Concrete is the most widely used building construction material, and it is the second most consumed substance on the planet after water. Twice as much concrete is used–nearly 35 billion tons, as all other industrial construction materials such as wood, steel, plastic, and aluminum combined due to its low cost, versatility, and durability. Concrete is a composite material made up of an aggregate phase and a binder (typically, Portland cement). The utilization of recycled concrete aggregates (RCAs) in concrete in lieu of natural aggregates has been extensively studied in the past two decades to avert the rapid depletion of limited natural resources as well as the issues associated with massive scale landfilling of construction and demolition waste. Once produced, RCA particles usually comprise a mixture of residual mortar and virgin aggregates; the quality of RCA largely depends on the properties of the parent concrete from which it is derived, relative amount of the attached old mortar, and the crushing process. The presence of RCA generally lowers the mechanical and durability performance of concrete compared to natural aggregates, mainly due to the inherent porosity of RCA particles caused by the microcracks in residual mortar.

Demolition of concrete structures

NO2 sequestered recycled concrete aggregates (NRCA) is an innovative waste cementitious material. It is the product in which demolished concrete is used as an absorbent for the removal of a primary air pollutant, NO2, without additives or catalysts. NO2 is a toxic gaseous compound that leads to the formation of ground-level ozone, acid rain, atmospheric particles, eutrophication, and various other toxic substances instigating environmental issues in addition to causing serious health issues. Current approaches in removing NO2 are typically expensive and cumbersome; in contrary, utilization of inexpensive adsorbents such as demolished concrete to capture NO2 has proven effective at eliminating overly complicated setup or control procedures.

The principal mechanism responsible for NO2 sequestration by concrete is the chemical neutralization between NO2 and surface alkaline hydrated products, such as calcium hydroxide (CH), and subsequent formation of Ca(NO2)2 and Ca(NO3)2. These nitrite/nitrate-based compounds are widely used as multi-functional concrete admixtures with corrosion inhibiting, set accelerating, anti-freezing, and as self-healing properties. Similarly, NRCA is found to be a promising constituent material that can improve both mechanical and durability properties of concrete, unlike conventional RCA. Notably, when incorporated as a fine aggregate, NRCA can mitigate chloride-induced steel corrosion in concrete similar to commercially available corrosion inhibiting chemical admixtures. Thus, incorporation of NRCA in producing concrete provides a new paradigm of turning solid waste material into a useful product. Even though coarse RCA have been used in various applications with much success, the use of fine RCA is usually limited owing to its unfavourable influence on performance of concrete. NRCA can be identified as an alternative and sustainable solution for this inherent problem associated with conventional fine RCA.

Further reading:

Dr. Erandi Ariyachandra,

PhD (Clarkson, USA), BSc. Eng. (Moratuwa, Sri Lanka)

Research Associate at Vinsi Partners Consulting Engineers

2.4/56 Delhi Rd, North Ryde NSW 2113


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