# Environmentally Friendly Smart Construction--Review of Recent Developments and Opportunities
### Use of Recycling Materials in Construction
#### 4.4.2 Geopolymer Concrete (GPC)  
Geopolymers are contemporary building materials made from various sources of aluminosilicates that offer environmental benefits and excellent durability. The term \"Geopolymer\" was coined to describe the polymerization reaction between an alkaline liquid (activator) and an aluminosilicate of a geological origin [261]. It is considered as a possible replacement for ordinary Portland cement as the third generation of cementitious materials. There are many examples of industrial waste materials that can be used as the binder material in geopolymer composites, including fly ash, furnace slag, glass waste and sludge. Low embodied energy and the possibility of obtaining the components of the concrete from recycling waste make geopolymers very attractive and environmentally friendly construction material. However, completely replacing cement with geopolymers is met with challenges, such as short setting time, the necessity for rigorous mix design, limited data on the long-term durability and a lack of standardized testing methods and regulations governing their use [262].  
Geopolymers consist of a source pozzolanic material and alkaline fluid. A pozzolan is an amorphous material rich in aluminium and silicon. This combination occurs in natural minerals such as clay and kaolinite and an industrial byproduct such as fly ash, silica fumes, blast furnace slag, rice husk ash and red mud. The pozzolanic material undergoes the polymerization reaction in the alkaline environment and forms an aluminium silicate gel, which is then converted into zeolite precursor gel used to form a geopolymer binder for the geopolymer concrete (GPC). The most common alkaline liquids used for the polymerization are sodium hydroxide, sodium silicate, potassium hydroxide and potassium silicate [52; 249; 263]. The quality of a geopolymer composite relies on the quality of gels produced through polymerization and pozzolanic reactions. Various factors contribute to the gel quality, including the fineness and silica content of the pozzolanic material, the alkalinity of the activator solution, the type and amount of the superplasticizer, the type and amount of fibre used and the curing conditions and duration. The use of various raw materials and alkaline fluids can produce considerable variations in GPC performance and mechanical properties.  
Many performance characteristics of GPC are still under investigation, but it has already been demonstrated that GPC exhibits superior corrosion resistance across a range of corrosive solutions [53; 264; 265], enhanced thermal and fire resistance in comparison to the ordinary Portland cement concrete (OPCC) [262; 263], high early strength [53; 263], and lower creep and shrinkage [53; 266]. The most attractive feature of geopolymers though is that they can provide high compressive and tensile strength to produce ultra-high strength/performance concrete (UHSC/UHPC) [267; 268]. To achieve a high strength in GPC, a greater proportion of pozzolan is required. However, adding more slag can impact the flowability of GPC due to the lubricating effect of the spherical particles. The sodium compound activators are typically used because they are more readily available and less expensive. Conversely, a use of potassium compounds can improve the compressive strength [268; 269]. UHPC- and UHSC-based geopolymers together with their enhanced corrosion resistance are well suited for offshore structures, protective barriers and high-rise buildings [249].