Abstract
The effect of calcination temperature on the pozzolanic activity of corn straw leaf ash (CSLA) was evaluated. The CSLA samples calcined at temperatures of 500, 700, and 850 °C were mixed in a portlandite solution for 6 h, and residual samples were obtained. The CSLA and residual samples were analyzed using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and a contact angle goniometer to determine the vibration bonds, minerals, phase composition, microstructure, Si 2p transformation behavior, and wetting behavior. The conductivity and loss of conductivity with mixing time of the CSLA-portlandite mixed solution was determined. The loss of conductivity of the CSLA prepared at 500 °C was high compared to that of the other calcination temperatures at the same mixing time, which was attributed to the higher amorphous SiO2 content in the CSLA at 500 °C. Calcium silicate hydrate was easily identified in the CSLA residual samples, and some dense small cubic and nearly spherical shaped calcium silicate hydrate particles were found in the CSLA residual samples at 500 °C. Based on the findings, it is recommended that CSLA be calcined at 500 °C using the cement system in view of higher pozzolanic activity but avoiding excessive agglomeration.
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Pozzolanic Activity of Corn Straw Leaf Ash Produced at Different Temperatures and Treated with Portlandite Solution
Tingye Qi,a,b Guorui Feng,a,b,* and Haochen Wang a,b
The effect of calcination temperature on the pozzolanic activity of corn straw leaf ash (CSLA) was evaluated. The CSLA samples calcined at temperatures of 500, 700, and 850 °C were mixed in a portlandite solution for 6 h, and residual samples were obtained. The CSLA and residual samples were analyzed using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and a contact angle goniometer to determine the vibration bonds, minerals, phase composition, microstructure, Si 2p transformation behavior, and wetting behavior. The conductivity and loss of conductivity with mixing time of the CSLA-portlandite mixed solution was determined. The loss of conductivity of the CSLA prepared at 500 °C was high compared to that of the other calcination temperatures at the same mixing time, which was attributed to the higher amorphous SiO2 content in the CSLA at 500 °C. Calcium silicate hydrate was easily identified in the CSLA residual samples, and some dense small cubic and nearly spherical shaped calcium silicate hydrate particles were found in the CSLA residual samples at 500 °C. Based on the findings, it is recommended that CSLA be calcined at 500 °C using the cement system in view of higher pozzolanic activity but avoiding excessive agglomeration.
Keywords: Calcination temperature; Pozzolanic activity; Corn straw leaf ash; Portlandite solution
Contact information: a: College of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, China; b: Shanxi Province Research Center of Green Mining Engineering Technology, Taiyuan 030024, China; *Corresponding author: fgr09000@126.com
INTRODUCTION
In China, the gross output of corn straw resources is approximately 2.4 billion tons (Vassilev et al. 2013; Zhang et al. 2016). In recent years, a high amount of corn straw, as a main source of fuel, has been transported to biomass power plants. However, a large amount of the corn straw ash waste needs to be managed with environmentally friendly and economically rational methods. Several studies have focused on the application of corn straw ash. It should be noted that ash with fusion on different components of corn straw possess chemical characteristics that lead to its application in different fields (e.g., fertilizer / adsorbent).
Corn straw is mainly made up of corn cobs, corn straw stems, and corn straw leaves. Corn cob ash is a useful material as a pozzolan for producing blended cement because it contains more than 70% total of SiO2 and Al2O3 (Adesanya and Raheem 2009, 2010). Corn stem ash (CSA) has been mixed with ground blast furnace slag, chrome slag, and pitch in different proportions as a coating material to increase adherence (Binici and Aksogan 2011). In addition, CSA was found to contain amorphous silica, which mixes in aqueous solution, and could be regarded as a potential pozzolanic material (Feng et al. 2020).
However, little has been reported on the application of CSLA. Whether or not CSLA has pozzolanic character should be investigated. Some kinds of biomass ash (BA) from agro-industrial by-products have long been known to possess pozzolanic activity, due to having large amounts of silica in amorphous form, which could be used in concrete or as a supplementary cementitious material (Morales et al. 2009; Shen et al. 2011; Frias et al. 2012). Rice husk ash (RHA), palm oil clinker powder, and wheat straw ash have been added in cement-based materials to improve the strength due to the dominant amorphous silica content in their BA, which is useful in pozzolanic materials that react with calcium hydroxide (CH) to produce calcium silicate hydrate (Soares et al. 2015).
There are many methods used to investigate the pozzolanic activity of BA (Villar Cociña et al. 2018). In general, BA is usually mixed in a saturated portlandite solution to assess the pozzolanic reactivity by measuring the electrical conductivity and testing other chemical properties.
Moraes et al. (2016) evaluated the electrical conductivity of CH and pozzolan suspensions to assess the pozzolanic reactivity of sugar cane straw ash and found that sugar cane straw ash is a good pozzolanic material. Van et al. (2014) found that the rate of decrease of the electrical conductivity was a suitable parameter for comparing the pozzolanic reactivity of silica fume and RHA. Impedance spectroscopy was used to characterize the pozzolanic activity of RHA. The method is based on the rate of the normalized conductivity change of the Ca(OH)2 and RHA paste during the first 24 h of hydration (Wansom et al. 2010). Vichan and Rachan (2013) blended calcium carbide residue with BA and mixed them in water. The Ca(OH)2 reacted with amorphous SiO2 from BA to produce pozzolanic products.
Table 1. Comparison of Pozzolanic Activity of Different Biomass Ash
Moreover, the flocculation or agglomeration properties are important for the mixing of BA in water (Wang et al. 2019). A high-quality fluidity of cemented-based slurry is required if the slurry is to be transported long distances through pipelines. If the raw materials show flocculation or agglomeration, then there is an increased blocking risk for transport through pipelines. Flocculation is related to the wetting behavior of the BA. A measurement of the contact angle was performed to reflect the wetting behavior of the BA. The interactions between the surface energies of the BA and water could explain the wetting behavior. If the mixed BA-water colloid was a thermodynamically unstable system with high surface free energy, it would be inclined to reduce the surface areas (wetting behavior) to reduce the surface energy. As a result, BA particles are likely to absorb other BA particles and reduce the surface free energy, which results in excessive agglomeration (Yang et al. 2019).
In this study, the pozzolanic properties of CSLA at calcined temperatures of 500, 700, and 850 ℃ were investigated. The electrical conductivities and loss of conductivity (LC) mixing the CSLA samples in CH suspensions for 0 to 6 h were measured. The Si4+ concentrations of CSLA mixing in KOH/NaOH solution were evaluated. The mineralogy, microstructure, wetting behavior, and shift of the Si 2p and Al 2p chemical bonds of the CSLA samples before and after mixing in the CH suspensions were investigated. These conclusions are useful to understand the pozzolanic activity behavior of CSLA at different calcination temperatures.
EXPERIMENTAL
Materials
Corn stalk leaves (CSL) were collected from Taiyuan city, Shanxi province, China. The CSL samples were washed with deionized water to remove dust and dried at 20 °C for 7 d. The dried samples were crushed to approximately 1 cm grain size to be able to fully combust them. The samples were calcined in a muffle furnace for 3 h at 500 (CSLA-500), 700 (CSLA-700), and 850 °C (CSLA-850).
Table 2. Chemical Composition of CSL Calcined at 500, 700, and 850 °C
According to previous research on the chemical compositions of CSLA-500, CSLA-700, and CSLA-850 (Feng et al. 2019), the chemical composition and physical characteristics of CSLA depend on the combustion temperature (Table 2). High proportions of SiO2, CaO, MgO, and K2O compounds were observed in the CSLA samples. The percentages of main pozzolanic composition (SiO2 + Al2O3 + Fe3O4) of CSLA-500, CSLA-700, and CSLA-850 were 70.18, 75.97, and 77.44 wt%, respectively.
The portlandite solution used in this study was a saturated Ca(OH)2 solution, which is prepared by dissolving a sufficient amount of calcium hydroxide powder with a minimum purity of 95% in pure water. Also, the powder was dissolved with the aid of magnetic stirring technology to promote the dissolution, and the stirring rate was 500 rev/min and the stirring time was 20 min.
Methods
Cooled CSLA samples were milled and sieved using a 0.074-mm membrane filter (Huafeng Hardware Instrument Co., Ltd., Shaoxing city, Zhejiang province, China) to maintain the particle size uniformity, and then characterized using a contact angle goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM).
The pozzolanic activity of CSLA was determined by evaluating the Si4+ concentration from CSLA mixed with 0.1 M and 0.5 M NaOH and KOH solution, respectively, at a liquid-to-solid (L/S) ratio of 0.13 L/g. The CSLA-NaOH (KOH) mixtures were mixed using a magnetic stirring apparatus and kept in a circulated oven at 20 °C for 3 d. The mixed suspensions were filtered using qualitative filter paper, and Si4+ concentration in the filter liquor were measured by inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8300 DV radical; Perkin-Elmer Inc., Waltham, MA, USA) to evaluate the pozzolanic activity of CSLA samples with different calcination temperatures.
The CSLA samples were mixed in a saturated Ca(OH)2 solution at a liquid-to-solid ratio of 0.13 L/g using a magnetic stirring apparatus that stirred the mix for 360 min at 500 rev/min. Simultaneously, the conductivity of the solution was checked at intervals of 1 min over the 6 h mixing time.
After mixing for 6 h, the mixed suspension was filtered, and absolute ethyl alcohol was used to wash the residual samples. The residual samples were dried in an oven at 80 ℃ for 6 h, and the dried CSLA-500 (CSLA-500-CH), CSLA-700 (CSLA-700-CH), and CSLA-850 (CSLA-850-CH) residues were obtained. “CH” here indicates the CSLA residue after mixing with calcium hydroxide (CH) solution. The FTIR, SEM, XRD, and XPS analyses were conducted on the dried residues.
The specific surface area of the CSLA samples was determined using a Malvern Laser Mastersizer S 2000 (Malvern Panalytical Ltd., Malvern, UK). The main chemical compositions of the CSLA (in the form of oxides) were measured using X-ray fluorescence (XRF, ZSX Primus II; Rigaku Co., Tokyo, Japan). The XPS analysis was performed using an AXIS UltraDLD instrument (Kratos Analytical Ltd., Manchester, UK) for surface characterization. Photoelectron emissions were excited using a monochromatic Al Kα line with a photon energy of 1486.67 eV. The microstructure analysis of the CSLA was performed using a JEOLJSM-IT200 electron microscope (JEOL Ltd., Tokyo, Japan) at 25 kV. The samples were gold-coated to prevent charging problems. The minerals of all the CSLA samples were characterized by XRD using an Ultima IV Rigaku diffractometer (Rigaku Co., Tokyo, Japan) with Cu Kα radiation (λ = 1.54178 Å), a generator voltage of 40.0 kV, and a current of 40.0 mA. The 2θ range of 5 to 80° was applied for all the powdered samples in the continuous scan mode at scanning steps of 0.02° and a rate of 10°/min. The conductivity value of the solution was measured using a Leici DDS-307A conductivity meter (INESA Scientific Instrument Co., Ltd., Shanghai, China), and the criterion pH value was adjusted before every test. Fourier transformed infrared spectroscopy was performed using a Bruker Tensor 27 (Bruker Co., Billerica, MA, USA) in a wavenumber range between 400 and 4000 cm-1. The contact angle between water and the CSLA was measured with the sessile drop method by using the contact angle goniometer, an OCA20 measuring system (DataPhysics Instruments GmbH, Filderstadt, Germany). The results presented were averages from three measurements. The amount of dissolved Si ions in the mixed solution was measured by ICP-OES. The intensity versus concentration calibration curve was obtained using a series of Si standard solutions that were prepared based on a conventional calibration process of ICP-OES prior to measurements. The lowest value of Si ions concentration in the mixed solution was measured at 2.476 mg/L.
RESULTS AND DISCUSSION
Dissolution of Si Ions
He et al. (1995) evaluated the pozzolanic activity of clay minerals by measuring the amount of dissolved Si ions in the KOH and clay mixed solution. In this study, the pozzolanic activity of CSLA samples were evaluated by Si4+ concentration of the CSLA and NaOH mixed solution. From Table 3, CSLA-500 showed a higher Si4+ concentration dissolving out from the mixed solution than the CSLA-700 and CSLA-850 samples, which showed that the CSLA-500 possessed higher pozzolanic activity than the CSLA-700 and CSLA-850. It was clear that the Si4+ concentration of CSLA mixed in 0.5 M KOH (NaOH) solution was higher than the 0.1 M KOH (NaOH) solution. The Si4+ concentration of CSLA mixing in the saturated Ca(OH)2 solution was below 2.476 mg/L because the Si4+ could react with Ca2+ and OH–, producing calcium silicate hydrate.
Table 3. The Dissolution of Si4+ of CSL Calcined at 500, 700, and 850 °C in the Alkaline Solution
*: The Si4+ concentration was below 2.476 mg/L
Conductivity Characteristic
Figure 1a shows that the electrical conductivity values decreased for the suspensions during the testing period. These reductions indicate that the chemical reaction progressed in the CSLA-CH suspensions. Moraes et al. (2016) concluded that the reaction of dissolved Ca2+ and OH– with pozzolan particles occurs to form stable and insoluble products. For the electrical conductivity, the sequence was CSLA-850 > CSLA-700 > CSLA-500, which could be explained by the presence of more soluble materials in the CSLA-850. The LC (%) is the main parameter for analyzing the degree of the reaction given in Eq. 1,
LC (%) = (C0 – Ct) / C0 × 100 (1)
where LC is the loss of conductivity (%), C0 is the initial conductivity (μS/cm), and Ct is the conductivity at a given time t (μS/cm).
Fig. 1. (a) Relationship between conductivity and steep time; (b) relationship between the loss of conductivity and the steep time for CSLA at different calcination temperatures
From Fig. 1b, the LC (%) value of CSLA-500 was higher than that of the other two samples, which signified that the CSLA-500 had the highest degree of reaction in the CH-suspension. It was found that CSLA-500 had the highest pozzolanic activity and the most amorphous SiO2 content, the results were the same as the Si4+ concentration of the CSLA-500 and NaOH (KOH) mixed solution.
Wetting Behavior
To investigate the surface properties of CSLA with different calcination temperatures, close contact angles were studied to illustrate the similar wetting behaviors of the CSLA microsphere in water. In Fig. 2 the contact angles between the CSLA microsphere particles and the water probes are shown.
Fig. 2. Contact angles between particles ((a) CSLA-500; (b) CSLA-700; (c) CSLA-850) and water probes
The contact angle of CSLA-500 was much greater than that of the CSLA-700 and CSLA-850 microspheres, indicating less hydrophilic behavior from the CSLA-500 particles. The wetting behavior of CSLA particles is related to the surface free energy or roughness, which can be explained by the excessive agglomeration of CSLA in water. The surface roughness affects the adsorption of water on the particle surface, hence the indirect correlation between agglomeration and contact angle.
When the CSLA was mixed with water, the liquid-vapor and solid-vapor interfaces decreased, and a new solid-liquid interface was created. The contact angle described the equilibrium state with the lowest possible total interfacial energy after wetting. It is described by the combination of Young’s and Wenzel’s equations, as given in Eq. 2,
-∆G = σLV [1 + (cosθ)/r] (2)
where -∆G is the change of the surface free energy (J/m2), σLV is the surface free energy of the liquid-vapor interface (J/m2), θ is the contact angle (°), and r is the “roughness factor” defined in Wenzel’s equation (Wenzel 1936, 1949) as the ratio of the actual surface area to the area of a smooth surface of the same geometric shape and size.
As shown by Eq. 2, a greater contact angle or smaller roughness factor results in a smaller -∆G. Hence, the -∆G of the wetting procedure of CSLA-500-water was much smaller than that of CSLA-700-water and CSLA-850-water. It is proposed that, as a result, a CSLA-500 particle tended to more easily absorb other CSLA-500 particles to reduce the surface free energy or roughness, which resulted in excessive agglomeration (Wang et al. 2019).
The contact angles of the CSLA decreased as the calcination temperature increased. Three factors could explain for this high wettability of the CSLA particles with the increasing calcination temperature in water: 1) The amount of organic ingredient containing nonpolar bonds decreased, and the soluble ionic crystals increased; 2) quartz interacted strongly with water molecules on its surface due to its highly hydrophilic property (Zdziennicka and Janczuk 2011), thus the content of SiO2 increased, leading to an increase in the wetting ability; 3) the particles with lower surface roughness values have higher hydrophobicities (Ulusoy et al. 2003). The CSLA-850 was observed to consist of fine particles with irregular shapes or rough surfaces based on the specific surface area results and SEM observations, which implied the good wettability of these fine particles (Li et al. 2011).
Microstructure
As shown in Fig. 3a, some large unburnt ash residues remained in the CSLA-500 samples because of inadequate conversion in the calcination process at 500 °C. The size of the particles ranged from 1 to 10 μm. The morphology was similar to that of RHA with inadequate conversion in the gasification process, and these particles were mainly composed of carbon as biochar (Yao et al. 2016).
Figure 3b shows a crisscrossed slab and porous structure in the middle of the CSLA-700 particles. As shown, some crystalline- and polygonal-shaped structures were present in CSLA-850, which revealed that CSLA-850 had a higher agglomeration tendency than the other two calcination temperature samples. In addition, the adhesion and slight caking of small particles were dispersed at the surface of the structure. Moreover, as shown in Table 2, the specific surface area increased as the calcination temperature increased, which can be attributed to crystalline growth leading to the formation of more broken smaller particles (seen in Fig. 3) and the removal of carbonyl and phenolic hydroxyl of the biochar, leading to cavity growth as the pyrolysis temperature increased (Bai et al. 2017).
