US EPA

Portland cement concrete is the most widely used construction material, and is a prevalent component of construction and demolition (C&D) waste. The large carbon footprint and energy content of Portland cement concrete; the constraints on its strength, durability and capability to encapsulate toxic elements; and the low value of C&D waste concrete have created a growing demand for alternative cementitious materials. The limitations of concrete are deep-rooted in Portland cement chemistry, which is based on calcium silicates forged at extreme temperatures through polluting and energy-intensive processes. The hydrates of these calcium silicates lack the integrated structure with primary bonds that provide rocks, ceramics and other materials with far superior engineering properties. The massive quantities of concrete consumed magnify the adverse economic and environmental consequences associated with these drawbacks.

 

Growing efforts have been devoted in recent years to the development and market transition of new concrete chemistries. A new class of concrete provides a technically and economically viable basis for overcoming the challenges of today’s concrete materials. This concrete relies upon chemically versatile aluminosilicates to render binding effects, which are formed using minimally processed and abundant waste products and possess extended 3D structures integrated with primary bonds. The energy content and carbon footprint of the emerging aluminosilicate-based concrete are an order of magnitude less than those of traditional concrete materials based on calcium silicate binders. The superior chemistry and structure of aluminosilicate-based concrete materials provide them with strength, impermeability, durability, and (hazardous element) encapsulation qualities that far surpass those of traditional Portland cement concrete. The versatile nature of aluminosilicate-based concrete materials enables effective and safe use of substantial quantities of C&D wastes and industrial byproducts. Finally, the aluminosilicate-based binders exhibit a tendency towards crystallization with aging, which raises the value of their demolition waste as quality aggregates when they reach the end of their extended service life.

 

The proposed project focuses on further lowering the cost and sustainability of aluminosilicate-based concrete materials through value-added use of C&D wastes and some abundant industrial byproducts. The resulting high-recycled-content geopolymer concrete materials could be tailored towards practically all applications of traditional concrete materials. The precast/prestressed concrete industry is the sector within the broader concrete industry that has been more open to the adoption of new technologies, and uses high-performance concrete on a routine basis. Metna Co. will work with both this sector and the broader industry towards development, pilot-scale implementation and market transition of high-recycled-content geopolymer concrete materials. The precast concrete and concrete pipe manufacturers, concrete materials suppliers, builders, and the waste management industry would be Metna Co.’s key partners in scale-up, field evaluation and market transition of the technology.

 

The key environmental benefits of the technology are: (1) close to 90 percent reduction in the carbon footprint of concrete, noting that production of today’s Portland cement for use in concrete accounts for about 7 percent of man-made CO₂ emissions; (2) value-added use in geopolymer concrete of major constituents of C&D waste (demolished concrete, waste glass, gypsum, waste concrete sludge) and market-limited industrial byproducts (foundry sand and growing volumes of clean coal combustion ash, which do not suit use in traditional concrete); and (3) effective encapsulation in geopolymer concrete of any toxic elements present in C&D waste, sludge, ash, foundry sand and other waste raw materials. These benefits have been shared with representatives from the concrete industry and other industries whose byproducts will find value-added applications in geopolymer concrete. The letters presented in the proposal indicate that the industry concurs with the envisioned environmental benefits of the technology.