Cement paste morphology

The differences in chemical composition are accompanied by differences in the morphology of the tobermorite gel. Spicular or cigar-shaped rolled sheets are formed in the normal plain hydrated cement paste, whilst in the presence of calcium chloride, thin crumpled sheets or foils are formed. It has been suggested [16] that either the high lime content or adsorbed chloride prevents rolling of the sheets. 

Fig. 7a-c show images of the products of hydration in the cement paste. Ettringite, hydrated calcium silicates and Ca(OH)2 were detected due to their typical morphologies. 

Fig. 4 shows the morphologies of fractured cement pastes with and without SBR dispersion or powder observed by ESEM. Irregular hydrates is observed on the surface of cement particles in the control paste after 10 minutes, and AFt or calcium aluminate hydrates with relative regular shape are also found (Fig. 4 (a)) while at this time a polymer-particle layer is seen on the surface of cement particles in the pastes with SBR dispersion or powder, and AFt or calcium aluminate hydrates appear among the polymer particles (Fig. 4 (b) (c)). 

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. 

Figure 6 shows the DSA results obtained during the hydration of Portland cement (PC-400) in water wjc = 0.3) under isothermal conditions at 35, 45 and 65°C, resp. The reactivity of the cement towards water was determined from the DSA results from the early stage of the interaction with water. Changes of surface and morphology taking place in the hydration products of cement have also been monitored by this method [36] under in-situ conditions of the setting of the cement paste. 

The C-S-H phase is an amorphous or nearly amorphous material of the general formula Ca0. Si02.H20, where both x and y may vary over a wide range. On the nanometer scale the C-S-H phase is stracturally related to the crystalline phases 1.4 nm tobermorite and jeimite. In cement pastes hmited amounts of foreign ions may be incorporated into the C-S-H phase. On the micrometer scale the C-S-H phase appears either as a dense amorphous mass or as a microciystalline material with an acicular or platelet-like morphology. The material contains pores with radii between about 1 and 10" nm, and exhibits a specific surface area exceeding 100 m /g. 

The monosulfate group, also known as the AFm phase, is represented by the formula C4ASH12 or C3A CS Hj2. AFm stands for Al-Fe-mono, in which one mole of CS is present. In portland cement this phase forms after the AFt phase disappears. This phase may constitute about 10% of the solid phase in a mature cement paste. In SEM, this phase has a hexagonal morphology resembling that of Ca(OH)2 and the crystals are of sub-micrometer thickness. The principle ionic substitutions in the AFm phase are Fe for AP+, and OH", C03 , Cl", etc. for S04. The density of this phase is 2.02 g/ml. The amount of crystalline hydrogamet present in cement paste is less than 3 %. It is of the type Ca3 Al2(OH)j2 in which part... 

Many of the properties of the cement paste are determined by its chemical nature and micro structure. Microstructure constitutes the nature of the solid body and that of the non-solid portion, viz., porous structure. Microstructural features depend on many factors, such as the physical and chemical nature of the cement, type and the amoimt of admixture added to it, temperature and period of hydration, and the initial w/c ratio. The solid phase study includes examination of the morphology (shape and size), bonding ofthe surfaces, surface area, and density. Porosity, pore shape, and pore size distribution analysis is necessary for investigating the non-solid phase. Many of the properties are interdependent, and no one property can adequately explain the physico-mechanical characteristics of cement paste. 

In hardened cement pastes, the microstructure can be nondescript and consist of equant or flattened particles (under 1000 A in largest dimension) such a morphological feature is described as T5q)e III. 

Fig. 2. TEM image of CSH gel in 1800 year-old cement paste from Hadrian s Wall showing both a fibrous aspect and a typical outer CSH porous honeycomb aspect in its morphology. Magnification...

Trtik, P., B. Miinch, P. Gassei A. Leemann, R. Losei R. Wepf and P. Lura. 2011. Eocussed Ion Beam Nanotomography Reveals the 3D Morphology of Different Solid Phases in Hardened Cement Pastes . Journal of Microscopy 241 (3) 234-242. 

Stubby rods of AFt phase are also seen (D25 D27). They are typically some 250 nm long and 100 nm thick. Studies using wet cells show them to occur both on the surfaces of the grains, and at some distance away (S41,S68) (Fig. 7.6b). They are probably more abundant near to the surfaces of the aluminate phase, and appear to nucleate in the solution and on the outer surface of a layer of gel. On drying, this layer shrinks, and the AFt crystals fall back onto the surfaces of the cement grains. The early products thus differ in morphology and composition from the exfoliating foils or honeycombs of C-S-H that have been observed in CjS pastes. 

The reaction of clinker phases with water, together with the crystalUzation of hydrates, brings about the substantial changes of rheological properties of the paste. The formation of hydrated phases and, according to Locher [28], the morphology of hydrates crystallized in the liquid phase or on the surface of cement grains, leads to the reduction of distance between them and the viscosity of paste is enhanced. At... 

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