Characterization of Alternative Concrete Binders
Research Problem Statement
Concrete is the most commonly used construction material, however, the production of ordinary Portland cement production contributes 2-3% of global energy use and 5% of world-wide carbon dioxide emissions (CO2). Furthermore, ordinary Portland cement may result in durability problems on concrete constructed in aggressive environments (e.g. with high sulfate concentrations) and thus, it may not be the ideal material for all types of construction. Modern construction practices push for a lower carbon footprint and alternative concrete binders could be a significant solution as many alternative binders consist of more waste products, use less energy, generate less CO2 during production, and improve concrete performance. Alternative binders have been around for a while, though have always faced the problem of cost and that many entities specify ordinary Portland cement concrete. These entities have begun to allow for higher levels of supplementary cementitous materials, usually waste materials, such as fly ash, ground granulated blast furnace slag, and silica fume, but ordinary Portland cement concrete is still generally the dominate portion of the blend. As the industry moves toward end result or performance-based specifications, alternative binders should become a more available and affordable option.
The chemistry of alternative binder systems is different than the ordinary Portland cement’s chemistry, thus the knowledge that comes through the research that has been conducted on concrete containing ordinary Portland cement may not be applicable for the alternative binder systems. Therefore, more research is needed to investigate the fresh and hardened properties.
The objectives of the proposed research are to characterize fresh and hardened properties of alternative concrete binders. The characterization should also include environmental impact and cost comparison to ordinary Portland cement production and construction use. Binders will be compared chemically as well as physically. Fresh concrete properties to be identified and compared include: temperature, slump, slump loss, unit weight, air content, and set time. Hardened properties to be identified and compared include: compressive strength gain, modulus, flexural strength gain, permeability / surface resistivity, shrinkage, porosity, efflorescence, carbonation, freeze/thaw durability, abrasion, resistance to sulfate attack, salt-scaling resistance, and resistance to alkali-silica reaction.
The list of potential binders / technologies is continually growing with the changes in technology. The following binders have the potential for inclusion into this study. Note that this list is not inclusive of all potential alternative binders available.
- Alkali activate binder (geopolymer): Alkali-activated binders are generally preferred due to their higher strength and durability, and lower environmental impact as compared with ordinary Portland cement.
- Aluminosilicate powders (generally fly ash, slag, or metakaolin) with alkali activating solution.
- Calcium aluminate cement: Calcium aluminate cement contains primarily monocalcium aluminate (CA) and sometimes C12A7 and/or CA2. There may be silica in small amounts in the form of C2S and/or C2AS (gehlenite). Calcium aluminate cements are generally used in refractory and building chemistry applications such as floor screeds and rapid-hardening mortars. Calcium sulfoaluminate cement: Calcium sulfoaluminate cements contain approximately C4A3S as a major constituent (30–70%). Calcium sulfoaluminate cement results in lower CO2 compared to ordinary Portland cement.
- Supersulfated cement: Supersulfated cements are generally comprised of blast furnace slag, calcium sulfate and an alkaline activator. A small quantity of Portland cement is often used as an alkaline activator. Supersulfated cements are binders free of ordinary Portland cement clinker. Supersulfated cements are often preferred due to their very low heat of hydration and their good durability in chemically aggressive environments.
- Generally blast furnace slag, calcium sulfate, and alkali activator.
- Magnesium phosphate cement: Magnesium phosphate cement provides a rapid setting and early strength development, thus it is generally used for emergency repairs of concrete pavements areas where it is not desirable to take the area out of service for a long period.Hydraulic lime cement.
- Portland limestone cement: Limestone has been used as a filler; however, in recent years, it is used as a partial replacement for ordinary Portland cement. Portland limestone cements are categorized based on the percentage of limestone added to the cement. Although, most Portland cement specifications allow the use of limestone up to 5%, Portland limestone cements with typically 5% to 35% of limestone are being produced and used in various countries around the world.