VGB POWERTECH 10 (2020) - International Journal for Generation and Storage of Electricity and Heat

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Wood fly ash as cement replacement VGB PowerTech 10 l 2020 the water soluble concentrations in Ta - b l e 2 shows that in WA1 17 % Na and 85 % K are soluble, whereas in WA2 >74 % Na and 58 % K are soluble. Steenari et al. [1997] reported that in several WA samples K- and Na-feldspars, such as KAlSi,O, and (Na,Ca)Al(Si,Al),O, were found. These minerals are the main constituents of gravel and sand, which contaminate the fuel. Thus, K and Na can occur in WAs both in the form of salts and feldspars, and the latter may be part of the insoluble fraction containing the two elements in the investigated WAs. The amorphous phase of the WAs might also contain K and Na, soluble as well as insoluble in water. The sum of concentrations of the water soluble Cl, SO 4 , Na and K (Ta b l e 2 ) account for 82 % and 77 % of the total soluble mass (also given in Ta b l e 2 ). Other possible soluble phases are different oxides and hydroxides seen from the high pH (>13.5), which shows that hydroxides are leached or formed as pH is measured in a suspension of WA in distilled water. 3.1.4 Mineral phases in WAs Ta b l e 4 shows the phases identified by XRD in the ashes. In cases with a concentration less than 5 % (calculated on basis of the Reference Intensity Ratio (RIR) given in Ta b l e 4 ) the approximate concentration is noted, and the presence of these phases must be regarded with some uncertainty. The investigated WAs contained quartz (SiO 2 ) and calcite (CaCO 3 ), which is in consistency with findings from other WAs [Berra et al. 2015; Olanders & Steenari, 1995, Steenari and Lindqvist, 1997]. In addition to CaCO 3 other Ca-containing phases (CaSO 4 , CaO or Ca(OH) 2 ) have been reported in [Berra et al. 2015; Ban & Ramli, 2011; Elinwa and Ejeh, 2004; Etiégni and Campbell, 1991]. CaSO 4 was not identified in the investigated ashes, and it shows that if present, the concentration of this phase is less than 1 % CaO was identified in WA2 and Ca(OH) 2 in WA1. This difference is due to the hydration of CaO to Ca(OH) 2 of WA1 from the water spraying just after the incineration. The soluble salts K 2 SO 4 and KCl were identified in both of the investigated WAs (Ta - b l e 4 ), though KCl in low concentration in WA2, which is in consistency with a lower Cl concentration in this WA (Ta - b l e 2 ). The two soluble salts have been reported in other WAs, as well, e.g. [Sigvardsen et al. 2019]. In summary, the mineral phases identified in the investigated WAs have previously been reported in other WAs. 3.1.5 Changes in WA phases from pretreatment Visually the ashes as received differed and WA1 contained larger lumps (of sizes up to 5 cm) of agglomerated ash particles (F i g - u r e 1 ). This was on the contrary to WA2, which was without such lumps, however, after wetting, lumps also formed in WA2. The lumps in WA1 were likely formed when the WA was wetted at the incineration facility. This is in consistency with previously reported by [Steenari et al. 1999]: a spontaneous agglomeration will occur during storage of the wetted ash in contact with air. The self-hardening property of WA is caused by formation of new phases [Steenari et al. 1997]: First hydration of CaO forming Ca(OH) 2 is a rapid and exothermic process. Carbonation of Ca(OH) 2 occurs in presence of water and carbon Tab. 4. Phases identified in the different ashes and pretreated ashes. Color identification: orange phase disappeared during pretreatment and green indicates a new phase formed during pretreatment. Quartz SiO 2 Calciumoxide CaO RIR WA1 Dried WA1 550° C WA1 Water wash WA1 Acid wash WA1 WA2 Hydra ed WA2 550° C WA2 Water wash WA2 3.11 X ~1 % X ~1 % X X X X X X 4.42 X X ~5 % Calcium hydroxide 3.5 X X X ~4 % X X ~3 % Ca(OH) 2 Calcite CaCO 3 3.21 X X X X X X X X X X Gypsum Ca(SO 4 (H 2 O) 0.5 ) Periclase MgO 3.04 X ~3 % X ~2 % X X X X X Magnesium hydroxide X X ~4 % Mg(OH) 2 Sylvite KCl 3.9 X X X ~4 % X ~5 % Arcanite K 2 SO 4 1.19 X X X X X X X Acid wash WA2 dioxide. The CaCO 3 hereby formed precipitates in a layer on the ash surfaces and in the pores. Ta b l e 4 shows that CaCO 3 was identified in both the investigated WAs. Ettringite formation contributes to the early solidification; however, if the pH is too low or soluble Al is lacking, CaSO 4 is formed instead [Steenari et al. 1997]. Hydration of amorphous silicate phases may also contribute [Etiégni & Campbel, 1991] and C-S-H gel was identified by SEM-EDX [Ramos et al. 2013] [Chea & Ramli, 2013]. Neither CaSO 4 nor C-S-H gels were though identified in the investigated hydrated WAs. The importance of each reaction and the relation between WA properties and its self-hardening behavior is not yet not fully understood [Illikainen et al. 2013], deeper knowledge is necessary in order to understand and evaluate the quality of WA for use in concrete, however, changes in mineralogy after wetting of WA must be expected. After wetting of WA2, the CaO disappeared and Ca(OH) 2 showed as a new mineral, which confirms the hydration. Thus after wetting WA2, the major Ca containing minerals were the same in WA2 as in the already hydrated WA1 (Ca(OH) 2 and CaCO 3 ). MgO, which was identified in WA2 (as received) and did not hydrate to Mg(OH) 2 during the wetting procedure. The XRD peak for KCl disappeared after wetting of WA2, which may indicate that Cl and K ions from this soluble salt precipitated again into other compounds. The drying of WA1, with an original water content of 19 % did, as expected, not change the phases present (Ta b l e 4 ). Heating to 550 o C was carried out to remove the organic fraction in the ashes, but it did also change the identified minerals in both ashes (Ta b l e 4 ). In WA1, the peak for Ca(OH) 2 had disappeared after the heating. The reaction Ca(OH) 2 CaO + H 2 O is reversible, and for a H2O partial pressure of 1 atm the correspondent equilibrium temperature is 510 to 512 °C [Schaube et al. 2012], i.e. lower than the temperature for the heating in the present investigation. The peak for CaO do though not appear in WA1 after the heating, and reveals that Ca forms other compounds. In WA1, the peaks for KCl and the small peak for SiO 2 also disappeared after the heating. There is no obvious explanation for the disappearance of the SiO 2 peak, and it might be due to variations within the WA, since the peak in the WA1 as received was very small to begin with. The melting point for KCl is 770 °C for KCl, i.e. higher than the current treatment and only hereafter mass loss from heating is seen [Li et al. 2019]. Thus, evaporation is not expected to explain the disappearance of KCl as mineral in the XRD measurements (Ta b l e 4 ). The two elements must participate in the formation of other phases, which can be amorphous, since no new peaks containing Cl or K appear. The same goes for K 2 SO 4 with a melting point of 1,069 °C. The heat- 54
VGB PowerTech 10 l 2020 Wood fly ash as cement replacement ing to 550 °C resulted in the CaO peak to disappear in WA2, which is on the contrary to [Lopinti et al. 2020], who saw beginning calcination (CaCO 3 transformation to CaO) already at 500 °C. The loss of CaO in WA2 after heating reveals that Ca enters into other phases. The washing in water and acid resulted in mass losses of 29 % and 37 % in WA1, respectively, and 13 % and 22 % in WA2. Washing in water and acid removed the soluble salts KCl and K 2 SO 4 from the ashes (Ta b l e 4 ). In the washed WA1, MgO and Mg(OH) 2 were identified, whereas no crystalline Mg was found before the washing, where the concentrations probably were too low for identification. When washing out part of the mass (the soluble fraction) the concentration of the non-soluble minerals increases. In the water washed WA2 Ca(OH) 2 appeared in a low concentration, and in WA1 the concentration of Ca(OH) 2 seemingly decreased. In the acid washed WAs, Ca(OH) 2 was not identified. 3.2 The investigated WAs in mortar 3.2.1 Workability issues during casting During the castings, it was observed that it was difficult to vibrate the fresh mortar well in the molds when WA was in the mixture, and it was increasingly difficult when increasing the replacement percentage of cement from 5 % to 10 %. The low workability resulted in the mortar not flowing well during the vibration and the casted mortar prisms did have larger visual pores than the references. Other researchers have reported similar finding, e.g. Udoeyo et al. [2006] reported slump test results of concrete containing varying percentages of WA and was evident that WA concrete mixes exhibited less workability than the reference. 3.2.2 Compressive strength (7 days) of mortar The screening of the effect from the WAs and pretreated WAs on the ease of casting and the 7 days compressive strength are in Ta b l e 5 and F i g u r e 2 , respectively. The general pattern is that the mortars with WAs have lower compressive strength than the reference mortar (F i g u r e 2 ). The casting of some of the mortars with WA was influenced by a lower workability (Ta - b l e 2 ), and the hence heterogeneities in the casted mortar prisms influenced the compressive strength. Thus, evaluating the compressive strength of the different mortar mixes includes both different characteristics for the WAs and treated WAs as well as the influence from the differences in workability. In order to evaluate the influence from the pretreatments and changes of phases, it is suggested to use plasticizer to have the same workability in future work, however, in this work, it was decided to investigate the effect of a 1:1 cement replacement with WA only. Tab. 5. Evaluation of the casting process 0 = similar to reference, - = slightly drier mix than reference but homogeneous prisms were obtained after vibration, -- = difficult to vibrate and the mortar prisms had larger airvoids, --- = impossible to vibrate and the mortar bars had many larger air voids and appeared very heterogeneous. The mortars with WA and pretreated WA1s all had lower compressive strength when replacing 10 % cement than 5 %. This was not the case for WA2 as received and hydrated WA2, but for the use of WA2 after the other pretreatments, as well. The casting of mortar with 10 % replacement was generally more difficult than casting mortars with 5 % replacement, replacement, however as this is not a focus area for this study, this was not investigated further. The compressive strength for the mortars with WA1 and WA1-Dried (5 % replacement) are very similar. The difference between the two recipes was the use of asreceived or dried WA1. The drying did not result in new mineral phases (Ta b l e 4 ), but it is likely that carbonation has taken place. The differences were in addition to a higher degree of carbonation also the content of WA1 dry matter and water. The water content of WA1 was 19 %, which means that the 5 % cement (22.5 g) was replaced with only 18.2 g WA1 dry matter and 2.3 g water, and for the 5 % replacement, this did not overall influence the compressive strength. For the 10 % replacement on the contrary, the compressive strength was highest when using WA1 as received, which is an indication of the slightly more water and better workability (Ta b l e 5 ). The changes in mineralogy (Ta b l e 4 ) needs to be considered when comparing the compressive strength of mortars with WA2 and WA2-hydrated. The compressive WA1 – 5 % WA1 – 10 % WA2 – 5 % WA2 – 10 % WA 0 - - 0 WA-dried 0 --- WA-hydrated - 0 WA-550°C 0 --- 0 0 WA-WW - --- 0 -- WA-AW -- --- -- --- (a) Compressive strength in MPa 50 45 40 35 30 25 20 15 10 5 0 Ref WA1 WA1- Dried WA1-550 WA1-WW WA1-AW (b) Compressive strength in MPa 50 45 40 35 30 25 20 15 10 5 0 5 % 10 % 5 % 10 % Ref WA2 WA2- hydrated WA2-550 WA2-WW WA2-AW Fig. 2. The 7days compressive strength of mortar prisms with the investigated ashes as received or pretreated. (a) WA1 and (b) WA-2. strength is higher for the mortar with WA2 than with WA2-hydrated (5 % replacement). It might indicate that the hydration occurring in the mortar mix when using the WA2 as received added to the strength, but the significantly higher compressive strength for the mortar with WA2-hydrated than WA2 for 10 % replacement indicates the opposite tendency, and no conclusion can be drawn. The compressive strength of the mortars with 5 % cement replacement WA-heated was high for both WAs (42 MPa and 41 MPa, respectively), which was only slightly lower than the compressive strength of the reference (43 MPa) corresponding to a 2 % decrease. The heating was performed to diminish the content of organic material, but from Ta b l e 4 it is also seen, that the mineral phases changed, not at least the Ca containing minerals. In common to the two heated WAs is that CaO was not detected by XRD, probably due to carbonation. During the casting it was noted that the mortar with WA1-heated (10 %) was impossible to vibrate into homogeneous prisms, even with higher frequency and longer duration of the vibration. This low workability contributes to almost halving the compressive strength for mortar with 10 % relative to 5 % cement replacement. The same pattern was not seen for heated WA2, where the compressive strength was similar for to the reference for the two percentages of replacement (Ta b l e 5 ). 55
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