CC BY
19Magazine of Civil Engineering. 2023. 124(8). Article No. 12405
Magazine of Civil Engineering issn
2712-8172
journal homepage: http://engstroy.spbstu.ru/
Research article UDC 691.3
DOI: 10.34910/MCE.124.5
Cementless binder based on high-calcium fly ash with calcium nitrate additive
K. Usanova1 B , Yu.G. Barabanshchikov1 , S. Dixit2'3-4
1 Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russian Federation
2 Division of Research, Uttaranchal University, Dehradun, India
3 Division of Research and Development, Lovely Professional University, Phagwara, Punjab, India
4 Khalifa University, Abu Dhabi, United Arab Emirates
M plml@mail.ru
Keywords: fly ash, microsilica, silica fume, early strength agent, calcium nitrate, X-ray diffraction analysis, strength, heat release, differential thermal analysis, heat of hydration, phase composition
Abstract. Fly ash from Berezovskaya Thermal Power Plant, containing a lot of CaO, in combination with silica fume does not expand and exhibits the properties of a binder. However, the strength of this binder is low. The addition of Ca(NO3)2 significantly increases the strength of the binder. The work aims to study the effect of Ca(NO3)2 additive on the strength, heat of hydration, and phase composition of hydration products of the binder based on high-calcium fly ash and silica fume. The results of X-ray diffraction and Differential Thermal Analysis show that the main phases formed during the hydration of binder with the calcium nitrate additive in various dosages are lime, calcium hydroxide, ettringite, CSH(II) type silicates and calcium aluminosilicates, corresponding to such minerals as gismondine, yugavaralite, goosecreekite. When hardening in water, the residual amount of lime after 7 days is sharply reduced. An increase in the dosage of Ca(NO3)2 from 1.5 to 11.8 % leads to a decrease in the CaO content by almost 2 times. With an increase in the dosage of calcium nitrate, the content of portlandite noticeably decreases, and a significant increase in the amount of calcium hydroaluminosilicates, especially the composition of CAS2H4, is observed. A study of the heat release process showed that calcium nitrate greatly accelerates the process in the first 60 minutes of hydration. However, then the composition with the addition of Ca(NO3)2 is inferior in the rate of heat release to binder without the additive. When testing a mortar with polyfractional sand, the addition of Ca(NO3)2 more than doubled the compressive strength of the mortar.
Funding: Russian Science Foundation No. 21-19-00324, https://rscf.ru/project/21-19-00324/
Citation: Usanova, K., Barabanshchikov, Yu.G., Dixit, S. Cementless binder based on high-calcium fly ash with calcium nitrate additive. Magazine of Civil Engineering. 2023. 124(8). Article no. 12405. DOI: 10.34910/MCE.124.5
1. Introduction
Numerous theoretical and practical studies, as well as world experience, show that industrial waste is a suitable raw material for replacing natural resources in the construction industry [1-7]. Such wastes include wastes from coal combustion at thermal power plants, blast-furnace slags and fly ash [8, 9]. A well-known solution to obtain cementless binders is also the use of fly ash [10-12].
Due to the limited supply of class "F" fly ash in the building materials market, there is a growing interest in high calcium fly ash [13, 14]. Recently, much research has been done on the use of high-calcium fly ash to produce cementless binders. Studies show that fly ash with a high content of calcium oxide has a low degree of polymerization of silicate anions and a high content of free lime, so it has a high hydration
© Usanova, K., Barabanshchikov, Yu.G., Dixit, S., 2023. Published by Peter the Great St. Petersburg Polytechnic University.
activity and a large volume expansion upon hydration. Because the free lime crystal size in the high calcium oxide fly ash is smaller and the lattice distortion is greater, the development of expansion during hydration of this fly ash is faster. It can be made into an effective additional binder with some appropriate technical measures [15].
It has been established that three types of fly ash from the Kansk-Achinsk fuel and energy complex provide the possibility of obtaining cementless compositions with a strength of up to 35-36 MPa. It has been shown that the additional introduction of silica fume and salt effluents into the ash stone modifies the phase composition of neoformations and the microstructure, transforming it into a denser one due to the formation of new compounds in the system - gyrolite and hydrochloraluminates [16]. The features of formation of the composition and hydraulic activity of high-calcium fly ash obtained by burning Kansk-Achinsk brown coals are studied. Fly ash, selected in a dry state, can be used as a kind of binding agents, provided that they are pre-treated, which reduces the uneven change in the volume of the fly ash paste during hydration [17].
Expansion studies of a mixture of cement and mineral admixtures show that "F" class fly ash increases the expansion of cement pastes, while "C" class fly ash and silica fume decrease expansion [18]. Studies of the fly ash hydration products from the circulating fluidized bed show that the expansion of the hardened paste occurs mainly due to the formation of dihydrate gypsum and ettringite. In addition, free lime indirectly causes expansion due to its effect on ettringite crystallization [19]. There are three hypotheses that explain the mechanism of action of fly ash to enhance the expansion of mixtures. The first hypothesis is that expanding mixtures containing fly ash produce more ettringite than mixtures without fly ash. The second hypothesis is that the lower stiffness of concrete containing fly ash results in higher expansion. The third hypothesis says that fly ash delays the hydration of expanding compounds in mixtures [20].
The reactions of hydration of expanding type K cement pastes and the effect of silica fume have been studied. In pastes with and without silica fume, expansion is associated with the formation of ettringite. The expansion continues until ettringite is formed. The rate and magnitude of expansion at an early age (the first two days of hydration) are higher when silica fume is added to the expanding cement paste. Solid phase analysis and rate curves of the pastes also show that the addition of silica fume accelerates the hydration of the paste [21]. Studies related to pore fluid analysis show that in expanding type K cement paste without silica fume, ettringite formation and subsequent expansion cease due to depletion of SO42-in the pore fluid after 11 days of hydration. In the presence of silica fume, the initial formation of ettringite is accelerated, which leads to an increase in the rate and magnitude of expansion [22]. However, as shown in [23], in the case of using high-calcium fly ash as an independent binder, silica fume is an agent that blocks the expansion of fly ash mixtures. The low hardening rate and low strength of the fly ash-silica binder suggests the study of additives that accelerate the hardening of cement compositions.
One of the accelerating agents is calcium nitrate, which effectively increases the setting and hardening rate of Portland cement. The mechanism of action of Ca(NO3)2 in the cement paste is to increase the concentration of Ca2+ ions, which contributes to the saturation of the solution and more rapid formation of Ca(OH)2 [24]. A comparison of three accelerating agents for pozzolanic Portland cement [25] shows that calcium nitrate is inferior in efficiency to calcium formate, which, in turn, is inferior to triethanolamine in the compressive strength of the cement mortar. It should be borne in mind that the effectiveness of calcium nitrate depends on the type of cement used. The results [26] show that calcium nitrate itself acts as a set accelerator, but it has a relatively small positive effect on the long-term development of mechanical strength. The combined addition of calcium nitrate and triisopropanolamine has shown significant and promising results at a very early age in terms of both setting acceleration and hardening acceleration.
The effect of calcium nitrate on white Portland cements is presented in [27] in various compositions of fresh cement paste after 7 and 28 days of hardening. The addition of calcium nitrate reduced the settling time and setting time of the specimens. In addition, the smaller size of the ettringite needles was established, and the plasticizing effect of calcium nitrate observed at an early stage of hydration. Flexural and compressive tests on cement specimens without the addition of sand or aggregates were carried out after curing at 7 and 28 days. The results showed a non-monotonic evolution of mechanical strength with calcium nitrate content. Diffractometry, SEM, and NMR relaxometry performed on specimens after final bending tests revealed changes in C-S-H gel morphology, an increase in the proportion of capillary pores, and an increase in the degree of carbonation with calcium nitrate content for older specimens.
Tests of commercial CEM I 52.5 R show that Ca(NO3)2 accelerates the silicate reaction and significantly influences the initial formation of the AFm and ettringite phases. In the presence of Ca(NO3)2, the dissolution rates of gypsum and anhydrite increase, and the formation of ettringite increases, especially at the highest analyzed dosage of 5 % Ca(NO3)2 by weight. In addition, significant amounts of NO3-AFm are formed in the presence of Ca(NO3)2. Calcium nitrate shortens the time it takes for alite to rapidly hydrate. In addition to a significant increase in Ca2+ ions, there is a decrease in the concentration of Al3+ in the pore solution, which can also accelerate the hydration of alite in the presence of Ca(NO3)2 [28]. An increase in
the amount of ettringite present in the cement structure in the form of non-oriented needles is evidenced by SEM micrographs taken on the 28th day of cement slurry curing using NC [29].
A study [30] considers class C fly ash in the presence of various concentrations of calcium nitrate. Tests have shown that calcium nitrate accelerates the hydration of the calcium aluminate phases with respect to dosage. The new products consist of calcium aluminate hydrate and nitrate. Evidence suggests that modified nitrate hydrates compete with or inhibit calcium aluminate sulfate hydrates.
The work aims to study the effect of Ca(NO3)2 additive on the strength, heat of hydration, and phase composition of hydration products of the binder based on high-calcium fly ash and silica fume.
Tasks of the research:
1. Determination based on the results of X-ray Diffraction Analysis (XRD) what interactions occur in the "fly ash - silica fume - Ca(NO3)2" system and how these interactions affect the hydration.
2. Differential Thermal Analysis (DTA) of cementless binder specimens with different content of Ca(NO3)2 additive.
3. Experimental study of the Ca(NO3)2 additive on the heat release of cementless binder.
4. Experimental study of the effect of the Ca(NO3)2 additive on the compressive strength of specimens from a mortar with polyfractional standard sand.
2. Materials and Methods
2.1. Characteristics of Fly Ash Collected from Electrostatic Precipitators
The work uses high-calcium fly ash from Berezovskaya Thermal Power Plant. The bulk density is 1.09 g/cm3, the true density is 3.09 g/cm3, the median particle size is 7.7 ^m, and the Blaine specific surface area is 2815 cm2/g. The chemical composition of coal fly ash from the Berezovsky deposit is given in Table 1.
Table 1. Chemical composition of the fly ash specimen.
CaO SiO2 AI2O3 Fe2O3 MgO P2O5 SO3 K2O Na2O Chlorine ion C Loss on ignition
42.2 26.8 6.49 6.09 7.05 <0.1 2.86 0.23 0.43 <0.05 2.33 3.44
The results of X-ray diffraction analysis (XRD) obtained earlier in [23] showed that about 15 % of free calcium oxide is contained in fly ash. In addition, the main minerals of this fly ash include aluminates, silicates, aluminosilicates, calcium ferrites, and aluminum and magnesium oxides (Table 2). TiO2, MnO2, P2O5 are present in small amounts. Minerals such as brownmillerite, andradite, merwinite, grossular, and quartz have been identified [23].
High-calcium fly ash, when mixed with water, seizes and hardens. However, due to the presence of a large amount of free lime, its hardening is accompanied by strong expansion and cracking.
Table 2. Composition of fly ash according to X-ray phase analysis.
Crystal phases Chemical composition of phases Conditional content [%]
Lime Ca O 62.39
Graphite C 12.81
Periclase Mg O 7.3
Brownmillerite 4CaOAhO3- Fe2O3 3.71
Andradite 3CaOFe2O3-3SiO2 3.62
Merwinite 3CaOMgO2SiO2 3.39
Millosevichite Al2(SO4)3 1.97
Calcium Aluminum Oxide 3CaOAl2O3 1.92
Grossular 3CaOAl2O3-3SiO2 1.06
Aluminum Oxide Al2O3 0.54
Yeelimite 3CaOAl2O3CaSO4 0.51
Kilchoanite 3CaO2SiO2 0.41
Quartz SiO2 0.38
The X-ray diffraction pattern corresponding to Table 2 is shown in Fig. 1.
Figure 1. X-ray diffraction pattern of fly ash.
2.2. Determination of Compressive Strength
The compressive strength of the specimens consisting of fly ash-silica fume binder with Ca(NO3)2 additive was determined on test cube with dimensions of 32*32*32 mm. 4 mixtures were tested, differing in the dosage of Ca(NO3)2 additive (from 1.5 to 11.8 % of the fly ash mass). The content of silica fume in the mixture was 30.4 % of the fly ash mass. The additive was introduced into the mixture in the form of an aqueous solution. The water content in the solution was considered in the calculation of the water/binder ratio (W/B). The water content in the finished mixture was selected so as to obtain mixtures with the same consistency. At the same time, W/B increases with increasing dosage of Ca(NO3)2 from 0.41 to 0.43.
6 specimens of each mixture were prepared. Demolding of the specimens was carried out after 2 days. Immediately after demolding, 3 out of 6 specimens were placed in water, and the other 3 were left to harden in sealed containers. This prevented the specimens from drying out and excluded their contact with the outside air in the laboratory, the temperature of which was regulated and amounted to (20±2) C. The specimens of both dry and wet hardening were tested for compressive strength after 7 days from the date of manufacture. Before testing, the state of the specimens was visually assessed, the presence of expansion and cracks.
The compressive strength of the specimens of fly ash-sand mortar (Table 3) was determined on test cube with dimensions of 7*7*7 cm. Specimens were tested at the age of 28 days, in the amount of 3 pieces per test. The specimens were stored in a laboratory room with a temperature of (20 ± 2) °C, for the first
7 days in covered forms, and the rest of the time in a desiccator above water.
Table 3. Compositions and designation of specimens for determining strength and heat release.
Material
Consumption of materials [kg/m3]
Q1
Q4
Q6
Fly ash Silica Fume MKU-85
Ca(NO3)2 (dry) Polyfractional sand Water Total
210
1645 318 2173
210 90 (42.9 %)
1476 340 2116
210 90 (42.9 %) 24.8 (11.8 %) 1451 330 2106
The compressive strength of all specimens was determined using a PGM-50MG4 hydraulic test press with a maximum force of 50 kN. During the test, the loading rate was maintained (50±10) kPa/s.
2.3. Influence of Calcium Nitrate on Phase Composition of Hydration Products of Fly Ash-Silica Fume Binder
The use of X-ray diffraction analysis and differential thermal analysis was aimed at establishing what interactions occur in the fly ash-silica fume- Ca(NO3)2 system and how they affect the hydration and strength of the fly ash stone. Both analyzes were performed as described in [23].
The analyses were performed on specimens of the fly ash-silica fume binder compositions listed in Table 4.
Table 4. Component content of binder.
Component content in composition of binder [%] Component type -
Mix 1 Mix 2 Mix 3
Fly ash 100 70 64.7
Silica Fume MKU-85 - 30 27.7
Ca(NO3)2 - - 7.6
Water-solid ratio 1.2 0.42 0.50
Semiquantitative analysis of crystalline phases in the specimens was carried out on a Dron 7 X-ray diffractometer produced by JSCE "Burevestnik"(Russia) with the following parameters: CuKa radiation, X = 0.15406 A, 20 shooting range from 8° to 94° with a step of 0.02°, and an exposure of 3 and 5 s.
Differential thermal analysis (DTA) was performed on the device "Termoscan" produced by LLC "Analitpribo" (Russia). The specimens for DTA had a mass of about 0.7-0.8 g. The specimens were heated to a temperature of 950-1000 °C.
2.4. Influence of Calcium Nitrate on Heat release of Fly Ash-Silica Fume Binder
The heat release of the specimens consisting of fly ash-silica fume binder and sand was determined experimentally by the semi-adiabatic (thermos) method according to EN 196-9:2010. The test procedure is described in detail [23]. To obtain comparable results, the heat release of concrete obtained by the semi-adiabatic method at an initial concrete temperature of 20 C was calculated to an isothermal hardening regime at a temperature of 20 C using the reduced time hypothesis [31].
The heat release of the binder was determined in the composition of a mortar with standard polyfractional sand on cylindrical specimens with a diameter of 62 mm and a height of 160 mm. Three compositions of the mortar were tested (designation Q1, Q4, Q6) with the same amount of fly ash, but with a different combination of additives (Table 3).
3. Results and Discussion
3.1. Compressive strength test results
The test results are shown in Table 5. For comparison Table 5 also shows the strength of the same fly ash-silica binder with the addition of MgCl2 according to [23].
Table 5. Compressive strength of specimens with different accelerating agents.
Ca(NO3)2 content [% by weight of fly ash]
Compressive strength [MPa]
Ca(NO3)2
MgCl2
dry
in water
dry
in water
I.5 4.7 8.2
II.8
3.5
2.7
8.8 10.4
0 0 0.38 1.15
2.9 8.3 9.8 15.2
0 0 1.92 2.3
The addition of Ca(NO3)2 showed a relatively low result compared to the addition of MgCh. With the same additive content of 11.8 %, the compressive strength of the specimens was 10.4 and 15.2 MPa, respectively.
The appearance of specimens with Ca(NO3)2 is shown in Fig. 2. It can be seen that the specimens of dry hardening with a low content of Ca(NO3)2 (1.5 % and 4.7 %) underwent cracking. In water, specimens of these compounds swelled and cracked even more. Specimens with a high content of Ca(NO3)2 (8.2 %-11.8 %) passed the test without cracking during dry hardening. In water, however, there was a slight expansion of these specimens and slight cracking.
The addition of Ca(NO3)2 is inferior in strength to magnesium chloride, but unlike chloride salts, it does not cause corrosion of steel reinforcement and therefore can be considered as an accelerator for reinforced concrete. Calcium nitrate does not cause cracking and expansion at high dosages (8.2%-11.8 %) and dry curing conditions, however, does not provide water resistance (Fig. 2). The softening coefficient is 0.04-0.11.
Figure 2. Specimens with Ca(NO3)2 additive. The top row is specimens hardening in dry conditions; bottom row is specimens hardening in water.
3.2. Results of X-ray diffraction analysis
X-ray patterns of the specimens of mixes presented in Table 4 after curing are shown in Fig. 3.
Figure 3. X-ray diffraction pattern of mixes after curing (Table 4).
A semi-quantitative analysis was carried out using the integral values of the intensity of the X-ray peaks. The obtained conditional values of the percentage of identified hydrated phases are given in Table 6.
Table 6. Conventional values of percentage of identified phases.
Phase Chemical Mix 1 Mix 2 Mix 3
composition of phase (28 days) (28 days) 1.5 h 2 days
Lime CaO - 12.8 43.0 30.2
Portlandite Ca(OH)2 85.4 41.8 37.2 35.0
Katoite C3AH6 11.1 - - -
Gismondine CAS2H4 - 27.5 11.2 18.9
Calcium Aluminum Silicate Hydrates CAS6H4 - - - 7.1
Katoite silication C3ASH4 - 2.7 - -
Calcium Silicate Hydrates CSH(II) - 15.2 - 8.7
Calcium Aluminum Oxide Sulfate Hydrate C3ACaSO4Hi3 3.5 - - -
Ettringite C3A-3CaSO4-H32 - - 8.6 -
Consideration of the data in Table 6 allows us to draw the following conclusions.
1. In the absence of silica fume and Ca(NO3)2 (mix 1), the interaction of fly ash with water for 28 days led to the complete hydration of CaOfree. At the same time, calcium hydroxide was formed in the overwhelming amount from the products of hydration. Its amount was 85.4 % of the mass of the identified phases. Hydrosilicates in crystalline form were not found. Of the calcium hydroaluminates, C3AH6 was present (11.1 %). A small proportion (3.5 %) was the low sulfate form of calcium hydrosulfoaluminate.
2. The addition of silica fume to the fly ash (mix 2) slowed down the hydration of lime. This corresponded to the effect of silica impurities on the rate of quenching of air lime. After 28 days of hydration, the fly ash still contained a significant amount of unreacted CaOfree (12.8 %) and the Ca(OH)2 content almost halved compared to mix 1 as a result of the action of silica fume. This was indicated by the significant content of calcium silicates and aluminosilicates formed. The effect of silica fume on the hydration of high-calcium fly ash was considered by us in more detail in [23].
3. The effect of Ca(NO3)2 (mix 3) was investigated for two hydration periods. After 1.5 hours, a significant amount of unreacted COfree (43 %) remained in the binder, and 37.2 % Ca(OH)2 was formed. Calcium silicate hydrates were absent, which was explained by insufficient time for reactions to occur and crystallization of hydration products. An analysis of the literature shows that, the interaction between Ca(OH)2 and SiO2 was noticeable approximately 3 days after the onset of hydration. Aluminosilicate CAS2H4, corresponding in composition to the natural mineral Gismondine, and calcium hydrosulfoaluminate of the trisulfate form (ettringite) were quite clearly identified. By the age of 2 days, calcium silicate hydrates of the tobermorite group CSH(II) according to Taylor were formed, the content of gismondine increased, and calcium aluminosilicate of composition CAS6H4 appeared.
A semi-quantitative X-ray phase analysis was also carried out for specimens of fly ash-silica fume binder with the calcium nitrate additive, which were previously tested for strength (Table 5). The results of X-ray phase analysis are given in Table 7, and the X-ray diffraction pattern is given in Fig. 4.
Table 7. Conventional values of percentage of identified phases resulting from hydration of binder consisting of fly ash and silica fume.
Phase Chemical composition of phase Dry curing Water curing
Ca(NO3)2 content [% by weight of fly ash]
1.5 4.7 8.2 11.8 1.5 11.8
Portlandite Ca(OH)2 47.4 34.0 24.4 20.8 33.6 32.1
Lime CaO 21.8 21.1 25.6 17.4 6.8 3.5
Calcite CaCO3 0.8 0.7 9.7 2.0 13.2 0.8
Ettringite C3A-3CaSO4-H32 8.4 9.4 14.2 9.3 10.8 10.8
Gismondine CAS2H4 - 8.7 23.9 37.2 8.2 21.5
Yugawaralite CAS6H4 7.9 - 2.2 1.0 2.9 3.9
Goosecreekite CAS6H5 - 16.6 - 12.4 9.5 7.7
Calcium Silicate Hydrate CSH(II) 13.8 9.6 - - 15.0 19.7
Table 7 shows only those phases that are formed in the binder as a result of hydration and accompanying reactions and which are most often repeated in the studied compositions. The total content of these phases is taken as 100 %.
The main identified phases that satisfy this condition are lime, calcium hydroxide, ettringite, CSH(II) type silicates, and calcium aluminosilicates corresponding to such minerals as gismondine, yugavaralite, and goosecreekite. The influence of Ca(NO3)2 on CaO hydration during dry hardening is manifested to a small extent. The residual content of free lime slightly fluctuates relative to the average value of 21.4 % with a tendency to decrease with an increase in the Ca(NO3)2 amount. With water curing, the amount of lime is sharply reduced to an average of 5.2 %. In this case, an increase in the dosage of Ca(NO3)2 from 1.5 to 11.8 % leads to a decrease in the CaO content by almost 2 times. With an increase in the dosage of calcium nitrate, the content of portlandite noticeably decreases, obviously due to the reaction with silica, and the amount of calcium hydroaluminosilicates increases significantly, the total content of which increased by 6.4 times during dry curing, and 1.6 times during water curing. However, calcium hydrosilicates during dry storage are observed only in the case of a low dosage of Ca(NO3)2 additive (1.5 % and 4.7 %). At higher dosages they are not identified. Specimens hardened in water show the presence of hydrosilicates in a significant amount and, the greater, the higher of the additive content. Ettringite does not show a pronounced dependence on the additive dosage and hardening conditions; its content in all compositions remained approximately the same. Thus, the addition of calcium nitrate mainly intensifies the formation of calcium aluminosilicate hydrates, especially of the composition CAS2H4. Reaction products of Ca(NO3)2 with calcium silicates and aluminates, such as calcium hydroxynitrates and hydronitroaluminates, which are usually formed during hardening of Portland cement with the calcium nitrate addition, are not found. Perhaps their formation occurs at the molecular level in the form of X-ray amorphous mass.
Figure 4. X-ray diffraction pattern of mixes with Ca(NO3)2 additive pre-tested for strength (Table 5).
3.3. Results of differential thermal analysis Fig. 5 shows the DTA curves for mixes 2 and 3 at different ages (Table 4).
0 100 200 300 400 500 600 700 600 900 1000
Temperature, X
Figure 5. DTA curves of mix 2 (without Ca(NO3)2) and mix 3 with Ca(NO3)2 additive of 11.8 % at different ages.
The thermogram of mix 2 shows 3 pronounced endothermic peaks at 210, 560, and 915 °C. The first of them may correspond to the loss of interlayer water in tobermorite. The effects at 560 and 915 °C refer to the decomposition of Ca(OH)2 and CaCO3, respectively. The exoeffect at 730 °C shows the presence of tobermorite in mix 2, which transforms into wollastonite — 5CaO6SiO2-9H2O.
Other thermograms show a number of effects caused by the Ca(NO3)2 influence. There are two endothermic peaks in the temperature range of 120-180 °C, which may be associated with the presence of gypsum and the reaction of ettringite formation. It is possible that gypsum is formed as a result of the reaction of Ca(OH)2 with aluminum sulfate. The intensity of the first endoeffect increases with the specimens age, which may indicate an increase in the ettringite amount. The decrease in the second peak intensity is associated with the consumption of gypsum for the reaction. In this temperature range, the loss of adsorbed and interlayer water by hydrosilicates also usually occurs. The effect at 170-180 °C may be associated with the dehydration of the CAS2H4 compound or other calcium alumina hydrosilicates. Endothermic peak at 280 °C observed after cement hydration for 5 hours, O.P. Mchedlov-Petrosyan et al. attribute the presence of C2AH7 [32]. The authors of [33] believe that at a temperature of about 280 °C, the process of dehydration of C2AH8 and CAH10 begins. Endothermic peaks at 140, 280, and 560 °C can also be attributed to calcium hydronitroaluminate C3ACa(NO3)2-10H2O.
A slight endothermic effect at 330-340 °C is associated with C3AH6 presence, and an exothermic peak at 380 °C indicates the transformation of soluble anhydrite into insoluble one.
The thermograms of mix 3 (Fig. 5) show almost no endothermic effect of Ca(OH)2 decomposition, although according to X-ray diffraction analysis, it should be present. At the same time, in the temperature range of 520-550 °C, small areas of exothermic peaks are observed, which are superimposed on the effect of Ca(OH)2 dehydration, blocking the heat input for the decomposition of portlandite. Since mix 2 does not show a similar effect, the addition of calcium nitrate can be considered responsible. Unfortunately, it has not yet been possible to decipher this exoeffect. Probably in this case, the burnout of unburned carbon particles occurs.
The thermograms shown in Fig. 6 characterize the effect of the Ca(NO3)2 dosage on the phase composition of the specimens consisting of fly ash, silica fume and Ca(NO3)2 additive.
0 100 200 300 400 500 600 700 300 900 1000
Temperature, °C
Figure 6. DTA curves of specimens with different content of calcium nitrate.
Fig. 7 shows that with an increase in the Ca(NO3)2 content, the depth of the endothermic peak sharply increases at a temperature of about 150 °C, which indicates an increase in the growth of hydrates of aluminates and calcium aluminosilicates in the presence of Ca(NO3)2. In this case, the intensity of the endothermic peak at 550 °C corresponding to the decomposition of Ca(OH)2 decreases. In these specimens, after 7 days of hydration, two endothermic effects at temperatures around 300 and 460 °C can be attributed to cubic C3AH6. The peak at 300 °C can also be seen as offset from the effect at 280 °C in the previous diagram (Fig. 5). On the thermograms in Fig. 7, two exothermic peaks are observed. The first one, at 410 °C, is especially pronounced on the curve for the mix with the highest content of the Ca(NO3)2 additive (11.8 %) during dry hardening. On thermograms with a lower content of calcium nitrate, this effect is more blurred and insignificant. A similar exothermic effect (also at 11.8 % Ca(NO3)2) is observed on thermograms for mix 3 at the age from 1.5 h to 28 days, however, at 280 °C (Fig. 5). The second exothermic peak on the thermograms of the dry curing specimens with the addition of 1.5-8.2 % Ca(NO3)2 at 610-620 °C is presumably associated with the burnout of carbonaceous particles.
3.4. Results of Heat Release Determination
The specific heat release of a mortar consisting of fly ash and sand, per 1 kg of fly ash at a temperature of 20 °C, depending on the composition, is shown in Fig. 7.
Figure 7. Specific heat release of mortar per 1 kg of fly ash at temperature of 20 °C, depending on composition: Q1 is mix without additives; Q4 is mix with MS additive of 42.9 %; Q5 is mix with additives: MS of 42.9 % and Ca(NO3)2 of 11.8 %.
Pure fly ash (curve Q1) reacts very violently with water due to the slaking of free lime. Approximately 1 day after intense heat release, the process slows down sharply and ends by the fourth day. In the
presence of silica fume (curve Q4), a sharp slowdown of the heat release process is observed. By the end of the second day, the reaction rate increases again and on the 10th day the heat release is higher than that of pure fly ash. The addition of Ca(NO3)2 (curve Q6) provides a reduced heat release of the mortar. In the initial period up to 1.5 days, curves Q4 and Q6 practically coincide. This indicates that the hydration process during this period is controlled by silica fume, and the retarding effect of calcium nitrate begins to affect later. At the same time, the heat release rate (the angle of inclination of the tangent to curves Q4 and Q6) for mix Q6 is higher than for mix Q4, and it should be expected that by a certain long term of hydration, the integral values of their thermal effects will become equal. Thus, the Ca(NO3)2 additive blocks the accelerating effect of silica, starting from 1.5-2 days of hydration.
Heat release experiments require a certain amount of time for preparing the mix, preparing the specimens, placing the specimens in thermoses, etc. In our case, this time is about 30 minutes. During this period, intense heat release occurs, however, it is not recorded in the experiment. Therefore, we made additional tests to increase the temperature of the paste based on fly ash and additives during the first 1 hour of hydration. The mixes of the test are given in Table 8.
Table 8. Mixes for determining temperature in initial period of hydration.
Consumption of materials [kg/m3] Material -
_T1_T4_T6_
Fly-ash 30 30 30
Silica Fume MKU-85 - 12.9 (42.9 %) 12.9 (42.9 %)
Ca(NO3)2 (dry) - - 3.54 (11.8 %)
Water 19.5 27.9 30.2
Total 49.5 70.8 76.64
The test procedure was presented in detail in [23]. Each mix was tested twice and the average value was taken. The results of measuring the temperature of the paste consisting of fly ash and silica fume are shown in Fig. 8.
40
35
U 30
25
- 20 u
L_
3
15
10
T6
T1
T4
/
r
10
20
30
40 50
Time, min
60
70
80
Figure 8. Temperature increase of specimens depending on mix: T1 is fly ash without additives;
T4 is fly ash with MS additive of 42.9 %; T6 is fly ash with MS additive of 42.9 % and Ca(NO3)2
additive of 11.8 %.
Fig. 8 shows that the temperature of the fly ash (mix T1) for 1 hour of hydration increased by 24.2 °C against the initial one. At the same time, in the presence of silica fume (mix T2), the hydration rate dropped sharply, and the temperature of the paste increased by only 8.7 °C. Here, the same regularity of silica fume effect was observed as in the initial period of hardening of the mortar consisting of fly ash and silica fume (Fig. 7). The introduction of the Ca(NO3)2 additive led to an exceptionally sharp rise in temperature in the first 15 min after mixing. The temperature increase during this time was 36.6 °C.
Thus, calcium nitrate is a strong accelerator of the initial stage of the hydration process, which can be estimated at 30-60 minutes. After the initial stage, up to a period of about 1.5 days, there follows a
period when the heat release of binder, both with and without the Ca(NO3)2 additive, proceeds practically at the same level. That is, the supplement during this period does not accelerate or slow down the hydration process. In the next third period, a low monotonous increase in heat release Q6 continues (Fig. 7), while the composition without additive shows a sharp increase in heat release (curve Q4)
3.5. Influence of Calcium Nitrate on Strength of mortar consisting of Fly Ash and Sand
The results of determining the compressive strength of the specimens are shown in the diagram in
Fig. 9.
m a-
tt E 41
41 >
a.
E o u
3,5
2.5
1.5
0.5
3.19
1.47
0.67
Q1
Q4
Composition
Q6
Figure 9. The results of determining compressive strength of specimens of mortar: Q1 is mix without additives; Q4 is mix with MS additive of 42.9 %; Q5 is mix with additives:
MS of 42.9 % and Ca(NO3)2 of 11.8 %.
Fig. 9 shows, that the silica fume additive increased the strength of the specimens by approximately 100 %, and when Ca(NO3)2 was added to the mortar consisting of fly ash, silica fume and sand, the strength increased by more than two times.
It should be noted the low strength of the tested mixes of the mortar, however, it must be borne in mind that the binder/sand ratio in these mixes ranges from 1:5 to 1:7.
4. Conclusions
This paper presented the results of experimental studies conducted to evaluate the influence of Ca(NO3)2 additive on the strength, heat of hydration, and phase composition of hydration products of the binder based on high-calcium fly ash and silica fume. The following key conclusions were drawn from this study:
1. The effect of Ca(NO3)2 (mix 3, Table 4) was studied using XRD and DTA. After 1.5 h of hydration, a significant amount of unreacted COfree (43 %) remains in the paste and 37.2 % Ca(OH)2 is formed. Aluminosilicate CAS2H4, corresponding in composition to the natural mineral gismondine, and calcium hydrosulfoaluminate of the trisulfate form (ettringite) are quite clearly identified. By the age of 2 days calcium silicate hydrates of the tobermorite group are formed, the content of gismondine increases, and calcium aluminosilicate of the composition CAS6H4 appears. The main phases identified in specimens aged 7 days at different Ca(NO3)2 content are lime, calcium hydroxide, ettringite, CSH(II) type silicates, and calcium aluminosilicates, corresponding to such minerals as gismondine, yugavaralite, and goosecreekite. The influence of Ca(NO3)2 on CaO hydration during dry hardening is manifested to a small extent. The residual content of free lime slightly fluctuates relative to the average value of 21.4 % with a tendency to decrease with an increase in the Ca(NO3)2 amount. With water curing, the amount of lime is sharply reduced to an average of 5.2 %. In this case, an increase in the Ca(NO3)2 content from 1.5 to 11.8 % leads to a decrease in the CaO content by almost 2 times. With an increase in the dosage of calcium nitrate, the content of portlandite noticeably decreases, obviously due to the reaction with silica, and a significant increase in the amount of calcium hydroaluminosilicates, the total content of which increased by 6.4 times during dry curing, and 1.6 times during water storage. The specimens hardened in water show the presence of hydrosilicates in a significant amount and in positive relation to the additive content. Thus, the calcium
nitrate additive mainly intensifies the formation of calcium aluminosilicate hydrates, especially of the composition CAS2H4.
2. With an increase in the Ca(NO3)2 content in the binder based on fly ash and silica fume after 7 days of hydration, the depth of the endothermic peak at 150 °C on the DTA curves increases, which confirms the intensive growth of hydrates of aluminates and calcium aluminosilicates in the presence of Ca(NO3)2. In this case, the intensity of the endothermic effect at 550 °C, corresponding to the decomposition of Ca(OH)2, decreases. Two endothermic peaks are observed at temperatures around 300 and 460 °C, which may belong to cubic C3AH6. There are also two exothermic peaks. The first one, at 410 °C, increases with the content of the Ca(NO3)2 additive. Its nature has not been elucidated. The second exothermic peak at 610-620 °C is presumably associated with the burnout of carbonaceous particles.
3. A study of the heat release process showed that pure fly ash reacts very violently with water, due to the slaking of free lime. During the first day, 85 % of heat is released from the value for 10 days. Silica fume significantly slows down the heat release of fly ash in the initial period of hardening up to 1.5 days, after which a sharp rise follows, and by the end of the experiment, the heat release of the binder exceeds the value for pure fly ash. Calcium nitrate greatly accelerates the process in the first 60 minutes of hydration. However, then the mix with the Ca(NO3)2 additive is inferior in the rate of heat release to binder without additive.
4. When testing a mortar consisting of fly ash, silica fume and sand, the Ca(NO3)2 additive more than doubled the compressive strength of the mortar.
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Information about authors Kseniia Usanova,
ORCID: https://orcid.org/0000-0002-5694-1737 E-mail: plml@mail.ru
Yuriy Barabanshchikov, Doctor of Technical Sciences ORCID: https://orcid.org/0000-0001-7011-8213 E-mail: ugb@_mail.ru
Saurav Dixit,
E-mail: sdixit@ricssbe. edu. in
Received 20.07.2023. Approved after reviewing 15.09.2023. Accepted 21.09.2023.
