Strength enhancement of concrete using incinerated agricultural waste as supplementary cement materials

The potassium (K) and sodium (Na) elements in banana are needed for hydration reaction that can enhance the strength properties of concrete. This research aims (a) to determine the material engineering properties of banana skin ash (BSA) and concrete containing BSA, (b) to measure the strength enhancement of concrete due to BSA, and (c) to identify optimal application of BSA as supplementary cement materials (SCM) in concrete. The BSA characterization were assessed through X-ray fluorescence (XRF) and Blaine’s air permeability. The workability, compressive strength, and microstructures of concrete containing BSA were analysed using slump test, universal testing machine (UTM) and scanning electron microscope (SEM). A total of 15 oxides and 19 non-oxides elements were identified in BSA with K (43.1%) the highest and Na was not detected. At 20 g of mass, the BSA had a higher bulk density (198.43 ± 0.00 cm3) than ordinary Portland cement (OPC) (36.32 ± 0.00 cm3) indicating availability of large surface area for water absorption. The concrete workability was reduced with the presence of BSA (0% BSA: > 100 mm, 1% BSA: 19 ± 1.0 mm, 2%: 15 ± 0.0 mm, 3% BSA: 10 ± 0.0 mm). The compressive strength increased with the number of curing days. The concrete microstructures were improved; interfacial transition zones (ITZ) decreased with an increase of BSA. The optimal percentage of BSA obtained was at 1.25%. The established model showed significant model terms (Sum of Squares = 260.60, F value = 69.84) with probability of 0.01% for the F-value to occur due to noise. The established model is useful for application in construction industries.

www.nature.com/scientificreports/ In material engineering properties, integration of natural fibres ash in concrete are continuously reported to be able to increase tensile 12 and compressive strengths with improved modulus of elasticity in concrete higher than glass, steel and polymers 13,14 . Approximately 95 million tons agricultural waste of banana origin was produced since 2012 15 . It showed that, this waste has a high disposal rate throughout the year providing a sustainable supply for the construction industries. Similar to coal ash, this agriculture waste has low density, appropriate stiffness and satisfactory durability 16,17 , possesses pozzolanic properties making it a good candidate for partial replacement of cement, admixture or supplementary cement material (SCM) in concrete 15 ; thus reducing construction costs. It has gained interest in civil construction and materials engineering fields ever since. Table 1 presents a comparison of strength enhancement using baby diapers polymers (BDP) 18 , coal bottom ash (CBA) 19 and banana by-products from literatures (e.g. banana leaves ash (BLA) 15 , banana skin powder (BSP) 20 and palm oil fuel ash (POFA) 20 , banana stem fibre (BSF) 14 ) with the current research.
Mohamad et al. 21 reported that the use of BSA in conventional concrete improved the tensile modulus for optimum use by 5% as well as flexural strength but with considerably lesser compressive strength. It is noted that, different properties of various natural fibres are able to increase the strength properties of composites accordingly 12 . The application of banana leaf ash is technically feasible because not only does it improve the concrete performance but also contribute to reduction of cement utilization in construction industries; approximately up to 10% leading to construction cost reduction 15 . The building material cost reduction has been in good agreement with Prakash et al. 14 who used banana stem fibres as an alternative to steel and artificial fibres to increase strength properties in mortar and concrete.
In addition, the current research presents observations on the microstructure improvement in the concrete after the integration of BSA via interfacial transition zone (ITZ). Response surface methodology (RSM) was used by Mohd Kamal et al. 19 in their study to optimize the amount of CBA and strength enhancement in concrete. Thus, current research presents studies on the strength enhancement of concrete using incinerated agricultural waste namely, BSA as SCM. The optimization on the application of BSA as SCM in concrete was modelled via RSM.

Methodology
Material collection and preparation. Banana skins locally called pisang nipah or Musa sp. were randomly collected from a local supermarket near Universiti Tenaga Nasional, rinsed several times with water, oven-dried (100 °C) for 24 h and incinerated at 300 °C for 30 min to produce banana skin ash (BSA). The resulting BSA was sieved to 75 μm sieve size. The preparation of BSA is shown in Fig. 1. Our study complies with relevant institutional, national, and international guidelines and legislation. Blaine's air permeability test or fineness test based on EN 196 ASTM C204 was conducted to obtain the fineness of BSA. The fineness of cement has a significant effect on the rate of hydration that will increase the concrete strength and the rate of heat evolution. Using Blaine's apparatus, the specific surface for banana peel ash and cement was obtained.
Engineering properties of concrete. Concrete workability. The slump test was conducted to assess the workability (or quality) of freshly made mix concrete focusing on the water-cement ratio based on ASTM C143 / C143M -20 standards. The mix concrete contained 0 (control mix), 1 and 2% of BSA.
Concrete compressive strength. The compressive strength (MPa) of concrete was determined based on ASTM C39/C39M-21 using a universal testing machine (UTM). The curing days were ranged from 0 to 28 days. The structural composition of concrete was observed and recorded using scanning electron microscope (SEM) (ZEISS GEMINISEM 500, Oberkochen, Germany). The interfacial transition zone (ITZ) was also observed. was used in response surface methodology (RSM) using the Design Expert ® software (Version 12) to assess the interrelation of process variables namely BSA composition in concrete and curing days with the response variable, compressive strength.
where Y = Predicted response variable, β = Regression coefficient, k = Number of factors or process variables in the experiment, e = Random error The ranges of process variables and code factors are shown in Table 2. The error percentage between experimental and predicted values was evaluated using Eq. (2).
Pearson's correlation coefficient (r) was used to measure linear association between two variables (Eq. 3) with the assumption that both variables are normally distributed. The correlation coefficient between variables can range from − 1 (shows negative linear correlation) to 0 (shows no linear relationship), to + 1 (shows positive linear correlation) 4,17,22,23 . where, r = Correlation coefficient, x i = values of the 'x' variable in a sample, x = mean of the values of the 'x' variable, y i = values of the 'y' variable in a sample, y = mean of the values of the 'y' variable.

Results and discussion
BSA characterization. The XRF analysis (wt. %) for BSA, (a) percentages of oxide elements (%) and (b) percentages of non-oxides elements (%), is shown in Table 3. The Class F fly ash (FA) comes from burning anthracite, possesses pozzolanic property, with little or no cementitious value (silicon dioxide (SiO 2 ) + aluminium oxide (Al 2 O 3 ) + iron (III) oxide (FeO 3 ) ≥ 80%); while Class C FA comes from lignite, and possesses both pozzolanic and cementitious properties (SiO 2 + Al 2 0 3 + FeO 3 ≥ 50%) 24 . As compared to other materials, the silicon dioxide (SiO 2 ) content of BSA (14.62%) was in the range close to OPC (20.6%). In the presence of heat during curing, the silicate-based material reacts with calcium hydroxide (Ca(OH) 2 ) generated by hydrating cement to form compounds possessing cementitious properties of better high strength performance 25 . Meanwhile, the calcium oxide (CaO) (4.12%) content of BSA in the current study was comparable to FA (3.32%) 17,23 , bottom ash (BA) (7.37%) 17,23 and class F FA (5%) 24 . The chemical compositions of BSA such as SiO 2 (14.62%) + Al 2 O 3 (0.26%) + FeO 3 (0.21%) was 15.09%. Accordingly, BSA is a pozzolan but cannot be classified according to ASTM standards classification for coal ash pozzolans because the percentage composition were below the standards. Nevertheless, BSA can be classified as SCM that contributes to the properties of hardened concrete through hydraulic or pozzolanic activity. SCM are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties. Banana skin is high in potassium (K) content followed by manganese (Mn), sodium (Na), calcium (Ca), iron (Fe), bromine (Br), rubidium (Rb), strontium (Sr), zirconium (Zr) and niobium (Nb) (mg/g) 12,21 . The 'K' content in the BSA was 43.15%, the highest compared to other non-oxides elements. The non-oxide element Na was not detected. However; the chemical composition was found to be lower than that reported by Anwahange et al. 26 . The 'K' and 'Na' are crucial for hydration reaction in cement mortar for the enhancement of strength properties. These chemicals react with calcium hydroxide to form potassium hydroxide (KOH) and sodium hydroxide (NaOH), which will accumulate in the aqueous phase of the solution and contribute to a change in the composition of the pore fluid. This leads to a change in the pH of the medium and accelerates the hydration of the cement 27 .
Blaine's air permeability results for BSA and OPC are shown in Table 4. OPC has high surface area (1470.8 ± 12.73) as compared to BSA (1091.2 ± 46.66). The OPC size or its fineness provides wider surface area for hydration that contributes to the hydration rate. It leads to the increasing in compressive strength development and heat evolution rate. It also increases the drying shrinkage of concrete 18,28,29 . The volume obtained for the BSA and OPC at 20 g of mass were 36.32 ± 0.00 cm 3 and 198.43 ± 0.00 cm 3 respectively. At the same mass, the BSA possessed a higher bulk density as compared to OPC. However, bulk density can be changed depending on the material's handling (i.e. loose or compact) [30][31][32][33] . Time taken for the manometer liquid to drop from the second to the third level of the manometer during the test (second, s) were 8.33 ± 0.70 s (BSA) and 15.10 ± 0.26 s (OPC).
(1) www.nature.com/scientificreports/ The value measured during testing is the time required to pass a certain volume of air through a packed bed of solids with given size and porosity. The surface area is directly proportional to √ t 34 . The higher the surface area, the shorter it takes for the time taken for air to flow through the compacted BSA and OPC beds.
Engineering properties of concrete. Concrete workability. The percentages of BSA (%), slump (mm) and types of slump are shown in Table 5. The control mix concrete (0% BSA) has the highest slump of more than 100 mm and can be classified as collapse type of slump. It showed that the workability of the control mix Table 3. X-ray Fluorescence (XRF) analysis for BSA, (a) percentages of oxide elements (%) and (b) percentages of non-oxides element (%).

(a) Percentage of oxides element (%)
Chemical formula Current research Mohamad 21 Kanning et al. 15 Beddu et al. 17  Theoretically, the increase of more water content in the concrete mix will not only increase its workability but also may increase the potential of segregation between coarse aggregate particles, bleeding, drying shrinkage and cracking as well as decrease in the concrete strength and durability 35 . Nevertheless, ash is a good water absorbent 16,23,36 . Adding ash will reduce water content in the concrete mix and its workability. Although workability of the concrete was reduced, further test such as compressive strength and SEM were conducted to measure the strength and to observe the microstructure changes upon BSA addition as SCM.
Concrete compressive strength. The percentages of BSA (%), ITZ (µm) and comparison of compressive strength (MPa) of mortar is shown in Table 6 and in Supplementary Material. The percentages of BSA against size of ITZ and compressive strength cover 0% (control mix), 1%, and 2% of BSA. As comparison of compressive strength (MPa) of mortar (0% (control mix), 1%, and 2% BSA) against curing days is shown in Fig. 2 Figure 3 shows SEM images of ITZ in concrete containing BSA at 2000 × magnifications. Concrete contains aggregates and hydrated cement paste. A water cement ratio in a concrete develops around the aggregate particles during casting, bearing a different pattern of microstructure around the hydrated cement paste. The interfacial transition zone (ITZ) can be observed around the aggregate 37,38 . It has a strong influence on the mechanical properties of mortar and concrete [37][38][39] . BSA contains potassium that can enhance the properties of mortars in fresh and hardened conditions such as microstructure. The addition of BSA to mixture enhances the properties of the microstructure as the thickness of ITZ is decreased (Fig. 3).
Model optimization. The 13 experimental runs pertaining to 2 process and 1 response variables were conducted for the RSM process optimization using CCD ( Table 7). The process variables were percentage of BSA (%) and number of curing days (day). The ranges of process variables were 0 (control) to 2% BSA and 7 to 28 for   www.nature.com/scientificreports/  . Quadratic model and model terms B, A 2 , and B 2 are significant (P-value < 0.05) (Fig. 4). The significant regression parameters of predictive models in terms of actual factors are shown in Eq. (4). The β coefficient for quadratic model and model terms of actual factors are shown in Fig. 4. Overall, the quadratic model is significant for the optimization purpose ( Table 9). The R 2 (0.9803) represents goodness of fit close to 1 40,41 .   www.nature.com/scientificreports/ The patterns of predicted versus actual values plot for compressive strength is shown in Fig. 5. The scatterplots were in a straight line showing a linear relationship and the proposed model terms were sufficient, and constant variance assumption was verified. Figure 6 shows the contour plots for BSA (%), curing days (day) and compressive strength (MPa). The compressive strength was found to increase with an increase in the number of curing days (7 to 28 days), which followed the usual strength development trend of concrete. The amount of BSA present in concrete also had an impact on its compressive strength. For a given days of curing, the compressive strength was highest at 1 to 1.5% BSA. Hence, 1.25% BSA has been adopted as optimum. Table 10 shows validation of the optimised model. In engineering applications, a variability of 20% of error percentage is often acceptable 42,43 . As discussed previously, the optimal percentage of BSA is equivalent to 1% and can be observed from the minimal error percentage obtained such as 2.4%, 14.3%, and 18.5% for 7, 14 and 28 days of curing accordingly. Although the error percentage for control mix design is the lowest compared to (4) Compressive strength (MPa) = 17.167 + 0.342091B + 1.77299A 2 + 0.007079B 2    www.nature.com/scientificreports/ 1% of BSA (4.1%, 2.4%, and 2.2% for 7, 14 and 28 days of curing accordingly), the microstructure was improved with concrete containing BSA. This improvement, reducing porosity and pore connectivity, leads to reduce the permeability and finally improve the durability 38 . The Pearson proximity matrices for BSA (%), curing days (day), experimental and predicted values for compressive strength (MPa) is shown in Fig. 7. The correlation coefficients obtained were highly positive between curing days (day) and experimental value for compressive strength (MPa) (R 2 : 0.955) as well as between curing days (day) and predicted value for compressive strength (MPa) (R 2 : 0.871). The experimental and predicted values were positively correlated with R 2 equals to 0.849 indicating the optimised model is validated.

Conclusion
Banana skin is an agriculture waste and BSA possesses pozzolanic property. BSA has high 'K' content crucial for hydration in concrete mix. The XRF analysed oxides and non-oxides elements of BSA that contribute to strength enhancement. Slump test showed that the workability of concrete reduced with addition of BSA. At the same mass of 20 g, BSA (36.32 ± 0.00 cm 3 ) has a higher bulk density than OPC (198.43 ± 0.00 cm 3 ) indicating large surface area for water absorption. The compressive strength of concrete increased with the number of curing days. The ITZ improved with the increased in BSA (control: 1.42 ± 0.38 µm, 1% BSA: 1.32 ± 0.33 µm, 2% BSA: 1.02 ± 0.15 µm). Overall, the optimal percentage of BSA was 1.25% at which the with the compressive strength was maximum for all curing days studied. The optimal number of curing days obtained was 28 days. The established model for strength enhancement of concrete using BSA as SCM was statistically significant and showed that it adequately represented the design space. The Pearson's proximity matrices showed correlation between curing days and compressive strength (both experimental and predicted). The established model can assist in the application of BSA in construction industries.