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19 November 2025

Impact of Low CO2 Footprint-Dissolution Treatment of Silica and Potassium-Rich Biomass Ashes on the Compressive Strength of Alkali-Activated Mortars

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ICITECH–Instituto Universitario de Investigación de Ciencia y Tecnología del Hormigón, Universitat Politècnica de València (UPV), 46022 Valencia, Spain
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Departamento de Ingeniería Minera y Civil, Universidad Politécnica de Cartagena Member of European University of Technology EUT+, 30203 Cartagena, Spain
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Author to whom correspondence should be addressed.
Sustainability2025, 17(22), 10359;https://doi.org/10.3390/su172210359
This article belongs to the Special Issue Circular Economy in Construction: Innovations, Challenges, and Sustainable Practices
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Abstract

Using almond shell biomass ash (ABA) as a potassium alkaline source and rice husk ash (RHA) as a soluble silica source to produce blast furnace slag (BFS)-based alkali-activated mortars offers a sustainable alternative to commercial activators. However, some thermal treatment is often needed to enhance ash dissolution, potentially increasing the CO2 footprint. In this study, we evaluated how a low-CO2-footprint thermal treatment for dissolving ABA, as well as RHA combined with ABA, affects the strength performance of binary (ABA/BFS) and ternary (RHA/ABA/BFS) alkali-activated mortars. This thermal treatment involved mixing the biomasses with hot water (85 °C) in a thermally insulated bottle (TIB). The binary alkali-activated mortar, cured for 7 days in a thermal bath at 65 °C, achieved 58.0 MPa in compressive strength, applying 1-h dissolution of ABA in a TIB. Additionally, the previous dissolution of RHA in conjunction with ABA for ternary alkali-activated mortar, cured also for 7 days in a thermal bath at 65 °C, resulted in mortars with a higher compressive strength, achieving 64.7 MPa. With the prior biomass dissolution method, the binary and ternary alkali-activated mortars cured at room temperature (20 °C) showed compressive strengths of 54.7 and 67.0 MPa after 28 curing days, respectively. Moreover, after 135 curing days, these mortars reached a compressive strength of 61.4 and 71.9 MPa, respectively. The BFS-alkali-activated binders with ABA and ABA plus RHA cut CO2 emissions by 86.8% and 85.7% compared to the OPC-based binder, respectively.

1. Introduction

The transition to a circular economy, understood here as a regenerative system that minimises waste and maximises resource efficiency [1], has emerged as a key strategy for reducing the environmental footprint of industrial production, especially in the construction sector. Central to this approach is the valorisation of industrial and agricultural residues, transforming them into valuable raw materials for construction rather than discarding them [2,3]. This concept aligns closely with the development of alkali-activated materials (AAMs), which have recently gained recognition as next-generation binders due to their ability to incorporate aluminosilicate-rich wastes [4,5,6]. The main products of AAMs depend on calcium content: high calcium leads to C-(N,K)-A-S-H gels, while low calcium results in (N,K)-A-S-H structures [7,8]. Secondary phases, such as hydrotalcite, AFm, and zeolites, may also form based on the raw material composition [8].
In this context, AAMs offer significant potential for reducing the carbon emissions associated with Portland cement production, which is responsible for an estimated 7–8% of global CO2 emissions [9,10]. The environmental benefits of AAMs could be further enhanced by substituting traditional alkaline activators, such as hydroxides, silicates, and carbonates, with alternatives that present a lower carbon footprint, considering that these traditional reagents inherently contribute to carbon emissions in a significant way [11,12,13].
In order to overcome this, in the last decade, researchers have been focusing on replacing commercial alkali activators with alkali- and silica-rich wastes [14]. Industrial wastes such as residual diatomaceous earth, silica fume and glass waste, as well as some biomass ashes like rice husk ash (RHA), sugar cane straw, bagasse ashes, and bamboo leaf ash, have been proposed as sustainable alternatives to silicate-based reagents [14,15]. Likewise, alkali sources can be derived from industrial wastes, such as Bayer liquor [16,17], red mud [18], calcium carbide residue [19,20], as well as alkaline cleaning solution [21]. In addition to industrial waste, agricultural biomass is increasingly being explored as a source of alkalis for AAMs, with several studies reporting promising compressive strength results that support their use as sustainable activators.
Within the parameters of this research field, Ban [22] examined high-calcium wood ash (HCWA) and pulverised fuel ash (PFA) as primary binders in cementless mortar blocks. Produced under pressure and cured for 7 days at 25 °C, and followed by 24 h of hydrothermal treatment at 60–100 °C, blocks containing 80% HCWA and 20% PFA achieved compressive strengths exceeding 5.20 MPa, demonstrating the viability of biomass ashes in binder systems. Additionally, Peys et al. [23] assessed potassium-rich agricultural ashes, including cotton stalk and shell ash, maize stalk and cob ash, and oak/beech ash, all of them as activators for metakaolin-based AAMs. In here, maize cob ash emerged as the most effective activator. Overall, the pastes prepared with a 0.9 ash-to-metakaolin ratio yielded a compressive strength of approximately 40 MPa. This was achieved through a pre-curing process at 20 °C for 24 h, followed by open curing for 48 h at 80 °C. Alternatively, Lima et al. [24] investigated coffee husk ash (CHA) as an activator for one-part blast furnace slag (BFS)-based AAMs. In this study, the CHA demonstrated a high K2O content (as potassium carbonate) and was compared to a commercial reagent (K2CO3) in various proportions to activate BFS. It must also be noted that a mortar containing 15% K2CO3 from CHA (relative to the mass of BFS) achieved a compressive strength of 40.9 MPa after 28 days of curing at 25 °C. This strength is comparable to the 47.0 MPa obtained with commercial K2CO3.
Several studies have also focused on the use of olive biomass ash (OBA) as an alkaline activator. Alonso et al. [25] compared olive biomass fly ash (OBFA) and bottom ash (OBBA) in BFS pastes (70% BFS, 30% ash), cured initially at 45 °C or 85 °C. OBFA showed superior strength (35 MPa vs. 21 MPa at 28 days), attributed to its higher K2O content, with little influence from curing temperature. Also worth mentioning is that Moraes Pinheiro et al. [26] evaluated OBA as either a partial BFS replacement (15–35%) or an addition (5–25%). At 7 days (65 °C), the maximum compressive strengths were 31.25 MPa for 30% replacement and 38.38 MPa for 25% addition.
Crucially, Font et al. [27] further advanced the concept by developing a ternary alkali-activation system using BFS, OBA, and RHA. In their studies, they prepared activating suspensions based on RHA and OBA. These suspensions were thermally (thermal bath) treated at 65 °C for 24 h to enhance silica solubilization. Also, a mortar composed of BFS and OBA in a 2:1 ratio, with ~9% RHA (relative to BFS + OBA), reached 67.4 MPa of compressive strength after 90 days of curing at 20 °C. This strength significantly surpassed that of binary systems (BFS + OBA) attributed to the formation of a potassium silicate-rich activating solution that promoted enhanced geopolymerization.
Soriano et al. [28] also carried out an investigation into alkali-activated mortars incorporating almond biomass ash (ABA) with BFS. Replacing 25% of BFS with ABA yielded a compressive strength of 35.2 MPa after 7 curing days at 65 °C, while adding 25% ABA (without replacing BFS) resulted in 45.2 MPa, highlighting the effectiveness of ABA as a performance-enhancing activator.
Across the previous studies utilising various biomass ashes as alkali or silica sources in AAMs production, thermal treatment has consistently been playing a pivotal role in enhancing compressive strength, particularly when promoting silica dissolution. These treatments have been shown to improve both the solubilization of potassium compounds from biomass ashes [25] and the release of reactive silica from RHA [29]. However, despite these benefits, the reliance on energy-intensive thermal processes raises sustainability concerns, especially when such systems are intended as greener alternatives to traditional cement. To address this, the present study suitably proposes a new, low-energy activation strategy for both binary (ABA/BFS) and ternary (ABA/RHA/BFS) alkali-activated mortars. In this approach, biomass ashes are preactivated by simply mixing with hot water in a thermally insulated bottle (TIB), vitally eliminating the need for external constant heating equipment such as the thermal bath used in the study carried out by Font et al. [27], which requires 24 h of operation. Moreover, Moraes et al. [30] showed that silica-rich biomasses undergo significant dissolution under the elevated alkalinity and temperature resulting from NaOH dissolution in water within TIB. In contrast, under our proposed approach, since ABA dissolution in water does not notably increase the water temperature, external heating remains necessary. The effectiveness of this simplified method is evaluated through compressive strength development, offering a clear insight into the dissolution efficiency and individual contributions of each component, with the broader goal of advancing more sustainable AAMs technologies.

2. Materials and Methods

2.1. Materials

BFS, employed as a precursor to producing binary and ternary alkali-activated mortars, was supplied by Cementval (Puerto de Sagunto, Valencia, Spain). The chemical composition of BFS is presented in Table 1. It has been clearly verified that BFS is mainly composed of CaO (40.15%), SiO2 (30.52%) and Al2O3 (10.55%). ABA, used as a potassium source to produce activating suspensions, had been supplied by Borges Agricultural & Industrial Nuts (Altura, Spain). The chemical composition of the raw material was measured by X-ray fluorescence (Philips MagiX Pro Spectrometer). As shown in Table 1, ABA presented a high K2O content, which was quantified at 49.73%. RHA, employed as the silica source for preparing the activating suspensions used in the production of ternary alkali-activated mortars, was supplied by DACSA S.A. (Tabernes Blanques, Valencia, Spain). The SiO2 content in RHA was 85.58% (Table 1), with 31.5% as amorphous silica determined by extraction using concentrated hydrochloric acid (37%) and 4 M KOH. The XRD patterns for all raw materials are presented in Figure 1. Calcium carbonates, as well as double calcium and potassium carbonate, were identified as crystalline phases in ABA. In contrast, RHA exhibited various SiO2 crystalline phases. Quartz and calcite were the predominant crystalline phases found in BFS.
Table 1. Chemical composition of the raw materials.
Figure 1. XRD patterns for (a) ABA, (b) BFS and (c) RHA. Legend: C for Calcite (CaCO3, PDFcard 050586), A for Arcanite (K2SO4, PDFcard 050613), P for Portlandite (Ca(OH)2, PDFcard 040733), F for Fairchildite (K2Ca(CO3)2, PDFcard 211287), B for Bütschliite (K2Ca(CO3)2, PDFcard 250626), N for Anhydrite (CaSO4, PDFcard 271496), K for Kovdorskite (Mg2(PO4)(OH).3H2O, PDFcard 330861), G for Gypsum (CaSO4.2H2O, PDFcard 330311), Q for Quartz (SiO2, PDFcard 331161), Tr for Trydimite (SiO2, PDFcard 181170), Cr for Cristobalite (SiO2, PDFcard 391425), S for Sylvite (KCl, PDFcard 411476).
All the raw materials (BFS, ABA, and RHA) were milled to increase their fineness. RHA was milled using an industrial machine, whereas ABA was ground in a laboratory ball mill (ash/ball mass ratio of 0.24) for 10 min. BFS was also milled in a ball mill for 30 min (ash/ball mass ratio of 0.35). The granulometric distribution of each one is presented in Table 2. The mean particle sizes of the BFS, ABA, and RHA are 30.4 µm, 21.5 µm, and 20.3 µm, respectively. Potassium hydroxide (KOH, 85% purity, pellets), potassium carbonate (K2CO3), and K2SiO3 aqueous solution (8% K2O, 20.8% SiO2 and 71.2% H2O), which were used to prepare reference mortars, were supplied by Panreac Quimica S.L.U. (Castellar del Vallès, Barcelona, Spain).
Table 2. Granulometric parameters of raw materials.

2.2. Methods

To evaluate the preactivation of ABA and RHA using TIB, the study was carried out in three stages, as shown in Figure 2. Firstly, the activating suspensions were characterised by measuring pH, viscosity, and FESEM-EDS analysis, as well as assessing their effect on the compressive strength of BFS alkali-activated mortars. In this stage, the mortars were cured in a thermal bath at 65 °C for up to 7 days to expedite the process. Next, secondly, using the optimal activator results, mortars were cured at room temperature, and their compressive strength was measured over 135 days. Finally, and thirdly, the carbon footprint of the binder mixtures was evaluated.
Figure 2. Methodology used to evaluate the preactivation of ABA and RHA in TIB to produce alkali-activated mortars.

2.2.1. Analyses of the Activating Suspensions

A study was undertaken utilising ABA and RHA in order to comprehend the impact of the thermal treatments on various properties of the activating suspensions. Several suspensions were prepared, and each composition, along with details of the thermal treatment and the corresponding analyses, is meticulously documented in Table 3. Each experiment was conducted with three replicates.
Table 3. Resume of the composition, thermal treatment, and analyses carried out on suspensions.
Here, to assess the influence of potassium solubilization on the alkalinity of the medium, the pH of the suspensions containing only ABA (s-ABA), only RHA (s-RHA), and a combination of ABA and RHA (s-ABA + RHA) was monitored over time. This analysis was conducted for suspensions maintained at 20 °C for 24 h or 85 °C for 2 h in a thermal bath. Furthermore, to measure the temperature increase resulting from the solubilization of potassium in ABA within the water, experiments were conducted at both room temperature (27 °C) and 85 °C. A suspension of s-ABA was prepared in a TIB, and temperature changes were tracked using a mercury-in-glass thermometer.
The change of viscosity of the suspensions s-ABA + RHA over a period of 24 h was measured. In addition, a suspension with double the RHA (s-ABA + 2RHA) was also under evaluation to help understand the effect of RHA dissolution on viscosity. In this context, viscosity was measured using a Bohlin Visco 88 BV viscometer (Bohlin Instruments Inc., Cirencester, UK), with a 30 mm measuring spindle set up for infinitely deep measurements and a rotational speed of 187 rpm. Initially, each suspension underwent a period of preparation of 1 h in a TIB with hot water at 85 °C. Then, since the viscosity should be measured at a constant temperature, the suspensions were cooled to 20 °C for approximately 10 min before each measurement. After that, the suspensions were maintained at 65 °C in a thermal bath for one, five, and 23 h to maintain the high-temperature conditions out of the TIB. In the process, the viscosity values were taken at each time interval, carefully repeating the same cooling process. The pH of each activating suspension was measured simultaneously with the viscosity using a Crison microPH2001 pH-meter (Crison Instruments, Alella, Spain).
In order to determine the chemical composition of the undissolved solids within the suspensions, suspensions of s-ABA and s-ABA + RHA were prepared using hot water in the TIB at 85 °C for 1 h. Subsequently, these suspensions were filtered using a filter paper with a grammage of 60 g/m2. The retained solids (undissolved fraction) on the filter paper were then dried out in an oven at 65 °C for 24 h. Additionally, the filtered suspensions underwent 24-h evaporation in an oven at 65 °C, resulting in a residual solid (dissolved fraction). The chemical compositions of the retained solid and the residual solid were also determined. Field Emission Scanning Electron Microscopy (FESEM) with Energy Dispersive X-ray (EDS) analyses were performed using a ULTRA55-ZEISS microscope (ZEISS, Madrid, Spain) with the samples coated with carbon. Also, the initial raw materials, ABA and ABA + RHA (same proportions equal to the suspension s-ABA + RHA), were FESEM-EDS characterised.

2.2.2. Preparation of the Activating Suspensions with ABA and ABA + RHA for Mortar Production

The activating suspensions were produced using only ABA for binary alkali-activated mortars, and combinations of ABA with RHA were utilised so as to prepare the suspensions in the case of ternary alkali-activated mortars. Also, the activating suspensions were prepared by mixing the biomass ashes with hot water in a TIB. Firstly, 180 (for ABA) or 225 mL (for ABA + RHA) of water was heated to 85 °C using a microwave oven (OMW 170G-W Model, 1200W, OK International Corporation, Suffolk, UK). Subsequently, the hot water was promptly mixed with the biomass ashes in a TIB. Such a temperature was set considering that a better solubilisation of potassium present in the biomass ashes is achieved at this temperature [25]. The biomass dissolution time in the TIB was set at 1 h and 24 h.

2.2.3. Production of Binary and Ternary Alkali-Activated Mortars

The proportions of all activating suspensions and solutions used to prepare the alkali-activating mortars are summarised in Table 4. The mass of ABA was fixed at 25% in addition to the mass of BFS. This percentage was determined based on the studies by Soriano et al. [28], in which it is shown that a better compressive strength can be achieved upon adding such a percentage of ABA. When combining RHA with ABA, the amount of RHA was fixed at 10% regarding the BFS mass, a similar value to the study by Font et al. [27]. In previous investigations [24,26,28], the reference mortar was prepared using KOH as the activating solution. In the present study, KOH is also used as the reference; however, K2CO3 is additionally incorporated as a commercial activator. This is because the composition of the ABA demonstrates the presence of potassium carbonate (Figure 1). Other researchers, such as Lima et al. [23], have shown that K2CO3 serves as an effective reference activator. In order for the activating solutions to be prepared with commercial activators, when using KOH or K2CO3, the solution was prepared at a K+ concentration of 5.3 mol·kg−1 for mortars with a water/BFS mass ratio of 0.4, and 4.2 mol·kg−1 for those with a ratio of 0.5. The masses of KOH and K2CO3 were accordingly chosen to approximate the hypothetical amount of potassium released from the solubilization of K2O present in the ABA used in mortar production. When adding RHA, it is demonstrated that potassium silicate forms [27]. For this reason, a reference activation solution is prepared using K2SiO3 and KOH. Also, for mix KS1-0.5, the added mass of K2SiO3 aqueous solution + KOH was 41.4% and 7.8% related to the BFS mass, respectively, providing a silicate species content equivalent to that from the complete dissolution of SiO2 in RHA (85.58%, Table 1). In contrast, for mix KS2-0.5, the K2SiO3 aqueous solution and KOH mass were set at 15.2% and 12.2% of the BFS mass, respectively, matching the amount of silicate species expected from the amorphous silica content in RHA (31.5%). In terms of SiO2/K2O molar ratio (ε), for mix KS1-0.5 and KS2-0.5, ε is equivalent to 1.52 and 0.5, respectively.
Table 4. Mixes of the alkali-activated mortars.
The precursor (binder) used in the mortar production was composed of 100% BFS, and the precursor-to-sand (B:s) mass ratio was fixed at 1:3. Each mortar was identified with an acronym, indicating the activating suspension/solution used. The acronyms used for the mortars’ identification are related to the activating suspensions compositions as follows: A for ABA, AR for ABA + RHA, K for KOH, KS1 and KS2 for KOH + K2SiO3, and KC for K2CO3. A number following each acronym also represents the water-to-BFS (w/B) mass ratio. While the w/B for binary alkali-activated mortars was fixed at 0.4, it was fixed at 0.5 for ternary alkali-activated mortars. Specific reference mortars were prepared with a w/B of 0.47 to analyse the influence of the water/solids ratio. For instance, the acronym AR-0.5 represents a mortar prepared with an activating suspension composed of ABA plus RHA and with a w/B of 0.5. Mortars with an acronym containing “rt” were exclusively cured at room temperature, while all others (without “rt”) were cured at 65 °C. In mortar R-A0.5-rt, RHA was directly mixed with BFS, and only ABA was pre-activated in TIB.
The activating suspensions/solutions were cooled to room temperature (20–25 °C) before mixing them with BFS and sand using a mechanical mortar mixer (Ibertest CIB-701). Firstly, the activated suspension/solution was mixed with BFS for 60 s; thereafter, sand was added and stirred for 180 s. The resulting fresh mortars were poured into metallic prismatic moulds measuring 40 × 40 × 160 mm3, according to UNE 196-1:2018 [31], and compacted using a vibration table (Toni VIB Model 5533, Toni Technik, Berlin, Germany)) for 60 s with an amplitude of 1 mm. Afterwards, the mortars were cured in a thermal bath (>90% RH) at 65 °C for 7 days, except those identified with the “rt” symbol. The latter were cured at room temperature (20 °C) under high moisture conditions (>90% RH) for 7, 28, and 135 curing days.

2.2.4. Compressive Strength of Alkali-Activated Mortars

The compressive strength of mortars was measured employing a universal testing machine, according to UNE 196-1:2018 [31]. For mortars cured in a thermal bath at 65 °C, the test was conducted after 7 curing days. Conversely, mortars cured at room temperature (20 °C) under high moisture conditions were assessed after 7, 28, and 135 curing days.

2.2.5. Carbon Emission Measurement of the Alkali-Activated Binders

The environmental impact of the binder’s mortars was calculated based on the carbon footprint. To calculate carbon emissions, the International Panel on Climate Change (IPCC) Equation (1) was applied.
E i = A i E F i
Here, carbon emissions ( E i ) are calculated by multiplying activity data ( A i ) by the emission factor ( E F i ). Activity data refers to the quantity of an activity or material that generates emissions, while the emission factor represents the average emissions produced per unit of an activity or material (typically expressed in kilograms or tonnes of CO2-equivalent per unit).

3. Results and Discussion

3.1. Characterisation of the Activating Suspensions

The pH of suspensions s-ABA, s-RHA, and s-ABA + RHA was continuously monitored under two conditions: (a) 24 h at 20 °C and (b) 2 h at 85 °C. The variation in pH over time for each suspension is illustrated in Figure 3. The initial pH value of 8.48 for these suspensions corresponds to the pH of the deionised water.
Figure 3. Monitoring of pH versus time for suspensions prepared with ABA (s-ABA), RHA (s-RHA) and ABA plus RHA (s-ABA + RHA) at (a) 20 °C and (b) 85 °C.
For the suspensions kept at 20 °C as seen in Figure 3a, notable pH shifts were observed within the first 10 min of initiating the test. The pH values increased from 8.48 to 9.95, 12.69, and 12.60 for the s-RHA, s-ABA, and s-ABA + RHA suspensions, respectively. This increase in pH was particularly prominent in the suspensions containing ABA (s-ABA and s-ABA + RHA). In these cases, the increased alkalinity is attributed to the dissolution of potassium compounds, as Alonso et al. [25] explained. The minor increase in pH (from 8.48 to 9.95) observed in the s-RHA suspension after the initial 10-min measurement period could be attributed to the dissolution of trace phases of potassium (3.32% in K2O, Table 1) present in RHA.
In the case of the s-ABA and s-ABA + RHA suspensions, a gradual pH increase was observed for 10 to 160 min. In the s-ABA suspension, the pH increased from 12.69 to 13.18, while in the s-ABA + RHA suspension, the pH increased from 12.60 to 13.10. Moreover, this pH increment must be attributed to the stepwise solubilization of K2O. After the 160-min mark, the s-ABA and s-ABA + RHA suspensions maintained a consistent pH level, averaging 13.20 and 13.14, respectively. Unlike the suspensions that include ABA, an opposite trend was observed in the pH of the s-RHA suspension. Over the time span of 10 to 220 min, the pH gradually decreased from 9.95 to 9.63. Following this initial period, the suspension’s pH plateaued at an average of 9.58, sustaining this level for the entire 24-h duration. This decrease in pH can be attributed to the pronounced acidic nature of RHA, a characteristic highlighted by Payá et al. [14].
In the s-ABA suspension subjected to the elevated thermal conditions of 85 °C (Figure 3b), the pH surged from 8.48 to 12.80 within 3 min, while for the s-ABA + RHA suspension, the pH increased to 12.27 during the same interval. Subsequently, the s-ABA suspension exhibited a gradual pH increment, ultimately stabilising at an average value of 13.10. In contrast, the pH of the s-ABA + RHA suspension followed a distinct trajectory, declining from 12.27 to 10.73 over 50 min. This lower pH was then sustained at an average of 10.73 for the subsequent 2 h. Remarkably, the s-RHA suspension maintained at 85 °C exhibited behaviour analogous to that at 20 °C, with an initial pH increase followed by a decrease, culminating in an average pH of 9.33 after 20 min, in a similar way to the pH of the s-RHA suspension at 20 °C (pH = 9.58). The decrease in pH after the third minute in the s-ABA + RHA suspension can be attributed to an acid-base reaction, but the interaction involving silica (acid) released during RHA dissolution in the highly alkaline medium generated by dissolution of potassium salts (base) from ABA [32]. Additionally, this pH decrease was exclusively evident in the s-ABA + RHA suspension maintained at 85 °C, underscoring the pivotal role of thermal treatment in inducing RHA dissolution.
In order to quantify the temperature rise attributed to the solubilization of K2O contained in ABA, two suspensions labelled s-ABA were meticulously prepared, one using room temperature water (27 °C) and another one using water at 85 °C. These suspensions were confined within a TIB, and temperature changes were continuously monitored using a mercury-in-glass thermometer. The evolution of suspension temperature with time is graphically presented in Figure 4. Upon mixing ABA with water, a noteworthy temperature rise was observed within the first 10 min. In the case of the s-ABA suspension formulated with room temperature water (27 °C), the temperature increased by 11 °C during this initial time frame. Conversely, the suspension prepared with water at 85 °C exhibited a temperature elevation of approximately 4 °C for the same period. This distinction in temperature increments, alongside the more pronounced pH changes experienced during the initial 10 min in the ABA-containing suspension, provides substantial evidence that a significant portion of the potassium compounds present in ABA has undergone solubilization by this point in time in an exothermic dissolution process [33].
Figure 4. Monitoring temperature versus time inside the TIB with suspensions based on ABA (s-ABA) prepared with water at 27 °C and 85 °C.
The viscosity measurements were conducted concerning s-ABA + RHA and s-ABA + 2RHA suspensions, as detailed in Section 2.2.1 and Table 4. The viscosity values versus the reaction time are depicted in Figure 5.
Figure 5. Viscosity of alkaline suspensions based on ABA and RHA at 20 °C.
Firstly, the viscosity of the suspension s-ABA was not measurable due to its high fluidity. However, the viscosity of the suspensions increased with the quantity of RHA, as well as the reaction time. Moreover, for both types of suspensions containing RHA, the viscosity displays a much faster rise within the initial six-hour period. Notably, in the first 24 h, the viscosity has risen more than twofold and fourfold for the s-ABA + RHA and s-ABA + 2RHA, respectively. Overall, the viscosity change was fittingly attributed to the jellification due to the potassium silicate-type gel formation and water absorption by the undissolved solids in the suspensions. Interestingly, a similar jellification behaviour was noted by Bouzón et al. [29], who observed the reaction during the extended dissolution of silica (from RHA) in a NaOH solution using a boiling reflux system.
FESEM-EDS analyses were conducted on both ABA and a mixture of ABA and RHA as pure raw materials. The latter mix used the same proportion for the s-ABA + RHA suspension (Table 4) and was milled to achieve proper homogenization. Additionally, FESEM-EDS analyses were performed on the retained solids after the filtration of the suspensions (1 h at 85 °C) and on the residual solids (i.e., dissolved solids) from liquid fractions after complete water evaporation. The percentages of retained and residual solids for each activating suspension are summarised in Table 5. FESEM micrographs of all these samples can be also found in Figure 6. Furthermore, the chemical composition based on EDS analyses for the ABA system and the ABA + RHA system is provided in Table 6 and Table 7, respectively.
Table 5. Proportions of the retained and residual solids after treatment in water at 85 °C in a TIB.
Figure 6. FESEM images of (a) ABA, (b) ABA-RHA, (c) retained solids from s-ABA suspension and (d) s-ABA + RHA suspension, (e) residual solids from filtered s-ABA suspension and (f) ABA + RHA suspension. Legend: A (unburned almond shell particles); R1 (RHA particles); and R2 (undissolved RHA particles).
Table 6. Chemical composition by FESEM-EDS of ABA and activator suspension based on ABA (retained and residual solids).
Table 7. Chemical composition by FESEM-EDS of milled solid (ABA/RHA) and activator suspension based on ABA/RHA (retained and residual solids).
For the s-ABA system, the sum of residual and retained solids was slightly higher than 100%, probably due to the formation of alkaline potassium soluble compounds (hydroxide, carbonate). However, for the s-ABA + RHA system, the sum was notably higher than 100%. The higher percentage of retained solids in the suspension s-ABA + RHA could be attributed to the undissolved solids from RHA/ABA and the water linked to potassium silicate gel associated with the jellification of the suspension [29].
As seen in Table 1, it is evident that ABA exhibits a notable loss on ignition (LOI) of 12.5%, primarily attributed to incomplete combustion of almond shell particles. These unburned particles are discernible in FESEM micrographs of ABA (labelled as A, Figure 6a), as well as in the micrographs of the undissolved fraction from the s-ABA suspension (Figure 6c). These same particles are also observable in both the mixture of ABA with RHA (Figure 6b) and the undissolved solid from the s-ABA + RHA suspension (Figure 6d). This structure had also been previously identified in FESEM micrographs of ABA by Soriano et al. [28]. In samples containing RHA, distinct structures associated with RHA are visible in the micrographs of the ABA + RHA mixture (labelled as R1, Figure 6b). Moreover, undissolved RHA particles can be discerned in the undissolved fraction from the s-ABA + RHA suspension (labelled as R2, Figure 6d). In Figure 6e,f, the formation of crystal structures is evident due to the evaporation of water and the subsequent crystallization of potassium compounds present in the liquid fraction (after filtration) of s-ABA and s-ABA + RHA, respectively.
As demonstrated in Table 6, the EDS analyses indicate that ABA has an average content of 61.84% K2O, 25.87% CaO, and 2.12% SiO2. However, noteworthy changes in the chemical composition are observed upon analysing the retained solids from the s-ABA suspension. The average content of K2O decreases significantly to 23.96%, representing a decrease of 61.20% compared to the original ABA composition. Conversely, the average content of CaO and SiO2 in the retained solids increases to 50.56% and 5.92%, respectively. From these observations, it can be inferred that approximately 61.20% of the total K2O content initially present in ABA was carried away with the filtrated suspension as solubilised potassium compounds. Furthermore, the residual solids from the filtrated suspension are primarily composed of K2O (96.0%) and a low percentage of SiO2 (4.0%). This reinforces the notion that the solubilization of other elements, such as calcium in ABA, is negligible. This suitably explains the increased CaO content observed in the undissolved fraction. The analysis is consistent with the understanding that the solubilization of certain elements, like calcium, in ABA is minimal, and it aligns with findings reported by Alonso et al. [25] regarding other biomass ashes (OBBA and OBFA).
The blend of RHA with ABA (ABA + RHA, starting material) led to changes in the oxide proportions compared to the system composed solely of ABA. As outlined in Table 7, the average percentages of K2O, CaO, and SiO2 in the mixture of ABA + RHA were 36.53%, 15.48%, and 40.78%, respectively. However, several significant changes in chemical composition are observed when analysing the retained solids from the s-ABA + RHA suspension. The average percentage of K2O decreased to 21.94%, indicating that approximately 39.90% of K2O, as solubilised potassium, was carried along with the suspension filtration. Interestingly, this value is lower than that observed for the system composed solely of ABA (61.20%). Furthermore, the residual solids from the filtrated suspension s-ABA + RHA were primarily composed of K2O (91.45%), with a minor amount of SiO2 (5.73%). Notably, despite the previous dissolution of RHA, the content of SiO2 in the filtrated suspension did not increase significantly compared to the system composed solely of ABA. This observation aptly supports the idea that dissolved silica reacts with potassium and produces potassium silicate gel, in line with the pH analyses. This phenomenon aligns with observations made by Bouzón et al. [29], who had reported analogous outcomes when filtering activating suspensions containing NaOH and RHA. According to their findings, the jellification process means that the silicate species formed were retained in the filter together with the non-dissolved RHA fraction.

3.2. Evaluation of Compressive Strength of Mortars

3.2.1. Influence of ABA on the Compressive Strength

The compressive strengths of mortars A-0.4, K-0.4 and KC-0.4, cured in a thermal bath at 65 °C for 7 days, are illustrated in Figure 7. While mortar A-0.4, prepared after 1 h of ABA preactivation in TIB, achieved compressive strengths of 58.0 MPa, the other one, prepared after 24 h of ABA preactivation, reached 56.2 MPa. Extending ABA preactivation from 1 to 24 h did not increase compressive strength. This finding showed that 1 h of ABA preactivation in TIB is enough to reach the best result in terms of compressive strength.
Figure 7. Compressive strengths at 7 curing days at 65 °C of binary (ABA/BFS) alkali-activated mortars (for 1 h and 24 h of ABA preactivation in thermally insulated bottle TIB) and reference alkali-activated mortars (KC-0.4: red line, and K-0.4: blue line).
A similar investigation was conducted by Soriano et al. [28], exploring the compressive strength performance of a “one-part” alkali-activated mortar applying BFS and ABA. In this study, the researchers had developed a mortar labelled as “Ad25,” where ABA was combined through dry mixing with BFS at a ratio of 25 wt.% in addition to the BFS. This mixture aligns with the composition of mortar A-0.4 in our current study. While the BFS employed for manufacturing the “Ad25” mortar shared the same chemical composition as the one employed in this study, the ABA was from a different source. Nonetheless, its chemical composition closely resembled the ABA used in the study. This allows for a reasonable comparison between the compressive strength performances of the “Ad25” mortar and mortar A-0.4. Following 7 curing days within a thermal bath set at 65 °C, the compressive strength of mortar “Ad25” yielded 45.2 MPa, whereas mortar A-0.4 exhibited a strength of 58.0 MPa (after 1 h of ABA preactivation). This highlights a substantial 23.8% enhancement in the compressive strength of alkali-activated mortars utilising these raw materials. Crucially, this improvement was achieved due to the prior dissolution of ABA, showcasing its influential role in enhancing the compressive strength development of these types of mortars.
For a comparative analysis, mortars K-0.4 and KC-0.4 were produced using KOH and K2CO3 as activators, respectively (Figure 7, K-0.4: blue line, and KC-0.4: red line). The results demonstrated a significant difference in compressive strength between the two. Specifically, the K2CO3-activated mortar exhibited a compressive strength of 59.8 MPa, markedly higher than the 44.8 MPa observed for the KOH-activated mortar. According to the literature, this disparity is attributed to the formation of additional binding phases in the case of the activation with alkaline carbonates, such as calcium carbonate, carboaluminates or hydrotalcite, among others, which densify the matrix [34,35]. When comparing mortars prepared with ABA to these two, their compressive strength performance is more similar to that of the K2CO3-activated mortar. This indicates that using ABA as an activator result in alkali activation comparable to that achieved with carbonate activators [36,37,38]. XRD analysis of ABA (Figure 1) supports these findings, showing that the mineralogical composition of ABA includes phases composed of potassium/calcium carbonates (K2Ca(CO3)2) such as fairchildite and bütschliite.
In this context, the results obtained are consistent with those reported by Lima et al. [21], who compared the use of mortars with blast furnace slag (BFS) as a precursor and K2CO3 or coffee husk ash as activators. These authors concluded that the mechanical properties of the mortars made with potassium-rich ash were comparable to those of the mortars using the commercial activator. Furthermore, in the case of the ash, the presence of insoluble carbonates or other insoluble compounds can enhance compressive strength through various physical effects, such as heterogeneous nucleation, acting as a filler, and reducing the water-to-solid ratio.

3.2.2. Effect of the Preactivation of RHA Combined with ABA

The comparison of compressive strengths of mortars AR-0.5, KS1-0.5, KS2-0.5, A-0.4 and A-0.47, cured in a thermal bath at 65 °C for 7 days, is illustrated in Figure 8. When RHA is preactivated (in TIB) in combination with ABA for 1 h, the mortar AR-0.5 achieved 64.7 MPa in compressive strength. Production of mortar AR-0.5 with 24 h of ABA + RHA preactivation was unsuccessful thanks to the extremely high viscosity caused by a jellification process, as illustrated in Figure 5. As demonstrated by Siddika et al. [39], both the rheological properties of activators and the attributes of precursors significantly influence the rheology of alkali-activated pastes. In line with this behaviour, the elevated viscosity of the ABA + RHA activating suspension after 24 h of preactivation negatively impacted the workability of the alkali-activated precursor materials.
Figure 8. Comparison of the compressive strengths at 7 curing days at 65 °C for ternary (AR-0.5), binary (A-0.4 and A-0.47) alkali-activated mortars (1 h biomass preactivation in TIB), and a reference alkali-activated mortar (KS1-0.5 and KS2-0.5).
Figure 8 shows that mortar AR-0.5 reached a compressive strength of 64.7 MPa, which is 75.7% of the value for mortar KS1-0.5 (85.5 MPa), and comparable to mortar KS2-0.5 (61.6 MPa). While mortar KS1-0.5 represents a hypothetical scenario in which the total silica content (both amorphous and crystalline—85.58%) in RHA is fully dissolved and reacts as silicate species, mortar KS2-0.5 denotes the strength level corresponding solely to the dissolution and reaction of the amorphous silica content in RHA (31.5%). Since AR-0.5 and KS2-0.5 mortars have similar compressive strengths, it suggests that ABA + RHA preactivation at least promotes reaction of the amorphous silica, achieving alkali activation comparable to commercial activators. Literature indicates that dissolving more than RHA’s amorphous silica requires higher alkalinity, temperature, or longer duration treatments [40,41].
Evaluating the compressive strength of mortars with RHA and ABA preactivated for 1 h shows that RHA enhances compressive strength compared to those without RHA (A-0.4). Comparing mortars, A-0.4 and AR-0.5, reveals an 11.6% strength increase due to RHA, despite the w/BFS ratio rising from 0.4 to 0.5. In this regard, a refined approach was adopted to facilitate a more distinct evaluation of the contribution of RHA to compressive strength. Mortar denoted as A-0.47 was meticulously prepared to enable a meaningful comparison with mortar AR-0.5. In this scenario, all these mortars share a uniform proportion of water concerning the initial solid content, denoted as the water-to-solid ratio (w/s), set between 0.37 and 0.38. The solid mass combines BFS, ABA, and RHA. Compared to the reference mortar A-0.47 (48.0 MPa), mortar AR-0.5 showed a significant 34.8% increase in compressive strength, reaching 64.7 MPa. This enhancement due to RHA is more notable than the 11.6% increase seen when comparing AR-0.5 to A-0.4.
Font et al. [27] used RHA preactivation with OBA via a thermal bath method (24 h at 65 °C) for BFS alkali-activated mortar. Their mortar, similar to our AR-0.5 (in terms of rich potassium biomass proportion) but with a 0.45 w/B ratio, showed a compressive strength of 58.4 MPa after 7 days at 65 °C. This ternary system outperformed binary BFS and OBA-based mortars due to silica dissolution in RHA facilitated by the potassium source from OBA, forming potassium silicate. Our findings agree with it, showing mechanical performance enhancement from ABA and RHA-based activators. Also, using hot water in TIB for preactivating biomass is as effective as a thermal bath but less energy-intensive.
A comparative analysis of electricity consumption was conducted to evaluate two preactivation methods: maintaining the activating suspension at 65 °C for 24 h in a thermal bath (SW22 model, Julabo, Seelbach, Germany) and heating water to 85 °C in a microwave oven for use in a TIB. Energy consumption was measured using an electricity monitoring device (PMB01 Model, Maxcio company). The results indicated that heating 1000 mL of water to 85 °C in a microwave requires 0.25 kWh, while maintaining the activating solution at 65 °C for 24 h in a thermal bath consumes 1.88 kWh per 1000 mL. These findings demonstrate that preactivating biomass ashes using preheated water in TIB is highly effective in terms of mortar’s mechanical strength and significantly more energy-efficient compared to sustained heating in a thermal bath.

3.2.3. Evaluation of the Compressive Strength Development at Room Temperature of Binary and Ternary Alkali-Activated Mortars

The compressive strength development of the mortars A-0.4-rt, AR-0.5-rt and R-A0.5-rt (Table 4), cured at room temperature (20 °C), was also evaluated. The biomass ashes were treated with water at 85 °C for 1 h before mortar production. Particularly, for the mortar R-A0.5-rt, RHA was dry mixed with BFS, being solely ABA previously preactivated. The compressive strength of these mortars was measured for 7, 28, and 135 curing days, and the results are summarised in Table 8. The relative compressive strength (RCS) of mortar AR-0.5 to mortars A-0.4 and R-A-0.5 was computed for a better analysis. These relationships have been depicted graphically in Figure 9.
Table 8. Compressive strength of the BFS alkali-activated mortars cured at room temperature (20 °C).
Figure 9. Comparison of compressive strength performance of mortar AR-0.5-rt regarding the mortars A-0.4-rt and R-A0.5-rt, based on relative compressive strength (RCS).
The rate of compressive strength development in mortars subjected to room temperature curing is notably slower than that of mortars undergoing curing at 65 °C. On examining the outcomes after 7 curing days in a 65 °C thermal bath, mortars A-0.4 and AR-0.5 (1 h for biomass ash preactivation) exhibited strengths of 58.0 and 64.7 MPa, respectively. In contrast, mortars A-0.4-rt and AR-0.5-rt reached 43.3 and 41.5 MPa, respectively. However, these mortars cured at room temperature achieved a comparable performance (54.7 and 67.0 MPa, respectively) after 28 curing days. This finding is consistent with the research conducted by Font et al. [27], which emphasised a notable increase in strength during the initial curing stages at 65 °C as opposed to 20 °C. The rapid development in compressive strength observed in mortars subjected to elevated curing temperatures can be attributed to the accelerated dissolution rate of BFS. This, in turn, leads to a more rapid formation of hydrated cementing gels [42].
When comparing mortar AR-0.5-rt to mortar A-0.4-rt, the relative compressive strength (RCS) ratios stand at 95.8%, 122.5%, and 117.1% for curing times of 7, 28, and 135 days, respectively. The lower RCS value at 7 days (95.8%) indicates that the initial strength development from RHA was insufficient to counterbalance the strength decrease stemming from the increased w/B ratio. Nonetheless, after 28 curing days, mortar AR-0.5-rt exhibited a notable 61.4% increase in compressive strength, in contrast to the 26.3% gain observed in mortar A-0.4-rt. This disparity underscores the significant role of RHA in compressive strength development, wherein the silicate species released from RHA dissolution primarily engage in reactions between the 7th and 28th curing days. During the span of 28 to 135 curing days, as the compressive strength gains for mortar AR-0.5-rt and A-0.4-rt amounted to 12.2% and 7.3%, respectively, it can be deduced that the strengthening effect is primarily attributed to the activation of BFS in both mortars.
The advantages of the previous dissolution of ABA facilitated through a TIB with heated water become apparent upon comparing the compressive strength of mortar A-0.4-rt, herein evaluated, with mortar Ad25, explored in the research by Soriano et al. [28]. The composition of mortar Ad25 closely resembles that of mortar A-0.4, as previously suggested, and suitably aligns with mortar A0.4-rt. In the mixture of mortar Ad25, ABA was dry blended with BFS, with subsequent curing at 65 °C. In this context, after a 7-day curing period at 65 °C, mortar Ad25 exhibited a compressive strength of 45.2 MPa, while mortar A-0.4-rt achieved 43.3 MPa following 7 curing days at 20 °C. Significantly, this underscores a noteworthy insight: using the previous preactivation of the ABA technique seems to compensate for the accelerated chemical reactions associated with curing in a 65 °C thermal bath. This compensation is particularly evident in terms of short-term strength performance.
To discern the specific contribution of RHA’s preactivation to the strength enhancement in mortars cured at room temperature, it is useful to compare the compressive strength of mortars AR-0.5-rt and R-A0.5-rt. Both mortars share the same mixture composition, as indicated in Table 4, with the key distinction of whether RHA is treated together with ABA or not in the preactivation process at 85 °C. Regarding RCS values, comparing mortar AR-0.5-rt to mortar R-A0.5-rt reveals ratios of 101.0%, 140.5%, and 128.2% for curing ages of 7, 28, and 135 days, respectively. These RCS values shed light on the favourable impact of the preactivation of RHA. For the initial 7-day curing period, the compressive strength of mortar AR-0.5-rt was 41.5 MPa, while mortar R-A0.5-rt achieved 41.1 MPa. This suggests that in the early stages of curing, RHA contributed similarly to the strength of both mortars, irrespective of RHA pre-treatment. However, the divergence becomes more apparent over 28 curing days. Mortar R-A0.5-rt exhibited a 16% strength increase from 7 to 28 curing days, whereas mortar AR-0.5-rt experienced a substantial 61.4% increase. This considerable gain must be primarily attributed to the slow dissolution of silicate species, which started with the preactivation and continued during the curing time. Comparing the strength gains from day 28 to day 135, mortars AR-0.5-rt (7.3%) and R-A0.5-rt (17.6%) show a similar mechanical development, mainly attributed to the prolonged activation of BFS. These findings further corroborate that the silicate species, resulting from the dissolution of RHA, primarily react between the 7th and 28th curing days.

3.2.4. Carbon Footprint Calculations

To enable a comparative assessment of the carbon footprint of mortars A-0.4-rt and AR-0.5-rt, reference mortars were established using Ordinary Portland Cement (OPC) (designated as OPC), apart from the reference mortars K-0.4, KC-0.4 and KS2-0.5. An additional reference was established to represent the scenario in which ABA and RHA were preactivated in a thermal bath (tb) at 65 °C for 24 h. This enabled a comparative assessment of the carbon footprint against the preactivation of biomass in the TIB method developed in this study. This reference was designated as AR-0.5-rt*tb.
For the calculation of emissions, a standardised binder content of 450 g per mortar was assumed for both OPC and BFS-based alkali-activated mortars. In the case of alkali-activated mortars, the dosage of commercial activators was set in Table 4. These values correspond to the hypothetical complete dissolution of K2O, which constitutes 49.73% of ABA by weight. Similarly, the required silicate content was calculated based on the assumed full dissolution of amorphous SiO2 content in RHA, which comprises 35.5%. These assumptions provide the basis for equivalence between the experimental and reference systems in terms of activator composition and enable a fair comparison of their associated carbon emissions.
The emission factor for OPC production was assumed to be 0.91 kg CO2 per kg of cement [43]. For BFS, ABA, and RHA, the emission factors were considered neutral, as these materials are by-products from other industrial processes. Water was also assumed to have no associated emissions. According to CarbonCloud data, the emission factors for KOH and K2CO3 were 1.48 kg CO2/kg [44] and 2.38 kg CO2/kg [45], respectively. Due to limited data on K2SiO3 solution production and its similarity to waterglass solution (Na2SiO3), the emission factor for K2SiO3 solution was assumed to be the same as that of waterglass, which is 1.2 kg CO2/kg. This value was sourced from the SimaPro 7.1 database (demo version) and agrees with the literature [46].
BFS and ABA were ground using a ball mill with a power rating of 0.3 kW. BFS was milled at a capacity of 450 g for 30 min, while ABA was milled at 300 g for 10 min. RHA was processed industrially in a mill with a 70 kg capacity and 2.5 kW power consumption over a period of 4 h [47]. For the preactivation of ABA and RHA, water was heated using a microwave, with an energy consumption of 0.25 kWh per 1000 mL, as calculated in Section 3.2.2. For the hypothetical preactivation using the thermal bath at 65 °C for 24 h, 1.88 kWh per 1000 mL is required, as detailed in Section 3.2.2. The emission factor for energy consumption was based on the national average provided by Instituto para la Diversificación y Ahorro de la Energía (IDEA), which is 0.25 kg CO2 per kWh [48].
Figure 10 presents a comparison of the total carbon emissions for each mortar, disaggregated by individual components and processing steps, thereby offering a comprehensive overview of the environmental impact associated with each system. The OPC binder emits 0.91 kg CO2 per kg binder, exhibiting the highest carbon emissions. This is followed by the alkali-activated binder synthesised using potassium silicate (mortar KS2-0.5), reaching 0.44 kg CO2 per kg binder, and the alkali-activated mortar activated with potassium carbonate (mortar KC-0.4), with 0.43 kg CO2 per kg binder. Alkali-activated binders generally emit much less CO2 than OPC binders, consistent with existing literature [49,50]. Additionally, carbon emissions from alkali-activated systems are largely due to the carbon footprint associated with commercial activators [51,52].
Figure 10. Comparison of total carbon emissions associated with each type of binder production.
In contrast, the binders A-0.4-rt and AR-0.5-rt, produced using biomass ashes preactivated with hot water, exhibit significantly lower carbon emissions, 0.12 and 0.13 kg CO2 per kg binder, respectively. These values correspond to emission reductions of 86.8% and 85.7% compared to the OPC-based binder. When biomass ash preactivation was considered to be carried out in a thermal bath at 65 °C for 24 h, the resulting binder (AR-0.5-rt*tb) displayed a carbon emission of 0.33 kg CO2 per kg binder, 2.5 times higher than the hot water-preactivated one (AR-0.5-rt), yet still 63.7% lower than that of the OPC-based binder. Notably, the biomass ash preactivation with hot water treatment used in mortars A-0.4-rt and AR-0.5-rt preparations contributes minimally to overall emissions, 0.0249-0.0311 kg CO2 per kg binder, while significantly enhancing the compressive strength of the resulting mortars. These findings fittingly highlight the environmental benefits of using alternative activators derived from agricultural by-products, such as ash from ABA and RHA, in alkali-activated materials and emphasise the effectiveness of the proposed hot water treatment as a low-impact method for biomass ash preactivation.
Additionally, the grinding process of BFS emerges as a major contributor to the carbon footprint of these systems. Optimisation through industrial-scale grinding will substantially reduce emissions, as evidenced by the lower emission factor associated with industrially ground RHA compared to lab-scale ground BFS.
The CO2 emissions per kilogram of binder per unit of compressive strength were calculated for each binder type and depicted in Figure 11. Compressive strength measurements were taken after 28 days of curing at room temperature. The compressive strengths recorded for OPC (w/b = 0.4), K-0.4, KC-0.4, and KS2-0.5 mortars were 76.4 MPa, 31.3 MPa, 63.3 MPa, and 62.3 MPa, respectively. The compressive strengths of mortars A-0.4-rt and AR-0.5-rt are presented in Table 8. For the reference sample AR-0.5-rt*bt, the compressive strength was thought to be the same as that of AR-0.5-rt.
Figure 11. Comparison of CO2 emissions per kilogram of binder per unit of compressive strength for each type of binder.
OPC remains the most polluting binder, with CO2 emissions of 0.0119 kg CO2/kg binder·MPa. This is followed by binders K-0.4 (0.0093), KS2-0.5 (0.0071), and KC-0.4 (0.0068). Among the binders using commercial activators, the KOH-activated binder is the least efficient, as it results in lower compressive strength compared to the others. In contrast, binders activated with ABA and a combination of ABA and RHA represent more sustainable alternatives, emitting only 0.0022 and 0.0019 kg CO2/kg binder·MPa, respectively. However, when thermal bath preactivation is applied, as in the AR-0.5-rt*bt reference binder, CO2 emissions increase to 0.0049, indicating a higher environmental impact compared to TIB preactivation.
In general, our findings agree with Fan et al. [51], who had conducted a life cycle assessment of alkali-activated binders in which 50–100% of the activator was replaced with ABA. Their conclusions indicate that these binders exhibit lower greenhouse gas emissions per unit of compressive strength and can achieve a reduction in GHG emissions ranging from 8% to 56%.

4. Conclusions

This study evaluated the preactivation of ABA and RHA in a TIB with hot water to produce binary (ABA/BFS) and ternary (RHA/ABA/BFS) alkali-activated mortars. Based on the results, the following findings are presented in what follows:
  • Based on the compressive strength of binary and ternary alkali-activated mortars, one-hour treatment was the optimal duration for preparing the activating solutions in a TIB filled with hot water (85 °C). For larger preactivation times, the viscosity of the activating suspensions increased due to a process of jellification, accompanied by water absorption by undissolved particles.
  • The dissolution of biomass ashes (ABA and RHA) employing the TIB with hot water technique enhances the compressive strength performance of binary and ternary alkali-activated mortars. Notably, when RHA was treated jointly with high-potassium biomass ash, the compressive strength performance of ternary alkali-activated mortars exceeded that of binary alkali-activated mortars by 34.8%.
  • The compressive strength development of binary and ternary alkali-activated mortars, when cured at room temperature (20 °C), exhibited a slower rate compared to those cured in a thermal bath at 65 °C. After 135 curing days at room temperature, the binary and ternary alkali-activated mortars reached 61.4 MPa and 71.9 MPa.
  • The preactivation of RHA with ABA played a crucial role in enabling RHA to exhibit reactive behaviour in the ternary alkali-activated mortar, which was cured at room temperature. The significant impact of RHA on the strength was particularly evident in the substantial increase in compressive strength, amounting to 61.4%, observed between the 7th and 28th curing days.
  • The BFS-alkali activated mortars made with ABA and ABA plus RHA reduce CO2 emissions by 86.8% and 85.7% compared to OPC-based mortars, respectively.
These data provide compelling evidence that the treatment of RHA alongside ABA, using a thermally insulated bottle (TIB) with hot water, represents a richly valuable technique for manufacturing alkali-activated mortars based on BFS and rich-potassium biomass ashes. The TIB with hot water technique proved to be an effective alternative for achieving the dissolution of rich-potassium and rich-silica biomass ashes while consuming less energy than a traditional thermal bath. Future studies should focus on scaling up the preparation of the activating solution by utilising larger-capacity, thermally insulated containers. Additionally, research should evaluate the efficiency and scalability of various filtration systems for solution purification to identify optimal approaches for removing impurities and improving process consistency.

Author Contributions

Conceptualization, D.B.I., J.M., M.M.T. and J.P.; methodology, D.B.I., L.S. and J.P.; validation, D.B.I., J.M. and M.V.B.; formal analysis, L.S., J.M. and M.M.T.; investigation, D.B.I., L.S., M.M.T. and J.P.; resources, L.S. and M.V.B.; data curation, D.B.I., J.M. and M.M.T.; visualization, D.B.I.; writing—original draft preparation, D.B.I., M.M.T. and J.P.; writing—review and editing, D.B.I., L.S., J.M., M.V.B., M.M.T. and J.P.; supervision, M.M.T. and J.P.; project administration, J.M. and M.V.B.; funding acquisition, J.M., M.V.B. and J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We thank the Electron Microscopy Service of the Universitat Politècnica de València. Mauro Mitsuuchi Tashima wishes to thank the Ramón y Cajal grant RYC2023-045079-I, funded by MCIU/AEI/10.13039/501100011033 and by FSE+.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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