Final Report: An Alternative Concrete Chemistry with Significantly Enhanced Durability, Sustainability, Economy, Safety and StrengthEPA Contract Number: EPD15036
Title: An Alternative Concrete Chemistry with Significantly Enhanced Durability, Sustainability, Economy, Safety and Strength
Investigators: Balachandra, Anagi
Small Business: Metna Co.
EPA Contact: Richards, April
Project Period: September 1, 2015 through February 29, 2016
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2015) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Building Materials
Portland cement concrete is the most widely used material of construction. The large carbon footprint and energy content of Portland cement concrete, and the constraints on its strength, durability and capability to immobilize toxic elements have created a growing demand for alternative cementitious materials. The limitations of concrete result largely from the chemistry of Portland cement that is based on calcium silicates forged at extreme temperatures via polluting and energy-intensive processes. Hydrates of these calcium silicates lack the integrated structure which provides rocks and ceramics with engineering properties that are far superior to those of Portland cement concrete. The massive quantities of concrete consumed (more than 4 tons/yr for every living human being) magnify the adverse economic and environmental consequences associated with these drawbacks of the conventional chemistry Portland cement. Growing efforts have been devoted in recent years to development and market transition of new concrete chemistries. A new class of concrete, generally referred to as inorganic polymer (geopolymer) concrete, provides a technically viable basis for overcoming the challenges of today’s concrete materials. The approach to production of this concrete, however, deviates from the broadly practiced methods of Portland cement concrete production. Today’s geopolymer concrete comprises an aluminosilicate precursor and alkaline solutions, which need to be mixed to produce a fresh concrete. This ‘two-part’ system realizes its full potential with thermal curing. Portland cement, on the other hand, is a ‘one-part’ hydraulic cement that hardens simply by the addition of water. Portland cement concrete also realizes its potential by room-temperature moist curing without resorting to elevated temperatures. The need for handling caustic solutions in field is another drawback of today’s geopolymer concrete. These drawbacks have seriously limited market acceptance of geopolymer concrete. The aluminosilicate-based chemistry of the binder in geopolymer concrete, on the other hand, offers significant performance and sustainability advantages; the robust nature of this chemistry also enables value-added use of significant quantities of industrial by-products and construction & demolition wastes which tend to be rich in aluminosilicates and alkali/alkaline earth metal compounds (reflecting the composition of the earth crust). This development effort has undertaken the challenge of developing an aluminosilicate-based hydraulic cement which combines the performance, sustainability and versatility advantages of (alkali activated) geopolymer binders with the constructability and economic benefits of Portland cement.
Summary of Findings
The Phase I project devised chemical and physical requirements which guided qualification and proportioning of raw materials for mechanochemical processing of aluminosilicate-based hydraulic cements. These requirements, which covered elemental compositions, mineralogical attributes, alkalinity, alkali-solubility and alkali-activation potential, were refined through implementation of an experimental program. Diverse C&D wastes and industrial by-products were evaluated as the prevalent raw materials for satisfying these requirements towards production of high-recycled content hydraulic cements based on the alkali aluminosilicate chemistry. These wastes/by-products included C&D brick, tile, gypsum and concrete wastes, different solid residues of conventional and clean (FBC, SCR) coal combustion as well as landfilled/impounded coal ash, and waste glass. Supplementary materials were also used to balance the chemistry and control the dissolution/hydration kinetics of the resulting hydraulic cements. These supplementary materials included sodium carbonate, calcium oxide, aluminum hydroxide and citric acid. Hydraulic cement formulations were developed around a prevalent waste/by-product that acted as the primary aluminosilicate precursor (e.g., impounded coal ash). For each primary (waste/by-product) aluminosilicate precursor, alternative formulations were devised to meet the specified requirements. Experimental studies were conducted to verify and refine these formulations, and to identify the more promising ones which were then subjected to more comprehensive performance, cost and sustainability analyses. Parallel analyses were also made on Portland cement in order to provide a basis to assess the competitive merits of the high-recycled-content hydraulic cements developed in the project. The effectiveness of one of the high-recycled-content hydraulic cements to immobilize heavy metals upon hydration was also evaluated. Preliminary studies were undertaken to assess the potential for production of high-recycled-content hydraulic cements with controlled variability, which consistently meet relevant standard requirements for the hydraulic cements used in concrete production.
Fundamental studies were conducted in order to gain insight into the mechanisms governing transformation of formulations developed around wastes/by-products into hydraulic cements. These studies included XRF, XRD, SEM and EDS analyses of waste raw materials, hydraulic cements, and hydration products. These investigations led to refinement and verification of hypotheses which explain processing of hydraulic cements in terms of activation and compounding of aluminosilicates via integration with alkali/alkaline earth metal cations.
Experimental results verified the theoretical potential to transform most of the C&D wastes/industrial by-products evaluated in the Phase I effort into hydraulic cements with performance characteristics matching or surpassing those offered by Portland cement and required by the performance-based ASTM standards. Most of the hydraulic cement formulations developed in Phase I project had recycled contents ranging from 75 to 95 wt.%, and made value-added use of wastes/by-products that failed to meet requirements for other applications (e.g., use as pozzolans in Portland cement concrete). Successful efforts led to tailoring of the dissolution/hydration kinetics of high-recycled-content hydraulic cements to match the fresh mix workability and constructability attributes offered by Portland cement, and the meet the requirement to undergo thorough hydration reactions at room temperature. Toxicity studies conducted with selected high-recycled-content hydraulic cements confirmed that the alkali aluminosilicate hydrates formed upon hydration of these cements effectively immobilize the heavy metal constituents of waste raw materials such as coal ash.
The prevalence of wastes/by-products among the raw materials of the hydraulic cements that are subject of this development effort, the use of low-cost and naturally available or recycled sources of alkali/alkaline earth metal cations and other supplementary materials/additives, and the economic and sustainability advantages of the cement manufacturing process developed in the project translate into major economic and sustainability benefits. Comparative initial and life-cycle analyses were performed on the cost, carbon footprint and energy content of selected high-recycled-content hydraulic cements versus Portland cement. Significant reductions in the initial carbon footprint, energy content and cost of concrete materials were found to result from replacement of Portland cement with any of the high-recycled-content hydraulic cements considered in this analysis. The superior stability and durability characteristics of high-recycled-content hydraulic cements based on the alkali aluminosilicate chemistry translate into increased service life of the concrete-based infrastructure. Life-cycle analyses demonstrated that the initial sustainability and economic advantages of the technology magnify when examined from a life-cycle point of view. The position of Portland cement production as a major source of carbon emissions and energy use, and the far-reaching economic and environmental benefits of a functional and long-lasting infrastructure further pronounce the initial and life-cycle sustainability and economic benefits that can be realized by implementing the technology. Diversion (or extraction) of large volumes of market-limited wastes from landfills for value-added use in construction is another environmental benefit of the technology.
The Phase I project was conducted in close collaboration with representatives of the waste management, power, cement manufacturing and concrete industries. These industrial firms were the sources of waste materials used in the project, and also guided the project team to undertake developments which maximize the use of market-limited wastes and address key market needs. A comprehensive patent search was conducted, through which the novel aspects of the approach were identified. Efforts were initiated to prepare a patent application based on the Phase I project outcomes. Successful efforts were undertaken in order to raise funds in support of the commercialization activities to be launched during the Phase II project. The collaborations with representatives of relevant industries led to the launch of efforts towards implementation of pilot-scale production and field demonstration projects based on the Phase I project outcomes.