Characterization and feasibility of using vegetable biomass ash in mortar

Caracterização e viabilidade de utilização de cinzas de biomassa vegetal em argamassa

Caracterización y viabilidad del uso de cenizas de biomasa vegetal en mortero

Civil construction is a segment that accounts for a high demand of raw materials, while releasing significant amounts of CO₂ into the atmosphere (Noor-ul-Amin, 2014; Berenguer et al., 2018). To minimize this problem, some agricultural residues have been incorporated by the construction industry (Chatveera and Lertwattanaruk, 2014). Moreover, much of this is because ash has pozzolanic properties, which plays a significant role when incorporated into cement (Hossain et al., 2016).

This property is identified in materials called pozzolanic that have silica or alumina in the state amorphous, which when in contact with water, react with the calcium oxide found in lime or in cement that give rise to a substance with cementicious properties (ASTM C 618-19; Farinha et al., 2018). To achieve this, in addition to a non-crystalline state, pozzolans must have a refinement of their particles, that is, they present a specific elevated surface area (Roselló et al., 2017).

The physical, chemical and mineralogical composition of these agro-industrial residues is varied, depending on the type of biomass, species, growth condition, harvest techniques, transport, storage, combustion process and many other conditions that can improve - or not - its content (Zajac et al., 2018). In general, it comprises aluminosilicates in the amorphous and crystalline phase, whereby silicon dioxide has the highest percentage in RHA and SCBA samples, as well as oxide of aluminum in ashes from wood origin (Farinha et al., 2018; Fernandes et al., 2016; Kazmi et al., 2017; Ukrainczyk et al., 2016).

Oxides of other metals are also present, such as iron, magnesium, calcium, and potassium.

In addition, there are carbonates and unburned carbon found in forest ash, such as ECA. Moreover, when applied in cementitious systems, they are responsible for a greater demand for water in the mixture, due to the high loss on ignition (Arif et al., 2016; Ban and Ramli, 2010; Garcia and Sousa-Coutinho, 2013; Ribeiro et al., 2017).

Despite this, the different levels of substitution can lead to an improvement of the properties of durability, resistance of the mortar, a decrease in the material cost in the construction (Hossain et al., 2016), good compaction and low evolution of the heat during hydration (Noor-ul-Amin, 2014). These are effective in controlling expansions of deterioration caused by alkali-aggregate reactions (Esteves et al., 2012) that can reduce the use of fossil fuels, form raw materials and generate income for workers involved in transportation, infrastructure, technological development processes and the application itself (Prasara-A and Gheewala, 2017).

Therefore, using ash as a partial substitute material for cement goes beyond sustainable issues, and encompasses economic and social issues (Prasara-A and Gheewala, 2017). However, commercializing waste and its application in construction is almost non-existent, even with the growing interest in supplementary cementitious materials from biomass in scientific research (Roselló et al., 2017), or with the possibility that new materials can improve the performance of buildings and materials, promoting greater quality control and minimizing the development of pathologies, for example. Therefore, further studies regarding the feasibility of using combustion residues in mortars and concrete should be conducted (Ukrainczyk et al., 2016).

These studies were developed because, in addition to the ashes showing good results when used as partial substitutes for Portland cement, they are by-products of cheap and abundant commodities worldwide, disposed of in landfills without any environmental concerns (Moraes et al., 2014; Zajac et al., 2018; Berenguer et al., 2018). In 2012, 34 million tons of RHA were discarded. In the following year, in the United States, it is estimated that 1.25 to 5.6 tons of SCBA were produced (Martirena and Monzó, 2017; Paris et al., 2016). The increased use of biomass for energy generation is one of the most important sources of renewable production and has projected growth for the future, also accounting for a greater availability of ash (Zajac et al., 2018).

There is confirmation that countries such as Brazil may have industries that may benefit from biomass residues for applications in construction, as discussed in the study by Moraes et al. (2014) and Regô et al. (2015), for example. The former analyzed the possible uses of by-products of the rice chain, and the latter characterized the RHA produced in the country and its application in cementitious matrices. As a result, the two authors cited found the same advantages mentioned in the article previously. They also identified that the ash produced in Brazil has a chemical composition that does not vary much, and this makes the material easier to use as a component of mortars.

Taking this into account, the aim of the present work is to verify the technical feasibility of using different types of biomass ashes - ECA, SCBA and RHA -, at different percentages of substitution, as supplementary cementitious material in mixed mortars, which contain two agglomerates in their composition, i.e., cement and lime, aiming at the partial reduction of Portland cement.

The physical and chemical characterization analyzed in this study, important for understanding the behavior of particles and their influence on pozzolanic reactions, was made from the results obtained for the particle density (PD), the fineness (F) of each element, water content (U) and loss on ignition (LOI). From the results shown in Table 3, it can be observed that the samples containing SCBA had a lower volume in their composition, as well as in the studies by Kazmi et al. (2017) as this quantity is inversely proportional to PD (Aprianti et al., 2016). Therefore, for ECA, the lowest PD value found, the volume is ten times greater than it would be if there were no substitution, according to Gluitz and Marafão (2013), and which will influence the strength of mortars as will be seen below.

Table 3
Physical characterization of the elements used

In addition, based on the PD, the process of accommodating the particles that make up the cementitious system can be understood. There is a better mechanical performance when they are strongly interlaced and where there are few voids. Thus, the best resistance results should be seen in the SCBA and RHA, because after being previously treated - ground and sieved - the samples showed a higher DP. Without this treatment for ECA, the strength or other property of the mortar could be negatively affected, making the substitution of cement by ashes, for example, inept (Martirena and Monzó, 2017; Farinha et al., 2018).

However, knowing that the behavior of a mortar is governed by other properties, grinding and sieving could make the ashes highly reactive as observed by Roselló et al. (2017), and with the pozzolanic activity taking place, the mechanical performance of the system can be satisfactory. Therefore, analyzing the results for the fineness index and following the specifications of ASTM C 618-19, SCBA is in fact a pozzolana, the maximum content of material retained in the 45 µm sieve is 34%, compared to 31, 72% accumulated. For NBR 12653 (ABNT, 2014) none of the ashes would be a pozzolanic material; the maximum content is 20%.

However, even though they are not pozzolans within the limits established by current regulations, the smallest particles, that is, not much material going through the 45 µm sieve, are concentrated close to the aggregate interface and cement matrix, causing a filler effect. This can contribute to resistance gain in the hardened state (Khan et al., 2017; Aprianti et al., 2016; Resende, 2013). This phenomenon is the same as that observed in cementitious systems containing hydraulic lime.

While for fresh mortar, the fineness index reveals those of lesser value, in the SCBA case, there is no change in workability for any substitution range because particles larger than 45 µm affect the plasticity of the mixture (Netto, 2006). Similarly, for the RHA, there is a greater loss of workability, especially when the increase in cement replacement is from 15 to 30%. These are assertions that are equally proven by the consistency index, which are within the limits established by NBR 13276 (ABNT, 2016), Figure 1.

Figure 1
Chart results for consistency index and w/c ratio.

From the fineness index test, a visual analysis of the samples retained in the sieve was also performed. For the ECA, applying sodium hexametaphosphate, a dispersing chemical compound (Mauri et al., 2011), the presence of small crystals was noted. These particles account for the high- water content in the sample, Figure 1, due to the high absorption capacity (Netto, 2006). This explains not only the accentuated volume of water, which favors the maintenance of workability (Ataie, 2016), but also the humidity, which is satisfactory during the mortar curing (Rajamma et al., 2009).

Due to the hygroscopic nature of these crystalline elements, with more water released, there is a greater loss of mass of the ECA when subjected to the loss on ignition test, and for this reason there was a 70.20% loss, the largest among the analyzed ash. In addition, this occurrence is also due to the presence of unburned organic matter (Prasara-a, 2017), which during heating in a muffle, releases another portion of CO₂ and water.

Nevertheless, the presence of these crystals is not favorable to the properties of the mortar. For Aprianti et al. (2016), amorphous elements are more reactive. However, due to the fact that a large part of the retained material is crystalline and that its fineness reached a value of 52.63%, it is possible that the rest of the sample, through-pass and amorphous material, compensated for the reactivity of the ash. Thus, for ECA, the considerable gain in strength is a function of the volume of ash added to the mortar due to its low PD, as previously indicated.

Regarding the water demand in the samples, using the consistency index, Figure 1, in relation to the CPR, there is a high amount of water for mixtures containing ash in partial replacement for cement, except for SCBA. The largest amount required was for the RHA30 trace with 20% more when compared to CPR. According to Ukrainczyk et al. (2016), this occurs because of the irregular and bulky shape of the ashes. Berra et al. (2015) show the high specific surface area of ash compared to Portland cement, in addition to the porous nature of its particles, as suggested by Arif et al. (2016).

These characteristics are other factors that also account for greater water absorption. Another is the presence of a large amount of organic matter available for hydration during the mortar hardening, as identified by Rajamma et al. (2009) and previously confirmed by the loss on ignition test. It can be observed, therefore, that the workability decreased when there was the presence of ash in partial replacement to cement, corroborating the results obtained by Belviso (2018), Aprianti et al. (2016), Ukrainczyk et al. (2016) and those discussed here.

Despite this, the decrease found was 2.3% compared to the 34.4% cited in the literature. This difference was due to the size of the selected particles. Ukrainczyk et al. (2016), for example, used materials with particles up to 80 µm. In this study, we opted for those with max. 75 µm, a value considered satisfactory by Ataie and Riding (2016) to ensure reactivity, which is inversely proportional to the particle density, and therefore positively influence the mechanical properties of the mortar.

In general, considering the analyses for ECA, which are inverse to those observed in SCBA and RHA, and following the requirements of NBR 12653 (ABNT, 2014), only the last two can be classified as class N pozzolans, for content of water, moisture and loss on ignition. Both reached a moisture content of less than 3% and did not exceed 10% of the loss on ignition value. Moreover, adding the behavior of the ash before the properties of the mortar in the plastic state, workability and water absorption, a better reactivity is expected in samples containing RHA.

Therefore, regarding the results, there was no significant gain in compressive strength at 14 days for any of the samples, Figure 2, as shown in Paris et al. (2016) and Abbas et al. (2017). CPDs with a 15% cement replacement content achieved better mechanical performance, especially for ECA15. This is related to factors such as the lower rate of cement replacement (Garcia and Sousa-Coutinho, 2013; Carrasco et al., 2014; Ukrainczyk et al., 2016), the greater amount of reactive material, the better workability, when compared to CDP, and greater water absorption.

Figure 2
Bar chart showing a comparison between the average resistance achieved in the simple compression test for each trace at 14 and 28 days (Tukey method p <0.05).

The cause of the poor performance of CPD is justified by the non-occurrence of the synergistic effect between cement hydration and the pozzolanic reaction, which directly influences the compression strength of the mortar (Isaia, 2003; Berra et al., 2015). On the one hand, the poor hydration action is possibly due to the use of wooden molds for molding the CDP, a material of a permeable nature, and little water retention during curing, as expected. On the other, the slow gray-cement reaction, Carrasco et al. (2014) and Rosales et al. (2017), suggest that long periods are necessary for there to be effects considered positive in resistance to compression.

At 28 days, the ECA30 did not show resistance gain as the causes are the same as those pointed out for the ECA15 at 14 days. This confirms that the ashes tend to contribute to the development of mechanical resistance due to their pozzolanicity and hydraulic activity, as in Berra et al. (2015). The best results were obtained for samples with a content of 15% and for RHA30. These were also identified by Rajamma et al. (2009) and Wang (2015). In RHA30, the sample with the greatest resistance gain is due to the high amount of silica in its composition (Fernandes et al., 2016), shown in Table 1, which can react more easily with the released CH, increasing the strength of mortars (Jamil et al., 2016).

Thus, it can be affirmed that, even without significant gain in compressive strength, the partial replacement of cement with biomass ash was considered acceptable for 15% by mass. The results identified in Figure 2 and those found by Rajamma et al. (2015), Garcia and Sousa-Coutinho (2013), Carrasco et al. (2014) and Ukrainczyk et al. (2016) point out that the resistance of samples containing ash is greater than that found in CPR for different curing times, that is, 28 days for those containing the biomass residue and 90 days for those without any additional cementitious material.

Analogously, analyzing the results obtained for flexural tensile strength, Figure 3, it was observed that only the SCBA30 and RHA30 traces did not exceed the CPR resistance and that for ECA30 the same value at 14 days was reached. For the age of 28 days, the traces containing ash obtained a better performance for tensile strength in flexion than for simple compression, confirming the delayed effect of gray-cement reactions, mainly for a 15% replacement.

Figure 3
Comparison between the average resistance achieved in the flexural tensile test for each trace at 14 and 28 days (Tukey method p <0.05).

However, the results for flexion differ from those obtained by Rajamma et al. (2009), where there was a gradual reduction in resistance with the increase in the percentage of ash and, consequently, an increase in pozzolanic reactions, especially for replacement rates greater than 20% (Chowdhury et al., 2015). This phenomenon can be controlled by re-burning residues and grinding processes, as was done for the ashes studied here, confirming the need to submit them to these types of pretreatments, as indicated by Jamil et al. (2016).

Finally, to identify the water absorption phenomenon, the specifications presented by NBR 15259 (ABNT 2005) were followed. It states that the capillarity absorption index must be calculated as the average of the three specimens submitted to immersion in water for 10 min and 90 min, Figure 4.

Figure 4
Absorption test at 28 days.

From the results found, the absorption rate decreases, even if expressively, as the cement replacement rate increases from 15 to 30%. Jamil et al. (2016) and Elinwa and Ejeh (2003) identified a reduction in water absorption when the mortar has 15% ash in its composition, at a rate of 0.8 to 1.25%. As a rule, it is accepted that this limit is less than or equal to 10%. However, it diverged from the studies by Chowdhury et al. (2015), in which the ash absorption and addition ratio is directly proportional.

For the highest values, ECA30 and ECA15, they occurred due to the presence of open pores in the structure, confirming the results for the granulometry and fineness index tests. In the presence of particles of different sizes, the water absorption capacity is increased. Therefore, due to the uniformity and fineness of the SCBA, the penetration of water in them has values close to those of the CPR. Thus, it is concluded that the absorption decreases as the amount of voids decreases, thus the SCBA can be used as filling material. It is also clear that, by increasing the ash percentages, the permeable voids will be filled, minimizing the absorption rate (Jamil et al., 2016; Rosales et al., 2017; Carrasco et al., 2014).

Concerning the cure, according to Aprianti et al. (2016), when incorporating fine material into cementitious systems, different procedures should be used to promote this concrete hydration. One option is to use superplasticizers (Ukrainczyk et al., 2016) to fix the w/c ratio and guarantee conditions of execution and performance (Carasek, 2010). For Ramos et al. (2013), Ataie and Riding (2016) the ideal value of the w/c ratio is equivalent to 0.4 and 0.45, respectively.

Given this, the ash with the greatest potential to improve the characteristics of mortar as a partial substitute for cement is RHA. Concerning ECA and SCBA, their potential as a filling material should be assessed, and a mortar that contains the three types together should be studied.

The use of ash as a component of mortar was clear. Materials that have this particularity are known as supplementary cementitious materials. All the data found in this research are also identified in the literature. The pretreatments reduced the granulometric variation of the ashes, increasing their specific surface, and the particles were better accommodated, showed greater reactivity. The content of unburned organic influenced the loss of ignition, but not enough to prevent better results in the hardened state when compared to the reference mortar.

It should be mentioned that from all the analyses made, mortars containing RHA achieved the best mechanical development. The behavior of ECA and SCBA suggests its application as a substitute for sand, for example, and may be the object of study in future research. The filling effect was identified by the resistance gain that occurred due to the behavior of the particles, like those of lime, occupying voids in the cement matrix. Using it as a substitute for cement should not be ruled out.

As for the substitution content, for the chosen bands, 15% is the ideal substitution content, however higher substitution contents may result in mortars with less mechanical resistance or more porous. The reference must therefore be the application of the mortar, whether laying or covering, whether external or internal area. Those studied here, due to their 1:1:6 trace and the results obtained, could be used for laying masonry or for ceramic tiles. However, the technical feasibility for these applications can only be proven by carrying out new studies.

Regarding water absorption, which was high for all ash contents, the mortar is highly porous. Ashes should be treated before being used (here they were ground and sieved). After doing this we suggest reburning the ash, that is, the residue needs to be ground, sieved and burnt. By doing this, the pozzolanic activity improves and voids that exist in the mortar will decrease. In addition, we suggest using an additive that will control the water-cement ratio.

Cite as: Gonçalves, C. F., Soares, A. F., Paula, H. M. (2021), “Characterization and feasibility of using vegetable biomass ash in mortar”, Revista ALCONPAT, 11 (2), pp. 1 – 16, DOI: https://doi.org/10.21041/ra.v11i2.484

Contribution of each author: In this work, the author C. F. Gonçalves was responsible for the analysis, discussion of the results, writing and review of the paper. Author A. F. Soares developed the experimental methodology, carried out experiments and collected data. The author H. M. Paula contributed with the original idea, coordination of experiments, guidance and supervision of all activities.

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Discussions and subsequent corrections to the publication: Any dispute, including the replies of the authors, will be published in the second issue of 2022 provided that the information is received before the closing of the first issue of 2022.

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5. ACKNOWLEDGEMENTS

The authors wish to thank the Brazilian National Council of Research and Development (CNPq) for financing this research (Project number 552118/2011-7).