Experimental study on the vegetation characteristics of biochar-modified vegetation concrete
Graphical abstract
Introduction
Vegetation concrete (mainly composed of cement, water and coarse aggregate) is always laid as a reinforcement base and covered by the soil layer with vegetation [1], [2], [3], [4], [5]. It is an environment-friendly material that has been increasingly used in pavements, parking lots and embankments to beautify the landscape, reduce environmental pollution, control storm water runoff and prevent landslides [6], [7], [8], [9], [10]. Current research [11], [12], [13] has shown that the plant compatibility and compressive strength of vegetation concrete are far below the requirements of artificial slope protection, road surface or other engineering construction, which has limited its application and must be improved. One of the most important factors is that plants cannot grow properly on vegetation concrete, as the most suitable soil for plant growth is slightly acidic, while the planting environment of vegetation concrete is mostly weakly alkaline. At present, many studies have been performed to reduce the alkalinity of vegetation concrete using low-basicity cement or by adding admixtures such as fly ash and slag to modify the plant growth environment [14], [15], [16], [17], [18], [19]. A few studies have been conducted on the direct promotion of plant growth by adding fertilizers or soil amendment in vegetation concrete. Gong et al. [20] proposed that the addition of urea as a raw material boosted the growth of plants, but nevertheless reduced the compressive strength of vegetation concrete to a certain extent. Yuan et al. [21] enhanced the adsorption capacity of heavy metal, organic matter and other pollutants, as well as the growth of plants to a degree, by adding activated carbon and other materials into the vegetation concrete. However, little work has been performed on biochar-modified vegetation concrete.
Biochar is a type of refractory fine granular charcoal generated by pyrolysis of vegetation or waste materials [22]. Compared with activated carbon, biochar is not only more prominent in improving soil properties as a soil conditioner but also more economical and practical in terms of environmental impact and cost [23]. Plenty of researches indicated that application of biochar as an amendment has the beneficial effects of improving soil fertility, which refers to the ability of soil to promote agricultural plant growth [24], [25]. Its well-developed pore structure, large specific surface area and high physical biological stability allow biochar to change the physical (e.g., soil available water content and water holding capacity), chemical (e.g., nutrient utilization efficiency and carbon sequestration) and biological (e.g., microbial abundance and activity) properties of soil within the plant root zone [26]. In acidic soil, biochar has an obvious effect on increasing crop yield and improving soil properties [27], [28], while it also has a promotion effect on the growth of crops in alkaline soil, it is not as obvious as seen in acidic soil [29], [30]. In recent years, there has been increased interest in research related to the absorption of heavy metals and organic pollutants by biochar. Mohamed [31] combined K3PO4 and plagioclase with biomass to reduce the plant toxicity and plant absorption of heavy metals. Kasak and Gupta et al. [32], [33] added ligneous biochar to artificially treated wetlands, which not only improved the efficiency of wastewater treatment but also promoted the development of organisms. The application of biochar in other building materials has also shown positive results. For example, as a modifier for asphalt cement, biochar has improved anti-aging performance and relieved temperature sensitivity [34].
These excellent qualities of biochar described above are not only expected to enhance the physical and mechanical properties of vegetation concrete but also to improve planting properties. Maria [35] added carbon nanotubes into cement mortar, while Gupta [36] used biochar, both of which improved the material’s mechanical properties. At the same time, it was also shown to have function to isolate carbon dioxide. Meanwhile, in work by Akhtar [37] the addition of biochar to concrete was able to improve flexural strength. However, there has not been a report on the study of biochar influencing plant growth on vegetation concrete. In this article, the variation of porosity, permeability, compressive strength, as well as the plant properties of vegetation concrete, were measured under different biochar mass contents; therefore, the most suitable grass for vegetation concrete slope protection was selected, and the optimal mix proportion for the fabrication of new biochar-modified vegetation concrete to satisfy current needs was also recommended.
Section snippets
Ordinary Portland cement
The ordinary Portland cement used in this study was produced in Tangshan, China, and had a strength of 42.5. The chemical composition of ordinary Portland cement is shown in Table 1 and its physical performance indexes are shown in Table 2.
Coarse aggregates
The coarse aggregates were a mixture of limestone and gravel produced in Beijing, China. The coarse aggregates were screened to an aggregate gradation of 10–25 mm with an apparent density of 2780 kg/m3 and crushing index of 8.5%.
Admixtures
Slag and fly ash were used as
Grass species selection
The germination rate, plant height and root length of ryegrass, tall fescue and Bermuda grass planted on vegetation concrete in the same proportion of 5.6 kg/m3 were measured in the experiment, as shown in Table 5. All three species could adapt to the growth environment produced by vegetation concrete, and the adaptability of ryegrass and Bermuda grass was superior to tall fescue according to the data. In addition, compared with Bermuda grass, ryegrass showed a higher germination rate of 78%,
Conclusions
To modify the plant adaptability in the environment of vegetation concrete, biochar is added as an amendment to fabricate vegetation concrete of different mix proportions, which has been illustrated in this research. The effects of biochar on porosity, permeability coefficient of vegetation concrete and the growth conditions of different grass species have been evaluated. It can be concluded that biochar bonded with some cementitious material fill in the pores of vegetation concrete, which
Conflict of interest
There is no conflict of interest to declare.
Acknowledgements
The authors would like to acknowledge the financial support from the National Key R&D Program of China (2017YFC0403600, 2017YFC0403602), and National Student’s Platform for Innovation and Entrepreneurship training program (201710019199).
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