Optimum gypsum

Although the maximum pore radius distribution in conventional gypsum microstructures usually lies within the range of capillary suction action (100 nm to 1 mm) [44, 52], the roughly 50% water absorption that occurs in untreated gypsum is often reduced to less than 5 wt% [44, 45] at standard market application rates of just 0.3 - 0.5 wt% H-siloxane. The optimum amount to use depends upon the gypsum raw material, the fineness of the grain in the plaster of Paris, its specific sur ce area, and the reaction temperature. 

Opinions differ as to how the H-siloxane works [44-46, 53]. Whereas one may consider hydrolysis to be the all-important active principle, in which surface-controlled processes involve the interaction of hydroxyl groups with the rehydrated gypsum hemihydrate [46, 45], another mechanism is based not on hydrolysis, but on the optimum distribution of inert methyl groups. Rapid condensation (with release of hydrogen) to amorphous polymethylsilicic acid substantially lowers the degree of hydrophoblcity [44]. 

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

Fig. 4.12 The optimum addition of the mixture of anhydrite with gypsum to clinker against tiicalcium aluminate reactivity (AC A) (after [28]), broken lines clinker with 0.3% SOy, full lines 1.0% SO,...

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])...

Marchiset, J. and others, "Influence of the Burning Conditions on the Optimum Gypsum," Proceedings of the 12th International Conference on Cement Microscopy, International Cement Microscopy Association, Vancouver, British Columbia, 1990, pp. 6-19. 

Improvement in filtration efficiency of gypsum upon flocculation is determined to be due to reduction in the blockage of filter media pores and in the filter cake resistance. An optimum in polymer dosage is explained in terms of stabilization of fines and deformability of the floes formed. Anionic polyacrylamides were found to be more effective filter aids than cationic polymers. 

Reaction Time and Retention Time - Somerville states that the average life of a phosphate rock particle in a typical reaction system may range from 15 seconds to 3 minutes. Nevertheless, retention times in industrial plants range from 1.5 to 12,0 hours or more 111]. One of the reasons relates to the formation of good gypsum crystals, as discussed previously. Another reason is the difficulty of close control of fi ee sulfuric acid (S04 2) content of the liquid phase when the reaction time is short. Close control of this value is extremely important. Although the optimum level of control may depend on the character of the rock, a level of about 1.5% is typical. Serious upsets can occur when the S04 level varies appreciably from the optimum. Obviously, the shorter the reaction time, the faster (and more often) problems can arise. The success of the IV -hour reaction time in the KeOogg-Lopker process is likely related to the use of... 

The production rate may vary widely, but a common design factor is 6.5 tonnes of P205/mVday. The filtration rate is affected primarily by the size and shape of gypsum crystals which, in turn, are affected by conditions in the reaction section including the type of phosphate rock, use of crystal shape modifiers, control of reaction conditions, etc. Insoluble impurities in the rock, such as clay, may affect filtration rates adversely [i9 The filtration rate is also affected by the temperature, concentration, viscosity of the acid, and the desired recovery. While many plants strive for maximum recovery, in specific plants there is often an economic optimum operating rate at which increased production is attained at some sacrifice of recovery. 

The first step after the formation of the three dried mixtures of clinker with red gypsum (see Table 1) was to detennine the appropriate water/cement (W/C) relation in weight terms for obtaining a normal consistency of the paste fonned, as defined in the UNE EN 196-3 regulation in the Spanish law. A normalized Vicat apparatus with a rod 50 mm in length and 10 mm in diameter was used for this purpose, and various moulds of the three mixtures were fabrieated with different W/C proportions. All the moulds were 40 mm in height, with a shape of a truncated cone with lower and upper diameters of 70 and 80 mm, respectively. For each mixture, the optimum W/C ratio corresponded to the penetration of the rod in the mould to a depth of 34 mm. 

The optimum W/C ratios for the three different cements formed with red gypsum are shown in table 5. This table also presents the optimum W/C ratio obtained for the commercial cement used as reference. All the W/C ratios are quite similar, with no significant differences found between the commercial and red gypsum cements. 

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