Ettringite retarded formation

The silica fume is said to decrease the formation of ettringite by reducing the concentration of calcium (Ca" +) and hydroxyl (OH ) ions involved in the formation of ettringite [146]. The expansion of lime-based admixtures is not affected by water-reducing admixtures to the same extent as observed for CSA-based admixtures. However, set retardation may result due to the increased amounts of calcium hydroxide produced in the presence of the WRA. 

Apart from the reaction of ettringite formation, the CaO and MgO are also applied as expansive components. The technology with CaO application, is more developed, particularly in Japan. In order to avoid the unfavorable properties of CaO, dne to its rapid or retarded hydration, the methods consisting in the anhydrite or C3S mixture manufacturing, as the matrix with calcium oxide inclusions, were developed. Dissolution or hydration of the matrix causes the gradual exposure of CaO crystals, which then can react with water. Kawano et al. [54] invented the... 

The effeet of KOH or C3 S on the formation of ettringite at different temperatures up to 80°C has been reported. Elevated temperatures do not prevent the formation of ettringite but the amounts depend on the eoncen-tration of other additives. Potassium hydroxide retards ettringite formation. At concentrations above 1 mol/1 syngenite is also formed. In the presence of C3S ettringite is formed at all temperatures. Only small amounts of ettringite are formed in the presence of 0.5 mol/1 of KOH and C3S. 

Hydration of C3S is known to be retarded by the superplasticizer. There is also evidence that the C/S ratio of the C-S-H phase is changed. In cements, the hydration of the C3S phase is retarded, but to a lesser extent than when hydrated in the pure form because part of the admixture is adsorbed by the C3A phase. Formation of ettringite may be accelerated or retarded by the superplasticizer, depending on the amoimt of alkali sulfates present in cement. 

An addition of 0.1% CLS may extend the initial and final setting times of cement mortar by two and three hours, respectively. The influence of 0.3% CLS on the hydration of cement is shown in Fig. 9. Thermograms indicate that the reference cement containing no admixture exhibits a broad endothermal peak below 200°C, representing the formation of both ettringite and C-S-H phase. These peaks increase in intensity as the hydration period is increased. The effect between 450 and 500°C is caused by the dehydration of Ca(OH)2 and its intensity indicates the extent to which the hydration of the C3S component has progressed. The cement hydration, in the presence of lignosulfonate, is retarded as seen by the lower intensity of the Ca(OH)2 decomposition peak. The low temperature effect below 300°C in the presence of CLS is not sharp as that obtained in the reference sample. 

Massazza and Costat l applied the DSC, XRD, and conduction calorimetric techniques to investigate the effect of a SNF type of superplasticizer on the hydration of CjA-gypsum mixtures. The results indicated that SNF retards the formation of ettringite and its conversion to the monosulfate form. 

An extensive work was carried out by Uchikawa l on the conduction calorimetry of superplasticizers, such as SNF (NS in the figure), lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC), and a three dimensional polycondensate product of aromatic aminosulfonic acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry corresponds to the heat of dissolution of alite, the heat of formation of the AFt phase, and the calcium hydroxide formation from free lime. The second peak corresponds to the heat of hydration of alite. The admixtures were found to accelerate the formation ofthe ettringite phase. At w/cratios ofO.3 and 0.5 and a later addition of the admixture, the appearance of the second peak was significantly delayed and the peaks were of lower intensity. Most retardation occurred with polycarboxylic acid and amninosulfonic acid-based admixtures (Fig. 11). DSC was used to determine the amount of lime formed at different times. The DSC results show that the addition of admixtures at different w/c ratios generally decreases the amounts of lime in the presence of superplasticizers (Fig. 12). 

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