Atomic bonds Cements

The types of bonds which can occur in cement paste have been analysed by numerous authors [28-32], There is a common opinion that two types of bonds can be consider the ionic-atomic bonds, acting only on a very small distance and the van der Waals forces. Initially a significance was attributed to the interlocking of needle-like, fibrous C-S-H particles. However, this was rejected because in the hardened, old pastes there is a very low content of fibrous components. The occmrence of these elongated needle-like C-S-H forms has been questioned recently (see Sect. 3.2.3). 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

We reported earlier the first-principles study on the crystal structure and hydraulic activity of P- and y-form belites (Sakurada et al. 2009a, 2009b). In this chapter, we present our DFT simulation results on the study of hydraulic properties of cement phases. Ca-Ca, Ca-0, and Si-0 bond lengths are chosen as a yardstick for making reliable prediction on the hydraulic activity of p- and Y-C2S phases. Moreover, to find a change in crystal structure of P-C2S on substitution of the strontium atom or barium atom for a calcium atom, a periodic boundary condition has been applied on a large supercell (504 atoms, a x 3, b x 3, c x 2) to remove any spurious interactions. 

Therefore, whether the mean Ca-Ca bond lengths have a correlation with the hydraulic activity of the cement clinker compounds was investigated. The Ca-Ca bond lengths within 4 A to the neighboring calcium atoms for all the independent calcium atoms within a supercell are computed. The calculated values of Ca-Ca bond length are listed in Table 13.3 and compared with experimental results reported by lost et al. (1977). 

Higher expansion of SrO polyhedra as a result of doping of an Sr atom into a CaO polyhedron causes shorter Ca-Ca interatomic distance in the configuration 2Sr-l in which two SrO polyhedra are independent and 2Sr-3, which forms an edge to edge bond between two Sr(l)0 polyhedra. Shorter Ca-Ca interatomic distance is likely to be favorable for the hydraulic activity of cement clinker compounds, which also compares well with the experimental observations. 

The C-S-H phase is the main binding agent in portland cement pastes. The exact structure of C-S-H is not easily determined. Considering the several possibilities by which the atoms and ions are bonded to each other in this phase, a model may be constructed. Figure 6 shows a number of possible ways in which siloxane groups, water molecules, and calcium ions may contribute to bonds across surfaces or in the interlayer position of poorly crystallized C-S-H material.1 1 In this structure, vacant comers of silica tetrahedra will be associated with cations, such as Ca". ... 

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