Cement paste volume changes

The concrete volume increase is directly dependent of the cement paste volume change. 

Fig. 47 Volume changes of cement pastes under near saturated conditions (Morgan).

During the hydration of cement paste, the gross volume of the mixture practically does not change, so that the initial volume, equal to the sum of the volumes of mixed water (V,) and cement (VJ is equal to the volume of the hardening product. As indicated in Figure 1.3 from Neville and Brooks [5], this consists in the sum of the volume of cement that has not yet reacted (Vuc)> hydrated cement (Vp + Vg ), the capillary pores that are filled by water (V ) or by air (VgJ. The volume of the products of hydration can be assumed to be roughly double that of the... 

It is recognized that chemical shrinkage is an indirect measure of the degree of hydration [9], However, in this case same assumptions should be made to use the chemical shrinkage as indicator of hydration rate of cement pastes incorporating SRA, which may be summarized in the following the volume decreases is only dependent on the reaction rate the reaction with water of the different anhydrous compounds of cement occur at the same rate and the density of combined and gel waters do not change in the presence of SRA. 

Fig. 5.51 Scheme of the volume changes of cement paste during hydration a closed system b open system, access of external water, (according to [114])... 

Ai tcin [125] reminds that the increase of coarse aggregate content and decrease of fine aggregate results in reduction of final shrinkage. However, it should be remember that the coarse aggregate content increase has no effect on the absolute volume change of cement paste. The latter one depends only on the w/c ratio. 

Cement pastes generally undergo small but distinct volume changes upon hydration and in the comse of curing. Volume changes of this kind are usually of little significance, bnt may become critical imder some special conditions. 

Figure 21.1 Schematic presentation of volume changes in an ordinary Portland cement paste due to hydration and curing. NUP, non-hydrated paste HP, air-cured hydrated paste WCHP, water-cured hydrated paste AHDP, air-dried hydrated paste.

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

Shrinkage of the cement-based matrix is the change in its volume due to moisture variation or chemical reactions and thermal effects during and after the hydration process. The cement paste is the source of shrinkage while other components are inert and only may control the deformations due to shrinkage. 

Pycnometry measurement of the change in weight of a sample of cement paste plus extra water (or oil) additional liquid is added to a constant volume before measurement... 

Figure 3.17 Chemical shrinkage of portland cement-quartz (PC-Qz) pastes (water-to-binder ratio [w/b] = 0.5, CEM I 52.5 R) hydrated at 20°C. Chemical shrinkage is given as volume change to compare with measured volume change of blank samples. Dilatometry was done using the Empa procedure (see Table 3.1) on three replicate samples the error bars indicate the standard deviation. (Courtesy of Axel Scholer.)...

A important experimental support of in situ hypothesis was presented by Cottin [76]. The mixture of calcium aluminate cement and gypsum is hydrating with too low water content. The change of volume calculated from the phase composition of the paste corresponds very well to the measured value. 

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