Workability
Flow table test
Figure 1 presents the results of the Flow Table test, indicating the flow value decreased as the percentage of biomass ash (BA) increased. Specifically, for samples CC20BA5, CC20BA10, and CC20BA15, the flow values dropped from 91.33% to 73.02% and 37.81%, respectively. Similarly, for CC15BA5, CC15BA10, and CC15BA15, the flow values were 100.00% to 50.59% and 35.61%, respectively. Lastly, the flow values for CC10BA5, CC10BA10, and CC10BA15 were 100.25%, 62.35%, and 43.60%, respectively. Generally, an increase in fly ash (FA) content enhances flowability due to the ball-bearing effect created by the spherical shapes of the fly ash particles21. However, in this study, the reduced flowability may be attributed to the corn cob ash, which absorbs more water than cement, resulting in lower slump values for fresh concrete compared to standard fresh concrete slump values22. Adesanya23 and Olafusi24 also noted that the decrease in slump values was linked to the larger size of irregularly shaped particles.
The overall flow table results.
Setting time
Table 3 presents the test results for the initial and final setting times of control cement paste samples and non-OPC binder including CC20BA5, CC20BA10, CC20BA15, CC15BA5, CC15BA10, CC15BA15, CC10BA5, CC10BA10, and CC10BA15. The initial setting times were found to be 97, 142, 65, 132, 271, 96, 123, 116, and 42 min, respectively, while the final setting times were 175, 233, 141, 205, 420, 210, 180, 195, and 135 min, respectively. It was observed that the BA content increased from 5 to 10%, the setting time increased, whereas further increasing BA to 15% resulted in a shorter setting time with both initial and final stages. This can be attributed to the imbalance between FA and BA content. Both materials contain reactive silica (SiO2) and calcium oxide (CaO), but BA is primarily composed of potassium oxide (K2O). Since the pozzolanic reactions in the binder mainly rely on FA dissolution, the decrease in FA and the increase in BA interrupt the dissolution of FA, leading to a slower pozzolanic reaction rate and extending the setting time. However, at 15% BA, the balance is restored, as the alkalis (K2O) enhances the dissolution of reactive components, creating an environment for continued pozzolanic reactivity between the SiO2 and CaO from both FA and BA25. This behavior was explained by Laskar and Talukdar26, who developed the geopolymer mortars prepared from ultrafine ground granulated blast-furnace slag incorporated with fly ash and alkali activators composed of (1) sodium silicate (SS) and sodium hydroxide (SH), and (2) only a sodium hydroxide (SH) solution. The results indicated that the setting times of mixes containing only SH as an alkali activator were longer than those containing SS and SH in combination as an alkali activator for the mixes containing a high amount of FA.
Compressive strength
The compressive strength test results, presented in Table 4 demonstrate the performance of samples CONTROL, CC20BA5, CC20BA10, CC20BA15, CC15BA5, CC15BA10, CC15BA15, CC10BA5, CC10BA10, and CC10BA15 at 3, 7, 28, and 56 days. The control samples exhibited the highest compressive strength at all ages compared to the other samples, which is typical due to the reduced clinker content limiting hydration reactions. However, pozzolanic activity still occurred, contributing to the compressive strength development, even though their strength levels remained significantly lower than the CONTROL sample.
Figure 2, derived from Table 4, provides clearer results. Most samples exhibited limited strength gain beyond a certain point. For instance, CC20BA5 demonstrated an early increase in compressive strength, reaching 15.35 MPa at 3 days, 20.54 MPa at 7 days, 23.73 MPa at 28 days, and 24 MPa at 56 days, which showed minimal further development. This is attributed to the high limestone powder (CC) content, which primarily acts as a filler rather than actively contributing to strength through the pozzolanic reaction of fly ash (FA). This effect is further evidenced by the slower strength progression in CC20BA15, which recorded the lowest strength development, increasing from 5.48 MPa at 3 days to only 13 MPa at 56 days. However, a distinct trend is found in mixtures containing BA10. CC20BA10 exhibited significant growth, increasing from 5.76 MPa at 3 days to 25.5 MPa at 56 days. Similarly, CC15BA10 improved from 4.52 MPa at 3 days to 28 MPa at 56 days. Notably, CC10BA10 achieved the highest compressive strength among the non-OPC binders, increasing from 4.7 MPa at 3 days to 29 MPa at 56 days. These results highlight the importance of achieving an optimal balance between pozzolanic activity and the reaction of limestone powder with aluminate hydrates to form carboaluminate phases, contributing to the strength development.
Compressive strength results of control paste, a CC20 binder, b CC15 binder, and c CC10 binder.
These results align with the findings of Ibrahim, et al.27, which studied the effect of alkaline activators and binder content of natural pozzolan-based alkali-activated concrete. The results show that the compressive strength development is significantly influenced by binder content and the sodium silicate-to-sodium hydroxide (SS/SH) ratio. Optimal strength gain occurs when sufficient alkaline activators promote the dissolution of reactive components. However, excessive binder content or extended curing may negatively impact strength due to microstructural instability or unreacted particles. This suggests the need for a balanced binder content to achieve the best results.
X-ray diffraction (XRD)
Figure 3 shows the XRD results of CC20BA10, CC15BA10, and CC10BA10 at 3, 7, 28, and 56 days which show the significance of the compressive strength results. CC (CaCO3), Gb (Al(OH)2), An (CaSO4), and Sy (K2Ca(SO4)2·H2O) are the significant mineral compositions found in the samples. The broader intensity of the CC peak and the ongoing development of compressive strength over curing age indicate active pozzolanic reactions despite the absence of portlandite (Ca(OH)₂) in the XRD results. Furthermore, carbonation reactions increase the intensity of CC peak, as portlandite reacts with CO₂ during curing, as illustrated in Eq. 3.28 At curing ages of 28 and 56 days, the hydrocalumite (Hy) phase, a layered-double hydroxide (LDH) material acting as a catalyst and CO2 capture agent, forms due to the alkaline activation of reactive alumino-silicate-based materials from the assistance of the gibbsite activator. The lamellar structure of hydrocalumite is similar to portlandite layers (Ca(OH)2), with part of the calcium being replaced by aluminum29,30,31, as demonstrated in Eq. 4. Similarly, the intensity of the Gb peak increases over the curing period, indicating the ongoing hydraulic reactions of calcium aluminates formed during initial hydration. These reactions gradually convert the calcium aluminate components into the stable garnet phase C₃AH₆ and gibbsite32.
$${\text{Ca}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ CO}}_{{2}} \to {\text{ CaCO}}_{{3}} + {\text{ H}}_{{2}} {\text{O}}$$
(5)
$${\text{3CaO }} + {\text{ Al}}_{{2}} {\text{O}}_{{3}} + {\text{ CaCl}}_{{2}} + { 1}0{\text{H}}_{{2}} {\text{O }} \to {\text{ Ca}}_{{4}} {\text{Al}}_{{2}} {\text{O}}_{{6}} {\text{Cl}}_{{2}} \cdot{1}0{\text{H}}_{{2}} {\text{O}}$$
(6)
XRD results of a CC20BA10, b CC15BA10, and c CC10BA10 with curing ages from 3, 7, 28, and 56 days; CC = Calcite: CaCO3, Gb = Gibbsite: Al(OH)2, An = Anhydrite: CaSO4, and Sy = Syngenite: K2Ca(SO4)2·H2O.
CO2 absorption
Figure 4 shows the DTA results of the CC20BA15, CC15BA15, and CC10BA15 after 24 h of CO2 exposure. The temperature range of 200–300 °C corresponds to the decomposition of gibbsite (Al2O3·3H2O)33, while the temperature range of 600–750 °C shows the decomposition of calcite (CaCO3)34. The results show a distinguished increase of CaCO3 at 2%, 2.7%, and 2.6% for CC20BA15, CC15BA15, and CC10BA15 respectively, which is the highest BA content among all the samples. The chemical composition of BA is mainly K2O and CaO, which potentially influence the carbonation of cement-based materials since the carbonation reaction in cement binder is known to occur within capillary pores with a pore solution at high pH35. In this research, BA likely enhances the reactivity of Ca2+ and CO2 from external sources, allowing them to enter and dissolve to form CaCO3 in the pore solution36.
TGA result of a CC20BA15, b CC15BA15, and c CC10BA15 before and after CO2 exposure.
Figure 5 shows the CO2 uptake after 24 h of carbonation test as a function of biomass ash (BA) and limestone powder (CC). The result confirms that increasing BA content leads to higher CO2 uptake, reaching up to 2.7% with the optimum percentage of BA of 15% which is due to the high alkalinity of BA. Furthermore, gibbsite in the non-OPC binder transforms into more stable calcium aluminate phases37, indirectly enhancing CO2 penetration by increasing porosity during carbonation. This result aligns with the study of Moro et al.38 that examines the impact of nano-TiO2 on CO2 sequestration in hardened cement paste, considering the water-to-cement ratio (w/c) and pore structure, samples with higher w/c ratios exhibit greater porosity, and the decrease in porosity due to nano-TiO2 does not notably restrict the space for carbonation. Consequently, the enhanced reactivity of calcium hydroxide (CH) from nano-TiO2 results in a greater CO2 uptake in these samples.
CO2 uptake (% by mass) after 24 h of exposure.
The CC15 and CC20 sample groups exhibited a wider green zone of the contour compared to the CC10 group, indicating their suitability as CO₂ absorption mixtures, with at least 1% CO₂ captured. This suggests the biomass ash (BA) and limestone powder (CC) content in these mixtures affect their carbonation potential, likely due to the synergistic effects of high alkalinity, increased porosity, and reactive phases among the mixed materials.
