ARTICLE pubs.acs.org/est
Use of Olive Biomass Fly Ash in the Preparation of Environmentally Friendly Mortars Manuel Cruz-Yusta,† Isabel Marmol,‡ Julian Morales,† and Luis Sanchez*,† †
Departamento de Química Inorganica, Facultad de Ciencias, Universidad de Cordoba, Campus de Rabanales, Edificio Marie Curie, 14071 Cordoba, Spain ‡ GRUPO PUMA, Avda. Agrupacion Cordoba No. 17, Cordoba, Spain ABSTRACT: The incorporation of fly ash from olive biomass (FAOB) combustion in cogeneration plants into cement based mortars was explored by analyzing the chemical composition, mineralogical phases, particle size, morphology, and IR spectra of the resulting material. Pozzolanic activity was detected and found to be related with the presence of calcium aluminum silicates phases. The preparation of new olive biomass fly ash content mortars is effective by replacing either CaCO3 filler or cement with FAOB. In fact, up to 10% of cement can be replaced without detracting from the mechanical properties of a mortar. This can provide an alternative way to manage the olive biomass fly ash as waste produced in thermal plants and reduce cement consumption in the building industry, and hence an economically and environmentally attractive choice.
’ INTRODUCTION Biomass is currently one of the major renewable energy sources in absolute terms. Thus, it accounts for more than 4% of the total energy consumption in the European Union (EU).1 The EU Commission has deemed an increase in the biomass share to 8.5% of the overall energy consumption essential.2 The primary sources for this type of energy are wastes from agricultural, liquid biofuel, and biogas production. Residual forest or agriculture biomass is used mainly for heat production in industrial processes at thermoelectric power plants and for heating in households. The current exhaustive use of natural resources and their management are posing serious problems that necessitate a prompt solution. Although biomass is widely used as an energy source much waste remains unprocessed. For example, only 50% of the amount of fly ash produced in thermal plants in Europe each year is reused.3 This waste has been used for a number of purposes including fertilization in agriculture,4 metal removal,5,6 atmospheric pollution control7 and in cement production.8 A new, interesting way of valorizing biomass fly ash—more specifically olive biomass—is by using it as a supplementary material in cement-based mortars, which are among the most widely used products in building and civil engineering works. The material construction industry could easily be one of the best targets of solid waste reconversion by virtue of the large amounts of raw materials it uses and the large volume of final products used in construction. Over the past decade, a variety of waste materials including construction rubble,9 tire rubber ash,10 blast furnace slag,11 silica fume,12 arsenical borogypsum,13 biological limestone,14 and granite sludge15 have been successfully used to r 2011 American Chemical Society
prepare mortars with proven advantages and benefits over existing alternatives. Most of the new binder/aggregate admixtures improve the mechanical properties of mortar and allow the proportion of cement added to be reduced. Some authors have examined the influence of fly ash addition on the mechanical strength of mortar and concrete via the characteristic properties, form, and size of their particles, and pozzolanic activity, with fly ash being the most common pozzolan material.16 20 Moreover, the binding properties of fly ash can be enhanced by mechanical or chemical activation.21,22 The use of fly ash in cement-based mortars is expected to provide significant environmental benefits, in addition to dispersing with the need for landfill disposal of ash products. Cement production is an environmentally relevant process accounting for 10% of all anthropogenic carbon dioxide emissions.23,24 Using fly ash as a replacement for cement in mortar formulations can thus help to minimize global CO2 emission. The use of olive biomass (more specifically the residues of the olive after pressing in the oil extraction) for cogeneration plants is an emerging activity for energy production. This activity in the Mediterranean countries produces vast amounts of inorganic wastes (fly ash and slag mainly). In this work, we focused on the valorization of fly ash from olive biomass (FAOB) combustion by incorporation as an alternative raw material into cement-based mortar formulations. The physicochemical and mineralogical characteristics of FAOB, and Received: March 23, 2011 Accepted: July 5, 2011 Revised: July 5, 2011 Published: July 05, 2011 6991
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Table 1. Percent Composition of the Mortars Studied mortar A
mortar B
mortar C
mortar D
mortar E
mortar F
mortar G
mortar H
mortar I
mortar J
quartz sand
65
65
65
65
65
65
65
65
65
65
dolomite sand
20
20
20
20
20
20
20
20
20
20
calcite
5.5
FAOB cement
9.5
4.4
3.3
2.2
1.1
1.1
2.2
3.3
4.4
9.5
9.5
9.5
9.5
the physical and mechanical properties of the new formulated mortars, were studied. This waste can replace ordinary limestone and cement in mortars, without detracting from their mechanical properties. The ensuing environmental benefits are analyzed.
5.5
5.5
5.5
5.5
5.5
0.95
1.9
2.85
3.8
9.5
8.55
7.6
6.65
5.7
Table 2. Chemical Analysis of the Cement and FAOB (%) cement
FAOB
loss of ignition sulfates, SO3
1.16 3.22
3.52 2.93
Al2O3
9.65
16.66
Fe2O3
3.43
6.50
Materials. The raw materials used to formulate the dry mortars
CaO
54.07
18.14
were Ordinary Portland Cement (CEM II/A-V 42.5 R), dolomite sand (CaMg(CO3)2; 0.4 2 mm in size), quartz sand as a source of SiO2 (0.2 0.6 mm) and calcite as filler (CaCO3; < 0.3 mm). Some were replaced partially or totally with FAOB ash to examine the effect on mortar strength. Fly-ash samples were collected from olive biomass incinerator electrical power plant of Nuestra Se~nora de Araceli Oleícola - El Tejar (Cordoba, Spain), in which the residues of the olive after pressing in the oil extraction—pulp and stones—were used as fuel. Characterization. The fly ash sample was analyzed as received. The chemical composition of this waste was determined by using different techniques. Alkali oxides, Na2O and K2O, were determined using atomic emission spectrometry while loss of ignition was analyzed by heating the sample in air at 975 ( 25 °C. Sulfates were determined by gravimetry, being the sulfate ions precipitated with barium chloride.25 CaO, MgO, Fe2O3, and Al2O3 were determined according to UNE EN 80230 standard.26 Calcium was determined by titration being Ca(II) ions precipitated as calcium oxalate in an ammonium oxalate solution. The precipitate was washed with hot water and dissolved with diluted sulfuric acid. Then the free oxalate ion was titrated with a KMnO4 standard solution. Mg (II) and Al(III) were determined by gravimetric analyses. The Mg (II) was precipitated as magnesium ammonium phosphate. The collected precipitate was calcined until constant weight, yielding magnesium pyrophosphate, from which the content of magnesium was calculated. The Al(III) was precipitated as Al(OH)3. Iron content was determined by titration with K2Cr2O7. The percentage of carbonate was determined using thermogravimetric analysis (TG) with a Setaram thermobalance (Setsys Evolution 16/18), heating from 25 to 1000 °C at a rate of 10 °C/min under ambient and inert conditions. Chloride content was determined by volumetry, by using a standard solution of AgNO3, according to UNE-EN 1744-1.27 Trace elements were determined by X-ray fluorescence analysis (XRF) using a Philips PW2404 X-ray spectrometer. The mineralogical characterization were done by X-ray diffraction (XRD) spectroscopy on a Siemens D5000 X-ray diffractometer using Cu KR radiation and the mineral phases identified by using the ICDD database (International Centre for Diffraction Data). Particle size distribution (PSD) was determined by laser diffraction, using Mastersizer 2000LF equipment from Malvern Instruments. Scanning electron microscopy (SEM) images were obtained with a Jeol JMS-6400 microscope. The FT-IR spectroscopy study was carried
MgO
1.41
10.00
free lime
1.03
0.26
SiO2
26.96
33.00
Na2O K2O
1.40
2.50 11.2
’ EXPERIMENTAL SECTION
out on an FOSS NIR System 6500 SII spectrophotometer. Pozzolanic activity was determined according to UNE EN 196-5.28 An amount of 20 g of test sample containing 80% of CEM-I and 20% of each pozzolan was mixed with 100 mL of distilled water. The samples thus obtained were allowed to stand in sealed plastic bottles that were placed in an oven at 40 °C for 8 days. Then the samples were vacuum filtered and allowed to cool to ambient temperature in sealed Buchner funnels. The filtrates were analyzed for [OH ] by titration against dilute HCl with Methyl Orange as indicator, and for [Ca2+] by adjusting the pH to 13 and titrating with EDTA in the presence of murexid as indicator. The results were plotted on a graph of [Ca2+] in CaO equivalents (mmol/L) on the y-axis versus [OH ], also in mmol/L on the x-axis. The solubility curve for Ca(OH)2 was then plotted and a control sample consisting of 100% CEM-I was compared to ensure that data points lay on the same saturation curve. Any test results lying below such a line were taken to indicate removal of Ca2+ from the solution and hence pozzolanic activity. On the other hand, any results lying on the line or above the line were taken to indicate the absence of pozzolanic activity. Specimen Preparation. The studied material consisted of masonry cement-based mortars produced by CEMKOSA (GRUPO PUMA).29 A reference mortar sample (Mortar A) was prepared from cement, quartz, calcite, and dolomite in a 9.5:65:5.5:20 weight proportion (Table 1). The reference mortar was classified as M5 according to UNE-EN 998-2,30 meaning that its compression strength after 28 days of curing exceeded 5 N/mm2. The composition of the Portland cement CEM II A/V 42.5 R used is shown in Table 2. Mortar samples were cast in 40 40 160 mm molds, using a water/binder ratio of 1.47, which produced normal consistency by flow table according to UNE-EN 1015-231 and acceptable workability. All samples were cured at 25 °C and 50 90% relative humidity (RH). Two types of mortars were prepared. Some were obtained simply by replacing calcite with ash and maintaining the cement, dolomite, and silica proportions (samples B F). 6992
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Figure 1. SEM images of biomass fly ash.
Figure 3. XRD pattern for FAOB.
Figure 2. FT-IR spectrum for FAOB as compared with that for a typical C type fly ash.
In others, cement was partially replaced with ash while maintaining the calcite, dolomite, and silica contents proportions (samples G J). Figure 4. Pozzolanic activity test result obtained for FAOB.
’ RESULTS AND DISCUSSION Fly Ash Olive Biomass Characterization. The major elements in our fly ashes were Si, Ca, and Al. Table 2 shows their main components, expressed as oxides. According to the ASTM C 618 norm,32 fly ash can be of the F type or C type. The FAOB composition was more similar to the C type; thus the total content of SiO2, Al2O3, and Fe2O3 exceeded 50% (about 56%) and the lime content was close to 20% (exactly 18.1%). On the other hand, the high proportions of Na2O and K2O are typical of biomass ashes33,34 and account for its use as fertilizer for olive trees. Carbonates and chlorides were present in 3.13 and 1.36% by mass, respectively. The amount of salt components in the FAOB does not preclude its use in commercial masonry mortars.30 Conversely, chromium (245 ppm) and copper (206 ppm) were detected as trace elements. Figure 1 shows scanning electron micrographs (SEMs) of FAOB powders. Their particle morphology is useful for understanding the physical properties of the ash. The microstructure of ash particles affects their binding and sorption properties.16,20 The morphology observed in the SEM images was similar to that of ashes from other sources. A significant fraction of particles was spherical and had a rough surface; others formed unevenly shaped aggregates. There morphological features are similar those of ash class C.35 Figure 2 compares the FT-IR spectrum for FAOB with that for a C type fly ash. As expected, the strongest bands were present in both spectra. One subtle difference was the presence of two weak
bands at 3640 and 1420 cm 1 due to portlandite and calcite, respectively, in the FAOB spectrum. The sharp bands in the 450/ 1300 cm 1 range are typical of calcium silico-aluminates phases, the ability of which to act as binders in mortar formulations is well-known.36 Figure 3 shows the XRD pattern for the FAOB sample. The predominant phases observed were quartz, sylvite, alkali sulfate, calcite, dolomite, hematite, and, also, some silicates (Mg2SiO4) and aluminates (Ca2Al2SiO7). The small deviation of the baseline from 20/40 2θ is typical of amorphous materials, which might be responsible for pozzolanic activity in the material.37 This activity was evaluated by using the accelerated method described in the Experimental Section. A low pozzolanic activity was confirmed by measuring the Ca2+ and OH concentrations in a solution containing CEM-I and GS wastes, which amounted to 2.2 and 64.9 mmol/L, respectively (Figure 4). The particle size distribution (PSD) of FAOB is shown compared with that of calcite filler and cement, Figure 5. Although unimodal distributions have been reported in some ashes,38 the PSD of FAOB, Figure 5a, was bimodal. Particle size ranged from 0.6 to 450 μm, with 26.3% ranging from 2 to 20 μm, and 68.7% from 20 to 300 μm. Such a high proportion of large particles may be due to the agglomeration observed in the SEM images (Figure 1). These aggregates allow the formation of elongated particles resulting and hence in larger diameters. The FAOB particle size is very different from that of the majority components, quartz and dolomite sand, in mortar (see Experimental Section). Figure 5b shows the particle size distribution of 6993
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Figure 7. Compression strength at 28 days for mortar with partial replacement of cement with FAOB.
Figure 5. Particle size distribution of (a) FAOB ash, (b) calcite filler, and (c) cement.
Figure 6. Compression strength at 28 days for mortars with partial replacement of calcite filler with FAOB.
the calcite filler, which was also bimodal, even though the lower size fraction as a whole was higher. Conversely, the distribution of the particles size for cement is show in Figure 5c. The distribution is multimodal with three main intervals, at 0.2 1 μm, 1 10, and the most populated 10 100 μm, being this material finest than FAOB. Considering that FAOB exhibits a PSD similar to that of calcite filler and pozzolanic activity, its incorporation in mortar will be studied by the gradual replacement of filler or cement. Valorization of Fly Ash Olive Biomass in Mortar. Figure 6 shows the compression strength values, at 28 days of curing, of mortars with partial replacement of calcite filler with FAOB. Using FAOB as filler increased the compression strength of the mortar, but no direct relationship between strength values and
FAOB content was found. In fact, complete replacement of the filler, F mortar, hardly altered the compression strength. This indicate that puzzolanic activity in the ash had little effect owing to the low activity and, especially, short curing time.39 On the other hand, the bulk density was similar in both raw materials (0.88 and 0.73 g/cm3 for CaCO3 and FAOB, respectively). These slight differences can be greater if the pristine filler is fully replaced. Because the grain size distribution of the aggregates is the most important attribute in relation to aggregate characteristics, an appropriate grain size distribution is required to ensure a high strength in the mortar.40 Considering this, the slight differences observed for the PSD (Figure 5) and bulk density may have an adverse effect on mechanical properties and account for the fact that mortar strength failed to increase by effect of FAOB completely replacing calcite in it. Once the effect of including FAOB as an admixture in the mortar formulation was confirmed, the compression strength of mortars in which cement was partially replaced with FAOB was measured (Figure 7). A continuous decrease in compressive strength with increase in FAOB content was observed. This loss of strength was to be expected since fly ash develops pozzolanic activity over long periods. Thus, Frías et al. using an accelerated reaction method found only 20% of lime to be fixed by fly ash at 28 days of curing.41 The commercial application of the M5 mortar used in this study requires fast hardening with the ability to reach values above 5 N/mm2 after 28 days of curing.30 Only the G formulation (with 10% of replacement) exhibited adequate resistance for commercial use. This behavior is attributed to the poor pozzolanic activity of this ash and the low percentage of small particles present in the substitute42 in comparison with cement. In this sense, the distribution of the particles size for cement is show in Figure 5c. The distribution is multimodal with three main intervals, at 0.2 1 μm, 1 10, and the most populated 10 100 μm, being this material finest than FAOB. Based on the previous results, cement can be effectively replaced with small amounts of FAOB. It would be interesting to increase the amount of cement to be replaced. A few years ago, heated calcium silico-aluminate phases were found to readily form cementitious phases.43 We followed this procedure to heat pristine FAOB at different temperatures for 4 h. The lowest temperature needed for cementitious phases such as Ca2MgSi2O7, KAlSiO4, or KAlSi2O6 to form was 950 °C. Using heated 6994
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Environmental Science & Technology FAOB to replace 20% of cement in the mortar formulation led to a compressive strength of 4.4 N/mm2, which is slightly higher than that of the H mortar. Thus, in spite of the high energetic process required for the chemical conversion, it was impossible to maintain appropriate strength values for the M5 mortar by replacing cement with FAOB in proportions above 10%. Replacing clinker with mineral components such as granulated blast furnace slag or fly ash in cement production is known to result in high savings in CO2.8 However, efficiently reducing the emission of greenhouse gases associated with the production of building materials requires not only improving cement production processes, but also optimizing its use in these materials. On this study, up to 10% of cement can in fact be replaced while maintaining the characteristics of M5 commercial mortar. Thus, using FAOB as cement replacement can be the starting point for preparing environmentally friendly mortars as this byproduct can be directly included in mortar formulations with no preconditioning to replace a substantial fraction of Portland Cement CEM II. This type of cement has a 57% market share and 24% of CO2 emissions associated to its production (729 kg per tonne of CEM II cement) in Europe.8 The new mortar with FAOB addition is seemingly thus a new environmentally friendly building material which enables valorization of fly ash byproduct and can help to mitigate CO2 emissions associated with cement production by reducing its use in the formulation. Also, its commercialization is feasible in terms of waste availability and customer acceptance. The FAOB studied here is produced at 15 plants located in Andalusia (Spain), which use around 5.8 106 olive biomass tonnes each year to generate electricity; this ensures the availability of vast amounts of fly ash for use in the production of building materials. Moreover, the studied mortar exhibits good workability, so it is bound to elicit customer satisfaction. In summary, a new way to manage olive biomass fly ash is here proposed through its incorporation as an alternative raw material into cement-based mortar formulations. Significant changes were not observed in the compression strength of the mortar when FAOB was used as filler. Even though FAOB exhibits pozzolanic activity, a continuous decrease in compressive strength was observed in mortars in which cement was partially replaced with this waste. Nevertheless, up to 10% of cement can in fact be replaced while maintaining the characteristics of the studied commercial mortar. Therefore, reducing the use of cement in building materials can help to mitigate the CO2 emissions associated with its production.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; phone: 34-957-218620; fax: 34-957-218621.
’ ACKNOWLEDGMENT This work was funded by Junta de Andalucía: Group FQM175 and P09-FQM-4764 project. We are grateful to Dr. Paya from Polytechnic University of Valencia providing a sample of C-type fly ash. ’ REFERENCES (1) International Energy Agency Statistic. Renewables Information; 2009; http://www.iea.org/publications/free_new_Desc.asp?PUBS_ID=2037.
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