Shrinkage-compensated cement

Expanding cement (shrinkage-compensated cement) do not shrink upon hardening. Addition of 10 to 15% of calcium sulfate or aluminate even effects an expansion ( managed gypsum bursting ). 

Fu, Y. et al. (1995a) Effect of chemical admixtirres on the expansion of shrinkage-compensating cement containing a prehydrated alttmina cemerrt-based expansive additive. Cement and Concrete Research 25,29-38. 

The expansion achieved in concrete containing these admixtures is dependent on the type of aggregates used in the mixture. Thus the desired level of shrinkage compensation should take into account the modulus of the aggregate to be used on the job. The effect of cement and water contents on the extent of expansion should also be considered. Minimum cement content required to achieve desired expansion should be determined through mix trials. Compatibility with other admixtures may also need trials and the manufacturer s recommendation should be followed [80]. 

The commonly used ratios of admixture to cement for the purpose of shrinkage compensation are 9-11 (admixture) to 91-89 (cement). At these ratios the properties of CSA concrete are similar to Portland cement concretes of similar mix proportions. At admixture dosages exceeding 11% however, concrete workability and strength decrease, while expansion and air entrainment increase. When expansion is umestrained and exceeds 0.3%, strength is reduced [74]. 

CSA admixtures are more prone to loss of activity due to C02 and moisture pick up than are shrinkage compensating or Portland cements [74,79, 85], Consequently, exceeding the shelf life may seriously reduce the expansive potential. The materials are therefore packed in water-proof bags which should always be stored in a dry... 

Fig. 9.7 Volume changes of expansive, shrinkage compensation and classic Portland cement...

Figure 21.2 Volume changes and stresses in OPC, shrinkage-compensated and self-stressing cement pastes due to hydration and drying.

Collepardi, M., Borsoi, A., Collepardi, S., Olagot, J. J. O., Troli, R. (2005) Effects of shrinkage reducing admixture in shrinkage compensating concrete under non-wet curing conditions, Cement and Concrete Composites, 27 704—8. 

Fu, Y., Xie, P., Gu, P., Beaudoin, J. J., Characteristics of Shrinkage Compensating Expansive Cement Containing a Pre-hydrated High Alumina Cement-based Expansive Additive, Cement Conor. 24 267-276 (1994)... 

High-temperature cement formulations are basically polymer concrete. Mainly unsaturated polyesters or vinyl ester resins with allylphthalate as vinyl monomer are in use [927]. Cming is achieved with peroxides, which decompose sufficiently fast at temperamres between 120° and 200° C. Polymer concrete requires additional materials, which compensate shrinkage. 

Addition of admixture to obtain higher strengths In this situation the small amount of reported work indicates that where the water-cement ratio is reduced, the shrinkage and creep of the concrete is also reduced. Figure 1.50 shows the effect of a hydroxycarboxylic acid plasticizer on the creep of the concrete where the material has been used to effect a reduction in the water- cement ratio without any other changes in the mix design. Thus it seems that the reduction of the water-cement ratio will compensate for the increases in creep observed in the data above. 

Most material studies reported in medical journals are of interest to those involved in mainstream plastic applications. Some medical plastics must perform under constant water immersion. It was reported that absorption of 1 % water reduces the fatigue life of PMMA by a factor of four, since bone cement can only be replaced by a surgical operation such a performance is clearly unacceptable. The use of silane to treat the hydroxyapatite filler in this material reduced water uptake. The water uptake increased with increased concentration of hydroxyapatite. In applications, such as dental fillings, increased water uptake is considered helpful since it compensates for the loss of volume due to shrinkage of the filling during curing. 

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