Portland cement paste chemical composition

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

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. 

The examination of chemical composition of slag glass surface at the early age of hydration has shown that it is modified immediately after the contact with the Uquid phase [6]. As the result of incongment dissolution of slag grains on their surface the l er of C-S-H is formed, with, however, lower C/S ratio than in the Portland cement paste. When the hydration of slag is activated by alkalis, this phase contains the Na+ orK+ ions. 

The C-S-H formed has lower C/S ratio than in the Portland cement paste. Take-moto and Uchikawa [23] studied the chemical composition of C-S-H phase binding C3S crystals with fly ash particles. They found a significant decrease of C/S ratio from approximately 2 in the vicinity of C3S to the very low value near the fly ash grain surface (Fig. 8.5). 

Deterioration may occur through a variety of chemical or physical processes. The rest of this chapter deals with these, with emphasis on chemical processes in which the cement paste is involved. For convenience, the various forms of attack are considered separately, but examination of deteriorated concretes often shows that more than one form of attack has been operative. This is often because one form of attack renders the concrete more susceptible to damage by another. Portland and composite cement concretes are considered calcium aluminate cements were discussed in Section 10.1.11. 

It can be concluded, that the phase composition of pastes from cements with fly ash is very similar to that of Portland cements, which is linked with significant similarity of chemical composition of both cements. 

Conduction calorimetric investigations have been earried out on Portland cement in whieh natural pozzolans have been incorporated, It has been found that in the pozzolan-containing pastes, the dormant period is deereased and the normalized height of the seeond main peak is greater than that given by the reference cement. This indieates that the pozzolan aecelerates the hydration of alite. It is possible that the aceeleration effect is due to a eombination of faetors, viz., higher specifie surface area of the pozzolan, its chemical composition, the physical state of the surface, and the rate of release of the alkalis. 

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