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20Magazine of Civil Engineering
journal homepage: http://engstroy.spbstu.ru/
ISSN 2071-0305
DOI: 10.18720/MCE.91.6
The fine high-calcium fly ash as the basis of composite cementing material
O.M. Sharonova*, V.V. Yumashev, L.A. Solovyov, A.G. Anshits
Federal Research Center «Krasnoyarsk Science Center of the Siberian Branch of the Russian Academy of Sciences», Krasnoyarsk, Russia * E-mail: sharon05@yandex.ru
Keywords: high-calcium fly ash, cementitious materials, superplasticizer, hydration, compressive strength, calcium compounds, X-ray diffraction
Abstract. The high-calcium fly ashes (HCFA) of Krasnoyarsk TPP-2, Russia were studied. The HCFA were selected from each of the 4 fields of the electrostatic precipitator. It was determined that the size distribution, chemical and quantitative phase composition vary significantly from 1st to 4th EF field. The fine high-calcium fly ash (dbo < 10 microns) selected from the fourth field of electrostatic precipitator was the source for high strength specimens. In the composition with a superplasticizer at a water:binder ratio of W/B = 0.25 the specimens were made and then cured from 1 to 120 days, with their compressive strength increasing from 17 to 72 MPa. The strength of these specimens is comparable to the strength of specimens based on Portland cement PC 42.5 N without superplasticizer. The methods of simultaneous thermal analysis (STA) and quantitative X-ray phase analysis (XRD) were used to study phase transformations of high-calcium fly ash in the process of hydration curing. The major newly formed phases are ettringite 3CaO^AbO3^3CaSO4^32H2O, as well as calcium carboaluminate hydrates Ca4Ah(OH)13(CO3)0.5^4H2O and Ca4Ah(OH)12CO3^5H2O with low crystallinity. The new phases can form a wide range of solid solutions by replacing Al 3+ with Fe 3+. The more the curing age was, the more transformations of calcium silicate amorphous substance contribute to form cryptocrystalline calcium hydrosilicates that increased the initial and long-term strength of the material. The phase transformations and strength indicators allow to use fine high-calcium fly ash of coal-fired power plants as an independent cementing material in modern technologies for producing building materials, in particular, in the technology of self-compacting composite concrete (SCC). The proposed alternative to cement contributes to the solution of a complex environmental problem: (1) in the heat power engineering the accumulation of fine ash particles can be lowered with consequent reduction of the pollution of water, soil and atmosphere with thin dust particles, and (2) in the construction materials industry a part of the cement can be replaced by the fine HCFA, that will save energy and natural resources.
Although the share of alternative energy sources is growing, the contribution of coal-fired power plants to electricity production remains quite high and is about 40 % in the world [1]. Coal fly ashes, a by-product of coal combustion, have a complex composition, and therefore differ in the properties and methods of utilization [2]. Recycling of coal fly ashes is constantly increasing, they are used in cement and concrete production, road base and pavement construction, soil amendment, zeolite synthesis, ceramic industry; as filler in polymer, but the current average utilization rate is don't exceed 25 % in the world [1-4]. High calcium fly ashes have a smaller contribution to ash wastes compared with aluminosilicate fly ashes, but they are more toxic when stored in ash dumps, since they form alkaline solutions with pH 11-13 [3].
1. Introduction
The use of coal ash in the construction industry as a substitute for cement and as an active mineral additive to concrete is the most promising trend of their large-scale use [1-7]. The HCFA are composed of crystalline phases with hydraulic activity (lime, aluminates, sulfates, sulfoaluminates, aluminoferrites of calcium), as well as active glass-crystal microspheres, due to the high content of Ca2+ cations in the glass [8-12]. For those reasons, in addition to pozzolanic activity, they have independent cementing properties. Usually 25-40 % of Portland cement (PC) is replaced with such ashes [13]. The use of high-calcium fly ash from the combustion of Kansk-Achinsk coal in the production of autoclaved and non-autoclaved aerated concrete, in addition to saving cement, leads to a decrease in shrinkage and an increase in the strength of products [12].The high-volume fly ash concretes where 50-70 % PC was substituted, are of great interest because they bring ecological, economical and technological benefits [2]. It was shown that it is possible to produce the concrete with the strength of 28-32 MPa by 28 days of hardening, with 100 % replacement of PC with HCFA, in composition with recycled glass [14].
The current trend in the energy industry is the modernization of thermal power plants, in order to increase the degree of exhaust gas purification to 99-99.7 %, including ash particles. This aim was achieved at a number of foreign and Russian thermal power plants by improving the operation of electrostatic precipitators or their combination with bag filters [15]. As a result, the capture of the smallest ash particles less than 10 microns in size and, especially, less than 2.5 microns was increased. Micro- and submicron ash particles can be effectively used in modern technologies to produce new materials, in particular, self-compacting concrete (SCC) with the compressive strength from 50 to 100 MPa [16]. The key element of the SCC technology is the use of polymer superplasticizer, which ensures the effective dispersion of fine cementing components in the liquid phase and the formation of a dense and durable microstructure due to close contact and interaction of the newly formed phases with each other and with unreacted components [16, 17].
High calcium fly ashes trapped in different fields of electrostatic precipitators vary dramatically in fractional composition and, to a considerable extent, in chemical and phase composition [9, 18-20]. It was found [9], that the dispersity increases significantly along the gas-dust flow, for example, 90 % of the fly ash particles from the 1st field are less than 40 microns in size; from the 2nd and 3rd fields are less than 12 microns; from the 4th field are less than 8 microns. It was noticed that the content of CaO, SiO2, and Fe2O3 phases decreases from the 1st to the 4th fields of the electrostatic precipitator, while the contents of CaSO4 and glass increase. Thus, HCFA obtained in different fields of the electrostatic precipitator are ash separation products significantly various in composition and, especially, particle size distribution, and, consequently, in their cementing properties. The main regularities to form the composition and properties of HCFA in the fields of electrostatic precipitators are described by the authors [19, 20]. Based on these data, it was proposed to use finely dispersed fly ashes of the last fields of electrostatic precipitators containing a smaller amount of free CaO, as a binder material.
The aim of this work is to study the dispersity and chemical and phase composition of high-calcium fly ashes, selected from each of the 4 fields of the electrostatic precipitator and to determine their cementing properties, which are promising for obtaining high-strength composite materials.
2. Materials and Methods
2.1. Materials
The objects of the study were high-calcium fly ashes (HCFA) from burning of pulverized low-ash (11 %) brown coal grade B2 of the Kansk-Achinsk basin, Russia. The coal was burned at the Krasnoyarsk TPP-2 (Krasnoyarsk, Russia) in boiler units of the BKZ-420 type with liquid slag removal at a temperature of 1400-1500 °C. High-calcium fly ashes (1-4) were selected from each of the 4 fields of electrostatic precipitator in a facility with an ash collection efficiency of >98 %. Portland cement PC 42.5N of the Krasnoyarsk cement plant was taken for comparison.
2.2. Methods
The macro component composition (components SiO2, AhO3, Fe2O3, CaO, MgO, SO3, Na2O, K2O, and TiO2) and the loss on ignition (LOI)) were determined by chemical analysis methods according to National standard GOST 5382-91. The particle size distribution was measured with a Fritsch Analysette 22 MicroTec laser particle analyzer (Germany) using a dry cell. The SEM images for fly ashes were taken with a Hitachi TM-1000 electron microscope (Japan).
X-ray powder diffraction quantitative phase analysis was carried out using the full-profile Rietveld method and the derivative difference minimization approach [21]. The diffractograms were recorded in the reflection geometry on a PANalytical X'Pert PRO diffractometer (Co Ka radiation, graphite monochromator, scan range 28 = 7-100°) equipped with a PIXcel detector. The weight fraction of crystalline and amorphous components was determined by the external standard method with corundum used as the standard. The overall absorption coefficients of the samples were calculated from the total elemental composition according
to the chemical analysis data. This method was successfully applied to determine the quantitative phase composition of high-calcium fly ash middlings [9] and narrow fractions of ferrospheres recovered from fly ashes [22].
The specimens shaped as 20x20x20 mm cubes were manufactured from 100 % of fly ash selected from each of the 3rd and 4th fields of electrostatic precipitator at a water/binder ratio W/B = 0.4. For comparison, the same specimens were made from 100 % Portland cement PC 42.5N taken from the Krasnoyarsk cement plant. For the finest fly ash of the 4th field of the electrostatic precipitator, the specimens were made at W/B = 0.25 with addition of the Melflux 5581F superplasticizer (0.12 wt %).
The concentration of superplasticizer was chosen according to the results of tests for fluidity. The fluidity was measured at 20 °C from the diameter of a spot of binder paste flowing from a tube with an internal diameter of 50 mm and a height of 51 mm according to JASS 15 M103 [23]. The spot diameter (F) was used to calculate the relative flow surface area (G) characterizing the deformability of a mixture by the equation G = F2/502 - 1. At W/B = 0.25, acceptable G = 8 was attained at a Melflux concentration of 0.12 %. The specimens were stored above the water layer in the desiccators for 1, 3, 7, 28, 60, 80, and 120 days. The strength tests of specimens were performed on an Instron 3360 tabletop dual-column testing machine (United States) at a return speed of 5 mm/min.
The simultaneous thermal analysis (STA) of hydrated specimens after drying for 2 h at 60 °C was performed on a Netzsch Jupiter STA 449C analyzer (Germany) with a Netzsch Aeolos QMS 403C mass spectrometer (Germany) in lidded Pt-Rh crucibles using a sample portion of 20.0 ± 0.1 mg. The measurement of the mass change (TG, DTG), the heat flux (DSC), and the composition of gaseous products (by Ar+, O2+ , CO2+ , CO+, H2O+, and SO2+ molecular ions) was performed in the mode of linear temperature increase at a speed of 10 °C / min within a temperature range of 40-1000 °C, supplying the 20 % O2 + 80 % Ar gas mixture (total flow rate, 50 cm3(NTP)/min).
3. Results and Discussions
3.1. Initial fly ashes
The studied high-calcium fly ashes of 1-4 fields of electrostatic precipitator significantly differ from each other by their fineness (Figure 1). About 90 % of particles (J90) of the fly ashes selected from the 1st and 2nd fields of the electrostatic precipitator are less than 40 microns, and 50 % of particles (dso) are less than 13 microns. The fly ashes of the 3rd and, especially, the 4th field of electrostatic precipitator have a higher dispersity, the value d9o of which is about 30 and 10 microns, and dso is about 9 and 4 microns, respectively. The particle size distribution for the PC 42.5 N (Figure 1) is somewhat shifted towards larger particles, the values of d9o and dso are 55 and 20 microns, respectively. At the same time, the content of particles less than 10 microns in cement is close to the fly ashes of the 1st and 2nd fields of the electrostatic precipitator.
Particle size, micron
Figure 1. Particle size distribution for HCFA selected from 1-4 fields of the electrostatic precipitator (1-4), and for Portland cement 42.5 N (PC).
The general view of the fly ashes taken from the 1st and 4th fields of the electrostatic precipitator is shown in Figure 2, from which it follows that the fly ashes consist of mainly microspheres with the different size and morphology. The studied high-calcium fly ashes were formed in a boiler with liquid slag removal at a temperature of 1400-1500 which led to the melting of the ash substance and the formation of melt drops in the gas-air flow, and during cooling, microspherical particles [11].
TM-1000_4368 xl.Ok 100 urn
Figure 2. SEM images for HCFA selected from 1st (top) and 4th (bottom) fields
of the electrostatic precipitator.
The chemical composition of fly ashes 1-4 (Table 1) is presented by predominant CaO (40-46 wt %), rather plentiful SiO2 (21-28 wt %), and a much lower content of other macro components (wt %): AhO3 (47); Fe2O3 (12-14); MgO (8-10) and SO3 (2-4). The composition of fly ashes 1-4 is close to PC [22, 23] by the content of SiO2 and AhO3 (Table 1), but differs from it by a lower content of CaO and a higher content of Fe2O3, MgO, and SO3.
Table 1. Chemical composition (wt. %) of the HCFA selected from 1-4 fields of the electrostatic precipitator and Portland cement (PC).
Components
HCFA . SO3 .
CaO S1O2 Al2O3 Fe2O3 MgO SO3 Na2O K2O T1O2 LOI
1 46.15 20.89 4.53 12.65 9.92 3.60 0.75 0.20 0.25 0.60
2 40.6 27.60 4.43 12.06 9.28 3.83 0.80 0.23 0.27 0.60
3 40.0 24.26 6.71 13.45 9.60 3.94 0.72 0.18 0.20 1.0
4 39.69 24.60 7.30 14.29 8.24 2.29 0.71 0.20 0.25 1.96
PC 42.5N 63.99 20.41 4.87 4.18 - 2.46 - - - -
PC [24] 66.4 21.9 5.7 3.2 1.2 0.4 0.7 0.3 -
PC [25] 67.17 22.14 3.12 2.51 - 2.13 - - 1.68
The major component in the phase composition of fly ashes 1-4 (Table 2) is a predominantly (Ca,Fe)-silicate amorphous phase (19-42 wt %). The clinker phases, such as tricalcium aluminate 3CaO^AbO3 (9.9-16 wt %) and calcium aluminoferrite Ca2FexAlyO5 (13.4-18.7 wt %) are in amounts comparable with the PC clinker. At the same time, the fly ashes do not contain the Ca3SiO5 and Ca2SiO4 calcium silicate phases, which attain 75-80 wt % in conventional PCs (Table 2) and cause their binding properties. Moreover, the fly ashes contain a considerable amount of free oxides CaO, MgO, SiO2 as well as the phases of CaCO3 and CaSO4. It should be noted that the content of amorphous phase grows from 19 to 42 wt %, while the content of free CaO decreases from 23.5 to 4.2 wt % in the series of fly ashes from 1 to 4 (Table 2). From a comparison of the chemical and phase composition of the HCFA, it follows that the predominant components of the amorphous phase are (in decreasing order) SiO2, CaO and Fe2O3. It should be noted also that the ratio of SiO2/CaO in its composition is significantly reduced (from 4.7 to 1.3) in a row of fly ash from 1 to 4. In the calculations of glass composition for the phase of calcium aluminoferrite Ca2Fex AlyO5, x = y = 0.5 was used.
Table 2. Phase composition (wt %) of HCFA selected from 1-4 fields of the electrostatic precipitator and Portland cement (PC).
Phases
HCFA O <D n
O CN < 3 <T <D F 2 O O 2 O OT 3 O3 C O OT 2 1 O O OT 3 O OT 2 s O
a a a g 1 a a ro a a <D LL
O O O 2 0 O 0 O 0 0
1 12.7 18.7 23.5 9.3 6.2 - 7.4 - - - 3.2
2 16.0 13.4 14.6 7.5 9.1 3.9 4.8 - - - 2.1
3 14.7 13.8 14.0 6.9 6.7 3.0 5.8 - - - 2.6
4 9.9 16.9 4.2 5.7 5.3 4.9 6.5 2.1 - - 2.4
PC 42.5N 5.8 13.2 - - 0.5 2.4 - - 64.5 9.2 -
PC [24] 7.9 10.1 0.6 0.2 - - - - 66.9 13.2 -
PC [25] 1.2 7.0 - - - 1.8 1.4 - 71.1 15.0 -
O
CM
H
LO
O
OT
a C
O
<N
H
CN
o w
a C
Cr O
m Ap
e s a
19.0 28.6 32.6
42.1
3.9 0.5
1.2
3.2. Compressive Strength of Cured Specimens
As follows from the data presented in Figure 3, the compressive strength (acomp) of specimens made from 100 % fly ash of the 3rd field at W/T = 0.4 (curve 1) is almost 2 times lower compared to specimens based on fly ash of the 4th field (curve 2). For curve 2, it can be seen that the acomp value increases from 11 to 22 MPa and then to 30 MPa with the curing age 3, 28 and 80 days, respectively. The fly ashes 3 and 4 (Table 1) have a very similar chemical composition, but significantly differ in particle size (Figure 1). It follows that the higher strength of the samples based on fly ash of the 4th field is presumably due to their higher dispersion (d9o < 10 microns), which contributes to an accelerated and more complete interaction of the components of microspheres (especially free CaO and amorphous phase) with the liquid phase to obtain strong structures. Strength tests of samples based on 100 % fly ashes of the 1st and 2nd fields seem to be irrational, since they are characterized by even lower dispersion, higher content of free CaO and, as expected, will have lower strength. It should be noted that the specimens based on fly ash 4 are significantly inferior to the comparison the specimens prepared from PC 42.5N at W/B = 0.4 (Figure 3, curve 3), for which the acomp value is 25, 48 and 61 MPa at a curing age of 3, 28 and 80 days, respectively.
The high tendency to agglomeration of finely dispersed HCFA does not allow to fully implementing the possibilities of their hydration interaction. The use of high-polymer polycarboxylate superplasticizers prevents
agglomeration, changes the surface properties and promotes the dispersion of micron and submicron particles, leading to the interaction of each individual particle with the liquid phase [16, 17]. The use of 0.12 wt % superplasticizer Melflux 5581F additive allowed reducing the value of W/B to 0.25, while maintaining the necessary fluidity. As a result, high-strength specimens were obtained based on 100 % fly ash of the 4th field (Figure 3, curve 4), the «omp value of which increases from 24 to 45 and 72 MPa on days 3, 28, and 120, respectively. Thus, the use of finest HCFA in a composition with 0.12 % polycarboxylate superplasticizer Melflux 5581F made it possible to produce specimens with the «comp value comparable to the strength of specimens based on PC 42.5 N without the addition of superplasticizer. High strength specimens based on HCFA of 4-field of the electrostatic precipitator with the addition of superplasticizer were studied by the CTA and XRD methods in detail. Some of the results of these studies were published in [26].
0 20 40 60 80 100 120
Curing age, days
Figure 3. Dependence of the compressive strength on a curing age for the specimens, made at W/B = 0.4 (1 and 2 - from HCFA of 3rd and 4th fields; 3 - from Portland cement 42.5N) and 4 - from HCFA of 4th fields with Melflux 5581F superplactisizer at W/B = 0.25.
3.3. STA analysis of Cured Specimens
From the data in Figure 4a for the specimens after three days of hardening it follows that intensive removal of bound water occurs at temperatures of 60-200 °C, the significantly lower water losses are observed at 200-300 °C. At temperatures of 410-520 °C, characteristic peak of portlandite decomposition by reaction (1) is observed.
Ca(OH)2 = CaO+H2O. (1)
Weight loss at the temperature 520-750 °C is caused by the decomposition of calcium carbonate by the reaction (2):
CaCO3 = CaO + CO2. (2)
With an increase in curing age up to 28 days (Figure 4b), water losses increase in all ranges, but to a greater extent at temperatures of 60-200 °C. The peak of water removal in the range of 320-410 °C also becomes more distinct.
Figure 4. STA data (TG, DTG, DSC and MS of H2O and CO2 ) for the specimens made from HCFA of 4th fields with Melflux 5581F superplactisizer at W/B = 0.25 at a curing age 3 days (a) and 28 days (b).
The data in Table 3 shows that the total loss of bound water ^(60-410 °C) increases from 5.4 to 12.7 wt % with an increase in the curing age from 1 to 120 days. The greatest contribution to water desorption is observed in the temperature range of 60-200 ^ (increasing from 3.6 to 8.1 wt %), and to less extent in the range of 200-320 ^ (increasing from 1.35 to 2.9 wt %). In the ranges 320-410 ^ and 410-520 the water losses increase during curing age 1-60 days, and decrease at 120 days. The losses of weight due to decarbonization of CaCO3 in the range of 520-750 °C increase for 28-days old specimens, and significantly decrease for the specimens of long-term curing (60 and 120 days).
Table 3. Weight loss (wt %) for the specimens produced from HCFA of 4th field of the electrostatic precipitator at different curing age.
Temperature range (°C)
Curing age, days 60-200 200-320 320-410 2(60-410) 410-520 520-750 2(60-750)
1 3.56 1.35 0.49 5.40 1.16 3.45 10.01
3 3.72 1.68 0.62 6.02 1.24 3.43 10.69
7 3.96 1.71 0.71 6.38 1.33 3.38 11.09
28 4.96 2.56 1.28 8.80 1.65 3.71 14.16
60 7.20 2.79 1.72 11.71 1.99 2.58 16.28
120 8.13 2.90 1.69 12.72 1.41 1.90 16.03
It is known for the systems of similar composition [24, 27], that endothermic peaks in the dehydration range below 200 ^ are correlated with the removal of the main part of water from ettringite, as well as from weakly crystallized and amorphous phases of calcium hydrosilicates. According to [24], the maximum intensity at 135-140 ^ corresponds to the dehydration of the ettringite, which is actively formed on the first day of Portland cement hydration, and the peak at 115-125 ^ corresponds to the dehydration of amorphous (cryptocrystalline) calcium hydrosilicates. Much lower water losses occur in the range of 200-320 ^ and correspond to the dehydration of AFm phases and more ordered structures of calcium hydrosilicates [27]. A weak endothermic peak with a water loss in the range of 320-410 ^ attributes to the dehydration of poorly crystallized boehmite AlO(OH) or other decomposition products of ettringite, as well as CaO • Al2O3 • 6H2O hydrates [28, 29]. An increase in the amount of Ca (OH)2, which is an indicator of pozzolanic reactions [30, 31], means that its formation during hydration of CaO exceeds its consumption in pozzolanic reactions during 1-60 days of curing, although this trend changes for 120 days of curing.
Thus, the STA study of specimens cured within 1-120 days made it possible to establish the forms of bound water and the tendencies of their change in curing age. It is known that the strength of cement stone grows along with increasing quantity of the hydrated substance [22]. As follows from Table 3, the amount of bound water, which is removed from crystalline hydrates and their thermal decomposition products in the range of 60-410 °C, is increasing from 5.4 to 12.7 wt. % for 1-120 days old specimens and correlates with the strength of the samples changing from 17 to 72 MPa.
3.4. Quantitative Phase Composition of Cured Specimens
The essence of hydraulic binders is to form strong structures of hydrate compounds as a result of their chemical and physicochemical transformations when they interact with water.
As follows from Table 4, only calcium aluminoferrite Ca2FexAlyO5 and calcium sulfate CaSO4 crystalline phases completely reacted on the 3rd day of hydration curing. The decrease in the content of CaO phase and the amorphous phase is observed. Ettringite (theoretical composition 3CaO • AhO3 • 3CaSO4 • 32H2O), calcium carbonate CaCO3 and portlandite Ca(OH)2 are actively formed.
On the 28th day of hardening, the main difference is a significant decrease in the amount of calcium carbonate and the formation of poorly crystallized hydrates of calcium carboaluminates Ca4Al2(OH)13 (CO3)0.5 • 4H2O and Ca4Ah(OH)12CO3 • 5H2O. In addition, the amount of portlandite significantly increases, but the content of the amorphous phase decreases. On the 120th day of curing, the content of portlandite decreases and the ratio for hydrates of calcium carboaluminates changes, the Ca4Ah(OH)12CO3 • 5H2O content increases with a simultaneous decrease in Ca4Ah(OH)13 (CO3)0.5 • 4H2O.
The hydration products of high-calcium fly ashes of Kansk-Achinsk coal [12, 32] contained tricalcium sulfoaluminate (ettringite 3CaO • Al2O3 • 3CaSO4 • 32H2O) and monocalcium sulfoaluminate 3CaO • Al2O3 • CaSO4 • 12H2O, as well as calcium hydrosilicate gel C-S-H and C-A-S-H. The formation of ettringite and monocalcium sulfoaluminate phases was accompanied by the disappearance of the anhydrite phase and a decrease of the line intensity for the phases CaO, aluminate and calcium aluminoferrite.
From the chemistry of hardening of Portland cements, it is known that ettringite is formed by the interaction of hydrated forms of calcium aluminate Ca3AhO6 and CaSO4 [24]. However, taking into account a small decrease in the Ca3AhO6 content for the 28-days and 120-days old specimens comparing to the initial one, the formation of ettringite during hydration of the HCFA occurs mainly due to the participation of calcium aluminoferrite Ca2FexAlyO5. It was also established [24, 30] that the hydration products of calcium aluminoferrite are in many respects similar to the hydration products of Ca3AhO6 aluminate, forming a wide range of solid solutions of hydrates of the type 4CaO (Al1-x, Fex) 2O3 • 19H2O or 3CaO (Ah-x,Fex)2O3 • 6H2O. Interacting with CaSO4, they can form almost continuous series of solid solutions of Al, Fe-ettringites.
Table 4. Phase composition (wt %) of the initial HCFA selected from of 4th field of the electrostatic precipitator and the specimens produced from this HCFA at curing age 3, 28 and 120 days.
Phase Initial HCFA 3 days 28 days [26] 120 days [26]
Crystalline phases
Ca2FexAlyÜ5 16.9 - - -
CaaAbOa 9.9 11.9 8.1 6.9
CaSO4 6.5 - - -
MgO 5.7 6.1 4.0 4.2
a-SiO2 (quartz) 5.3 5.6 5.7 6.3
CaCO3 4.9 12.6 2.1 3.0
CaO 4.2 3.3 2.3 1.4
Ferrospinel 2.4 2.9 2.7 2.9
Ca(OH)2 2.1 4.0 9.6 8.1
Ca6Al2(SO4)3(OH)i2-26H2O (ettringite) - 8.6 12.4 15.5
Ca2SiO4 (larnite) - 5.7 3.4 4.0
Z 57.9 60.7 50.3 52.3
Poorly crystallized and amorphous phase
Ca4Al2(OH)i3(COa)o.5^4H2O - 2.0 14.1 5.9
Ca4Al2(OH)i2COa^5H2O - - 5.3 12.7
Amorphous phase 42.1 37.3 30.3 29.1
Z 42.1 39.3 49.7 47.7
About 21 % and 27 % of the Ca2FexAlyO5 phase is consumed on the formation of ettringite according to the data for 28 and 120 days. Under the conditions of sulfate ion deficiency, the significant part of Ca2FexAlyO5 is actively involved in the formation of calcium carboaluminate hydrates, which belong to the so-called AFm phases. It is known [24] that hydrates of calcium aluminoferrites in the presence of Ca(OH)2 and CaSO4 can form AFm phases of the general composition [Ca2 (Al, Fe) (OH) 6] • X • XH2O, where X is one singly charged anion or 0.5 doubly charged anion. The anions X can be OH-, SO42- and CO32-. It should be noted that Al- and Fe-substituted AFm phases are isostructural analogues and, depending on temperature, they form solid solutions of various degrees of substitution. In the presence of Ca (OH)2 these phases are weakly crystallized and closely mixed with each other and with other phases, in particular, with the calcium silicate hydrates [24]. Thus, the phases of ettringite and calcium carboaluminate hydrates are solid solutions due to the isomorphic substitution of Al3+ for Fe 3+. It was not possible to assess the degree of substitution by XRD, mainly due to the low crystallinity of these phases.
The main source of silicates in the HCFA is the amorphous phase of microspheres, which reactivity, as known [8, 24], grows with increasing calcium content. In the initial period, the microspheres having the highest calcium content will participate in the dissolution. Later, as the pH of the solution and the reaction time will increase, the microspheres with lower calcium content and higher silicon content will interact. The result of long hydration time is an increasing amount of calcium hydrosilicates, which usually very weakly crystallize under ordinary conditions. It is also known [24] that during hardening of Portland cement, calcium silicate hydrates are cryptocrystalline substances that serve as a cementing material, linking unhydrated clinker particles and large crystals of Ca(OH)2, ettringite, etc. In the 28-days and 120-days old composite specimens, the amount of the amorphous phase decreased from 42 % to 30 % and 29 % compared to the initial fly ash and, apparently, a significant part is newly formed calcium silicate hydrates, the proportion of which increases with a curing time.
Thus, the high-strength specimens were produced from the finest high-calcium fly ash of the 4th field of the electrostatic precipitator in the composition with the Melflux 5581F superplasticizer at W/B = 0.25. Their compressive strength increased from 17 to 72 MPa as the curing age increased from 1 to 120 days. The strength of these composite specimens is comparable to the strength of specimens based on PC 42.5 N without the addition of superplasticizer. The close mixture of newly formed hydrated phases (ettringite, hydrates of calcium carboaluminates, portlandite and amorphous calcium hydrosilicates) ensures high strength of the material.
4. Conclusion
1. The high-calcium fly ashes (HCFA) selected from each of the 4 fields of the electrostatic precipitator of Krasnoyarsk TPP-2 were studied. It was established that the dispersity and composition of the HCFA vary considerably.
2. Compressive strength tests of the specimens produced from 100 % of the HCFA of the 3rd and the 4th fields at a water/binder ratio of W/B = 0.4 were performed. It was established that the strength of the 4th field specimens is 2 times higher compared to the 3rd field specimens and 2 times lower than the strength PC 42.5N specimens.
3. High-strength specimens were manufactured from the fine high-calcium fly ash of the 4th field (dgo < 10 microns) in composition with a polycarboxylate superplasticizer (Melflux 5581F) at a ratio W/B = 0.25. The compressive strength of the specimens increased from 17 to 72 MPa at a curing age from 1 to 120 days. The strength of these specimens is comparable to the strength of specimens based on PC 42.5 without superplasticizer.
4. Synchronous thermal analysis (CTA) and quantitative X-ray phase analysis (XRD) were used to study the phase transformations of the HCFA components in the process of hydration curing, and also to establish the main newly formed hydrate phases providing early and long-term strength. The results on the quantitative phase composition of high-strength specimens create additional opportunities to control the hardening processes of HCFA based binding materials.
5. Acknowledgments
The study was carried out due to the basic research project of the SB RAS V.45.3.3 at Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center «Krasnoyarsk Science Center of the Siberian Branch of the Russian Academy of Sciences».
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Contacts:
Olga Sharonova, +7(923)3697036; sharon05@yandex.ru Vladimir Yumashev, +7(962)0780323; yumashev_vlad@mail.ru Leonid Solovyov, +7(906)9732912; leosol@icct.ru Alexander Anshits, +7(950)4370735; anshits@icct.ru
© Sharonova, O.M., Yumashev, V.V., Solovyov, L.A., Anshits, A.G., 2019
Инженерно-строительный журнал
сайт журнала: http://engstroy.spbstu.ru/
ISSN
2071-0305
DOI: 10.18720/MCE.91.6
Тонкодисперсная высококальциевая летучая зола как основа композитного цементирующего материала
0.М. Шаронова*, В.В. Юмашев, Л.А. Соловьев, А.Г. Аншиц
Федеральное государственное бюджетное научное учреждение «Федеральный исследовательский центр «Красноярский научный центр Сибирского отделения Российской академии наук», г. Красноярск, Россия * E-mail: sharon05@yandex.ru
Ключевые слова: высококальциевая летучая зола, цементирующие материалы, суперпластификатор, гидратация, прочность на сжатие, соединения кальция, рентгенофазовый анализ
Аннотация. Исследованы высококальциевые летучие золы (ВКЛЗ), отобранные с каждого из 4-х полей электрофильтров (ЭФ) Красноярской ТЭЦ-2. Определено, что распределение по размерам, химический и количественный фазовый состав значительно изменяются от 1 к 4 полю ЭФ. Образцы высокой прочности были получены на основе тонкодисперсной ВКЛЗ с 4 поля ЭФ (dgo < 10 микрон) в композиции с суперпластификатором при соотношении вода:связующее В/С = 0.25, их прочность на сжатие возрастала от 17 до 72 МПа при отверждении от 1 до 120 суток. Прочность этих образцов сопоставима с прочностью образцов на основе портландцемента ПЦ 42.5Н без добавки суперпластификатора. Образцы высокой прочности исследованы методами синхронного термического анализа (СТА) и количественного рентгенофазового анализа (РФА). Установлено, что главными новообразованными фазами являются эттрингит 3CaO^AhO3^3CaSO4^32H2O, а также гидраты карбоалюминатов кальция Ca4Ah(OH)i3(CO3)0,5^4H2O и Ca4Ah(OH)i2CO3^5H2O с низкой окристаллизованностью. За счет замещения Al3+ на Fe3+ новые фазы могут образовывать широкие ряды твердых растворов. Из совокупности данных методов СТА и РФА можно утверждать, что при увеличении сроков твердения возрастает вклад превращений кальцийсиликатного аморфного вещества с образованием скрытокристаллических гидросиликатов кальция, повышающих начальную и долговременную прочность материала. Фазовые превращения и показатели прочности позволяют использовать тонкодисперсные высококальциевые летучие золы угольных ТЭЦ как самостоятельный цементирующий материал в современных технологиях получения строительных материалов, в частности, в технологии самоуплотняющихся композитных бетонов (СУБ). Предложенная альтернатива цементу способствует решению комплексной экологической проблемы: (1) в области теплоэнергетики уменьшается накопление тонкодисперсных частиц золы, а, следовательно, уменьшается загрязнение вод, почв и атмосферы тонкими пылевидными частицами и (2) часть цемента в промышленности строительных материалов замещается на ВКЛЗ, что позволит сэкономить энерго- и природные ресурсы.
Литература
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Контактные данные:
Ольга Михайловна Шаронова, +7(923)3697036; эл. почта: sharon05@yandex.ru Владимир Витальевич Юмашев, +7(962)0780323; эл. почта: yumashev_vlad@mail.ru Леонид Александрович Соловьев, +7(906)9732912; эл. почта: leosol@icct.ru Александр Георгиевич Аншиц, +7(950)4370735; эл. почта: anshits@icct.ru
© Шаронова ОМ, Юмашев В.В., Соловьев ЛА, Ah^ML( A.T., 2019
