Role of agro waste in geopolymer concrete: strength, durability and microstructural properties

The morphology of GGBS can provide information about its physical structure when seen under a scanning electron microscope (SEM) as shown in figure 2(a). This information is essential for comprehending the material's characteristics and possible uses. Under a SEM, GGBS usually shows an amorphous or glassy structure. This structure is the result of the slag components' not crystallizing due to the quick cooling method used during manufacture. The form and distribution of GGBS particle sizes can be seen in SEM pictures. The irregular morphologies of the particles might vary from angular to more irregular shapes with sharp edges, contingent on production-related and post-processing parameters including the rate of cooling.

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The morphology of SBA particles can differ, ranging from asymmetrical to elongated shapes. The size and form of SBA particles usually fluctuate, as shown in figure 2(b). Pores or cavities inside SBA particles can be seen using SEM imaging. Properties like permeability and water absorption capacity can be impacted by variations in the size and distribution of these pores. Factors like the degree of ash treatment and the combustion process have an impact on the porosity of SBA particles. For sugarcane bagasse ash to be optimally utilized as a supplemental cementitious material in concrete and other building applications, its shape must be comprehended through SEM examination.

The SEM images of MBS shows that the particles are typically in the micron range, with diameters varying from a few micrometres to tens of micrometres as shown in figure 2(c). Different shapes, such as spherical, angular, or irregular forms, may be displayed by the particles. The kind of biomass source, the parameters of the micronization process, and any further treatments or adjustments all affect the distribution of particle size and form. The pore structure and porosity of micronized biomass silica particles can be understood using SEM investigation.

A total of 7 mixes are considered for the study and each mix is represented as M1 to M7. The mix details are given in table 3 and the reference mix contains 100% GGBS. The GGBS is initially replaced by SBA from 5 to 10% at 5% offset. The optimum mix is found as 10% GGBS replacement by SBA and the further replacement with MBS is done at GGBS-BA mix with 10% replacement. The MBS is added from 4 to 12% at 4% offset for further studies.

The compressive strength and flexural strength of specimens were examined to analyze the durability of the geopolymer concrete cubes after 7 and 28 days of curing, following the IS: 516–1959 standard [26]. The geopolymer concrete was cured using the ambient curing method. Compressive strength tests were conducted on concrete cubes measuring 100 mm × 100 mm × 100 mm using a digital compression testing machine with a 2000 kN capacity at a consistent load rate of 25 N mm⁻²/min. Additionally, the flexural strength of 500 mm × 100 mm × 100 mm prisms was also determined.

A research was conducted to measure the water absorption and sorptivity of geopolymer concrete using 100 mm × 100 mm × 100 mm cubes, following the guidelines outlined in ASTM C-64213 [27]. The geopolymer concrete specimens were placed immersed in normal water up to 3 mm depth. The gain in mass is evaluated to an accuracy of 0.01 g in at stipulated intervals. The sorptivity index was calculated as in equation (1). The capillary rise versus square root of time graph has been drawn and the sorptivity index was measured by normalizing the slope. From the regression equation, primary and secondary sorptivity were derived.

$$\begin{eqnarray}&&S=\frac{I}{{t}^{\frac{1}{2}}}\end{eqnarray} \tag{ 1 }$$

Where, I - The amount of water retained per unit area of the inflow surface as shown in equation (2)

$$\begin{eqnarray}&&I=\frac{W}{A\ast D}\end{eqnarray} \tag{ 2 }$$

S - Sorptivity (mm/min^1/2);

t - time elapsed (min).

W - change in weight

A - surface area of specimen through which water penetrated (mm²)

D - Density of Water

The carbonation of the samples was carried in accordance with RILEM TC056-CPC-18 refers to the Technical Committee 056 of RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) which deals with Carbonation of Concrete. Allowing the specimens to cure under controlled conditions until they reach the desired age 28 days. Exposing the specimens to a controlled environment with a known concentration of CO₂ (4%) with 70% relative humidity for a period of 22 days of exposure. This can be done in a carbonation chamber where the CO₂ concentration, temperature, and humidity are regulated according to the requirement.

Specimens in the shape of cubes, with dimensions of 150 mm × 150 mm × 150 mm, were placed in sulfuric acid solutions with a pH of 0.8 in order to investigate the durability of materials against acid corrosion. The acid solutions and their concentrations were selected based on the practical application of concrete in industries such as sewage pipes, mining, and food processing. The pH value was used to verify that fresh solutions replaced the acid. To confirm the deterioration of the specimens, optical microscopic images were utilized, and the compressive strength was assessed after 28 and 90 days of exposure.

The geopolymer concrete specimens achieved a compressive strength of 38.23 after 7 days, and 43.21 after 28 days, as illustrated in figure 3. The strength gain rate was approximately 88.47%, achieved within 7 days, which may be attributed to the intense geopolymer reaction during the early stages [28]. The control mix only with GGBS shown a 28 days strength of 43.21 MPa. This my be attributed to tiny and angular structure of GGBS particles, rich calcium content and the rapid reaction it also (GGBS) lessen the need for an elevated curing temperature [29]. The primary outcome of the reaction is the formation of C-A-S-H gel, a mineral that contributes to strength, as evidenced by the SEM images.

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The mix with SBA of more than 10% showed a reduced strength this may be due to the reduction in the GGBS content and increase SBA which exhibits low reactivity [30]. It can be attributed to the fact that the alkaline-activated solution did not dissolve the SBA, thereby preventing it from contributing to the polymerization process. Even though there was unreacted SBA remaining in the geopolymer matrix, the unreacted SBA particles did not have a significant impact on strengthening the material [31].

The geopolymer concrete with 10% SBA and 8% MBS content exhibited better compressive strength than the reference mix M1. Maximum compressive strength is observed for the mix with 10% SBA and 8% MBS and is 45.98 MPa, which is around 6.41% more compared to the reference mix. This may be attributed to the compact and dense microstructure developed due to the incorporation of the MBS content with SBA [20]. Similar to SBA an addition of MBS beyond 8% significantly reduced the compressive strength. The fineness of SBA and MBS tends to absorb the free water available in the mix. As the percentage of SBA and MBS increases the water-to-binder ratio decreases and it affects the compressive strength.

It has been discovered that when unreacted silica accumulates in SBCA, it results in SiO₂. Additionally, the levels of Al₂O₃ increase, which has an impact on the compressive strength. Nevertheless, the MBS demonstrated greater strength than anticipated, reaching 63.48 MPa with 10% replacement and 45.98 MPa for M6. This increased strength of MBS is attributed to the higher presence of calcium silicate hydrate gel and primary alumina silicate gel. As the volume of MBS increases, the workability of the concrete decreases. The desired consistency of the concrete can be achieved by adding an extra alkali-activated solution. The capacity of tiny shapeless MBS particles to absorb also requires a more alkaline solution [20].

The data shown in figure 4 illustrates the bending strength of the standard concrete and the geopolymer concrete. When the SBA content exceeds 10%, flexural strength decreases. After 28 days of curing, the mix with 10% SBA (M3) exhibited the highest flexural strength, 11.32% lower than the unblended mix. The increase in MBS content at 8% enhanced flexural strength. The reduction in strength diminished beyond 8% blending MBS. When the percentage of SBA increased beyond 10% and MBS more than 8% the strength reduced due to the weaker bond strength. Maximum flexural strength is obtained for the geopolymer with the ternary blend of 10% SBA and 8% MBS (M6) and has a 7.54% increase in strength compared to the reference mix (M1) for 28 days of curing. The presence of an additional calcium silicate hydrate phase led to a significant rise in flexural strength, which is influenced by the degree of aluminosilicate polymerization. The molar concentration, type of alkali activation solution (sodium hydroxide and silicate), crystallinity of the materials, and Si/Al ratio all affect the degree of aluminosilicate polymerization [32]. Increasing the MBS content even more led to a decrease in flexural strength. The flexural strength was reduced when MBS was added beyond its limit, as shown in a previous study [33].

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The figure 5 shows the water absorption results. The control mix M1 generate more heat attributed to the GGBS induced polymer reaction which results in the cracks as shown in SEM and EDS images which results int more water absorption [34]. The SBA nullify the thermal effect of GGBS as a result less heat paved the way for dense matrix [35]. The water absorption has reduced as the percentage of SBA increased to 10%. Compared to the reference mix M1 the water absorption has reduced to 4.62%. Similarly, the increase in MBS up to 8% tends to decrease in water absorption compared to reference mix since the pores are occupied by the C-A-S-H gel [36]. However, increasing the percentage of SBA beyond 12% resulted in a non-homogenous structure due to the occupation of more spherical particles in the pores and resulted in increased water absorption. The lowest water absorption rate is 25.49% and is observed for the geopolymer concrete with 10% SBA and 8% MBS which supports the compressive strength values. This can be due to the fact that fine MBS particles obstruct the pores due to the micro filler effect, resulting in solid concrete structures with less ability for liquid penetration .The increase in MBS content beyond 8% range however increased the water absorption which is well supported by the reduction in strength this can be attributed to the hindrance action shown by the MBS on the polymeric aluminosilicate gel which affects the compact and denseness of the geopolymer matrix [20].

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Figure 6 shows the Initial sorptivity versus mix designation. In terms of absorption, micro/nano silica performed marginally better than SBA [37]. The sorptivity value was reduced by around 3.07, 9.23, 3.58% less compared to control mix when SBA was introduced in the mix at 5, 10 and 5% respectively. M3 mix with 10% of SBA shown better results in terms of capillary absorption this may be attributed to blocking of pores by the filler effect of ground SBA [38]. Increase in the SBA beyond 10% resulted in the increase of pores and adverse effect on capillary absorption. The MBS incorporation into the mix along with SBA produced positive effect on sorptivity by reduced water absorption for M6 mix which consist of SBA (10%) and MBS (8%) due to the fine particle occupied in the pores. The incorporation of only SBA may lead to the more pores but when it is combined with MBS it leads to the denser matrix reducing the pores. The strong bridging and filling effects of micro/nanomaterials may cause a decrease in permeable gel pores when the curing time and proportion of micro/nanoparticles are increased [37]. The sorptivity reduces to 27.03% compared to conventional concrete for an optimum mix M6 as shown in figure 6. The incorporation of excess of MBS into the matrix creates an effect on packed aluminosilicate polymer gel's ability to form was likely impeded by the high MBS content, which had a negative impact on the GPC's structural suitability. The capillary suction increased as a consequence [20].

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Table 4 presents the data from the RCPT test for the GPC mixes on the 28th day. Concrete's pore structure and connectivity are essential in determining how aggressive ions can enter the material if exposed to harsh conditions, such as in a sea environment. Concrete offers increased resistance to water absorption when its sorptivity is reduced. A high sorptivity coefficient indicates the pore network has minimal tortuosity or a well-connected porous structure. Electric charges transmitted through specimens with varying compositions - including 100% GGBS, 5, 10, and 15% of SBA were measured at 1762, 1989, 2339, and 2787 Coulombs, respectively. A decrease in the charge received was observed for the M6 mix, which contained 10% SBA and 8% MBS, registering 1713 Coulombs. The charge transmitted through the control specimen was similar to that of the M6 mix. Here is the reworded text: According to ASTM C 1202, the electric charge passing through the specimen is utilized to assess the low chloride penetrability of sample M6. A moderate level of chloride permeability was observed for SBA replacement at 10% and 15%. The pore structure plays a crucial role in determining the permeability of concrete. The impact of MBS particles can be characterized as a decrease in the penetration of chloride ions when MBS is present in optimal proportions. The arrangement of pores is a crucial factor in determining the permeability of concrete. The impact of MBS particles can be characterized as a decrease in the penetration of chloride ions when MBS is used in the right amounts. Because it is porous and has high density, concrete has low permeability to liquids, resulting in MBS-filled concrete absorbing less water. However, if an excessive quantity of MBS rich in silica is introduced, it disrupts the SiO₂ to Al₂O₃ ratio, impacting the formation of the geopolymer and causing increased ion transfer in the concrete. Excess silica-rich MBS addition may lead to an improper SiO₂ to Al₂O₃ ratio, impacting geopolymer formation and increasing ion transfer in the GPC. The MBS pozzolanic reaction can enhance chloride resistance by improving pore refinement and filler effects [20].

The enhanced ability to withstand chloride is linked to the enhancement of the pores and the impact of the MBS pozzolanic reaction. The combined influence of fly ash and bagasse ash with MBS boosts the compaction of solid materials by occupying the small pores in geopolymer concrete [30].

The control mix M1 with only GGBS exhibited more coloured area which is the noncarbonated zone this may be attributed to the fact that calcium rich precursor had improved the pore structure this is in line with the result observed by [39]. The mix M4 with 15% SBA showed increase in the carbonation depth attributed to the fact the blending of SBA will increase the porosity which will favours the condition for carbonation as shown in figure 7. The physical alterations in the mortar caused by the addition of SBA content included a steady increase in the number of pores and voids in the matrix along with unreacted particles, which raised porosity and lowered compressive strength similar results were observed.

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The incorporation of MBS along with SBA in the mix M6 showed the results in which the coloured area is more compared to other mix as discussed already the colored area is non-carbonated region. This can be due to the fact that the optimal addition of the MBS will reduce the voids and increase the compactness of the structure there by reducing the carbonation. Increase in both SBA and MBS beyond 15 and 8% (M7) respectively into the GGBS mix increased the carbonation due to the increase in the voids which is supported by reduced the compressive strength.

The carbonation usually depends on the factors such as pH, porosity and humidity [40]. The conventional concrete forms a dense CaCO₃ layer as a result of the carbonation reaction, which causes CO₂ infiltration to the concrete surface to decrease with the length of the exposed period. Here, the present study utilised GGBS, SBA, and MBS, which is being activated by the alkaline solution makes up the majority of the ingredients in most of the mixes varieties of concrete, and sodium carbonate (Na₂CO₃) are the main by-products of the carbonation reaction similar results were observed by [41].

7.5.1. Mass loss percentage

When geopolymer concrete is exposed to an acid erosion medium for an extended period, the mortar will separate from the matrix, exposing the aggregates, which can result in concrete mass loss and structural damage. Figure 8 depicts the mass loss rate of all GPC when exposed to a sulfuric acid solution for 28 and 90 days. As illustrated in the figure, the mass loss rates of every GPC specimen rapidly increased over time in immersion. The geopolymer concrete with MBS replacement experienced a lower mass loss than the control geopolymer concrete with GGBS. The experiment's results indicate that the acid resistance of GPC based on MBS and GGBS was significantly lower than that of GPC based on GGBS. As the duration of exposure lengthened, the weight of all GPC samples exhibited a significant drop after 28 days.

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The mass loss rate of GGBS-MBS-based GPC was more than 15% after a 90-day exposure period, which is consistent with the observed phenomenon depicted in the figure. The specimens' mass loss rate for the GGBS-based GPC stayed below 8% after 90 days of exposure. However, the M6 sample experienced a mass loss rate of only 4.44%, the lowest value observed after exposure to the acid solution for 90 days. According to the findings regarding mass loss rate, it was observed that the GPC specimen labelled as M6 exhibited the most effective resistance to sulfuric acid corrosion. The weight reduction is linked to the extent of neutralising various GPCs when exposed to the sulfuric acid solution.

7.5.2. Compressive strength after acid exposure

Figure 9 illustrates the reduction in GPC's compressive strength following exposure to the sulfuric acid solution. GPC that included GGBS, SBA and MBS in the alkaline mixture had initially exhibited higher compressive strengths than those that didn't include MBS, since the formation of geopolymer gel was enhanced due to the reactive silica present in MBS. After being exposed for 90 days, the GPC specimens with GGBS failed but maintained some of their compressive strength. The specimens containing high-silica MBS and activated with high concentrations of alkali activators exhibit higher compressive strength than the others. This is due to MBS acting as a filler, blocking the pores of specimens, and thus reducing the infiltration of the harsh chemical through the pore network. Visual analysis of the GGBS-based GPC specimens from optical microscopic images revealed in figure 10.

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The deterioration starts at the Interfacial Transition Zone, which enhances the neutralization depth and mass loss and leads to reduced compressive strength. M6 exhibited the minimum deterioration of all the specimens, where the MBS acts as a filler, and the neutralization depths are reduced. Here, the bond between mortar and aggregate is not affected much. Therefore, the strength reduction is not significant.

The x-ray diffraction of different geopolymer concrete is given in figure 11. It is clear from the figure that various minerals are formed as a result of chemical reactions.

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The mix M1 showed the mineral Q quartz at 26.22° which is crystalline in nature the zeolite and albite minerals are also formed at 27.64°, and 54.48° respectively. The Calcium silicate hydrate C-A-S-H is responsible for the strength development formed at 29.06° 2θ [33, 34]. The mix M₃ also showed a similar trend but an additional mineral calcium silicate hydrate CSH mineral is formed as a secondary strength forming mineral due to the incorporation of MBS at 50.06° 2θ apart from C-A-S-H at 29.36°. The mix M4 Showed minerals such as Quartz and Mullite at 26.52°, 20.67° 2θ respectively which is due to the incorporation of SBA into the matrix. this mix depicted enhancement in the peak of C-A-S-H and C-S-H gel due to the hybrid reaction of geopolymer and conventional reaction. Mix M6 showed a diminished in the intensity of zeolite and mullite minerals but the strength forming C-A-S-H and C-S-H peaks are well established in this mix at 29.59 and 50.36 respectively which is supported by the compressive strength results [42].

The SEM images of different Concretes as shown in figure 12. The concrete M1 shows the formation of the dense matrix, portlandite, void, and cracks. It also exhibits thermal cracks due to GGBS in the matrix [34], as already discussed in the water absorption test results. The formation of portlandite reduced drastically in rest of the mix due to the formation of secondary strength forming minerals such as C-S-H which consumes portlandite and silica in presence of moisture [43], which was observed in the XRD test results for the same. The Mix M3 exhibits partially reacted and no reacted SBA the incorporation of SBA into the matrix reduces the fissures or cracks due to reduction in GGBS which is responsible of heat inception, it also witnesses the formation dense glassy matrix which contributes to the strength. The Mix M6 who's strength resembles the control samples compact topography and traces of reacted and inert SBA and MBS, SBA sometimes led to the formation of voids [44], in the mix which is countered by the MBS due to its micronized silica particles. Hence the combination of SBA and MBS proved to be vital in enhancing the strength parameter of geopolymer concrete.

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The EDS of M1, M3, and M6 are summarised in the table 5. The Ca/Si ratio for M1 is 0.60 and the same ratio forM3 and M6 was 0.68 and 0.59 respectively this can be due to incorporation of silica rich SBA in M3 and blending of SBA, MBS in M6 which contributes to the strength forming minerals and also reduces thermal cracksas shwon in figure 13.The Ca/Al ratio for M1 M3 and M6 are 1.82, 2.26 and 1.78 respectively which indicate towrds reduction in the calcium content due to replacement of GGBS with SBA in M3 and blending of SBA, MBS in M6 and also some tracesesof alumina present in SBA.

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