Plaster-based magnetite composite materials in construction

Calculation and experimental data demonstrate the possibility of using iron-ore concentrate of Lebedinsky Mining and Processing Plant (Lebedinsky GOK) in the production of plaster concrete. Their physical-mechanical, thermal and radiation protective properties were studied. Structurization mechanisms in plaster magnetite systems depending on the type of plaster binder, textures and the structure of plaster crystals providing for the design of composite materials with predetermined properties are suggested. Composite materials to ensure protection against X-ray radiation are obtained.


Introduction
At present, the search for new lost circulation materials and new binding agents for the production of composite materials that ensure protection against ionizing radiation remain critical. Materials on the basis of plaster binders (construction plaster and anhydrite binder) and iron oxides (hematite, goethite, maghemite, magnetite) present a certain interest in this respect.
Ferriferous minerals are widespread in nature, they are available, have low cost, developed specific surface and high sorption capacity, can be modified and regenerated, and are non toxic. The sorption capacity of magnetite is defined by structural oxygen atoms breaking the surface, which are partially hydrated in water and capable to absorb metal cations [1,2]. High thermal and electrical conductivity of ferriferous lost circulation materials allows eliminating the main disadvantage of standard concrete materials caused by their low thermal conductivity that makes the removal of heat, generated in structural protection, complicated.
Plaster binders have a number of advantages compared to other types of binders. First of all, the hydration products of plaster binders contain 19-20 wt.% of water, which leaves the material at a temperature ranging within 60-200 °C. Heating of plaster materials leads to transformation of crystallization water into steam with high energy absorption that improves considerably the fire resistance of materials. Besides, this type of binding agents allows producing environmenally safe, healthy, free-cutting materials and articles [3].
New types of highly effective composite materials for the protection against ionizing radiation causes the need to improve the theory and practice of their design. The materials applied in the protection against radiation shall bear high radiation resistance and rather high gamma radiation attenuation factor, and besides, they shall have decent cost.
The given work covers these problems [4,5] and is devoted to production of composite materials to ensure protection against X-ray radiation on the basis of plaster binders and iron-ore concentrate of Lebedinsky GOK, which is mainly composed of magnetite.

Materials and methods
The study was focused on the application of iron-ore concentrate (IOC) of Lebedinsky GOK as a raw material. IOC chemical composition, wt. %: G-4 cement plaster (СP) of LLC Unistrom-Trading with the following characteristics was used: R сomp . = 4-5 MPa, R bend = 2.0 -2.5 MPa, setting time was 8-13 min. рН of water suspensions of construction plaster (W/S=12.5) equaled 7.4. Thermal insoluble anhydrite (AnII) was obtained via compression of natural plaster at a temperature of 650 °C within 3 hours. рН of its water suspensions equaled 11.2. K 2 SO 4 and (NH 4 ) 2 SO 4 equal to 2 wt.% were used as the activators of anhydrite hardening. Raw materials were crushed in a vibration mill. An electrometric method was used to study the hydration kinetics. The studied suspensions with W/S = 12.5 were stored in sealed containers.
Strength characteristics of materials were determined using 2×2×2 cm samples in 2 and 7 days of hardening. Samples were dried within 2 hours at 60 °C before tests. Thermal conductivity of materials was measured via the continuous heat flux method according to GOST 7076-99 and radiation protective properties were measured on the basis of linear and mass attenuation factor of γ-radiation by means of the DKS-96 dosimeter-radiometer. The normal consistency was defined in accordance with GOST 23789-79.

Main part
One of the ways to increase strength of low-grade plaster binders is to modify them with various microfillers. IOC of Lebedinsky GOK was studied as such filler.
AnII suspension has an alkaline reaction of environment (рН = 11.2) due to water polarization with SO 4 2ions [7,8]: рН of CP lies within the interval from 5.5 to 7.5, which is equivalent to water polarization with Ca 2+ ions: IOC of Lebedinsky GOK polarizes water with OHion: When mixing suspensions of calcium sulfate and iron oxide, depending on acid condition of the environment, calcium ferrites, hydrated calcium ferrites and hydrated calcium sulfoferrites can be formed. Such reactions were studied on the basis of calculation of standard Gibbs free energy (∆G o 298 ) [9,10] . 1), the formation of hardly soluble hydrated calcium ferrites in alkaline environment is more preferable than the formation of ferrites (reactions 1-6). With the reduction of acidity hydrated calcium ferrites are destroyed alongside with the formation of Fe(OH) 3 , which can colmatage plaster stone pores thus improving its structure. Thus, it may be concluded that from the thermodynamic point, the reactions of formation of hydrated calcium ferrites and hydrated calcium sulfoferrites are possible. At the same time, these reactions also depend on kinetic factors. To check the above-mentioned assumptions, short-term (1 hour) and long-term (30 days) potentiometric study of processes within plaster-magnetite systems were performed.
It is found that IOC with a weakly alkaline reaction of the environment reduces the setting time of gypsum plaster: the initial setting decreases from 10 to 8.5 min. and the final setting -from 18 to 12.5 min. It also slightly reduces К red . of gypsum plaster-based materials. Thus, in case of plain samples, К red. = 0.408, while pilot samples -0.367. IOC additives increase рН of dehydrate and CP and decrease рН of AnII within initial hydration. In longer setting times, the рН of plaster-magnetite systems reduces, which serves as indirect confirmation of the fact that reactions 7-14 may take place (Tab. 1).
Long-term kinetic study showed that already in the first week, the IOC water suspensions are characterized by brown residue, which is caused by oxidation of Fe +2 into Fe +3 and interaction of the latter one with water molecules. Soon after, the same brown residue is formed in systems on the basis of (plaster + IOC) and (AnII + IOC). On the contrary, systems on the basis of CP and IOC are not The iron-ore concentrate of Lebedinsky GOK with alkaline reaction of the environment has considerable influence on hydroxylated solid plaster particles, thus displacing their acid-base balance, which will ultimately affect structurization of plaster stone. This is confirmed by potentiometric study.
At present, the most widely spread gypsum binder is gypsum plaster. That is why the study was initially focused on G-4 gypsum plaster. Tab. 2 shows its compositions. The amount of hardening water was separately calculated for CP and IOC. As shown in Tab. 2, with the increase in the amount of IOC additive in a mix, the W/S decreases from 0.55 to 0.36 for CP and from 0.40 to 0.28 -for An II.
According to obtained results (Fig. 2 a), 10-40 wt. % of IOC may be introduced into gypsum plaster. In this interval of IOC volume, the R сж of a binder is stabilized being at the level of 12-13 MPa. The strength of pure gypsum plaster under the same hardening conditions makes 17 MPa, and density amounts to 1,367 kg/m 3 . The density of filled IOC samples increases to 1,700 kg/m 3 . Small IOC addition slightly reduces CP strength (up to 5%).
Anhydrite cement was also studied as a binder alongside with gypsum plaster in order to obtain composite materials. Hardening of materials on the basis of anhydrite cement was ensured within the steam-air medium. Similar to the previous case, the IOC volume varied from 10 to 80 wt. %. The obtained data (Fig. 2 b) show that materials on the basis of anhydrite binder have higher strength properties than similar materials on the basis of gypsum plaster. a b Figure 2. Influence of IOC additives on strength: а) CP; b) An II The R comp dependence on IOC volume is close to linear. The R comp curve trend in gypsummagnetite compositions allows assuming that the material structure, i.e. type of base unit packing, size and shape of crystals, plays a leading role in strength stabilization. SEM imaging of the material microstructure was studied to confirm this assumption. The microstructure of SG-50 composition (Fig.  3 a) presented by thin, small, extended prismatic crystals differs from the microstructure of AM-50 composition (Fig. 3 b). CP-based compositions are exposed to fast hydration and hardening of a binder. At the same time, small extended prismatic crystals up to 8 µm long and 0.4-0.8 µm thick (Fig.  3 a) are formed, which, when overlapping, create the primary structure of a material due to crystallization. Larger magnetite particles are captured in voids between plaster crystals. Such structurization mechanism illustrates the curve trend with regard to material strength change in gypsum plaster (Fig. 2 a).
Up to 40 wt. % of IOC may be introduced into voids between CP crystals not damaging crystallization. A further increase in IOC volume will lead to damage of crystallization and a decrease in material strength. Plaster crystals cover magnetite particles. It is also confirmed by material color. Materials on the basis of G-4 are gray, while materials on the basis of anhydrite binder are intense black. a b Figure 3. Microphotographs of composite materials: a -SGM-50; b -AM-50