Gypsum addition

In the oil-weU drilling industry, the barite suspension used as drilling mud can be destabilized by the presence of soluble materials such as gypsum. Addition of barium carbonate precipitates the gypsum, inhibits coagulation, and thus permits the mud to retain the desired consistency and dispersion. 

Fig. 4.2 Effect of gypsum addition on the concentration of Ca ions in the solution and the formation of new phases during cement hydration (according to [3]) a 9% and 2.3 % gypsum addition, b without gypsum...

Fig. 4.10 Optimum gypsum addition as a function of compressive strength vs. different period of maturing (according to [35])...

The optimmn gypsum content is influenced also by the fineness of cement and the content of alkali. The gypsum addition increases with the specific surface area of cement S (by Blaine method, in m /kg) and with the Na O, content. The following correlation has been proposed by Ost [40] ... 

The alkali content in clinker is usually on the level of 1 % Na O with dominating soluble alkali sulphates. In the solution rich in CH, they increase the solubility of gypsum, while the solubility of calcium hydroxide decreases. In this condition the SO4 ions can be bonded with C-S-H. That is why the gypsum addition should be higher. When the specific surface area of cement is high and the reaction of tricalcium aluminate is therefore accelerated, the gypsiun addition should be higher too. 

The best workability or the minimnm water content to assure the required consistency of paste (Fig. 4.18) are attribnted to such gypsum addition which gives simultaneonsly a good set controlling effect. Obviously, at too low gypsum content... 

Optimum gypsum addition increases with temperature in which the concrete is matured (Fig. 4.19) [41]. The process of CjA hydration is then accelerated, according to the Arrhenius rule, and the solubility of Ca(OH)2 is reduced (see p. 4.1.3.4). 

Fig. 4.19 Effect of temperature on the optimum gypsum addition to cement (Kuhs, Oberhausen, according to [41])...

Slota, R. J., and Lewandowska-Kanas, A. (1986) Modification of clinker phase composition and strength properties of a high-alkali cement clinker by gypsum additions, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 2, pp. 205—210. 

The resistanee of slag concrete to sulfate is increased if more than 65% eement is replaeed by slag at a constant gypsum addition of 5%. The high resistanee to sulfate attack can be attributed partly to lower permeability of eonerete. 

The exact mechanism by which gypsum addition to portland cement controls setting behavior is not known although much work has been reported in this field. It is clear, however, that interactions between C3A and gypsum are integral to the stiffening and hardening processes. 

The influence of a gypsum addition is also illustrated in Fig. 17. The increase in the quantity of ettringite, the decrease of gypsum, and the appearance of monosulfate hydrate are indicated by the relevant peaks at 120°C (270°C), 160°C, and 200°C respectively. The presence of hemihy-drate resulted in the formation of both ettringite and gypsum (confirmed by XRD). The peaks in the system containing hemihydrate in Fig. 17 (120-140°C, 270°C) support the XRD results. 

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