# Influence of the Composition of the Activator on Mechanical Characteristics of a Geopolymer
## 1 Introduction  
Some old structures, such as the Egyptian pyramids or Roman constructions have lasted more than 2000 years and are still in good condition, showing a remarkable durability. [PERSON] argued that many of them were made of aluminosilicate calcium hydrates similar to those of Portland cement and also of crystalline phases of an calcite, a natural rock. These constructions have been shown to have outstanding behavior, not only concerning the mechanical performance, but also in terms of durability and resistance to weathering [1; 2; 3]. The (re)discovery of these materials could be very important for the construction industry. In fact, in addition to the higher strength properties shown, these materials might solve some important limitations of ordinary Portland cement (OPC): environmental problems with the release of carbon dioxide (CO\({}_{2}\)) during the production process, and also performance due to aggressive environmental conditions [4; 5; 6; 7]. Some authors claim that the technology reduces CO\({}_{2}\) emissions caused by the cement and aggregates by 80% to 90% when compared to OPC [8; 9; 10]. Other authors found that the difference in CO\({}_{2}\) emissions is not so high, but they confirm that the technology has clear advantages in such aspect [11]. Therefore, the need to promote research for materials alternative to OPC seems clear, and such material must be also competitive in its mechanical behavior. A possible solution seems to bethe use of a binder obtained by alkaline activation. In general, when compared to OPC, these materials have shown acceptable mechanical strength (sometimes the strength is higher than that of normal concrete structures, made from OPC). The development of the strength can be very quick, which might speed up the construction rate. Durability and stability are also acceptable. Understandably, these characteristics depend on the mix proportions of the components of the concrete mass. In general, the alkali activation is a hydration reaction of aluminum silicate with alkali or alkaline-earth type materials, such as: hydroxides (ROH, R(OH)2), weak acid salts (R2 CO3, R2S, RF), strong acid salts (Na2 SO4, CaSO4.2H2O) or silicate salts such as R2(n) SiO2, where R is an alkaline ion of Na, K or Li type, or alkali earth such as Ca [12; 13]. Basically, the process consists of mixing an alkaline activator with a binder rich in alumino-silicates. First, the alumino-silicates undergo a heat treatment, involving loss of water, and modification of the coordination of the aluminum ion with oxygen. This helps to enhance the results since the material shows a great susceptibility to combine chemically [13; 14]. Slags from blast furnaces, fly ash from coal burning in thermoelectric power plants, volcanic ash with natural heat treatment, tile or brick powder, and metakaolin obtained from kaolin (natural alumino-silicates), are examples of potential materials for the alkali activation. Alkali activators are obtained by the use of alkali metals such as sodium and potassium. Within a relatively large group of alkaline activators, two can be distinguished: a simple activation and a combination of activators. A simple activation is obtained by a single activator, i.e., just one chemical compound, for example sodium hydroxide (NaOH) or potassium hydroxide (KOH). Combination of activators is also possible and might involve hydroxide and silicate of the alkali metal in appropriate proportions. According to [PERSON] [13], geopolymer mass activated with a simple activator is very plastic and shows low workability due to the high viscosity of the activator. This viscosity increases considerably when the concentration of hydroxide increases. The composite activators are slightly less viscous and, therefore, they would lead to higher workability. Using an activator based on sodium or potassium eventually leads to similar results [15]. Simply use of sodium silicate might be sufficient depending on the curing temperature.