44. The fineness of grinding and particle size distribution (grain) composition of the cement

The finely ground cement has higher strength, the thinner it grinding - the higher strength. This applies particularly to the early strength, in subsequent periods differences in the strength decrease. Large cement particles react with water in the main surface. 1.13 The table shows data on different kinds of specific surface Blaine cements. When the cement grinding process is gypsum is ground and it accumulates more easily in the fine fraction, it is also possible during the grinding of clinker ash. Granulated blast furnace slag on the contrary - a solid, crushed harder than clinker and accumulated in the coarse fractions.

The early strength of cement clinker provide the finest fraction (0 - 3 microns). The largest cement fraction of particles larger than 50 microns solidify so slowly that they are sometimes considered almost inert.

Grinding aids also contribute to the formation of a narrow particle size distribution. Influence of the distribution of grain to increase the technical strength cements and concretes is not always clear.

Grinding aids can produce in the Federal Republic of Germany cements with a specific surface area of ​​about> 3500 cm 2 / g. They are surface-active agents (surfactants) are added to the material to be ground in a small dosage to make the process more efficient milling. Their usefulness and efficiency increases with the fineness of cement. For the same cement (the same fineness) intensify the additive in an amount of 0.01-0.1% may increase the throughput (throughput) of the mill to 10-50%.

Intensifying additives do not promote corrosion of rebar in concrete. While a positive effect for the grinding of Portland cement was indisputable, only small benefits observed with slag cement grinding. Special effective intensify the surfactant additive include glycols (e.g., polyethylene, polypropylene) calcium stearate, and ethanolamines (as triethanolamine, trietanolaminatsetat) which are generally added in an amount less than 0.05%. Their use increases the specific surface of the cement to an average of 800 cm2 / g. grinding effect is primarily that the surfactant molecules form on the surface of cement particles comminuted thin film which prevents sticking of particles to each other (aggregation) and comminuted adhering cement particles onto the surface of grinding bodies and bronefuterovki. The phenomena of adhesion and aggregation, resulting in fine grinding of cement prevents grinding process. Furthermore, increasing the flowability and mobility of the ground material increases the particle separation efficiency in the separator and increase mill throughput. In plants typically used intensify the additive from the group of glycols and ethanolamines, with normal concentrations which have no adverse effect on the setting time and the cement hardening process. Furthermore, long-term tests of concrete showed that the optimal dosage of grinding aids do not adversely affect the strength of [107]. High amounts of additives (> 0.2%) can lead to a reduction and early 28-day strength. Long-term use of grinding aids showed that they do not adversely affect the strength of the concrete during long periods of hardening and durability of concrete.

In the CIS countries as intensifiers cement grinding process, the greatest use is made of cationic compounds - technical lignosulfonates (LST) (formerly known as RRT), triethanolamine (TEA), a mixture of triethanolamine with LST in the ratio 1: 3 - 1: 5 and soap stock, lignin , oil soap. When administered in an amount of TEA 0.015 ... 0.03% by weight of cement mill productivity increased by 15 ... 35%, the specific energy consumption is reduced by 10 ... 30%. Intensify the process of grinding of cement and coal additives, carbon black (0.3%), coke dust (2 ... 3%), tripoli (1 ... 2%).

The efficiency of grinding aids, and depends on the method of their introduction into the mill. Most plants surfactant additives are added by the expiration of the simplest to the material on the belt conveyor or nutritional clinker plate. This method is not very effective, as long as the surfactant additive is uniformly distributed over the entire surface of the material will require considerable time. The material will have time to pass into the second chamber pulverizing. In these conditions the function of the additive will be reduced only to eliminate buildup on the balls.

More effective is the introduction of an aqueous surfactant solution in atomized form into the second chamber cement mill. Enhancer molecules from the beginning to the grinding process again sopri¬kasayutsya razmalyvaemo¬go bare surfaces of the material are adsorbed on them, and act as ponizite¬li hardness. With this method of introducing a surfactant into the mill their optimum dosage will be many times less than used in conventional feeding them with the nutrient materia¬lom on the plate.

Easy to spray aqueous surfactant solution ensures contact with a large surface intensi¬fikatora milled material. This method is the use of surfactants is effective also because mi¬neraly cement clinkers are highly hydrophilic and strongly adsorbed water on the newly formed po¬verhnostyah. Even the water itself causes a significant decrease in strength adsorbtsion¬noe clinker minerals in their iz¬melchenii. The adsorbed surfactants creates a shell around the particles and thereby inhibit aggregation and sticking of cement.

Small amounts of moisture without surfactant intensify cement grinding. It was found that the moisture content of milled material has a significant impact on energy consumption for cement grinding in a ball mill. When moisture milled material is 1 ... 1.5% of electricity consumption was 34 ... 36 kWh / t, when ground is absolutely dry material consumption was 43 kWh / ton, humidity 2% - 43 kWh / t.

Introduction to the mill for grinding clinker water in the range of 1% to the weight of the feed to the mill charge umenshaet¬ or completely prevent adhesion and aggregation of small particles of cement. Water vapor reduces the electrical resistance of the medium in a mill and thus reduce the electrostatic forces of interaction of positively charged particles and negatively cement za¬ryazhennymi bronefuterovkoy and grinding bodies, thereby preventing them from sticking to the particles. Water vapor, washing cement particles form a temporary "mosti¬ki" is a kind of conductor, through which the neutralization of electrostatic charges. As a result, the phenomenon of adhesion and aggregation are reduced or even eliminated.