Mixing, pastes preparation

Before proceeding further it is well to consider the term cement, for its definition can be the source of some confusion. Both the Oxford English Dictionary and Webster give two alternative definitions. One defines a cement as a paste, prepared by mixing a powder with water, that sets to a hard mass. In the other a cement is described as a bonding agent. These two definitions are quite different. The first leads to a classification of cements in terms of the setting process, while the second lays emphasis on a property. In this book the term cement follows the sense of the first of these definitions. 

BS crystals are obtained during paste preparation at temperatures higher than 80°C [51]. This technology yields long, well-shaped, 4BS crystals with a thickness that varies depending on the time of paste mixing and the temperature of paste preparation. When the paste is mixed for 10-20 min, the crystals are 2-4 pm thick and require 20-30 h for formation. The PAM... 

It is a monoclinic crystaUine substance built of long tbin crystals. It is formed when PbO is mixed with H2SO4 solution during paste preparation, within a very narrow pH interval 8—15 wt% H2S04/leady oxide. Monobasic lead sulfate is obtained also during soaking of tbe cured... 

Figure 6.8 illustrates the effect of BaS04 and of expander (1% of sodium lignosulfonate, SL) on the phase composition of pastes prepared using ball mill leady oxides with different H2S04/Pb0 ratios and mixed at 35 °C for 30 min [13]. 

When the paste is prepared at 70 °C, considerable amounts of 3BS crystals remain unreacted even after 30 min of mixing. The situation is different when the process of paste preparation is carried out at 90 °C. In this case, the whole paste is converted into 4BS crystals within the first 10 min. At 80 °C, the transformation of 3BS to 4BS is completed within 30 min. This indicates that the process of nucleation and growth of 4BS crystals is very sensitive to the temperature of paste preparation. In order to produce 4BS paste within 30 min the temperature of paste preparation should be above 90 °C. 

Figure 6.17c shows that, when a mixture of 50% tet-PbO plus 50% orthorhomb-PbO is used, 3BS forms within the first couple of minutes already, while formation of 4BS starts after the 5th min of paste stirring. After 20 min, the content of 4BS is as high as 65% against 20% in the paste prepared from tet-PbO alone. Our investigations have proved that the maximum rate of 4BS formation is achieved when the leady oxide mix contains 75 to 80% tet-PbO. The recommended proportion of the two PbO modifications in the initial lead powder for preparation of 4BS pastes is 20% orthorhomb-PbO + 80% tet-PbO. 

The relatively high temperature of paste preparation and mixing accelerate the above reaction. 

A sample scheme of the successive technological procedures involved in the process of 3BS paste preparation is presented in Fig. 6.32. An example of a formulation for 3BS paste preparation could be LO (78% PbO) — 500 kg, H2O — 65 L, H2SO4 (1.4 g cm ) — 39 L, fibres — 0.35 kg. For the conversion of the 3BS paste into 4BS during the plate curing procedure, some 6—7 kg of tetrabasic lead sulfate nucleants should also be added to the initial paste mix. 

Paste preparation starts with the introduction of fibres or (fibres + 4BS nucleants) in the paste mixer. Then water is added and the suspension is mixed for 1 min. The next step is to add the leady oxide (LO) and mix for another 3—4 min until a homogenous mass is obtained. Then the sulphuric acid solution is added slowly, for about 10—12 min. The temperature is monitored continuously and kept below 50 °C. If needed, the cooling system of the paste mixer is activated. After adding the whole amount of H2SO4 solution, the paste is mixed for 15 more minutes to allow the 3BS crystals to grow. Finally, samples are taken from the paste to measure its density and consistency. If these parameters meet the technological requirements, the paste... 

The paste formulations and the durations of the various procedures given in the schemes are only illustrative. They vary depending on mixer type and design, paste hatch volume, degree of oxidation of the LO as well as on the quality of the initial materials used for paste preparation, the temperature of mixing, etc. 

Operation of the paste preparation station is under continuous hardware and software control. The sequence of the various procedures of paste preparation, weighing and dosing of flie individual paste formulation components and the duration of mixing (of each component of the dry mix and of the paste) are controlled automatically. The changes in temperature and viscosity of the paste are also under continuous control. 

There is no need to add water during paste preparation when mixing 1kg LO with 200.5 mL H2SO4 solution of 1.18 gem... 

H2SO4/LO = 6% by weight should be mixed with 200 ml H2SO4 solution with density 1.18 gcm or with 84.5 ml of H2SO4 solution (1.40 g cm ) plus 115.5 mL of H2O, so as to produce paste with density 4.10 gem. During the process of paste preparation the temperature should not exceed 50 °C in order to obtain 3BS phase. 

Alternatively the two pastes can be extruded onto a conventional mixing pad and blended by hand with a spatula. Even when prepared in this way, it is desirable for the freshly mixed paste to be placed by syringe, so from the mixing pad, it must be loaded into an appropriate tip for delivery. Mixing in this way carries the risk of incorporating air into the paste, which in the form of bubbles can weaken the set material [30]. 

S. The pastes without third letter are the reference pastes, without SRA. One example is Al+G that corresponds to the paste prepared with cement A1 and the SRA G. The eight pastes were prepared using W/C=0.4 and the mixing procedures followed the EN-196-1, but without sand addition. SRA was added 20 seconds after starting the mixer, which had already cement and water. 

A paste prepared from 4-ethoxymethylene-2-phenyl-5-oxazolone and tetrahydro-furan mixed with indoline 4-(5-indolinylmethylene)-2-phenyl-5-oxazolone. Y 80%. F. e. and steps s. H. Behringer and P. Duesberg, B. 96, 377 (1963). 

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