Hydrated cement

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

Recent applications of relaxation dispersion measurements to concrete or cement-based materials are promising for characterizing reactive nanopor-ous materials, the structure of which may evolve over time (75-78). The MRD profiles have provided, for the first time, a direct means for characterizing the specific surface area, Sp, of a hydrated cement-based material (79), without exposing the sample to extremes of temperature or pressure (80-83). The interest in such a surface area is to provide information on the microsctruc-ture and its impact on macroscopic or structural properties. The method is based on a clear separation of surface and bulk contributions of the overall... 

Water-reducing admixtures are not adsorbed equally by the various anhydrous and hydrated cement constituents and in studies with calcium lignosulfonate, the approximate maximum adsorption figures shown in Table 1.5 have been obtained [38,39], In addition, adsorption isotherms have been studied at various ages of C3A hydration [36] and it has been shown that it is the initial hydration products (less... 

The amount of calcium lignosulfonate adsorbed on to hydrating cement is almost independent of initial water-cement ratio within the range 0.4 to 1.5 [34]. 

Fig. 1.22 The level of various anionic species present in the aqueous phase of hydrating cement pastes.

Fig. 1.25 The heat evolved from hydrating cement with and without the addition of calcium lignosulfonate as a function of combined water (Khalil).

Earlier work [37] by one of the authors indicated that superplasticizers of the SNF and SMF type were less strongly adsorbed onto the hydrating cement than normal water-reducing agents and this was used to explain why there was less retardation by the superplasticizers. This... 

SMF and SNF superplasticizers are adsorbed rapidly onto hydrating cement but this net effect is made up of very rapid adsorption by C3A and slower adsorption by the silicate phases, as shown in Fig. 2.9. [39],... 

Desorption experiments have shown that SMF and SNF are irreversibly adsorbed on to hydrating cement. 

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. 

The hydration reactions may also be influenced by changes in the pH value of the solution in contact with hydrated cement, which may alter the solubility or stability of some hydrated cement compounds or inhibit the formation of protective coatings. 

Since solution phenomena are dominant in the very early stages, the influence of various admixtures on early hydration reactions may be reflected by changes in the composition of the liquid phases in contact with hydrated cement, e.g. only extremely small amounts of AI2O3, Si02 and other oxides have been... 

The solubility of heavy-metal-containing solid phases (1) does seem to be a limiting factor with regard to the second and third mechanisms (Glasser 1994), so only ions that do not precipitate as oxides will be incorporated in or sorbed to hydrated cement minerals to a significant degree. 

Because of the complexity of hydrated cement pastes and the variety of possibilities for binding, a study of the binding of metal and metalloid ions to specific cement minerals is advantageous. The binding to single components of the hydrated cement paste can be compared to the teachability of an ion bound in a hydrated cement paste and deductions made as to the dominant mechanism that limits the solubility in the porewater. 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

For concrete at early ages the most important is the effect of chemical processes on its transport and strength properties, e.g. porosity n=f(Thydr)> intrinsic permeability T=f(Thydr,d), and mass source related to the hydration tn-hydr = f (Thydr) Creep of concrete is modeled by means of the solidification theory [17], where the degree of cement hydration Thy dr is used as the volume fraction of the load-bearing portion of hydrated cement. 

Figure 1. Schematic of hydrating cement grains, illustrating growth of C-S-H gel around them ana construction of interconnected, water-filled interstices. Matrix is enlarged relative to scale.

Whatever the fine structure and reactions associated with hydrating cement paste, the permeability of the hardened matrix will depend on the sizes of interconnected capillary openings remaining after hydration. 

Matrix Bonding Infrared Absorption Studies. It is possible that some form of reaction could occur between liquid sulfur and the hydrated cement phases to produce a chemical bond which would contribute to the strength of the composite, perhaps more noticeably with filling of the... 

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