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Article

Effects of Ca/Si and Si/Al Ratios on the Wood Biomass Ash-Based Alkali-Activated Materials with Pozzolanic Additives

by
Yiying Du
,
Ina Pundienė
* and
Jolanta Pranckevičienė
Laboratory of Concrete Technology, Institute of Building Materials, Vilnius Gediminas Technical University, Linkmenų Str. 28, LT-08217 Vilnius, Lithuania
*
Author to whom correspondence should be addressed.
Buildings 2026, 16(1), 51; https://doi.org/10.3390/buildings16010051
Submission received: 4 November 2025 / Revised: 12 December 2025 / Accepted: 20 December 2025 / Published: 22 December 2025
(This article belongs to the Special Issue Sustainable Concrete: Research on Waste Utilization and Performance Optimization)
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Abstract

Wood biomass ash (WBA) is a by-product from biofuel energy plants. The disposal of this waste is connected with numerous environmental concerns. A more sustainable choice is to recycle it as a raw material for building and construction materials. However, due to its unstable characteristics, its application in alkali-activated materials (AAM) poses a challenge. One issue is the development of the mechanical properties. To improve them, pozzolanic additives, including coal fly ash (CFA), metakaolin (MK), and natural zeolite (NZ), were added at replacement ratios of 10–40%. Calcium hydroxide, sodium hydroxide, and sodium silicate were used together as ternary activators. The samples were cured at 60 °C for the first 24 h and for the remaining 27 days at room temperature. Mechanical behavior, water absorption, and chemical compositions were examined. The results obtained from XRF were compared with the calculation results of the chemical compositions based on the mix design and oxide compositions of the raw materials. The results show that the respective optimum replacement ratios were 30% CFA, 20% MK, and 20% NZ, with the highest compressive strength corresponding to 22.71, 20.53, and 24.33 MPa, and the highest flexural strength of 4.49, 4.32, and 4.21 MPa. NZ was the most effective in AAM, due to the highest Si/Al ratio in the Ca-rich ambient. Then, CFA contributed less, and MK was the least efficient when used in combination with WBA in AAM. The reduction of Ca/Si ratios in the AAM caused by the pozzolanic additives favors the formation of a binder system made of different hydrates and facilitates the strength enhancement when the Ca/Si ratio is lower than 0.35.

1. Introduction

Alkali-activated (geopolymer) concretes encompass a wide range of sustainable formulations by using lightweight aggregates, waste-based materials, and modified activation conditions. Lightweight geopolymer concrete incorporating pumice and waste rubber demonstrated reduced density and a favorable waste tire rubber ratio of 10% [1]. Pumice and expanded perlite have also been used to produce low-density fly ash-based geopolymer concretes with adequate mechanical strength and favorable microstructural characteristics [2]. Waste-aluminum–based and perlite-based alkali-activated systems further highlight how activator concentration, pore-forming agents, and silica enrichment influence strength, porosity, and thermal properties [3,4]. Additional studies on recycled rubber-incorporated geopolymer composites show that full replacement of fine aggregates with waste rubber is feasible in the production of sustainable composites with enhanced toughness and strain capacity [5]. Together, these works illustrate the application of alkali-activated concretes, ranging from lightweight and insulation materials to rubberized composites, all contributing to reduced environmental impact through waste utilization and cement-free binder systems.
The arrival of biofuel energy brings a problem with waste management of wood biomass ash (WBA), which is mainly disposed of in landfills. Considering its environmental concerns, one sustainable way can be the valorization of it in building and construction materials, which reduces the environmental burden due to the disposal of this waste and allows the transformation of a valueless by-product into a sustainable material with practical implications. This is of great ecological significance. For the utilization of WBA, there have been studies in Portland cement (PC)-based materials. For instance, Hamid and Rafic [6] studied the partial replacement of PC via WBA, reporting that small substitutions—particularly around 10%—can improve strength in some cases, although variations in ash composition and reduced workability limit its broader applicability. Vu et al. [7] researched WBA–PC particleboards, indicating that incorporating up to 30% WBA as a partial PC replacement can maintain acceptable mechanical performance while improving thermal insulation. Bhat [8] studied self-compacting concrete incorporating WBA as partial PC replacement, showing that up to 10% substitution can produce comparable strength to conventional concrete, offering an eco-friendly and sustainable alternative for reducing CO2 emissions. These results show that only with a small addition of less than 30%, has WBA positive effects. Another alternative is to use WBA in AAMs. Abdulkereem et al. [9] investigated the blended WBA–coal fly ash geopolymer mortars, pointing out that partial substitution of fly ash at a maximal 20% by WBA enhanced early-age strength, while a 10% substitution yielded optimal 28-day performance due to the newly formed calcium polysialate and calcium silicate hydrates. Cheah et al. [10] studied WBA and pulverized fuel ash hybrid geopolymers. Results show that combining these ashes at ratios between 70:30 and 50:50 can produce low-alkalinity mortars with sufficient strength, stiffness, and durability for load-bearing masonry, attributed to the formation of potassium polysialate and gehlenite phases. Du et al. [11] developed an AAM with 100% WBA as the only precursor. However, the highest compressive strength only reached 6.3 MPa. These results show that there is a possibility to incorporate a higher amount of WBA in AAMs, but there is still the challenge of limited strength when it is added in a large amount.
One solution can be the incorporation of chemically active materials as binary precursors in small amounts, such as the introduction of pozzolanic materials into the WBA-based alkali-activated binder. One example is coal fly ash (CFA). As an industrial by-product, its use in geopolymer has been widely investigated, and some efforts have been made on the hybridization of CFA and WBA [9,12], demonstrating its feasibility. In addition to the industrial wastes, commercial materials are also incorporated as precursor materials for geopolymers. Metakaolin (MK) is one such common precursor material. There have already been some efforts made to include WBA in MK-based geopolymers, but no uniform conclusion has been achieved. Some studies reported a continuous loss of strength [13] with an increase in the WBA ratio, while others noted that WBA usage at less than 50% favored strength development [14]. This can be attributed to the wide range of WBA physicochemical properties that influence its effects on geopolymers [15]. Natural zeolite (NZ) is another precursor source that has acquired much attention in recent years due to its unique microstructure. It is a crystalline, hydrated aluminosilicate material with an open, three-dimensional framework of silica and alumina tetrahedra with shared oxygen atoms. In its structure, intercrystallite channels and chambers with ions and water molecules form a web-like morphology. Studies have demonstrated its positive effects when used in geopolymer materials [16,17,18], but regarding its hybrid utilization with WBA, there has been no relevant research.
The elemental compounds in the produced AAMs significantly influence microstructural development and the formation of hydrate products. Wang et al. [19] compared the structure of calcium silicate (aluminate) hydrate (CS(A)H), finding that a lower Ca/Si ratio at 0.8, in contrast to that of 1.2, contributed to longer silica chain length and less structural disorder. The introduction of Al, decreasing the Si/Al ratio, was key to the introduction of additional phases and especially the formation of strätlingite and Al(OH)3. Wang et al. [20] also pointed out that the micromechanical behavior of CSH is governed largely by its Ca/Si ratio, the degree of Al incorporation, and the presence of secondary phases. Lowering the Ca/Si ratio promotes greater silicate chain polymerization, produces a more compact layer arrangement, and decreases porosity. Al additions further modify the structure by generating cross-linked Q3(Al) units, which substantially improve the rigidity and overall mechanical performance of CSH. Thus, understanding the compositions of Ca, Si, and Al in the materials is important in analyzing their property evolution.
Additionally, incorporating recycled fine aggregates could further enhance sustainability. Natural aggregates are non-renewable natural resources. Around 48.3 billion tonnes are used in the making of concrete per year [21], which imposes a significant burden on the conservation of natural resources. Meanwhile, the mining and processing of them are associated with large energy consumption [22]. Thus, some researchers have introduced recycled aggregates to partially or fully replace natural aggregates, highlighting their increasing sustainability [23,24].
The research gap existing is that, in most of the studies, WBA was used in a small amount to partially replace other precursor materials due to its low chemical reactivity, and when it is used alone, there remains a challenge to guarantee the mechanical properties. Considering this and the sustainability of WBA valorization, which was discussed in former investigations [11,25], the objective of this study was to utilize a large amount of WBA to produce an AAM with good engineering properties. To achieve this, pozzolanic additives were introduced as binary precursors. Another existing research gap is that there have been limited investigations addressing the feasibility of the co-utilization of a large amount of WBA together with CFA, MK, and NZ, especially for the hybrid utilization of NZ with WBA, where there has been no relevant research. Thus, the objective was also to analyze the feasibility of their co-usage with WBA, and the mechanism behind it.
Therefore, in this study, for the purpose of widening the application of WBA in AAMs, two types of WBAs (WBBA and WBFA) were valorized as the primary precursor materials. To improve the mechanical properties of WBA-based AAMs, pozzolanic additives CFA, MK, and NZ were applied at 10, 20, 30, and 40% of total precursor mass. Calcium hydroxide (CH), sodium hydroxide (SH), and sodium silicate (SS) were activators. Recycled sand (RS) was utilized too as a fine aggregate. Mechanical strength, water absorption, chemical compositions via XRF, and phase analysis using XRD were performed to characterize the produced mortar. Meanwhile, the results obtained from XRF were compared with the calculation results of the chemical compositions based on the mix design and oxide compositions of the raw materials. This provided an insight into the possible application of WBA in AAMs.

2. Materials and Methods

2.1. Raw Materials

The precursor materials of AAM included WBA (WBFA and WBBA), as well as MK, CFA, and NZ, as pozzolanic additives. RS from biofuel plants was used as a fine aggregate. WBAs and RS were collected from a thermal plant and supplied by Ekobaze Waste Management in Vilnius, Lithuania. The difference between WBBA and WBFA is that WBBA is made up of heavier unburned or partially burned materials that settle at the base of the furnace, while WBFA consists of the fine particles carried upward in the exhaust stream and captured by air-pollution control equipment [10]. RS comes from natural river sand which is burnt together with the wood biomass in the thermal plants. After collection, the ash was pre-treated using a ball milling equipment for 24 h. A 0.5 mm sieve was also used on WBBA. RS was pre-processed with a 1.25 mm sieve. CFA was collected from an energy producing factory in Krakow, Poland. Before use, it was also milled for 24 h. Commercial MK was produced by Bauchemie GmbH (Bottrop, Germany). The ball mill equipment used was DECO-JM. The volume was 1.5 L. The rotation speed was 480 rpm, and the ball-to-powder ratio 2:1. The duration of the milling for all three ashes (WBFA, WBBA, and CFA) was 24 h.
Alkaline activators included liquid water glass (WG), powdery sodium hydroxide (SH), and calcium hydroxide (CH). They were from Lerochem (Klaipeda, Lithuania). CH was used as a dry powder at 10% by precursor weight, while SH was dissolved in tap water to obtain an SH solution with a concentration of 7 mol/L. WG contained 50% sodium silicate (SS) and 50% water by weight, and its density was 1388 kg/m3. The molar ratio of SiO2/Na2O of WG was 3.2. In the alkali-activation solution containing SH solution and WG, the molar ratio of SH/SS was 3, and the molar ratio of SiO2/Na2O was 2.5. The dry powder of SH was added to half the weight of WG to maintain a dry powder weight ratio of SS to SH at 1. The pH value of sodium silicate solution was 11.3 and that of sodium hydroxide solution was 13.4. The pH value of the activating solutions was 13.2. Boric acid (BA) from Lerochem (Klaipeda, Lithuania) was added as a retarder at a concentration of 1% by precursor weight.
The properties of the raw materials are summarized in Table 1. The lightest materials were NZ and MK, with bulk densities of 0.24 and 0.43 g/cm3, respectively. RS and WBBA had the highest density, exceeding 1 g/cm3. WBFA exhibited a great loss of ignition, related to its high carbon content, which negatively influences the long-term strength development and durability [26]. That is one of the factors making it difficult to be utilized. According to the chemical composition of the raw materials, wood biomass wastes (WBA and RS) are primarily composed of SiO2 and CaO, whereas CFA, MK, and NZ are aluminosilicate-rich materials.
Figure 1 shows the particle size distributions. CFA, MK, and NZ showed the highest number of comparatively finer particles, with most particle sizes distributed between 1 and 10 μm. The particle diameters of WBBA and WBFA were mainly centered within the range of 10 to 100 μm, whereas most RS particles had a size of over 100 μm. The D50 values of the raw materials were 33.9 (WBBA), 16.2 (WBFA), 227.4 (RS), 3.7(CFA), 6.2 (MK), and 6.0 (NZ). After the same pretreatment, WBFA exhibited a smaller D50 than WBBA, with a reduction of 52.2%, indicating that WBFA may be a more effective precursor material than WBBA, as demonstrated in previous research [11].
Figure 2 displays the crystalline phases. SiO2 was identified in all the raw materials. In WBA, WBBA also contained CaCO3, Ca(OH)2, and CaSiO3, while WBFA consisted of Ca(OH)2, CaCO3, and CaO. RS consisted of CaSiO3. For the other precursor materials, kaolinite (Al2Si2O5(OH)4), mullite (Al6Si2O13), and clinoptilolite were detected in MK, CFA, and NZ, respectively. Compared with the results of the oxide composition of the materials, although there was a high amount of SiO2 in both wood biomass wastes and other precursor materials (MK, CFA and NZ), the different phases determined different chemical reactivity.

2.2. Mix Proportions and Sample Casting

In Table 2, the mix design is specified. Four groups were prepared and investigated. WBFA and WBBA were added at the same amount, and the replacement ratios of WBA with CFA, MK, or NZ, were 10%, 20%, 30%, and 40%. The fine aggregate/precursor ratio was 2 due to a consideration of strength development, which was tested in the preliminary experiments. The water/precursor ratio was 0.55. The liquid/binder ratio was 0.44. Na2O equivalent percentage was calculated according to the formula Na2O eq = Na2O + 0.658 × K2O.
To prepare the alkali-activator solution (AAS), SH was dissolved in tap water at 60 °C to obtain an SH solution, because SH dry powder has a higher dissolution degree in hot water. Then, after being cooled to room temperature, it was mixed with WG and stirred for 1 min. The dry powders of WBFA, WBBA, CH, BA, and RS were weighed and mixed for 2 min. The dosage of BA was around 2.65 g per kg of mortar, which had an effect on retarding the reaction and thus prolonging the initial setting. Then, the AAS was poured into it and homogeneously stirred for 3 min. Afterwards, it was placed into steel prism molds with dimensions of 160 mm × 40 mm × 40 mm, and stacked in several layers. The molds were then shaken for 2 min. There was no bleeding or segregation, due to the lower flowability of the produced materials compared to the traditional PC-based materials. During the casting, qualitative observation of the workability was made. The molds were subjected to a 60 °C heat treatment for 24 h before demolding. Usually, the temperature treatment for AAMs is set between 60 °C and 100 °C for 24 h [15]. In this investigation, a comparatively lower temperature of 60 °C was selected in consideration of a decrease in energy consumption. The demolded samples were labelled and placed in laboratory settings with plastic film for 28-day curing at room temperature (22 ± 2 °C) and ambient humidity with no specific humidity control.

2.3. Testing Methods

The particle size distribution was examined via a “Cilas 1090 LD” (Orleans, France) particle size analyzer with a diameter range of 0.01–500 μm in wet mode. The flexural and compressive strengths were tested via a WDW-20 universal testing apparatus. Flexural strength was evaluated using a three-point bending test at a loading rate of 50 N/s, while compressive strength was measured at a loading rate of 400 N/s. Four samples were subjected to flexural strength tests, measuring 160 mm × 40 mm × 40 mm in size. Six samples were prepared in compressive strength tests with a sample size of 50 mm × 50 mm × 50 mm. The surfaces of the samples were polished to be even before the testing. Afterwards, the fractured pieces of five representative samples (REF, F30, M20, Z20, and P10) were collected for further microstructural analysis. The crystalline phases and mineralogical compounds were identified using an X-ray diffractometer from SmartLab (Rigaku, Tokyo, Japan), with a 2-theta range up to 80°, to obtain X-ray diffraction (XRD) patterns. The equipment has an X-ray tube with a 9 kW rotating Cu anode and a step size of 0.02°. The scanning rate is 1 s per step. The chemical composition of the raw materials and samples was identified using a high-performance multichannel sequential wavelength dispersive X-ray fluorescence (XRF) spectrometer (AXIOS-MAX, Panalytical, Eindhoven, The Netherlands). The operation wavelength was 1440 s. The elemental ratios were calculated based on the oxide compositions in the XRF results. Before XRD and XRF analysis, the samples were ground into a fine powder of less than 0.063 mm. The water absorption was tested by immersing the samples in water. The weight of the samples was recorded per 24 h until the weight changes were smaller than 1%. Before immersing into water, the samples were oven-dried under 60 °C and the mass change was inspected every 24 h until it was less than 1%. The water was at room temperature (22 ± 2 °C). The data of equilibrium saturation was collected, and the calculation followed the equation below:
W = m 1 − m 2 m 2 × 100 %
where W is the water absorption of the samples, %; m1 is the mass of the immersed sample, g; m2 is the mass of the dried sample, g.

3. Results and Discussions

3.1. Strength Development

The 28-day compressive strength results are illustrated in Figure 3. The ANOVA F-value is 30.677. Tukey HSD does not identify distinct pairwise differences. The reference sample with only WBAs had a compressive strength of 15.44 MPa. The replacement with CFA, MK, and NZ at 10 to 40% of the precursor weight showed positive effects on the strength increment, ranging from 3.11% to 57.62%. The highest strength values were recorded at 22.71, 20.54, and 24.33 MPa, respectively, representing improvements of 47.09%, 33.03%, and 57.62% over the initial values, as seen in Table 3. This is attributed to the rich aluminosilicate minerals in CFA, MK, and NZ, as shown in Figure 2, which act as reactive precursor sources and contribute to the formation of increasing amounts of geopolymeric gels, as reported in other research [27,28,29]. Along with the replacement ratio growing to 40%, the same strength evolution pattern was observed for CFA, MK, or NZ-incorporated samples. The compressive strength was enhanced to the highest values, then reduced after the optimal ratio was attained at 30, 20, and 20%, respectively, indicating the existence of a maximal compacity of pozzolanic additive utilization in the WBA-based binder system. Furthermore, the unreacted particles can absorb water in the matrix given a large specific surface area and a porous nature, thereby interfering with the hydration process. Well-crystallized hydration products in the shape of plates and small needles, belonging to CSH and ettringite, were observed in the sample with solely biomass ashes. However, there was a considerable number of microcracks and pores, which hindered the strength growth of the sample. In contrast, the addition of CFA or MK or NZ precursors visibly densified the microstructure of AAMs, as observed in the SEM micrographs in previous research [30]. Previous research utilizing CFA, MK, or zeolite products together with biomass ashes reported an optimal replacement dosage in the improvement of mechanical properties at 30–40%, 50%, and 1–5%, respectively [10,14,31]. These vary from the results obtained in this investigation, mainly influenced by the alkalinity, the degrees of the curing conditions, and particularly the characteristics of biomass ashes, which significantly differ due to the biomass source and combustion settings [15].
A similar development of flexural strength is observed in Figure 4 as the compressive strength. The ANOVA F-value is 2.924. Tukey HSD does not identify distinct pairwise differences. The reference sample strength reaches a value of 3.58 MPa, and the substitution of WBA with CFA, MK, and NZ contributes to an increase in strength ranging from 0.84% to 25.53%. The highest flexural strength was observed at a replacement ratio of 30% CFA, 20% MK, and 20% NZ, with strengths of 4.49, 4.32, and 4.21 MPa, respectively, representing increases of 25.53%, 20.67%, and 17.60%, as shown in Table 3. An improvement in the strength was recorded with the growth of the supplementary precursor amount before the optimal content was achieved. This indicates the positive effects of adding pozzolanic additives composed of aluminosilicate-based minerals (Figure 2) to the WBA-based mortar, enhancing the amount of active Si and Al. After that, when the replacement ratio exceeded the thresholds of the binder system, the strength began to decrease, as indicated by the compressive strength results. As a commonly used geopolymer precursor, MK is usually very effective in promoting geopolymerization. However, in this study, CFA outperformed MK in improving the mechanical properties, even though the aluminosilicate content in the raw material MK was higher than that in CFA. Similar strength loss at a high amount of MK usage was also observed by other researchers [13,14], primarily due to the structural defects caused by the unreacted MK particles [32].

3.2. Water Absorption

In Figure 5, the bulk density varied around an average value of 1891.8 kg/m3, with a fluctuation of 3.93%. The alternation of the sample bulk density is determined by the particle size (Figure 1), the density, and the physical properties (Table 1) of the raw materials, as well as the pore structure development of AAM. As the pozzolanic additive materials are comparatively lighter than WBBA and have a more porous microstructure, the partial replacement of WBA with these materials, as their content increases, can lead to a reduction in the mixture density. This explains the overall decrease in mortar density when the CFA amount increases from 30 to 40%, MK grows from 10 to 30%, and NZ increases from 10 to 40%. On the other hand, the supply of pozzolanic sources in the binder system improves the reaction degree, which can form a denser mortar microstructure due to the production of a greater amount of gel products, thereby enhancing the bulk density [33]. This corresponds to the observed results, where samples achieve higher bulk density compared to REF after using a secondary precursor, especially for those with NZ.
Water absorption is affected by the porosity and pore structure of the materials, particularly the existence of open pores. Similar to existing research results, the AAMs produced reached water saturation within 24 h [34]. In Figure 5, the evolution of the water absorption aligns with the changes in the bulk density. The correlation coefficient R2 between water absorption and bulk density was −0.41, indicating a weak correlation. REF showed a water absorption of 10.20%. After the use of an extra precursor, the water absorption values exhibited an enhancement of between 0.33% and 18.20%. One factor that can be attributed to the introduction of lighter and more porous precursor materials, with a finer particle size distribution, is the generation of microvoids in the mortar system. Meanwhile, the greater specific surface area and microstructural characteristics determine that the particles of CFA, MK, and NZ tend to absorb more water, which also accounts for the increase in water absorption [28,29].
When CFA was added from 10 to 40%, the water absorption increased by 1.95% to 12.15% at 20% CFA usage, then it decreased slightly to 11.67% at 40% CFA. This indicates that although the use of CFA can increase water demand in the mortar system, its increasing addition promotes the activation reaction, producing more hydration products that effectively densify the mortar microstructure. For mixes with MK and NZ, a similar increment in water absorption was observed at the beginning, reaching 12.59% and 13.50%, respectively, at a content of 30%. The water absorption value then maintained the same level when the amount of MK and NZ increased to 40%. It reveals that, unlike CFA, the continuous addition of MK or NZ due to the zeolitic structure, which captures water, is less effective in improving the reaction degree of the mortar binder, as indicated by the strength results, where the largest strength is achieved at 20% MK or NZ. The growing use of MK or NZ exceeds the capacity of the binder system, contributing to insufficient reaction and the presence of unreacted precursor particles. Also, due to the structure of NZ that tends to capture water, the higher the amount of NZ added, the more the amount of water was absorbed.

3.3. Chemical and Phase Composition

In Figure 6, furthermore, XRF results for the primary elements in four representative samples are presented, along with the ratios between the main compounds (SiO2, CaO, Al2O3, and Na2O). Compared to REF, the utilization of CFA, MK, and NZ enhanced the contents of SiO2 and Al2O3, while the amount of CaO was decreased. Particularly in samples F30 and Z20, the Si/Na and Al/Na ratios appear to have increased. It confirms the positive effects of active Si and Al contents, resulting from the use of pozzolanic additives, which favor the production of NASH. As a polymeric gel, NASH in the mortar can further improve the mechanical properties, contributing to higher strength.
The chemical content ratios of Si/Na, Ca/Na, and Al/Na in the representative samples calculated based on the XRF results are presented in Figure 7, with correlation coefficients of 0.74, −0.01, and 0.5, respectively. The calculations were as follows: Si/Na = ((SiO2)%/60.08)/2((Na2O)%/61.98); Ca/Na = ((CaO)%/56.08)/2((Na2O)%/61.98); Al/Na = 2((Al2O3)%/101.96)/2((Na2O)%/61.98). The calculation of the data in Figure 8, Figure 9, Figure 10 and Figure 11 followed the same procedure. In the F30 sample, all three ratios remained at higher values when compared to others. This indicates that the utilization of CFA at 30% introduces a significant amount of aluminates and silicates, and at the same time, can serve as an additional source of Ca, related to its effective improvement of strength.
Figure 8 and Figure 9 illustrate the relationship between the strength and Ca/Si and Si/Al ratios, calculated from the material composition, which significantly influence the reaction degrees and product categories [28]. Due to the significant amounts of CaO and SiO2 in the sample compositions, an evident trend was observed in Figure 8, which was further confirmed by the XRF results. As the Ca/Si ratio decreases, the strength increases, especially when the content of Ca surpasses a certain amount (Ca/Si ratio 0.35). This aligns with previous research, which suggests an optimal amount of Ca in the binder system; then, when the increment of Ca content exceeds this amount, a decrease in compressive strength is recorded [11].
Within the Ca/Si ratio range of 0.2–0.6, variations in phase assemblage and mechanical performance demonstrate that an optimal Ca/Si ratio of approximately 0.3–0.35 corresponds to the highest compressive strength. The intermediate C–S–H phase facilitates a gradual and regulated release of Ca2+ ions [35]. As the Ca/Si ratio increases up to 0.35–0.4, the morphology of the CSH phases progressively transforms from needle-like to plate-like, ultimately developing into an interwoven, network-type microstructure [36]. This microstructural evolution is accompanied by a notable enhancement in compressive strength. Furthermore, the influence of the Ca/Si ratio (0.3–0.4) on the mechanical performance can be ascribed to the pozzolanic interactions occurring within alkali-activated matrices [37]. The pozzolanic reaction in such systems proceeds through a dissolution–reorganization–precipitation sequence. Upon exposure of CFA to alkaline activators, hydroxide ions (OH−) attack the amorphous silica network, cleaving Si–O–T bonds and generating reactive silicate monomers. These monomers subsequently undergo polymerization into extended chain structures, with aluminum species substituting for silicon in tetrahedral coordination. In Ca-enriched environments, such as those characteristic of wood bottom ash WBA-based AAMs, the liberated Ca2+ ions contribute to the stabilization of the aluminosilicate frameworks. This process promotes the coexistence of CSH, CASH, and NASH gels, thereby resulting in significant improvements in the mechanical properties of the material.
The correlation between compressive strength and Si/Al ratio is comparatively weak (Figure 9), indicating that the reaction participated by Ca-containing compounds dominates the development of the material properties. In this case, relatively low SiO2 and high Al2O3 contents can have a positive impact. Based on the calculated Si/Al ratio, except for sample Z20, the strength increases as the ratio decreases. In the reference sample, REF, as it contains a minimal amount of Al2O3, the Si/Al ratio is high. It explains why CASH crystals were not identified in such a system. Conversely, increasing aluminosilicate sources, especially the Al contents in the Ca-rich environment, promotes the formation of Al-containing hydrates CASH, instead of CSH, and triggers geopolymerization to generate polymeric products, which significantly facilitates strength development. This effect has also been mentioned by other researchers, who point out that the increase in Ca sources favors the dissolution of Si-O-Si and Si-O-Al bonds in aluminosilicates, generating oligomers that interact with Ca2+ and produce aluminate silicate hydrate (ASH) and calcium aluminate hydrate (CAH) [38]. The dissolution of Ca2+ in the binder system promotes a rapid geopolymerization between silicates and aluminates, producing more NASH gels [39]. In addition, the presence of aluminate contents in higher proportions facilitates the dissolution of active aluminosilicates in the alkaline environment, which favors the polymerization, releasing free [SiO4]− and [AlO4]− tetrahedral units [40]. For samples with NZ addition, it is worth noting that NZ, primarily composed of clinoptilolite, has an open framework structure made up of interconnected [SiO4]4− and [AlO4]5 tetrahedra with negative charges. They attract sodium and calcium ions at ambient temperature. Thus, although the Si/Al ratio of Z20 is high, it still achieves the greatest compressive strength.
The relationship between the compressive strength and Ca/Si or Si/Al ratios of the samples with 30% of pozzolanic additives are displayed in Figure 10 and Figure 11, respectively. The usage of pozzolanic additives led to the reduction of Ca/Si ratios in the AAM, which favors the formation of a binder system composed of various categories of hydrates and promoted the development of the strength when the ratio was lower than 0.8. However, on exceeding this, strength loss was detected. Another point influencing the strength development is the introduction of the Al content which contributes to the decrease of the Si/Al ratio and facilitates improved mechanical properties. Nonetheless, sample Z30 fails to follow this tendency, due to the unique microstructure of NZ which results in a compact three-dimensional web structure when added as a precursor, impressively improving the compressive strength. Unlike other silicate structures, NZ has large vacancies (one-point defect in a crystal) in tits structure that enable big cations, such as sodium or calcium ions, to bind with water molecules, i.e., to hydrate. The voids in the minerals can be connected, thus forming channels of different sizes. This capacity of cation exchange for clinoptilolite-based NZ is of a high value of 72.51 mg-ekv/100 g [41]. It means that sodium and calcium ions from the activator solution and other components can be captured in the NZ structure, creating a synthesis product, i.e., an NZ-geopolymer composite binder.
The XRD was further tested and the results are exhibited in Figure 12. Five major crystalline phases were identified (quartz, calcite, portlandite, CSH, and CASH). The last two are hydrate products. An amorphous region was also detected at the 2θ degree between 25 and 35°, assigned to the gel hydrate products CSH, CASH, and NASH [42]. Wollastonite and lime in the raw materials were absent in the AAM samples, highlighting their active participation in the activation process. Also, in the XRD pattern of Z30 with 30% NZ, the intense peaks belonging to clinoptilolite in the raw material NZ almost disappeared. NZ effectively dissolved into the alkaline environment and transformed into activation products. The absence and weakening of clinoptilolite peaks have also been reported in other investigations, particularly with the adoption of SH as an activator, which significantly promoted the dissolution of NZ [17,43]. The use of NZ, on the one hand, promotes the hydration of WBA. On the other hand, as an active pozzolanic source, it reacts with alkali-activators to produce NASH geopolymeric gels. The same outcomes were observed for samples F30 and M20.
The disappearance of phases of mullite and kaolinite indicates the effective release of aluminosilicate content in the raw materials to form hydrate products. It is worth noting that although CH was added as an activator, the peaks of portlandite in the AAM were very weak and narrow, which testifies to its consumption in the binder system. Intensive peaks of quartz were detected in the mortar samples, which remained unreacted, as reported in existing studies [44,45]. The significant amount of quartz in the WBA negatively affects its chemical reactivity and increases the challenges of its utilization. Another point worth noting is the increasing intensity of peaks belonging to hydrates, especially CASH, in samples with pozzolanic additives CFA, MK, and NZ, in contrast with REF. The utilization of pozzolanic additives significantly improves the reactivity of the binder and facilitates the hydration reaction and explains the further enhancement of the mechanical properties.

4. Conclusions

In this study, an AAM was produced by incorporating pozzolanic additives (CFA, MK, and NZ) with a maximum content of 40% and WBA as the primary precursor source. As a valueless waste from biomass combustion sites, the recycling of WBA to be reused in AAMs, instead of being disposed of in landfills, greatly contributes to environmental conservation and is of great ecological significance. Due to the consideration to reduce energy consumption, a more ecological curing method was adopted with only 24 h of temperature treatment at 60 °C, instead of the traditional 100 °C. Meanwhile, until the 28 days, the samples were cured at ambient settings, instead of curing chambers, which contributes further to the much improved ecoefficiency. This material was designed to be pre-fabricated in the factory and cast with recycled coarse aggregates, allowing for the production of sustainable, green alkali-activated concrete for applications in low-load-bearing structural elements, such as lintels of windows and doors. Also, it can be applied as cold-bridge materials and furnishing materials for exterior or interior decorations.
The greatest compressive strength was 24.33 MPa, recorded for the sample Z20 with 20% of NZ, this value being 57.58% larger than that of REF. The highest flexural strength was 4.49 MPa in sample F30, which contained 30% CFA, and was 25.42% bigger than that of REF. The water absorption of the AAMs with pozzolanic additives showed different degrees of enhancement, primarily influenced by the incorporation of lighter materials with a finer particle size distribution.
The chemical compounds of the raw materials impacted the property development of AAMs. By introducing pozzolanic additives, the element ratios of the samples were altered. A strong correlation was observed between compressive strength and the Ca/Si ratio. The optimal Ca/Si ratio was 0.3–0.35, which corresponds to the highest strength development. The Si/Al ratio showed a weaker influence. These alternations in the chemical compounds influenced the production types of hydrates from solely Ca-containing hydrates to coexistence with Al-containing hydrates, especially when the Si/Al ratio was low, i.e., when more Al was added into the binder system.
In the context of this research, attention was paid to the impacts of the chemical-mineral characterization of the AAMs after the use of pozzolanic additives; however, analysis of the microstructure and pore structure was not conducted, which is important for revealing the reaction mechanisms. Also, it is suggested to conduct water permeability tests, which are very useful for investigating the pore structure of the samples, especially together with mercury intrusion porosimetry and water absorption. The quantitative analysis of the amorphous phases regarding the formation of CASH, CSH, and NASH were also not included. It is suggested to have quantitative XRD analysis in the future study and to focus more on the quantitative analysis of workability. In this study, the main content focuses on the effects of chemical compositions, such as Ca/Si and Si/Al ratios, on the mechanical and physical properties of the samples; therefore, the content related to pore structure is not included. In future studies, the target can be on microstructural analysis, combined with porosity and pore structure analysis. Additionally, more focus can be placed on early-stage property development and reaction mechanisms.

Author Contributions

Conceptualization, I.P. and Y.D.; methodology, I.P. and Y.D.; software, J.P.; validation, I.P. and J.P.; formal analysis, I.P. and Y.D.; investigation, Y.D.; resources, I.P. and J.P.; data curation, J.P. and Y.D.; writing—original draft preparation, Y.D.; writing—review and editing, I.P. and J.P.; visualization, Y.D. and J.P.; supervision, I.P. and J.P.; funding acquisition, I.P. and J.P. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the Research Council of Lithuania within the project “Development of water treatment systems that counteract the eutrophication process of lakes based on zeolites obtained from industrial by-products” under the international programme M-ERA.NET 3 Cofund (Grant No. S-M-ERA.NET-24-2).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors offer their sincere gratitude to all the staff in the Institute of Building Materials, Vilnius Gediminas Technical University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Buildings 16 00051 g001
Figure 1. Particle size distribution of raw materials.
Figure 1. Particle size distribution of raw materials.
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Figure 2. Phase compositions of raw materials.
Figure 2. Phase compositions of raw materials.
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Figure 3. Compressive strength, ANOVA F-value: 30.677.
Figure 3. Compressive strength, ANOVA F-value: 30.677.
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Figure 4. Flexural strength, ANOVA F-value: 2.924.
Figure 4. Flexural strength, ANOVA F-value: 2.924.
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Figure 5. Water absorption and bulk density.
Figure 5. Water absorption and bulk density.
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Figure 6. Primary elemental composition of the representative samples.
Figure 6. Primary elemental composition of the representative samples.
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Figure 7. Si/Na, Ca/Na, and Al/Na ratios of the representative samples.
Figure 7. Si/Na, Ca/Na, and Al/Na ratios of the representative samples.
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Figure 8. Ca/Si ratio of the reference sample and samples with 20% of pozzolanic additives.
Figure 8. Ca/Si ratio of the reference sample and samples with 20% of pozzolanic additives.
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Figure 9. Si/Al ratio of the reference sample and samples with 20% of pozzolanic additives.
Figure 9. Si/Al ratio of the reference sample and samples with 20% of pozzolanic additives.
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Figure 10. Ca/Si ratio of the reference sample and samples with 30% of pozzolanic additives.
Figure 10. Ca/Si ratio of the reference sample and samples with 30% of pozzolanic additives.
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Figure 11. Si/Al ratio of the reference sample and samples with 30% of pozzolanic additives.
Figure 11. Si/Al ratio of the reference sample and samples with 30% of pozzolanic additives.
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Figure 12. XRD patterns of AAM (SO2, ICDD 00-046-1045; CaCO3, ICDD 00-005-0586; CH, ICDD 00-004-0733; CSH, ICDD 00-034-0002; CASH, ICDD 00-080-1579).
Figure 12. XRD patterns of AAM (SO2, ICDD 00-046-1045; CaCO3, ICDD 00-005-0586; CH, ICDD 00-004-0733; CSH, ICDD 00-034-0002; CASH, ICDD 00-080-1579).
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Table 1. Material properties of raw materials in the study.
Table 1. Material properties of raw materials in the study.
Material PropertiesWBBAWBFARSCFAMKNZ
Physical parameters
Mean diameter (μm)56.6227.62273.206.068.639.66
Bulk density (g/cm3)1.030.571.230.7940.430.24
Chemical contents (wt%)
SiO250.2622.9162.0337.0054.471.5
Al2O39.202.634.6522.8031.913.1
CaO19.9931.5014.141.922.823.22
Fe2O31.902.330.713.630.421.66
MgO6.443.575.500.880.440.81
Na2O1.190.260.610.505.460.60
K2O5.714.016.422.960.502.99
P2O53.342.713.400.730.130.03
SO30.510.881.160.180.130.01
Loss of ignition2.7326.930.965.201.10-
Table 2. Mix composition (unit: kg/m3) (Fine aggregate/precursor ratio—2; water/precursor ratio—0.55; liquid/binder ratio—0.44; CFA/WBA ratio—0.1, 0.2, 0.3, 0.4; MK/WBA ratio—0.1, 0.2, 0.3, 0.4; NZ/WBA ratio—0.1, 0.2, 0.3, 0.4).
Table 2. Mix composition (unit: kg/m3) (Fine aggregate/precursor ratio—2; water/precursor ratio—0.55; liquid/binder ratio—0.44; CFA/WBA ratio—0.1, 0.2, 0.3, 0.4; MK/WBA ratio—0.1, 0.2, 0.3, 0.4; NZ/WBA ratio—0.1, 0.2, 0.3, 0.4).
GroupMixWBBAWBFACFAMKNZRSSH
Solution		WGCHBABinderNa2O eq%
1REF273 273 -- -1094 301168 55 5 10759.36
2F10246 246 55 --1094301 168 55 5 10759.44
F20219 219 109 --1094 301 168 55 510759.27
F30191 191 164 --1094 301 168 55 510759.23
F40164 164 219 --1094 301 168 55 5 10759.18
3M10246 246 -55 -1094301 168 55 5 10759.40
M20219 219 -109 -1094 301 168 55 510759.47
M30191 191 -164 -1094 301 168 55 510759.52
M40164 164 -219 -1094 301 168 55 5 10759.57
4Z10246 246 --55 1094301 168 55 5 10759.32
Z20219 219 --109 1094 301 168 55 510759.28
Z30191 191 --164 1094 301 168 55 510759.24
Z40164 164 --219 1094 301 168 55 5 10759.19
Table 3. Percentage enhancement of the strength of the optimal mixes compared to the reference samples (significant at α = 0.05).
Table 3. Percentage enhancement of the strength of the optimal mixes compared to the reference samples (significant at α = 0.05).
Optimal MixFlexural Strength (%) p-ValuesCompressive Strength (%)p-Values
F3025.530.0098847.150.00278
M2020.880.0187333.050.01218
Z2017.780.0080557.620.0000067
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Du, Y.; Pundienė, I.; Pranckevičienė, J. Effects of Ca/Si and Si/Al Ratios on the Wood Biomass Ash-Based Alkali-Activated Materials with Pozzolanic Additives. Buildings 2026, 16, 51. https://doi.org/10.3390/buildings16010051

AMA Style

Du Y, Pundienė I, Pranckevičienė J. Effects of Ca/Si and Si/Al Ratios on the Wood Biomass Ash-Based Alkali-Activated Materials with Pozzolanic Additives. Buildings. 2026; 16(1):51. https://doi.org/10.3390/buildings16010051

Chicago/Turabian Style

Du, Yiying, Ina Pundienė, and Jolanta Pranckevičienė. 2026. "Effects of Ca/Si and Si/Al Ratios on the Wood Biomass Ash-Based Alkali-Activated Materials with Pozzolanic Additives" Buildings 16, no. 1: 51. https://doi.org/10.3390/buildings16010051

APA Style

Du, Y., Pundienė, I., & Pranckevičienė, J. (2026). Effects of Ca/Si and Si/Al Ratios on the Wood Biomass Ash-Based Alkali-Activated Materials with Pozzolanic Additives. Buildings, 16(1), 51. https://doi.org/10.3390/buildings16010051

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