Geopolymer Concrete
Concrete, as one of the most widely used building materials, is known in civil engineering projects worldwide. Considering the high consumption of concrete and the increasing demand for cement production, attention to the environmental detrimental effects of this substance, including its contribution of about 7% to carbon dioxide emissions in the atmosphere and considerable energy consumption such as electricity and fossil fuels, is inevitable, and offering alternative products in the path of sustainable development is considered essential. Geopolymer concrete can be a suitable substitute for Portland cement concrete as a scientific and practical solution. Geopolymer cements require less energy for production and reduce carbon dioxide emissions by 22 to 72% compared to Portland cements. Therefore, one of the solutions to produce environmentally-friendly concrete is the use of pozzolanic and supplementary cementitious materials and reducing the consumption of Portland cements.
What is the history of the term geopolymer?
In the 1950s, Viktor Glukovskii from the Soviet Union developed cementitious materials initially known as “silicate earth concretes” and “earth cements.” However, since the introduction of the concept of geopolymers by Joseph Davidovits in 1991, this term became widely recognized. Other titles used for the term geopolymer include “alkali-activated cement,” “geocement,” and “alkali-bonded ceramic.” Despite the diverse nomenclature, all these terms describe materials that undergo complex reactions of alkali dissolution and precipitation.
Geopolymers are a new type of adhesive material that replaces Portland cement and is produced by the reaction of an aluminosilicate material like slag with an alkaline solution, resulting in a type of concrete known as geopolymer concrete. Adhesives are classified based on the nature of their adhesive components into high-calcium materials like slag and low-calcium materials like Class F fly ash. Some advantages of geopolymer concrete over Portland concrete include high compressive strength, higher resistance to chemical attacks and chloride ion penetration, resistance to freeze-thaw cycles, resistance to abrasion especially when filled with polytetrafluoroethylene fillers, heat resistance, non-release of toxic vapors, low thermal conductivity, adhesion to old, fresh, steel, and ceramic surfaces, inherent steel protection due to high residual pH, low chloride diffusion rate, and disadvantages include rapid setting and early loss of effectiveness, high drying shrinkage, and high carbonation rates.
Non-structural research on geopolymer concrete and its applications in non-structural fields have been widely spread throughout the world; however, research on the structural behavior of this concrete, especially in Iran, is limited.
Polymers are based on organic materials (carbon-based) or mineral polymers (silicon-based).
Organic Geopolymers: Natural Geopolymers (rubber, cellulose) Artificial Organic Geopolymers (textile fibers, plastics, elastomers, etc.)
Biological Natural Polymers (biological, medical, pharmaceutical)
Inorganic Geopolymers: Inorganic Geopolymers are usually ceramic, aluminum silicate materials that form covalent and non-crystalline (amorphous) bonds.
Commercial Geopolymers may be used for fire and heat-resistant coatings and adhesives, pharmaceutical applications, high-temperature ceramics, and more. They can also be used as new adhesives for fire-resistant fiber composites, encapsulation of toxic and radioactive waste, and new cements for concrete.
Performance: Increasing the concentration of alkali activator in slag activation reduces performance. Silicate modulus can also be effective in concrete performance, as a modulus of less than 0.5 reduces performance, while a modulus between 0.5 and 1 improves performance.
Setting time: The setting time of geopolymer cement paste depends on its temperature, and the rate of temperature increase should be controlled to prevent cracking due to rapid water loss.
Compressive strength: The compressive strength of geopolymer concrete in the first three days can reach up to 90%. The type of activator material and the processing method in geopolymer concrete are important for the compressive strength, with longer durations and higher temperatures during processing resulting in higher compressive strength.
Adhesive strength: The adhesive strength of geopolymer concretes is higher than that of ordinary concrete, which directly relates to their compressive strength. The factors present in geopolymer concretes that affect their compressive strength also increase their adhesive strength.
Shrinkage: The shrinkage in geopolymer concretes varies significantly; some researchers report very low shrinkage, while others report significant shrinkage depending on the mix design. This variation is due to the materials used in geopolymer concretes.
Microstructure: Geopolymer concretes have fine cracks in their structure in the early ages, which decrease as geopolymerization progresses and with increasing age up to 28 days, the structure becomes denser and less porous.
Durability: Geopolymer concretes have a cohesive and low porosity structure, offering low permeability and good durability.
Acid attack resistance: Due to their composition based on silicate-aluminates, geopolymer concretes have good resistance to corrosion.
Low energy consumption: Geopolymer concrete is made from by-products of industries such as fly ash, blast furnace slag, etc., requiring very minimal energy consumption for its production. Additionally, no energy is required for processing these concretes as they can set at ambient temperature.
Benefits of Geopolymer
Increasing the lifespan of deteriorated structures to over 50 years Better physical performance compared to traditional Portland cement Significant reduction in greenhouse gas emissions Higher chemical resistance to corrosion, acidic and alkaline attacks High resistance to temperature and fire Suitability for various industrial markets, including power generation, chemical industries, and oil and gas
Implementation Challenges of Geopolymer Concrete Alongside the advantages and features of polymer concrete, it should be noted that the implementation of this concrete comes with challenges. Common challenges in implementing geopolymer concrete, based on Wang’s research, include:
Quick setting Formation of efflorescence on the surface Variability in strength Alkaline reaction with aggregates and others…
Geopolymers as a Suitable Replacement for Portland Cement The growing global demand for concrete presents an excellent opportunity for the development of geopolymer cements, which emit significantly less CO2.
In 2021, it was found that geopolymer mortars provide compressive strength and flexural resistance.
These mortars are sufficient for construction applications. They also offer the advantages of stability in addressing global warming potential. This indicates that they are good alternatives to conventional Portland cement.