The Compressive Strength and Microstructure of Alkali-Activated Mortars Utilizing By-Product-Based Binary-Blended Precursors

: Researchers have investigated the feasibility of using ultraﬁne palm oil fuel ash (u-POFA) as a cement replacement material because of its potential to reduce the environmental impact of concrete production. u-POFA, a by-product of palm oil fuel combustion, is a suitable replacement for Portland cement in concrete mixes because of its sustainability and cost-effectiveness. This study investigated the microstructural and compressive strengths of alkali-activated mortars (AAMs) based on ﬂy ash (FA) and granulated blast-furnace slag (GBFS) being added with varying percentages of u-POFA. The mixture samples were prepared in eighteen mortars using sodium metasilicate (Na 2 SiO 3 ) as the source material and sodium hydroxide (NaOH) as the alkaline activator. This study used ﬁeld-emission scanning electron microscopy coupled with energy-dispersive X-ray spectrometry, X-ray diffraction, X-ray ﬂuorescence, and Fourier-transform infrared spectroscopy to characterize the binary-blended mortars after 28 days of curing and determined the strength of the FA+GBFS (87.80 MPa), u-POFA+GBFS (88.87 MPa), and u-POFA+FA mortars (54.82 MPa). The mortars’ compressive strength was inﬂuenced by the CaO/SiO 2 and SiO 2 /Al 2 O 3 ratios in the mixture, which was directly due to the formation rate of geopolymer products of the calcium–alumina–silicate–hydrate (C–(A)–S–H), aluminosilicate (N–A–S–H), and calcium–silicate–hydrate (C–S–H) phases. Based on the contents of FA and GBFS, u-POFA signiﬁcantly enhanced concrete strength; therefore, u-POFA used in a suitable proportion could enhance binary-blended AAMs’ microstructure.


Research Background
Palm oil fuel ash (POFA) is a waste product from burning palm oil for energy. It is a fine, powdery material comprising a mixture of silica, calcium oxide, and other minerals. It is used as a soil amendment and fertilizer in agriculture, a construction material, and a fuel source in industry. In addition to reducing the amount of required cement, it has the beneficial properties of improving concrete strength, durability, and workability [1]. Zero-cement concrete is an improved version of geopolymer concrete, where the thermal activation of the binding materials, such as fly ash (FA) and granulated blast-furnace slag (GBFS), is combined with alkaline solutions, namely sodium hydroxide (NaOH) and sodium silicate

Literature Review
Ultrafine palm oil fuel ash (u-POFA) is a by-product of palm oil fuel combustion. It is a fine, light-colored powder composed of silica, alumina, and other minerals. Researchers have conducted extensive investigations of the pozzolanic properties of u-POFA. Studies have shown that pulverized fuel ash (PFA) improves the strength and durability of concrete and mortar and reduces concrete permeability [6]. PFA also reduces the water absorption of concrete and mortar and concrete shrinkage. In addition to its pozzolanic properties, researchers have investigated the potential of using PFA as a cement replacement. Using PFA as a partial cement replacement in concrete and mortar improves its strength and durability. PFA also reduces the hydration heat of concrete and mortar and alkali-silica reaction [7].
In a previous study, a geopolymer mixture treated for 28 days had a strength of 51 MPa. The mixture was treated with a GBFS/FA hybridization ratio of 20/80 using Na 2 SiO 3 /NaOH of 1.5 and 40% alkali activator liquid [8]. Furthermore, u-POFA can be utilized as a raw material for blended geopolymers or alkali-activated synthesis. Southeast Asian countries, including Indonesia, Malaysia, and Thailand, have a rich source of POFA and can use it as a raw material for geopolymers or alkali-activated mortars [9]. The pre-treatment steps, such as sieving, calcination, and grinding, influence the effects of the waste materials utilized as raw materials for geopolymers [10]. Research is currently focused on the synthesis of blended geopolymers or alkaline-activated geopolymers from POFA and GBFS [6,11], but POFA and FA require improvement in their effectiveness. Si and Al provide POFA with good pozzolanic properties, which allow it to be used at 30% via binder weight replacement in PC concrete and up to 60% when being used to treat POFA [11]. POFA concrete has a high compressive strength of 104 MPa after 28 days of treatment. Recently, a group of researchers investigated using POFA as a complementary material blend with other aluminosilicate materials to produce geopolymer cement pastes or mortars [12]. For instance, blending low-calcium FA with POFA resulted in concrete with a compressive strength of 30 MPa after 28 days of curing [13]. The compressive strength of treated palm oil fuel ash (TPOFA)-based geopolymer mortar was 62.52 MPa after 120 days of curing [14].

Motivation for the Research
Based on the literature review, the research motivation for using u-POFA with cement mixtures is to gain insight into the potential of using u-POFA as a partial cement replacement in concrete. It is crucial to research the effects of u-POFA on the mechanical properties of concrete, the optimal dosage of u-POFA for different concrete mixtures, and the environmental benefits of using u-POFA in concrete. Similarly, there is a need to explore the potential of using u-POFA as a pozzolanic material in other applications, such as mortars and grouts. Previous research has shown that adding u-POFA in concrete improves its mechanical properties, reduces the amount of the required cement, and reduces the carbon footprint during the concrete production process.

Research Objective
The first objective of this study is to explore the potential of u-POFA as a base material for alkali-activated and geopolymer products. This focused on POFA properties and the feasibility of adding POFA to alkali-activated and geopolymer products. It also investigated the effects of u-POFA on the compressive strength and microstructure of binary blends containing GBFS and FA binders. The researchers also determined the effects of binder proportions and modification of the binary-blended materials on the mechanical properties of the synthesized alkali-activated mortars and employed XRD, FT-IR, FESEM, and EDX to characterize the resulting AMMs.

Palm Oil Fuel Ash
The POFA used in this research was from United Oil Palm Industry in Penang, Malaysia. The first step in preparing the POFA was drying the ash in a 105 • C oven for 24 hours to remove its inherent moisture since the POFA was placed outside the mill. The POFA was sieved using a 300-mesh sieve to remove coarser particles of the partially burnt ashes from fibers and palm kernel shells, following the work by Elbasir et al. in 2017 [15]. The POFA was then ground using a ball mill fitted with 150 steel balls and rotated at 180 rpm for eight hours to obtain 10 µm particles. Unburnt carbon was removed by heating the ground POFA in a 550 • C furnace for 90 minutes. The resulting POFA was designated treated t-POFA. The fine POFA, which was designated f-POFA, was obtained by grounding the t-POFA for an additional eight hours to increase its fineness, and the f-POFA was ground for another eight hours to obtain u-POFA (ultrafine POFA) with an average particle size of 1.1 µm.

Fly Ash
The fly ash obtained from Lafarge Malaysia Berhad (Rawang Plant, Selangor, Malaysia) followed the ASTM: C618-12a specification and was separated into two groups of lowcalcium FA (Class F, CaO < 10%).

Ground Blast-Furnace Slag
YTL Cement Technical Center in Pulau Indah, Selangor, Malaysia, provided the ground blast-furnace slag (GBFS) with an exact gravity of 2.89.

Aggregates
The fine aggregates with a 100 percent content were from river sand and had a fineness modulus of 1.85 and a specific gravity of 2.62 under saturated and surface-dry (SSD) conditions. The ratio of sand to raw materials (FA, u-POFA, and GBFS) was maintained at 1.5.

Alkaline Activator
The alkali activator for the mortar mixtures (BR1 to BR18) was synthesized using 10 M concentration of NaOH and sodium silicate with an initial silica modulus of 2.2 (Ms = SiO 2 /Na 2 O). It contained sodium silicate with 10 M sodium hydroxide.

Methods for Determining Material Properties
This research first analyzed the physicochemical properties of the base materials, where a Malvern 3000 laser diffraction particle size analyzer (Grovewood Road, Malvern, Worcestershire, WR14 1XZ, United Kingdom) was used to analyze the particle-size distributions and surface area. A Rigaku RIX3000 instrument was used to carry out X-ray fluorescence (XRF) to determine the chemical composition of the base materials, and a Zeiss SupraTM 35VP was used to investigate their morphology via FESEM. Table 1 presents the materials for preparing the different binary blends, namely FA+GBFS, u-POFA+GBFS, and u-POFA+FA, for developing the mortars. Thus, it was necessary to mix the alkali activator solution with water and sand. GBFS, FA, and u-POFA in the raw materials had a ratio of 2.5 to the alkali activator (Na2SiO3/NaOH). All mixtures were added with a minimum of 5 wt% water of the alkali-activated mortars [16]. The literature review showed that the optimum ratio for fabricating the mortars was 1.5 for the sand/binder ratio [17]. The absolute volume method was used to obtain the raw material, sand, alkali activator, and extra water proportions for the AAMs, as presented in Table 1.

Design of the Mixtures
To ensure consistency and reproducibility, control mortars that represented different raw materials were prepared to ensure that the mortar mixtures were developed appropriately (FA: GBFS from BR14 and BR18; u-POFA: GBFS from BR8 and BR13; and u-POFA: FA from BR1 and BR7).   Table 1). The binary blends were mixed in a 4.73 L mixer for two minutes to remove air pockets, and the raw materials were blended with sand, NaOH, and Na 2 SiO 3 for ten minutes. The mortars were cast in double layers in an oil-smeared 5 × 5 × 5 cm steel mold. The samples were preserved in vinyl bags for 12 h at 25 • C to minimize moisture loss. Finally, the samples were de-molded, placed in a heat-resistant vinyl sheet wrapping, and cured in an oven at 75 • C for 24 h [10].

Analysis and Testing
The mortar samples were tested for compressive strength after 7, 14, and 28 days following the ASTM C109 (ASTM, 1999) [11]. The tests were conducted using three samples of each mortar. FESEM, FTIR, and XRD were employed to characterize the mortar samples with the optimum mix.  Table 2 presents the chemical composition of the base materials determined through XRF. The primary base materials were CaO, SiO 2 , and Al 2 O 3 . All base materials, namely u-POFA (64.595%), FA (49.053%), and GBFS (36.83%), had high levels of SiO 2 . The percentages of Al 2 O 3 were 5.851% for u-POFA, 14.44% for GBFS, and 23.516% for FA; the percentages for CaO were 39.85% for GBFS, 9.293% for u-POFA, and 5.080% for FA. The loss of ignition (LOI) for GBFS, u-POFA, and FA was 0.60%, 2.50%, and 2.13%, respectively. Figure 1 shows the particle size of the raw materials. The median particle size for the GBFS, u-POFA, and FA was 14.20 µm, 1.1 µm, and 9.8 µm, respectively. Compared to other base materials, the u-POFA had the highest surface area of 1871 m 2 /kg. This research produced 18 binary-blended trial mixtures by adding varying amo of GBFS, u-POFA, and FA. Three of the (AAM) mixtures, namely u-POFA+FA, GBFS and u-POFA+GBFS, had seven different mixing ratios of (90:10), (70:30), (25:75), (0 (50:50), (80: 20), and (100:0) ( Table 1). The binary blends were mixed in a 4.73 L mixe two minutes to remove air pockets, and the raw materials were blended with sand, N and Na2SiO3 for ten minutes. The mortars were cast in double layers in an oil-smeare 5 × 5 cm steel mold. The samples were preserved in vinyl bags for 12 hours at 25 minimize moisture loss. Finally, the samples were de-molded, placed in a heat-resi vinyl sheet wrapping, and cured in an oven at 75 °C for 24 hours [10].

Analysis and Testing
The mortar samples were tested for compressive strength after 7, 14, and 28 following the ASTM C109 (ASTM, 1999) [11]. The tests were conducted using three ples of each mortar. FESEM, FTIR, and XRD were employed to characterize the m samples with the optimum mix. Table 2 presents the chemical composition of the base materials determined thr XRF. The primary base materials were CaO, SiO2, and Al2O3. All base materials, na u-POFA (64.595%), FA (49.053%), and GBFS (36.83%), had high levels of SiO2. The per ages of Al2O3 were 5.851% for u-POFA, 14.44% for GBFS, and 23.516% for FA; the per ages for CaO were 39.85% for GBFS, 9.293% for u-POFA, and 5.080% for FA. The lo ignition (LOI) for GBFS, u-POFA, and FA was 0.60%, 2.50%, and 2.13%, respectively ure 1 shows the particle size of the raw materials. The median particle size for the G u-POFA, and FA was 14.20 µm, 1.1 µm, and 9.8 µm, respectively. Compared to other materials, the u-POFA had the highest surface area of 1871 m 2 /kg. , cristobalite (SiO 2 ), and quartz (SiO 2 ), while the FA was made up of sillimanite (Al 2 SiO 5 ), mullite (Al 6 Si 2 O 13 ), and quartz (SiO 2 ). These results were consistent with the findings of previous studies [18,19]. Based on the chemical compositions provided by the ASTM C618 [20], POFA was a Class F mineral admixture.  Figure 2 shows the XRD patterns of the raw materials (GBFS, u-POFA, and FA GBFS displayed some amorphous stages, with broad humps between the 20-40° o and other crystalline stages, namely spurrite Ca5(SiO4)2CO3, hatrurite (Ca3SiO5), q (SiO2), and anorthite CaAl2Si2O8. The u-POFA primarily comprised calcite (CaCO3), sium aluminum phosphate (K3Al2(PO4)3), cristobalite (SiO2), and quartz (SiO2), wh FA was made up of sillimanite (Al2SiO5), mullite (Al6Si2O13), and quartz (SiO2). The sults were consistent with the findings of previous studies [18,19]. Based on the che compositions provided by the ASTM C618 [20], POFA was a Class F mineral admix  Figure 3 shows the FA, GBFS, and u-POFA particle morphology determine FESEM. Figure 3a shows that the irregular-shaped particles of the u-POFA have irre and porous surfaces. Figure 3b shows that the spherical particles of the FC range be 10 and 200 µm. The individual FA particle is hollow and most likely contains other sm particles. Figure 3c shows the morphology of the GBFS, which has diamond-and sq shaped particles.  Figure 3 shows the FA, GBFS, and u-POFA particle morphology determined via FESEM. Figure 3a shows that the irregular-shaped particles of the u-POFA have irregular and porous surfaces. Figure 3b shows that the spherical particles of the FC range between 10 and 200 µm. The individual FA particle is hollow and most likely contains other smaller particles. Figure 3c shows the morphology of the GBFS, which has diamond-and squareshaped particles.

Compressive Strength
3.2.1. The Effects of FA on the u-POFA-based Mortars Figure 4 shows the effects of u-POFA, curing time (7-28 days), and the FA ratio in the alkali-activated mortars (BR1 to BR7) on the compressive strength of the fabricated mortars. The highest mortar strength for the mortar with 75 wt% FA (BR6) was 54.82 MPa at 28 days, which is comparatively more favorable than the unblended FA (BR7, 53.20 MPa at 28 days). Adding u-POFA and FA increased the strength of the binary-blended, alkaliactivated mortar. Table 3 shows that the compressive strength of the geo-polymeric system was directly influenced by the whole blended AAM with varying SiO2/Al2O3 ratios. Compared to lower SiO2/Al2O3 ratios, by including the BR2 to BR7, the BR1 (unblended u- Adding u-POFA and FA increased the strength of the binary-blended, alkali-activated mortar. Table 3 shows that the compressive strength of the geo-polymeric system was directly influenced by the whole blended AAM with varying SiO 2 /Al 2 O 3 ratios. Compared to lower SiO 2 /Al 2 O 3 ratios, by including the BR2 to BR7, the BR1 (unblended u-POFA) had the lowest compressive strength with the most significant SiO 2 /Al 2 O 3 ratio of 11.3. As observed by other researchers [12,21], Al(OH) 2 −4 species were permitted to condense within the mortar materials during the initial stages due to the lower Si and higher Al levels within the blended alkali-activated mortars, AAMs BR4-BR7. The phenomena allowed these mortars to build greater strength of compression than the BR1 mortar.

Compressive Strength
3.2.1. The Effects of FA on the u-POFA-based Mortars Figure 4 shows the effects of u-POFA, curing time (7-28 days), and the FA ratio in the alkali-activated mortars (BR1 to BR7) on the compressive strength of the fabricated mortars. The highest mortar strength for the mortar with 75 wt% FA (BR6) was 54.82 MPa at 28 days, which is comparatively more favorable than the unblended FA (BR7, 53.20 MPa at 28 days). Adding u-POFA and FA increased the strength of the binary-blended, alkaliactivated mortar. Table 3 shows that the compressive strength of the geo-polymeric system was directly influenced by the whole blended AAM with varying SiO2/Al2O3 ratios. Compared to lower SiO2/Al2O3 ratios, by including the BR2 to BR7, the BR1 (unblended u-POFA) had the lowest compressive strength with the most significant SiO2/Al2O3 ratio of 11.3. As observed by other researchers [12,21], Al(OH)2−4 species were permitted to condense within the mortar materials during the initial stages due to the lower Si and higher Al levels within the blended alkali-activated mortars, AAMs BR4-BR7. The phenomena allowed these mortars to build greater strength of compression than the BR1 mortar.    Figure 5 shows the compressive strength of the distinct blends of u-POFA+GBFS mortars (BR8 to BR13) treated for 7 to 28 days. The maximum compressive strength of the unblended u-POFA (BR1) was 37.52 MPa after 28 days of treatment. Increasing the GBFS ratio from 10 to 75% increased the compressive strength of the binary-blended u-POFA+GBFS mortar from 21.42 to 185.6%. The BR12 with 75 wt% GBFS, cured for 28 days, had a higher compressive strength of 88.78 MPa than the unblended GBFS mortar, which had a compressive strength of 86.79 MPa. u-POFA significantly increased the strength of the alkali-activated GBFS, or geopolymer, during the blending process. GBFS is vital in enhancing the compressive strength of alkali-activated u-POFA. The higher percentages of GBFS in the mixture increased the compressive strength of the chemical-activated mortar. The change in compressive strength was consistent with the findings of other investigations [6,22]. The BR12 mixture had the highest strength ( Figure 5). The unblended GBFS's compressive strength decreased without u-POFA. Compared to u-POFA, GBFS contains a significant amount of Ca and Al. As a result, the continuous rise in the mortar's achievable compressive strength is supported by Ca and Al [23]. Since GBFS serves as a viable source of Ca, it is directly associated with changes in the compressive strength of alkali-activated mortars.  Figure 5 shows the compressive strength of the distinct blends of u-POFA+GBFS mortars (BR8 to BR13) treated for 7 to 28 days. The maximum compressive strength of the unblended u-POFA (BR1) was 37.52 MPa after 28 days of treatment. Increasing the GBFS ratio from 10 to 75% increased the compressive strength of the binary-blended u-POFA+GBFS mortar from 21.42 to 185.6%. The BR12 with 75 wt% GBFS, cured for 28 days, had a higher compressive strength of 88.78 MPa than the unblended GBFS mortar, which had a compressive strength of 86.79 MPa. u-POFA significantly increased the strength of the alkali-activated GBFS, or geopolymer, during the blending process. GBFS is vital in enhancing the compressive strength of alkali-activated u-POFA. The higher percentages of GBFS in the mixture increased the compressive strength of the chemical-activated mortar. The change in compressive strength was consistent with the findings of other investigations [6,22]. The BR12 mixture had the highest strength ( Figure 5). The unblended GBFSʹs compressive strength decreased without u-POFA. Compared to u-POFA, GBFS contains a significant amount of Ca and Al. As a result, the continuous rise in the mortar's achievable compressive strength is supported by Ca and Al [23]. Since GBFS serves as a viable source of Ca, it is directly associated with changes in the compressive strength of alkali-activated mortars.   Figure 6 shows the effects of the GBFS mortars (BR14 to BR18) on the compressive strength of the FA-based mortar. Each mortar's compressive strength raised the strength of alkali-activated, binary-blended, FA+GBFS-based mortars that were treated for a duration of 7, 14 and 28 days. A compressive strength alteration was set up by the investigation for a distinct blend of FA, with rising levels of GBFS. Compared to those treated for 7-14 days, the mortars treated for 28 days had the highest compressive strength. The maximum compressive strength of 53.88 MPa of the BR7 (unblended FA, 28 days) increased to 87.8 MPa when mixed with 75 wt% of GBFS. The binary blending of GBFS and FA significantly increased their strength. The higher compressive strength of the FA-based geopolymer was due to the higher percentage of GBFS, which was consistent with the results reported by Xu et al. [7]. The high compressive strength of the geopolymer concrete was due to the GBFS replacement ratio from 50 to 80%, subjected to wet or air curing [24]. for 7-14 days, the mortars treated for 28 days had the highest compressive strength. The maximum compressive strength of 53.88 MPa of the BR7 (unblended FA, 28 days) increased to 87.8 MPa when mixed with 75 wt% of GBFS. The binary blending of GBFS and FA significantly increased their strength. The higher compressive strength of the FA-based geopolymer was due to the higher percentage of GBFS, which was consistent with the results reported by Xu et al. [7]. The high compressive strength of the geopolymer concrete was due to the GBFS replacement ratio from 50 to 80%, subjected to wet or air curing [24].

Mineralogical Analysis
The best binary-blended mortars were (BR18; FA+GBFS), (BR6; u-POFA+FA), and (BR12; u-POF+GBFS). Figure 7 shows the XRD diffractograms for the alkali-activated mortars, namely (BR13; GBFS), (BR1; u-POFA), and (BR7; FA). The crystalline features with high-intensity peaks in the diffractograms are apparent for calcite (CaCO3), albite Na(AlSi3O8), hillebrandite (Ca2SiO3(OH)2, mullite (3Al2O3SiO2), analcime (NaAlSi2O6·H2O), and quartz (SiO2). The crystal stage indicates that different active reactions occurred during mortar fabrication. The primary mineral structures in the BR6 are analcime, mullite, and quarts, where analcime and mullite are the precipitations of sodium-alumina-silicate-hydrate (N-A-S-H) present in the base materials. The main stages of the BR12 are quartz, hillebrandite, and calcite. The main stages of the BR18 are albite, anorthite, and quartz. The peak of the anorthite stage and that of calcium-aluminosilicate-hydrate (C-(A)-S-H) [25] are similar. The primary stage in the BR18, BR6, and BR12 is quartz. Each mortar appeared to go through distinct reaction stages in its development, as indicated by the peak intensity of the corresponding mineral stages of the mortars [12]. The geopolymer binders are of high quality because of the presence of hillebrandite and albite in the main steps of the mortars [26,27]. The albite stages are from aluminum silicate. Si is in a tetrahedral form in one chain, and Na and Al are in the octahedral structure in their crystalline structure stages. The hillebrandite phase,

Mineralogical Analysis
The best binary-blended mortars were (BR18; FA+GBFS), (BR6; u-POFA+FA), and (BR12; u-POF+GBFS). Figure 7 shows the XRD diffractograms for the alkali-activated mortars, namely (BR13; GBFS), (BR1; u-POFA), and (BR7; FA). The crystalline features with highintensity peaks in the diffractograms are apparent for calcite (CaCO 3 ), albite Na(AlSi 3 O 8 ), hillebrandite (Ca 2 SiO 3 (OH) 2 , mullite (3Al 2 O 3 SiO 2 ), analcime (NaAlSi 2 O 6 ·H 2 O), and quartz (SiO 2 ). The crystal stage indicates that different active reactions occurred during mortar fabrication. The primary mineral structures in the BR6 are analcime, mullite, and quarts, where analcime and mullite are the precipitations of sodium-alumina-silicate-hydrate (N-A-S-H) present in the base materials. The main stages of the BR12 are quartz, hillebrandite, and calcite. The main stages of the BR18 are albite, anorthite, and quartz. The peak of the anorthite stage and that of calcium-aluminosilicate-hydrate (C-(A)-S-H) [25] are similar. The primary stage in the BR18, BR6, and BR12 is quartz. Each mortar appeared to go through distinct reaction stages in its development, as indicated by the peak intensity of the corresponding mineral stages of the mortars [12]. The geopolymer binders are of high quality because of the presence of hillebrandite and albite in the main steps of the mortars [26,27]. The albite stages are from aluminum silicate. Si is in a tetrahedral form in one chain, and Na and Al are in the octahedral structure in their crystalline structure stages. The hillebrandite phase, Ca 2 SiO 3 (OH) 2 , is an inherent stage of the CaO-SiO 2 -H 2 O ternary system, with a high level of calcium silicate hydrate (C-S-H). The calcite (CaCO 3 ) stage in the BR13 and BR6 was produced via the reaction between CO 2 and CaO during AAM development [28].  [29,30]. The symmetric stretching vibration (O-Si-O or Si-O-Si and) is within 730.95-777 cm −1 , and the AlO 2 functional group is within 650.12 cm −1 -695 cm −1 [12,29]. The bands for sodium bicarbonate deformation were responsible for the peaks in the vibration bands at 874.70-876.70 cm −1 [30,31]. The asymmetric Si-O-T (T = Al or Si) vibration within the geopolymer gels or zeolite is suggested to be the cause of the peaks of the vibration bands at 1006.72-1044.42 cm −1 [32][33][34].  [25]. The peaks share a likeness with the characteristic stretching vibrations of the -OH of water's hydrogen bonding integrated within the structure of a geopolymer mortar [35]. The FTIR spectroscopy showed the reaction products formed from the generation of mortars with functional groups. The time for hydration to obtain the hydrated matrixes of the mortars, which are made up of O, Ca, and Si, may be increased by the functional groups' presence.

Fourier-Transform Infrared (FTIR) Spectroscopy
Appl. Mech. 2023, 4, FOR PEER REVIEW 10 Ca2SiO3(OH)2, is an inherent stage of the CaO-SiO2-H2O ternary system, with a high level of calcium silicate hydrate (C-S-H). The calcite (CaCO3) stage in the BR13 and BR6 was produced via the reaction between CO2 and CaO during AAM development [28].   [12,29]. The bands for sodium bicarbonate deformation were responsible for the peaks in the vibration bands at 874.70-876.70 cm −1 [30,31]. The asymmetric Si-O-T (T = Al or Si) vibration within the geopolymer gels or zeolite is suggested to be the cause of the peaks of the vibration bands at 1006.72-1044.42 cm −1 [32][33][34].  [25]. The peaks share a likeness with the characteristic stretching vibrations of the -OH of water's hydrogen bonding integrated within the structure of a geopolymer mortar [35]. The FTIR spectroscopy showed the reaction products formed from the generation of mortars with functional groups. The time for hydration to obtain the hydrated matrixes of the mortars, which are made up of O, Ca, and Si, may be increased by the functional groups' presence.  Figure 9 shows the microstructures of the binary-blended, alkali-activated mortars (BR1; u-POFA), (BR7; FA), and (BR13; GBFS) and the optimal combination for the binaryblended mortars (BR6; u-POFA+FA), (Br12; u-POF+GBFS), and (Br18; FA+GBFS) after 28 days of treatment. The figure also shows the FESEM images of (BR1; u-POFA), (BR7; FA), and (BR13; GBFS) used to examine the morphological properties of the FA-, u-POFA-, and GBFS-based alkali-activated mortar samples. The FESEM micrograph for the u-POFA alkaliactivated binder illustrates an enhanced look with a more compact, dense, and discretely uniformed cross section because of the higher ratio of surface area to volume, showing a comparatively smoother structure than fly ash. The microstructure of the FA ash particles shows that it contains reacted and unreacted fly ash portions, which have numerous minute pores. The micrographs feature GBFS-bonding features in the interfacial zone between the slag paste and the alkali-activated binder. A transition zone formation, which is uniform and dense, appears. There is a gradual increase in pore refinement and advancement in the matrix, and the matrix-aggregate interface can be observed to have higher GBFS contents. Hence, these results are consistent with the improvement in compressive strength that is needed to obtain the best combinations of binary-blended mortars BR6 (u-POFA+FA), BR18 (FA+GBFS), and BR12 (u-POF+GBFS). The microstructure of the BR6 primarily comprises FA, which is the congested nanofiber bulk on the surface of the unreacted FA. Silva et al. [20] showed that hydrolysis reactions occurred on the surfaces of solid particles. The BR6 has many particles with a highly dispersed matrix of small pores. Figure 9 shows that the pores contain unreacted particles [33]. u-POFA altered the microstructure of the POFA/FA-based alkali-activated mortar, as indicated by "b" in Figure 9. The BR12 displays substantial morphological changes caused by the alkali-activated reaction between POFA (25%) and GBFS (75%). The morphology shows small cracks, and the intact particles are smooth and spherical. The BR18 contains 75% GBFS and 25% FA, and the morphology produces products with extensive alkali-activated reactions. The particles in the BR18 were from the FA, and these particles are primarily spherical and irregular-shaped GBFS particles.

Field Emission Scanning Electron Microscopy (FESEM)
Appl. Mech. 2023, 4, FOR PEER REVIEW 11 Figure 8. FTIR for alkali-activated mortars at 28 days. Figure 9 shows the microstructures of the binary-blended, alkali-activated mortars (BR1; u-POFA), (BR7; FA), and (BR13; GBFS) and the optimal combination for the binaryblended mortars (BR6; u-POFA+FA), (Br12; u-POF+GBFS), and (Br18; FA+GBFS) after 28 days of treatment. The figure also shows the FESEM images of (BR1; u-POFA), (BR7; FA), and (BR13; GBFS) used to examine the morphological properties of the FA-, u-POFA-, and GBFS-based alkali-activated mortar samples. The FESEM micrograph for the u-POFA alkali-activated binder illustrates an enhanced look with a more compact, dense, and discretely uniformed cross section because of the higher ratio of surface area to volume, showing a comparatively smoother structure than fly ash. The microstructure of the FA the surfaces of solid particles. The BR6 has many particles with a highly dispersed matrix of small pores. Figure 9 shows that the pores contain unreacted particles [33]. u-POFA altered the microstructure of the POFA/FA-based alkali-activated mortar, as indicated by "b" in Figure 9. The BR12 displays substantial morphological changes caused by the alkali-activated reaction between POFA (25%) and GBFS (75%). The morphology shows small cracks, and the intact particles are smooth and spherical. The BR18 contains 75% GBFS and 25% FA, and the morphology produces products with extensive alkali-activated reactions. The particles in the BR18 were from the FA, and these particles are primarily spherical and irregular-shaped GBFS particles.  Figure 9. FESM for the alkali-activated mortars at 28 days.

Conclusions
The ultrafine palm oil fuel ash enhanced the strength of granulated-blast-furnaceslag-and fly-ash-based, binary-blended, alkali-activated mortars. Adding 25 wt% u-POFA enhanced the compressive strength of the GBFS and FA mortars. This enhancement was due to the optimal CaO/SiO2 and SiO2/Al2O3 ratios in the mortars, which were directly associated with the formation of calcium-silicate-hydrate (C-S-H), calcium-alumina-silicate-hydrate (C-(A)-S-H), and sodium-alumina-silicate-hydrate (N-A-S-H). It was Figure 9. FESM for the alkali-activated mortars at 28 days.

Conclusions
The ultrafine palm oil fuel ash enhanced the strength of granulated-blast-furnaceslag-and fly-ash-based, binary-blended, alkali-activated mortars. Adding 25 wt% u-POFA enhanced the compressive strength of the GBFS and FA mortars. This enhancement was due to the optimal CaO/SiO 2 and SiO 2 /Al 2 O 3 ratios in the mortars, which were directly associated with the formation of calcium-silicate-hydrate (C-S-H), calcium-aluminasilicate-hydrate (C-(A)-S-H), and sodium-alumina-silicate-hydrate (N-A-S-H). It was also directly associated with the transformation of the mortars' microstructure. As such, a better-quality binary-blended AAM using the appropriate ratio of GBFS or FA may be produced through abundantly accessible POFA.