Physicochemical and physicomechanical studies of dolomite binder

To obtain dolomite cement, it is proposed to use screenings of dolomite stone and coal waste. Both physicochemical and physicomechanical studies of cement samples are carried out. It is shown that, in contrast to the existing dolomite binders, the proposed cement has the ability to harden when mixed with water.

IOP Publishing doi:10.1088/1757-899X/1164/1/012085 2 minerals, and the hydration processes proceed without dissolution thereof (topochemical mechanism) [5]. Michaelis did not deny the occurrence of crystalline hydrates formation in the process of cement hardening, but paid less attention thereto. In 1923 A.A. Baikov tried to combine these theories (unified colloid chemical theory) [6]. He identified three periods: dissolution (before the start of setting); colloidation (hydration) and crystallization with the formation of a crystalline aggregate. The main position of Baikov's theory, current theoretical concepts are adhered to, is the ability of a significant part of the binder to hydrate according to the topochemical scheme. P.A. Rehbinder divides the hardening process into three stages: dissolution of unstable clinker phases in water and separation of crystals; the formation of a coagulation structure; growth and aggregation of crystals [7].
The composition of the end products in magnesia cement is determined by ratio of initial components, since under insufficient solution content in conditions of high density of structure and under a significant change in MgCl2 concentration due to crystalline hydrates formation, the phase transitions of metastable compounds into stable ones may be stopped at one of the stages, and only 3MgO•MgCl 2 •11H 2 O or 5MgO•MgCl 2 •13H 2 O may be the end products, as well as mixtures thereof or mixtures of these oxychlorides with Mg(OH) 2 or MgCl 2 .
In the majority of scientific works devoted to magnesian binder based on dolomite and materials based on it, much attention is paid to the problems of improving the quality of binders and materials .
The use of saline solutions negates the advantages of dolomite binder production due to the high price of this salt compared to water.
Modern theoretical and practical data suggest research on obtaining a magnesia binder that can harden in water.

Research hypothesis
Under aqueous conditions, magnesium oxide hydrates so slowly that this method has not found wide application, but when it is mixed with saline solutions (most often with a solution of MgC12 6H2O), the process is accelerated. An increase in the solubility of the Mg (OH) 2 layer appearing on the MgO surface and the involvement of magnesium oxide in the hydration process can be achieved due to the formation of minerals in the binder during its heat treatment, obtained by joint firing of dolomites and coal waste. The minerals formed during firing play the role of a salt mixture, and the interaction reactions proceed through the formation of calcium and magnesium hydrosilicates, which take part in the hardening process.
Possible reactions in the mixture of dolomite and coal enrichment wastes are presented below:  Table 1 shows the results of the thermodynamic identification of the Gibbs energy of these reactions. The dependence of the Gibbs function change in the temperature range of (900 -1500) K is represented by a diagram in Figure 1. An analysis of the results obtained shows that there is a thermodynamic probability of the formation of compounds during firing, capable of solidifying when mixed in water.

Experimental results
The prepared composition of DSS to WAC being as 1:1; 1:2, 1:3 was isothermally fired in a muffle furnace at 1000°C for 1 hour. After being fired the resulting material was cooled down in the muffle to 150-180°С for 6-8 hours. Afterwards the material was grounded in a ball mill up to a specific surface area of 3200-3300 cm 2 /g. The X-ray diagram of the obtained dolomite binder is shown in Figure 4. The results of X-ray analysis show that the binder contains compounds capable of hydrating when tempered with water, and having hydraulic properties. Then the binder and sand mixture was mixed with water. The water-cement ratio was taken considering cone flow of 109-110 mm on the shaker table.
The strength properties were determined with 40x40x160 mm and 31.6x31.6x31.6 mm samples with a binder-to-sand composition as 1:3. The samples were heat-and moisture-treated at 95°C according to 2+8+2h steam curing regime, with the prior exposition thereof to air for 2-3 hours. The physical and mechanical properties of the binder are given in table 2.
Dolomite binder samples with DSS-WAC composition of 1:3 are seen from table 2 to have the best strength properties characterized by strength of 4.0MPa after 28 days of leaving in moist room, and that of 21.0MPa after heat-and moisture-treated. Therefore, the usage of DSS -WAC mixture under the heat treatment enabled to get a binder capable of hardening when mixed with water.

Results
 1. Principles of binding properties found in the dolomite binder when mixed with water are theoretically verified and experimentally confirmed.  2. The research in making a dolomite binder by mutual firing of dolomitic stone screenings and waste coal with further grinding thereof to a specific surface area of 3200-3300 cm 2 /g. The minerals capable of interacting with water and acting as a saline grout are found in the products of firing.  3. The dolomite binder was mixed with water at a water-cement ratio of 0.4-0.45.  4. Both physical and mechanical and physical and chemical studies have been carried out. The compressive strength of a dolomite binder in the cement to sand composition as 1:3 after being heat-and moisture-treated is 14-21 MPa.