Cementation effect, hydrate

The study of the effect of the addition of PVA and clay to the cement-water hydration was done. A general and empirical method for determining the kinetic parameters was used. The apparent activation energy was higher for the case of the polymer and polymer-clay addition than clay and pristine cement paste. The maximum adiabatic temperature reached was for the case of cement-water and clay sample. The polymer swells and produces a film around the cement and delay the cement hydration. Instead, clay acts as nucleating agent and produces a high hydration rate. The addition of clay to PVA did not affect the hydration kinetics. 

Portland cement clinker normally contains not more than about 0.2% P2O5, and further small additions to anhydrous portland cement before hydration and setting are reported to inaease the hardening rate and the ultimate strength. Larger additions (certainly >1.0%) have a retarding effect and can have disastrous results, but the subject is somewhat controversial [37,38]. 

The admixture of gypsum in production of cement is necessary to decrease the early hydration rate and to adapt it to the technology for handling and placing the fresh mix. The rate of hydration is indicated in regulations for different kinds of cement and on average it may be assumed that hydration is achieved six hours after addition of water to cement. In that period, only about 15% of cement volume is effectively hydrated and these processes... 

Chemical admixtures in small amounts are added to concrete to enhance the physical, mechanical, chemical, and durability characteristics of concrete. Superplasticizers are admixtures that have the ability to increase the workability of concrete (for easy placement), but also produce high strength concretes. The relative rates of hydration of cement containing superplasticizers at different temperatures are conveniently followed by conduction calorimetry. In Fig. 11 both the rates of hydration and the cumulative amounts of heat developed in cement pastes hydrated at temperatures of20,40, and 55" C are plotted as a function of time. In the figure, SMF refers to the superplasticizer based on sulfonated melamine formaldehyde. The addition of the superplasticizer retards the hydration of cement. The retardation increases with the dosage of the superplasticizer. Also, the retardation effect becomes less significant as the temperature of hydration is increased. 

In cements, the hydration of C3S is retarded, but not to the same extent as when it is used in a pure form. In cement, part ofthe superplasticizer is adsorbed by the C3A phase so that only smaller amounts are available to retard the hydration of the silicate phase. The rate of hydration of cement in the presence of superplasticizers has been studied by many thermal techniques. The effect of SMF on the hydration of cement can be followed by conduction calorimetry (Fig. 8). ] The peak occurring at 5 hours in the... 

Gu, P., and Beaudoin, J. J., Effect of Lithium Salts onPortland Cement/ High Alumina Cement Paste Hydration, J. Mater. Sci. Lett., 14 1207-1209(1995)... 

Skibsted, J. 1988. High-speed Si and Al MAS NMR studies of Portland and high alumina cements Effects of microsilica on hydration reactions and products. In Geopolymer 88 Proceedings of the First European Conferenoe on Soft Mineralurgy, Davidovits, J., and J. Orlinski (eds.), 2, 179-196. Universite de Technologic de Compiegne. 

Water is essential to the reaction and cements are not formed when the acid is present in an organic solvent rather than in aqueous solution. Water acts as an effective reaction medium and probably hydrates reaction products. 

Hydrates quickly, responsible for strength of cement in early stage setting time can be controlled by addition of gypsum Responsible for strength in all stages Responsible for final strength Little effect on physical properties... 

Dead burned magnesium oxide is suitable as an expanding additive [368-370]. The expansion occurs by a hydration mechanism. The additive is particularly effective when used at setting temperatures greater than approximately 150° C. Enhanced adhesion of expanded cements can be achieved by the addition of urea-formaldehyde resins [1720]. 

V.Alunno Rosseti, F. Medici, Inertization of toxic metals in cement matrices effects on hydration and hardening, Cem. Conor. Res 25(6), 1147-1152, 1995. 

The various types of water-reducing admixtures possess different but characteristic adsorption isotherms which qualitatively reflect their effect on cement hydration kinetics, as shown in Fig. 1.17. 

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],... 

There is little published data on the effect of air-entraining agents on the chemistry and morphology of cement hydration. However, the limited studies [15] indicate that the normal hydration pattern under isothermal conditions for ordinary Portland cement shown in Fig. 3.14 is modified as follows ... 

Addition of dampproofers based on caprylic, capric or stearic acids, stearates or wax emulsions do not have any effect on the setting characteristics of hydration products of Portland cement. However, the unsaturated fatty acid salts, such as oleates, although not affecting the tricalcium silicate hydration, have a marked effect on the ettringite and monosulfate reaction [12] and this is illustrated in the isothermal calorimetry results in Fig. 4.4. It is possible that a calcium oleoaluminate hydrate complex is formed involving the double bond of the oleic acid. 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

The properties of a latex depend on the nature of polymers in the latex, particularly the monomer ratio in copolymers and the type and amount of plasticizers. The monomer ratio affects the strengths of the latex modified mortars to the same extent as the polymer-cement ratio [87, 92]. Mechanical and chemical stability, bubbling and coalescence on drying all depend on the type and amount of surfactants and antifoamers and the size of dispersed polymer particles. It is important that the use of selected antifoamers and surfactants as stabilizers or emulsifiers produces no adverse effect on cement hydration. 

Surfactants enable the polymer particles to disperse effectively without coagulation in the mortar and concrete. Thus, mechanical and chemical stabilities of latexes are improved with an increase in the content of the surfactants selected as stabilizers. An excess of surfactant, however, may have an adverse effect on the strength because of the reduced latex film strength, the delayed cement hydration and excess air entrainment. Consequently, the latexes used as cement modifiers should have an optimum surfactant content (from 5 to 30% of the weight of total solids) to provide adequate strength. Suitable antifoamers are usually added to the latexes to prevent excess air entrainment increased dosages causes a drastic reduction in the air content and a concurrent increase in compressive strength [87, 92-94]. 

Fig. 6.18 The effect of varied concentrations of the stabiiizer on the rate of cement hydration.

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