C-S-H phase

As it results from the ESCA studies the C/S in the about 2 tun thick surface layer is decreasing rapidly, as in the case of C3S hydration [31]. 

Basic properties of Portland cement pastes are attributed to the C-S-H gel. Therefore this phase is a field of interest and the subject of numerous investigations [32]. However, in spite of this, the structme and chemical composition of C-S-H cause several discussions. It is the effect of colloidal constitution of this phase and variable, not well defined composition, depending on liquid phase composition, primarily of calcium ions concentration. Moreover, the morphology of this phase transforms as a function of hydration or maturing time of the samples. 

First of all the C/S molar ratio changes depending on the saturation of the liquid phase in concrete, in relation to Ca(OH)2 (Fig. 3.17). Especially in the case of cements with mineral additions, particularly siliceous fly ash or ground granulated blastfurnace slag, this ratio decreases and the sections of chains composed of [SiO ] tetrahedra become longer [32]. 

The adjective tobermorite-like has been used to C-S-H for a long time, more or less between the Congresses in London and Tokyo (1953-1969) by many authors. This term was subsequently rejected because of the significant differences in chemical composition, degree of crystallinity and the silicate anion stmctirre. From 

C-S-H in equilibrium with the liquid phase in which the calcium ions concentration is increasing shows a variable C/S ratio, rising with this concentration. The change of C/S ratio in relation to the increase of calcium ions concentration in solution are shown in Fig. 3.17. Taylor [33] divided the C-S-H phase on two types C-S-H (I) with C/S ratio 1.5 and C-S-H (II) with C/S ratio 1.5. This classification can be primarily explained by a discontinuity of C/S ratio with the change of calcimn ions concentration in the solution [34]. The other discontinuity has been foimd by Damidot and Nonat at calcium ions concentration corresponding to 22 mmoles CaO/kg, in the C/S range fiom 1.5 to 1.8. Therefore they proposed the occurrence of three types of C-S-H, which for avoiding mistakes they called as follows a-C-S-H (0.66 C/S 1), p-C-S-H (KC/S 1.5), y-C-S-H (1.5 C/S 2). 

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In the MC modified mortar, polymer films are also detected, stretched in the open pore spaces. In Fig. 2, the polymer film is partially intermingled with the ettringite needles and other cement hydrates [3]. Additionally, polymer films are observed between the ettringite needles at the air void surface in MC modified mortar (Fig. 3). The films cover the C-S-H phase, giving it a more smooth and bright texture. 

Figure 3 Polymer film, covering the C-S-H phase between ettringite needles in 1 % MC modified...

Figure 4 C-S-H phase on top of an aggregate in 1% PVAA modified mortar before storage under water (left). After storage under water for 7 d, a polymer film covers the C-S-H phase (right).

However, the changes in the ettringite content are less clear. Overall the trend seems to oppose that of portlandite. That is, more ettringite is formed in water than in POW, but the changes in content are less pronounced. It might be that the reduced availability of calcium ions, due to increased portlandite precipitation in POW, causes less ettringite to precipitate in early hydration times, but no such trend is observed for the C-S-H phase tobermorite. Also there is a higher consumption of clinker phases in POW, which also seems to be activated by the hydroxide ion content. 

The siheate ions are released into solution and react with the calcium ones giving C-S-H phase. It is the end of induction period interfering with Ca ions concentration decrease in the hqnid phase due to the C-S-H crystallization. The phenomenon... 

Further densification of C-S-H phase and transformation to the type HI and also IV, in the form of compact gel, leads to the significant reaction rate decrease. The formation of inner C-S-H begins. This is also the consequence of lower content both of anhydrous phases and of the liquid phase. The further growth of Ca(OH)2 crystals is observed. Finally, one can conclude that there is less experimental data relating to the post-induction period. 

Fig. 3.17 CaO/SiOj molar ratio in the C-S-H phase as a function of Ca concentration in the liquid phase...

Fig. 3.18 C/S molar ratio of C-S-H phase according to the classification proposed by Nonat. (After [35])...

With trimethylsilylation procedure the presence of [Si207] dimmers, as a main structural component of C-S-H phase, and a low amount of longer linear chain sec-... 

For this reason some hypothetical models have been proposed, as a solution of this problem. In the oldest one the C-S-H phase was considered as a solid solution of Ca(OH)2 in tobermorite [58]. In the second the C-S-H phase was presented as a defected 1.4 nm tobermorite stracture, with the Ca + and OH ions between the layers [59]. 

Structurally to jennite. Taylor [63] assumed that similar to the tobermorite-like and jennite-like areas can be quite freely limited and penetrate each other, even within the single layers. Therefore the base of this model relates to the very disordered C-S-H phase stmcture. 

Nonat [35, 39] gives the thermodynamic supplement to this model basing on the mass action law, related to the solution in equilibrimn with the C-S-H phase and to the smface of this phase. The thermodynamic calculations comply well with the experimental results, as it is shown in Fig. 3.31. 

Another approach to the C-S-H phase modelhng was proposed by Jetmings [91], which was taking into accoimt the discrepancies occurring when the microstractme of this phase was evaluated based on the specific surface area and pore stmcture. It... 

Richardson and Groves [66, 67] stodied the solid solutions in the C-S-H phase. The AP+ ions are the most important they substitute silicon exclusively in the... 

Finally, Ulm et al. [82] have shown that the concrete and primarily the C-S-H phase, controlling its properties, can be classified as a porous material, complying the rules governing the mechanics of porous bodies. Nowadays the properties of these materials can be examined in the nanometric scale, as it has been shown by Nonat [80] and Plassard [81]. They are elastic bodies to which the Biot modulus and coefficient (between 0.61 and 0.71 for C-S-H) could be applied [82]. 

There is a significant number of metastable calcium aluminate lydrates. Then-identification is difficult because of a laige number of polymorphs and high susceptibility to the formation of caiboaluminates under the influence of CO2, although they occur as well ciystallized hydrates. In cement paste they can also form the nanometric mixtures with the C-S-H phase. Therefore it will be convenient to begin the discussion of calcium aluminate hydration from the presentation of the CaO-Al203-H20 system (Fig. 3.34). 

The hydration, or rather hydrolysis, of orthosilicates is more complex. The C-S-H phase is poorly crystallized and reveals variable C/S ratio this is not a ciystalhne phase but rather a gel. The heat of adsorption of water on the C-S-H gel interferes with the heat of reaction. Nowadays, it is assumed that the hydration of orthosilicate phases are linked with the following heat of reaction ... 

Fig. 4.65 The strength of autoclaved composites vs. the tobermorite and C-S-H phase content (according to [178])...

There is relatively an unanimity opinion about the role of paste phase composition on the strength of concrete. The high strength is attributed to the C-S-H phases, as in the case of ambient temperature, as well to 1.1 nm tobermorite [136, 155]. On the other hand the a-C2SH is causing the significant strength decrease [1, 156, 178, 199], For this reason the 10-20% quartz addition to the paste causes the decrease of the mechanical properties of material, however, the quartz addition of 30-40% is favourable [1]. 

There are two schools first of Bmnauer and the second one of Feldman and Sereda which, on the basis of different hypotheses, hy to explain the differences of cement paste porosity, measured with HjO sorption or other gases, principally N2 [35, 43, 48, 54]. These Itypotheses are connected rigorously with the proposed stmctural models for C-S-H phase. 

The Feldman and Sereda hypothesis [35, 43, 48, 57, 62, 64] is based on the assumption that the exchange of interlayer water in the C-S-H phase occurs. The volume of this water is equivalent to the volume of adsorbed water reduced by the volume of adsorbed nitrogen The measurements of helium volume... 

The detailed studies of the stracture of cement paste and its influence on the physical properties we owe to Powers [31, 36]. According to his assumption, the cement gel , it means the C-S-H phase composed of colloidal particles, is a basic binding component of the paste. This colloidal status of C-S-H is proved by an... 

The effect of porosity on the strength of material was studied also by Dyczek and Petri [86], on the pressed samples produced from tobermorite, xonotlite and C-S-H phase. 

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