Feasibility of using pozzolanicity tests to assess reactivity of wood biomass fly ashes

Pozzolanic activity of materials can initially be assumed from the quantity of the pozzolanic oxides SiO2, Al2O3 and Fe2O3. Some of the methods for assessing pozzolanic reactivity of materials include measurement of CaO consumption (Frattini test), decrease in electrical conductivity and pH of a solution containing pozzolanic material, or measurement of an increase in strength of mortar with replacement of cement by pozzolanic material (strength activity index, SAI). The above-mentioned pozzolanicity tests are used in this study to evaluate reactivity of wood biomass fly ashes (WBA-F). The results presented in the paper show that only the SAI method can fully reveal the hydraulic and pozzolanic activity of WBA-F.


Introduction
The use of wood biomass as a renewable energy source (RES) results in generation of wood biomass ash (WBA) as waste in power plants. Many researchers compared WBA fly ash (WBA-F) with coal fly ash and expected a positive pozzolanic reaction [1,2]. According to the literature review, the sum of pozzolanic oxides in wood biomass fly ash can vary between 19.8 % [3] and 80.66 % [4], implying that some of the WBAs studied may be pozzolanically reactive. However, studies on the use of WBAs in cement mixes have shown that some WBAs exhibit both hydraulic and pozzolanic behaviour [1]. During chemical analysis of 28 different wood and woody biomass samples, Vassilev et. al. [5] established that CaO is dominant in the composition, with an average value of 43 wt. %. Several authors [6] have shown that, besides pozzolanic behaviour, a hydraulic behaviour can also be expected due to high CaO content among the tested WBAs compared to the reactivity of known mineral admixtures. The estimation of pozzolanic reactivity by various authors often leads to contradictory results. For example, pozzolanic activity of bottom WBAs and three fly WBAs tested with the Frattini method according to  was evaluated negatively [9], regardless of a significant amount of pozzolanic oxides in one of the ashes (56.01 %) [10]. Rajamma et al. [7] showed the pozzolanic activity of fly WBA with a 53.2 % pozzolanic oxide content. Testing the pozzolanic reactivity with indirect methods (Strength Activity Index) showed a positive [11] and negative result [12], depending on characteristics of WBA used in the test. It can be concluded from this that the reactivity of WBA with a high calcium content, and its influence on the properties of cementitious materials, has not as yet been systematically researched. When Portland cement reacts with water, C-S-H gel is formed as the main hydration product. During hydration, portlandite is also formed, which contributes to high alkalinity of the solution and protects reinforced concrete from corrosion at the macro level. A hydraulic binder is defined as a material which, when reacting with water, leads to the formation of hydration products and to the transition to a solid structure (hardening in reaction with water) [13]. The estimation of the amount of portlandite (CH) as hydration product is used to estimate the degree of hydration of the hardened cement paste, and it provides information on possible influence on mechanical properties after hardening. Pozzolanic material is characterized by its ability to react with portlandite and form additional C-S-H gel, which fills cavities between cement particles and aggregate, and contributes positively to the mechanical and durability properties of cement composites. In the scope of this research, WBA-Fs from three different power plants were tested with two standardized and one nonstandardized pozzolanic test methods, in order to evaluate their potential as a supplementary cementitious material. The standardized methods included a direct method -the Frattini test, which measured the Ca(OH) 2 reduction over time caused by pozzolanic reaction, and an indirect method (strength activity index). A non-standard method was used to measure the change in electrical conductivity of a saturated Ca(OH) 2 solution with the addition of biomass ash. The aim of this study was to evaluate feasibility of these common pozzolanic tests to indicate the reactivity of wood biomass fly ashes (WBA-Fs).

Materials
Pozzolanic reactivity was investigated at three different WBA-F, which were collected at different power plants in Croatia. General information about power plants from which these ashes were generated (type and temperature of combustion, capacity, and type of biomass) is given in Table 1. A detailed characterization of WBAs used is given in [14]. A summary of chemical composition of WBA-Fs, cement, and particle size distribution, is shown in Table 2 and Figure 1.  Feasibility of using pozzolanicity tests to assess reactivity of wood biomass fly ashes For benchmarking purposes, coal combustion fly ash (FA) [15], commercial metakaolin (MK), and silica fume (SF) [16], were used in certain experiments, in addition to biomass ash, to assess pozzolanic activity. It can be seen from chemical composition (

Frattini test
The Frattini test, as described in EN 196-5:2011 [8], was used to assess pozzolanicity of WBA-F as a supplementary cementitious material and as a pozzolanic cement. In the first case, the test was carried out with the 85 % cement and 15 % WBA-F mixture, while in the second case the test was conducted with pure WBA-F. As WBA-Fs have a high content of CaO, it was hypothesized that they are mainly hydraulically active and that a super-saturation of the available calcium ions could occur when they are added to Portland cement. Therefore, the Frattini test was performed on WBA-F following the procedure similar to that used for pozzolanic cements. 20 g samples (cement/WBA-F mixture or pure WBA-F) were diluted in 100 ml of distilled water and conditioned at 40°C, with the test being carried out after 7, 15, and 37 days. The sample was vacuum filtered through Büchner funnel using filter paper with a pore diameter of 2 µm to separate solids from aqueous substances. 50 ml of the filtrate was homogenized with five drops of the methyl orange indicator prepared beforehand. According to the standard, the titration of standard 0.1 mol/l of hydrochloric acid in the burette of known volume reaches the end point when the indicator in solution changes from clear to light yellow, indicating pH neutralization. This allows calculation of free hydroxyl ions in solution before titration from known volumes and the molar concentration of hydrochloric acid. To determine [Ca 2+ ] in solution, the alkalinity was adjusted to pH 12.5 ± 0.2 by adding NaOH. EDTA solution and 1.0 g murexide indicator were used in the second titration to determine the CaO concentration in ml/l, analogous to the determination of [OH -]. The colour change in the first titration was pale, but the reaction endpoint between EDTA and CaO exhibited a significant change from pink to violet colour. The test results are expressed as the concentration of calcium ions (expressed as calcium oxide) and the alkalinity of the aqueous solution in contact with the cement hydrate after a fixed period of time, compared with the saturation concentration of calcium ions (expressed as calcium oxide) as a function of the hydroxyl ion concentration at 40° C. Test results below the portlandite saturation curve indicate the removal of Ca 2+ from the aqueous solution (the solution becomes unsaturated in portlandite), which is then attributed to the pozzolanic activity of the material added to the cement. Results lying on the line indicate zero pozzolanic activity, while results above the line do not correspond to pozzolanic activity. It should be noted that this procedure assumes that no other source of soluble calcium is present in the system, since leaching of calcium would invalidate this approach.

Electrical conductivity (EC)
If a reactive pozzolanic material is added to the saturated lime solution, the conductivity is expected to decrease over time due to ion consumption by pozzolanic reaction [17][18][19]. Based on this hypothesis, an experimental indirect method for rapid detection of the pozzolanic activity of natural pozzolans was developed [19]. The method involves periodic monitoring of the electrical conductivity and pH of a saturated Ca(OH) 2 solution into which a controlled amount of pozzolanic material has been added. This indirect method of assessing pozzolanic activity starts with preparation of a saturated calcium hydroxide solution. Ca(OH) 2 was mixed with distilled water and added to oven where it was heated to a temperature of 40 ± 1°C. Samples of 4g BWA-Fs were homogenized with a 40 mL solution. During a period of four hours, a Lab 850 Schott pH-meter was used to determine the pH-value, while the PCD 650 Oakton was applied to measure electrical conductivity of the solution. A drop in pH value indicates a reaction between WBA-F and OHanions, and the EC value depends on the concentration of free Ca 2+ and OHions in the solution. A decrease in these values over time can be attributed to the pozzolanic reaction between SCM and free ions.

Strength activity index (SAI) test
The test procedure is described in European Standard EN 450-1 [20] for coal fly ash. The test result is expressed as a compressive strength ratio between mortar prisms with supplementary cementitious material (A) and reference prisms made of Portland cement (B). The strength activity index SAI = A/B*100 is expressed in percent. In this research, mortar prisms 40 x 40 x 160 mm were cured at 7, 28, and 90 days and, after each period, A and B prisms of the same age were subjected to pressure to determine the strength. According to EN 450-1, the SAI test of no less than 75 % at 28-day compressive strength, and 85 % at 90-day compressive strength, with cement substitution of 25 %, indicates pozzolanic reactivity. According to ASTM C618, an activity index greater than 75 % after 7 and 28 days, for 20 % cement substitution with coal fly ash, is interpreted as pozzolanic reactivity. According to BS 3892, a result of more than 80 % for 30 % replacement [21] is considered a proof of pozzolanic reactivity. In this study, mortar prisms were made with CEM I 42.5 and standard quartz sand in a ratio of 1:3 with a water-binder ratio (w/b) of 0.50, while 15 % of cement was replaced by WBA-Fs. It is obvious that this is a lower substitution level compared to the substitutions of 20 to 30 % of cement by coal fly ash as required by the above mentioned standards [20]. However, due to workability issues, 15 % of cement replacement by WBA-F was a maximum that could be reached without the use of chemical admixtures, as these admixtures would hinder the effects of WBA-F [6]. In addition, the authors of a previous research [22][23][24] concluded that 10-15 % of the cement replacement by WBA-F was the maximum value, due to the higher amount of free CaO and free MgO in WBA-F samples. However, since the calculated SAI values were not obtained on the same quantities of cement substitute as required by the standards, the SAI values in this study were used only for comparison purposes and not for acceptance according to these standards.

Frattini test
Frattini test results at 7, 15 and 37 days for F1 ash (results labelled respectively as MF1-7, MF1-15, and MF1-37), for F2 ash (results labelled respectively as MF2-7, MF2-15, and MF2-37) and for F3 ash (results labelled respectively as MF3-7, MF3-15, and MF3-37) as a 15 % cement replacement, are shown graphically in Figure 2. According to the standard, the tests should be performed after 7 and 15 days. However, in this study, the measurement was continued until the sample age of 37 days since WBA delays setting times and extends heat of hydration development [2,10,25].  [26]. The MF3 blend was slightly below the line after 37 days, while MF2 was still above the portlandite saturation line after 37 days. The highest removal of Ca 2+ ions was therefore registered in the case of F1, the ash with the highest content of pozzolanic oxides (54.68 %). This higher amount of pozzolanic oxides was able to bind calcium ions from the solution, which indicates pozzolanic activity of this ash after 37 days. The same can be observed with ash F3, but to a lesser extent. Tested WBA-Fs have a large amount of free calcium oxide in their chemical composition, which is larger than usual for pozzolanic materials. When they were added to replace cement, the amount of available calcium oxide was potentially too high to be absorbed by a relatively small amount of pozzolanic oxides available in these ashes. The question remains as to what happened before 7 days, since there seems to be less Ca 2+ ions GRAĐEVINAR 72 (2020) 12, 1145-1153 Feasibility of using pozzolanicity tests to assess reactivity of wood biomass fly ashes in samples F2 and F3 after 7 days, although they have a higher amount of free lime available. Since the hydraulic reaction is fast [27], faster than the pozzolanic reaction, it is possible that these calcium ions were already absorbed during the hydraulic reaction between free lime and water. In any case, it can be concluded from the results of the Frattini test carried out on WBA-F and cement blend that certain pozzolanic reactivity does exist in the case of F1 ash and perhaps even in F3 ash. However, if pozzolanic reactivity is found, it is rather slow. The addition of biomass ash to Portland cement showed a supersaturation of calcium ions which could not be absorbed by available pozzolanic oxides. It was therefore decided to test the ashes according to the procedure for pozzolanic cements, i.e. to test the pure samples of WBA-F itself according to the Frattini method. The results for the ashes F1 and F2 after 7 and 15 days are shown in Figure 3. The results for ash F3 are not shown in the Figure, as they did not stabilize over time and the values were well outside the solubility curve. As can be seen in Figure 3, F1 ash exhibits a significant CaO + reduction, which is higher than when mixed with cement ( Figure  2). This can be explained by an excessively high CaO content in a blend of ash and cement. Based on the results for pure sample, it can be assumed that F1 initially reacted alone with the waterconsuming free lime to form portlandite, and that portlandite subsequently reacted with available pozzolanic oxides. On the basis of the results, it could therefore be concluded that F1 meets requirements for pozzolanic cements. The results for F2 exceeded the value of 90 mol/l [OH -], for which a solubility curve is given in the standard EN 196-5:2011 [26]. Nevertheless, the values for F2 were near the saturation line after 7 days and moved below the line after 15 days, which also indicates a hydraulic reaction due to high calcium content and potential pozzolanic reaction between residual CaO reacting with unbound pozzolanic oxides at later ages (15 days).

Figure 3. Results of Frattini test for pure samples
Based on the comparison of the ash-cement mixture and pure ash results, it can be concluded that an excessive amount of CaO was available and reactivity was impaired when these

Electrical conductivity
Pozzolanic reaction leads to a reduction of free Ca 2+ and OHions, which should lead to a decrease in the pH-value and EC pH-values [18]. For this test, silica fume, metakaolin, and coal fly ash, were used to compare the trend of behaviour with the WBA-F samples tested. The electrical conductivity monitoring results are shown in Figure 4, while the pH monitoring results are given in Table 4. Table 4 also shows changes of pH value from time zero (Δ).

Figure 4. Electrical conductivity test results
Three distinct types of behaviour can be defined based on results shown in Figure 4: -Silica fume and metakaolin, as typical pozzolanic materials, showed an immediate decrease in electrical conductivity and pH over time, which is consistent with previous studies [17,19], -FA and F1 showed similar behaviour in this test, a slight and slow decrease in electrical conductivity and pH values over time, although the proportion of pozzolanic oxides in the FA sample is greater compared to the F1 sample [15,28], -F2 and F3 showed an increase in electrical conductivity and pH value during the first 5 hours, and a slight decrease in both values after 5 hours, indicating an initially high availability of free Ca 2+ and OHions and, finally, slow reaction during later hours. However, if we look at only the part of the curve after 5 hours, when the values of electrical conductivity start to decrease, this decrease was higher in F2 compared to F1 and F3.
In previous studies [29], this method was evaluated as not being fully reliable for assessing the pozzolanic activity of FA, because of its high content of soluble salts, mostly Na 2 SO 4 , K 2 SO 4 and CaSO 4 , which cause high conductivity. Potentially similar effect could be expected for F2 and F3, due to their higher SO 3 content. In any case, the method of monitoring changes in electrical conductivity and pH over time revealed that changes Feasibility of using pozzolanicity tests to assess reactivity of wood biomass fly ashes in the values were significantly different compared to classical pozzolanic materials, due to high amount of available CaO. Any pozzolanic reactivity available in WBA-F samples was slow and only evident later during the testing period.

Strength activity index test
Average compressive strength test results and calculated SAI values for three samples, with 15 % WBA-F used as cement replacement and reference mixture with only cement (M0), are given in Table 5. The results are obtained as an average of three specimens for each mix, with average values given in Table 5 and standard deviation indicated in Figure 5. It should be noted that, due to workability issues, only 15 % of cement was replaced with WBA-F, which is lower than the values given in well-known standards for the use of coal fly ash [20,21]. The results presented in Table 5 show that, after 7 days, all samples with 15 % cement substitution with WBA-F had lower compressive strength compared to the cement sample. WBA-F mixes reached between 75 and 83 % of mortar strength, which is even lower than the reduced amount of cement in the samples. The trend of SAI for up to 7 days followed the trend in the amount of free CaO and particle size distribution. Mainly, F3 ash that has the highest amount of free CaO

Conclusion
Tested WBA-F show a lower amount of pozzolanic oxides compared to the criteria for the assessment of the pozzolanicity of fly ash from coal combustion plants specified in standard EN 450-1 [20]. Based on the sum of the pozzolanic oxides of WBA-F alone, it could be assumed that they do not contribute to the pozzolanic reaction and formation of C-S-H gel. In the present work, pozzolanicity tests (standardised and non-standardised) were applied to WBA-F to verify this assumption.  Notwithstanding their lower amount of pozzolanic oxides, a direct test measurement (Frattini method) showed that WBA-F has both hydraulic and pozzolanic properties. When the Frattini test was carried out on a WBA-F and cement mixture, supersaturation with CaO was found, and the pozzolanic reaction was hindered by the amount of CaO available. Nevertheless, after 37 days of measurement, the pozzolanic activity was obvious but slow, exceeding the test time predicted by the standard. When the Frattini test was performed with pure WBA-F, a rapid decrease in the available amount of CaO was observed after 7 and 15 days, due to the hydraulic activity at an early age, and pozzolanic activity at later age. The trend of CaO reduction in the period of up to 7 days correlated well with the first SAI results obtained with mortars containing 15 % cement replacement with WBA-F. Mortars with WBA-F, which showed a higher CaO reduction at an early age, also showed a higher compressive strength at the same age, which was due to their hydraulic activity. In addition, ashes with a higher CaO reduction between 7 and 15 days also showed a higher strength gain between 7 and 28 days. However, strength development at later ages (90 and 365 days) cannot correlate with the Frattini test. Therefore, the Frattini test, as a direct method for measuring pozzolanicity, cannot be used with certainty to fully describe the hydraulic and pozzolanic reactivity of wood biomass ash, since most of its pozzolanic activity is rather slow and becomes apparent at a later age only. The behaviour of WBA-F in monitoring electrical conductivity also differed compared to classical pozzolanic materials such as silica fume and metakaolin, and was similar to fly ash from coal combustion. In WBA-F, the electrical conductivity increased significantly in the first hours, possibly indicating an increase in Ca+ and Ca-OH ions. After some time, the electrical conductivity began to decrease. The trend of a decrease in electrical conductivity can be correlated to obtain compressive strengths at the end of the test. WBA-F with the highest decrease in specific electrical resistance showed the highest compressive strength after 90 and 365 days. However, it seems difficult to explain how a decrease in electrical conductivity during 35 hours could serve as an indication of an increase in strength at a later mortar age, and this is certainly something that merits further investigation.
In conclusion, only the direct method of measuring compressive strength was able to fully demonstrate the activity (hydraulic and pozzolanic) of fly ash from wood biomass. If the fly ash activity were based solely on the sum of pozzolan oxides, CaO reduction, or loss of electrical conductivity, its compressive strength at a later age would certainly be underestimated. To confirm the hypothesis made in this paper, and to try to fully understand the reactivity of WBA-F, it is proposed to use alternative methods to analyse the reactivity of materials. In the continuation of this research, it is proposed to evaluate the reactivity with the R3 test for the evaluation of pozzolanic and/ or hydraulic reactivity, which is validated within the framework of RILEM Technical Committee 267 -TRM. It is also proposed to study hydraulic properties of the WBA-F together with the XRD analysis of mineral phases occurring at different sample ages. Finally, there is an interesting SAI trend for mixtures with WBA-F after 365 days. The results showed a compressive strength gain, which could be explained by a positive contribution of the use of WBA-F. This contribution of WBA-F needs to be further investigated on a microstructural level for the later age of cement composites with WBA-F as cement substitute.