Geopolymer Concrete: A Sustainable Alternative to Portland Cement

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 10 2024

Portland cement, widely used in concrete manufacturing, is one of the most used products globally. The production of Portland cement requires a lot of energy and raw materials and contributes to global warming through significant CO₂ emissions.¹ Recently, geopolymer concrete has emerged as a sustainable alternative to Portland cement, promoting environmentally friendly building materials.²

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Geopolymers are semi-crystalline or amorphous three-dimensional aluminosilicate structures formed by combining [SiO₄]^4- and [AlO₄]^5- tetrahedra. The source materials are industrial residues such as fly ash and blast furnace slag, activated by alkali binders to produce geopolymers.^1,2 This article explores the application of geopolymers in the construction sector, replacing harmful traditional Portland cement while helping industrial waste management.

Production of Geopolymer Concrete

Geopolymers are inorganic polymers fabricated by the polycondensation reaction of aluminosilicates with alkalis.³ Commonly used aluminosilicates include silica fume, fly ash, metakaolin, kaolinite, feldspar, red mud, calcined clays, mining waste, waste glass, metallurgical slag, zeolite, rice husk ash, etc.^1,3 Alternatively, the common alkali activators used in geopolymers are sodium hydroxide, potassium hydroxide, sodium silicate, and potassium silicate.³

These aluminate and silicate species are dissolved in concentrated alkaline solutions followed by reorganization, condensation, and polymerization. Several oligomers rope together to form geopolymers, releasing free water as the by-product.² The reaction of aluminosilicates with alkali solution depends on their chemical and physical attributes such as fineness, glassy phase composition, and mineralogy.³

Using fine resource materials improves the mechanical properties such as compressive strength of the geopolymer. The properties of geopolymer can be further enhanced for a specific application by upgrading the reaction mixture and post-processing treatments.³ Heat treatments and curing methods are employed to enhance geo-polymerization.

Geopolymer pastes cured in a microwave oven exhibit higher compressive strength. However, the penetration depth of microwaves is only a few centimeters, limiting its effectiveness. Alternatively, novel and eco-friendly solar curing can effectively cure structures with high surface areas.¹

Properties and Advantages

Geopolymer concrete possesses promising engineering properties as a sustainable alternative to conventional concrete.¹ Compared to ordinary Portland cement (OPC) concrete, it can reduce up to 80% embodied carbon.³ Moreover, the CO₂ footprint of geopolymer concrete has been estimated to be 9% less than concrete containing 100% OPC. Thus, geopolymer concrete can help effectively recycle solid waste, saving a lot of resources, and simultaneously reducing CO₂ emissions.¹

The starting materials govern the physical properties of geopolymers. For instance, slag-based geopolymer concrete exhibits low workability and flowability. Alternatively, silica fume with fine particles produces concrete with good workability but requires high water proportion. Moreover, the high reactivity and large surface area of silica particles accelerate the reaction process and decrease sitting time during concrete formation.²

The compressive strength of geopolymer cement pastes is generally higher than that of OPC paste.¹ Additionally, geopolymer concrete strength increases with time similar to that of Portland cement. This is attributed to the continuous polymerization and condensation of the precursors.³

Furthermore, geopolymers offer the advantages of excellent stability at high temperatures (applicable as fire-resistant material), resistance to chemical attacks, and resistance to freeze-thaw cycles.^1,2 Steel bars reinforced in geopolymer concrete demonstrate a lower corrosion rate than those in Portland cement concrete.¹

Applications of Geopolymer Concrete

Owing to its attractive properties such as fire resistance, heat resistance, corrosion resistance, and overall mechanical performance, geopolymer concrete can replace OPC in several applications.¹ It is most popular in precast applications due to the relative ease of handling sensitive materials and controlled high-temperature curing. Consequently, geopolymer concrete is used in fabricating railway sleepers and sewer pipes.³

Geopolymers exhibit piezoelectric behavior under load. Thus, the geopolymer concrete acts as a self-sensing material by incorporating elements like graphene. This can help sense the deterioration of civil structures with time, prompting required measures to ensure their safety and durability.¹

In aggressive environments such as tunnel segments and sewer pipes, geopolymer concrete is more suitable than OPC. The former also offers high resistance to chloride, making structures more durable in the areas where salt is used to melt ice cover. Additionally, it is favorable for structures in constant attack from saltwater such as piers, coastal bridges, and underwater constructions.³

Geopolymers are modified with other materials to produce multifunctional concrete. For example, concrete incorporated with photocatalytic materials like titania and zinc oxide exhibits self-cleaning, providing cleaner and aesthetically pleasing structures. Such materials are great alternatives to traditional materials used to restore and preserve cultural heritage buildings.¹

Challenges

Despite several advantages, the adoption of geopolymer concrete over traditional concrete is limited due to several challenges. The primary one is the lack of data on the long-term durability of geopolymer-based structures. Additionally, the geo-polymerization mechanism is not fully understood due to the wide range of raw materials used, which contain a variety of impurities.¹

No standard mix design and testing method exist for geopolymer concrete. Thus, comprehending the effect of production materials and procedures is difficult.³ Moreover, geo-polymerization occurs under high pH conditions, requiring high curing temperatures to obtain the desired strength. Consequently, room-temperature-cured geopolymer concrete needs to be developed to reduce power consumption.¹

Efflorescence is common in geopolymer concrete structures due to the high alkalinity and mobility of binders used during polymerization. This affects the structure’s surface and also reduces its strength. The sealed curing method can avoid this phenomenon. However, it decreases the early-age compressive strength of the concrete. Thus, further improvement is required in formulating and manufacturing geopolymers.¹

The sustainability of geopolymer concrete is also generally questioned. This is due to the simplified assessments and indicators used to evaluate concrete materials.

These assess metrics like carbon footprint and energy efficiency. However, factors like water usage, air pollution, and ecosystem impact are not adequately considered. Moreover, universal standards for evaluating the sustainability of concrete materials have not been established yet.⁴

Future Prospects

Novel methods are emerging to enhance the usability and sustainability of geopolymer concrete. For instance, a recent study in Discover Materials explored using agro-waste in geopolymer concrete. Agricultural waste is rich in aluminosilicate, improving quality and reducing the cost of geopolymers. Agro-waste ash-based geopolymers have the potential to be the most durable, inexpensive, user-friendly, and eco-beneficial construction materials in the long term.⁵

Overall, well-established regulatory standards are necessary to make geopolymer technology adoptable by industries.¹ Additionally, geopolymer concrete’s environmental, economic, and social impacts should be comprehensively analyzed with a focus on long-term performance. This would help enhance awareness among the stakeholders and propel its application as an eco-friendly alternative to OPC.³

References and Further Reading

1. Singh, N. B., & Middendorf, B. (2020). Geopolymers as an alternative to Portland cement: An overview. Construction and Building Materials, 237, 117455. DOI: 10.1016/j.conbuildmat.2019.117455

2. Ahmed, L.A.Q., . Frayyeh, Q. J.,  & Ameer, O. A. A. (2022). Geopolymer as a Green Concrete Alternative to Portland Cement Concrete: Article review. Journal of Al-Farabi for Engineering Sciences, 1(2). DOI: 10.59746/jfes.v1i1.16

3. Almutairi, A. L., Tayeh, B. A., Adesina, A., Isleem, H. F., & Zeyad, A. M. (2021). Potential applications of geopolymer concrete in construction: A review. Case Studies in Construction Materials, 15, e00733. DOI: 10.1016/j.cscm.2021.e00733

4. Kanagaraj, B., Anand, N., Samuvel Raj, R., & Lubloy, E. (2023). Techno-socio-economic aspects of Portland cement, Geopolymer, and Limestone Calcined Clay Cement (LC³) composite systems: A-State-of-Art-Review. Construction and Building Materials, 398, 132484. DOI: 10.1016/j.conbuildmat.2023.132484

‌5. Alaneme, G. U., Olonade, K. A., & Esenogho, E. (2023). Eco-friendly agro-waste based geopolymer-concrete: a systematic review. Discover Materials, 3(1). DOI: 10.1007/s43939-023-00052-8

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