Comparison of the Properties of Calcium Silicates Derived from Different Raw Materials

. The limited deposits of natural wollastonite in our country and the labor intensity of its extraction and processing, make promising the development of methods of its synthesis based on various types of domestic plant and fossil raw materials, especially industrial waste. The use of rice husk ash and calcium oxide in the solid-phase synthesis of calcium silicates ensures the content of β -wollastonite in their composition at the level of naturally-occurring Miwoll10-97. The obtained results indicate greater efficiency of amorphous silicon dioxide for the synthesis in the solid phase of calcium silicates. Synthetic calcium silicates, like naturally occurring wollastonite, have a pronounced alkaline surface nature, with the acid-base characteristics not influenced significantly by the sintering temperature, as well as the type and ratio of initial components. Calcium silicate produced from zeolite-siliceous rock has the smallest particle size and the narrowest particle size distribution, i.e. the most homogeneous structure. All investigated calcium silicates increase the wear resistance of epoxy materials, and low-porous silica sand-based fillers are more effective, providing high resistance of filled epoxy materials to climatic factors as well.


Introduction
Calcium silicates are effective fillers in a number of building and polymer materials [1].Natural calcium silicate -wollastonite is widely used [2,3] in the production of ceramics, protective and decorative coatings, rubber, composite materials based on thermo-and thermosetting plastics, etc.
Wollastonite is also used for porcelain, fire-retardant and acid-resistant materials, asbestos-cement products (slate, etc.), in metallurgy, as a coating for welding electrodes, in production of special radio ceramics, insulators with low dielectric losses, and sanitary products [4,5].
At present the need of domestic industry in wollastonite is covered by imports.For the purpose of import substitution, the limited deposits of naturally-occurring wollastonite in our country and the labor intensity of its extraction and processing [6] necessitate the development of methods of its synthesis based on various types of domestic plant and fossil raw materials, including waste production [7].
As synthetic calcium silicates can essentially differ in structure and properties both from natural wollastonite and among themselves, it is practically important to make the comparative research of silicate fillers received from various raw components.

Materials and methods
In the present work, the following silicate fillers were studied: -natural long-needle wollastonite Miwoll 10-97 (Technical Specification 577-006-40705684-2003), produced by ZAO "Geocom", with a characteristic length-to-grain diameter ratio of 10:1; -calcium silicates (CS) synthesized by solid-phase method in a muffle chamber furnace [8] based on amorphous silicon dioxide from rice husk ash (RHA), burnt at 800°C and limestone (calcium carbonate) (CS1), burnt for 3 hours at 1100°C and ash and calcium oxide, obtained by thermal decomposition of limestone at 9000C for 3 hours, at 900°C (СS2), with a molar ratio of silicon and calcium-containing components 1: 1.2 [9]; -synthetic calcium silicate (CS3), obtained by fusion of quartz sand and blast furnace slag (BS) at a SiO2:BS ratio of 1.07:1 at 1300°C -1 hour; -synthetic calcium silicate (CS4), obtained by fusion at 1500°C of quartz sand and limestone at a ratio of components SiO2:CaCO3: 1.13:1 and isothermal ageing of these components at 1500°C-1 hour; -synthetic calcium silicate (CS5) obtained by solid-phase synthesis from zeolitesiliceous rock (ZSR) [10] and limestone at a ratio of these components: 1:0.56 at a temperature of 1175°C for 3 hours.
Samples of calcium silicates CS3 and CS4 were provided for testing by Dr. Povolotsky A.D. (South Ural State National Research University).
Synthetic calcium silicates were grinded in a porcelain mortar and passed through a 64 mcm sieve before the study.
X-ray quantitative phase analysis of calcium silicate samples was performed on a multifunction Rigaku SmartLab diffractometer at acquisition parameters: angular interval from 3° to 65° with a scanning step of 0.02.
Filler particle size was determined by laser diffraction in accordance with GOST R 8.777-2011 on a Horiba LA-960 laser particle analyzer.
Determination of porosity and specific surface area of silicates was carried out using low-temperature gas sorption of nitrogen by Brunauer, Emmet and Teller methods (BETmethod ISO 9277:2010) and gas absorption (ISO 15901-2:2006) on the device Quantachrome Nova 1200e.
Filler particle size was measured by laser diffraction in accordance with GOST R 8.777-2011.
pH of aqueous suspensions of fillers was determined by a SevenMulti pH-meter at 20°С.

FORM-2023
https://doi.org/10.1051/e3sconf/202341001001 E3S Web of Conferences 410, 01001 (2023) Wear resistance tests of epoxy materials were carried out on vertical optimeter IZV-1 under the following regime: specific pressure of the counter-body on the tested surface of the sample P = 1 MPa, sliding speed Vsl = 1 m/sec, without lubrication.
Weather resistance of filled epoxy materials was evaluated, according to ASTM G154, by testing in the climatic test chamber with UV irradiation QUV 80-spray for 96 hours at a temperature of 60°C and the radiation power of 1.38 Watt/m2 (double noon).

Results and discussion
The phase composition of natural Miwoll 10-97 wollastonite and samples of synthetic calcium silicates obtained on the basis of different raw materials, according to Quantitative Phase Analysis by X-Ray Diffraction data (Fig. 1-3), significantly differs (Table 1).
The phase composition of natural Miwoll 10-97 and CS2 synthesized on the basis of Q-PXRD and calcium oxide contains approximately the same (about 80 %) amount of needlelike β-wollastonite, which has a triclinic syngony.
Impurity in CS2 is larnite (island calcium silicate), and in natural wollastonite are crystalline silicon dioxides and clay minerals.
The target component in CS-composition is β-wollastonite -mineral with needle-like structure, which presence, according to the literature data [8,12], provides high efficiency of modifying action of filler.It is known [13] that the rate and intensity of the reaction in the solid phase depend on numerous factors, the main ones being the phase structure of the components of the initial mixture and their degree of dispersion, as well as the temperature and time regimes of sintering.
The use of calcium oxide (CS2) instead of its carbonate (CS1) in the synthesis of SC on the basis of RHA provides a much higher content of silicate filler β-wollastonite (Table 1   Obtaining calcium silicates on the basis of natural quartz sand requires a higher temperature of solid-phase synthesis (1300-1500°C), associated with its crystalline structure.
In addition, according to the data obtained, CS3 contains a large content of pseudo wollastonite, which is an undesirable component.Therefore, the use of this component is not rational, both in terms of energy saving and the cost of the final product, and its percentage.The synthesis of calcium silicate (CS4) at 1500°C from quartz sand and limestone is also irrational, due not only to increased fusion temperature of the components, energy consumption, and low wollastonite yield, but also to limited and high cost of equipment for the solid phase reaction at such high temperatures [13].
In addition, the phase composition of CS3 and CS4 wollastonite content is lower (table 1) than when used for the synthesis, as the initial components -RHA and calcium oxide (CS2) or CSR and limestone (CS5).
The above results of the analysis of synthetic calcium silicates and initial materials for their synthesis, indicate the greater efficiency of the use of raw materials containing amorphous silicon dioxide for the synthesis of CS [12].
Synthetic calcium silicates, as well as natural wollastonite, have a pronounced alkaline nature of the surface [14].Acid-base characteristics of synthetic CS surface do not depend significantly on the temperature of their production, as well as the type and ratio of the initial components (table 2).Natural wollastonite, CSR-based CS5, and synthetic silicates produced with silica sand have lower pH values.The particle size of synthetic calcium silicates based on RHA is significantly larger than that of natural Miwoll 10-97.The average pore diameter of these calcium silicates is also slightly higher (Table 3).Total pore volume of CS 3 and CS 4 produced at high temperatures (1300-1500°C) is by an order smaller than that of natural wollastonite and calcium silicates based on rice husk ash (Table 3).The lower porosity of these fillers is related, as we assume, to the high synthesis temperature as well as the nature of the initial components [15].
Blast-furnace slag contains diopside whose Ca 2+ and Mg 2+ cations can break the silicaoxygen chains in the melt, reducing its viscosity and accelerating the diffusion processes [16].Perhaps this is the reason that the temperature of obtaining CS 3 , when using blast furnace slag is lower than in the case of limestone.Natural wollastonite and synthetic calcium silicates derived from rice husk ash have bimodal particle size distribution, while CS based on zeolite-siliceous rock and quartz sand have unimodal distribution.
It is important to mention that the smallest particle size and the least scattered particle size distribution has CS 5 (table 3 and fig.4), obtained on the basis of CSR, that is, this filler has the most homogeneous structure.Besides, CS 5 and CS 2 have the highest wollastonite content of all investigated synthetic calcium silicates.It is at a level of natural Miwoll10-97.Thus, the use of zeolite-siliceous rock as a raw material makes it possible to obtain silicate filler of optimal phase and particle size composition.
However, in this case a higher temperature of solid-phase synthesis is required in comparison with the use of RHA as a source of silicon dioxide.At the same time, the reserves of zeolite-silica deposits in Russia far exceed the volumes of rice husk ash production, which strongly depend on the rice crop, i.e. on weather factors.
The modifying effect of natural wollastonite and synthetic calcium silicates was studied by us by the example of filling epoxy compositions with them.It is established (table 4) that all investigated fillers increase wear resistance of epoxy materials, and in the greater degree reduce wear of CS obtained with application of silica sand.
It can be connected with higher temperature of their synthesis, causing lower porosity, in comparison with other investigated samples of calcium silicates (table 3).It should be noted that the content of pseudo wollastonite having monoclinic syngony (table 1) in large quantities in the composition of CS 3 and CS 4 almost has no negative effect on the wear resistance of epoxy materials filled with it.Lower specific surface area of micropores and smaller total pore volume, as well as perhaps the morphology of the particles, make a greater contribution.
The wear resistance rate of calcium silicate-filled epoxy materials after keeping them in the climatic test chamber practically does not increase.This index insignificantly grows only for compositions containing CS synthesized with RHA (table 4).It can be connected with increased porosity of these silicate fillers.
The received results testify about high resistance of epoxy materials filled with CS to the influence of climatic factors and perspective of their exploitation in external conditions.

Conclusions
Reducing the energy expenditures and the cost of production of calcium silicates by solidphase synthesis becomes possible with the use of domestic local mineral raw materials and by-products of rice production.
The use of silicon dioxide of amorphous structure in the synthesis of calcium silicates leads to the intensification of solid-phase synthesis processes, reducing as a result the sintering temperature of the components and increasing the yield of needle structure wollastonite.
At the same time, the relatively small volume of rice husk production in our country limits the production capacity of synthetic calcium silicates and requires the search for alternative sources of amorphous silicon dioxide.
The use of zeolite-siliceous rock makes it possible to obtain silicate fillers of an optimum phase and grain-size composition, and the considerable quantity of this fossil raw material in Russia opens up the possibility of their multi-tonnage production.
By the example of the application of calcium silicates as fillers of epoxy materials, the prospects of their use for the creation of weather-resistant wear-resistant coatings are shown.

Table 1 .
The phase composition of natural wollastonite and synthetic calcium silicates.

Table 2 .
pH of calcium silicates surface.

Table 3 .
Porosity characteristics and average particle size of synthetic calcium silicates and natural wollastonite.

Table 4 .
Wear resistance of epoxy composition with different silicate filler additives