [3] C. Alexandridou, G.N. Angelopoulos, F.A. Coutelieris, Mechanical and durability performance of concrete produced with recycled aggregates from Greek construction and demolition waste plants, J. Clean. Prod. 176 (2018) 745–757. [4] H.J. Du, K.H. Tan, Use of waste glass as sand in mortar: part I – Fresh, mechanical and durability properties, Cem. Concr. Compos. 35 (2013) 109– 117. [5] J.T. Dang, J. Zhao, W.Y. Hu, Z.H. Du, D.Y. Gao, Properties of mortar with waste clay bricks as fine aggregate, Constr. Build. Mater. 166 (2018) 898–907. [6] M.Z. Chen, J.T. Lin, S.P. Wu, C.H. Liu, Utilization of recycled brick powder as alternative filler in asphalt mixture, Constr. Build. Mater. 25 (2011) 1532– 1536. [7] Q. Liu, T. Tong, S.H. Liu, S.H. Liu, D.Z. Yang, Q. Yu, Investigation of using hybrid recycled powder from demolished concrete solids and clay bricks as a pozzolanic supplement for cement, Constr. Build. Mater. 73 (2014) 754–763. [8] H.J. Du, S.D. Pang, Value-added utilization of marine clay as cement replacement for sustainable concrete production, J. Clean. Prod. 198 (2018) 867–873. [9] I.H. Song, J.S. Ryou, Hybrid techniques for quality improvement of recycled fine aggregate, Constr. Build. Mater. 72 (2014) 56–154. [10] J.A. Bogas, J. de Brito, D. Ramos, Freezee-thaw resistance of concrete produced with fine recycled concrete aggregates, J. Clean. Prod. 115 (2016) 294–306. [11] L. Li, J.Z.J.X.Lu. Bao, C.S. Poon, Systematic evaluation of the effect of replacing river sand by different particle size ranges of fine recycled concrete aggregates (FRCA) in cement mortars, Constr. Build. Mater. 209 (2019) 147–155.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of waste clay bricks as fine aggregate on the mechanical and microstructural properties of concrete Juntao Dang a, Jun Zhao b,⇑ a b
School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China School of Mechanics and Engineering Science, Zhengzhou University, Zhengzhou 450001, China
h i g h l i g h t s Different additional water volumes of waste clay bricks (WCBF) were added into concrete. Macroscopic and microscopic characterization of concrete with WCBF were studied. Mechanical properties of concrete deteriorated with increasing of additional water. WCBF are effective in enhancing the compactness of interfacial transition zone.
a r t i c l e
i n f o
Article history: Received 18 March 2019 Received in revised form 28 July 2019 Accepted 18 August 2019
Keywords: Waste clay brick Fine aggregate Recycled concrete Mechanical properties Microstructure
a b s t r a c t To satisfy the demand of modern concrete and environmentally friendly society, the value-added utilization of waste clay bricks could be effectively improved without sacrificing the strength of sustainable concrete. This paper investigated the influence of variable replacement ratios (0%, 25%, 50%, 75% and 100%) and additional water volumes (no extra water, partially extra water and totally extra water) of fine aggregates made of waste clay bricks (WCBF) on the properties of recycled concrete (RBC). Based on the pore structure, microscopic morphology and chemical element of microscopic test results, the mechanism of the WCBF on the properties of concrete were analyzed. The results indicated that the density of RBC was gradually reduced with the increasing of additional water volume and replacement ratio of WCBF. The compressive strength of RBC with no extra water and partially extra water within 50% replacement ratio was comparable with normal concrete (NC), but the compressive strength of RBC with fully extra water and partially extra water beyond 50% replacement ratio was degraded. The splitting tensile strength of RBC was comparatively increased or similar with the decrease of additional water volume, while the RBC yielded lower elastic modulus especially beyond 50% replacement ratio. The microscopic results revealed that the RBC porosity and total pore volume, as well as the pore content with diameter higher than 100 nm of RBC were increased. In addition, the interfacial transition zone was compacted because of the pozzolanic activity together with the penetration of cement matrix into the surface of the WCBF by about 120 mm. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The booming development of the global economy and construction industry, as well as the frequent occurrence of natural disasters have generated tremendous amounts of construction and demolition wastes (CDW), which contain concrete, brick, glass, plastic and ceramic [1–4]. Particularly in China, with the accelerated urbanization, the generation of CDW from rural region was
⇑ Corresponding author at: School of Mechanics and Engineering Science, Zhengzhou University, Science Road 100, Zhengzhou 450001, Henan, China. E-mail address: [email protected] (J. Zhao). https://doi.org/10.1016/j.conbuildmat.2019.116757 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
mainly composed of waste clay bricks and waste mortar. The traditional way of disposing CDW was physical landfill, which resulted in a secondary pollution of the environment. Furthermore, the urbanization process has facilitated the requirement for construction, and numerous building materials (such as concrete, mortar and brick) were consumed. Additionally, the considerable quantities of non-renewable resource (such as virgin aggregate and cement) were depleted which further aggravated the burden of environment [5]. Currently, the natural aggregate and cement were substituted by the recycled materials like supplementary cementitious materials [6–8], recycled fine aggregate [9–12], and recycled coarse aggregate [13–16] which were able to prepare
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J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757
sustainable concrete or mortar after processing. These methods not only resulted in the reduction of excessive excavation of natural aggregate, which contributed to alleviate the hazard of environment and reduced the CO2 emission, but also satisfied the requirement of construction and expansion of urbanization. In recent years, the different particle sizes of waste clay bricks as alternative materials were prepared by sorting, screening and crushing like recycled coarse aggregate (with particles larger than 5 mm in diameter) [17,18], recycled fine aggregate (with particles lower than 5 mm in diameter) [19,20], and supplementary cementitious material (with particles lower than 0.125 mm in diameter) [21,22]. Zong et al. [23] researched the influence of the replacement ratio (30%, 40% and 50%) of recycled coarse aggregate from waste bricks on the mechanical properties and durability. It was concluded that due to the lower hardness of waste clay bricks, a considerable reduction on RBC strength were achieved. Rashid et al. [24] studied the fresh and compressive strength of recycled concrete prepared with coarse ceramic aggregate. It was showed that the incorporation of coarse ceramic aggregate with 30% replacement ratio into concrete obtained the higher compressive strength but lower environmental impact, which satisfied the requirement of sustainable recycled concrete. Additionally, Ge et al. [25] and Qu et al. [26] investigated that the recycled waste clay powder partially replaced the cement without sacrificing the performance because of the pozzolanic activity of micro-powder from waste clay bricks. With respect to the fine ceramic recycled aggregates and fine aggregate from waste clay bricks (WCBF), Siddique et al. [27] studied the effect of different replacement ratios (0–100%) of fine ceramic aggregate on the mechanical properties of recycled concrete with various water to binder ratio. It was found that fine ceramic aggregate resulted in an increase of mechanical strength and enhanced the generation of hydration products, which indicated that fine ceramic aggregate was feasible for preparing the concrete. Alves [28] and Vieira [29] have evaluated the performance of RBC with incorporation of fine ceramic aggregate with 20%, 50% and 100% replacement ratio. The result illustrated that the performance of concrete with fine ceramic aggregate was in accordance with the NC. However, Gonzalez-Corominas and Etxeberria [30] shown that compared to the NC, the RBC with less than 30% replacement ratio of fine ceramic aggregate resulted in the similar performance or slightly improved the performance of concrete. Dang et al. [5] have studied the effect of particle size distribution (PSD) and water content of WCBF on the properties of mortar. The result revealed that the WCBF with partially water content and 0–5 mm PSD had a beneficial improvement on the strength, while the WCBF with totally water content and 0.15–5 mm PSD had an adverse effect on the strength. In general, the waste clay bricks were simply backfilled due to the substantial defect of coarse aggregate from waste clay bricks, which was not suitable for preparation of concrete. On the other hand, both the economic value and the utilization rate were reduced due to the comparatively insufficient research with respect to the WCBF. Hence, the influences of variable replacement ratios (0%, 25%, 50%, 75% and 100%) and additional water volumes (no extra water, partially extra water and totally extra water) on the properties of concrete at different ages have been investigated. Based on the mechanical characterization of concrete, the microscopic tests have been carried out to further analyze the mechanism of the WCBF on recycled concrete. 2. Experimental investigations 2.1. Materials In this study, the crushed stone as coarse aggregate mainly constituted of limestone, as well as possessed the consecutive gradation of 5–20 mm, the water absorption of 0.9% and the apparent density of 2771 kg/m3. The natural fine aggre-
gate (NF) was river sand and the physical properties are presented in Table 1. In addition, Table 2 shows the chemical composition of cement and WCBF, while Table 3 shows the mechanical properties of 42.5 Ordinary Portland cement. Both the mixing water and additional water used for casting concrete were tap water. The slump of the NC was adjusted to 180 ± 20 mm by incorporating the polycarboxylate superplasticizer about 5.14 kg/m3. The WCBF included clay bricks and waste mortar derived from the CDW of masonry structure as shown in Fig. 1. The physical properties and chemical composition of the WCBF are illustrated in Tables 1 and 2, respectively. The grading curves of WCBF and NF are presented in Fig. 2. It can be observed that the fineness modulus of WCBF after processed is comparably consistent with the NF. While after mechanical crushing, the micro-powder content of WCBF is considerably increased up to 18.2% compared with the NF. The microscopic morphology of the WCBF through the SEM image is showed in the Fig. 3. It can be concluded that WCBF formed rough surface and loose texture.
2.2. Mixing proportion and specimen preparation As shown in Table 4, the mix proportion of concrete is designed in compliance with JGJ55-2011 [31]. The water cement (w/c) ratio of concrete is 0.35. The NF is substituted by WCBF at the replacement ratio of 0%, 25%, 50%, 75% and 100% by absolute volume, which can be represented by W0, W25, W50, W75 and W100. The additional water volume is determined by the water absorption of WCBF, which is represented by D (no extra water was the dry state of WCBF), T (totally extra water was 100% saturated surface dry state of WCBF), and P (partially extra water was 75% saturated surface dry state of WCBF). For example, CW25T represents the specimen of 25% replacement ratio, and totally extra water volume of WCBF. The mixing procedure of NC and RBC is presented in Fig. 4. The total water of recycled concrete included mixing water for conventional concrete and additional water volume of WCBF. The specimen was compacted by vibrating table. After casting, the specimen was kept a day in ambient temperature. Then the demolded specimens moved to the curing room (95% relative humidity and 20 ± 2 °C temperature).
2.3. Testing methods 2.3.1. Density The density of concrete was carried out in accordance with BS EN 12390-7 [32], which measured the saturated density and oven-dry density at 28 and 90 days.
2.3.2. Investigation of the mechanical properties The mechanical properties of concrete were evaluated according to GB/T500812002 [33], including the compressive strength, splitting tensile strength and modulus of elasticity of concrete. The compressive strengths of concrete at 3 d, 7 d, 14 d, 28 d, 56 d and 90 d were determined by cubic samples of 100 mm 100 mm 100 mm. The splitting tensile strengths of concrete at 28 d and 90 d were assessed by cubic samples of 100 mm 100 mm 100 mm. The modulus of elasticity of concrete at 28 d and 90 d were conducted on prismatic samples of 150 mm 150 mm 300 mm. 2.3.3. Investigation of the microstructure After 90 days’ curing, the dimension of 3–5 mm sample without coarse aggregate was taken out from the core of concrete specimen and immediately soaked in anhydrous alcohol to stop hydration. Then the sample was dried in the oven at 50 °C for 72 h before testing. The pore structure of concrete was characterized by mercury intrusion apparatus (MIP) with the tested pore size range of 0.003–1000 mm. After spraying with carbon, the microscopic morphology and interfacial transition zone (ITZ) of concrete were observed by scanning electron microscope (SEM) at different magnification. The elemental analysis of concrete was measured by energy dispersive X-ray spectroscopy (EDS) at the accelerating voltage of 20 kV based on the SEM. The sample was grounded, polished and sprayed with carbon before quantitative line scanning.
Table 1 Physical properties of fine aggregates. Sample
NF
WCBF
Density (kg/m3) Loose bulk density (kg/m3) Compact bulk density (kg/m3) Loose porosity (%) Compact porosity (%) Crush index (%) Saturated surface dry absorption (%) Fineness modulus SO3 content (%)
2596 1474 1560 43.2 39.9 6 1.1 2.86 0.2
2548 1170 1305 54.1 48.8 39.4 8.6 2.9 0.56
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J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757 Table 2 The chemical composition of cement and WCBF.
Cement WCBF
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
K2O
TiO2
19.24 66.26
4.08 16.42
3.25 6.18
62.47 5.97
4.81 1.36
4.19 –
1.38 2.63
– 0.93
Table 3 Properties of 42.5 Ordinary Portland cement. Setting time/min Initial setting 158
Final setting 198
Flexural strength/MPa
Compressive strength/MPa
3d 6
3d 23.6
28 d 8.7
28 d 48.9
Fig. 3. The SEM image of WCBF.
Fig. 1. The WCBF with the particle size of 0–5 mm.
especially on the oven-dry density. Furthermore, the density of RBC is slightly declined with the increasing of replacement ratio. In addition, the additional water volume of WCBF plays a crucial role in the RBC density. It can be clearly suggested that there is a liner reduction in RBC density with the increasing of additional water volume. The influence of the WCBF with no extra water on the density of RBC is negligible. However, compared to the NC, the saturated density and oven-dry density of RBC comparatively decreased with the increasing of additional water volume, achieving a maximum reduction of 6% and 5% respectively, when totally extra water was 100% SSD (T). On the one hand, this is attributed to the porosity of both waste brick and mortar yielded an inferior density of WCBF than the NF, resulting in a lower density of RBC. On the other hand, the effective w/c ratio is responsible for the density of concrete. The effective w/c ratio of RBC is gradually increased with the increasing of additional water volume, thus reducing the compactness of RBC and eventually decreasing the density of RBC. 3.2. Compressive strength
Fig. 2. Grading curves of fine aggregates.
3. Results and discussion 3.1. Density Fig. 5 presents the effect of WCBF with different additional water volumes and replacement ratios on density of concrete. At 28 and 90 days, the oven-dry density and saturated density of NC are 2328–2331 kg/m3 and 2416–2419 kg/m3, respectively, while the oven-dry density and saturated density of RBC are 2180– 2299 kg/m3 and 2299–2386 kg/m3, respectively. It can be observed that NC with higher density is less influenced by curing age, but the RBC with lower density is affected by curing age to some extent,
The effect of WCBF on the compressive strength of concrete is illustrated in Fig. 6 and Table 5. As shown in the figure, the increasing tendency of compressive strength of RBC is consistent with the NC, which increases with the increase of curing age. The different additional water volumes of WCBF have a remarkable impact on the compressive strength compared to replacement ratio. In general, the compressive strength of RBC with no extra water of WCBF is the maximum, followed by RBC with partially extra water of WCBF, being the compressive strength of RBC with fully extra water of WCBF the minimum. When the WCBF with no extra water is incorporated in the concrete, the RBC with 25% replacement ratio is slightly increased by 6% and 9% respectively at 28 and 90 days compared to the NC. As the replacement ratio increases up to 50% of WCBF with no extra water, the compressive strength of RBC is comparable to the NC. Nevertheless, the compressive strength of RBC is gradually decreased with the increase of additional water volume. When the additional water volume of WCBF is partially extra water, the compressive strength of RBC within 50% replacement ratio is
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Table 4 Mix proportion of concrete (kg/m3). Sample
W/C
Water (kg)
Cement (kg)
Coarse aggregate (kg)
River sand (kg)
WCBF (kg)
Additional water (kg)
Wa/C
CW0 CW25D CW50D CW25T CW50T CW75T CW100T CW25P CW50P CW75P CW100P
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
180 180 180 180 180 180 180 180 180 180 180
514 514 514 514 514 514 514 514 514 514 514
1052 1052 1052 1052 1052 1052 1052 1052 1052 1052 1052
672 504 336 504 336 168 0 504 336 168 0
0 165 330 165 330 495 660 165 330 495 660
0 0 0 14 28 43 57 11 21 32 43
0 0 0 0.03 0.06 0.08 0.11 0.02 0.04 0.06 0.08
Note: Wa is additional water introduced to WCBF.
Fig. 4. The mixing procedure of concrete, (a) recycled concrete, (b) normal concrete.
Fig. 5. Effect of WCBF on density of concrete, (a) oven-dry density, (b) saturated density.
Fig. 6. Effect of WCBF on compressive strength of concrete.
approximately identical to the NC. For the replacement ratio beyond 50%, there is a gradual reduction of compressive strength. At 28 and 90 days, the maximum reduction of compressive strength of RBC is 7% and 6%, respectively. When the additional water volume of WCBF is fully extra water, the compressive strength of RBC is decreased by a maximum of 11% and 8% respectively at 28 and 90 days compared to NC. This is due to the WCBF with no extra water can absorb free water from the internal concrete, thus reducing the effective w/c ratio. Whilst the incorporation of WCBF may create the pozzolanic activity, generating more stable and uniform hydration products, as well as leading to the formation of a compactness ITZ and dense cement matrix. Moreover, the combination of the irregular surface of WCBF and the penetration of the cement matrix to the surface of the WCBF contribute to enhance the adhesion force of ITZ further, which is fundamental to improve the compressive strength. This is confirmed by the microscopic morphology of the ITZ from SEM test.
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J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757 Table 5 Mechanical properties of concrete at different ages. Sample
CW0 CW25D CW50D CW25T CW50T CW75T CW100T CW25P CW50P CW75P CW100P
Compressive strength (MPa)
Splitting tensile strength (MPa)
Modulus of elasticity (GPa)
3d
7d
14 d
28 d
56 d
90 d
28 d
90 d
28 d
90 d
52.8 57.6 54.4 47.3 47.5 47.8 46.2 48.9 51.9 50.3 49.0
53.2 58.0 57.7 49.6 52.2 52.8 50.1 53.9 55.8 54.2 54.9
56.6 63.8 60.9 50.3 54.4 56.6 53.7 57.3 59.9 60.9 58.0
63.4 67.5 64.3 56.4 59.1 59.4 58.8 60.7 60.8 61.6 59.0
63.8 70.4 65.5 59.5 59.8 59.7 58.8 62.1 63.4 63.6 59.4
66.9 73.1 67.9 62.5 65.0 61.9 61.7 66.8 68.5 64.5 62.9
4.65 5.15 5.08 4.75 4.80 4.87 4.77 4.77 4.69 4.80 4.85
4.80 5.20 5.10 4.93 4.90 4.91 4.87 4.87 4.90 4.87 5.00
35.4 34.7 34.5 31.5 30.5 30.3 31.6 32.0 30.6 32.4 31.5
41.2 41.8 41.3 39.4 38.5 37.2 33.0 38.4 40.1 37.7 34.6
However, with the increasing of additional water volume and replacement ratio, the effective w/c ratio of RBC is progressively increased. Therefore, the influence of the inferior properties of WCBF is gradually increased so as to offset the beneficial effect of WCBF, thus leading to a degradation of the compressive strength. In addition, as shown in Table 6 and Fig. 3, the significant increase of total pore volume and porosity are justified by the presence of WCBF, which possess both porous structure and the micro-crack because of the mechanical crush. Hence, the compressive strength of RBC is reduced. 3.3. Splitting tensile strength Fig. 7 and Table 5 display the influence of WCBF on the splitting tensile strength of concrete at different ages. As shown in figure, the increasing trend of splitting tensile strength of RBC is in accordance with NC. Moreover, the splitting tensile strength of RBC is slightly increased at 90 days. In general, it is worthwhile to noted that the splitting tensile strength of RBC is comparatively enhanced compared to NC. This is attributed to the rough irregular surface of WCBF, which improves the adhesion force and compactness of ITZ. The different additional water volumes of WCBF have slightly effect on the splitting tensile strength of RBC. As shown in the Fig. 7, the splitting tensile strength of RBC incorporating no extra water of WCBF is the maximum, followed by the RBC with partially extra water of WCBF, being the splitting tensile strength of RBC with fully extra water of WCBF the minimum. When the WCBF with no extra water is incorporated in the RBC, the splitting tensile strength of RBC is relatively higher than that of NC, but there is a negligible reduction of the splitting tensile strength with increasing of replacement ratio. At 28 and 90 days, the splitting tensile strength of RBC is maximally increased by 11% and 8%, respectively. When the additional water volume of WCBF is partially extra water, the splitting tensile strength of RBC is increased by a maximum of 4%. While the addition water volume is increased up to fully extra water, the splitting tensile strength of RBC is increased by 5% and 3% maximally at 28 and 90 days. This result
Fig. 7. Effect of WCBF on splitting tensile strength of concrete.
shows that the splitting tensile strength of RBC is marginally decreased with the increasing of additional water volume. The favorable performance of RBC with WCBF is attributed to three reasons as follows. First, the porous structure of WCBF can absorb part of the free water, which reduces the effective w/c ratio and improves the hardness of cement matrix and ITZ. Second, the pozzolanic activity of WCBF contributes to the formation of hydration product, which can enhance the mechanical interlocking of interface. Finally, the micro-powder of WCBF enables to fill the void and crack in the vicinity of WCBF, which can promote the generation of a dense structure. Hence, the multiple effects of WCBF overwhelm the detrimental effect of the porous structure of itself, which improves the splitting tensile strength. This result is further demonstrated by the close interlocking between the WCBF and the cement matrix from the SEM observation.
Table 6 Result of MIP analysis for concrete at 90 days. Samples
CW0 CW50D CW50T CW100T CW50P CW100P
Total pore volume (mL/g)
0.0577 0.062 0.077 0.0916 0.0622 0.079
Porosity (%)
12.57 13.02 15.28 17.51 13.11 15.66
Pore size distribution (mL/g) <4.5 nm
4.5–50 nm
50–100 nm
>100 nm
0.0037 0.0035 0.0041 0.0048 0.0029 0.0037
0.0215 0.0181 0.0305 0.0405 0.0223 0.0302
0.0038 0.0086 0.0025 0.0023 0.0020 0.0039
0.0287 0.0318 0.0399 0.0440 0.0350 0.0412
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3.4. Modulus of elasticity Fig. 8 and Table 5 depict the effect of WCBF on the elastic modulus of concrete at different ages. It can be observed that the increasing trend of elastic modulus of RBC is consistent with NC. At 28 days, compared to the NC, the replacement ratio of WCBF has negligible effect on the elastic modulus, while the elastic modulus of RBC is slightly affected by the WCBF with different additional water volumes. With the increasing of curing age, the elastic modulus of RBC is gradually increased. Moreover, the influence of replacement ratio and additional water volume on the elastic modulus of RBC is progressively obvious. It is noted that there is a linearly worsening tendency on the elastic modulus of RBC in compliance with the order of no extra water, partially extra water and fully extra water of WCBF. When the additional water volume of WCBF is no extra water, the elastic modulus of RBC is comparable with NC. Because of the WCBF with higher absorption capacity, there is a reduction of the effective w/c ratio of RBC, which basically compensates the degradation induced by the porous characteristic of the WCBF. However, it is clearly observed that the elastic modulus of RBC is gradually decreased with the increasing of additional water volume. When the additional water volume of WCBF is partially extra water, the elastic modulus of RBC is marginally decreased by 3% with the replacement ratio increases up to 50% at 90 days compared to the NC. However, when the replacement ratio increases up to 100%, the elastic modulus of RBC is comparatively reduced by 16% compared to the NC. When the additional water volume of WCBF is fully extra water, the elastic modulus of RBC exhibits a linearly descending trend with the increasing of replacement ratio at 90 days. And the WCBF with fully extra water has a little adverse impact on the elastic modulus of concrete with the replacement ratio no larger than 50%, while the elastic modulus of RBC with 100% replacement ratio can be achieved a maximum reduction of 20% at 90 days. Generally, the poorer properties of WCBF result in both weak stiffness and lower strength, which may be responsible for the deterioration of elastic modulus of RBC. In addition, the effective w/c ratio of RBC is gradually increased with the increasing of additional water volume, which increases the porosity of RBC, thus resulting in an abruptly decrease in the elastic modulus of RBC. Although the WCBF can enhance the compactness of ITZ and cement matrix, it cannot compensate the defect caused by its own porous structure. Particularly, when the replacement ratio is beyond 50%, the WCBF has a considerably unfavorable impact on
Fig. 8. Effect of WCBF on modulus of elasticity of concrete.
the elastic modulus of RBC. As shown in Table 6, based on the pore structure of concrete by MIP test, the incorporation of WCBF brings about a significantly increase in the pore content with diameter higher than 100 nm, which plays a predominant role in the reduction of elastic modulus of recycled concrete.
3.5. Investigation of the microstructure 3.5.1. Mercury intrusion porosimetry The effect of WCBF on the pore structure of recycled concrete at 90 days is presented in Table 6. As shown in the table, the porosity and total pore volume of NC is 12.57% and 0.0577 mL/g, respectively, while the porosity and total pore volume of RBC presents the values of 13.02–17.51% and 0.062–0.0916 mL/g, respectively. It is noted that the porosity and total pore volume of RBC increases with the addition of WCBF. The porosity and total pore volume of CW50D and CW50P increases by 8% and 4% compared to NC. Nevertheless, with the increase of additional water volume, the pore characteristic of recycled concrete is obviously worsened. The total pore volume and porosity of CW50T is maximum increased by 33% and 22%, respectively, compared to NC. When the replacement ratio of WCBF is increased up to 100%, the porosity and total pore volume of recycled concrete are increased prominently. Compared to the NC, the total pore volume and porosity of CW100P are increased by 37% and 25%. When the additional water volume of WCBF is rose up to the fully extra water, the total pore volume and porosity of CW100T are characterized by a maximum increase of 59% and 39%, respectively. It can be illustrated that the WCBF has prominent detrimental effect on the pore characteristic of RBC. Furthermore, the pore structure of RBC is gradually deteriorated with the increase of additional water content and replacement ratio. Fig. 9 shows the effect of WCBF on cumulative pore volume of concrete. The cumulative pore volume curves of CW50D and CW50P are basically consistent with the normal concrete. The WCBF is preferably embedded into the cement matrix with low w/c ratio, thus improving the compactness of ITZ and impeding the entry of mercury. However, compared to the NC, the cumulative pore volume curves of CW50T, CW100P and CW100T are overall upward shifted. The result exhibits that the deterioration of pore characteristic is proportional to both the replacement ratio and additional water volume of WCBF. According to the investigation of Metha and Monteiro [34], the pore size distribution of concrete is classified according to the following pore diameter intervals: less than 4.5 nm, 4.5–50 nm, 50–100 nm, and higher than 100 nm. As shown in Table 6, it is noted that the pore content
Fig. 9. Effect of WCBF on cumulative pore volume of concrete.
J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757
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Fig. 10. The SEM images of normal concrete at 90 days, (a) magnified 500 on CW0, (b) magnified 2000 on CW0.
with diameter higher than 100 nm is prominently increased with the addition of WCBF. When the WCBF replacement ratio is 50%, the pore content with diameter higher than 100 nm of CW50D, CW50P and CW50T is increased by 11%, 22% and 39% respectively compared to the normal concrete. When the replacement ratio of WCBF is increased up to 100%, the pore content with diameter higher than 100 nm of CW100P and CW100T is significantly increased by 44% and 53% respectively compared to the normal concrete. It can be illustrated that the pore content with diameter higher than 100 nm of RBC is substantial higher than NC due to the presence of WCBF. 3.5.2. Scanning electron microscope Figs. 10 and 11 present the ITZ microscopic morphology of normal concrete and recycled concrete at 90 days. It is noted that there is an obvious micro-crack between NF and cement matrix because the micro-bleeding of NF forms a relative higher w/c ratio around the ITZ. Nevertheless, there is no visible distinctive debonding between WCBF and cement matrix, which is substantially different to the ITZ between NF and cement matrix. This is attributed to the porous structure of WCBF as well as the mitigation of wall effect caused by the absorption of free water around the ITZ. Therefore, the ITZ between WCBF and cement matrix is well-bonded together, thus enhancing the compactness of ITZ. Through the observation with different magnification, it is clearly indicated that the WCBF is wrapped or embedded in the cement matrix, which is tightly integrated with the cement matrix. Compared to the inert NF, the pozzolanic reaction between WCBF and Ca(OH)2 from the cement matrix contributed to the formation of hydration products (C-S-H, AFt and C-A-H) which uniformly wrapped on the surface of WCBF and filled in the vicinity of cement matrix and ITZ. In addition, the powder of WCBF not only has a better micro-filling effect, but also has a larger specific surface area
which is readily penetrated by the alkaline solution, leading to adequately exert on pozzolanic reactivity. Additionally, the rough irregular surface of WCBF effectively enhances the mechanical interlocking of ITZ between WCBF and cement matrix, which is essential in improving the splitting tensile strength. However, only an amount of hydration products is detected in a certain range on the surface of WCBF. It can be deduced that the limit depth of WCBF is penetrated by cement matrix, which is not effectively penetrated in the core of WCBF. Therefore, the detrimental effect on the strength of RBC cannot be offset sufficiently by the modified boundary of WCBF, which further demonstrate that the WCBF result in the deterioration of the pore structure of recycled concrete. The most prominent representation of deterioration is given by the increase in pore content of pore diameter higher than 100 nm. 3.5.3. Energy dispersive X-ray spectroscopy Figs. 12 and 13 present chemical element analysis around the ITZ of concrete. As shown in Fig. 12, it is noted that the element composition is obviously distinguished in the adjacent NF for normal concrete. The NF mainly contains Si element, but the cement matrix includes Si, Al and Ca element. Compared to the normal concrete, there is no variation in the element types of CW50P, while the WCBF contains a large amount of Si and Al elements. This is consistent with the result of the X-ray fluorescence test analysis. The chemical composition of SiO2 and Al2O3 in the WCBF account for 82.68% as presented in Table 2. In addition, as shown in the Fig. 13, the distribution of Si and Al element components in the WCBF is gradually decreased from the interior of WCBF to the cement matrix, while the content of Ca element is progressively stabilized from the boundary of WCBF to the cement matrix. Furthermore, it is clearly observed that a relative amount of Ca element is well concentrated on the boundary of
Fig. 11. The SEM images of recycled concrete at 90 days, (a) magnified 500 on CW50P, (b) magnified 2000 on CW50P.
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Fig. 12. The SEM image and element distribution of concrete with CW0.
Fig. 13. The SEM image and element distribution of concrete with CW50P.
the WCBF compared to the NF. Therefore, it is evident that the Ca element component is penetrated from cement matrix to the WCBF about 120 mm. This is attributed to the porous structure and rough surface of WCBF generates the adhesion force to the cement slurry, which causes the cement slurry to penetrate into the WCBF. Whilst the pozzolanic reactivity between the active component of WCBF and Ca(OH)2 from cement hydration will take place near the boundary of WCBF, thus facilitating the generation of the C-A-H and C-S-H of hydration products within the reaction rim. Moreover, this result is basically consistent with SEM images from Fig. 11, which suggest that the amounts of hydration products of Ca element fill the void in a certain thickness in the vicinity of WCBF. Hence, due to the multiple effect of mechanical interlocking and the homogeneous hydration product from pozzolanic reaction, the strength of ITZ is enhanced, and the porosity and micro-crack in the thickness of the WCBF rim is filled. These beneficial effects are fundamental to obtain the acceptable mechanical properties of RBC, especially the enhancement of splitting tensile strength. 4. Conclusion This paper studied the influence of additional water volume and replacement ratio of WCBF on the density and mechanical properties. Based on the SEM, EDS and MIP test results, the morphology of
microstructure, the chemical element analysis and the pore structure of concrete have been researched. (1) The density of RBC is gradually declined with the increasing of replacement ratio. The influence of the WCBF with no extra water on the concrete density is negligible, but the density of RBC is comparatively reduced with the increasing of additional water volume. (2) The additional water volume and replacement ratio of the WCBF play a predominant role in the compressive strength. The compressive strength of RBC with no extra water of WCBF is similar or higher than the NC. When the additional water volume of the WCBF is partially extra water, the compressive strength of RBC within 50% replacement ratio is comparable with NC, while the compressive strength of RBC beyond 50% replacement ratio is slightly decreased. Additionally, the compressive strength of RBC with the fully extra water of WCBF results in a maximum reduction of 8%. (3) The incorporation of WCBF marginally strengthens the splitting tensile strength, while the WCBF with no extra water enhances the splitting tensile strength of RBC with the maximum increase of 8%. (4) The WCBF within 50% replacement ratio has minor effect on the elastic modulus of concrete. But with the increasing of additional water volume and replacement ratio, the WCBF
J. Dang, J. Zhao / Construction and Building Materials 228 (2019) 116757
decrease the elastic modulus of concrete with a maximum loss of 20%. (5) The porosity and total pore volume of recycled concrete is significantly increased with the incorporation of WCBF, especially the pore content of pore diameter higher than 100 nm. Whilst the satisfactory compactness of ITZ between the WCBF and cement matrix is improved due to the rough irregular surface and pozzolanic activity of WCBF. In addition, the cement matrix can effectively penetrate into the surface of the WCBF by about 120 mm, which promotes the generation of more stable and dense hydration products and enhances the strength and compactness of the ITZ.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Research reported here was supported by the National Key R&D Program of China (2016YFE0125600) and Program for Innovative Research Team of Education Ministry of China (IRT_16R67). References [1] X.S. Shi, F.G. Collins, X.L. Zhao, Q.Y. Wang, Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete, J. Hazard. Mater. 237–238 (2012) 20–29. [2] M. Bravo, J. de Brito, L. Evangelista, J. Pacheco, Durability and shrinkage of concrete with CDW as recycled aggregates: benefits from superplasticizer’s incorporation and influence of CDW composition, Constr. Build. Mater. 168 (2018) 818–830. [3] C. Alexandridou, G.N. Angelopoulos, F.A. Coutelieris, Mechanical and durability performance of concrete produced with recycled aggregates from Greek construction and demolition waste plants, J. Clean. Prod. 176 (2018) 745–757. [4] H.J. Du, K.H. Tan, Use of waste glass as sand in mortar: part I – Fresh, mechanical and durability properties, Cem. Concr. Compos. 35 (2013) 109– 117. [5] J.T. Dang, J. Zhao, W.Y. Hu, Z.H. Du, D.Y. Gao, Properties of mortar with waste clay bricks as fine aggregate, Constr. Build. Mater. 166 (2018) 898–907. [6] M.Z. Chen, J.T. Lin, S.P. Wu, C.H. Liu, Utilization of recycled brick powder as alternative filler in asphalt mixture, Constr. Build. Mater. 25 (2011) 1532– 1536. [7] Q. Liu, T. Tong, S.H. Liu, S.H. Liu, D.Z. Yang, Q. Yu, Investigation of using hybrid recycled powder from demolished concrete solids and clay bricks as a pozzolanic supplement for cement, Constr. Build. Mater. 73 (2014) 754–763. [8] H.J. Du, S.D. Pang, Value-added utilization of marine clay as cement replacement for sustainable concrete production, J. Clean. Prod. 198 (2018) 867–873. [9] I.H. Song, J.S. Ryou, Hybrid techniques for quality improvement of recycled fine aggregate, Constr. Build. Mater. 72 (2014) 56–154. [10] J.A. Bogas, J. de Brito, D. Ramos, Freezee-thaw resistance of concrete produced with fine recycled concrete aggregates, J. Clean. Prod. 115 (2016) 294–306. [11] L. Li, J.Z.J.X.Lu. Bao, C.S. Poon, Systematic evaluation of the effect of replacing river sand by different particle size ranges of fine recycled concrete aggregates (FRCA) in cement mortars, Constr. Build. Mater. 209 (2019) 147–155.
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