Alkaline activation of high-calcium ash and iron ore tailings and their recycling potential in building materials

Ativação alcalina de cinzas com alto teor de cálcio e rejeito de minério de ferro e seu potencial de reciclagem para utilização como material de construção

Abstract

Alkali-activated materials are agglomerates obtained from the alkaline activation of aluminum- and silicone-rich precursors. The most popular precursors for this type of activation are low-calcium fly ashes, blast furnace slag, and metakaolin. However, both high-calcium ashes (HCAs) and iron ore tailings (IOTs) are interesting wastes that can be investigated as precursors because of their available volume and environmental impact during their final deposition. Therefore, by performing tests of compressive strength, FTIR spectrometry and scanning electron microscopy, we sought to identify the products formed during HCA and IOT activation without thermal treatment. Nine mortar formulations with different HCA and IOT proportions were developed using sodium hydroxide and sodium silicate as activators. Thus, using FTIR spectrometry, we observed the reaction between the industrial wastes. Additionally, the compressive strength result suggested that the material could be used as compressed blocks in masonry walls for the development of more environmentally friendly building materials, which would mitigate the impact of waste disposal and convert industrial solid waste into value-added products.

Keywords:
Alkaline activation; High-calcium ash; Iron ore tailings; Compressed blocks; Waste valorization

Resumo

Materiais álcali ativados são aglomerantes obtidos a partir da ativação alcalina de precursores ricos em alumínio e silício. Os precursores mais comuns utilizados para ativação são as cinzas com baixo teor de cálcio, as escórias de alto forno e o metacaulim. Contudo, a cinza de cavaco de eucalipto (CCE) e o rejeito de minério de ferro (RMF) são resíduos interessantes para ser estudados como precursores devidoao volume disponível e ao impacto ambiental advindo de sua deposição final. Diante disso, por meio de ensaios de resistência à compressão, espectrometria de FTIR e microscopia eletrônica de varredura buscou-se identificar os produtos formados pela ativação alcalina da CCE e RMF. Para isso foram desenvolvidas nove pastas com proporções diferentes de CCE e RMF, utilizando como ativadores o hidróxido de sódio e o silicato de sódio. Como resultado observou-se pela espectrometria FTIR a reação entre os resíduos industriais. A resistência à compressão sugere que o material álcali ativado pode ser utilizado como blocos compactados para a alvenaria de vedação, de forma a contribuir para o desenvolvimento de materiais mais ecológicos, mitigando os impactos ambientais causados pela disposição e transformando os resíduos sólidos industriais em produtos com valor agregado.

Palavras-chave:
Ativação alcalina; Cinzas com alto teor de cálcio; Rejeito de minério de ferro; Blocos compactados; Reciclagem

Introduction

Alkali-activated materials are potential substitutes for Portland cement because of their mechanical properties, durability, and low environmental impact and because they do not emit CO₂ during their production. (PACHECO-TORGAL; CASTRO-GOMES; JALALI, 2008; PROVIS, 2018). Such binders are obtained via the polymerization of aluminum- and silicone-rich precursors that are dissolved in highly alkaline solutions (YIP; LUKEY; VAN DEVENTER, 2005; FOUCHAL et al., 2015).

Several authors have stated that alkaline activation products depend on the precursors and activators used (ISMAIL et al., 2014; PROVIS; BERNAL, 2014). Thus, the variability and complexity of the resulting products are still a challenge for researchers (MYERS et al., 2015; PROVIS; PALOMO; SHI, 2015). According to Provis et al. (2015), high-calcium precursors produce calcium aluminosilicate hydrate (C-A-S-H) with a tobermorite-like structure as the main geopolymerization product, whereas precursors with a low calcium content are more likely to form aluminosilicate alkalis (N-A-S-H) that have a high crosslinking density. These geopolymerization products are directly related to the mechanical resistance of the alkali-activated material (PUERTAS et al., 2011; REDDEN; NEITHALATH, 2014). According to Redden and Neithalath (2014), higher alkali concentrations in an activator increases the generation of N-A-S-H. Puertas et al. (2011) concluded that C-A-S-H formed from NaOH presents a structure similar to C-S-H with tobermorite and chain lengths between 5 and 14. When the activator is sodium silicate, tobermorite with chain lengths of 11 and 14 also exists.

The main reports on this subject have studied low-calcium fly ash, iron ore tailings (IOTs), and metakaolin as precursors for alkali-activated materials (PROVIS; BERNAL, 2014). However, high-calcium ashes (HCAs) for use as eucalyptus chip ashes appear to be an interesting waste for investigation because of their volume and chemical composition. According to Myburg et al. (2014) and Larcombe et al. (2015), eucalyptus is the most cultivated wood in the world (i.e., the main reforestation wood) with approximately 20 million hectares planted in more than 100 countries across six continents. However, the steel industry demands large volumes of charcoal and due to social and environmental aspects, it uses reforestation wood for production (GONÇALVES et al., 2014).

According to the work by Amaya et al. (2015), when burning charcoal, approximately 1 % of the material is transformed into ash. Thus, in the 20 million eucalyptus hectares planted, it is estimated that approximately 74x10⁵ t of ash are produced, according to the average Brazilian productivity level; however, the productivity may vary according to the type of soil, climate and spacing between plants (INDÚSTRIA…, 2017).

High-calcium ashes from eucalyptus have already been proven to be viable cementitious materials that can replace Portland cement in proportions up to 15 % (RESENDE et al., 2014). It is believed that above this replacement content the ash will not behave similar to other ashes, such as sugar cane bagasse or rice husk ashes, because of its low silicon oxide content (PARIS et al., 2016; BEZERRA et al., 2017).

Although wood is a renewable energy source and aids in capturing carbon, 70 % of wood ash is usually disposed of in landfills (CHOWDHURY; MISHRA; SUGANYA, 2015). Ashes treated as waste increase the landfill rate and impose costs on the incinerator, which often renders biomass-based energy production as economically unfeasible (OBURGER et al., 2016). Moreover, wood ashes have a high alkalinity, and depending on the extent of changes in the soil solution chemistry, pronounced changes in soil pH may also have an undesirable effect that leads to increased leaching of pollutants and nutrients. Additionally, some of the inorganic compounds could become toxic at high concentrations (WIKLUND, 2017). Thus, the EPC (EUROPEAN…, 2002) waste framework (Decision 1666/2002/EC) has set up a hierarchy of waste treatment options, preferring prevention, reuse and recycling over disposal.

IOTs can also be used as a precursor, and it is a waste that is generated by the mining sector (BASTOS et al., 2016). Global iron ore production is 2,230 billion metric tons (Gt) of usable ore and 1,360 billion metric tons of iron content. Global iron ore production in 2016 on a usable-ore basis was led by the countries listed in Table 1, which together accounted for 93.59 % of global production (U.S. GEOLOGICAL…, 2017).

Table 1
Iron ore production in 2016

Country	Production (Gt)
Australia	825
Brazil	391
China	353
India	160
Russia	100
South Africa	60
Ukraine	58
Canada	48
United States	41
Iran	26
Sweden	25

Based on data from global iron ore production (U.S. GEOLOGICAL…, 2017) and the generation of IOTs in China in 2010 (MENG; NI; ZHANG, 2011), it is estimated that 3,360 billion metric tons of IOTs were generated in 2014 around the world.

IOTs are usually disposed of as a dam (BASTOS et al., 2016; PAPPU; SAXENA; ASOLEKAR, 2007). Dams have a high implementation impact due to their high volume and may cause environmental and social impacts when disruption occurs (PASSOS; COELHO; DIAS, 2017). Thus, the safety of tailing dams is currently a challenge for engineering. Dams are designed to last indefinitely; however, in recent decades, accidents have occurred that have devastated the surrounding ecosystem and victimized hundreds of species (AIRES et al., 2018). In 1998, a dam failure in Aznalcollar, Spain released 2 million m³ of toxic mud into the environment, resulting in the contamination of aquifers and surface water and the death of several aquatic species (DAVIES, 2002). In 2003, a major disaster of this type occurred in the Republic of Macedonia, releasing 100,000 m³ of damaging tailings with heavy metals in the Kamenica River. The height of the tailing flow was approximately 10 meters and the length was 12 km (VRHOVNIK et al., 2013).

Additionally, mine tailings are generally linked to elevated heavy metals (JUWARKAR; JAMBHULKAR, 2008), a lack of organic matter and nutrients (LANGE et al., 2012), an acidic pH (CHATURVEDI; AHMED; DHAL, 2014), a poor water holding capacity (MOUAZEN et al., 2014), a high bulk density (SAKAI et al., 2008) and a high salinity (LI et al., 2014).

Because of the impact of its deposition, some authors have already studied the reuse of IOT. Zhang, Ahmaris and Zhang (2011) showed that IOTs are a promising construction material if utilized with geopolymerization technology. Bastos et al. (2016) also investigated the potential of IOTs for infrastructure materials when properly stabilized. Recently, concretes and mortars with IOTs were investigated (CARRASCO et al., 2017; SANTANA FILHO, et al., 2017; FONTES et al., 2016; HUANG et al., 2013). Cui et al. (2017) utilized IOTs for autoclaved aerated concrete in China due to the billions of tons generated there. Additionally, in Australia, Kuranchie et al. (2015) studied IOTs as aggregates in concrete.

Thus, this study aimed to understand the chemical interactions to produce scientific knowledge that could reduce the impact of deposition of these industrial wastes in the future by incorporating them into building materials. Thus, the products formed from the activation of the two wastes without thermal treatment were identified, IOTs and HCAs, and we correlated them with the compressive strength obtained from using different proportions. The phases were identified using FTIR spectrometry and scanning electron microscopy.

Materials and methods

IOT samples were collected from the pulp filter at the end of the beneficiation process of iron ore. The HCA sample was obtained at the bag filter of boilers used for steam production. The tailings and ashes endured a milling process in the Pulverisette 5 planetary mill, Fritsch, at 300 rpm for 10 min. After milling, the particle size distribution of the tailings and ashes were determined using a laser granulometer, the Cilas 1090 laser particle size analyzer.

The chemical compositions of IOTs and HCAs were determined via a casting method using lithium tetraborate in X-ray fluorescence spectrometers (PANalytical and Axios Fast and Axios models, respectively). The mineralogical composition was determined via X-ray diffraction using an XRD-7000 instrument from Shimadzu at 40 kV and 30 mA using Cu radiation. The angular speed was 0.24°/min, and the scan interval was from 2θ = 5° to 80°. To analyze the diffractogram, the demonstrative version of the Match! software and the Crystallography Open Database, revision 204654 from January 2018 was used.

For specimen molding, Brazilian standard sand, NBR 7214 (ABNT, 2015), was used in medium-fine (# 50) and fine (# 100) particle sizes. As activators, we used sodium silicate from Imperial Química with a SiO₂/Na₂O = 2.15 ratio in solution and 47.19 % of solids and 98 % pure solid sodium hydroxide. Additionally, all mixtures used the superplasticizer additive, Silicon ns high 400, which is based on polycarboxylates modified with stabilized nanosilica at a pH of 3.0±1.0, a specific mass of 1.09+0.02 g/cm³ and a chloride content of < 0.15 % (SILICON, 2017). Despite the polycarboxylic-ether-based superplasticizer not being efficient for this type of material (LASKAR; BHATTACHARJEE, 2013), the plasticizer was used because of its silica-containing chemical composition that was in the proportion of 2 % of the precursors’ mass; the activator/precursor ratio was 0.7. The mortars were developed from these materials as shown in Table 2. Four specimens for each proportion and age were molded, following the recommendations of NBR7215 and ASTM C109/C109 M (ABNT, 1996; AMERICAN…, 2016) standards in cylindrical 25 x 50 mm (diameter x height) PVC molds. Curing occurred at room temperature. The specimens were tested for compression at 7 and 28 days using an EMIC universal testing machine at a 0.25 + 0.05 MPa/s loading speed according to the testing recommendations of the NBR7215 standard (ABNT, 1996). Table 3 shows the amount of each activator used and the amount of water required to maintain the same activator/precursor ratio in all mixtures. To obtain insight into the physicochemical behavior of this system, FTIR was used to confirm the existence and formation of the phases during the geopolymerization phase. The infrared spectra were recorded using an IR-Prestige 21 from Shimadzu using the KBr method. For the morphological characterization, fragments of the broken specimen were imaged using a HITACHI SEM, model TM 3000, with a back-scattered electron detector.

Table 2
Material Ratios

Mortars	HCA (%)	IOT (%)	SiO2/Na2O	Activator/precursor	Plasticizer
HCA70 IOT30 SN1.00	70	30	1.00	0.7	2 % mass of precursors
HCA50 IOT50 SN1.00	50	50
HCA30 IOT70 SN1.00	30	70
HCA70 IOT30 SN1.55	70	30	1.55
HCA50 IOT50 SN1.55	50	50
HCA30 IOT70 SN1.55	30	70
HCA70 IOT30 SN1.85	70	30	1.85
HCA50 IOT50 SN1.85	50	50
HCA30 IOT70 SN1.85	30	70

Table 3
Activator ratio

SiO2/Na2O	IOT+HCA (g)	SiO2Na2O (g)	Na(OH) (g)	Water (g)	Water/activator (g)	Activator/ precursor
1.00	80.00	10.00	2.25	43.75	3.57	0.70
1.55		18.12	1.37	36.51	1.87
1.85		21.00	0.67	34.33	1.58

For statistical evaluation of the compressive strength results, we used the "Anova: Simple factor" method using Microsoft Excel software and a significance level of 5 %. The results of the ANOVA were verified by the hypothesis that the results of the compressive strength of the specimens are exactly equal in the gap, i.e., the results can be considered similar. For the analysis, we took into consideration the source of variation between groups and within groups. For the P value test, P values smaller than the degree of significance rejected the equality between the resistors. Another method of analysis was to compare the value of the critical region boundary (critical F) and the value of F (F). If "F" exceeded "critical F", the hypothesis needed to be rejected.

Results

The chemical compositions of the IOTs and HCAs obtained via XRD are shown in Table 4. As shown, the main constituents of the IOTs are Fe₂O₃ (65.78 %), SiO₂ (22.3 %), and Al₂O₃ (4.25 %). Other components appear in quantities below 1 %. HCA is chemically composed mostly of CaO (43.1 %), MgO (8.24 %), Al₂O₃ (4.94 %), K₂O (4.52 %), P₂O₅ (3.24 %), and Fe₂O₃ (2.42 %). The HCA showed a loss on ignition of 29.66 %, and much of this loss may be due to the high carbon content of HCA. This loss on ignition is higher than that reported in the literature (PARIS et al., 2016), and because the HCA is originated from biomass, it contributes even more to the capture and removal of CO₂ mentioned above (ZHAO, 2018; ROGELJ et al., 2016). The main minerals present in the IOTs and HCAs, as shown in Figure 1, were identified in the diffractogram as showed in Table 5.

Figure 1
Diffractogram of (a) IOTs and (b) HCA

Table 4
Chemical composition

Material	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	Na2O	K2O	P2O5	MnO	LOI
IOT	22.30	4.25	65.78	0.01	0.14	0.21	<0.10	0.04	0.15	0.50	6.10
HCA	1.75	4.94	2.42	43.10	8.24	0.50	0.28	4.52	3.24	0.39	29.66

Table 5
Chemical composition

IOT	HCA
quartz (SiO2) (cod 9005019)	calcite (CaCO3) (cod 1010962),
hematite (Fe2O3) (cod 9000139)	calcium oxide (CaO) (cod 1011095)
goethite (FeOOH) (cod 2211652)	sylvite (KCl) (cod 1000050)
gibbsite (Al(OH)3) (cod 1200016)	periclase (MgO) (cod 9006747)
kaolinite (Al2Si2O5(OH)4) (cod 9009230)	portlandite (Ca(OH)2) (cod 9000113)

The main minerals present in the IOTs and HCAs, as shown in Figure 1, were identified in the diffractogram as showed in Table 5.

The FTIR spectra of the industrial wastes are shown in Figure 2 and Figure 3. As shown in the IOTs, the band at 3650 cm-1 corresponds to kaolinite (DICK et al., 2008), the band at 3150 cm-1 corresponds to goethite (WECKLER; LUTZ, 1998), the bands in the range of 800-1200 cm-1 correspond to vibrations of Si-O bonds (DICK et al., 2008), and the band at 530 cm-1 corresponds to hematite (YANG et al., 2010).

Figure 2
FTIR of the IOTs

Figure 3
FTIR of the HCAs

For the HCAs, in Figure 3, the band at 3640 cm-1 corresponds to portlandite (HORGNIES; WILLIEME; GABET, 2011), the bands at 1400 and 880 cm-1 correspond to calcite (HORGNIES; WILLIEME; GABET, 2011), and 1100 cm-1 corresponds to vibrations of Si-O bonds (PALOMO et al., 2007).

As shown in Figure 4, most (i.e., 90 %) of the IOT and HCA samples had particles with diameters less than 36.63 µm and 25.03 µm, respectively. The average particle diameters obtained in the test were 14.13 µm for the tailings and 12.30 µm for the ashes.

Figure 4
Diffractogram of (a) IOTs and (b) HCA

The results of the compressive strength tests are shown in Figure 5. The mortar with the higher compressive strength at 7 days was HCA70IOT30SN1.85, but the result was quite close for all mixtures with 1.85 and 1.55 SiO₂/Na₂O ratios. At 28 days, there was an increase in the strength, and the mixtures with 1.85 and 1.55 ratios continued to have the best performances. However, the mortar with a 1.00 ratio showed better compressive strength with 30 % HCA and 70 % IOT. With an increase in the HCA content, the compressive strength of the mixture decreased. The compressive strength for this material is superior to that found by Silva et al. (2015), who also utilized industrial waste and geopolymeric stabilization to make compressed blocks. The minimum value typically recognized for compressive earthen materials is 2 MPa at 28 days (HOUBEN; GUILLAUD, 2008). In fact, international documents regulating compressive earth block masonry construction can be less demanding. For instance, the New Zealand NZS 4298 standard (STANDARDS…, 1998) requires a minimum compressive strength of approximately 1.52 MPa. The Spanish standard, UNE 41410 (ASOCIACIÓN…, 2008), follows a similar trend since it requires that the lowest class provided (BTC 1) delivers a normalized compressive strength above 1.3 MPa. Thus, these mixtures can also be used for the production of compressed blocks, contributing to valorization of non-hazardous waste and reducing its deposition impact.

Figure 5
Compressive strengths

In addition to the geopolymerization, the amount of water present in each mixture contributed directly to the compressive strength. The trace with the highest water content (SiO₂/Na₂O = 1.00) was the one with the lowest strength. Thus, increasing the water/activator ratio implies an increase in the porosity of the mortar and a reduction in the compressive strength (WINNEFELD et al., 2010).

In the statistical analysis, the averages of the compressive strength tests for the mixtures with SiO₂/Na₂O ratios of 1.55 and 1.85 were considered similar for both 7 and 28 days, except for the mixture of HCA/IOT at a ratio equal to 50/50 at 28 days. For the average compressive strength of the mixture at a SiO₂/Na₂O = 1.00 ratio, the other admixture was not considered statistically similar, without any exception. This could also be supported by the largest amount of water in this mixture since this admixture has over 90 % and 125 % water than those with SiO₂/Na₂O = 1.55 and 1.85 ratios, respectively. However, that with the SiO₂/Na₂O 1.55 ratio had only 18 % more water than that with the 1.85 ratio.

The FTIR spectra of the mortars show that there was a reaction between the industrial wastes, resulting in the formation of new structures and consumption of others. According to Fig. 6, the mortars with the 1.00 SiO₂/Na₂O ratio maintained the bands for goethite and calcium carbonate, but they have a lower intensity, and the wavenumber that is most expressive is at 1006 cm-1. That wavenumber corresponds to calcium aluminosilicate hydrates and aluminosilicates (KAPELUSZNA et al., 2017), which provide compressive resistance to the material.

Figure 6
FTIR spectra of the mortars with a 1.00 SiO₂/Na₂O ratio

In the case of the mortars with 1.55 and 1.85 SiO₂/Na₂O ratios, shown in Figure 7 and 8, the same bands could be found: goethite (WECKLER; LUTZ, 1998), calcium carbonate (HORGNIES; WILLIEME; GABET, 2011) and calcium aluminosilicate hydrates and aluminosilicates (DICK et al., 2008). However, the mixtures exhibited similar behaviors, which could explain the more homogeneous mechanical performance.

Figure 7
FTIR spectra of the mortars with a 1.55 SiO₂/Na₂O ratio

Figure 8
FTIR spectra of the mortars with a 1.85 SiO₂/Na₂O ratio

Morphologically, we observed higher IOT percentages in the mortars and the occurrence of crystalline lamellar structures composed of overlapping planes, as shown in Figure 9. This structure can be associated with kaolinite because of its morphology (SILVA FILHO et al., 2015). In higher HCA-content mortars, we found fewer kaolinite-like structures. For the mortars with SiO₂/Na₂O ratios between 1.55 and 1.85, such structures were smaller and more dispersed, as shown in Figure 10. Therefore, the absence of kaolinite in the mortars with a higher HCA content may be associated with the HCA reaction with calcium and thus the development of more resistant structures. Higher levels of calcium oxide result in a higher efficiency of kaolinite dissolution with lower Na₂O/SiO₂ ratios (WHITTINGTON, 1996).

Figure 9
Kaolinite-like structure of the HCA30IOT70

Figure 10
Micrographs of HCA50IOT50 (a) SN1.55, (b) SN1.85

Cube-like structures were found in mortars with SiO₂/Na₂O ratios between 1.55 and 1.85, as shown in Figure 11. These structures can be linked to zeolite A because of its morphology (SILVA FILHO et al., 2015) and the increase in the mortars’ strength because they have been observed more clearly in formulations that have higher compressive strengths (AYELE et al., 2015). This could also be confirmed by the FTIR spectra in which mortars with the more expressive peaks for aluminosilicates exhibited the best compression strength values, except for mortars with a SiO₂/Na₂O, ratio = 1.00, which is due to the high amount of water that showed an unsatisfactory behavior for performance as a compressed block.

Figure 11
Micrograph of the HCA70IOT30SN1.55 mortar

It is important to highlight that these aluminosilicates structures were formed at room temperature. Tang, Guo and Wang (2013) previously confirmed the interactions between kaolinite and calcium at room temperature, which produce crystalline aluminosilicates.

Conclusion

This study investigated the potential of solid waste alkaline activators as a building material. The results showed that high-calcium ashes and iron ore tailings are potential materials for the development of environmentally friendly building materials. Moreover, when the solid industrial wastes are reused, IOTs will no longer be disposed of in the form of dams and no longer contaminate the soil or subsoil, which is safer for society and the environment. HCAs will no longer be disposed of in landfills, avoiding possible soil pH changes, its leaching and loss of nutrients that can harm agriculture. Through alkaline activation, the wastes could be stabilized without the use of cement, which contributes to reducing the greenhouse effect.

Using alkaline activation, we observed the reaction between the industrial wastes. The FTIR spectra showed that calcium aluminosilicate is the most expressive phase, which ensures the compressive resistance of the material. Morphologically, the presence of kaolinite was identified as a constituent of IOTs in the mortars. The kaolinite may have reacted with the calcium from the ashes to form the calcium aluminosilicates. Zeolite A was also observed and is an example of aluminosilicate structures identified in the FTIR spectra.

Regarding the compressive strength, all formulations except for that with a 1.00 SiO₂/Na₂O ratio have the potential for application in compacted blocks since the minimum strength for these blocks at 28 days is 2 MPa. Furthermore, other aspects need to be assessed in future investigations, such as the non-mechanical features of the masonry system. These include features, such as the acoustic and thermal performance of the masonry walls, the development and study of compatible renderings, and a durability assessment. In addition, for the production of masonry elements, the use of two wastes without thermal processing and one of them originating from biomass and having approximately 30 % of carbon in its constitution will lead to the reduction of the carbon footprint of the constructions that use these elements.

Acknowledgments

The authors would like to express their gratitude to the Minas Gerais State Research Foundation (FAPEMIG APQ3739-16), the National Council for Scientific and Technological Development (CNPq), and the Brazilian Federal Agency for the Support and Evaluation of Graduate Education (CAPES) for financial support and scientific initiation scholarships.

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Publication Dates

• Publication in this collection
  10 June 2019
• Date of issue
  Jul-Sep 2019

History

• Received
  24 June 2018
• Accepted
  28 Oct 2018