CRUSHING CHARACTER OF SAND-SODIUM-SILICATE MIXTURES STRUCTURED BY STEAM-MICROWAVE TREATMENT

Л.І. Солоненко, С.І. Реп ́ях, К.І. Узлов, Т.В. Кимстач. Характер руйнування піщано-рідкоскляних сумішей, що структуруванні паро-мікрохвильовою обробкою. На сьогоднішній день з числа відомих способів структурування піщанорідкоскляних сумішей найменш дослідженим, але найбільш перспективним, є спосіб паро-мікрохвильового затвердіння. Мета роботи – встановити характер руйнування піщанорідкоскляних сумішей, структурованих за способом паро-мікрохвильового затвердіння. У роботі використовували кварцовий пісок, плакований натрієвим рідким склом. Структурування суміші (плакованого піску) проводили мікрохвильовим випромінюванням при номінальній потужності магнетрона 700 Вт і частотою випромінювання 2,45 ГГц, використовуючи наважку води в суміші масою 1 г. Структури зразків суміші досліджували на растровому електронному та оптичному мікроскопі. Встановлено, що зі збільшенням масового вмісту рідкого скла для плакування кварцового піску від 0,5 до 6 % (за масою), а також зі збільшенням тривалості обробки суміші способом паро-мікрохвильового затвердіння межа міцності суміші при стисненні зростає по залежності близької до статечної. Характер руйнування структурованої суміші, при цьому не залежить від вмісту рідкого скла, який пішов на плакування кварцового піску, але залежить від тривалості паро-мікрохвильового затвердіння зі збільшенням якого переходить від адгезійного до змішаного або від адгезійного до змішаного і когезійного. Ключові слова: піщано-рідкоскляна суміш, рідке скло, паро-мікрохвильове затвердіння, адгезія, когезія, міцність


Introduction
Casting molds and cores (CMC) structuring by thermal drying is consequence of certain heat-and mass-transferring processes realization in them. This results the new structures appearance at the binder material contacts boundaries and mixture refractory filler grains, binder material cuffs between adjacent filler grains appearance, binder material compaction and solidification in cuffs, etc [1 -6].
Phases cyclic arrangement in structure of such mixtures causes the fact that, finally, their strength is determined by the value of both -binder adhesion forces to the surface of sand grains (σ ADH ) and cohesion value (σ COH ) -strength of both: CMC binder and refractory filler grains [7,8]. In the first approximation, depending on σ ADH /σ COH ratio value, the nature of this or that mixture destruction could be estimated. Moreover, for some types of mixtures, strictly specific destruction character is inherent. In particular, authors [9] CMC, according to destruction nature, distinguish sandy-clay mixtures as typical, in which destruction is exclusively adhesive in nature (σ ADH /σ COH > 1), sand-resin mixturesare of cohesive nature (σ ADH /σ COH < 1), sand-sodium-silicate mixtures -are of mixed (adhesivecohesive) character (σ ADH /σ COH ≅ 1).

Recent publications analysis
Many scientists and researchers have been involved in theoretical calculations of structured CMC ultimate strength. The best known are theoretical strength calculations results of Lyass A.M. [10], Zykova A.P. and Minaev G.I. [11] for mixed nature of CMC destruction, of Degtyarenko G.I. and Stolyar E.A. [2] for cohesive destruction nature.
These theoretical studies general result is the statement that with density of binder material dry residue increasing, tensile strength of CMC decreases, and with binder content and mixture specific density increasing, CMC tensile strength increases. At the same time, in these authors' calculations, there are parameters, which real values for structured CMC are not constant and varying in very wide range. For this reason, present studies have outstanding theoretical meaning.
Velikanov G.F. and Brechko A.A. in [12] presented simplified formula for CMC compression ultimate strength calculation: where σ ADH -adhesive strength; σ COH -cohesive strength; N S -number of contacts per area unit; S -contact area. Formula (1) takes into account contacts (cuffs) number and area, with increase in which strength increases, and indicates its dependence on sand fractional composition and binder amount used in mixture. But formula (1) does not take into account the effect of structuring method on adhesive and cohesive strength values, which has significant influence on CMC strength and follows from histogram in Fig. 1, based on [2] data.
Problem establishing. To date, among the known methods of sand-sodium-silicate mixtures (SSSM) structuring, the least studied, but the most promising is steam-microwave solidification (SMS) method. But at present there are no data on SSSM, structured by the SMS-process, destruction nature. Effect of sodium silicate solute (SSS) mass content in mixture and its structuring time have not been studied.
Purpose -to establish SSSM structured by SMS-process destruction character.
Investigation tasks -to investigate the fractography of SSSM structured by SMS method fracture surface, to determine the influence of microwave radiation treatment duration and sodium silicate solute, which has been cladded with quartz sand, mass content on the nature of such mixtures destruction.  Research object. Structure of sand-sodium-silicate mixtures, structured by the PMO method, fracture surfaces.

Materials and methodology
Quartz sand with clay content of less than 0.5 % (by weight) and with predominant grain size of 0.23 mm has been used in this study. For use, sand has been cladded with sodium silicate solute (GOST 13078-81) with silicate modulus of 2.8...3.0 and specific density of 1.43...1.46 g/cm 3 . Mixture structuring has been carried out by microwave radiation at nominal magnetron power of 700 W and radiation frequency of 2.45 GHz using water portion in mixture of 1 g. Ultimate strength in compression (σ COMP ), tensile (σ TEN ) and static bending (σ BEND ) of structured mixtures have been determined according to generally accepted methods on samples of structured mixture with diameter of ∅30×30 mm in compression tests, on samples of beam type with dimensions 15×15×160 mm under static bending and on specimenseights in tension tests according to GOST 23409.7-78. Samples have been tested on LRu-2e device. Gas permeability of structured mixtures has been determined on samples with diameter of ∅50×50 mm using bell-type device model 04315 according to GOST 23409.6-78.
Samples broken surfaces structural peculiarities have been investigated according to fractography analysis results of fractures with scanning electron microscope SEM-106 I at accelerating voltage from 20 to 30 kV and beam current from 4 µА to 30 µА in secondary electrons with probe spot diameter from 1000 to 200 nm and with MBS-10 optical microscope.
As standard samples for fracture sites images of structured mixtures identification within one grain (sand grain) obtained with scanning microscope at magnification ×100 have been used. Fractography of these microstructures is shown in Fig. 2.

Investigation results
Values of ultimate strength in compression (σ COMP ), tensile (σ TEN ) and bending (σ BEND ), as well as their ratios for SSSM, structured by the SMS-process, are given in Table 1. According to Table 1 data, average values of ratios are σ COMP / σ TEN = 5.55, σ COMP / σ BEND = 3.68, and values of relative deviations of σ TEN and σ BEND calculations, using these coefficients, respectively, does not exceed 7 and 5 %. This means that by determining, for example, SSSM structured according to the SMS process value of ultimate compressive strength, it is possible with a high degree of reliability for engineering calculations to calculate its tensile and static bending strength. Based on obtained ratios and relatively small values of deviations between studied mixtures strength indicators, to determine mixtures destruction characters, compressive strength values have been used. Such decision has been explained by the fact that among strength indicators presented in table, exactly results of multiply repeated experimental determination of compressive strength have been characterized by smallest obtained values "scattering". Dependences of SSSM structured by SMS-process ultimate strength in compression on duration of microwave treatment and on SSS mass content used for quartz sand cladding are shown in Fig. 3. Curves 4 and 5 (see Fig. 3) have been plotted according to theoretical calculations results presented [13] in non-relative coordinate system and are purely qualitative in mixture ultimate strength changing vs. binder content and mixtures destruction character.
From dependences in Figure 3 analysis, it follows that with SSS mass content from 0 to 6 % increasing, as well as microwave radiation duration increasing, mixture ultimate strength in compression increases according to dependence close to exponential law. At the same time, regardless of SSS content used for quartz sand cladding, mixture processing according to SMS-process for 2 minutes leads to such mixtures destruction per adhesive mechanism. At the same time, mixture processing according to SMS-process for 3 and 4 minutes with SSS content of up to 1.5 % leads to such mixtures destruction per cohesive mechanism. In all other cases there is mixed (adhesive-cohesive) structured mixtures destruction character. It should also be noted that cladded quartz sand structuring at least with SSS content of 0.5 to 2.5 % allows obtaining structured mixtures with ultimate strength in compression level sufficient for their using as CMC. Reason for mixtures structured by SMS method such high strength properties, comparing to other structuring methods, is, apparently, dense and more perfect structure of sodium silicate cuffs between mixture sand grains, which follows from images in Fig. 4 analysis. It should be noted that SSS solidification with water of 55...65 % (by weight) content by microwave radiation has been accompanied by its foaming and foam structures of dehydrated sodium silicate [14,15] forming. This, although it makes it possible to obtain CMC with high strength values, but requires manual or automated mixture compacting and is characterized by structured mixtures gas permeability low level. In case of SSSM structuring according to SMS-process, mixture compaction is carried out on vibrating table for 30...60 sec, while ensuring structured mixture gas permeability at level of 170...230 units and compressive strength of 1.0...1.5 MPa at SSS mass content in mixture of 0.5 % and 4.2...5.6 MPa (see Fig. 3) -at 5 % SSS.
Samples fracture surfaces appearances obtained using scanning electron and optical microscopes at ×20 magnifications are shown in Fig. 5 -9.  9. Samples fracture surfaces appearances (adhesive fracture character, see point K in Fig. 3) with SSS mass content of 5.0 % (by weight), structured for 2 min by SMS-process, on scanning electron (a) and optical (b) microscopes, ×20 From Fig. 5 -9 analysis it follows that data on samples destruction character, obtained from theoretical studies [13] results, coincide with visual assessments results, which confirms dependence of destruction character on SSS mass content and samples time processing with microwave radiation by SMS-process.

Conclusions
With SSS mass content from 0.5 to 6.0 % (by weight) for cladding quartz sand increasing, as well as with mixture by SMS-process treatment duration increasing, mixture ultimate strength in compression increases in close-to-exponential dependence. Structured mixture destruction character, in this case, does not depend on SSS content used for quartz sand cladding, but depends on SMS-process duration and with its time increasing changes from adhesive to mix or from adhesive to mixed and cohesive.