Reuse of woody biomass fly ash in cement-based materials

Construction and Building Materials 76 (2015) 286–296

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Reuse of woody biomass fly ash in cement-based materials Mario Berra a,⇑, Teresa Mangialardi b, Antonio Evangelista Paolini b a b

Ricerca sul Sistema Energetico – RSE S.p.A., Via Rubattino, 54, 20134 Milan, Italy Department of Chemical Materials Environment Engineering, University of Rome ‘‘La Sapienza’’, Via Eudossiana, 18, 00184 Rome, Italy

h i g h l i g h t s  Physico-chemical and mineralogical characterisation of pure woody biomass fly ash (WBFA).  Treatments of WBFA to reduce detrimental effects on the technological properties of cement mixes.  Technological feasibility of WBFA reuse as partial cement replacement material in cement mixes.  Technological feasibility of WBFA reuse as a filler/partial sand replacement material in concrete.  Hydraulic and pozzolanic activity of WBFA.

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 9 October 2014 Accepted 26 November 2014

Keywords: Woody biomass fly ash Mineral admixture Filler Blended cement Concrete Workability Setting Compressive strength

a b s t r a c t The reuse of woody biomass fly ash (WBFA) as a mineral admixture or as a filler/partial sand replacement material in cementitious mixes was investigated. Three different WBFAs were used, two coming from virgin wood and one from treated wood combustion. The physical and chemical characteristics of these ashes and the technological properties (workability, setting, compressive strength) of cementitious mixes incorporating WBFA were evaluated. It was found that, in spite of the satisfactory technological properties exhibited by most related blended cements, the studied WBFAs did not meet the UNI EN 450-1 requirements for reuse as mineral admixtures, even if they were subjected to a preliminary water-washing treatment. The reuse of raw wood fly ash as a filler/partial sand replacement material was found to be satisfactory and possible for low-quality concrete. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The use of biomasses in place of traditional fuels represents a suitable way of reducing greenhouse gas emissions, in the general policy towards a highly energy-efficient, low-carbon economy. The most important biomasses are the residues from woodworking or forest activities, the wastes from farms and agro-business, the organic fraction of municipal solid wastes, and the plants deliberately grown for energetic purposes. They represent a high potential of burnable biomass and their fast increasing use in biomass-based thermal plants has called for the disposal problems associated with the ash production. According to the European Waste Catalogue and hazardous residues list [1], both bottom ash and fly ash coming from the

⇑ Corresponding author. E-mail addresses: [email protected] (M. Berra), teresa.mangialardi@ uniroma1.it (T. Mangialardi), [email protected] (A.E. Paolini). http://dx.doi.org/10.1016/j.conbuildmat.2014.11.052 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

combustion of untreated wood are classified as non-hazardous wastes. Woody biomass bottom ash may be reused as a building material for replacing granular material in geotechnical works, like road foundations [2]. Its application to agricultural or forests soils has also been proposed [3,4]. Reuse of Woody Biomass Fly Ash (WBFA) in agricultural and/or industrial applications could pose environmental problems related to higher content and higher leachability of heavy metals of this fly ash [5–8], as compared to bottom ash. Published work [7,8] has shown that certain types of WBFA do not meet the limit concentrations of heavy metals established by Dutch or Austrian regulations for reuse of biomass ashes as mineral fertilisers. Leaching tests on WBFA samples [6] have shown that there is a leachant pH range (below 7.5) where the release of heavy metals of particular environmental concern is above the limits for disposal of WBFA into non-hazardous waste landfills. This leaching behaviour could be incompatible with the reuse of WBFA in agricultural applications, while it would be compatible with a safe

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reuse of this ash in cement-based materials, as a mineral admixture (partial cement replacement material) and/or as a filler material. From the environmental point of view, the reuse of WBFA as a mineral admixture would produce several beneficial effects such as (1) a significant reduction of CO2 emission related to industrial cement production from traditional raw materials (limestone and clay), (2) the preservation of the natural resources involved in cement production, and (3) the solution of the environmental problem related to the WBFA disposal. The last two beneficial effects would also be attained by using WBFA as a filler in concrete. However, the reuse of WBFA as a partial cement replacement material is not allowed by ASTM C 618 [9] and UNI EN 450-1 [10], that are the current standards governing the use of fly ashes as mineral admixtures in concrete. Indeed, such regulations preclude the use of any material not derived from coal combustion. It is reasonable to think that a future extension of the current regulations to the reuse of fly ash from pure biomass combustion in concrete should be limited to those samples capable of meeting the physical and chemical requirements as specified in the ASTM C 618 or UNI EN 450-1 standards. This may be the reason for which the research on the reuse of biomass fly ashes in cement-based materials has been mostly focused on co-fired fly ashes (fly ashes coming from combustion of coal and biomass blends) and, only in minor part, on pure biomass fly ashes. At present, a conspicuous number of papers dealing with the reuse of woody biomass fly ashes in concrete or mortar is, however, available in the literature [11–42]. Most of these papers are focused on the reuse of WBFA as a partial cement replacement material, while only a few papers deal with the reuse of biomass fly ash as a filler [11,13,33,36]. Literature survey [27,30,33] has shown that the performance of woody biomass fly ash as mineral admixture is strongly dependent on its physico-chemical characteristics that, in turn, depend on the type of woody biomass and the type of combustion adopted in the thermal plant [43]. The content of unburned carbon and inorganic compounds in the WBFA samples could significantly affect the concrete properties (workability, setting, mechanical strength), as well as the presence of a considerable amount of heavy metals could pose severe limitations to reuse of this waste in cementitious mixes (excessive delay of cement hydration and/or excessive heavy metal leaching). The feasibility of using biomass fly ash as a filler has been recently demonstrated by Cuenca et al. [36] in their experimental study on the performance of self-compacting concrete. The present study was undertaken in order to achieve the following three objectives: (1) to evaluate the suitability of three different types of WBFA coming from the electrostatic precipitators of Italian wood burning plants as partial cement replacement materials; (2) to assess the feasibility of a preliminary washing treatment of WBFA with deionised water as a means of improving the chemical characteristics of such wastes; and (3) to evaluate the suitability of WBFA for reuse as a filler/partial sand replacement material in concrete. The first two objectives were accomplished by evaluating the physical and chemical characteristics of raw and washed fly ashes, and the technological properties (workability, setting, pozzolanic activity, and mechanical strength) of blended cements prepared at different replacement levels of Portland cement with unwashed or washed WBFA. The third objective was attained through measurements of workability and mechanical strength on concrete mixes containing one type of the studied fly ashes. This ash was selected on the basis of the results collected in the first phase of experimentation.

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2. Materials and methods 2.1. WBFA characterisation and washing treatment The three types of wood fly ash used in this study were labelled as WBFA1, WBFA2 and WBFA3. Samples WBFA1 and WBFA2 were obtained from the combustion of chestnut or poplar virgin wood chips, respectively. Sample WBFA3 resulted from the combustion of production scraps of treated wood. An aliquot of as received fly ashes (raw ashes) was first dried in a laboratory oven at 80 °C and then analysed for particle size distribution through dry sieving and laser diffraction technique. The real density of each ash (dry mass per unit solid volume excluding open porosity) was determined by a pycnometer for solids. The chemical composition was determined by X-ray fluorescence (XRF) for major elements and by atomic absorption spectrophotometry (AAS) for trace metals. The latter analysis was performed on the liquid phase resulting from nitric acid/hydrogen peroxide hot digestion of each sample of WBFA. The same solution was analysed also for sulphate and chloride contents by ionic chromatography (HPLC). The contents of unburned carbon and inorganic carbon of fly ash were evaluated by simultaneous thermo-gravimetric analysis/differential scanning calorimetry (TGA/DSC) with a thermo-analyser operating under static air at a heating rate of 10 °C/min over a temperature range from 25 °C to 1100 °C. The crystalline phases of each WBFA were identified by X-ray diffraction (XRD) analysis by using a Ni-filtered Cu Ka radiation (40 kV, 30 mA). Prior to use of WBFA as a mineral admixture in blended cements, each type of raw fly ash was dried and then sieved on 150 lm sieve. The retained portion on 150 lm sieve (30, 15 and 47 wt.% for WBFA1, WBFA2 and WBFA3, respectively) was ground to fineness below 45 lm and then mixed with the other portion of ash (combined WBFA). Thus, the retained portion of the combined WBFA did not exceed 30 wt.% when wet sieved on 45 lm sieve. An aliquot of each combined WBFA was also subjected to a two-step washing treatment with deionised water (liquid/solid ratio = 25 L/kg; contact time = 30 min/step). After each step, the suspension was filtered and the two filtrates were combined and analysed for chloride and sulphate ions by HPLC and for sodium, potassium and calcium ions by AAS. The overall weight loss of fly ash, L.O.W. (Loss On Washing), was also determined after drying of the solid residue to a constant weight at 80 °C. The combined WBFA after washing treatment was referred to as washed WBFA, while the combined WBFA not subjected to washing was indicated as unwashed WBFA.

2.2. Preparation and testing of blended cements The ash–cement blends (blended cements) were made using unwashed or washed WBFA and Portland cement, CEM I 42.5R, the latter being also referred to as PC. The chemical and mineralogical compositions of Portland cement are given in Table 1. The blended cements, also indicated as WBFA-PC cements, were made at Portland cement replacement levels of 15 and 30 wt.% and were identified with the wt.% content of WBFA. These cements were characterised for their pozzolanic activity and technological properties.

2.2.1. Pozzolanicity tests Each blended cement was tested for its pozzolanic activity by using the UNI EN 196-5 test method [44]. According to this method, the pozzolanic activity is assessed on an aqueous suspension of the test cement (water/cement ratio of 5 ml/g) by comparing the concentration of calcium hydroxide (expressed as CaO) in the aqueous solution in contact with the hydrated cement, after a fixed period of curing at 40 °C (14 days), with the concentration of calcium hydroxide capable of saturating a solution of the same alkalinity, the latter being expressed as OH ion concentration. The test cement is classified as a pozzolanic cement if the concentration of calcium hydroxide in the solution is lower than its saturation concentration.

2.2.2. Physical tests on fresh cement pastes The water demand of the blended cements was evaluated through workability measurements on cement pastes made with blended cement or Portland cement (control) as a binder and deionised water as mixing water. The water/binder weight ratio (w/b) was varied over the range from 0.45 to 0.75, and the workability of these pastes was evaluated through the use of a mini-slump test [45]. With this test, the mix workability was expressed in terms of the area Acp (cm2) of the paste collapsed without shocking from a truncated cone open at both ends (upper diameter = 20 mm; base diameter = 40 mm; height = 60 mm) and preliminarily filled with the test sample. The setting behaviour of blended and Portland cements was evaluated by measuring the initial and final setting times of pastes made with normal consistency (w/b ratio from 0.35 to 0.38, depending on the type and content of WBFA), according to UNI EN 196-3 test method (Vicat apparatus) [46].

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Table 1 Chemical and mineralogical composition of Portland cement. Oxide component

wt.% of dry solid

Heavy metal

mg/kg dry solid

Mineralogical constituents according to Bogue

wt.%

SiO2 Al2O3 Fe2O3 CaO Free CaO MgO SO3 SrO Mn2O3 P2O5 TiO2 Na2O K2O Na2Oeq.

20.03 5.04 2.44 63.03 0.91 1.35 3.36 0.21 0.15 0.13 0.22 0.30 1.12 1.04

Cd Cr Cu Ni Pb Zn

3 68 32 7 110 460

C3S C2S C3A C4AF

53.71 16.90 9.22 7.42

L.O.I. at 950 °C (%) Particle density (g/cm3)

2.40 3.15

2.2.3. Mechanical and chemical tests on hardened cement pastes and mortars The pozzolanic activity of the studied WBFAs was also evaluated through determinations of the portlandite (CH) content on cubic specimens (40 mm side) of blended or Portland cement pastes (w/b = 0.50) after different curing times (7– 90 days) at 20 °C and 100% RH. Each sample of paste was preliminarily pulverized, treated with acetone and ethyl ether in order to stop cement hydration, and then subjected to DSC–TGA analysis. The portlandite content (LCH) was evaluated from the weight loss on the TGA thermogram occurring between 400 and 500 °C (endothermic peak corresponding to CH dehydration on DSC thermogram). The effect of WBFA addition on the mechanical strength development of cementbased materials was evaluated on mortar specimens made with standard mix proportions (w/b = 0.50 by mass; sand/binder = 3.0 by mass; quartzitic sand with 0.1– 4.0 mm gradation). Compressive strength measurements were performed on three replicate cubic specimens (40 mm side) after curing times of 7, 14, 28, 60 and 90 days at 20 °C and 100% R.H. according to UNI EN 196-1 [47]. A constant displacement-rate compressive testing machine with a maximum load capacity of 60 kN was used. The coefficient of variation of strength measurements was about 3%.

Table 2 gives the compositions of the two concrete mixes used for the experiments. In this table, the dosages of the various ingredients are reported in terms of volume and weight of component per unit volume of concrete. The concrete mixes were prepared with a laboratory mixer according to the following procedure: mixing of the dry ingredients for 5 min, adding 70% of the total water and mixing for 3 min, adding the rest of the water containing superplasticiser and mixing for 2 min. The as prepared concrete mixes were first tested for the workability level according to UNI EN 12350-2 (slump test) [48]. Next, from each mix, cubic specimens (150 mm side) were cast and, after 1 day of curing at 20 °C and RH >95% within the moulds, they were demoulded and stored at 20 °C and 100% RH. After 7, 28 and 90 days of curing, three replicate specimens were tested for compressive strength according to UNI EN 12390-2 [49], and the strength values were averaged. The coefficient of variation of these measurements was about 7%. The density of concrete specimens was evaluated from weight measurements.

2.3. Preparation and testing of concrete mixes

3.1. Physical and chemical characteristics of as received WBFA samples

Concrete mixes were prepared using CEM I 42.5 R (Table 1), deionised water and a calcareous aggregate coming from an Italian quarry. This aggregate was available in both coarse and fine grain sizes, with a maximum particle size of 31.5 mm. The dry particle densities of fine aggregate (sand) and coarse aggregate (gravel) were 2610 and 2703 kg/m3, respectively. As explained later (Section 3.4), on the basis of the particle size distributions of the woody biomass fly ashes investigated, the raw WBFA2 sample (real density = 2350 kg/m3) was selected and used in concrete mixes as a filler material and, in almost equal part, as a partial sand replacement material (ash concrete). A commercial polyacrylic-type superplasticiser, in the form of aqueous solution (density = 1.1 g/cm3; dry active matter = 30 wt.%), was also used. A reference concrete mix was prepared by replacing the amount of fly ash used as filler with a conventional filler such as ground limestone (real density = 2740 kg/ m3), and by using fine calcareous aggregate as total sand. Two different dosages of superplasticiser (0.5% by weight of Portland cement for ash concrete and 0.75% for reference concrete) were needed to obtain mixes with the same workability level. The amount of total water needed for each concrete mix was calculated taking into account the water absorption of calcareous sand (1.61 wt.%), gravel (1.0 wt.%), and 0.063–0.6 mm WBFA2 ash (10.2 wt.%), and the water content of the commercial superplasticiser solution.

3.1.1. Particle size distribution and density Fig. 1 shows the particle size analyses of the as received, dried WBFA samples. These analyses revealed that raw WBFA2 was the most finely grained material, with 50 wt.% of particles having sizes (d50) of less than 60 lm, against d50 values of about 100 lm for raw WBFA1 and 140 lm for raw WBFA3. The average particle size was 135, 86 and 179 lm for raw WBFA1, WBFA2, and WBFA3, respectively. The maximum particle size was 450 lm for raw WBFA1 and about 600 lm for raw WBFA2 and WBFA3. The real density of the studied raw WBFAs ranged from 2.35 g/cm3 (WBFA2) to 2.76 g/cm3 (WBFA3), with the density of WBFA1 (2.40 g/cm3) being similar to that of WBFA2. These values of density were comparable to those exhibited by coal fly ashes [50] and were significantly lower than those of neat cements. Thus, the partial replacement of Portland cement with

3. Results and discussion

Table 2 Composition of the ash and reference concrete mixes. Component

Portland cement Free water Calcareous sand (0.063–4 mm) Wood fly ash sand (0.063–0.6 mm) Wood fly ash filler (0–0.063 mm) Ground limestone filler (0–0.063 mm) Calcareous gravel (4–31.5 mm) Superplasticiser (dry matter) Entrapped air Total

Concrete with WBFA2

Reference concrete

L/m3

kg/m3

L/m3

kg/m3

133 175 270 38 43 – 326

420 175 716 79 101 – 890 2.1 – 2383

133 175 308 – – 43 326

420 175 816 – – 118 890 3.2 – 2422

15 1000

15 1000

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phases, the XRD patterns were corrected for the contribution of amorphous phase to background line. Fig. 2(a)–(c) shows the corrected XRD patterns of the as received WBFA1, WBFA2 and WBFA3, respectively. The main crystalline phases identified in all fly ashes were lime, quartz and calcite, accompanied by minor amounts of gehlenite (2CaOAl2O3SiO2) and goethite (FeO(OH)). In the WBFA3 sample, anhydrite and lead oxides such as plattnerite (Pb2O3), minium (PbOPb2O3) and anglesite (PbSO4) were also identified.

3.2. Reuse of WBFA as partial cement replacement material

Fig. 1. Particle size distributions of the as received WBFA1, WBFA2, and WBFA3 samples.

wood fly ash would produce a significant reduction in the unit weight of cementitious material. 3.1.2. Chemical composition of as received WBFA samples Table 3 gives the chemical characteristics of the as received, dried WBFA samples. These data revealed that SiO2 (29.88–40.38%) and CaO (20. 76–33.13%) were the main oxide components in each raw WBFA. The fly ashes were also rich in alkalies (3.72–5.91% as Na2Oeq), especially in K2O (2.08–6.70%), and contained significant amounts of chlorides, sulphates and magnesium oxide. All fly ashes were characterised by a significant presence of heavy metals of particular environmental concern, such as As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn. In every raw WBFA, zinc was the predominant heavy metal (2274 mg/kg for WBFA1, 636 mg/kg for WBFA2, and 17470 mg/kg for WBFA3), while mercury was the heavy metal with the lowest concentration (<1.0 mg/kg in all fly ashes). The presence of heavy metals in raw WBFA1 and WBFA2 samples probably arose from adsorption of heavy metals by plants in soils containing phosphate fertiliser and sludge prior to woody biomass combustion. In the case of raw WBFA3 sample, that was characterised by the highest heavy metal content, significant metal contribution also resulted from industrial wood treatments. 3.1.3. Crystalline phases of raw WBFA samples When subjected to XRD analysis, all the as received fly ashes revealed the presence of a large amount of amorphous phase (above 45%). Therefore, for an easier identification of the crystalline

3.2.1. Compliance of raw WBFAs with physical and chemical requirements As anticipated in Section 1, the reuse of fly ash from pure biomass combustion as a mineral admixture in concrete where cementitious or pozzolanic action, or both, is desired, is not considered by the current international regulations. In USA, the current restrictions concern the fly ashes coming from combustion of fossil fuels (bituminous and sub-bituminous coal, peat, and lignite), and these restrictions (physical and chemical requirements) are specified in the ASTM C 618 standard. In Europe, the current restrictions concern both the fly ashes coming from fossil fuels and those resulting from the combustion of biomass and fossil fuel blends (co-firing), with a biomass (cocombustion material) content of not higher than 20 wt.%. These restrictions are specified in the UNI EN 450-1 standard. With regard to the fineness requirements, the amount of fly ash retained on 45 lm sieve shall not exceed 40 wt.% (Category N) or 12 wt.% (Category S) according to UNI EN 450-1, and 34 wt.% according to ASTM C 618. It can be noted from Fig. 1 that none of the as received WBFA samples was able to meet the ASTM or EN fineness requirements, the retained ash portion on 45 lm sieve being equal to 80, 61 and 77 wt.%. for WBFA1, WBFA2 and WBFA3, respectively. It follows that, without preliminary grinding, only a minor part of raw fly ash (23–39 wt.%, depending on the type of WBFA) could be used as a mineral admixture in blended cement formulations. Therefore, it was necessary to preliminarily grind the retained portion of each WBFA sample on 150 lm sieve and to combine this ground portion with the other ash portion, as already described in Section 2. Table 4 compares the chemical characteristics of the as received WBFA samples to the chemical requirements prescribed for reuse of fly ashes resulting from fossil fuels (ASTM C 618) or coal and co-combustion materials (UNI EN 450-1) as partial cement replacement materials.

Table 3 Chemical analyses of as received, dried WBFAs. Chemical species

CaO SiO2 Al2O3 Fe2O3 K2O Na2O Na2Oeq MgO MnO P2O5 SO3a Cl a

wt.% of dry solid

Chemical species

WBFA1

WBFA2

WBFA3

33.13 29.88 9.58 5.79 3.64 1.32 3.72 3.50 0.77 2.33 2.90 1.07

23.76 36.44 8.96 6.39 6.70 1.49 5.91 3.52 0.52 1.90 4.68 1.90

20.76 40.38 9.54 6.09 2.08 3.04 4.41 3.00 0.15 0.68 9.30 1.74

Acid soluble sulphates expressed as SO3.

mg/kg dry solid WBFA1

WBFA2

WBFA3

Unburned organic carbon Inorganic carbon As Cd Cr Cu Hg Ni Pb Zn

0.50 1.09 18 9.0 101 175 0.2 41 177 2274

0.40 0.75 15 7.6 18 48 0.3 50 39 636

0.59 0.44 45 60 124 920 0.4 102 5318 17470

L.O.I. at 975 °C (%) L.O.W. (%)

4.50 14.0

3.64 11.2

3.13 10.3

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In the present study, the contents of reactive SiO2 and CaO were not determined, so that these two parameters were not considered for the above comparison. It can be noted from Table 4 that, exception made for the loss on ignition (L.O.I.), the American specifications are less severe than the European prescriptions. In particular, ASTM C 618 does not restrict chloride, reactive silica, free and reactive calcium oxides, magnesium oxide, phosphate, and total alkalis contents. Moreover, the sulphate limit (5 wt.% for both classes C and F) is less than the maximum allowable (3 wt.%) by European regulation. All raw WBFA samples were found to fail the UNI EN 450–1 prescriptions. In particular, all fly ashes did not meet the prescription for the sum (% SiO2 + % Al2O3 + % Fe2O3) and free CaO. Furthermore, raw WBFA1 failed for chloride content, raw WBFA2 for the contents of chlorides, sulphates and total alkalis (acid-soluble alkalis determined according to UNI EN 196-2 [51]), and raw WBFA3 for chloride and sulphate contents. According to the ASTM requirements, only raw WBFA2 was found to be suitable for reuse as mineral admixture and it was classifiable as Class C fly ash. Raw WBFA1 was found to be unsuitable for the sum (%SiO2 + %Al2O3 + %Fe2O3), while raw WBFA3 (classifiable as Class C) failed for sulphate content. It must be recognised that the presence of a significant amount of water-soluble compounds, such as chlorides and alkalis, could promote the formation of a high porosity within the hardened cementitious mixes, thus penalising mechanical strength development and durability. High contents of water-soluble chlorides can also be deleterious for steel reinforced concrete, since they will promote the corrosion of iron reinforcing bars. Furthermore, high contents of available alkalis (i.e., the amount of alkalis released into the pore liquid of cementitious matrices) are responsible for the development of deleterious expansion associated with alkali-silica reaction (ASR) in concretes made with aggregates containing some alkali-reactive forms of silica and/or silicate. Deleterious expansive phenomena in concrete can also arise from very slow dissolution of significant amounts of sulphates or magnesium oxide, with subsequent precipitation of very expansive phases, such as ettringite (C3A3CaSO432H2O) or brucite (Mg(OH)2).

Fig. 2. X-ray diffraction patterns of as received WBFA1 (a), WBFA2 (b) and WBFA3 (c) samples.

3.2.2. Chemical characteristics of washed WBFA samples As shown in Table 4, the chemical species of raw WBFAs failing the ASTM or EN requirements were mostly water-soluble species, such as alkalis and chlorides, and less water-soluble species as sulphates.

Table 4 Comparison between the chemical characteristics of as received WBFA samples and the chemical requirements for reuse of fly ash as mineral admixtures (European and American Standards). Chemical characteristic (wt.%)

WBFA1

WBFA2

WBFA3

Chemical requirement UNI EN 450-1

Loss on ignition (L.O.I.) max. SiO2 + Al2O3 + Fe2O3 min. Reactive SiO2 min. Reactive CaO max. Free CaO max. MgO max. Chloride max. Sulphate (as SO3) max. Tot. alkalis (Na2Oeq)b max. Water-sol. phosphate as P2O5 max. Total phosphate as P2O5 max.

4.50 45.25 n.d. n.d. 2.40 3.50 1.07 2.90 3.72 0.003 2.33

n.d. = Not determined. a Category C (5 wt.% for Category A; 7 wt.% for Category B). b %Na2Oeq = %Na2O + %K2O0.66. c Acid-soluble alkalis (UNI EN 196-2). 1 Class F. 2 Class C.

3.64 51.79 n.d. n.d. 2.45 3.52 1.90 4.68 5.91 0.008 1.90

3.13 56.01 n.d. n.d. 2.25 3.00 1.74 9.30 4.41 0.006 0.68

a

9.0 70 25 10 1.5 4.0 0.10 3.0 5.0c 0.01 5.0

ASTM C618 6.0 701; 502 – – – – – 5.0 – – –

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The XRD patterns of the raw fly ashes (Fig. 2(a)–(c)) did not reveal the presence of highly water-soluble crystalline phases such as halite (NaCl) or sylvite (KCl). In the case of WBFA3, anhydrite was identified as a crystalline sulphate phase (Fig. 2(c)). A two-step washing treatment of WBFA with deionized water was used in order to reduce the contents of chlorides, sulphates and alkalis in each ash. Table 5 gives the chemical characteristics of the washed WBFA samples. The contents of calcium oxide, alkalis, sulphates and chlorides were obtained on the basis of the determinations of Na+, K+,  Ca+2, SO2 4 , and Cl ion concentrations in the filtrate resulting from the combination of the two steps of washing, and taking into account the overall weight loss, L.O.W., of each fly ash (Table 3). The sum (%SiO2 + %Al2O3 + %Fe2O3), as well as the MgO content of each washed fly ash was calculated using the data in Table 3 and assuming no dissolution of SiO2, Al2O3, MgO and Fe2O3 during the washing treatment [52]. Irrespective of the fly ash considered, high percentage releases of chlorides (74.1–93.6%) and total alkalis (63.3–84.7% as Na2Oeq) were always obtained. In contrast, sulphates were released to a minor extent (24.5–69.2% as SO3), especially in the case of WBFA2 (30.8%) and WBFA3 (24.5%). Relatively low calcium release was always observed (5.4–11.8% as CaO). In spite of the improved quality of all the washed fly ashes, washed WBFA3 still failed the ASTM specifications for sulphate content. On the other hand, washed WBFA1 resulted to be suitable for reuse as Class C. None of the washed fly ashes was able to meet the UNI EN prescriptions: WBFA1 failed only for the sum (SiO2 + Al2O3 + Fe2O3), while the other two ashes also failed for chlorides and sulphates. Thus, the two-step washing treatment adopted in this study proved to be insufficient for converting raw wood fly ash into a material conforming to UNI EN 450-1 requirements, for reuse as mineral admixture in concrete. In the light of the above findings, the subsequent study on the technological properties of blended cements was to be limited to the blends prepared with washed WBFA1. However, this study was also extended to all unwashed fly ashes (WBFA1, WBFA2 and WBFA3) in order to evaluate eventual detrimental effects of such ashes on the technological properties of cement pastes and/ or mortars, as a consequence of their adverse compositional characteristics. 3.3. Technological properties of blended cements 3.3.1. Workability of cement pastes Fig. 3(a) and (b) shows the results of the mini-slump tests on the pastes made with blended or Portland cements. In these figures, the area Acp of the collapsed sample is plotted as a function of the w/b ratio for each type of paste investigated. In the case of no paste collapse, the value of Acp was about 12 cm2. As shown in Fig. 3(a), at a fixed w/b ratio, the partial replacement of Portland cement with unwashed WBFA1 decreased the mix workability (Acp reduction), and this effect was more pronounced at the higher ash content (30 wt.%). As a result, at a fixed

Table 5 Chemical characteristics of washed WBFA samples. Chemical species (wt.%)

WBFA1

WBFA2

WBFA3

CaO Chlorides Sulphates as SO3 Total alkalis as Na2Oeq MgO SiO2 + Al2O3 + Fe2O3

33.97 0.08 1.04 0.66 4.07 52.62

25.32 0.47 3.65 2.12 3.96 58.32

21.03 0.50 7.80 1.80 3.34 62.43

291

Fig. 3. Effect of WBFA addition on the workability of cement pastes with different w/b ratios (mini-slump tests).

workability level, the water demand of unwashed WBFA1-PC blends was found to be higher than that of Portland cement. Over the range of Acp values from 40 to 100 cm2, water demands relative to PC of 119–120% and 122–129% were calculated for the blended cements made with 15 and 30 wt.% WBFA1, respectively. This increased water demand agreed with those reported by other researchers, according to which the lower workability of WBFAPC blends was attributable to the irregular shape and the higher specific surface area of porous wood fly ash particles, in comparison to PC particles [15,19,21,25,32,35,39]. The use of washed WBFA1 in place of unwashed WBFA1 increased the water demand of blended cements relative to PC to a minor extent (102–104%), and this increment was little affected by the dose level of washed WBFA1. The water demand of washed WBFA1-PC blends was also lower than the maximum water requirement established by ASTM C 618 for mineral admixtures (105–115%, depending on the class of mineral admixture). These data evidenced a beneficial effect of the water-washing treatment of WBFA1 sample on mix workability. This result was in contrast with what was reported in our previous paper dealing with the water-washing treatment of municipal solid waste incinerator (MSWI) fly ashes [52]. For such wastes, the washing treatment was found to reduce the average size of the ash particles and to increase the porosity (mesoporosity) and the specific surface area of the ash. These physical modifications produced an increase in the water demand of the washed ash that was more pronounced for the MSWI fly ashes characterised by higher L.O.W. values (20–30 wt.% ash dissolution). On the other hand, the washing treatment was also able to remove the amount of floating carbon contained in raw fly ash, thus eliminating the water demand associated to this material. Therefore, the observed beneficial effect of the water-washing treatment of WBFA1 sample on cement mix workability (Fig. 3(a)) could be related to a modification of the chemical

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composition of this fly ash rather than to changes in its physical properties. As shown in Fig. 3(b), the use of unwashed WBFA2 or WBFA3 increased the mix workability, thus reducing the water demand of the blended cements relative to PC (88–98%). In particular, the water demand reduced with increasing ash content and, at a fixed ash dose level, this reduction was more pronounced for WBFA3-PC blends. With respect to unwashed WBFA1-PC pastes, the observed higher workability of the cement pastes containing unwashed WBFA2 or WBFA3 could be related to the lower values of L.O.I. (lower unburned carbon content) and of L.O.W. (lower ash dissolution) exhibited by the last two fly ashes (Table 3). The conflicting data of Fig. 3(a) and (b) proved the great variability in the flow properties of blended cements made with different types of wood fly ash. On the other hand, contradictory results have also been reported in the literature about the effect of woody biomass fly ash on the workability of mortar or concrete. In particular, at cement replacement levels of 15–30 wt.%, wood fly ash was found to significantly reduce [15,16,21] or markedly increase the slump of concrete [34,40]. No change in workability was also reported in the case of mortar samples containing 10% or 20 wt.% wood fly ash [41].

3.3.2. Setting of cement pastes Table 6 gives the initial (ti) and final (tf) setting times of blended or Portland cement pastes with normal consistency. With respect to the control mix (ti = 290 min; tf = 450 min), a little retarding effect on setting was produced by partially replacing Portland cement with 15 or 30 wt.% unwashed WBFA1, and this effect was more evident for initial setting. No substantial change in setting behaviour was detected when washed WBFA1 was used in place of unwashed WBFA1 at a dose level of 15 wt.%. In contrast, the use of washed WBFA1 at a dose level of 30 wt.% significantly accelerated the initial and final setting in comparison to PC paste. The use of unwashed WBFA2 or WBFA3 at a dose level of 15 wt.% only produced a moderate delay in final setting. Conversely, a strong delay of both initial and final setting was recorded for a dose level of 30 wt.%, and this effect was more pronounced with the use of WBFA3 (delays of 330 and 510 min for initial and final setting). The observed highest retarding effect of WBFA3 could be ascribed to its highest contents of sulphates and heavy metals (particularly, Zn, Pb and Cu) as compared to WBFA2 and WBFA1 samples. Exception made for the blended cements made with 30 wt.% of unwashed WBFA2 or WBFA3, the delay of initial setting observed for all the other blended cements made with unwashed fly ashes was less than 120 min (maximum delay allowed by UNI EN 4501 for a cement paste made with 25 wt.% fly ash). A moderate prolongation of the setting time could also be considered a desirable property of blended cements when longer times are required in which the concrete mix has to be workable.

3.3.3. Compressive strength of mortar specimens and activity index of WBFAs Fig. 4 shows the compressive strength development of the mortar specimens prepared with unwashed or washed WBFA1-PC blends (ash dose levels of 15 wt.% and 30 wt.%; w/b = 0.50) and with Portland cement, when these specimens were cured at 20 °C and 100% R.H. Over the whole range of curing times investigated (7–90 days), the compressive strengths of the mortar specimens containing unwashed or washed WBFA1 were always lower than those exhibited by the control mix at the same curing time. The use of washed WBFA1 in place of unwashed WBFA1 reduced to a greater extent the compressive strength of control mortars when the curing time was less than 40–45 days. At longer curing times (60–90 days), an opposite result was observed, especially for the ash dose level of 30 wt.%. However, the differences between the compressive strengths of mortars containing washed or unwashed WBFA1 were clearly evident (well above the variation coefficient value) only when a dose level of 30 wt.% and a curing time of 90 days were considered. Fig. 5(a) and (b) shows the effect of increasing the ash content on the 28-day or 90-day compressive strengths of the mortar specimens made with unwashed WBFA2-PC or unwashed WBFA3-PC cements. In these figures, the changes of compressive strength with ash content for unwashed WBFA1-PC and washed WBFA1-PC cement mortars are also reported. Irrespective of the curing time and ash content considered, the highest strength reductions relative to PC mortars (0% ash content) were always observed for the mortars containing unwashed WBFA3. For these specimens, the compressive strength reduced almost linearly with ash content, and percentage strength reductions of about 60% at 28 or 90 days were recorded for a dose level of 30 wt.%. For the mortars containing unwashed WBFA2, strength reductions of 27–30% were observed. The lowest strength reductions were found for the mortars containing unwashed or washed WBFA1 (percentage strength reductions of 13–15% for unwashed ash and 3–18% for washed ash). It was noteworthy that, especially at longer curing times (90 days), the compressive strength of the mortars containing washed WBFA1, unwashed WBFA1 or WBFA2 only slightly reduced as the ash content was increased from 15 to 30 wt.% (Fig. 5(b)). This could be ascribed to the capability of such fly ashes of contributing to development of mechanical strength as a result of their pozzolanic and/or hydraulic activity.

Table 6 Setting times of cement pastes. Mix

Initial setting time (min)

Final setting time (min)

PC (control) 15% Unwashed WFBA1 30% Unwashed WBFA1 15% Washed WBFA1 30% Washed WBFA1 15% Unwashed WBFA2 30% Unwashed WBFA2 15% Unwashed WBFA3 30% Unwashed WBFA3

290 360 350 375 230 290 500 270 620

450 460 490 440 415 520 800 650 960

Fig. 4. Development of compressive strength for the mortars containing unwashed or washed WBFA1, in comparison with the control mortar.

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3.3.4. Hydrating behaviour of blended cements In order to detect an eventual pozzolanic activity by the WBFAs selected, pozzolanicity tests on WBFA-PC blends were performed according to UNI EN 196-5. Moreover, the portlandite (CH) content of hydrated pastes made with blended or Portland cement was determined after different times (7–90 days) of curing at 20 °C and 100% RH. (a) Pozzolanicity test results

Fig. 5. Effect of WBFA content on the compressive strength of mortars cured for 28 days (a) or 90 days (b) at 20 °C and 100% RH.

Compressive strength reduction of mortars or concretes containing WBFA have mostly been reported in the literature [30,33]. However, in some cases, no strength variation [41] or strength increase [38] was found. In order to assess the behaviour of the studied WBFA samples, their activity index was determined as specified in the UNI EN 450-1 standard for coal or co-combustion fly ashes. The activity index is defined by this European Standard as the ratio between the compressive strength of the standard mortar prepared with 75 wt.% Portland cement – 25 wt.% fly ash and the compressive strength of standard mortar prepared with 100 wt.% Portland cement (control mortar). A fly ash sample is suitable for use in concrete if an activity index of not less than 0.75 at 28 days and of not less than 0.85 at 90 days is obtained. Using the curves of Fig. 5(a) and (b), the 28-day and 90-day compressive strengths of the mortars containing 25 wt.% fly ash were obtained by interpolation of the experimental data, and the activity index values at 28 and 90 days were then calculated for each WBFA. It was found that only washed WBFA1 and unwashed WBFA1 (activity indexes of 0.85 at 28 days for both ashes, and 0.94 and 0.86 at 90 days) were able to go beyond the required activity indexes, with the highest index value being obtained for washed WBFA1 at 90 days. The activity indexes of unwashed WBFA2 (0.71 at 28 days and 0.79 at 90 days) were slightly lower than the corresponding limit values. The results obtained for unwashed WBFA3 (activity indexes of 0.49 at 28 days and 0.50 at 90 days) evidenced the inadequacy of this ash for reuse in cement-based materials. This was attributable not only to the lack in pozzolanic and/or hydraulic activity of WBFA3 but also to its strong retarding effect on cement hydration, as suggested by the setting data of Table 6. Therefore, the subsequent investigation on the hydrating behaviour of blended cements was limited to the blends made with unwashed and washed WBFA1, and unwashed WBFA2.

Fig. 6 shows the results of the pozzolanicity tests for the blended cements investigated, together with the test result for the control (Portland cement). As can be noted, all the points representing the blended cements were placed above the curve of the calcium hydroxide solubility, as the Portland cement point did. This means that none of the studied WBFAs had sufficient pozzolanic activity, at least for the dose levels of ash investigated (15 and 30 wt.%). This result could be ascribed to an insufficient content of the three ‘‘pozzolanic oxides’’, SiO2, Al2O3 and Fe2O3 in the studied fly ashes. However, using the pozzolanicity test (UNI EN 196-5), Rajamma et al. [26] reported a pozzolanic behaviour for a wood fly ash designated as F1 and characterised by a sum of the three ‘‘pozzolanic oxides’’ (52.9%) comparable to that of unwashed WBFA2 (51.8%) (Table 4). But, fly ash F1 had a much lower CaO content (11.4%) as compared to WBFA2 (23.76%) (Table 3). The different behaviour of the two ashes could be explained considering that: (1) the pozzolanic activity of wood fly ash is not directly proportional to the sum of the three ‘‘pozzolanic oxides’’ as determined by the chemical analysis but to the amount of these oxides in the amorphous phase of the fly ash, and (2) the pozzolanicity test UNI EN 196-5 could be inadequate for testing pozzolanic materials rich in CaO. (b) Portlandite contents of hardened cement pastes Table 7 gives the portlandite contents (LCH) of hydrated cement pastes, determined from the TGA/DSC thermograms and expressed as g CH/100 g anhydrous binder (Portland or blended cement). In this table, the Portland cement contribution (L0 CH) to portlandite content of blended cement pastes is also reported. The L0 CH values were calculated assuming an inert behaviour of WBFA. At low curing times (up to 14 days), the portlandite content of blended cement pastes was found to be higher than the corresponding L0 CH value. These results suggested an early contribution of fly ash to portlandite content of cement pastes, as a result of the hydration reaction of lime coming from unwashed WBFA1 and WBFA2 (XRD patterns of Fig. 2(a) and (b)) or directly from washed WBFA1. Indeed, the XRD pattern of washed fly ash (not reported here) showed the presence of portlandite and the disappearance of lime contained in raw WBFA1. In the case of washed WBFA1PC blends, the increased CH content relative to PC paste could also be due to a little acceleration of cement hydration, as suggested by the setting data (Table 6). At longer curing times (28–90 days), the LCH values of the blended cement pastes were found to be lower than the corresponding L0 CH values. However, the CH consumption was relatively low and did not significantly increase with increasing ash content. These data were indicative of a low pozzolanic activity of the studied wood fly ashes, thus indicating that the negative results of the pozzolanicity test (Fig. 6) were principally attributable to an insufficient amount of the ‘‘pozzolanic oxides’’ in the amorphous phase of the studied WBFAs. Therefore, the observed little reduction in compressive strength of the mortars containing unwashed or washed WBFA1, or unwashed WBFA2, with increasing ash content (Fig. 5(a) and (b))

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Fig. 6. Results of pozzolanicity tests on the blended cements investigated and on Portland cement as control.

Table 7 Portlandite contents of hydrated cement pastes. L0 CH

Time (days)

LCH (g CH/100 g anhydrous binder) PC

15% Unwashed WBFA1

30% Unwashed WBFA1

15% Washed WBFA1

30% Washed WBFA1

15% Unwashed WBFA2

30% Unwashed WBFA2

15% WBFA

30% WBFA

7 14 28 90

20.7 21.3 22.7 25.7

18.8 18.2 15.0 14.5

17.5 16.4 14.0 13.6

19.1 18.0 14.9 14.0

18.0 17.5 15.0 13.6

19.0 18.7 15.0 13.8

18.3 17.8 14.7 14.0

17.6 18.1 19.3 21.8

14.5 14.9 15.9 18.0

could be ascribed to the capability of such ashes of developing hydraulic reactions and, only to a minor extent, to their pozzolanic activity. This interpretation of the WBFA behaviour was supported by the values of the hydraulic index, K3, calculated on the basis of the chemical composition of the studied fly ashes. This hydraulic index is defined as (%CaO + %MgO + %Al2O3)/(%SiO2) and represents one of the several chemical hydraulic indexes proposed as a quality criterion for reuse of ground granulated blast furnace slag as a partial replacement of Portland cement [53]. Values of K3 above 1.0 are indicative of good hydraulic properties. On the basis of the data in Tables 3 and 5, K3 values of 1.55, 1.42 and 0.995 were calculated for unwashed WBFA1, washed WBFA1 and unwashed WBFA2, respectively. If this index was also calculated for unwashed WBFA3, a K3 value of 0.82 was obtained, indicating low hydraulic activity for this ash. The last prediction agreed with both the experimental data of Fig. 5(a) and (b) and the values of the activity index obtained for WBFA3 according to UNI EN 450-1 procedure (Section 3.3.3). 3.4. Reuse of WBFA as filler/partial sand replacement material in concrete In spite of the good technological properties evidenced by the blended cements made with washed or unwashed WBFA1 and, to a minor extent, with unwashed WBFA2, and the related beneficial effects on the environment, the reuse of such WBFAs as mineral admixtures in blended cement formulations would be precluded on the basis of their physical and chemical characteristics (Tables 3 and 5) and the requirements specified in the ASTM C 618 or UNI EN 450-1 standards (Table 4). This is also taking into account the excessive costs needed for the preliminary treatments

of such fly ashes (ash grinding and washing) as well as for the treatment of the resulting wastewater, in order to remove the amount of heavy metals released from fly ash during the washing step. According to UNI EN 12620 [54], the studied raw WBFAs, especially WBFA1 and WBFA2, could be used in concrete mixes as a partial sand replacement material and, in part, as a filler for unreinforced concrete and no requirements category (e.g., ASNR for acidsoluble sulphates). The contribution of each ash as a filler was evaluated from the data in Fig. 1 as the percent cumulative passing on 63 lm sieve. This contribution resulted to be about 27, 53 and 30 wt.% for dried raw WBFA1, WBFA2, and WBFA3, respectively. Considering the highest contribution given by raw WBFA2 as a filler, this fly ash was selected for the experimentation on concrete mixes. As evidenced by Table 2, the dose level of superplasticiser needed to achieve a fixed workability level (slump = 190 mm; class S4) for the concrete containing WBFA2 (ash concrete) was significantly lower than that needed for reference concrete (0.5 wt.% against 0.75 wt.%). This was consistent with the results obtained on cement pastes showing an increase of mix workability by WBFA2 addition (Fig. 3(b)). As confirmed by the density measurements, the ash concrete was characterised by a lower density (2363 kg/m3) as compared to the reference concrete (2412 kg/m3). Fig. 7 compares the compressive strength development of ash and reference concretes. It can be noted that the average compressive strengths of the ash concrete were equal to or greater than the ones of the reference concrete. However, the differences between the compressive strengths of the two types of concrete were within the variability

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Fig. 7. Compressive strength development of concrete containing WBFA2 or powdered limestone as fillers.

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On the basis of these technological results, it would be worth considering whether a European Technical Approval for construction products is worthwhile in view of reusing virgin wood fly ashes in blended cement formulations. The unsatisfactory chemical composition of raw WBFAs represents only a limitation for their reuse as filler/partial sand replacement material in concrete. Due to their high contents of chlorides and sulphates, raw WBFAs may be reused only for unreinforced concrete and no requirements category (low-quality concrete). The technological feasibility of such a reuse has been demonstrated in this study. In particular, replacing conventional filler (ground limestone) and an aliquot of natural sand with raw virgin wood fly ash (WBFA2) resulted in no modification of the compressive strength development of concrete mixes and in a significant reduction of the amount of superplasticiser needed to achieve a fixed workability level. Such a reuse of wood biomass fly ash would also alleviate waste disposal issues. Acknowledgement

range of measurements (variation coefficient value of about 7%). Anyway, these data proved the suitability of raw WBFA2 for reuse in concrete as a filler/partial sand replacement material. As suggested by the results of previous leaching tests on hardened raw WBFA2-PC mixtures [55], the reuse of this fly ash should not pose environmental problems related to excessive release of heavy metals.

This research has been funded by the Research Fund for the Italian Electrical System under the Contract Agreement between RSE (formerly known as ERSE) and the Ministry of Economic Development – General Directorate for Nuclear Energy, Renewable Energy and Energy Efficiency, stipulated on July 29, 2009 in compliance with the Decree of March 19, 2009. References

4. Conclusions The physical and chemical characteristics of the woody biomass fly ashes (WBFA) investigated, two samples coming from virgin wood (WBFA1 and WBFA2) and one from treated wood (WBFA3) combustion, do not fulfil the ASTM and EN requirements specified for reuse of coal fly ashes as mineral admixtures in concrete. This is because of an excessive particle size, an insufficient ‘‘pozzolanic oxides’’ content and an excessive sulphate and chloride content of raw WBFAs. Therefore, the studied raw WBFAs could not be reused as mineral admixtures in concrete, even if the ASTM C618 and UNI EN 450-1 requirements would also be extended to fly ash samples not derived from coal combustion. Grinding and water-washing treatments of raw WBFAs may improve their physico-chemical characteristics, so that two of the three studied WBFAs were able to meet the ASTM requirements for Class C coal fly ash. Anyway, the UNI EN requirements were not fully satisfied. Exception made for WBFA3 characterised by excessive contents of sulphates and heavy metals, the other two types of fly ashes, WBFA1 and WBFA2, behave satisfactorily as mineral admixtures, in spite of their not fully conforming physico-chemical characteristics. In particular, little reduction or increase in the mix workability, limited setting delay and moderate compressive strength reduction were observed when ground unwashed or washed WBFA1 or ground unwashed WBFA2 were used at cement replacement levels of 15 wt.% or 30 wt.%. As suggested by compressive strength measurements, pozzolanicity test results and portlandite determinations on hardened pastes, as well as by the chemical composition of the studied fly ashes, the capability of the two virgin wood ashes of contributing to the mechanical strength development of cement-based materials is principally related to their hydraulic activity and, to a minor extent, to their pozzolanic activity. In this regard, the highest activity index (0.94 at 90 days) was obtained for washed WBFA1.

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