Cement paste chemical shrinkage

Water held by capillary tension in pores of diameters smaller than 50 nm wiU evaporate at lower values of the relative humidity as the diameter of the pores decreases. Indicatively, values of relative humidity from 95 % to 60 % are required when the diameter of capillary pores decreases from 50 nm to 5 nm [17]. In this case, evaporation can produce significant shrinkage of the cement paste. In addition, the mobihty of ions (thus the electrical conductivity of the solution in these micropores) is affected by chemical and physical interactions between the Hquid and the solid and is therefore lower than that of a solution of the same composition. 

Keywords Shrinkage reducing admixtures, chemical shrinkage, cement pastes, compressive strength. 

In order to evaluate de delay of hydration due to the presence of SRA, the degree of hydration of cement pastes with SRA is analyzed, using the chemical shrinkage test, for two different cements and three polymers. The compressive strength of mortars was also used as an indirect method of estimation of relative degree of hydration. 

Fig. 2 show the chemical shrinkage results for cements A1 and Se. As can be seen, the values are not too different when adding the SRAs. However, small changes are noticeable, which depends on the type of cement and on the type of SRA. To put that into evidence. Fig. 3 show individually the chemical shrinkage of reference paste and SRA paste. In this figure is evident the higher influence of SRA on pastes with cement Se. 

Comparing the chemical shrinkage of the two reference pastes, without SRA, the values of cement Se are higher than the correspondent ones of cement Al, at the same age, which is in... 

Figure 2 Chemical shrinkage of pastes made with cement A1 and cement Se...

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. 6 shows the compressive results of the eight mortars. The results also indicate less influence of SRA S on the performance of the material, which may be related with degree of hydration. The relative influence of the three SRA on chemical shrinkage and compressive strength was similar for pastes made with cement Se. Compressive strength results on pastes made with cement A1 also... 

The influence of SRA products in chemical shrinkage of pastes depends on the cement used and on the type of polymer, and no systematic tendency was observed. However, the values obtained in this study generally indicate a small reduction of chemical shrinkage when using these polymers, mainly at early age. 

Wild, S., Khatib, J.M., and Roose, L.J. (1998) Chemical shrinkage arrd autogenous shrinkage of Portland cement-metakaolin pastes. Advances in Cement Research 10, 109-119. 

Just like the hydration of the cementitious component, the reaction of the ejq)ansive component with water is accompanied by a chemical shrinkage. Thus the resultant external expansion of the paste, which accompanies the hydration of an expansive cement, cannot be explained by an increase of the volume of the reactants formed in this chemical process. It is obvious that an expansion of the system would not take place if the hydration occurred in a simple through-solution reaction, in which the anhydrous starting component was readily dissolved and the hydration product precipitated randomly from the liquid phase. To produce an expansion a particular reaction mechanism seems to be required. Several hypothesis have been put forward to ejqrlain the expansion process ... 

Justines, H. et al. (1997) Influence of measuring method on bleeding chemical shrinkage values of cement pastes, ia Proceedings 10th ICCC, Goteborg, paper 2ii069. 

Koster, H., and Odler, 1. (1986) Investigation on the stracture of fully hydrated Portland cement and tricalcium siUcate pastes. 1. Bound water, chemical shrinkage and density of hydrates. Cement and Concrete Research 16,207-214. 

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. 

Figure 1.37. The chemical shrinkage by hydration of cement paste can be measured by the simple setup shown the water level in the narrow neck of the flask over a specimen of cement paste directly indicates the shrinkage of volume. The specimen is photographed 0 hours and 48 hours, respectively, after casting.

The chemical shrinkage has the consequences that a mixture of Portland cement and water - cement paste - has a natural tendency to imbibe water from the environments during hydration. If this water imbibition is prevented, capillary tensile stresses wiU arise in the pore water. 

Figure 1.38. A Danish equipment for measuring chemical shrinkage in hardening cement paste. With this equipment, which is thermostatically controlled, simultaneous registration of chemical shrinkage in up to 6 specimens of cement paste is possible (Conometer of the make Loik).

Figure 1.39. Chemical shrinkage measured on hydrating cement paste. Cement type PC(R/HS/EA/ W), 20°C, w/c ratio 0.50 (Geiker M Studies of Portland Cement Hydration).

Phase transformations play an important role within the science of construction materials. Different phenomena such as hardening of steel, freezing damage of concrete, moisture absorption in wood, and chemical shrinkage of hardening cement paste, are all connected with phase transformations in systems of matter. Table 4.1 contains an overview of the most important phase transformations between substances in the solid, liquid, and gaseous states. 

Lura, R, F. Winnefeld and S. Klemm (2010). Simultaneous measurements of heat of hydration and chemical shrinkage on hardening cement pastes . Journal of Thermal Analysis and Calorimetry 101(3) 925-932. 

Figure 3.2 Principle of the development of the volumetric proportions of the constituents of the cement paste during hydration. CS chemical shrinkage. Volumes is for cement V p, capillary water gel water, V, solid hydration products. (Based on Powers, T. C., Industrial and Engineering Chemistry, 27(7), 790-794, 1935.)...

Figure 3.3 Principle of measurement of chemical shrinkage (CS) as volume decrease of a saturated cement paste to follow the overall development of hydration as a function of time.

As far as the author knows, Le Chatelier (1900) was the first to report measurements of chemical shrinkage in cement paste he used dilatometry. This measuring principle is used in the ASTM standard for testing early-age hydration, ASTM C1608 (2012), Test method for chemical shrinkage... 

Chemical shrinkage is computed as the measured millilitres of sorbed water, i.e. the volume decrease per gram of cement in the paste sample. The mass of cement powder in the vial is given by (assuming density of water at 1000 kg/m3) (ASTM 2012) as... 

Figure 3.11 Chemical shrinkage of cement paste samples varying in height and w/c (20°C). Left graphs ordinary portland cement (PC) right graphs rapidhardening Portland cement (RPC) (Geiker 1983).

Figure 3.15 Effect of amount and composition of fluid on top of cement paste on measured chemical shrinkage, w/c = 0.30 25 g paste water deionised water, syn. PS synthetic pore solution, CH PS calcium hydroxide saturated pore solution. (After Sant, G., P. Lura and J. Weiss, Journal of the Transportation Research Record, 1979, 21-29, 2006.)...

The composition and the amount of fluid applied on top of the cement paste affect the measured chemical shrinkage. The larger the amount and the more diluted the fluid, the higher the initial rate of chemical shrinkage. (Sant et al. 2006) see Figure 3.15. To reduce the possible impact of the fluid, the excess fluid should be limited see Table 3.2 and the guidelines in Section 3.9. 

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.)...

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