Mechanical Properties of Cement Mortar Containing Fine-Grained Fraction of Fly Ashes


This paper presents the effect of fly ash grain-size fractions on Portland-fly ash cement hydration and its properties. Siliceous fly ashes of size fraction of 0 - 16 and 16 - 32 μm, separated from initial fly ash samples from 1st, 2nd and 3rd hopper in ESP system, were analysed. Cement hydration was investigated by determination of hydration heat and content of Ca(OH)2 and C3S in cement samples. Water to cement ratio and initial setting time of cement pastes as well as compressive strength and microstructure of cement mortars were also analyzed. Results showed that the same amount of the same size ash fraction can give cement of lower or higher early strength and its lower or higher increase with time. Incorporation of 20 wt% of ash fraction of 0 - 16 μm can produce Portland-fly ash cement CEM II/A-V of strength class 42.5R (from 2nd hopper) or 52.5N (from 3rd hopper). Cement containing 40 wt% of ash fraction of 0 - 16 μm from 2nd and 3rd hopper can be classified as pozzolanic cements CEM IV/A-V of strength class 42.5 and normal or rapid early strength, respectively. Different development of strength of cement with addition of the same size ash fraction separated from the initial ash sample from the next hopper in ESP system is connected with higher depolymerization degree of SiO4 units in ash glass, resulting from the greater amount of AlO4 units replacing SiO4 units. Ash fraction of 16 - 32 μm shows lower depolymerisation of glass network and as a consequence lower hydration degree of C3S to portlandite and calcium silicate hydrates (C-S-H).

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E. Tkaczewska, "Mechanical Properties of Cement Mortar Containing Fine-Grained Fraction of Fly Ashes," Open Journal of Civil Engineering, Vol. 3 No. 2A, 2013, pp. 54-68. doi: 10.4236/ojce.2013.32A007.

1. Introduction

Siliceous fly ash, named as class F according to ASTM C618-12, has been used as component of blended Portland cement and as mineral additive in the concrete. Wide use of fly ashes in cement and concrete is determined by their pozzolanic activity, defined as ability to react with calcium hydroxide, a by-product of Portland cement hydration process, to form additional calcium silicate hydrates (C-S-H) and other cementitious compounds, that is, calcium aluminate hydrates (C2AH8 and C4AH13) and calcium aluminosilicate hydrates (C2ASH8 and C3AS3-C3AH) [1]. Scientific literature shows that rate of pozzolanic reaction is attributed to many factors such as particle size distribution, specific surface area, chemical and mineral composition of the fly ashes [2-7], which should be taken into account for predicting contribution of these materials in cement. Work [8] shows that fineness of ashes is more important factor and gives much effect on pozzolanic properties of them. Finer fly ashes reveal greater amount of glassy component than their coarse fractions, which result from higher amount of grains below 45 μm, especially below 10 µm [1-4]. Ash fraction of 0 - 45 μm is characterized by the greatest amount of active components, that is, SiO2 and Al2O3, whoes total content is approximately 15% - 20% according to ASTM C379-65T. The coarse ash fraction lowers content of active silica and aluminum oxide in it, which generally does not exceed 10% in case of fraction above 100 μm. Works [4,8-10] presented that fly ashes separated from different hoppers, attached to an electrostatic precipitator system after coal combustion, have variable physical and chemical properties. Ashes collected from 3rd hopper have improved fineness and particle size distribution in form of a normal distribution curve. Also, they show lower amount of SiO2, but greater amount of Al2O3 and alkalies (Na2O and K2O) [2,6,7]. Consequence of that is increase in content of active chemical components in ash from 3rd hopper, especially the largest increase is observed for active Al2O3 [4]. According to Tkaczewska [2], summarized amount of active chemical components in ash of 0 - 16 µm is higher than that in traditional siliceous fly ashes and difference could be as much as 1.5 times. Of course, this is due to greater fineness fly ash of 0 - 16 µm, but also can be resulted from different chemical composition and structure of glassy component. Glass former oxides in siliceous fly ash, independent on their fineness, are SiO2 and Al2O3. Glass network is mostly composed of SiO4 tetrahedra and each is linked with bridging oxygens Si-O-Si. The Al ions also occupy middle of some of tetrahedra. Negative charge created by substitution of AlO4 tetrahedra for SiO4 units is balanced by exchangeable cations, for example Na+ and K+. Consequence of that is increase in depolymerization of Si-OSi network [11]. Works [2,9,12] indicate that the same fly ash fraction, but selected from 3rd hopper, reveals higher depolymerization degree of glass as a result of higher content of Na and K ions in it [13]. In conclusion, ash of 0 - 16 μm from 3rd hopper reveales the highest pozzolanic activity, resulting not only from great fineness of it, but also from the highest depolymerization degree of SiO4 tetrahedra in glass network [14,15]. Due to amphoteric properties, the Al ions in glass structure can be both as network formers (coordination 4) and as network modifiers (coordination 6). According to Bumrongjaroen et al. [13], function of aluminum ions in glass depends on its chemical composition, in particular, on value of aluminum saturation index (parameter ASI), defined as the charge ratio of Al2O3/(Na2O + K2O + 2CaO). In glass with ASI value less than 1, the Al ion is only network former (AlO4 units). However, amount of tetrahedral Al ions in fly ash glass is limited by content of Na and K ions [16,17]. Results [7] indicate that in glass with ASI value greater than 1, the Al ion is network former and network modifier (AlO6 units). Increase in content of active Al2O3 in fine grains of ashes is directly connected with greater amount of AlO4 units in their glass structure, which in turn results in greater amount of alkali metal ions [12]. It can be assumed that if ashes contain much more Al2O3 in relative to content of Na2O and K2O, the residual Al ions can be acting in the form of AlO6 octahedra. Results [15] indicate that pozzolanic activity of siliceous fly ashes increases with content of AlO6 units in glass structure (octahedral Al ions easily go into solution than tetrahedral Al ions).

Introduction of fly ashes to cement modifies its hydration process and its properties, that is, setting time and compressive strength. It is known that siliceous fly ashes are inert addition not showing hydraulic properties as cementitious materials. Introduction of ashes to cement reduces content of Portland clinker in mixture and consequently reduces amount of alite (C3S) (dilution effect). Cement containing fly ashes has lower hydraulic activity in comparison to that of Portland cement, that is, hydration process of fly ash cement is a relatively slower. Lower heat evolution rate of fly ash-Portland clinker mixture is confirmed by longer induction period and lower intensity of main peak on microcalorimetric curve [1,3-6,18-22]. Fly ashes delay initial and final setting times of cement pastes [1,23,24] and decrease early strength of cement mortars, mainly due to dilution effect as well as slower rate of pozzolanic reaction at initial stage, to about 28 days. With time of hydration, after about several weeks, progress of pozzolanic reaction is significant, what improves properties of fly ash cement, especially its compressive strength, which is comparable to or even higher than that of Portland cement [1,20,25]. Formation of hydration product and, consequently, development of hardened matrix in cementitious system with ashes is generally slower than in reference paste, but after 90 or 180 days microstructure of fly ash cement shows better compactness due to presence of higher content of C-S-H, both from hydration of Portland cement and from pozzolanic reaction. As a result, content of Ca(OH)2 is reduced, although hydration process of fly ash cement is still progressing [24]. Of course, the greater reduction in Ca(OH)2 content the greater increase in C3S hydration degree in cement [23]. Results [9,18,21] show that cement containing fine-grained fly ashes, especially fraction below 45 μm of them, reveals much better mechanical properties in comparison to those of cement containing unsorted ashes. Ash fraction less than 20 μm is the most desirable from point of view of application as cement component. Addition of 20 wt% of ash fraction of 0 - 20 μm can give cement of strength class 42.5R or even strength class 52.5N [2,21].

2. Materials and Testing Procedures

2.1. Characterization of Raw Materials

Fly ashes used in experiment were from the bituminous coal-fired power plant in Poland. Fly ash samples were collected from three different hoppers attached to electrostatic precipitator (ESP) system after coal combustion. Ashes generated from 1st hopper, situated closest to the inlet, are named as F1 sample. Ashes collected from 2nd hopper are named as F2 sample, whereas ashes from 3rd hopper located at outlet are named as F3 sample. Fly ash samples were separated in two particle size fractions in range of 0 - 15 μm (fraction A) and 15 - 30 μm (fraction B). These fly ashes are not typically used in cement production, but they show very good pozzolanic properties, resulting from their finer grain-size distribution [4,18].

Particle size distributions of ashes, measured using a Malvern Mastersizer 2000 laser diffraction particle size analyzer (Malvern Instruments Ltd., Malvern, UK), are shown in Figure 1. Table 1 shows volume content of selected particle size fraction of fly ashes and their

Figure 1. Particle size distribution curves of fly ashes of F1, F2 and F3.

Table 1. Particle size distribution and Blaine surface area of fly ashes F1, F2 and F3.

Blaine surface area. As it is shown, the 52.8, 61.0 and 71.1 vol% of ash particles that were obtained respectively in 1st, 2nd and 3rd hopper are passing through a 45 μm mesh sieve by wet sieving in accordance with Polish Standard PN-EN 451-2:1998. This means that F2 and F3 are classified as fly ashes of fineness Category N according to Polish Standard PN-EN 450-1:2012. It can be noticed that maximum particle size of ashes becomes smaller and range of their particle size distribution gets narrower, as distance from boiler increases (Figure 1). The F1 ash gives particle size distribution curve with two maximum frequency diameters, at 5 and 45 μm, whereas the F3 ash shows a normal particle size distribution. Blaine surface of ashes increases from 1st hopper towards 3rd one, with a value of more than 480 m2/kg for F3 ash.

Table 2 shows chemical composition of fly ashes. Main compounds of ashes are SiO2, Al2O3 and Fe2O3, which summarized content is over 85 wt%, so meets requires minimum of 75 wt%. Loss on ignition content is much lower than 5 wt%, so these ashes can be classified as Category A according to Polish Standard PN-EN 450- 1:2012. The F3 ash shows lower silica content and higher aluminum content than F1 ash and difference is respectively 9.7% and 13.4%. Total amount of Na2O and K2O is changed from the 3.3 wt% (F1 ash) to 4.7 wt% (F3 ash). Significant increase in content of alkalies, by 42% for F3, may suggest a change in chemical composition and structure of glassy phase. Consequence of that may be grater pozzolanic activity of F3 due to presence in its glass structure not only units of SiO4 and AlO4, but also units of AlO6 [13]. The SO3 content is comparable for all ashes.

The X-ray patterns of ashes, carried out using Philips X’Pert Pro MD diffractometer, are presented in Figure 2. Glassy and two crystalline phases—quartz b and mullite —are presented in ashes. Intensity of crystalline phases, expressed as counts per second, changes from 1st hopper towards 3rd one. Intensity of quartz line (at 2q = 26.5˚) decreases from 128 to 70 cps, whereas intensity of mullite peak (at 2q = 26.5˚) changes from 31 to 16 cps. Finer ashes have more glass, which is confirmed by an increase in board diffraction effect in range of 18˚ - 28˚ 2q.

Table 3 shows Blaine surface area and chemical composition of fractions of 0 - 16 and 16 - 32 μm of F1, F2 and F3 ashes. Blaine surface area of ash of 0 - 16 µm is generally two times higher than that of 16 - 32 µm, what is primarily associated with greater amount of grains below 10 µm in ashes of 0 - 16 µm or with presence of dehydrated clay minerals fromcoal gangue (with a highly developed surface area). The same ash fraction, but separated from initial ash sample from 3rd hopper, is characterized by much higher content of alkalies, whereas content of SiO2 and Al2O3 varies slightly. For F3-A and F3-B content of alkalies increases by up to

Conflicts of Interest

The authors declare no conflicts of interest.


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