Paste hydration

In the past, hydrates have been associated with significant movement of earth (seafloor slumps) in deepwater ocean environments. Notably, Bugge et al. (1988), Schmuck and Pauli (1993), MacDonald et al. (1994), and Pauli and Dillon (2001) have described both large and small Earth movements associated with hydrates. Concerns have been expressed (Campbell, 1991) about the effect of hydrates on foundations of platforms and pipelines, as well as offshore drilling. Hydrates as geohazards are discussed in Section 7.8. 

The conclusion is that both the body structure and the surface structure of tobermorite are highly reproducible. Whether we use tricalcium silicate or fi-dicalcium silicate as starting solids, whether we use a water to solid ratio of 0.7 or 9.0, whether we use paste hydration or ball-mill hydration or a third type which I have not discussed (which gave the six other points on the curve), we wind up with a tobermorite having very nearly the same body structure and surface structure. 

Paste hydrated 2 months at indicated temperature, remainder of time at room temperature. 

Stoichiometry of Hydration Reactions. The stoichiometry of the hydration reactions has a significant influence on surface formation. In previous investigations of Ca3Si05 and / -Ca2Si04 pastes hydrated at room temperature for long times (3, 5), the stoichiometry of the hydration process was found to be represented by the following equations ... 

The calcium silicate hydrate formed on paste hydration of C3S or P-CjS is a particular variety of C-S-H, which is a generic name for any amorphous... 

Fig. 10.5 shows portions of fracture surfaces of some pastes of Ciment Fondu examined by secondary electron imaging in the SEM. At 22 h. in pastes hydrated at 20 C, fibrous material is abundant, together with plates of CjAHg. The fibrous material is probably partially dehydrated CAH,(,. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

Fig. 11.5 Rales of heat output from CjS pastes hydrated at 25 C and w/e ratio 0.6, with and without CaCl, in a concentration of 0.0204 mol...

FTIR characterization. The FTIR spectra (Fig. 6) of unmodified and modified cement pastes are compared after 4 days of hydration. Analysis of pristine PVA and nanoclay was also made. In the range between 3100-3700 cm the H2O and OH stretching bands appear [29]. They are also present in the spectrum of the anhydrite paste, because of the hygroscopicity of the cement. A small increase of the OH band, associated with the Ca(OH)2 molecules, is recorded at -3643 cm when compared to the anhydrite paste. The cement samples have a higher peak than the paste with PVA and PVA/clay. A cement paste hydrated for 28 days has a higher peak than the peak hydrated for only 4 days. 

Fig. 5.63 Effeet of eement paste hydration progress on its permeability at different w/e. (aeeording to [134])...

The paste hydration of Portland cement at ambient temperatures is characterized by several stages. 

Upon mixing with water, alite-fluoroaluminate cement exhibits very rapid hydration. In paste hydration at ambient temperature about 40-50% of the CjjA2.CaF2 is hydrated... 

The hydration of CA is accompanied by a liberation of heat of hydration. In paste hydration at ambient temperature the rate at which heat is liberated is slow initially, but a heat hberation maximum develops at the end of an induction period, after which time the rate of the hydration reaction is significarrtly accelerated. The time at which the maximttm heat output (ranging up to about 800 W/kg Edmonds and Majirmdar, 1988) is reached becomes extended as the temperature increases from 0°C up to about 30°C, indicating a gradual slow-down of the hydration reaction. At even higher temperatures. 

Out of the two calcium ferrite phases, C2F hydrates appreciably faster than CF. In paste hydration about 65% of the former and 10% of the latter l drate within 28 days at ambient temperature. Initially, low-basicity hydroferrites are formed as products of hydration, which contribute to a moderate strength of the hardened paste. However, after a few days of hydration these convert into cubic C3FHg and amorphous FefOHlg, and this conversion is associated with an almost complete loss of bonding properties. 

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. 

The rate of hydration of C3S has been determined by following the consumption of C3S, non-evaporable water or Ca (OH)2 with time. In Fig. 1, the general form of curve relating the fraction of C3S consumed as a function of time in a paste hydrated at a W/S ratio of 0.5 is given.1 1 Various stages of reaction could be related to the conduction calorimetric curves. At 120 days, more than 80% hydration has taken place. At one year presumably all C3S has been consumed. 

Tricalcium aluminate reacts with water to form C2AH8 and C4AH13 (hexagonal phases). These products are thermodynamically unstable so that without stabilizers or admixtures they convert to the C3AH5 phase (cubic phase). In a paste, hydration is slightly retarded in the presence of CH. In dilute suspensions the first hydrate formed is C4AH19. 

Feldman and Beaudoin correlated strength and modulus of elasticity for several systems over a wide range of porosities. The systems included pastes hydrated at room temperature, autoclaved cement paste with and without additions of fly ash, and those obtained by other workers. Porosity was obtained by measurement of solid volume by a helium pycnometric technique and apparent volume, through the application of Archimedes principle. 

Kantro, D. L., Weise, C. H., and Brunauer, S., Paste Hydration of B-Dicalcium Silicate, Tricalcium Silicate and Alite, Symp. Structure of Portland Cement Paste and Concrete, pp. 309-327, Highway Res. Board (1966)... 

Compared to the extensive thermal investigations of C3S hydration in the presence of CaCl2, only a limited amount of work has been reported on the effect of calcium chloride on the hydration of C2S. Calcium chloride accelerates the hydration of dicalcium silicate. In the thermograms of C2S paste hydrated for 1-3 months, lower amounts of calcium hydroxide are formed in the presence of calcium chloride, compared to that hydrated... 

Figure 15. DTA estimation of lime in cement pastes hydrated with triethanolamine.

Figure 21. Relationship between strength and calcium hydroxide content in cement paste hydrated at -5°C...

Figure 8. Influence of sodium carbonate (NLS) and sodium lignosulfonate + sodium carbonate (NC) on the paste hydration of C3S (80%)-C3A (20%).

Copper refining wastes containing Cr and Sb were added to cements and the rate of hydration was followed by DTA, TG, and DTG. There was a significant decrease in the amount of Ca(OH)2 in pastes hydrated with the copper refining waste, suggesting that it is a good retarder.I l... 

Table 4. Amounts of Calcium Hydroxide Formed in a Cement Paste Hydrated in the Presence of SMF Superplasticizer...

Figure 8. Amount of calcium hydroxide formed in cement-silica fume pastes hydrated for different times.

Figure 11. DTA of eement pastes hydrated to different times with 30% siliea fume.

Sellevold, E. J., Eager, D. H., Klitgaad, J. K., and Knudsen, T., Silica Fume-Cement Pastes Hydration and Pore Structure, Report BML 82 610, pp. 19-50, The Norwegian Inst. Of Technol, Trondheim, Norway (1982)... 

Modulus of elasticity versus porosity curves for Reg Set cement paste hydrated in the presence of CaCl2 are all higher than the curve for Reg Set cement paste with 0% CaCl2. All curves for Reg Set cement paste are below the curve for hydrated portland cement paste. 

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)... 

Figure 2. DTA and XRD data for a Secar 71 paste hydrated at 10°C for 24 hours.l ...

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