## 1 Introduction  
Soil constitutes of clay minerals, which are classified according to their internal structure as 1:1 (one tetrahedral silicate sheet bonded to one octahedral hydroxide sheet) or 2:1 (one octahedral hydroxide sheet sandwiched between two tetrahedral silicate sheets). The latter group consists of illites, Kaolins, smectites, and others. Montmorillonite (MMT) is a dioctahedral smectite (one SiO\({}_{2}\) platelet/layer sandwiched between two Al\({}_{2}\)O\({}_{3}\) platelets/layers). It can be found in pure form or in the form of bentonite, in which 50%-80% MMT is intermixed with chlorite, muscovite, illite, and kaolinite. If bentonite undergoes a purification process with the use of sodium ions, \"sodium bentonite\" is produced, which practically is pure MMT. Montmorillonite is; therefore, a two-dimensional naturally hydrophilic stack of platelets of nanosized thickness [1]. The lateral dimensions vary from a few nanometers to a few hundreds of micrometers [2]. Given that the European Commission in 2011 has specified that \"nanomaterial\" means a natural, incidental, or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1-100 nm [3]. It is only natural that an extensive number of scientists are involved into the separation of the platelets in order to produce individually reactive platelets of nanosized thickness, generally termed as \"nanoclay.\" If an organic surface modifier is inserted in the MMT layers to separate them, then an organomodified MMT (OMMT) is produced and is considered to be nano-Montmorillonite (nMt). Then, nanoclays or nMt's can be dispersed in ceramic, metallic, biopolymeric, or polymeric matrixes or in water. The preferred manufacturing and fabrication method depends on the purpose of use of the nanoclays or nMt's. Ultrasonic sonication, planetary centrifugal mixing, magnetic dispersion, roll milling, and combinations of them have been investigated. For example, a significant number of papers is published on the effect of the nanoclay dispersion methods on the mechanical behavior of E-glass/epoxy nanocomposites [4; 5] or clay-polymer nanocomposites [6; 7; 8]. The use of nanoclays is widespread in environmental science; for example, for removal of contaminants in water [9], remediation of contaminated soils [10] or biodegradation of polyolefins [11], in medicine for drug delivery [12], and in other fields. The use of nanoclays in cement science is limited, notwithstanding the size compatibility between the main hydration product of cement, C-S-H, which is nanosized [13] and nMt, which is in the same order of the nanocrystalline structure of C-S-H [14]. In fact, most studies investigate the effect of the addition of thermally- and mechanically-treated (calcined at temperatures above 700 \({}^{\circ}\)C) nanoclays, predominantly kaolinite [15], MMT's or micro-sized OMMT [16], despite the environmental cost due to the calcination process [17]. Others, report results on the use of industrial nMt's (in liquid form purchased from J Nano Technology Co., Ltd.) used as an admixture without providing any characterization of the nanoparticles [14]. In recent years a number of studies are suggesting the replacement of Portland cement by an amount of nanoclays or nMt's in composite cements in an effort to produce \"green\" cementitious nanohybrids (i.e., nanohybrids that contain minimum amount of Portland cement clinker), which is accountable for almost 8% of the global manmade CO\({}_{2}\) emissions. As summarized in Figure 1, binary cements comprising PC and LS should not contain less than 65% PC. The same is valid for composite cements containing PC and/or LS/Fly ash/blast furnace slag/pozzolanas/silica fume, according to the Eurocodes [18].