The Behavior of Structural Elements of Geopolymer Concrete

Today, the construction industry is one of the fastest-growing industries in the world. However, this necessity to construct has resulted in the yearly utilization of over 4 billion tons of cement worldwide. Even more, this use is expected to increase rapidly in the coming years.

However, while cement remains an inevitable component of the industry, due to its constitution — Limestone — it continues to pose a significant threat to the environment. Worse, the constant use of Limestone is not sustainable.

As such, an alternative material became imperative. And in response to this current need, Geopolymer concrete was adopted. Its composition meant the world could reduce pollution of its environment, ensure the sustainable use of Limestone, and still satisfy the world’s construction needs.

However, since it remains a somewhat new binder, it becomes necessary to examine its behavior. In light of this, this essay examines the behavior of structural elements of Geopolymer concrete.

The Geopolymer concrete is a synthetic binder that was identified in 1978 by Professor Davidovits as an alternative to the traditional Portland cement. It involves a replacement of the Portland cement with aluminosilicate filled materials. These materials include rice husk, fly ash (the most commonly used material), blast furnace slag, red mud, and metakaolin.

This material is then activated through an alkaline solution. In turn, the chemical reaction produces an inorganic amorphous three-dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds (Davidovits 1991). Also, this cement possesses certain features that make it desirable. They include high mechanical strength, low shrinkage, excellent acid resistance, high temperature (800 °C), fast setting, and exceptional durability.

Geopolymer concrete comprises three elements, namely — Fly Ash, Sodium Hydroxide or Potassium Hydroxide, and Sodium silicate or Potassium silicate. Also, the polymerization process of Geopolymer concrete does not include water. Instead, water is expelled during the curing and drying process. This, in turn, ensures that the chemical and mechanical properties of a Geopolymer concrete are distinct from that of a traditional Portland cement. Precisely, the Geopolymer concrete possesses more resistance to heat and other inorganic solvents, alkali-aggregate reactivity, water ingress, and several types of chemical attacks. It is also impermeable and possesses up to 1.5 times higher compressive strength than the traditional Portland cement.

However, while the Geopolymer concrete is an upgrade to the traditional Portland cement, it still has some limitations. These limitations include transporting the fly ash to the specified locations, the high cost of acquiring the alkaline solution, the safety risk inherent in the high alkalinity of the solution, and the difficulties in applying high-temperature curing or steam curing process to the solution.

Although there are several studies on the structural intensity of the traditional concrete, research on the behavior of the structural elements of Geopolymer concrete is limited. As such, the behavior of Geopolymer concrete is examined through its bond strength and shear behavior.

The bond behavior of concrete is the relation between the concrete and the reinforcing steel. It involves the transfer of force from reinforcing steel to the concrete in its environs, usually through adhesion between the concrete and the reinforcing steel.

The level of resistance is then determined by factors such as the tensile and compressive intensity of the concrete, geometry of the reinforcing steel, and its surface condition, among others. Noteworthy, the level of bond strength between a concrete and steel reinforcement greatly determines how well the structural elements perform.

Concerning Geopolymer concrete, it has a greater normalized bond strength compared to the traditional Portland cement. This is because Geopolymer concrete possesses higher tensile strength than the conventional Portland cement with similar compressive strength.

Further, in an experiment, 12 beams of 200 mm × 300 mm in cross-section and 2500 mm in length were tested in the laboratory. Then the concrete compressive strength was set between 29 and 55 MPa. Subsequently, the beams were then subjected to a pure bending moment in the spliced region.

However, the Geopolymer concrete splits in the sliced area, just like the traditional Portland cement (Chang et al., 2009). As such, the existing design codes — American Concrete Institute Code ACI 318 — can be utilized for the conservative design of the bond strength of Geopolymer concrete.

According to (Sarker et al., 2013), the fracture energy of Geopolymer concretes increases when there’s a corresponding increase in the concrete compressive strength. And the Geopolymer concrete possesses more fracture energy than the traditional Portland cement. However, due to its higher tensile and bond strength, the failure modes of Geopolymer concretes are higher compared to the conventional Portland cement.

Also, the Geopolymer concrete possesses a dense interfacial transition zone that results in a higher critical stress intensity. And a more brittle failure-type with a smoother fracture plane than the traditional Portland cement. Similarly, Geopolymer concrete has a higher shear strength ranging from 4.5 to 23 % than the traditional Portland cement. As such, it is usually reliable for the construction of low-rise shear walls. In the same vein, American Concrete Institute Code ACI 318 and the Australian Standards 3500 can be used to determine the shear behavior of a Geopolymer concrete.

The Geopolymer concrete constitutes a viable solution to the problems associated with the traditional Portland cement. Its structural elements can efficiently and effectively satisfy the construction industry’s needs with little or no damage to the environment. It also possesses high resistance to heat, low shrinkage, and longer durability that makes it preferable to the traditional Portland cement. However, it is not without its limitations. Still, it represents a welcome development, more so the continued research to improve the geopolymer system.