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4RESEARCH INTO INFLUENCE OF ULTRA- AND NANODISPERSE SIZE ADDITIVES ON THE STRUCTURE AND PROPERTIES OF HEAT INSULATING AUTOCLAVED AERATED CONCRETE
Akbarova Gulnoza Muhammad qizi
Teacher of chemistry at school 55,
Nurabad district, Samarkand region.
https://doi.org/10.5281/zenodo.8269212
Qabul qilindi: 21.08.2023
Crossref DOI: 10.24412/cl-37059-2023-08-49-58
UDK: 666.972
Abstract: The paper presents the results of the research into the influence of various ultra- and nanodisperse size additives on the structure and mechanical-and-physical properties of heat insulating autoclaved aerated concrete (AAC). Research into interaction mechanisms of such additives as silica gel, high active metakaolin and multi-walled carbon nanotube dispersion (MWCNTs) with heat insulating AAC components consisted in the evaluation of the dynamics of the cellular concrete plastic strength increase of porous mixture, degree of cellular concrete blowup and comparing the strength and density of the check sample and the modified one. The studies found that the most effective method of obtaining the structural strength of AAC is by the use of the MWCNTs. They contribute to obtaining the optimum viscoplastic properties of bulk concrete and stabilizing of the pore formation process with simultaneous formation of the solid homogeneous hexagonal structure.
Keywords: Ultra-and nanodisperse size, ultrafine additives, construction polymers, ammophos, wollastonite, Reactivity, Hydration, Strength.
1. INTRODUCTION.
One of the most important directions in the modern building material science is development and introduction of new effective heat insulating materials, which is mainly due to the growth of electricity rate and cost of energy needed for heating of buildings. As for the energy efficiency, increasingly stringent requirements to the thermistance of enclosing structures and controlled environment improvement in buildings are worth mentioning. Autoclaved aerated concrete (AAC) with low density is an example of the material with efficient thermal characteristics. Insufficient structural composite strength at low values of its average density is a major obstacle to the wide use of this material as heat insulation [1,2,3]. A solution to the problem is the use of ultra- and nanodisperse size additives, which can influence the mineralization processes and increase the structural strength of AAC interporous walls [4]. The use of fine ground mineral additives, which act as crystallization centers of new growths in AAC, is the most common way of structuring aimed at binding the hydration hardening [5,6]. Ultrafine additives containing active micro-silica (silica gel) are of great current interest [7,8,9]. Another active mineral additive commonly used to increase the physical and mechanical characteristics of concretes is high active metakaolin [10,11]. Works by Russian and foreign researchers on the possibility of significant change in the strength characteristics of concrete with different density through the use of multi-walled carbon nanostructures in ultralow doses (0.002- 0.005% by the weight of the binder) are known [12,13,14]. However, the efficiency of the modifiers described above was � evaluated in relation to either heavy cement concrete or cellular concrete with average density of 500 kg/m3 . Thus, research into the influence of ultra- and nanodisperse size additives on the processes of structure formation and improvement in the physical and mechanical characteristics of low density AAC (250 kg/m3 or less) is relevant.
2. MATERIALS AND METHODS OF RESEARCH.
To obtain heat insulating AAC, cement CEM I of 32,5-42,5 class (GOST 31108) produced by OAO �Gornozavodskcement�; 2nd grade lump lime by OAO �PZSP�: (GOST 9179); quartz pit sand (Proletarskoe field, Perm) containing unbound SiO2 of no less than 85% (GOST 8736); water from the central water source (GOST 23732) were used. �Stapa Alupor N905� specialized blowing agent manufactured by �Eckhart� corporation (Germany) was chosen as a pore agent [2]. Superplasticizer based on �Melflux 5581F� polycarboxylate esters produced by �BASF Construction Polymers� corporation (Germany) was used to reduce the water content and stabilize viscoplastic properties of the aerated concrete mix. To improve the structural strength of the heat insulating AAC, the following ultra- and nanodisperse size additives have been used: - The silica gel (SiO2), produced by OAO �Ammophos� (Cherepovets, Vologda region). The silica gel is produced as a result of hydrofluosilicic acid reaction with aluminum hydroxide at the temperature of 90-95�C in the process of aluminum fluoride manufacture. High activity of silica gel is due to the content of active silica particles characterized by high specific surface (S = 15000cm2 /g) and high reactive capacity. Based on the literature data and the results of this additive testing on cementation systems [7,9,15], when selected, the working range of varying dosages of silica gel was taken from 4% to 10%. - The �HAM-40� high active metakaolin, manufactured by �Synergy� mining company, (Magnitogorsk, Chelyabinsk Region). It is a product with high pozzolanic activity and maximal (97-99%) amorphization aluminosilicate structure. As for the granulometric composition, the �HAM-40� is powder with the average particle diameter of less than 15 microns for 50% volume product weight. The mass fraction of particle size less than 2 microns is 20%. The metakaolin used has a high whiteness and a high specific surface (S = 13000 cm2 /g). Based on data in the literature [10,11] and the recommendations of the manufacturer, the dosage range is taken from 4% to 10%.
- The �FulVec� multi-walled carbon nanotube dispersion was produced by �Novy Dom� (Izhevsk, Republic of Udmurtia) and developed by the scientists of the Kalashnikov Izhevsk State Technical University. This dispersion contains 2% (equivalent mass) of carbon nanotubes. Multi-walled carbon nanotubes (MWCNTs) made by plasmaarc low-temperature pyrolysis and manufactured by �Arkema Co� French company (France) are used for making this dispersion. The initial components are premixes under the �Masterbatch CW2-45� trade name, treated with surfactants (surfactant with anticoagulation properties based on carboxymethylcellulose) in the form of granules. The quantity of nanotubes used ranged 0.001-0.005% by the weight of the binder in the study of the MWCNTs dispersion influence on the structure formation process and the final properties of concrete mix [16,17]. The quantity of the AAC main components used was selected based on the data obtained in the previous stages of research [1,2] taking into account the CR 277-80 requirements. The research into interaction mechanisms of the ultra- and nanodisperse size additives with the components of porous mix was based on the evaluation of the dynamics of the bulk plastic strength increase, the degree of the cellular concrete blowup, and on the comparison of strength, density and thermal conductivity in the check sample and the modified one [18,19]. The additive effect testing was conducted on the aerated concrete mixtures based on heat insulation chosen at the previous stages of the research. The sample formation was carried out in metal satellite molds, 400?400?400 mm of size, under production conditions at the OAO �PZSP� plant (Perm). Further on, to research into the structure and properties of the resulting bulk concrete, 100?100 mm cylinders were drilled. The degree of bulk concrete blowup was found with a rule. The plastic strength of the AAC was determined at the upper central portion of the article with a conical plastomer, 45� apex angle, 1kg of weight. The compressive strength, thermal conductivity, and the average density were determined according to GOST 10180 and GOST 12730.1 and GOST 7076, respectively. �FEI Quanta 200�, �XL 30 ESEM-FEG� scanning electron microscopes manufactured by PHILIPS company were used to analyze the micro- and macrostructure of the MWCNT modified concrete. The X-ray diffraction analysis was performed by �DRON-3� general-purpose diffractometer, while the �Netzsch STA Luxx 409 PC�, the instrument for simultaneous thermal analysis, was used to perform the differential thermal and thermal gravimetric analysis in the temperature range of 20 - 1500�C. The quantitative analysis of aerated concrete pore structure was carried out with the use of �PoreMaster-6B� porosimeter.
3. RESULT DISCUSSION.
3.1. Determining of the most effective modifier.
The analysis of the mix rheological properties showed that silica gel addition to aerated concrete had no positive influence on the process of pore formation and viscoplastic characteristics of aerated concrete. The mix modified with silica gel addition had the increased density, hereupon the process of pore formation and plastic strength increase became more difficult. The temperatures of aerated concrete pouring and heating were decreased, which suggests a weaker calcium hydroxide and aluminum interaction and insufficient hydrogen formation. This effect can be explained by high activity of micro silica which binds a large amount of calcium hydroxide at the initial stage of aerated concrete structure formation. But at the same time the residual amount of free calcium hydroxide is not enough for the interaction with aluminum particles which determines the amount of hydrogen generated and, as a result, the strength of bulk concrete blowup. Based on the deviation of the structure formation process from the optimal one for the heat-insulating aerated concrete described above, it was concluded that silica gel addition had no positive effect on physical and mechanical properties of the material. On the contrary, the density of the samples was significantly increased, which also led to strength increase
The analysis of the experiment results did not show the considerable improvement of AAC characteristics either. When increasing the dosage of HAK- 40 by the amount of Portland cement, it was observed that the samples increased their strength due to the compaction and porosity reduction, and, as a consequence, the material density and its thermal conductivity increased too. The greatest increase of the modified sample strength in comparison with the check sample was equal to 3.5, which was connected with the AAC increase in density by 1.75 times. With the increase in HAM-40 quantity, the bulk concrete blowup degree decreased, which led to inadmissibly high values of heat insulating aerated concrete density. In its turn, the degree of bulk concrete blowup directly depends on such rheological properties as viscosity and plastic strength of the mix (yield strength). The values of the mix rheological properties increase with the metakaolin addition to the mix in the quantity of 4 � 10% by the cement weight. Thus, the 4% HAM-40 addition causes a sharp rise in the viscosity and yield strength. Further on, these characteristics change in a different way, though, with a regular increase in plastic strength. Viscosity and plastic strength increase monotonically when changing the HAM quantity from 4 to 10%. It depends on the fact that metakaolin has a very high specific surface area and part of mixing water is used for its hydration. The best effect on physical, mechanical and rheological properties of cellular concrete and a concrete mix is produced when a modifying additive, MWCNT �Ful Vec�, is introduced into a concrete mix. The results of modified MWCNT sample tests are given in Table 3.
On the basis of the data obtained it can be concluded that the introduction of the MWCNT component into the aerated concrete mix made it possible to increase the compressive strength of the material throughout the range of additive variation. However, the best effect, for the average composite density reduction, was obtained in the MWCNT dosing range from 0.001 to 0.003% by the cement weight. A significant increase in concrete strength, while maintaining or even slightly reducing its average density became possible with 0.002% additive rate. A qualitative change of the rheological properties of the concrete mix should also be noted. The process of bulk concrete blowup proceeded intensively, with no visible ruptures and deformations. Retention of ultra light gas mass and the maintenance of optimal pore structure became possible owing to timely bulk concrete increase in necessary plastic and structural strength [20,21]. On the basis of the data obtained, it was concluded that the most effective way of strengthening the interporous wall of the solid phase heat insulating AAC was the introduction of modified multi-walled carbon nanotubes �Ful Vec� into the raw mix.
3.2. Research of the multi-walled carbon nanotube dispersion influence on the processes of heat insulating AAC structure formation
The micro structural analysis of pore sizes by mercury porosimetry showed that the addition of a specialized gasifier, superplasticizer �Melflux 5581F� and MWCNTs �Fulvec� dispersion to the mix improved the nature of gas silicate solid phase pore structure, i.e. the number of micropores was reduced, the content of capillaries increased, and the density of interporous aerated concrete walls became higher. At the same time, the use of modifiers made it possible to stabilize the aerated concrete macrostructure changing it from a cubic scattered cubic structure to a homorgenious hexagonal one (Figure 1), which, in turn, had a positive impact on physical, mechanical and heat insulating characteristics of aerated concrete [4].
Fig. 1. Pore size distribution in the overall structure of the modified and check AAC samples.
The comparative analysis of the data obtained shows that the optimization of pore formation process and the acceleration of plastic strength increase by AGC are connected with the hydration process intensification of the Portland cement minerals owning to their active interaction with MWCNTs, which provides the required strength of cellular concrete at the stage of bulk concrete blowup. At this stage, spatial framed cells in the structure of a modified cement matrix are formed. A large number of point contacts provides the formation of a cellular structure in which the group transition to adhesion in the near order of causes hardening of the modified matrix structure due to the formation of spatial packing [17]. As a result, the plastic viscosity of the mix increases and the value of limit shear stress decreases.
Earlier at the previous stage of the research [23] evaluation of the structural and phase changes in the AAC samples modified by MWCNTs was performed using the X-ray diffraction method (XRD). The comparison of X- ray diffraction patterns of AAC samples produced on the basis of aluminum powder (? = 250-300 kg/m3) and the samples obtained with the use of a specialized blowing agent and the MWCNTs dispersion (? = 150-200 kg/m3) determined that the free lime content increased with the decrease in density and increasing porosity of aerated concrete. This fact contributes to the intensification of calcium hydroxide and a silica component interaction. On this basis, the modified samples are distinguished by a large content of xonotlite and other calcium hydrosilicates with low basic capacity, the existence of which determines the structural strength of the AAC solid phase. The modified MWCNTs dispersion of the sample indicated the presence of a significant number of the diffraction reflections of wollastonite and hydrogarnets. The latter, having, as it is known, a cubic system are actively involved in the process of AAC structure formation.
On the basis of the X-ray diffraction analysis, it was found that the modified samples observed redistribution of xonotlite and tobermorite content in favor of the latter. The researchers [19,22] found that xonotlite resulting from the autoclaving process could be partially or completely transformed into tobermorite 11,3A at a temperature reduction, which increased the volume of the �binder-water� system. Such processes can improve the AAC performance property.
Thus, it can be assumed that the introduction of MWCNTs dispersion into the AAC mix promotes a qualitative structural change of AGC new growths, which provides reinforcement of interporous partitions and increases the strength of aerated.
The analysis of the TGA and DTA curves confirms the fact that the amount of water adsorbed by the material increases with the increase of its porosity. The temperature ranges in which the removal of a moisture absorbent (T= 90 � 100 �C) and physico-chemically and chemically bound water (T = 100 � 220 �C) are greater for the modified sample that those for the check sample.
The weight loss on the TGA curve in the temperature interval of 738-747 �C (dehydration of calcium hydrosilicates) in the check sample was 1.78 wt, % and 3.54 wt, %. In the nanomodofied sample, which confirmed a higher content of calcium hydrosilicates in the modified sample.
CONCLUSIONS.
The research into the influence of silica gel, high active metakaolin, and MWCNTs dispersion on the structural strength and density of heat insulating AAC has shown that the most effective way of strengthening interporous walls of this material is by introducing the �Ful Vec� multi-walled carbon nanotube dispersion into the raw material mixture. The additive content of 0.001-0.003% by weight of cement made it possible to significantly increase the AAC strength, while maintaining, and even slightly decreasing its average density.
The MWCNTs introduction resulted in qualitative change in the rheological properties of the aerated concrete mix. The process of bulk concrete blowup proceeded intensively, without visible ruptures and deformations. Due to the timely increase in the bulk concrete plastic and structural strength it became possible to maintain the ultra light bulk concrete and retain the optimum pore structure. It was found, that the MWCNTs introduction into the mix changed the nature of the solid phase AAC microporous structure: the number of micropores was reduced, the content of capillary density and the interpore partition density increased. Also, the use of modifiers made it possible to stabilize the concrete microstructure, thus improving the uniformity of pore shape and size, which positively affected the durability and heat insulating properties of AAC.
The introduction of �Ful Vec� MWCNTs dispersion into the concrete with the proportion of 0.001-0.003% by weight of cement resulted in the qualitative change in the morphology of new growths, having interlocking plate and needle-like crystals of low basic calcium hydrosilicates which form strong crystalline frame providing the AAC performance characteristics and enhancing the interpore partition density.
As a result of research, heat insulating concrete with the grade class of B0,5, average grade density D200 was obtained.
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