## 1 Introduction  
Geopolymer, which was introduced by [PERSON] in 1976 [1], is an inorganic polymeric material of great interest in the last few decades because of its environmentally friendly, cement-free properties and wide potential applications such as construction materials, adsorbents, and so on [2; 3; 4]. Geopolymer is a type of alkali-activated material that possesses a three-dimensional network structure composed of SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedra sharing an oxygen atom as a linking bridge. In general, the formula of geopolymer is M\({}_{n}\)[-(SiO\({}_{2}\))\({}_{x}\)-AlO\({}_{2}\)]\({}_{n}\).mH\({}_{2}\)O, where M is the ion used to balance the charge such as Na\({}^{+}\), K\({}^{+}\), etc., z is the Si/Al molar ratio; n is the degree of polymerization, and m is the amount of water. Geopolymer material is generally produced by the polymerization of aluminosilicate sources exposed to an alkaline activator solution. The alkaline activators are normally alkaline hydroxides such as NaOH, KOH, and water glass solution. The source materials could be natural minerals such as clays [5; 6] and kaolin [7] or waste/by-product materials such as ashes [8; 9], slag [10; 11], rice hush [12; 13], red mud [14; 15; 16; 17], etc. One source material or an admixture of more than one source material could be used to prepare geopolymers. In the particular case of red mud, which is the major waste of alumina production from bauxite via the Bayer process [18] and has a quite low Si/Al ratio in composition, the admixture of red mud and other source materials such as fly ash [16; 19], metakaolin [20; 21], slag [22; 23; 24], biomass fly ash, and metakaolin [14] should be used to prepare geopolymers with sufficient qualities. For application purposes, a lot of effort has been made by scientists to improve the properties of geopolymers. Recently, researchers have found that the calcium component (Ca\({}^{2+}\)) could have a distinct influence on the properties of geopolymer products. [PERSON] et al. [2] fabricated a micropore-foamed geopolymer composed of fly ash, ZSM-5 zeolite, and calcium hydroxide. According to the authors, the addition of calcium hydroxide brought a positive effect on enhancing the mechanical properties of geopolymer materials obtained. The formation of N-A-S-H and C-A-S-H, which were crystalline phases as unreacted filler derived from materials, was observed. [PERSON] et al. [25] investigated the effect of calcium on the existing form of the gel product of fly ash-based geopolymers and found that the appropriate amount of added calcium hydroxide could enhance the mechanical properties of geopolymers. [PERSON] et al. [26] found out that fly ash-based geopolymer blended with kaolin had higher resistance to chemical attack than geopolymer material with 100% fly ash. The authors supposed that the development of N-A-S-H and C-A-S-H in geopolymer could create strong resistance to acid attack. [PERSON] et al. [27] produced the geopolymer brick using wastes of aggregate industries, sand, and calcium hydroxide. They found that the geopolymer brick prepared at 8 M NaOH concentration had higher compressive strength than those prepared at 4 M and 12 M NaOH. Curing geopolymer specimens at temperatures from 70 \({}^{\circ}\)C to 105 \({}^{\circ}\)C would also enhance the compressive strength of geopolymer brick specimens. [PERSON] et al. [28] found that the addition of 10% wt. of calcium oxide dramatically increased the compressive strength of metakaolin geopolymer by 105%. It has been demonstrated that adding a calcium component during the preparation of geopolymer can form more complex calcium-containing sodium aluminosilicate (C,N-A-S-H), hydrated calcium silicate (C-S-H), and hydrated calcium aluminosilicate (C-A-S-H) [29]. The positive effect of the calcium component on the properties of geopolymer materials has mainly been reported based on the formation of C-S-H and/or C-A-S-H, which act as fillers and help bridge voids and gaps in the material [30].