Concrete is by far the most common building material for civil and military applications due to its mechanical stability and durability

Concrete is by far the most common building material for civil and military applications due to its mechanical stability and durability; however, it is also subject to several disadvantages. First of all, it may negatively affect the environment due to the considerable amount of CO2 emitted from the production of cement. Note that CO2 released from such sources may constitute around 6% of the total global CO2 emission (Valipour et al., 2013). The concrete currently used also has some structural defects related to its durability such as shrinkage, frost attack, and chemical attacks which can deteriorate concrete structures. Besides, the expenses of maintaining, protecting, repairing, and rehabilitating existing concrete structures cannot be neglected. In order to mitigate or even avoid these problems, several realizable solutions are suggested. One of these solutions is to improve the mechanical structure as well as durability properties of concrete during the manufacturing process. Another way is to reduce the consumption of portland cement which is a hydraulic material used as a main ingredient of concrete. These two methods can be achieved by optimizing particle packing density of concrete and by partially replacing portland cement in concrete with other materials (including industrial by-products and natural pozzolans). Since the replacement of portland cement replacement affords economical, environmental, and quality benefits, it has intensively been studied.
Natural zeolites are crystalline alumio-silicates with uniform micropores, channels, and cavities. There are three components in zeolite composition such as extra framework cations, framework, and sorbed phase (Mx/n(AlO2)x(SiO2)y•nH2O). Zeolite has been used for various purposes in diverse fields of applications including wastewater treatment (Wang and Peng, 2010), gas purification (Sircar and Myers, 2003), and construction industry (Küçüky?ld?r?m and Uzal, 2014; Ahmadi et al., 2018). Because of unique structure and advanced properties, the natural zeolite has a good potential in concrete industry. First, the addition of this material as a kind of pozzolanic material in concrete can strengthen the stability of concrete mixture (Perraki et al., 2010, Vejmelková et al., 2015). Furthermore, in case of concrete with a low water/binder ratio (wbr), zeolite particles with high adsorption capacity can act as internal water curing agents to help increase the durability and permeability of concrete (Thang et al., 2016; Zhang et al., 2018). In addition, this microporous material is utilized in aggregate forms for lightweight concrete (Karakurt et al., 2010). The addition of natural zeolites positively influenced the mechanical properties and durability of concrete. Concrete containing zeolite has thus been proven for the higher resistance to sulfate attack (Janotka and Krajci, 2008; Vejmelková et al., 2015) and freeze-thaw (Nagrockiene and Girskas, 2016).
In this review, we will discuss the basic properties of natural zeolite and its application toward concrete industry based on the related literatures recently published. Adding natural zeolite to concrete will also be evaluated following three aspects: cost, environmental effects, and performance. Additionally, this review will describe the present challenges of natural zeolites for the construction applications, tackling those problems, and future prospect.

2. Characteristics of natural zeolite
2.1. Structure
Natural zeolite is crystalline aluminum-silicates having a 3D honeycomb structure which is formed by TO4 (T: Si, Al) linked with other tetrahedrons by four shared O atoms. This structure is described in Figure 1.
Each AlO4 tetrahedron carries the negative charge, which leads to the presence of an extra – framework cation in order to keep the framework neutral. The amount of Al within the framework can change variously, with Si/Al = 1 to ? to make polymorphs of SiO2 (Payra and Dutta, 2003). In general, natural zeolites possess a relatively low Si/Al ratio since organic structure-directing agents are absent. The zeolite structure contains channels and cages occupied by exchangeable charge-balancing cations and water molecules. The zeolitic properties are not affected by cations (such as K+, Na+, Ca2+ or Mg2+) that can be replaced by other extra–framework cations. Besides, under the high temperature, the H2O molecules can be taken away from the framework, which does not result in any structural alteration of zeolite.
Zeolites are primarily constituted of tetrahedrons of which the geometric arrangements are defined as secondary building units (SBUs). The number of zeolite structure types is determined by the possible ways to form polyhedra through linking between SBUs. There are seven groups of SBUs (Breck, 1974) and around 40 types of zeolites in nature (Christie et al., 2002). Figure 2 depicts the types of the SBUs in zeolites in relation to the positions of only T (Si and Al).
Oxygen ring systems including 6, 8, 10, 12, and 14 membered units form interconnected channels and pores in zeolite structure with the molecular dimensions from 3 to 10 Å. These sizes are affected by position, size, and coordination of the extra-framework cations; therefore, ion exchange can control the pore opening. Table 1 gives the information of structural properties of the most popular natural zeolites (Inglezakis and Zorpas, 2012).