1 Extended Abstract

Nanotechnology is widely used in construction to advance high performance and durability materials, as represented by concrete, in which nanomaterials such as nanosilica (SiO2), nano-alumina (Al2O3), and nanotitanium oxide (TiO2) are used to replace cement as a partial binder. Suitable amounts of adding these nanomaterials are beneficial with regard to the mechanical, economic, and environmental aspects of concrete. For example, the pores in the cement matrix can be filled by nano-SiO2, which has been demonstrated to enhance by ≈10% the comprehensive strength and by 25% flexural strength of the concrete [1]. Nano-TiO2 concrete shows outstanding effective self-cleaning because TiO2 nanoparticles can trigger a photocatalytic degradation of gas emissions (e.g. NOx, CO and VOCs), reducing the environmental impact of such concretes in a promising way [2].

To seek better performance of the concrete with the least amount of virgin material and address the global issues of construction and demolition waste and emissions, some researchers have introduced both recycled aggregate and nanomaterials into concrete, but environmental evaluations of such concrete products are scarce. Therefore, we assessed a broad range of environmental impacts of recycled aggregate concrete in a comprehensive literature review of sustainable concretes containing recycled aggregate and nanomaterials to establish an experimental database. Only concretes with recycled aggregate and nano-SiO2 were included after the review process. These concrete products were evaluated by life cycle assessment (LCA), considering both volume and compressive strength.

The LCA model fitting sustainable concretes incorporating recycled aggregate and nanomaterials was modified from the framework of Xing et al. [3], based on the normal conditions and current technology applied to the Australian concrete industry. The functional unit was 1 m3 of ready-mix concrete, considering its 28-day compressive strength, and the system boundary was from cradle to gate. The LCA methodology was CML 2002, but focused primarily on carbon emissions and energy consumption.

The literature review process resulted in 92 mix designs containing recycled aggregate and nano-SiO2 and their carbon emissions and energy consumption were quantified by the LCA model, as shown in Fig. 1. Generally speaking, the incorporation ratio of nano-SiO2 was limited to 15% whereas virgin aggregate could be fully replaced by the recycled aggregate. The carbon emission and energy consumption of the concretes examined were 91.6–607.1 kg CO2 eq., and 611.6–2453.6 MJ, respectively. With the development of compressive strength, both the carbon emissions and energy consumption of the concrete increase. When other supplementary cementitious materials are simultaneously used in the concrete mix, the concrete products perform much better.

Fig. 1
2 scatterplots with positive correlations of 8 nano silica ratios from 2% to 15%. 1. Carbon emission, kilograms of carbon dioxide equivalent, versus compressive strength in megapascals. 2. Energy consumption mega pascals versus compressive strength.

Carbon emissions and energy consumption of concretes

Full size image

Compared with the effect of nano-SiO2 on the strength and environmental impact of the concrete, substituting recycled aggregate was less effective. We therefore suggest introducing nanomaterials into recycled aggregate concrete for higher potential regarding the sustainability of the concrete.

Our study quantified the effects of nanomaterials and recycled aggregate to the overall environmental impacts of designed concrete products and estimated the ecological performance based on the mix design and compressive strength.