Date of Award
Julio F. Davalos
Concrete, Sustainability, Materials Characterization, Structures
Concrete is the most used material in the world, second only to water. Conventional concrete is produced with Portland cement (PC). The production of cement is an energy intensive operation that has raised significant environmental concerns, since one ton of cement generates an equal ton of CO2. In the USA about 90 million tons of cement are used annually, with 3 million tons used in New York. Most contemporary concrete applications for buildings and infrastructure use high-performance concrete (HPC) mixtures which are produced by replacing a percentage of cement with supplementary cementitious materials (SCMs), consisting mainly of fly-ash (FA) and ground granulated blast furnace slag (GGBS or Slag). The addition of SCMs is necessary to achieve high strength, increased durability, and lower permeability of deleterious substances such as salt solutions. The added benefit of SCMs is the production of “Green Concrete” by reducing CO2 footprint.
The two most used SCMs are fly-ash and slag. The proper availability and cost of these materials are paramount for the concrete industry, and recently, there has been an inconsistent supply of FA, which is a residue from the combustion of coal burning plants. In the USA coal-powered plants are shutting down or converting to cheaper and cleaner natural gas, due to environmental regulations. The scarcity of FA is compounded by price and supply instability of imported slag, which is a byproduct from steel mills mainly outside the USA. Thus, in response to the urgent need for an effective and economical SCM to support the concrete construction, this work has been focused on the evaluation of a new glass pozzolan (GP) from recycled post-consumer glass.
The GP branded as Pozzotive® has been developed by Urban Mining Northeast as a proven product used in concrete-block, cast-in-place and pre-stressed concretes. Pozzotive® conforms to ASTM C-618 specifications for pozzolans and is produced from recycled mixed-color glass, which currently has no value-added real market application except as filler material. In the USA about 12 million annual tons of glass are discarded (~30% recycled). In NYC about 200,000 tons per year (~50% recycled) is available and represents an exceptional sustainable resource for large-scale production of GP.
With the goal of contributing significantly to the implementation of sustainable concrete, this research focuses on the development of mixture designs for different applications, characterization of concrete mixtures and evaluations of fresh and hardened properties with different percentages of cement replacement by GP (between 20% and 40%). Concretes with GP were tested at macro- and micro-structure levels. Evaluation of fresh properties, such as setting time, workability and pump-ability, is essential for concrete plants and construction planning, and it showed that GP is similar to slag due to the fineness of the particles and their angular shape. In relation to slag, which is cementitious in nature, the mechanical properties (strength and stiffness) of pozzolanic GP showed lower values at early age due to delayed reactivity; however, between 28 and 56 days all concretes with GP achieved the same or slightly higher values than their references with slag, fly-ash, and cement. Durability properties were evaluated and concretes with GP showed very low permeability and high freeze-thaw resistance, demonstrating undoubtedly better performance than the reference concretes. Chemical and physical properties characterizations were performed on cementitious materials (FA, Slag, GP, and cement), cementitious pastes, and concretes mixes with GP and FA, slag, and cement. The results showed that GP acts both as a filler and as a pozzolanic material. Substituting higher cement percentages with GP increases the porosity, however it results in more refined pore structure of cementitious matrix. Adding more GP results in smaller spacing factor and higher surface area of air voids, which improves resistance to freeze-thaw and thus increases durability.
This research was applied in field projects for sidewalks and high-rise buildings. It is significant that this work also contributed to correlate changes in microstructure to achieve target performance and durability characteristics for the development of recently approved ASTM C-1866 Standard Specification for Ground Glass Pozzolan for Use in Concrete (2020), which will allow wide acceptance and applications of GP and provide benefits to both the Concrete and Solid Waste industries.
In the East coast of the USA there is an estimated one to two million tons of mixed-color glass, which does not need to be separated by color to be used for producing glass pozzolan. This material as well as other large quantities in urban communities in the country can be efficiently transformed into value-added GP, which is an inert and environmentally friendly product that can contribute to CO2 reduction worldwide.
The outcomes of this research, supported by field application projects, and the publishing of the research and field results, have facilitated the development of ASTM guidelines for using GP. For the approval of the ASTM C-1866 Standard Specifications, it was necessary to demonstrate that this material is as good as or better than other SCMs, not just in the laboratory but also when applied in actual projects. The future goal is to also contribute to the development of standard guidelines for NYC-DDC and -DOT who anticipate being able to prescribe percentages of glass pozzolan in construction projects in New York. Once it is shown that this product works successfully in the New York area, we believe that this experience can be applied elsewhere worldwide.
Krstic, Marija, "Macro - and Microstructure Evaluation and Field Applications of Concrete with Recycled Glass Pozzolan" (2020). CUNY Academic Works.