Residual fly ash from pyrometallurgical processes as a partial replacement for Portland cement in mortars: a study of structural evolution and determination of compressive strength

One of the main objectives in the manufacture of concrete and mortar in recent years is to reduce thca. amount of cement used during their manufacture [1]. In this way, in addition to stone aggregates, residues from pyrometallurgical processes are also used as raw materials to produce concrete [2, 3]. Fly ash is a solid waste obtained by electrostatic precipitation or by the mechanical collection of dust that accompanies the combustion gases of the burners of thermoelectric plants fed by pulverized coals [4]. Fly ash has been studied as an additive or substitute for Portland cement in concrete to improve its compressive strength due to its pozzolanic activity, which is attributed to the presence of SiO₂ and Al₂O₃, mineral species that react with calcium hydroxide during the cement hydration to form calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), which in turn provide a denser metrix, greater resistance, and reduced concrete porosity [5–7]. Specifically, the strength of mortar depends on the cohesion of the cement paste on its adhesion to the aggregate particle, and to a certain extent, on the strength of the aggregate [8]. Attempts have been made to develop the strength in mixtures of mortar by substituting the Portland cement with fly ash. Chindaprasirt and Rukzon [9] investigated the strength, porosity, and corrosion resistance of mortars made with ternary blends of ordinary Portland cement, ground rice husk ash, and classified fly ash, showing that the use of a ternary blend improved the strength at low concentrations of rice husk ash and fly ash and the later age in comparison to that of ordinary Portland cement mortar. Supit et al. [10] evaluated the effect of ultrafine fly ash (UFFA) on the compressive strength of mortars containing high-volume class fly ash as a partial replacement for cement, revealing that cement mortars with 8% UFFA exhibited higher compressive strength at 7 and 28 days than control mortars. Chindaprasirt, Homwuttiwong, and Sirivivatnanon [11] reviewed the influence of the fineness of fly ash on water demand and some of the properties of hardened mortar and found that the use of fly ash significantly improved the drying shrinkage with the coarse fly ash showing the least improvement primarily due to the high water-to-binder ratio of the mix. Cheerarot and Jaturapitakkul [12] replaced the Portland cement with 10%, 20%, and 30% fly ash by weight of cementitious material, showing that the compressive strengths of the fly ash mortars at 7 days were higher than 75% of the standard mortar and higher than 100% after 60 days. Rukzon and Chindaprasirt [13] studied the strength and chloride resistance of mortars made with ternary blends of ordinary Portland cement, ground palm oil fuel ash, and 0–40 wt% classified fly ash; and they determined the use of ternary blended cement produces mortars with good strength. Fu et al. [14] studied the influence of clinker, activators, and fly ash on the properties of blended cement with a high fly ash content and found that the main hydration product of the fly ash–blended cement was CSH gel, ettringite, and a small amount of Ca(OH)₂. Elahi [15] evaluated the performance of high-performance concretes, replacing Portland cement with up to 15% silica fume, 40% class F fly ash, and 70% ground granulated blast-furnace slag and found that a combination of 40% fly ash, 7.5% silica fume or 50% ground granulated blastfurnace slag, and 7.5% silica fume improved the hydration properties and compressive strength of cement paste.

A high percentage of cement substitution by residual fly ash tends to decrease the compressive strength because there is a saturation of the mixture due to these residues [16]. One method for measuring the consumption of Ca(OH)₂, the main reactant of pozzolanic reactions, is X-ray diffraction analysis, which is relatively accurate in evaluating the reactivity of fly ash [16]; this method requires a minimum of 28 days for sufficient fly ash pozzolanic reactions to occur. In this sense, this article presents as a scientific contribution, the development of a serious and systematic investigation of the microstructural evolution and mechanical performance specifically of mortars in which the Portland cement was substituted with 10% and 15% fly ash. The results will allow the generation of basic science that contributes to the enrichment of the information found in the literature regarding the structural evolution of mortars in which residual fly ash from pyrometallurgical processes as a partial replacement for Portland cement has been used. The structural evolution of the mineralogical phases was analyzed by X-ray diffraction and scanning electron microscopy– energy-dispersive spectroscopy (SEM-EDS) stopping the hydration process at 3, 7, 14, and 28 days and the subsequent evaluation of the mechanical resistance to compression.

The source materials were composite Portland cement (CPC-30R) with a median particle size of 5.67 μm, sand with an average particle size of 4 mm, residual fly ash, and water. The physical properties of the Portland cement (CPC-30R) and fly ash were determined according to standardized Mexican standards [17–20]. Initially, Portland cement and fly ash were structurally characterized by X-ray diffraction (diffractometer Equinox 2000 Cu Ka) and morphologically using a scanning electron microscope (JEOL JSM-IT300). The particle size distribution was also determined (Beckman and Coulter LS13320). The amount of fly ash used was 0, 10%, and 15% by weight of the Portland cement, the water-to-binder ratio was maintained at 0.64, and 63% sand was used [21]. The standard mortar sample (sample 0) was prepared without the addition of fly ash.

Seventy-two specimens were manufactured, of which 24 were for the standard mortar sample (sample 0), 24 samples with 10% fly ash, and 24 samples with 15% fly ash. The mortar samples were prepared in bronze cubic molds (50 mm per side) with a base with smooth and rigid walls that were previously covered with a thin layer of lubricating oil to prevent sticking. The sand and 50% of the water were mixed before the addition of Portland cement, fly ash, and the remaining 50% of water and stirred until a homogeneous mixture was obtained. The mold was half-filled with the mixture and compacted with a piston 32 times in 10 seconds. This compaction was performed in four cycles of eight consecutive blows distributed uniformly on the surface of the mortar, and each cycle was perpendicular to the previous one. Immediately afterward, the second half of the mortar was added, and a second compaction was performed. Excess mortar on the surface of the mold was spread and leveled with a screed, applying zigzag movements with a 15°inclination, as stated in the standard [22]. The samples remained in the molds at room temperature for 24 hours, then were removed and immersed in tanks with clean water at room temperature where they remained for 3, 7, 14, and 28 days until the compressive strength test. The tank water was renewed every 72 hours. The measurement of the mechanical resistance to compression (sc) of the mortar samples was conducted according to the standard NMX-C-486-ONNCCE [23] using Controls Pilot equipment, model 50 – C43C04, with a load capacity of 150 kN and a load application speed of 2.55 (kg/cm²)/s. The structural evolution of the mortars was evaluated and monitored using X-ray diffraction and SEM-EDS from the remaining samples from the compression test [24, 25].

The physical properties of the Portland cement (CPC-30R) and fly ash are shown in Table 1. The fineness of fly ash was 302 m²/kg, which was about the same as that of Portland cement. It is important to know that the fineness of the Portland cement largely determines the rate of hydration, the development of the heat of hydration, the shrinkage, and the acquisition of strength of the cement. On the other hand, the specific gravity of the fly ash was 2.02 while that corresponding to Portland cement was 3.10. The passing 45 μm sieve (%) for cement and fly ash were 98% and 79%, respectively.

3.1.

Characterization of Portland cement and residual fly ash

3.1.1.

Residual fly ash

The X-ray diffraction diffractogram of residual fly ash is shown in Figure 1, showing the predominance of a combination of the mineralogical species quartz (SiO₂) (JCPDS 120708), calcium, and iron oxide (Ca_0.15Fe_2.85O₄) (JCPDS 460291), hematite (Fe₂O₃) (JCPDS 160653), and alumina Al₂O₃ (JCPDS 350121), in line with the X-ray characterization study of mullite in aluminosilicate fly ash [26]. Figure 2a is the backscattered electron (BSE) image of the fly ash showing the particles composed mainly of O, Fe, Si, and Al (Fig.2b) that are preferentially spherical with a median particle size of 13.07 μm (Fig.2c). This composition is consistent with that reported by Kutchko and Kim [27], who determined that the surface and internal structure of most fly ash particles comprised mainly amorphous aluminum-silicate spheres and a smaller amount of iron-rich spheres. It has been reported that the particle size of the fly ash can positively impact the compressive strength of concrete since it reduces the porosity of the material and the fineness of fly ash, not the chemical composition, which is the major factor affecting the strength activity index of fly ash cement mortar [28, 29].

Fig. 1.

Fig. 2.

3.1.2.

Portland cement CPC-30R

Figure 3 shows the X-ray diffraction diffractogram of Portland CPC-30R cement comprising mainly alite (Ca₃O₅Si) (JCPDS 961540705), calcite (CaCO₃) (JCPDS 969007690), Brownmillerite (AlCa₂FeO₅) (JCPDS 961008725), and tricalcium aluminate (Al₂Ca₃O₆) (JCPDS 969014360), similar to that reported by Young and Yang [30].

Fig. 3.

Figure 4a presents the BSE image of Portland cement CPC-30R, showing that the material is composed mainly of O, Ca, Si, C, Mg, and Al (Fig.4b). The morphology is characterized by large crystalline particles of undefined angular shapes with a median particle size of 5.67 μm (Fig.4c) on a visible, amorphous material adhered to the surface of the large particles.

Fig. 4.

3.1.3.

Sand

Figure 5 shows the X-ray diffraction diffractogram for the sand used in the manufacture of mortars mainly composed of calcite (CaCO₃) (JCPDS 057690). The presence of this mineralogical specie gives the particles a morphology of flat angular surfaces with a brittle type of fracture (Fig.6a). The average particle size was 4 mm, and the dispersive energy analysis (EDS) indicated that they were composed of Si, Ca, Mg, K, C, O, and Al (Fig.6b), consistent with the XRD results.

Fig. 5.

Fig. 6.

3.1.4.

X-ray diffraction of mortar samples

Cement hydration depends on reactions between cement minerals and water, as well as the presence of gypsum. The hydration products are hydrated calcium silicates, hydrated calcium hydroxide, and calcium sulfoaluminate [7, 8]. Figure 7 shows the X-ray diffraction diffractograms for the standard mortars (no added residual fly ash) at 3, 7, 14, and 28 days of curing time. The mineralogical species identified were as follows: portlandite (CaOH₂O) (JCPDS 020969), calcite (CaCO3) (JCPDS 471743), ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂) (JCPDS 371476), iron oxide (Fe₂O₃) (JCPDS 540489), silicon oxide (SiO₂) (JCPDS 882487), and sillimanite (Al₂SiO₄) (JCPDS 831562), which are characteristic of a typical mineralogical composition of Portland cement during hydration [31]. Portlandite (calcium hydroxide) is the mineral phase responsible for maintaining the high pH of the mixture and protecting against electrochemical corrosion. It is the first mineral to decompose at high temperatures (350–460°C). which can be reduced with the addition of pozzolans such as fly ash [32]. Ettringite (calcium trisulphoaluminate) is the mineral species that gives cement greater cohesion and is generally produced later (more than 28 days) by the reaction between gypsum and water [32], as evidenced in the spectrum corresponding to 28 days of curing time (Fig.7), in which there is an increase in the intensity of the peaks corresponding to this phase located in 2q ≈27, 44, 48, and 71°, which is indicative of the consolidation of the ettringite phase with increasing curing time.

Fig. 7.

Figures 8 and 9 show the X-ray diffraction diffractograms for the mortar mixes substituted with 10% and 15% residual fly ash at 3, 7, 14, and 28 days cure time. The main mineral species identified were the following: portlandite (CaOH₂O) (JCPDS 020969), calcite (CaCO₃) (JCPDS 471743), ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂) (JCPDS 371476), iron oxide (Fe₂O₃) (JCPDS 540489), silicon oxide (SiO₂) (JCPDS 882487), sillimanite (Al₂SiO₄) (JCPDS 831562), and, additionally, the magnetite species (Fe₃O₄) (JCPDS 110614). These results indicate that the substitution of Portland cement with fly ash does not significantly modify the composition of the mineral species formed, except for the identification of a magnetite phase (Fe₃O₄) (JCPDS 110614) in the 10% and 15% fly ash mortar mixtures. The physical and chemical characteristics of fly ash allow it to develop the pozzolanic function in the mortar and generate a dense microstructure with a discontinuous pore network that makes it difficult for chlorides to pass through [5, 7]. It is also noted in Figures 8 and 9 that the peaks corresponding to the main hydration products, portlandite (CaOH₂O) (JCPDS 020969) identified in positions 2q ≈ 21, 39, 55, and 59° and ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂) identified in positions 2q ≈ 27, 44, 48 and 71 ° exhibit an increase in their intensities with increasing curing time from 3 to 28 days attributed to an increase in the consolidation of mineral species.

Fig. 8.

Fig. 9.

3.1.5.

Morphology of mortars

Figure 10 shows the detailed SEM images for the standard (a), 10% (b, c), and 15% (e, f) fly ash mortar mixtures at 28 days of curing time. The standard mortar sample containing mainly portlandite physically arranged in the form of thin hexagonal sheets or platelets did not show important morphological changes over the 28 days of curing. Ettringite (ettringite positive), is characterized by elongated rods arranged in fibers around the aggregate particles. It has been reported [33] that this type of oriented growth of ettringite generates an expansion effect and depends on the curing conditions. As mentioned in the previous XRD results (Figs. 7, 8, and 9), this species also increases the cohesion of the mixture. However, ettringite identified within the fissures (Fig. 10b) or the pores generates expansion and consolidates in the late stage of curing (28 days) [33].

Fig. 10.

The 10% and 15% residual fly ash mortar mixtures at 28 days of curing time also contained particles coated with the hydration products portlandite and ettringite (Fig. 11a), particles with a smooth surface (Fig. 11b), and residual fly ash particles that showed signs of attack on their surface (Fig. 11c). These particles have previously been reported at 28 days of curing [34] in an investigation of the effect of fly ash on the microstructure of blended cement paste.

Fig. 11.

3.1.6.

Compressive strength of mortars

The mechanical resistance to simple compression of the mortar (sc) is defined as the ability to support a load per unit area and is expressed in terms of stress, usually in kg/cm², MPa, or pounds per square inch (psi). The results of sc obtained for the mortar samples at 3, 7, 14, and 28 days of age after manufacture are shown in Figure 12. In general, the sc increased with curing time and the substitution of Portland cement by residual fly ash from 3 to 28 days and from 0% to 15%, respectively, obtaining the maximum values at 28 days. The sc for the sample containing 15% residual fly ash and the standard sample at 3 days of age was 12.06 MPa and 13.63 MPa, respectively, with a difference of 1.57 MPa that decreased to 0.07 MPa at 28 days of age. Regarding the samples with 10% residual fly ash, the sc was 12.57 MPa at 3 days of age and 17 MPa at 28 days of age, with a difference of 1.06 Mpa and 0.38 Mpa with respect to the standard sample at 3 and 28 days of age. It is generally accepted that the pozzolanic reaction in the fly ash/cement systems is important at 28 days [35–37], and the reaction between the fly ash and the calcium hydroxide (CH) forms calcium silicate hydrates (CSH) which have lower calcium-to-silicate ratios (C/S) [38]. The increase in sc of the mortars evaluated in this work could be attributed to an improved bond between the hydrated cement matrix and the sand due to the conversion of CH, which tends to form on the surface of aggregate particles to CSH.

Fig. 12.

The conclusions derived from this study are described as follows. The main mineralogical species formed during the Portland cement hydration process identified for the standard mortar mixtures (sample 0% residual fly ash), at 3, 7, 14, and 28 days of curing time were the followoing: portlandite, calcite, ettringite, iron oxide, silicon oxide, and sillimanite. These species were also identified in the mortar mixtures with Portland cement substitution of 10% and 15% residual fly ash and additionally, the species magnetite was identified. The addition of fly ash as a replacement for Portland cement did not drastically influence the modification of the composition of the mineral species formed. The SEM images obtained showed that the samples of mortars without the addition of fly ash contained mainly Portlandite and ettringite (ettringite positive). In the samples with cement substitution of 10% and 15% of residual fly ash at 28 days of curing, particles of fly ash coated with hydration products (portlandite and ettringite), particles with a smooth surface, and particles of fly ash with signs of attack on their surface were also observed. An increase in the value of sc was observed when the age of the mortar and the substitution of Portland cement by fly ash was increased from 3 to 28 days and from 0% to 15%, respectively. The maximum value of sc was 17.38 MPa, and it was registered for the mortar mixtures with Portland cement substitution by 15% fly ash at 28 days of curing time.