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Rice husk ash and spent diatomaceous earth as a source of silica to fabricate a geopolymeric binary binder
Accepted Manuscript Rice husk ash and spent diatomaceous earth as a source of silica to fabricate a geopolymeric binary binder Johanna M. Mejía, Ruby Mejía de Gutiérrez, Carlos Montes PII:
S0959-6526(16)00092-5
DOI:
10.1016/j.jclepro.2016.01.057
Reference:
JCLP 6645
To appear in:
Journal of Cleaner Production
Received Date: 3 August 2015 Revised Date:
28 November 2015
Accepted Date: 23 January 2016
Please cite this article as: Mejía JM, de Gutiérrez RM, Montes C, Rice husk ash and spent diatomaceous earth as a source of silica to fabricate a geopolymeric binary binder, Journal of Cleaner Production (2016), doi: 10.1016/j.jclepro.2016.01.057. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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RICE HUSK ASH AND SPENT DIATOMACEOUS EARTH AS A SOURCE OF SILICA TO FABRICATE A GEOPOLYMERIC BINARY BINDER
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*Johanna M. Mejía, Ruby Mejía de Gutiérreza, Carlos Montes b.
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Composites Materials Group (CENM), Universidad del Valle, Colombia
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Institute for Micromanufacturing, Louisiana Tech University, United States of America *[email protected]
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Abstract
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The goal of this research is to propose the use of industrial by-products as solid sources of silica in replacement of commercial sodium silicate (SS) for the manufacture of geopolymeric binary binders. The result of this approach is a binder with good mechanical properties and environmental benefits, which can be used in civil engineering applications such as the manufacturing of masonry elements.
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A Colombian fly ash (FA) and a commercial metakaolin (MK) were used as geopolymer precursors. Rice husk ash (RHA), spent diatomaceous earth (SDE) and commercial sodium silicate were used as sources of silica to adjust the SiO2/Al2O3 molar ratio. These silica sources were mixed with a sodium hydroxide (NaOH) solution. Three alkaline solutions were used to activate the binary system FA/MK; SN (sodium silicate and sodium hydroxide), RN (rice husk ash and sodium hydroxide) and DN (spent diatomaceous earth and sodium hydroxide). A FA/MK proportion of 70/30 by weight, and the respective SiO2/Al2O3 and Na2O/SiO2 molar ratios of 4.4 and 0.2 were kept constant for all experiments. Mechanical and microstructural analysis were conducted on the three systems, and compressive strength was measured at 28, 180 and 360 days. The reaction products were characterized by using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM).
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Results show that it was feasible to utilize RHA and SDE as a source of silica for geopolymerization. The use of these alternative silica sources delayed the initial and final setting time at 60°C of a FA/MK-based binary alkali activated geopolymer system. Regardless of the source of silica used, the mechanical strength of the resulting geopolymer binders did not show a significant change over time. The FA/MK-SN system exhibited a final compressive strength of 75 MPa at 360 days, and at the same age FA/MK-DN and FA/MK –RN both reached final compressive strength values of 38 MPa.
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Key words: silica source, metakaolin, fly ash, geopolymer, diatomaceous earth, rice husk ash.
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1. Introduction
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The coal combustion process generates waste materials such as fly ash and bottom ash. In Colombia, the main thermoelectric plants and some industrial boilers generate approximately 600 kt / y of fly ash, whose quality does not permit its use as an active addition in concrete. This requires disposal in landfills and generate environmental problems due to their mismanagement. The high content of alumina and silica make the fly ash a suitable material for being used as raw material in the production of simple and binary geopolymers (Andini et al. 2008, Chindaprasirt et al. 2009, Mejia et al. 2013, Mejia et al. 2015). The geopolymerization process helps to reduce the environmental impact, given the lower energy consumption, saving space required for the disposal of waste, low processing temperatures, at the same time develop materials with good mechanical and thermal properties with a broad spectrum of applications. According to McLellan et al. (2011) and Yang et al. (2013), geopolymers may have during their life cycle, environmental issues similar to those of Portland cement in terms of CO2 footprint and energy requirements. According to Tempest et al. (2009), the major part of the emissions and energy consumption of geopolymers can be attributed to the sodium silicate and sodium hydroxide activators. Sodium silicate, for example, is produced from natural resources (silicon oxide and sodium carbonate) at high temperatures (1300 ° C), so that their production is energy intensive and generates CO2 in the atmosphere. Habert G. et al. (2011) proposed interesting strategies to minimize the adverse effect of sodium silicate in the alkaline activation process. The first of these strategies is to minimize the SiO2/Al2O3 molar ratio in order to reduce the amount of sodium silicate added to the system. The second strategy is to use a mixture of ground blast furnace slag and activated clays as raw material, which will also result in a geopolymer with a low SiO2/Al2O3 ratio. As a contribution to improve the ecological properties of geopolymer, Bernal et al. (2015) and (2012), He et al. (2013), ), Mejia et al. (2013), Puertas and TorresCarrasco (2014) proposed to use a supplementary source of silica in alkali activated systems, such as rice husk ash or glass wastes to produce silicate solutions via chemical reaction with NaOH or KOH. Bernal et al. (2011) and Kim et al. (2013), suggested another way to avoid the use of sodium silicate by using CaO and alkali sulfates as a low cost alternative to classic alkali activators.
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According to the mentioned above, this study promote the use of a supplementary sources of silica instead sodium silicate to adjust the SiO2/Al2O3 molar ratio of a geopolymer based on fly ash (FA) and metakaolin (MK). The silica sources namely, spent diatomaceous earth (SDE) and a rice husk ash (RHA). Commercial diatomaceous earth (DE) comes from a single-celled algae named diatoms, when diatom algae die the silica shell or skeleton, also known as frustules, collects at the ocean floor, forming fossils. After millions of years, these fossilized cell walls become diatomaceous earth (DE) or diatomite. DE is composed from amorphous nano-silica particles, and consists of 87– 91% silicon dioxide (SiO2) and a small amount of alumina (Al2O3) and iron oxide (Fe2O3). Commercial DE because of its characteristics, such as porous structure, high silica content, low density, thermal resistance, is widely used as filter, catalytic support, biological support, functional filler, and adsorbent. (Ibrahim et al. 2012). In this study the DE was used in a brewery company, after it was reused several times, it eventually lost its properties and became a waste named spent diatomaceous earth (SDE). 2
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Nowadays it has been identified that decreasing the consumption of natural resources and improving the use of wastes and by-products as a raw material for several industrial process will highly contribute to the sustainable development and will benefit the environment by preserving natural resource, air, soil and earth for future generations. Thus the use of wastes and by-products such as RHA, SDE and FA among others as geopolymer feedstocks may help companies to produce high added value materials and low CO2 footprint. On the other hand, is being carrying out a solution for companies concerned to waste managment issues by including a new stage in their production process, recycling and transforming waste and by-products into a new material with a commercial value.
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The performance of silicate solutions prepared from these wastes (RHA and SDE) was compared to that of a commercial sodium silicate solution. Three alkaline solutions were produced in the laboratory to alkali activate a binary system based on FA/MK. Setting time, mechanical strength and microstructural development were evaluated. X-ray fluorescence (XRF), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed as characterization methods.
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RHA is an agro-industrial by-product generated during the combustion process of rice husk, this sustainable biomass fuel is used to produce electricity during the rice drying stage in a milling rice, it consists of approximately 40 wt.% cellulose, 30 wt.% lignin, and 20 wt.% silica (He et al. 2013).
2. Experimental Procedure
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2.1. Materials
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A Colombian fly ash (FA) from a thermoelectric power station, and a commercial metakaolin (MK) (Metamax) were used as geopolymeric precursors. The binary system was designed using a FA/MK proportion of 70/30, with SiO2/Al2O3 and Na2O/SiO2 molar ratios of 4.4 and 0.2 respectively.
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A commercial sodium silicate (SS) with 32.4 wt.% SiO2, 13.5 wt.% Na2O, and 54.1 wt.% H2O was used in the study. Rice husk ash was obtained directly from a Colombian rice milling factory, where rice husk was used as a source of energy in a combustor. The total silica content was 90.91% and 36% corresponds to amorphous silica. The spent diatomaceous earth (SDE) was obtained from a Colombian brewing company, where it was used as a filter in one of the beer 3
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production stages. The total amount of silica in SDE was 91.86 and 43% corresponds to amorphous silica.
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The FA, RHA and SDE were subjected to conditioning treatments. FA, had an initial particle size of 63.9 µm, it was ground in a ball mill during 90 minutes to reduce its particle size to 19.8 µm. The RHA with an initial particle size of 136 µm, first it was ground during 60 minutes in a ball mill, until its particle size was reduced to 28 µm. Then, its particle size was further reduced to 5 µm by an attrition mill. The spent diatomaceous earth (SDE) was modified by a calcination process, in order to remove the organic compounds (yeast). Thermal treatment was conducted at 400°C for 3 hours.
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Table 1 summarizes the chemical composition, loss of ignition (LOI) and physical properties of the source materials. The chemical analysis of the geopolymer precursors and the two sources of silica was determined by X-ray Florescence (XRF). It is worth noting that the fly ash contains 14% of residual carbon, which is usually considered as low quality, since this carbon content is outside ASTM C 618 requirements. However, results from previous studies (Mejia et al. (2015), Mejia et al. 2014, Rivera et al. 2014) have demonstrated that it is feasible the use of this high LOI fly ash as a geopolymer precursor. Table 1. Chemical analysis of source materials and physical properties
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FA
MK
53.7 52.4 21.5 44.3 4.5 0.47 0.8 0.02 1 1.92 1.4 0.15 0.6 0.02 0.6 0.18 0.5 0.07 0.3 0.31 0.2 0.01 14.8 1.10 Physical properties
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Chemical compounds (%) SiO2 Al2O3 Fe2O3 CaO TiO2 K2O SO3 MgO P2O3 Na2O Sr L.O.I
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Density (g/cm3) Surface área BET (m2/g) Particle size (µm)
RHA SDE
90.93 0.11 0.19 0.36 1.97 0.15 0.33 0.67 0.02 5.10
91.86 3.23 1.56 0.57 0.17 0.51 0.02 0.29 0.45 0.63 0.68 2.06
2.21
2.5
2.04
14.8
12.7
11.35 12.99
19.8
7.76
4.6
24.69
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Fly ash and metakaolin were used as a geopolymer precursors, since both are aluminosilicate materials with a vitreous phase that allows silica and alumina to be dissolved in alkaline environments. XRD pattern from FA (figure 1) shows more crystalline compounds, such as mullite (2Al2O3∙SiO2; pattern diffraction file (PDF) #00-015-0776), hematite (Fe2O3; PDF# 00001-1053), and quartz (SiO2; PDF #00-005-0490) in the fly ash compared to the metakaolin. MK has a broad halo between 20 and 35° 2θ, which represents the amorphous phase of the material, and some small crystalline peaks associated to mullite and quartz.
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Figure 1. XRD patterns of the raw material. a) Fly ash and b) Metakaolin 2.2. Alkaline solutions preparation and synthesis
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Three alkaline solutions were used to activate the binary system FA/MK; SN (sodium silicate and sodium hydroxide), RN (rice husk ash and sodium hydroxide) and DN (diatomaceous earth and sodium hydroxide). The solutions were prepared by mixing the source of silica with analytical grade sodium hydroxide (99% purity). The liquid/solid ratio was determined depending on the workability of the paste. The ratios were, 0.3 for the paste prepared with SN solution and 0.45 for the paste prepared with RN and DN solutions. The sodium silicate solution (SN) was mixed with the sodium hydroxide solution, and prepared the same day it was used. The RN and DN solutions were prepared by adding and dissolving either rice husk ash (RHA) or spent diatomaceous earth (SDE) in a sodium hydroxide solution. The last two solutions were kept in constant stirring during 24 hours. (Figure 2)
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Figure 2. Alkaline solutions made from alternative silica sources a) RN solution and b) DN solution Geopolymer pastes were prepared by mixing the solid precursors (FA/MK 70/30 by weight) with the alkaline solutions SN or RN or DN for 10 minutes in a Hobart Mixer. The slurry was cast inside cubic molds (2x2x2 cm). The specimens were oven cured for 24 hours at 60°C and 90% of relative humidity. After curing, the samples were kept at ambient temperature until they reached the testing age. Table 2 shows the three geopolymer systems with their respective work conditions: alkaline solutions, molar (SiO2/Al2O3 and NaO2/SiO2) and liquid/solid (l/s) ratios.
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Table 2. Geopolymer systems, alkaline solutions, molar and liquid/solid ratios
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Alkaline solution SS+NaOH solution RHA+NaOH solution SDE+NaOH solution
SiO2/Al2O3
NaO2/SiO2
l/s
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0.2
0.35
4.4
0.2
0.42
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2.3. Techniques
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Setting time was determined using a Vicat needle as it is described in ASTM C191. Compressive strength testing was conducted at the ages of 28, 180 and 360 days, using a compression testing instrument (Universal INSTRON 3369), at a displacement speed of 1 mm/min. Density, absorption and voids were measured as described in ASTM C642. The microstructural characterization of the binary alkaline activated systems was carried out using instrumental techniques such as Scanning Electron Microscope (SEM), using a JEOL JSM6490LV microscope under a high vacuum (3x10-6 torr), fitted with an OXFORD INTRUMENTS 7573 INCA PentaFETx3 detector. The samples were covered with gold with a Denton VacuumDesk IV unit in order to increase the resolution of the microphotographs. X-ray diffraction (XRD), was conducted on a PanAnalytical X´Pert MRD diffractometer using a K∝ radiation, a copper X-ray tube, in a range 2θ of 5 to 60°.
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3. Experimental results and discussion
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3.1. Alternative sources of silica characterization
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Rice husk ash and spent diatomaceous earth (as shown in Table 1) are mainly composed of silica. The X-ray diffraction (XRD) shows the pattern of RHA and SDE (Figure 3) it exhibits an intense peak of tridymite (American Mineralogist Cristal Structure Database AMCSD, 0000531) and cristobalite (AMCSD, 0001629), both SiO2 polymorphs formed during the combustion process of the RHA. The absence of a halo between 15-30º 2θ, indicates the lack of amorphous phase. On the other hand SDE has a halo between 15-30º 2θ which represents the amorphous phase of the material. Also, a few intense peaks were found, which belong to crystalline phases such as hematite (Fe2O3; PDF# 00-001-1053), quartz (SiO2; PDF #00-005-0490) and cristobalite (AMCSD, 0001629).
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Figure 3. XRD patterns of Rice husk ash (RHA), and Spent diatomaceous earth (SDE)
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Figure 4 shows the particle morphology of the silica source (RHA and SDE). RHA acquired an irregular shape after the milling process, but the original rice husk ash sample had a honey-comb morphology with a cellular porous surface. On the other hand, different shapes characteristic of the diatomaceous shape can be identified in Figure 2b. Most of the particles look like tubes with small holes on the surface, which is the reason why SDE is used as a filter.
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Figure 4. SEM images- a) rice husk ash and d) spent diatomaceous earth 3.2.Geopolymeric binder characterization 7
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3.2.1. Setting time Figure 5 shows the initial and final setting time for each geopolymeric binder cured at 60ºC. Perna I, and Hanzlícek T., (2015), mention that additives and the mixing method partially modify the geopolymer properties and the setting time, thus in the present study it was found that also the reactivity of the silica source and the amount of water in the system also plays an important role.
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The FA/MK-SN binder, set faster than the other geopolymeric binders made with an alternative source of silica. Its initial and final setting time was 55 and 70 minutes, respectively. The FA/MK-RN and FA/MK-DN systems showed nearly the same initial and final setting times, with a difference of approximately 25 minutes between both materials. The initial and final setting time for FA/MK-RN 74 and 86 minutes, respectively, and for FA/MK-DN, 100 and 110 minutes, respectively.
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The SiO2/Al2O3 molar ratio of 4.4 and the Na2O/SiO2 molar ratio of 0.2 respectively was kept constant for all systems. When silica from sodium silicate was replaced by either RHA or SDE, the amount of NaOH was increased to adjust the Na2O/SiO2 molar ratio to 0.2. Thus, the hydroxyl ion concentration in the solution increased, which helped dissolve the silica from the alternative sources and to dissolve the aluminum and silica from FA and MK. This chemical reaction took longer time than the chemical reaction that took place in the FA/MK-SN system.
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Regarding to water requirements, FA/MK-SN required a lower liquid/solid ratio than FA/MKRN and FA/MK-DN. Both FA/MK-RN and FA/MK-DN had a liquid/solid ratio of 0.42, which is likely due to the honeycomb-like morphology and to the porous surface of the RHA and SDE, respectively (Detphan and Chindaprasirt et al. 2009, Tanakorn et al. 2013). This kind of morphology reduces workability and increases water demand, which may have contributed to the delay of the setting time of these geopolymer systems, compared with the initial and final time of the binder made with sodium silicate.
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Figure 5. Setting time measures for FA/MK-SN, FA/MK-RN and FA-MK-DN
3.2.2.
Mechanical, microstructure and physical analyses.
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The results provided in Figure 6 show the compressive strength of the specimens at 7, 28, 180 and 360 days. An interesting observation is that the compressive strength did not change significantly between 28 and 360 days for any of the systems. The highest compressive strength at 360 days was 75 MPa, which was achieved by the geopolymer made with commercial sodium silicate (FA/MK-SN). However, an average compressive strength value of ~ 38 MPa was achieved by FA/MK-RN and FA/MK-DN, which suggests them as suitable materials for civil engineering applications.
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Geopolymer binders made with RHA (FA/MK-RN) and SDE (FA/MK-DN) exhibited the half of the compressive strength compared to the reference system FA/MK-SN. This results suggest that the source of silica and its amorphous phase has an important influence on the mechanical development. Bernal et al. (2015) state that, the mechanism of activation of the precursor is dominated by the different rates of release of silicate from the different source of silica. For instance, Bernal et al. (2012) used RHA with a high amorphous silica content and silica fume (SF) as alternative source of silica, and they reached comparables mechanical strength values to those synthesized from commercial silicate solutions. Previous studies of geopolymers based on granulated blast furnace slag and fly ash using two types of RHA as silica source, with an amorphous silica content of 94.7 and 27.7%, showed differences in compressive strength of approximately 10 MPa and up to 35 MPa compared to activated sodium silicate geopolymer (Mejia et al, 2013)
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Figure 6. Compressive strength of the different alkaline systems FA/MK-SN, FA/MK.RN and FA/MK-DN at 7, 28, 180 and 360 days
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Both RHA and SDE have crystalline phases that are basically silica compounds such as quartz, trydimite and cristobalite which have low reactivity in alkaline solutions. At early and advanced ages (7-180 days), FA/MK-RN shown similar values of about 26 MPa. After 360 days, the material increased its strength by 10 MPa, reaching a final strength of 36 MPa. This behavior was totally different from that of FA/MK-DN, which only had a small increase in strength from 7 to 360 days (4 MPa) but its compressive strength at 7 days was 34 MPa. This result at early ages is related with the higher amorphous silica content of SDE. It is assumed that part of the crystalline silica from RHA dissolves over time to participe into geopolymerization (Duan et al. 2015, Rajamma et al. 2012). Trydimite and cristobalite from RHA appear to be more susceptible to being dissolved by an alkaline solution than quartz and cristobalite from SDE, then quartz is a
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non-reactive compound and does not contribute to the chemical reactions of geopolymer (Autef et al. 2012).
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The XRD spectra in Figure 7 show crystalline compounds in agreement to Autef et al. 2012, which states that all geopolymeric binders display peaks due to crystalline phases coming from the raw material (FA, MK, RHA and SDE), such as mullite (2Al2O3∙SiO2; pattern diffraction file (PDF) #00-015-0776), quartz (SiO2; PDF #00-005-0490), cristobalite (AMCSD, 0001629) and tridymite (AMCSD, 0000531). Both quartz polymorphs (trydimite and cristobalite) appeared in FA/MK-RN and just cristobalite in FA/MK-DN. An arising small hump is observed around 2θ= 22º and 2θ=33º for all geopolymeric systems at 28 (figure 7 a) and 360 (figure 7 b) days, indicating the presence of an amorphous phase. It is also worth noting that the binary system synthesized from sodium silicate as silica source presents a sharp increase of that hump, which point out, the higher is the reactive silica in the silica source, the amorphous gel formation is increased.
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Figure 7. XRD patterns for geopolymer systems. a) 28 days and b) 360 days
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Figure 8 shows SEM images of the three geopolymer matrices at 28 days. FA/MK-SN made with commercial sodium silicate has a dense and homogeneous matrix (Figure 8 a), which may have contributed to the high mechanical strength shown in figure 5. As it was explained before, the sodium silicate solution dissolves rapidly, bonding SiO2 and Al2O3 species from FA and MK that were previously attacked by OH- ions from NaOH. Porosity is rapidly filled with gel as soon as the liquid phase is able to reach the precursor particles (Al Bakri et al. 2011). Some embedded un-reacted and partially reacted fly ash particles were also found on the surface. It is expected that after 360 days SiO2 and Al2O3 species from the surface continue to dissolve and link into the geopolymer network, improving the compressive strength at advanced ages as was shown in Figure 5.
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Figure 8. SEM images at 28. a) FA/MK-SN, b) FA/MK-RN and c) FA/MK-DN
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He et al. (2013) and Kusbiantoro et al. (2012) stated that the incomplete dissolution of RHA and SDE results in a lower density of the Si–O–Si bonds in the geopolymer, affecting gel formation, material porosity and, mechanical properties. FA/MK-RN and FA/MK-DN both have a porous and inhomogeneous microstructure, as it is shown in Figures 8b and 8c. The compressive strength of ~36 MPa reached by the mixtures made with alternative silica, compared with 75 MPa from FA/MK-SN suggests that, there was a lower formation of reaction products.
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To complement the results above, density, absorption and voids were determined for all three systems, and are shown in Table 3. The values obtained are consistent with compressive strength and SEM micrographs. FA/MK-RN and FA/MK-DN exhibited similar microstructure and mechanical strength development. Both of them have lower density than FA/MK-SN, therefore greater absorption and voids percentage than FA/MK-SN. Considering the physical properties shown in Table 3, low bulk density and high percentage of voids specifically in those materials made from alternatives source of silica (RHA and SDE) are considered lightweight geopolymers. This classification is consistent with other studies where SDE and RHA were used (Detphan and Chindaprasirt et al. 2009, Tanakorn et al. 2013). The values obtained in this work range between 1320 and 1550 kg/m3 and are similar than those reported by Cioffi et al (2003) (1200-1600 kg/m3) and lower than those found by Andini et al (2008) (1480-1740 kg/m3) for geopolymers based on MK or FA respectively. Table 3. Density, absorption and voids
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Absorption (%)
Voids (%)
FA/MK-SN
1550
15.52
25.26
FA/MK-RN
1370
23.48
33.03
FA/MK-DN
1320
26.77
35.91
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4. Conclusions From the results of the research, it can be concluded that rice husk ash and spent diatomaceous earth offer a feasible alternative to conventional sodium silicate as a reactive silica source. The 11
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In terms of linking knowledge to action, challenges to bring this technology to market include a comprehensive study of the variability of rice husk ash and spent diatomaceous earth source materials and its effect in the final properties of the geopolymer pastes, mortars and concrete produced, in order to establish of tolerance limits on their chemical and physical properties to insure a consistent product quality. Another challenge includes the conditioning treatment that is necessary to make on the rice husk ash a spent diatomaceous earth before it is utilized as a source material for geopolymers, since feasibility and cost studies are required before large scale production. 5. Acknowledgments
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This study was funded under the Colombian Institute for the Development of Science, Technology, and Innovation COLCIENCIAS Project Hybricement (Contract N° 0638-2013). Authors would like to thanks to Louisiana Tech University, in which part of the experimental work was carried out as part of a research internship. Authors also thank GENSA Thermo electrical Company for providing the fly ash, La Esmeralda Colombian milling rice and SABMiller Colombian Brewery for providing the rice husk ash and spent diatomaceous earth, respectively.
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change in source of silica, from sodium silicate to rice husk ash and spent diatomaceous earth, influences the setting time, the mechanical behavior and physical properties. The setting time results show a possibility of using rice husk ash and spent diatomaceous earth as an additive to extend the setting time of the geopolymer based on FA/MK for a particular application. The alternative systems FA/MK-RN and FA/MK-DN had a compressive strength at 28 days of up to 27 and 36 MPa and at 360 days of 36 and 38 MPa respectively, suggesting that the material can be a cementitious binder comparable with Portland cement. Concerning to the physical properties, the porous particles from the rice husk ash and spent diatomaceous earth helped to decrease the bulk density and to increase the porosity of the geopolymer. These properties suggest the use of the binder as a lightweight material with high mechanical strength (~38 MPa) for the construction industry.
6. References Andini, S., Cioffi, R., Colangelo, F., Grieco, F., Montagnaro, F., Santoro, L., 2008. Coal fly ash as raw material for the manufacture of geopolymer-based products. Waste Manag. 28, 416–42. Al Bakri, A. M., Kamarudin, H., Bnhussain , M., Khairul, I., Mastura, W.I.W., 2011. Mechanism and Chemical Reaction of Fly Ash Geopolymer Cement- A Review. J Asian Sci Res. 1 (5), 247-253. Autef, A., Joussein, E., Gasgnier, G., Rossignol, S., 2012. Role of the silica source on the geopolymerization rate. J. of Non-Crystalline Solids. 358, 2886–2893. Bernal, S,A., Rodríguez, E,D., Mejía de Gutiérrez, R., Provis, J,L., 2015. Performance at high temperature of alkali-activated slag pastes produced with silica fume and rice husk ash based activators, Mater Const. 65, 318, e049.
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Bernal, S., Herfort, D., Skibsted, J., 2011. Hybrid binders based on alkali sulfate-activated Portland clinker and metakaolin, 13th International Congress on the Chemistry of Cement. 141147, Madrid, Spain. Bernal, S,A., Rodríguez, E, D., Mejía de Gutiérrez, R ., Provis, J,L., Delvasto, S., 2012. Activation of Metakaolin/Slag Blends Using Alkaline Solutions Based on Chemically Modified Silica Fume and Rice Husk Ash, Waste Biomass Valoriz. 3, 99–108. Chindaprasirt, P., Jaturapitakkul, C., Chalee, W., Rattanasak, U., 2009. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manag. 29, 539–543. Cioffi, R., Maffucci, L., Santoro, L., 2003. Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue. Resour Conserv Recycl. 40, 27–38. Detphan, S., and Chindaprasirt, P., 2009. Preparation of fly ash and rice husk ash geopolymer. Int J Min, Metall and Mater. 16. (6), 720-726. Duan, P., Yan, C., Zhou, W., Luo, W., Shen, C., 2015. An investigation of the microstructure and durability of a fluidized bed fly ash–metakaolin geopolymer after heat and acid exposure. Mater Des. 74, 125–137. Habert, G., d’Espinose de Lacaillerie, J, B., Roussel, N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. J Clean Prod. 19, 1229-1238. He, J., Jie, Y., Zhang, J., Yu, Y., Zhang, G., 2013. Synthesis and characterization of red mud and rice husk ash-based geopolymer composites. Cem Concr Compos. 37, 108–118. Ibrahim, S.,,and Selim, A, Q., 2012. Heat treatment of natural diatomite. Physicochem Probl Miner Process. 48(2), 413−424. Kim, M,S., Jun, Y., Lee, C., Oh, J,E., 2013. Use of CaO as an activator for producing a pricecompetitive non-cement structural binder using ground granulated blast furnace slag. Cem Concr Res. 54, 208–214. Kusbiantoro, A., Nuruddin, M, F., Shafiq, N., Qazi, S, A., 2012. The effect of microwave incinerated rice husk ash on the compressive and bond strength of fly ash based geopolymer concrete. Constr Build Mater. 36, 695–703. McLellan, B, C., Williams, R, P., Laya, J., van Riessen, A., Corder ,G, D., 2011. Costs and carbon emissions for geopolymer pastes in comparison to ordinary portland cement., J Clean Prod. 19, 1080-1090. Mejía, J,M., Mejía de Gutiérrez, R., Puertas, F., 2013, Rice husk ash as a source of silica in alkali-activated fly ash and granulated blast furnace slag systems. Mater Constr. 63, 311, 361375. Mejía, J, M, Rodríguez, E, D., Mejia de Gutierrez, R., 2014. Potential Utilization of a Low Quality-Fly Ash as an Aluminosilicate Source in the Production of Geopolymers. Ingeniería y Universidad. 18 (2), 309-327. Mejía, J, M., Rodríguez, E, D., Mejia de Gutierrez, R., Gallego, N., 2015, Preparation and characterization of a hybrid alkaline binder based on a fly ash with no commercial value. J. Clean Prod. 104, 346-352. Perna, I., Hanzlícek, T., 2015, The setting time of a clay-slag geopolymer matrix: the influence of blast-furnace-slag addition and the mixing method. J. Clean Prod. http://dx.doi.org/10.1016/j.jclepro.2015.05.069 Puertas, F.,Torres-Carrasco, M., 2014. Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterization. Cem Concr Res. 57, 95– 104.
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Rajamma, R., Labrincha, J, A., Ferreira V. M., 2012. Alkali activation of biomass fly ash– metakaolin blends. Fuel. 98, 265–271. Rivera, J., Mejia, J., Mejia de Gutierrez, R., Gordillo, M., 2014. Hybrid cement based on the alkali activation of by-products of coal. J. Constr. 13(2), 31 – 39. Tanakorn, P-N., Chindaprasirt, P., Sata, V., Sinsiri, T., 2013. High calcium fly ash geopolymer containing diatomite as additive. Indian J Eng Mater Sci. 20, 310-318. Tempest, B., Sanusi, O., Gergely, J., Ogunro, V., Weggel, D., 2009. Compressive strength and embodied energy optimization of fly ash based geopolymer concrete. World of Coal Ash Conference 2009 in Lexington, KY: USA, 1-17. Yang, K, H., Song, J, K., Song, K, I., 2013. Assessment of CO2 reduction of alkali-activated concrete. J. Clean Prod. 39, 265 -272.
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Highlights
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Rice husk ash and spent diatomaceous earth were used as a source of silica. Rice husk ash and spent diatomaceous earth delay the setting time of the geopolymer. Similar compressive strength were achieved in the geopolymer by using RHA and SDE. To use rice husk ash and spent diatomaceous earth reduce the geopolymer bulk density.
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• • • •
S0959-6526(16)00092-5
DOI:
10.1016/j.jclepro.2016.01.057
Reference:
JCLP 6645
To appear in:
Journal of Cleaner Production
Received Date: 3 August 2015 Revised Date:
28 November 2015
Accepted Date: 23 January 2016
Please cite this article as: Mejía JM, de Gutiérrez RM, Montes C, Rice husk ash and spent diatomaceous earth as a source of silica to fabricate a geopolymeric binary binder, Journal of Cleaner Production (2016), doi: 10.1016/j.jclepro.2016.01.057. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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RICE HUSK ASH AND SPENT DIATOMACEOUS EARTH AS A SOURCE OF SILICA TO FABRICATE A GEOPOLYMERIC BINARY BINDER
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*Johanna M. Mejía, Ruby Mejía de Gutiérreza, Carlos Montes b.
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Composites Materials Group (CENM), Universidad del Valle, Colombia
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Institute for Micromanufacturing, Louisiana Tech University, United States of America *[email protected]
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Abstract
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The goal of this research is to propose the use of industrial by-products as solid sources of silica in replacement of commercial sodium silicate (SS) for the manufacture of geopolymeric binary binders. The result of this approach is a binder with good mechanical properties and environmental benefits, which can be used in civil engineering applications such as the manufacturing of masonry elements.
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A Colombian fly ash (FA) and a commercial metakaolin (MK) were used as geopolymer precursors. Rice husk ash (RHA), spent diatomaceous earth (SDE) and commercial sodium silicate were used as sources of silica to adjust the SiO2/Al2O3 molar ratio. These silica sources were mixed with a sodium hydroxide (NaOH) solution. Three alkaline solutions were used to activate the binary system FA/MK; SN (sodium silicate and sodium hydroxide), RN (rice husk ash and sodium hydroxide) and DN (spent diatomaceous earth and sodium hydroxide). A FA/MK proportion of 70/30 by weight, and the respective SiO2/Al2O3 and Na2O/SiO2 molar ratios of 4.4 and 0.2 were kept constant for all experiments. Mechanical and microstructural analysis were conducted on the three systems, and compressive strength was measured at 28, 180 and 360 days. The reaction products were characterized by using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM).
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Results show that it was feasible to utilize RHA and SDE as a source of silica for geopolymerization. The use of these alternative silica sources delayed the initial and final setting time at 60°C of a FA/MK-based binary alkali activated geopolymer system. Regardless of the source of silica used, the mechanical strength of the resulting geopolymer binders did not show a significant change over time. The FA/MK-SN system exhibited a final compressive strength of 75 MPa at 360 days, and at the same age FA/MK-DN and FA/MK –RN both reached final compressive strength values of 38 MPa.
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Key words: silica source, metakaolin, fly ash, geopolymer, diatomaceous earth, rice husk ash.
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1. Introduction
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The coal combustion process generates waste materials such as fly ash and bottom ash. In Colombia, the main thermoelectric plants and some industrial boilers generate approximately 600 kt / y of fly ash, whose quality does not permit its use as an active addition in concrete. This requires disposal in landfills and generate environmental problems due to their mismanagement. The high content of alumina and silica make the fly ash a suitable material for being used as raw material in the production of simple and binary geopolymers (Andini et al. 2008, Chindaprasirt et al. 2009, Mejia et al. 2013, Mejia et al. 2015). The geopolymerization process helps to reduce the environmental impact, given the lower energy consumption, saving space required for the disposal of waste, low processing temperatures, at the same time develop materials with good mechanical and thermal properties with a broad spectrum of applications. According to McLellan et al. (2011) and Yang et al. (2013), geopolymers may have during their life cycle, environmental issues similar to those of Portland cement in terms of CO2 footprint and energy requirements. According to Tempest et al. (2009), the major part of the emissions and energy consumption of geopolymers can be attributed to the sodium silicate and sodium hydroxide activators. Sodium silicate, for example, is produced from natural resources (silicon oxide and sodium carbonate) at high temperatures (1300 ° C), so that their production is energy intensive and generates CO2 in the atmosphere. Habert G. et al. (2011) proposed interesting strategies to minimize the adverse effect of sodium silicate in the alkaline activation process. The first of these strategies is to minimize the SiO2/Al2O3 molar ratio in order to reduce the amount of sodium silicate added to the system. The second strategy is to use a mixture of ground blast furnace slag and activated clays as raw material, which will also result in a geopolymer with a low SiO2/Al2O3 ratio. As a contribution to improve the ecological properties of geopolymer, Bernal et al. (2015) and (2012), He et al. (2013), ), Mejia et al. (2013), Puertas and TorresCarrasco (2014) proposed to use a supplementary source of silica in alkali activated systems, such as rice husk ash or glass wastes to produce silicate solutions via chemical reaction with NaOH or KOH. Bernal et al. (2011) and Kim et al. (2013), suggested another way to avoid the use of sodium silicate by using CaO and alkali sulfates as a low cost alternative to classic alkali activators.
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According to the mentioned above, this study promote the use of a supplementary sources of silica instead sodium silicate to adjust the SiO2/Al2O3 molar ratio of a geopolymer based on fly ash (FA) and metakaolin (MK). The silica sources namely, spent diatomaceous earth (SDE) and a rice husk ash (RHA). Commercial diatomaceous earth (DE) comes from a single-celled algae named diatoms, when diatom algae die the silica shell or skeleton, also known as frustules, collects at the ocean floor, forming fossils. After millions of years, these fossilized cell walls become diatomaceous earth (DE) or diatomite. DE is composed from amorphous nano-silica particles, and consists of 87– 91% silicon dioxide (SiO2) and a small amount of alumina (Al2O3) and iron oxide (Fe2O3). Commercial DE because of its characteristics, such as porous structure, high silica content, low density, thermal resistance, is widely used as filter, catalytic support, biological support, functional filler, and adsorbent. (Ibrahim et al. 2012). In this study the DE was used in a brewery company, after it was reused several times, it eventually lost its properties and became a waste named spent diatomaceous earth (SDE). 2
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Nowadays it has been identified that decreasing the consumption of natural resources and improving the use of wastes and by-products as a raw material for several industrial process will highly contribute to the sustainable development and will benefit the environment by preserving natural resource, air, soil and earth for future generations. Thus the use of wastes and by-products such as RHA, SDE and FA among others as geopolymer feedstocks may help companies to produce high added value materials and low CO2 footprint. On the other hand, is being carrying out a solution for companies concerned to waste managment issues by including a new stage in their production process, recycling and transforming waste and by-products into a new material with a commercial value.
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The performance of silicate solutions prepared from these wastes (RHA and SDE) was compared to that of a commercial sodium silicate solution. Three alkaline solutions were produced in the laboratory to alkali activate a binary system based on FA/MK. Setting time, mechanical strength and microstructural development were evaluated. X-ray fluorescence (XRF), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed as characterization methods.
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RHA is an agro-industrial by-product generated during the combustion process of rice husk, this sustainable biomass fuel is used to produce electricity during the rice drying stage in a milling rice, it consists of approximately 40 wt.% cellulose, 30 wt.% lignin, and 20 wt.% silica (He et al. 2013).
2. Experimental Procedure
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2.1. Materials
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A Colombian fly ash (FA) from a thermoelectric power station, and a commercial metakaolin (MK) (Metamax) were used as geopolymeric precursors. The binary system was designed using a FA/MK proportion of 70/30, with SiO2/Al2O3 and Na2O/SiO2 molar ratios of 4.4 and 0.2 respectively.
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A commercial sodium silicate (SS) with 32.4 wt.% SiO2, 13.5 wt.% Na2O, and 54.1 wt.% H2O was used in the study. Rice husk ash was obtained directly from a Colombian rice milling factory, where rice husk was used as a source of energy in a combustor. The total silica content was 90.91% and 36% corresponds to amorphous silica. The spent diatomaceous earth (SDE) was obtained from a Colombian brewing company, where it was used as a filter in one of the beer 3
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production stages. The total amount of silica in SDE was 91.86 and 43% corresponds to amorphous silica.
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The FA, RHA and SDE were subjected to conditioning treatments. FA, had an initial particle size of 63.9 µm, it was ground in a ball mill during 90 minutes to reduce its particle size to 19.8 µm. The RHA with an initial particle size of 136 µm, first it was ground during 60 minutes in a ball mill, until its particle size was reduced to 28 µm. Then, its particle size was further reduced to 5 µm by an attrition mill. The spent diatomaceous earth (SDE) was modified by a calcination process, in order to remove the organic compounds (yeast). Thermal treatment was conducted at 400°C for 3 hours.
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Table 1 summarizes the chemical composition, loss of ignition (LOI) and physical properties of the source materials. The chemical analysis of the geopolymer precursors and the two sources of silica was determined by X-ray Florescence (XRF). It is worth noting that the fly ash contains 14% of residual carbon, which is usually considered as low quality, since this carbon content is outside ASTM C 618 requirements. However, results from previous studies (Mejia et al. (2015), Mejia et al. 2014, Rivera et al. 2014) have demonstrated that it is feasible the use of this high LOI fly ash as a geopolymer precursor. Table 1. Chemical analysis of source materials and physical properties
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FA
MK
53.7 52.4 21.5 44.3 4.5 0.47 0.8 0.02 1 1.92 1.4 0.15 0.6 0.02 0.6 0.18 0.5 0.07 0.3 0.31 0.2 0.01 14.8 1.10 Physical properties
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Chemical compounds (%) SiO2 Al2O3 Fe2O3 CaO TiO2 K2O SO3 MgO P2O3 Na2O Sr L.O.I
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Density (g/cm3) Surface área BET (m2/g) Particle size (µm)
RHA SDE
90.93 0.11 0.19 0.36 1.97 0.15 0.33 0.67 0.02 5.10
91.86 3.23 1.56 0.57 0.17 0.51 0.02 0.29 0.45 0.63 0.68 2.06
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Fly ash and metakaolin were used as a geopolymer precursors, since both are aluminosilicate materials with a vitreous phase that allows silica and alumina to be dissolved in alkaline environments. XRD pattern from FA (figure 1) shows more crystalline compounds, such as mullite (2Al2O3∙SiO2; pattern diffraction file (PDF) #00-015-0776), hematite (Fe2O3; PDF# 00001-1053), and quartz (SiO2; PDF #00-005-0490) in the fly ash compared to the metakaolin. MK has a broad halo between 20 and 35° 2θ, which represents the amorphous phase of the material, and some small crystalline peaks associated to mullite and quartz.
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Figure 1. XRD patterns of the raw material. a) Fly ash and b) Metakaolin 2.2. Alkaline solutions preparation and synthesis
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Three alkaline solutions were used to activate the binary system FA/MK; SN (sodium silicate and sodium hydroxide), RN (rice husk ash and sodium hydroxide) and DN (diatomaceous earth and sodium hydroxide). The solutions were prepared by mixing the source of silica with analytical grade sodium hydroxide (99% purity). The liquid/solid ratio was determined depending on the workability of the paste. The ratios were, 0.3 for the paste prepared with SN solution and 0.45 for the paste prepared with RN and DN solutions. The sodium silicate solution (SN) was mixed with the sodium hydroxide solution, and prepared the same day it was used. The RN and DN solutions were prepared by adding and dissolving either rice husk ash (RHA) or spent diatomaceous earth (SDE) in a sodium hydroxide solution. The last two solutions were kept in constant stirring during 24 hours. (Figure 2)
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Figure 2. Alkaline solutions made from alternative silica sources a) RN solution and b) DN solution Geopolymer pastes were prepared by mixing the solid precursors (FA/MK 70/30 by weight) with the alkaline solutions SN or RN or DN for 10 minutes in a Hobart Mixer. The slurry was cast inside cubic molds (2x2x2 cm). The specimens were oven cured for 24 hours at 60°C and 90% of relative humidity. After curing, the samples were kept at ambient temperature until they reached the testing age. Table 2 shows the three geopolymer systems with their respective work conditions: alkaline solutions, molar (SiO2/Al2O3 and NaO2/SiO2) and liquid/solid (l/s) ratios.
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Table 2. Geopolymer systems, alkaline solutions, molar and liquid/solid ratios
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Alkaline solution SS+NaOH solution RHA+NaOH solution SDE+NaOH solution
SiO2/Al2O3
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2.3. Techniques
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Setting time was determined using a Vicat needle as it is described in ASTM C191. Compressive strength testing was conducted at the ages of 28, 180 and 360 days, using a compression testing instrument (Universal INSTRON 3369), at a displacement speed of 1 mm/min. Density, absorption and voids were measured as described in ASTM C642. The microstructural characterization of the binary alkaline activated systems was carried out using instrumental techniques such as Scanning Electron Microscope (SEM), using a JEOL JSM6490LV microscope under a high vacuum (3x10-6 torr), fitted with an OXFORD INTRUMENTS 7573 INCA PentaFETx3 detector. The samples were covered with gold with a Denton VacuumDesk IV unit in order to increase the resolution of the microphotographs. X-ray diffraction (XRD), was conducted on a PanAnalytical X´Pert MRD diffractometer using a K∝ radiation, a copper X-ray tube, in a range 2θ of 5 to 60°.
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3. Experimental results and discussion
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3.1. Alternative sources of silica characterization
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Rice husk ash and spent diatomaceous earth (as shown in Table 1) are mainly composed of silica. The X-ray diffraction (XRD) shows the pattern of RHA and SDE (Figure 3) it exhibits an intense peak of tridymite (American Mineralogist Cristal Structure Database AMCSD, 0000531) and cristobalite (AMCSD, 0001629), both SiO2 polymorphs formed during the combustion process of the RHA. The absence of a halo between 15-30º 2θ, indicates the lack of amorphous phase. On the other hand SDE has a halo between 15-30º 2θ which represents the amorphous phase of the material. Also, a few intense peaks were found, which belong to crystalline phases such as hematite (Fe2O3; PDF# 00-001-1053), quartz (SiO2; PDF #00-005-0490) and cristobalite (AMCSD, 0001629).
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Figure 3. XRD patterns of Rice husk ash (RHA), and Spent diatomaceous earth (SDE)
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Figure 4 shows the particle morphology of the silica source (RHA and SDE). RHA acquired an irregular shape after the milling process, but the original rice husk ash sample had a honey-comb morphology with a cellular porous surface. On the other hand, different shapes characteristic of the diatomaceous shape can be identified in Figure 2b. Most of the particles look like tubes with small holes on the surface, which is the reason why SDE is used as a filter.
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Figure 4. SEM images- a) rice husk ash and d) spent diatomaceous earth 3.2.Geopolymeric binder characterization 7
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3.2.1. Setting time Figure 5 shows the initial and final setting time for each geopolymeric binder cured at 60ºC. Perna I, and Hanzlícek T., (2015), mention that additives and the mixing method partially modify the geopolymer properties and the setting time, thus in the present study it was found that also the reactivity of the silica source and the amount of water in the system also plays an important role.
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The FA/MK-SN binder, set faster than the other geopolymeric binders made with an alternative source of silica. Its initial and final setting time was 55 and 70 minutes, respectively. The FA/MK-RN and FA/MK-DN systems showed nearly the same initial and final setting times, with a difference of approximately 25 minutes between both materials. The initial and final setting time for FA/MK-RN 74 and 86 minutes, respectively, and for FA/MK-DN, 100 and 110 minutes, respectively.
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The SiO2/Al2O3 molar ratio of 4.4 and the Na2O/SiO2 molar ratio of 0.2 respectively was kept constant for all systems. When silica from sodium silicate was replaced by either RHA or SDE, the amount of NaOH was increased to adjust the Na2O/SiO2 molar ratio to 0.2. Thus, the hydroxyl ion concentration in the solution increased, which helped dissolve the silica from the alternative sources and to dissolve the aluminum and silica from FA and MK. This chemical reaction took longer time than the chemical reaction that took place in the FA/MK-SN system.
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Regarding to water requirements, FA/MK-SN required a lower liquid/solid ratio than FA/MKRN and FA/MK-DN. Both FA/MK-RN and FA/MK-DN had a liquid/solid ratio of 0.42, which is likely due to the honeycomb-like morphology and to the porous surface of the RHA and SDE, respectively (Detphan and Chindaprasirt et al. 2009, Tanakorn et al. 2013). This kind of morphology reduces workability and increases water demand, which may have contributed to the delay of the setting time of these geopolymer systems, compared with the initial and final time of the binder made with sodium silicate.
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Figure 5. Setting time measures for FA/MK-SN, FA/MK-RN and FA-MK-DN
3.2.2.
Mechanical, microstructure and physical analyses.
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The results provided in Figure 6 show the compressive strength of the specimens at 7, 28, 180 and 360 days. An interesting observation is that the compressive strength did not change significantly between 28 and 360 days for any of the systems. The highest compressive strength at 360 days was 75 MPa, which was achieved by the geopolymer made with commercial sodium silicate (FA/MK-SN). However, an average compressive strength value of ~ 38 MPa was achieved by FA/MK-RN and FA/MK-DN, which suggests them as suitable materials for civil engineering applications.
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Geopolymer binders made with RHA (FA/MK-RN) and SDE (FA/MK-DN) exhibited the half of the compressive strength compared to the reference system FA/MK-SN. This results suggest that the source of silica and its amorphous phase has an important influence on the mechanical development. Bernal et al. (2015) state that, the mechanism of activation of the precursor is dominated by the different rates of release of silicate from the different source of silica. For instance, Bernal et al. (2012) used RHA with a high amorphous silica content and silica fume (SF) as alternative source of silica, and they reached comparables mechanical strength values to those synthesized from commercial silicate solutions. Previous studies of geopolymers based on granulated blast furnace slag and fly ash using two types of RHA as silica source, with an amorphous silica content of 94.7 and 27.7%, showed differences in compressive strength of approximately 10 MPa and up to 35 MPa compared to activated sodium silicate geopolymer (Mejia et al, 2013)
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Figure 6. Compressive strength of the different alkaline systems FA/MK-SN, FA/MK.RN and FA/MK-DN at 7, 28, 180 and 360 days
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Both RHA and SDE have crystalline phases that are basically silica compounds such as quartz, trydimite and cristobalite which have low reactivity in alkaline solutions. At early and advanced ages (7-180 days), FA/MK-RN shown similar values of about 26 MPa. After 360 days, the material increased its strength by 10 MPa, reaching a final strength of 36 MPa. This behavior was totally different from that of FA/MK-DN, which only had a small increase in strength from 7 to 360 days (4 MPa) but its compressive strength at 7 days was 34 MPa. This result at early ages is related with the higher amorphous silica content of SDE. It is assumed that part of the crystalline silica from RHA dissolves over time to participe into geopolymerization (Duan et al. 2015, Rajamma et al. 2012). Trydimite and cristobalite from RHA appear to be more susceptible to being dissolved by an alkaline solution than quartz and cristobalite from SDE, then quartz is a
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non-reactive compound and does not contribute to the chemical reactions of geopolymer (Autef et al. 2012).
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The XRD spectra in Figure 7 show crystalline compounds in agreement to Autef et al. 2012, which states that all geopolymeric binders display peaks due to crystalline phases coming from the raw material (FA, MK, RHA and SDE), such as mullite (2Al2O3∙SiO2; pattern diffraction file (PDF) #00-015-0776), quartz (SiO2; PDF #00-005-0490), cristobalite (AMCSD, 0001629) and tridymite (AMCSD, 0000531). Both quartz polymorphs (trydimite and cristobalite) appeared in FA/MK-RN and just cristobalite in FA/MK-DN. An arising small hump is observed around 2θ= 22º and 2θ=33º for all geopolymeric systems at 28 (figure 7 a) and 360 (figure 7 b) days, indicating the presence of an amorphous phase. It is also worth noting that the binary system synthesized from sodium silicate as silica source presents a sharp increase of that hump, which point out, the higher is the reactive silica in the silica source, the amorphous gel formation is increased.
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Figure 7. XRD patterns for geopolymer systems. a) 28 days and b) 360 days
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Figure 8 shows SEM images of the three geopolymer matrices at 28 days. FA/MK-SN made with commercial sodium silicate has a dense and homogeneous matrix (Figure 8 a), which may have contributed to the high mechanical strength shown in figure 5. As it was explained before, the sodium silicate solution dissolves rapidly, bonding SiO2 and Al2O3 species from FA and MK that were previously attacked by OH- ions from NaOH. Porosity is rapidly filled with gel as soon as the liquid phase is able to reach the precursor particles (Al Bakri et al. 2011). Some embedded un-reacted and partially reacted fly ash particles were also found on the surface. It is expected that after 360 days SiO2 and Al2O3 species from the surface continue to dissolve and link into the geopolymer network, improving the compressive strength at advanced ages as was shown in Figure 5.
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Figure 8. SEM images at 28. a) FA/MK-SN, b) FA/MK-RN and c) FA/MK-DN
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He et al. (2013) and Kusbiantoro et al. (2012) stated that the incomplete dissolution of RHA and SDE results in a lower density of the Si–O–Si bonds in the geopolymer, affecting gel formation, material porosity and, mechanical properties. FA/MK-RN and FA/MK-DN both have a porous and inhomogeneous microstructure, as it is shown in Figures 8b and 8c. The compressive strength of ~36 MPa reached by the mixtures made with alternative silica, compared with 75 MPa from FA/MK-SN suggests that, there was a lower formation of reaction products.
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To complement the results above, density, absorption and voids were determined for all three systems, and are shown in Table 3. The values obtained are consistent with compressive strength and SEM micrographs. FA/MK-RN and FA/MK-DN exhibited similar microstructure and mechanical strength development. Both of them have lower density than FA/MK-SN, therefore greater absorption and voids percentage than FA/MK-SN. Considering the physical properties shown in Table 3, low bulk density and high percentage of voids specifically in those materials made from alternatives source of silica (RHA and SDE) are considered lightweight geopolymers. This classification is consistent with other studies where SDE and RHA were used (Detphan and Chindaprasirt et al. 2009, Tanakorn et al. 2013). The values obtained in this work range between 1320 and 1550 kg/m3 and are similar than those reported by Cioffi et al (2003) (1200-1600 kg/m3) and lower than those found by Andini et al (2008) (1480-1740 kg/m3) for geopolymers based on MK or FA respectively. Table 3. Density, absorption and voids
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System
Bulk Density (kg/m3)
Absorption (%)
Voids (%)
FA/MK-SN
1550
15.52
25.26
FA/MK-RN
1370
23.48
33.03
FA/MK-DN
1320
26.77
35.91
350 351 352 353 354
4. Conclusions From the results of the research, it can be concluded that rice husk ash and spent diatomaceous earth offer a feasible alternative to conventional sodium silicate as a reactive silica source. The 11
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In terms of linking knowledge to action, challenges to bring this technology to market include a comprehensive study of the variability of rice husk ash and spent diatomaceous earth source materials and its effect in the final properties of the geopolymer pastes, mortars and concrete produced, in order to establish of tolerance limits on their chemical and physical properties to insure a consistent product quality. Another challenge includes the conditioning treatment that is necessary to make on the rice husk ash a spent diatomaceous earth before it is utilized as a source material for geopolymers, since feasibility and cost studies are required before large scale production. 5. Acknowledgments
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This study was funded under the Colombian Institute for the Development of Science, Technology, and Innovation COLCIENCIAS Project Hybricement (Contract N° 0638-2013). Authors would like to thanks to Louisiana Tech University, in which part of the experimental work was carried out as part of a research internship. Authors also thank GENSA Thermo electrical Company for providing the fly ash, La Esmeralda Colombian milling rice and SABMiller Colombian Brewery for providing the rice husk ash and spent diatomaceous earth, respectively.
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change in source of silica, from sodium silicate to rice husk ash and spent diatomaceous earth, influences the setting time, the mechanical behavior and physical properties. The setting time results show a possibility of using rice husk ash and spent diatomaceous earth as an additive to extend the setting time of the geopolymer based on FA/MK for a particular application. The alternative systems FA/MK-RN and FA/MK-DN had a compressive strength at 28 days of up to 27 and 36 MPa and at 360 days of 36 and 38 MPa respectively, suggesting that the material can be a cementitious binder comparable with Portland cement. Concerning to the physical properties, the porous particles from the rice husk ash and spent diatomaceous earth helped to decrease the bulk density and to increase the porosity of the geopolymer. These properties suggest the use of the binder as a lightweight material with high mechanical strength (~38 MPa) for the construction industry.
6. References Andini, S., Cioffi, R., Colangelo, F., Grieco, F., Montagnaro, F., Santoro, L., 2008. Coal fly ash as raw material for the manufacture of geopolymer-based products. Waste Manag. 28, 416–42. Al Bakri, A. M., Kamarudin, H., Bnhussain , M., Khairul, I., Mastura, W.I.W., 2011. Mechanism and Chemical Reaction of Fly Ash Geopolymer Cement- A Review. J Asian Sci Res. 1 (5), 247-253. Autef, A., Joussein, E., Gasgnier, G., Rossignol, S., 2012. Role of the silica source on the geopolymerization rate. J. of Non-Crystalline Solids. 358, 2886–2893. Bernal, S,A., Rodríguez, E,D., Mejía de Gutiérrez, R., Provis, J,L., 2015. Performance at high temperature of alkali-activated slag pastes produced with silica fume and rice husk ash based activators, Mater Const. 65, 318, e049.
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Bernal, S., Herfort, D., Skibsted, J., 2011. Hybrid binders based on alkali sulfate-activated Portland clinker and metakaolin, 13th International Congress on the Chemistry of Cement. 141147, Madrid, Spain. Bernal, S,A., Rodríguez, E, D., Mejía de Gutiérrez, R ., Provis, J,L., Delvasto, S., 2012. Activation of Metakaolin/Slag Blends Using Alkaline Solutions Based on Chemically Modified Silica Fume and Rice Husk Ash, Waste Biomass Valoriz. 3, 99–108. Chindaprasirt, P., Jaturapitakkul, C., Chalee, W., Rattanasak, U., 2009. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manag. 29, 539–543. Cioffi, R., Maffucci, L., Santoro, L., 2003. Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue. Resour Conserv Recycl. 40, 27–38. Detphan, S., and Chindaprasirt, P., 2009. Preparation of fly ash and rice husk ash geopolymer. Int J Min, Metall and Mater. 16. (6), 720-726. Duan, P., Yan, C., Zhou, W., Luo, W., Shen, C., 2015. An investigation of the microstructure and durability of a fluidized bed fly ash–metakaolin geopolymer after heat and acid exposure. Mater Des. 74, 125–137. Habert, G., d’Espinose de Lacaillerie, J, B., Roussel, N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. J Clean Prod. 19, 1229-1238. He, J., Jie, Y., Zhang, J., Yu, Y., Zhang, G., 2013. Synthesis and characterization of red mud and rice husk ash-based geopolymer composites. Cem Concr Compos. 37, 108–118. Ibrahim, S.,,and Selim, A, Q., 2012. Heat treatment of natural diatomite. Physicochem Probl Miner Process. 48(2), 413−424. Kim, M,S., Jun, Y., Lee, C., Oh, J,E., 2013. Use of CaO as an activator for producing a pricecompetitive non-cement structural binder using ground granulated blast furnace slag. Cem Concr Res. 54, 208–214. Kusbiantoro, A., Nuruddin, M, F., Shafiq, N., Qazi, S, A., 2012. The effect of microwave incinerated rice husk ash on the compressive and bond strength of fly ash based geopolymer concrete. Constr Build Mater. 36, 695–703. McLellan, B, C., Williams, R, P., Laya, J., van Riessen, A., Corder ,G, D., 2011. Costs and carbon emissions for geopolymer pastes in comparison to ordinary portland cement., J Clean Prod. 19, 1080-1090. Mejía, J,M., Mejía de Gutiérrez, R., Puertas, F., 2013, Rice husk ash as a source of silica in alkali-activated fly ash and granulated blast furnace slag systems. Mater Constr. 63, 311, 361375. Mejía, J, M, Rodríguez, E, D., Mejia de Gutierrez, R., 2014. Potential Utilization of a Low Quality-Fly Ash as an Aluminosilicate Source in the Production of Geopolymers. Ingeniería y Universidad. 18 (2), 309-327. Mejía, J, M., Rodríguez, E, D., Mejia de Gutierrez, R., Gallego, N., 2015, Preparation and characterization of a hybrid alkaline binder based on a fly ash with no commercial value. J. Clean Prod. 104, 346-352. Perna, I., Hanzlícek, T., 2015, The setting time of a clay-slag geopolymer matrix: the influence of blast-furnace-slag addition and the mixing method. J. Clean Prod. http://dx.doi.org/10.1016/j.jclepro.2015.05.069 Puertas, F.,Torres-Carrasco, M., 2014. Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterization. Cem Concr Res. 57, 95– 104.
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Rajamma, R., Labrincha, J, A., Ferreira V. M., 2012. Alkali activation of biomass fly ash– metakaolin blends. Fuel. 98, 265–271. Rivera, J., Mejia, J., Mejia de Gutierrez, R., Gordillo, M., 2014. Hybrid cement based on the alkali activation of by-products of coal. J. Constr. 13(2), 31 – 39. Tanakorn, P-N., Chindaprasirt, P., Sata, V., Sinsiri, T., 2013. High calcium fly ash geopolymer containing diatomite as additive. Indian J Eng Mater Sci. 20, 310-318. Tempest, B., Sanusi, O., Gergely, J., Ogunro, V., Weggel, D., 2009. Compressive strength and embodied energy optimization of fly ash based geopolymer concrete. World of Coal Ash Conference 2009 in Lexington, KY: USA, 1-17. Yang, K, H., Song, J, K., Song, K, I., 2013. Assessment of CO2 reduction of alkali-activated concrete. J. Clean Prod. 39, 265 -272.
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Highlights
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Rice husk ash and spent diatomaceous earth were used as a source of silica. Rice husk ash and spent diatomaceous earth delay the setting time of the geopolymer. Similar compressive strength were achieved in the geopolymer by using RHA and SDE. To use rice husk ash and spent diatomaceous earth reduce the geopolymer bulk density.
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