# Effect of Admixtures on Selected Properties of Fly Ash-Based Geopolymer Composites
## 1 Introduction  
Geopolymers (composites based on alkali-activated binders) can be an alternative to cementitious composites, which are the most commonly used materials in construction. In an era of global warming, geopolymer technology is a promising concept for replacing traditional cement production and reducing CO\({}_{2}\) emissions. When waste materials are used as the base for geopolymers, economic and ecological benefits can be obtained [1; 2; 3; 4; 5]. The properties of geopolymer composites depend on the composition, especially [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]:  
* Type of binder;
* Type of activator;
* Concentration of sodium hydroxide liquid (NaOH);
* Sodium silicate to sodium hydroxide liquid ratio by mass. * Additionally, different curing conditions affect the final properties of the geopolymer. Binders most commonly used in geopolymers include metakaolin, fly ash, and ground, granulated blast furnace slag. Metakaolin-based geopolymers are characterized by their consistent properties, but their water demand can be a problem in their production [6; 11]. Fly ash-based geopolymers show better mechanical properties [6]. Slag-based composites, on the other hand, have better resistance to aggressive environments compared to the other two [6]. The origin of the binder itself is also of great importance. The oxide composition of fly ash, for example, varies depending on the combined heat and power plant from which it was obtained. The ratio of alumina to silicon dioxide is particularly important in the production of geopolymer composites [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The most used alkali activators are sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)), and potassium silicate (K\({}_{2}\)SiO\({}_{3}\)), in most cases mixtures of those substances. In most studies, concentrations ranging from 4 to 14 M wereused [5; 7; 8; 12]. In most cases, the molar ratio of sodium silicate to sodium hydroxide was mainly tested at 2.0 or 2.5 [5; 9; 12; 13; 14; 16]. The concentration of sodium hydroxide, either used as a standalone activator or combined with sodium silicates, also has a strong influence. In most studies, concentrations ranging from 4 to 14 M were used [5; 6; 8; 9; 12; 14; 16]. Many researchers have examined the impact of different maturation conditions [6; 7; 8; 9; 10; 12]. In the production of geopolymers, elevated temperatures are necessary to allow them to harden. The temperature range most commonly used varied from 40 to 85 \({}^{\circ}\)C. Samples were treated at a higher temperature usually for 24 h.  
The effect of admixtures on geopolymers is not yet resolved. Especially since, for example, the effect of plasticizing admixtures in cement concrete is well known, but in geopolymer concrete, it may be quite different. The effects of admixtures on workability and mechanical properties were mainly checked. Sometimes they were only added as an additive to improve consistency, without assessing the impact on properties [9; 11; 16; 17; 18; 19]. Geopolymer composites based on fly ash (FA) and ground granulated blast-furnace slag (GGBS) are usually tested. Several studies have not found a clear improvement in properties through the use of various admixtures, including vinyl copolymer and polyacrylate copolymer, naphthalene-based, melamine-based, lignocellulosic, and modified polycarboxylates [9; 12; 16; 17; 18]. [PERSON] et al. [20] studied the effect of a polycarboxylic ether-based high-range water-reducing superplasticizer on geopolymer concrete specimens, where FA and GGBS were used in a 50:50 ratio. The most favorable results were obtained with an admixture of 3% by weight of the binder. A 15% increase in compressive strength and a reduction in water absorption of approximately 12% compared to concrete without the admixture were found.