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Using recycle concrete aggregate coating agent for improving concrete microstructure and hardened characteristics
Lapyote Prasittisopin1, Phattarakamon Chaiyapoom1, Chawis Thongyothee1, and Chalermwut Snguanyat1
1
Siam Research and Innovation company, Siam Cement Group – Cement and Building Material, Saraburi, 18260, Thailand; Phone (+66)-36240888; E-mail: lapyotep@scg.co.th Abstract
Recycled concrete aggregates (RCA) have been widely used as concrete aggregates in construction. This is due to economic and environmental benefits of using RCA in concrete. Unlike natural aggregate (NA), the use of RCA in concrete seems not promising to the users implementing it, especially in structural purposes. The RCA concrete is reported to have lower quality than NA concrete, especially when a high RCA content is used. The product improvement to ensure the quality of RCA-based concrete is needed. This work aims to examine the microstructures of different RCA concretes. A new surface coating agent has been developed to improve paste-aggregate bonding. After casting coated-RCA (C-RCA) concrete specimens with the replacement level of 30%, interfacial transition zone (ITZ) between C-RCA particle and cement paste was characterized using a Scanning Electron Microscope with Energy-Dispersive X-ray methods. Results indicated a reduction of ITZ region when using the C-RCA concrete and, consequently resulting in improved hardened performance characteristics. The compressive and flexural strengths of the C-RCA concrete were similar to the NA concrete and were higher than the RCA. The use of surface coating agent for RCA is one of the promising alternatives such that the use of RCA can be broadened in the construction industry.
Keywords: Recycled concrete aggregate, Interfacial transition zone, Microstructure, Scanning electron microscope, Hardened characteristics.
Introduction
A consumption of recycled concrete aggregates (RCA) is ubiquitous in today’s construction. RCA differs from natural aggregate (NA) in that RCA composes of two materials of different natures: NA and attached cement mortar. Latter material is a cause of different and deteriorated properties of RCA: lower density and higher absorption. These result in negative effect on RCA concrete quality, mainly affecting to properties related with mechanical properties and durability [1]. One aspect that influences on old mortar content is the number of crushing processes in the production plants. Increasing the number of crushing processes, attached mortar content can be reduced, and consequently leading to aggregate quality can be improved. However, this causes an increase of production and maintenance cost, so that an optimization between number of stage-crushing processes and aggregate quality is required to assess [2-3].
This work reports an alternative method to improve RCA quality by using cement-based coating agent. The coating agent was reported elsewhere [4-6]. A microstructure or interfacial transition zone (ITZ) of coated-RCA (C-RCA) concrete is analyzed here using a Scanning Electron Microscope with Energy-Dispersive X-ray (SEM/EDX). Mechanical characteristics of C-RCA concrete including compressive strength and flexural strength are determined compared with the NA and RCA concretes.
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Materials.
Ordinary portland cement was used in all concrete mixtures. Fly ash for concrete mixtures was obtained from electrical plant in Lampang, Thailand. The RCA was obtained from concrete precast elements. It was crushed into small fragments using an industrial crusher with the maximum jaw size of 5.08 cm. Limestone NA was procured locally from Saraburi, Thailand. Its specific gravity was 2.7 and absorption was 0.38%. The specific gravity and absorption were determined following ASTM C12. Fine aggregate was procured from a local source in Saraburi, Thailand and met ASTM C33 requirements. Specific gravity of the fine aggregate was 2.6. Tap-water was used throughout this experiment.
Microstructure analyses.
Cross-sections of the NA, RCA, C-RCA concretes were observed using SEM/EDX instrument (FEI Quanta FEG 450). The SEM images were observed at 1000X magnification. Ca and Si elements were line scan analyzed using EDX. Before analyzed, the specimens were cut in half to observe the ITZ between old NA and new mortar, and cured with epoxy. The cut specimens were surface-ground with sand papers and Al powders. The surface-ground specimens were then kept in a desiccator prior to the SEM/EDX analyses. The EDX was normalized with each element itself. The Ca and Si were analyzed to investigate hydrating phases of cement around the ITZ. Higher Ca(OH)2 concentration results in higher values of the weight ratio of Ca and Si (Ca/Si). The bulk hydrating cement region, on the other hand, contains higher C-S-H concentration. This C-S-H attributes to lower Ca/Si, which tends not to be present in the ITZ region. Different Ca/Si are used to classify the Ca(OH)2 phase in the ITZ region with the C-S-H phase in the bulk hydrating cement region. The approximated size of the ITZ region for each system was reported here.
Concrete testing.
Concrete specimens were prepared following a mix design used for a pre-stressed concrete panel. Mix design of 1 cu. m. includes: cement: 186 kg; fly ash: 95 kg; fine aggregate: 910 kg; coarse aggregate: 1070 kg; and water = 170 kg. Its slump was controlled ranging from 14 to 15 cm. Three concrete types (NA, RCA, and RCA concrete) were investigated. The C-RCA concrete was evaluated at the replacement level of 30%. The data was normalized by the maximum value in the data set. Triplicate samples were conducted for each condition. Results
Microstructure analyses.
The SEM/EDX in Figure 1 is the NA concrete and shows regions of the hydrating cement paste (left-bottom of the figure), the NA (right-top of the figure), and the ITZ which is in the paste region in a vicinity to the NA. The EDX analyses indicate higher Ca/Si is aggregate composition and lower Ca/Si is paste composition. The image shows that the point where the Ca/Si begins to reduce is the surface of aggregate. Reduced Ca/Si indicates less amount of Ca(OH)2 and likely higher amount of C-S-H. The EDX shows that a region between the aggregate and paste which approximates 26 microns far from the aggregate. The SEM/EDX images of the RCA concrete show in Figure 2. Its regions consist of hydrating cement paste
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(right-bottom of the figure), the aggregate (left-top of the figure), and the ITZ. The EDX analyses show that the ITZ thickness is approximately 63 microns. Figure 3 shows the cross-sectional SEM/EDX images of the C-RCA concrete which the region of higher Ca/Si is aggregate. The region that is closed to the aggregate is coating agent. The ITZ region is the regions where the Ca/Si is decreasing and to the point where the Ca/Si begins increasing again. This ITZ region is 18 microns far from the aggregate. Based on the EDX analyses, the ITZ region of the RCA concrete is larger than the ITZ regions of the NA and the C-RCA concretes, respectively. Smaller ITZ region of the C-RCA concrete is believed to have improved concrete performance.
Concrete testing.
Results in Figures 4a indicate that the 7- and 28-day compressive strengths of the C-RCA concretes are similar to the NA and higher than the RCA concretes, respectively. Figure 4b shows that the 7-day flexural strength of the C-RCA concrete seems to be similar to the NA but 13% higher than the RCA concretes. The 28-day flexural strength of the NA concrete is 8% and 13% higher than the C-RCA and the RCA concretes, respectively. Both results are corresponded to that using the coating agent improves the concrete performance. One reason is because the smaller ITZ region of cement paste and aggregate.
Figure 1. SEM/EDX images for the NA concrete at 1000X magnification level.
Figure 2. SEM/EDX images for the RCA concrete at 1000X magnification level.
Aggregate
Paste
Aggregate
Paste
Paste ITZ Aggregate
lower Ca/Si higher Ca/Si
Paste
Aggregate ITZ
lower Ca/Si higher Ca/Si
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Figure 3. SEM/EDX images for the M-RCA concrete at 1000X magnification level.
Figure 4. a) Compressive strength; b) flexural strength of NA, RCA, and C-RCA concretes at 7 and 28 days.
Conclusion
Analyzes of the ITZ microstructure of the C-RCA concretes were carried out in this work. The results of SEM/EDX indicated that using this coating agent method for the RCA led to a smaller ITZ region. The smaller ITZ region in concrete microstructure could result in improved concrete performance: increased compressive strength and flexural strength. The C-RCA could be used as coarse aggregate for making a good quality concrete, instead to the NA. This method can be one of the promising technologies to broaden the applications of RCA concrete. Research on long-term concrete performance and evaluation of additional cost is critical for verification.
References
[1] Fathifazl, G., et al. (2011). Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate. Cement and Concrete Composites, 33, 1026-1037.
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Morphology, ACI Materials Journal, 112(3), 385-392.
[3] Halim, S. C. (2008) Application of reactive and partly soluble nano-materials. PhD
Dissertation, ETH Zurich, Zurich, Switzerland.
[4] Zhao, Z., et al. (2013). Evaluation of pre-coated recycled aggregate for concrete and mortar. Construction and Building Materials, 43, 191-196.
Aggregate
Paste
Paste Aggregate
ITZ
lower Ca/Si higher
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[5] Li, J., et al. (2009). Influence of coating recycled aggregate surface with pozzolanic powder on properties of recycled aggregate concrete. Construction and Building
Materials, 23, 1287-1291.
[6] Prasittisopin, L., et al. Coarse Aggregate Coating Substance and the Method of Coating Thereof,” Patent application pending, 2015, TH1501001431.
