Recycled concrete fine powder (RFP) as cement partial replacement: Influences on the physical properties, hydration characteristics, and microstructure of blended cement
Introduction
With increasing urbanization worldwide, much construction and demolition (C&D) waste has been generated [[1], [2], [3]]. Currently, annual C&D waste production in the United States and the European Union is about 700 and 800 million tons. It exceeded 1.8 billion tons per year in China [[4], [5], [6], [7]]. The recycling rate of C&D waste is more than 70% in some developed countries and regions such as Japan, Europe, and the United States. At the same time, it is currently less than 10% in China. Generally, lots of C&D waste is accumulated in landfills with landfill disposal [4,8,9]. In addition, cement is used in concrete for about 300–400 kg/m3 and consumes 4–6 billion tons per year [[10], [11], [12]]. Cement is also a significant contributor to the carbon footprint of concrete, and its carbon emission in concrete production accounts for about 80% and accounts for 5–7% of global CO2 emissions [[13], [14], [15], [16], [17]].
Sustainable development is a focal point worldwide today. The increasingly stringent environmental regulations and requirements of international agreements are the current trends, with significant economies developing relevant laws, regulations, and future strategic deployments [11,18,19]. For example, the Chinese government has committed: Carbon peaking by 2030 and carbon neutrality by 2060. It is such a piece of encouraging news. In the current background, the construction industry must balance social, economic, and environmental aspects to achieve sustainable development [20]. The cement industry actively seeks alternative materials to achieve sustainable development, circular economy, and green development [21].
During the life cycle of concrete buildings, especially in the demolition period, a large amount of concrete powder is generated [20,22]. RFP, including hydrated cement mortar and some C2S, C3S, SiO2, and free CaO, are potentially active [10,23]. Recycling and turning (C&D) waste into RFP for reusing concrete is excellent potential [5]. Effective use of RFP could replace part of the cement, help solve the disposal challenges of construction waste, reduce the carbon footprint of the concrete industry, and conserve natural resources. It is in line with the requirements of the cement sustainability plan and has high economic and environmental benefits [22,24,25].
Research on the application of RFP in concrete has been conducted, mainly focusing on the macroscopic properties of RFP on concrete. In addition, most of the work on RFP in cement-based materials has been done from the perspective of activating the potential activity of RFP and relatively few other microscopic aspects. It still faces the problems of challenging activation of RFP activity and the high cost of most excitation methods, which are not conducive to the level of resource utilization of RFP. Nevertheless, RFP has partially available in finer particles. Presently, there is a lack of deep investigation on the role of the particle advantage of RFP in cementitious materials.
In recent years, some important studies have also been on using construction and demolition waste and microstructural analysis of materials for environmental protection and resource conservation [[26], [27], [28], [29], [30]]. The RFP has an irregular microstructure containing hydration products, CaCO3 and SiO2 [31], so the calcium-rich RFP (>20% CaO) may have the potential for alkali excitation [32]. In most studies, the addition of RFP reduced new hydration products in cementitious materials; tests combining water, chemical shrinkage, and calcium hydroxide consumption indicate that RFP was significantly weaker than fly ash and slag in terms of volcanic ash activity [33]. Adding an appropriate amount of RFP can reduce the capillary water absorption and carbonation depth of mortar [34]; the minimum negative effect on the compressive strength of the self-compacting mortar is achieved at approximately 20% RFP content [35]. Based on the current state of research, this study was conducted to investigate further the advantages of particle filling of RFP in the mortar and explore the hydration model construction of RFP blended cement.
This study examines the effect and working mechanism of RFP replacement levels on cement. The physical characteristics, mechanical properties, and action mechanisms were explored at macroscopic and microscopic levels of cement. Further work included the hydration process of cement with RFP hydration kinetics modeling. Besides, an advanced NMR technique is also used to investigate the pore structure of the blended cement mortar. This paper investigates the particle advantage and potential activity of RFP to use RFP as a material with volcanic ash properties to achieve cement savings and increase the resource reuse of construction waste. The gist of this paper contributes to further research on the properties of the cement containing RFP, which aims to advance the utilization of RFP for practical engineering applications and reduce C&D waste and its associated complexity problems.
Section snippets
Cement
Portland cement was a grade 42.5 silicate cement produced by Fushun Cement Co., Ltd. of China (conforming to standard GB8076-2008), with a specific surface area of 354 m2/kg. The chemical composition and properties of cement are shown in Table 1, Table 2. The particle size distribution is shown in Fig. 1, and the median particle size distribution is 22.7 μm. The scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) at different magnifications are shown in Fig. 2.
Recycled concrete fine powder
RFP
Standard consistency and setting time
The standard consistency water consumption (SCWC) of blended cement is shown in Fig. 6. The experimental results show that the SCWC of blended cement obeys a decreasing law with the increasing RFP content of blended cement. The SCWC of blended cement has almost unchanged when the RFP content is less than 10%. However, the SCWC of blended cement decreases by 0.7% and 1.5% when the RFP content is 30% and 50%, respectively.
Fig. 7 presents the initial setting time (IT) and the final setting time
Hydration heat releases
Isothermal calorimetry was used for the hydration tests. The exothermic hydration curve of blended cement is shown in Fig. 9.
The exothermic hydration curves of blended cement with RFP (0–90%) are shown in Fig. 9. Isothermal calorimetry was used for the hydration tests of the blended slurry. According to Fig. 9(a), the cumulative heat release of blended cement (hydration 70 h) with 10%, 30%, 50%, 70%, and 90% RFP, respectively decreases by 10.96%, 23.53%, 36.91%, 41.16%, and 75.22%, compares to
Flexural and compressive strength
The effects of the different RFP content on the cement mortar are shown in Fig. 11. When the increase of RFP admixture, the flexural and compressive strength of mortar at the same age for all shows a decreasing trend. At 3d age, the flexural strength of the mortar with 10% RFP (5.7 MPa) increased more than that of pure cement mortar (5.4 MPa) by about 5.56%. In comparison, the flexural strength of the mortar with 30% and 50% RFP is 4.5 and 3.5 MPa, which decreased by 16.67% and 35.19%,
XRD analysis
The XRD pattern of the mortar with RFP is shown in Fig. 12. The peaks of pure cement mortar are mainly the diffraction peaks of SiO2 with higher intensity and CH. Besides, it also found that a new CaCO3 peak appears, SiO2 peak intensity increases, and CH peak reduces for mortar as the RFP increases from 10% to 50%, compared with pure cement mortar.
TG-DTG analysis
The TG-DTG curves of blended cement mortar are shown in Fig. 13. The thermal decomposition of the powder is divided into three stages corresponding
Pore structures of blended cement containing RFP
The advantage of the NMR technique is that it is fast, non-destructive, reproducible, quantitative, and accurate [49]. It is based on the signal volume being proportional to the amount of hydrogen-containing fluid in the test for the same detection parameters. Its main principles are as follows. A curve of the amount of fluid versus the NMR signal volume is fitted by testing a set of porosity calibration samples with a known amount of hydrogen-containing fluid. The measured signal volume of the
Conclusion
This paper focuses on the effect of different replacement levels of RFP on the properties of cement and the mechanism of RFP action. Various technical tools were applied to experimental studies in macroscopic and microscopic aspects. The main findings are as follows.
- (1)
The high specific surface area and water absorption of RFP particles cause the fluidity of cement paste to increase first and then decrease. For cement, an appropriate amount of RFP (not more than 30%) shortens the setting time and
CRediT authorship contribution statement
Zhong Li: Methodology, Investigation, Writing – original draft, Data curation. Yadong Bian: Conceptualization, Methodology. Jihui Zhao: Writing – review & editing, Validation, Formal analysis. Yiren Wang: Investigation, Writing – review & editing. Zhenxia Yuan: Investigation.
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.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51908568, No. 52002410), Natural Science Foundation of Guangdong Province (No. 2019A1515011981), Zhuhai Social Development Field Science & Technology Project (No. ZH22036203200015PWC), and State Key Lab of Subtropical Building Science, South China University of Technology (No. 2022ZB20).
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Co-first author, this author contributes to this manuscript equally.