Cement paste swelling

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. 

Fig. 4.20 Effect of gypsum content on the initial set and swelling of cement pastes with variable CjA content (schematically) (according to [41 ])...

It is assumed that cement paste plays a role of semi-permeable membrane which is impermeable for the silicate anions formed as a result of reaction. Some authors postulate that this semi-permeable membrane is composed of sodium and potassium sUicates gel formed during reaction. A concentration gradient appears and linked with him the osmotic pressure. Recently, the difference in discussions of swelling mechanism, attributed to the sorption of water or to the osmotic pressure, became insignificant. There is a view that sodium and potassium ions can diffuse through the semi-permeable membrane more easily than the calcium ions. 

Adsorption of water in hardened cement paste - shrinkage and swelling... 

A hygroscopic material expands - swells - when adsorbing moisture from the air, and it contracts - shrinks - when it gives off moisture. These moisture movements can provoke stresses that may result in shrinkage cracks in materials that are embedded in a structure. This phenomenon is well-known from concrete and other cement-bound materials where the binder phase consists of cement paste. 

Most practical cements contain Mg " which is less strongly bound to the polyacrylate than Zn (Gregor, Luttinger Loebl, 1955a). Magnesium oxide forms a paste with PAA which sets to a plastic mass this is not hydrolytically stable, for when placed in water it swells and softens (Hornsby, 1977 Smith, 1982a). Moreover, if ZnO powder contains more than 10% MgO, the resultant cement deteriorates under oral conditions. 

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