**1. Introduction**

Technical advances in building industry in association with progress in knowledge collected on the chemistry of cementitious materials, requirements of global sustainable development serve as a moving force in further development of the alkali-activated materials [1–3].

Earlier, according to traditional views, free alkalis and compounds of alkali metals were excluded from traditional hydraulic cement compositions because of their high solubility which resulted in the worser durability and properties.

However, the studies held on ancient cements and concretes showed that their excellent durability could be attributed to the presence of aluminosilicates of alkali metals – analogs of natural zeolites – in their hydration products. Alkalis play an

important role in mutual transformations of minerals in the Earth's crust. It should be also mentioned that aluminosilicates of alkali metals (first of all, feldspars) are known to be more stable and durable compared to those of calcium [1].

In 1957 Viktor Glukhovsky made an attempt to model natural processes of formation of the aluminosilicates of alkali metals at different temperatures and made an assumption that the compounds of alkali metals (Li, Na, K, Rb, Cs) – the elements of Group 1 of the periodic table of elements, exhibited hydraulic binding properties similar to compounds of the alkaline earth metals (Mg, Ca, Sr, Ba) – the elements of Group 2 [4].

An important conclusion was also made that the increase of temperature promoted a smooth dehydration process and subsequent re-crystallization of the hydration products into stable anhydrous aluminosilicates of alkali metals. This conclusion was later confirmed [5–7]. Unique service properties of materials predetermine its application for development of a wide area of modern composite materials for the building industry such as protective coatings, inorganic glues, heatinsulating materials, thermo resistance composites, etc. [8–14].

Extensive works carried out have allowed to revise views of regularities of exhibiting binding properties by mineral substances and to prove that, in parallel with the compounds formed by the elements of Group 2 (alkaline earth elements) of the periodic table of elements together with complex-formers of Groups 3, 4, 5 or 6, the compounds formed by elements of the main Subgroup of Group 1 together with twin complex-formers of the Groups 3 and 4 (aluminosilicates) possess binding properties. Such products can be formed through a combination of other amphoteric and acid compounds [4, 8, 9].

The established regularities governing the formation of a mineral-like cement matrix based on the compounds of alkali metals taken alone and in combination with the compounds of alkaline earth metals were established and studied in details (hardening processes, principles of compositional structure and prediction of their properties) and used for development of a new class of hydraulic binders known nowadays under a name of alkali-activated cements or alkali-activated aluminosilicate cements [10–17].

The idea behind the alkali activated cements is modeling of the minerals of the Earth's crust in the system of Me2O-Me2O3-SiO2-H2O (Me2O- Li2O, Na2O, K2O, Rb2O, Cs2O; MeO-CaO, MgO; Me2O3-Al2O3, Fe2O3, Cr2O3). For example, aluminosilicate rocks of feldspar composition are broken up by chemical wind erosion to a dispersion state or to clay minerals. These transformations are accompanied by hydration of anhydrous minerals composed from the compounds of alkali metals, the decrease of alkali content in hydrated new formations; the replacement of alkalis by hydrogenous ions or H3O<sup>+</sup> -groups; the transition of aluminium from IVcoordination to VI-one; the partial removal of silicic acid, i.e. by processes occurring under hydration and hardening of building cements. Gel-like silicate and aluminosilicate substances are formed as a result of chemical erosion of feldspars in an erosion crust, in the zones of hydrothermal metamorphism proceeding at relatively low temperatures and pressures. These substances react with compounds of alkali metals brought by circulating superheated aqueous solutions. As a result, aluminosilicate hydrates of alkali metals of zeolite type are formed. These minerals have a very low water solubility despite the fact that their composition includes strongly soluble compounds of alkali metals [5–8].

In particular, analcime (Na2OAl2O34SiO22H2O) is formed at 303 K (29.85°C) at the sea bed through a coagulation of silica and alumina sols with their concurrent adsorption of ions og alkali metals from environment.

Geological data suggest that on numerous occasions aluminosilicates of alkali metals of zeolite type or sodalite, feldspathoid, feldspar types (these minerals are

### *Genesis of Structure and Properties of the Zeolite-Like Cement Matrices of the System… DOI: http://dx.doi.org/10.5772/intechopen.97520*

structurally analogous to zeolites) which are the rock-forming minerals, were formed at the expense of chemical interaction between a clayey substance and compounds of alkali metals in an erosion burst. But these processes in nature proceed very slowly, during geological periods. The presence of an alkaline medium is the determining factor for the progress of these processes.

The processes in natural or compositionally analogous to natural, artificial aluminosilicates of alkali metals can be accelerated to limits wherein aluminosilicates can be used as hydraulic binders. Similar to the production of Portland cement clinker, it is possible through the conversion of these substances from the stable crystalline state into a more active metastable one, including a glassy state, or through the external introduction of compounds of alkali metals [5].

Extensive experimental studies confirmed the proposed theoretical bases and allowed to prove that caustic alkalis; salts of alkali metals and weak acids; silicates, aluminates and aluminosilicates reacting in the presence of alkalis under condition that their concentration in the system is sufficient react with natural clay minerals or undead burnt ones; with natural and artificial aluminosilicate glasses, among them metallurgical slags, fuel ashes and slags. The processes can take place in natural conditions and under steam curing and a water-resistant stone with the hydration products that are analogous to natural minerals of zeolite type is formed [3, 4, 8, 9].

Main criteria to be applicable to a choice of starting materials for the zeolite-like cement matrices are: high contents of silica and alumina in them which promote a synthesis of the zeolite-like phases of the (Na, K)2О-Al2O3-SiO2-H2O system and some content of calcium oxide which will serve as an inhibitor of the process of zeolite synthesis [7–9]. The most appropriate for these goals are natural clays in natural and dehydrated state, volcanic rocks or industrial by-products like fuel ashes and slags, metallurgical slags, red muds etc. [3, 4, 9, 17, 18].

Large differences can be observed between microstructures of these cement matrices. A microstructure of the metakaolin derived cement stone, for example, was investigated by systematic variation of activator composition and related to mechanical strength [18–31]. The most important factors affecting a phase composition and, correspondingly, properties of the material are: curing conditions and type and concentration of the alkaline activator solution (that is, the ratios between main oxides (Na2O/Al2O3 and SiO2/Al2O3) as well as a water to solid (W/S) ratio [4, 9, 11, 18–23, 30, 31]. It was observed that with the Si/Al ratio increase, a porosity of the microstructure changed from large pores to more homogenous structure with small pores. This observation was linked to a strong correlation with the Young's modulus and mechanical strength increase [22].

The majority of researchers [32–34] made a conclusion that the structure formation processes in the alkali activated aluminosilicate systems were determined by a required constitutional composition and flowed step-by-step with the formation, depending on a temperature, of the amorphous, glassy or crystalline zeolite-like cement matrices.

For example, reaction products of the interaction of clay minerals with NaOH at 100–300°C are hydronephelines with a structure of sodalite, and those of the interaction with КОН – caliophilites containing zeolitic and adsorbed water.

Hydronepheline and caliophilite themselves can act in a ceramic matrix as structure forming binders.

Low-temperature ceramics produced from them is characteristic of the enhanced resistance to alkalis. With further curing temperature increase of the mineral system to be synthesized the process will flow under a dehydrationcondensation scheme. Hydrates that are formed at low temperatures start to dehydrate, their crystal lattices destroy and the substance becomes amorphous. Further transformations of the dehydration products are attributed, in case of the sodium compounds, to their transformation at 100–800°C into nepheline, at 800–1100°C into albite under condition of required amount of silica available in the system, in case of the potassium compounds – first, into anhydrous caliophilite, afterwards – into orthoclase. The formation of a hydrate phase as a result of the interaction of minerals of unfired clays with salts of alkali metals in normal conditions is rather difficult to identify. However, being subjected to heating, they transform into anhydrous minerals of nepheline, leucite, afterwards – into albite and orthoclase. Their synthesis is preceded by the processes of dehydration of the clay minerals and thermal dissociation of the alkali metal salts.

A character and temperature at which the products are synthesized is determined by a structural type of a clay mineral, type of compound of alkali metal, and nature of siliceous component.

The phases composed from aluminosilicates of alkali metals of the albite and orthoclase type are the most intensively crystallized in the compositions which contain bentonite, and with the lowest intensity in the compositions which contain kaolin. The additives of mineralizers, amorphous silica, α-cristobalite and preliminary dehydration of the clays render an intensifying action on crystallization of these phases.

In the mixed binding systems of the Na2О(СаО)-Al2O3-SiO2-H2O composition the structure formation processes are determined by crystallization of the phases which are analogs to natural plagioclases.

A purpose of the paper is to make a systematic analysis of transformation of phase and properties of the zeolite-like cement matrices of the system Na(K)- Al2O3-SiO2-H2O in relation to various factors, the most important of them are: temperature of curing, reactive silica content (SiO2/Al2O3 ratio), initial alkalinity (Na2O/Al2O3 ratio), cation type (Na or K, and Ca), type of aluminosilicate component (metakaolin or fuel ash (fly ash).
