# Influence of the Composition of the Activator on Mechanical Characteristics of a Geopolymer
## 1 Introduction
According to [PERSON] [16], the molar concentration 12 M sodium hydroxide is the lowest concentration able to promote the alkaline reaction. [PERSON] [17] suggested that the maximum concentration would be 20 M from which the identification of the alkaline reaction would no longer be possible. In his study of the alkaline activation of metakaolin by using an alkaline solution with sodium silicate, [PERSON] [13; 18] reported an increase in mechanical strength both for compression, with values from 30 to 60 MPa, and for tensile bending, with values between 5 and 7 MPa. [PERSON] and [PERSON] [19] reported that the use of a solution consisting of sodium hydroxide (NaOH) and sodium silicate (Na2 SiO3) as the alkali activator, rather than only NaOH caused an increase of 40 to 90 MPa after curing for one day. [PERSON] and [PERSON] [15] studied the effect of temperature (at room temperature and at 80 \({}^{\circ}\)C), and the strength development of slag mortars alkali-activated with Na + and K + hydroxide and with sodium silicate. To obtain a high strength, these authors found that both activators are required for curing at room temperature. At 80 \({}^{\circ}\)C sodium silicate is essential and sufficient as the NaOH solution is not needed to the gain in strength. This is because at this temperature the dissolution of the slag is sufficiently high. At room temperature, NaOH concentration affects directly the rate of increase of the strength, as it affects the rate and amount of dissolution of the slag. At 80 \({}^{\circ}\)C, the amount of KOH was shown to be more effective than NaOH. The behavior of each component of the activator is not yet fully understood. In the alkaline activation process, the sodium silicate acts as a binder while sodium hydroxide acts more in the dissolution process of the raw materials [20]. However, this proposal does not pose the possibility of Na2 SiO3 playing both roles: providing at first the dissolution of aluminum silicate (breakdown of siloxane bonds) and afterwards acting as a binder, rearranging the structure [7]. Also interesting are recent studies on some mechanical properties of geopolymer composites reinforced by ultra-high-molecular-weight polyethylene fibers and on the strength of powder-based three dimensional (3D) printed geopolymer samples [21; 22]. Past research was concentrated on material studies. However, if the geopolymers are to replace to a certain extent the cement-based materials, the mechanical behavior of such types of material needs to be fully understood. Structural members made from geopolymers need to be constructed and tested. For instance, the deformability of members is of great importance in the structural design of continuous beams and prestressing systems are often used to overcome structural concrete difficulties [23; 24]. In normal concrete, the modulus of elasticity does not vary too much with the strength. Moduli of elasticity of geopolymers can be very different from those of normal concrete and this will pose new structural problems that need to be studied. Cracking behavior of members is of great importance too, because of its influence on the corrosion of the steel bars of the reinforced elements and also because of its influence on the deformability of the members [25]. The combination of geopolymers with other materials, such as steel and timber for structural applications also need to be studied. These studies might follow similar laboratorial or computational steps as those applied to combinations of concrete with steel or with timber [26; 27]. There are already some few studies on the structural behavior of members made with geopolymers. Some reports of tests on columns and beams were presented in Chapter 26 of the book _Concrete Construction Engineering Handbook_[10]. Tests on columns were also performed by [PERSON] et al. [28]. More structural examples are reported by [PERSON] et al. [29]. The bond between steel bars and geopolymers was studied by [PERSON] et al. [30].