Experimental investigation on trinary blended geopolymer mortar synthesized from Industrial-agro and municipal solid waste ash subjected to different acid exposure

Geopolymer binders prove to be a reliable option to avoid dependency on conventional binders, and reduce the burden on the environment. The current study assesses the durability characteristics of a developed mortar made from a combination of Granulated blast furnace slag (GBFS), Sugar cane bagasse ash (SCBA), and Municipal solid waste ash (MSWA). The Geopolymer mortar specimens were cured under ambient conditions after the ternary blended mix had been activated by a solution of sodium silicate and sodium hydroxide of 8 M (SS/SH-2.5). Mass loss and compressive strength were measured at various time regimes of acid attack studies under mild concentrations of 3% sulphuric acid, hydrochloric acid, and nitric acid. The durability, macroscopic, microscopic, and morphological, studies were carried out. The results indicated that trinary blended geopolymer mortar exposed to sulphuric acid showed less mass loss and high compressive strength retention compared to the control mortar. The nitric acid exposure exhibited comparable strength loss for the trinary blend with control mortar and hydrochloric acid showed negligible effect on mortar. Mass loss was more significant in sulfuric acid than the hydrochloric and nitric acid. RCPT and Carbonation tests revealed that geopolymer mortar exhibits moderate chlorine ion penetration and the ingress of CO2 was less in the trinary blend mix. The mineralogical studies showed the formation of gypsum, nitrate, and chloride products. The optical microscopic image revealed the ingress of sulfuric acid is more intense in the mortar compared to hydrochloric and nitric acid and microstructural studies witnessed the degradation of C-A-S-H supported by EDX.


Introduction
The construction industry particularly concrete infrastructure had an immense contribution to the environmental carbon footprint.Ordinary Portland cement is responsible for exhibiting approximately 8% of the global CO 2 emission [1].To reduce the consumption of cement various supplementary cementitious materials are used viz., Fly ash, Rice husk ash (RHA), sugar cane bagasse ash (SCBA), GGBS, etc. Presently researchers are aiming at binders with zero cement product development.With the help of geopolymer material, this can be achieved which offers several advantages over traditional cement-based materials, including lower carbon emissions during production and greater strength development [2].Geopolymer binder is formed through a chemical reaction between source materials rich in silica and aluminum, such as fly ash, metakaolin slag, or clay, and alkaline activators like sodium or potassium silicate solutions [3,4].Agro and industrial waste in geopolymer proved to be a promising material for the reduction of dependency on conventional binders.
The sugarcane bagasse is derived from the sugar cane industry.The handling of this ash becomes a serious issue which is being addressed by disposing of it to the nearest land in turn resulting in environmental problems [5].India produced 405.5 MT of sugar cane during 2021 [6].The SCBA is rich in silica which can be incorporated into cement as supplementary material [7].The reactive silica from the SCBA will improve the C-S-H gel attributed to the pozzolanic reaction [8].The blending of SCBA into the cement matrix can significantly improve the compressive strength and reduce the porosity and heat of hydration compared to the conventional binder [9].
Yadav et al (2020) showed that the incorporation of SCBA into MK-based geopolymer mortar exhibited 25 Mpa of compressive strength for 0.5, and 2.5 l/B SS/SH ratio respectively [10].Babu and Thangaraj showed that 10% of SCBA can effectively be utilized in the GGBS-based geopolymer [11].Zafar et al (2022) The incorporation of 20% thermo-mechanical activated SCBA into FA-based geopolymer mortar block showed 20.7 Mpa compressive strength attributed to an increase in amorphous silica content supported by XRD results [12].
The sludge residue is generated during the treatment of sewage sludge in the sewage treatment plant it consists of bio-organic matter regarded as a precious resource that might replace fossil fuel.And also consists of organic matter and heavy metal pollutants which are harmful to the environment [13].Approximately 3955 thousand metric tonnes of dry sewage sludge are produced annually in India out of 62000 minimal liquid discharge (MLD) [14].The incinerated MSWA possesses chemicals such as SiO 2 , CaO, Al 2 O 3, etc which possess the pozzolanic properties [15].The 15% incorporation of calcinated (700 °C-800 °C) MSWA can produce a superior quality of concrete when compared to conventional concrete [16].The rate of heat evolution was increased by the MSWA blending into the cement mix compared to the fly ash [17].
Durability is mainly the resistance of a material against an aggressive environment which affects its serviceability [18].When the hydrates of the concrete stability are disturbed by the low pH the calcium is leached out and forms an amorphous hydrogel ascribed to acid action upon the concrete thus alkaline nature of the concrete is diminished by acid [19].Its effects mainly include the formation of calcium salts of acid gypsum which is formed by the decomposition of CSH or portlandite.The action of an aggressive environment can be anything but usually indicates towards acid attack [20], deterioration by sulfate [21], and chloride-induced corrosion [22].The dissolving of CO 2 , SO 2, and NO x into the precipitation reduces the pH to 4-5, and such acidic precipitation when coming in contact with the concrete deteriorates severely [23].The significant difference between the mechanism of degradation of fly ash-based geopolymer material exposed to the sewer system and 1.5% sulfuric acid was established [24].Concrete under the sewer system confronts sulfuric acid, initially, sulfates are converted into the hydrogen sulfide H 2 S by the sulfate-reducing anaerobic bacteria later aerobic Thiobacillus bacteria will convert H 2 S into sulfuric acid on exposure through oxidation in the sewer system [25].These H 2 S, CO 2 , and other acidic substances can reduce the concrete pH, and in the marine region, the concrete is subjected to the corrosion of reinforced steel bars which reduces the load-carrying capacity of the structure attributed to the migration of chlorides commuted through water into the concrete pore system [26].The acid attack significantly contributes to global economic loss [27].In this context, it is important to develop the durability of concrete which is capable of resisting various acid attacks, especially in sewer systems.The chlorine ions play a vital role in the corrosion of reinforcement in seawater It can be controlled by incorporation of a Ground granulated blast furnace salg (GGBS) which Countians alumina that will hinder the impact of chlorine ions [28].The literature study carried out on geopolymer mortar/concrete with different mineral admixtures and alkaline activators subjected to an acid environment is presented in table 1.
Zaidi et al (2021) proved geopolymer material shows excellent strength durability, and workable properties, especially in marine environments [37].Wan et al (2021) studied the behavior of metakaolin-based geopolymers with different water content and concluded that water content has a significant influence on the gel structure, strength, and porosity [38].Pather et al (2021) studied the effect of acid on fly ash-based geopolymer concrete with different types of aggregate, the granite or silicious aggregate is suitable for acid-resistant geopolymer concrete [39].Bakharev (2014) observed that procuring of mortar samples for a longer duration before thermal curing was beneficial in the strength development of fly ash-based geopolymer material [40].Ariffin et al (2013) Observed the superiority of pulverized fuel ash (PFA) and palm oil fuel ash (POFA) Geopolymer concrete over conventional concrete when subjected to a 2% sulfuric acid solution for 1 year [41].
Ozcan and Karakoç (2019) studied GGBS and ferrochrome slag incorporated geopolymer exposed to different acids (5%) and concluded that as the amount of ferrochrome slag in the concrete samples increased, so did their resistance to acid assault [42].Wu et al (2021) observed that adding calcium aluminate cement up to 10% improved the alkali-activated metakaolin's H 2 SO 4 (pH = 2.0) resistance [43].This was related to a decrease in permeability void volume and enhancement in the neutral pH.Huseien et al (2018) studied the effect of replacing fly ash with POFA in fly ash/slag blended mortar [44].The results indicated that the increased POFA content led to a reduction of Al content and enhancement in the Ca/Al ratio which negatively affected the H 2 SO 4 (10%) resistance of tested samples.
From the several previous studies, it is evident that most of the research work focused on fly ash or GGBSbased binder with consumption of energy for calcination under one or two acid exposures, but still, there is a gap in how the materials behave under ternary blended form from different sources and different acid exposure.The main objective of this research is to evaluate ternary blended geopolymer synthesized from GBFS, SCBA, and MSWA with the least energy consumption and adopting ambient curing to achieve sustainability when subjected to different acids such as sulphuric, hydrochloric, and nitric acid solutions of 3% by wt.The mass loss, visual appearance, and compressive strength were the properties studied.To analyze and understand the mechanism of the acid resistance of geopolymer mortar x-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), Optical microscope imaging, Depth of acid penetration using Image J software, and Scanning electron microscopy (FESEM), were considered in this research.

Materials
The Experimental investigation was carried out to understand the behavior of geopolymer mortar samples, their strength development, and resistance against damage owing to the immersion in different acids.The impact of alkali activating solution and replacement of GBFS by SCBA and MSWA were also taken into consideration.
GBFS was supplied by Astra Chemicals Chennai (TN), and SCBA was collected from Vellore Sugar Mills, Tamil Nadu.The MSWA was collected from the sewage treatment plant in Warangal (TS).The precursors were oven dried at 100 °C for 24 h sieved (300 μ) to remove the unwanted particles or matter and milled to the required fineness.The chemicals like sodium hydroxide pellets (97% purity) and sodium silicate (solution) were purchased at Harish Lab, Vellore.

Physical properties
The physical properties of the materials used in the present investigation are summarised in table 2. The fine aggregate purchased from the local market confirms IS 383-2007 the water absorption of fine aggregate (FA) was 1.46%, and the finesse modulus was 3.1 confirming Zone II.
The loss on ignition (LOI) for three materials is 1.41, 4.32, and 3.53% by wt for GBFS, SCBA, and MSWA respectively the SCBA possess high unburnt carbon content after burning of SCB at fuel stock at a temperature of 500 °C-550 °C hence exhibits combustible matter as shown in figure 1.The Sludge in the sewage treatment plant While Geopolymer mortar (GPM) made from fly ash (85% GGBS (15%) mix exhibited excellent resistance against acid solutions.Mohamed et al (2022) [30] Three GGBS: fly ash mortar considered 3:1, 1:1, and 1:0 ratio, exhibited resistance against H 2 SO 4 acid.The compressive strength of these mortars increased when exposed to air.Khan et al (2022) [31] The test program results showed that replacing fly ash (FA) with 10%-20% GP (Glass powder) diminished the physical, mechanical, and morphological deterioration of the specimens caused by acid (H 2 SO 4 ).Yang et al (2021) [32] The optimum performance was found in Geopolymer cement (GPC) made with fly ash consisting of high calcium and 12 M NaOH solution.Zhang et al (2020) [33] Halloysites incorporated with Si and P (phosphoric acid) into the geopolymer matrix with Si-O-P-O-Al networks at higher phosphoric acid concentrations, increased compressive strength.Vafaei et al (2019) [34] The Geopolymer cement (GC) developed from low calcium fly ash and calcium aluminate cement showed noticeably better resistance, in pH 3 HCL acid.Sturm et al (2018) [35] Micro silica, Rice husk ash, and Silica from chlorosilane production were added in place of Ground granulated blast furnace slag (GGBFS) into the mortar mix.Mortars exhibited a high resistance against sulfuric acid attacks.Karakoc et al (2016) [36] Ferrochrome slag (FS) was used to create a new geopolymer binding material.The crushed sand aggregates incorporated concrete cubes showed a minimum reduction in compressive strength when exposed to a 7% MgSO 4 solution.

Mix design
The mortar mix was prepared according to ASTM C311-04 [50].A total of 4 mixes were prepared by varying % by weight of SCBA and MSWA.The fine aggregate-to-binder ratio was considered 2.75 and the liquid-to-binder ratio was 0.50 for all mixes based on the codal provision ASTM C311-04.Based on the preliminary test carried out on the different sodium hydroxide molarities of 4, 6, 8, and 10 for SS/SH ratio of 1 and 2.5.The optimum values i.e., 8 M of sodium hydroxide and 2.5 ratio SS/SH were selected for the present investigation [11], and mix details are summarised in table 4. The cubes were covered with polyethylene sheets to avoid moisture loss during the reaction.The curing regime was an ambient condition and all specimens were tested at time intervals of 28, 56, and 90 days of acid immersion.

Mortars preparation and compressive strength
The digital mortar mixer is used for mixing the material according to the requirement.Moulds with internal dimensions of 50 × 50 × 50 mm were used.Compressive strength was determined using a 2000KN compression testing device.The loading rate was kept under control at around 75 KN/min, which is within the parameters set forth by the ASTM C109 standard [51].The corresponding compressive strength was calculated by using the formula load by the area of the specimen and validated with mineral and morphological results.

Durability properties
RCPT was conducted wherein a cylindrical disc of 50 mm height and 100 mm dia was coated with epoxy on its side and sandwiched between two chambers of a test cell one containing 3% of NaCl and the other 0.3 N of NaOH the charge passed (60 volts) measures the samples electrical conductivity throughout the test, which lasts about 6 h, 30 min intervals are used to record the current.ASTM C 1202-07, specifies the chloride ion penetrability based on the charge transmitted.To determine the carbonation of the mortar cube of 50×50 mm, the samples used in the accelerated carbonation process were kept in a carbonation chamber with an average temperature of 28 °C, a CO 2 content of 4% an interior relative humidity of 70%, and a 22-day exposure period was maintained during the experiment in accordance to RILEM TC056-CPC-18.One percent phenolphthalein solution in ethanol is required for the indicator solution.This is made by combining 1 g of phenolphthalein powder with 100 ml of a solution that contains 30 ml of de-ionized water and 70 ml of ethanol.
Compliance with the ASTM C267-2006 [52].Standard Test Methods for the Chemical Resistance of Mortars were tested.The Specimens were immersed in a 3% concentration of sulfuric (H 2 S0 4 ), hydrochloric (HCL), and nitric acid (HN0 3 ), having a pH of 1.20, 2.60, and 1.35 respectively 3% was selected depending upon the concrete material subjected to an acidic environment in the sewage pipeline, mining, and food processing industry.The 3% of the solution was monitored every week by adding acid to the required pH and it was replaced by a fresh solution Every month pH was checked using a portable digital pH meter.The mortar samples were visually observed and images were taken before and after acid exposure.The mass loss of the samples was calculated by taking the initial weight of mortar before acid immersion and the final weight of mortar after acid immersion, before taking the weight of mortar, samples were taken out of the acid solutions were lightly brushed with a soft plastic brush under running water before being measured to eliminate any loose surface debris and free surface water.

Microscopic and macroscopic properties
The Optical microscopic images were taken on an Olympus-BX53M model optical microscope.Image J software was used to determine the depth and area of acid ingress.The mortar after the test was machine sliced in the form of a thin square section of size 1 cm x 1 cm ground to smoothen the surface oven dried at 105 °C then used for the optical microscope.A field emission scanning electron microscope (FE-SEM) was carried out on a Thermo Fisher FEI QUANTA 250 FEG for morphological studies of materials that operate at a voltage range of 5-30 kV, a resolution of 1.2 nm at 30 kV at high vacuum conditions.The crushed chip samples after the compressive test were taken and kept in an isopropanol solution to arrest the reaction for 24 h oven dried then utilized for the FESEM test.

Results and discussion
The geopolymer mortar mix was prepared, cast, and tested for compressive strength, durability, and microstructural properties for SS/SH-2.5, and 8 M of sodium hydroxide solution under an ambient curing regime.

Mass loss of geopolymer mortar subjected to acid exposure
The mass loss change (%) for the geopolymer mortar at 28, 56, and 90 days of three acid exposures is depicted in figure 3. The sample before and after acid exposure was weighed and the respective mass loss for each specimen after acid exposure was calculated.The samples exposed to sulfuric acid lost their mass as time progressed compared to the other two acids.Control mortar CM-2.5 severely damaged the edges got opened up which allowed the acid solution to get inside the matrix with ease attacking the polymer chain.This can be related to more amount of calcium-based binding chemicals breaking down quickly in acidic environments, cement matrixes gradually deteriorate until they are completely damaged [53].Control mortar exhibits more calcium aluminate silicate hydrates (C-A-S-H) when compared to other mixes, which are not chemically stable in acid conditions, they experience severe mass losses.The CMB-2.5 sample observed less deterioration against the sulfuric acid attributed to the less calcium content present in it.The development of gypsum as a result of the chemical reaction between the calcium compounds in the C-A-S-H gel and the sulfate anion in sulfuric acid can be explained by the larger change in mass losses in sulfuric acid for the mortar compared to hydrochloric and nitric acid.In comparison to hydrochloric acid attack at the same pH, the internal dissolving forces brought on by the formation of gypsum minerals formed in degrading mortar might produce cracking and exfoliating of the top surface [54,55].The mortar sample containing SCBA, SCB-2.5 showed a reduction in the mass loss in sulfuric acid due to the presence of ample voids in the matrix.The Mortar sample with MSWA depicted mass loss for this can be due to the acidic nature of the precursor MSWA obtained from sludge and the presence of more sulfur content from XRF results which intensified the acid attack on the mortar.
In geopolymer samples exposed to hydrochloric acid after 56 days, there is an increase in the mass due to maturity effects in the sample [56] supported by compressive strength results wherein the 56 days strength is more than 28-and 90-day strength.However, 90 days of mass loss was reduced significantly this may be because of the expelling of CaCl 2 which is water soluble salt and loosely bonded unreacted precursor through the capillary pores because of prolonged immersion supported by XRD results depicting less crystalline peak at 90 days.In the SCB-2.5 mortar exposed to HCL and HNO 3 the moss loss was less significant compared to H 2 SO 4 attributed to the fact that the replacement of GGBS with SCBA increases the pH value [57].The MSWA incorporated mortar MSW-2.5 exposed to HCL showed minor loss of mass attributed to its weak acidic nature and less pH 2.6 compared to the other two acids, however, HNO 3 exposed mortar exhibited mass loss which is comparable to the sulfuric acid attack as the pH of these two acids were comparable 1.20 and 1.35 for sulfuric and nitric acid respectively, and the mechanism of deterioration was similar as observed in the H 2 SO 4 acid wherein the calcium salt is highly soluble.The long-term acid resistance of the CMB-2.5 mortar sample for all three acids was comprehensively improved and outperformed compared to the control mortar attributed to low calcium content and more stable cross-linked polymer.During acid exposure, the mass loss of various mortar samples caused by acid exposure was observed.Figure 4 shows the findings of mass loss against exposure time.As can be observed, sulfuric acid exposure caused substantially larger mass losses than hydrochloric and nitric acid exposure.

Compressive strength for geopolymer mortar
The ordinary Portland cement when exposed to acid attack mainly deteriorated because of reaction products' susceptibility to dissolution and breakdown when exposed to acidic conditions, especially Ca-rich products.Figure 4 shows that in alkali-activated binders, there is an initial loss of strength in all samples, submerged in acids, followed by a significant increase in compressive strength over time in samples immersed in HNO 3 (c) or HCl (b), this may be due to protective ambience in a closed environment was established by the water in the acid solution due to prolonged immersion of mortar, which resisted the acid attack by densification of pore system.This effect on the pore system and improvement in strength was well documented in [58].which is also supported by the increase in weight for the HCL, and HNO 3 immersed sample.The H 2 SO 4 immersed samples experienced a decrease in compressive strength as shown in figure 5(a) due to the formation of gypsum as a result of SO 4 2− attack on C-A-S-H.The control mortar consisting of 100% GBFS exhibited high vulnerability to the H 2 SO 4 acid solution exposure attributed to the Ca + leaching and dealumination followed by the formation of gypsum crystals [56].The acid resistance of SCB-2.5 at 90 days was improved and it is comparable to strength after 28 days of exposure, especially for H 2 SO 4 and HNO 3 acid solution attributed to the saturation of acid attack.The enhancement in the strength because of polymeric reaction in a prolonged immersion of acid can be one more reason [58].The strength at 90 days of H 2 SO 4 acid exposure for SCB-2.5 and CMB-2.5 comprehensively improved because of the closed curing effect on the pore system, and due to polymeric reaction [19].The Mortar Sample incorporated with MSWA showed a reduction in strength for sulfuric and nitric acid at 90 days of acid exposure this can be due to the source from which it is generated usually the sludge is acidic which boosted the acid attack and the presence of sulfur content in MSWA.The Combined sample CMB-2.5 showed less variation in the loss of strength at 28, 56, and 90 days for all three-acid exposure attributed to the incorporation of SCBA  and MSWA enhanced the resistance against the acid attack [59].Strength enhancement was observed for mortar samples at 90 days, exposed to sulfuric acid due to the activating of polymeric reaction after the acid attack reached saturation state.
The control mortar samples subjected to HCL and HNO 3 acid exhibited a better resistance against acid attack at 90 days compared to 28, and 56 days due to maturation effects in the sample [56], high alkalinity of the solution, and low CaO /SiO 2 in alkali-activated systems [60].The mortar sample SCB-2.5 and MSW-2.5 were exposed to HCL and HNO 3 with almost similar trends of reduction in strength but HNO 3 exposed sample was damaged more compared to HCL which indicates the mortars had better resistance against HCL than HNO 3 .The CMB-2.5 mortar sample exhibited a gain in strength after 56 Days of HCL and HNO 3 acid exposure compared to 28 and 90 days.The better acid resistance of mortar samples is because of the hardened alumina silicates or the development of alumina silicates from chemical reactions [61,62].
In slag-based geopolymer ternary binders that are attacked by H 2 SO 4 acid solution, experience the resultant dealumination of the binding gel and leaching of soluble elements.The material cracked, allowing SO4 2-to enter the corroded layer, and gypsum crystals are created by the reaction of Ca 2+ with SO4 2-in the mortar sample.Nevertheless, as declining in the leaching solution was undersaturated concerning this phase, the concentration of the H 2 SO 4 solution to maintain the 3% value caused the elimination of Calcium and sodium ions from the binding agent rather than promoting the production of gypsum in the samples [63], this phenomenon resulted in the better performance of CMB-2.5 against acid solutions after 56 days.
The pH of the acid solution replicates its strength or the dissolution rate, strong acid exhibits lesser pH value and weak acid shows higher pH values [58].The pH will also have impact on the strength reduction.The overall strength reduction of the mortar sample subjected three acids reveals that sulfuric acid is more severe than the nitric acid, and hydrochloric.The hydrochloric acid being higher pH value produced least effect on mortar strength.
The % loss in the strength of geopolymer mortar revealed that the mortar subjected to sulfuric acid experienced severe loss of strength compared to the other two acids.The control mortar lost 64% of its strength at 90 days of exposure as shown in table 5.The SCB-2.5 and MSW-2.5 mortar samples lost about 40% of their strength at 90 days of sulfuric acid exposure but the rate of loss of strength was very slow especially after 28 days of acid exposure, mainly due to replacing calcium rich GBFS material and CMB-2.5 showed the least strength loss of 30.13% at 90 days of acid immersion.The mortar samples exposed to HCL were found to be the least affected, CMB-2.5 mortar sample's loss of strength at 90 days was better than the control mortar which is 5.52% for CMB-2.5 and 7.08% for the control sample, and gain in strength was observed for most of the sample after 56 days due to the maturity effect.The mortar samples immersed in HNO 3 experienced strength loss similar to sulfuric acid, especially for SCB-2.5 and MSW-2.5 mortar samples, however, the 90 days strength loss of the CMB-2.5 sample was comparable to the control mortar strength loss.

Rapid chloride penetration test for geopolymer mortar
The RCPT test was carried out according to ASTM C 1202 [64].Table 6 displays the chloride ion penetration for the mortar samples.The Geopolymer CM-2.5 mortar imparted a moderate resistance against the ingress of chloride ions.The control mortar exhibited a 2575 Col charge passed, this can be attributed to the fact the reactivity of GBFS is high and produces more heat of hydration thereby influencing the cracks initiation in the matrix and resulting in the positive effect on the permeability of the mortar sample [65].One more reason may be the presence of portlandite from the FESEM images in figure 10(a), which inhibits the resistance against the chloride ion ingress.Incorporation of SCBA proved to be moderate since the calcium content is reduced but SCBA contains voids and absorbs moisture which improves the ionic moment inside the matrix Similar results were observed by Akhtar A, & Sarmah A K [66].The MSW-2.5 exhibited better resistance against chloride penetration 2160 coulombs This can be ascribed to dense and compact matrix with less void resulting in improving the compressive strength and less calcium content reduced thermal cracks.On the other hand, ternary blended sample CMB-2.5 demonstrated a better resistance against the chloride ion penetration due to the less calcium content present in the matrix resulting in less heat of hydration which resulted in less crack formation and may be due to less or no portlandite formation supported by FESEM images in the section 3.8.
It can be concluded that the decrease in chloride ingress is mainly due to the diminishing of the pore connectivity system which is due to the compact and dense geopolymer matrix results in the reduction of ionic mobility inside the mortar, and the addition of precursor materials brought the change in the ionic concentration.

Carbonation test for geopolymer mortar
Figure 5 depicts the geopolymer mortar subjected to carbonation, wherein the colored region exhibits the intact area from the carbonation attack and the colourless area shows the attacked region.The carbonation usually depends on factors such as concentration humidity of the surroundings, pH, porosity of the sample, etc.The mechanism of carbonation is given in the equations (1)-( 3) [67].
The control CM-2.5 after carbonation show more colored area after the test which is the intact zone from carbonation combined blended mortar CMB-2.5 showed similar results subjected to carbonation exhibited better resistance this may be because the calcium-rich precursor enhanced the pore structure of the geopolymer blends as Slag and MSWA both possessed calcium content which is evident from XRF results similar results were observed by author Rajagopalan et al [68].The porosity of geopolymers increased with the 10% addition of SCBA in the mix enhancing the carbonation, which also resulted in a change in compressive strength for the SCB-2.5 sample.Figure 5 provides typical images illustrating the carbonated areas of various mixes after 22 days of rapid carbonation.It is evident that in SCB-2.5 the borders between the colored and uncolored zones are discernible when using the phenolphthalein indicator to determine the carbonation area of mortar samples.The addition of SCBA content had an impact on the physical changes in the mortar the pores, and voids in the matrix gradually increased along with unreacted particles, which increased porosity and decreased the compressive strength [69], which led to enhancement in the carbonation.This is mostly explained by variances in the precursors' calcium, silica, aluminum, and magnesium content, which led to discrepancies in the hydration products.The incorporation of MSWA into the MSW-2.5 showed that the effect of carbonation on the mortar sample was similar to that of control and trinary blended samples this can be due to the dense and compact structure resulted in better resistance against ingress of CO 2 .
The freshly prepared conventional concrete usually has a pH more than of 13 and the carbonated concrete will have a pH of less than 9.In a fresh mortar sample formation of calcium hydroxide and C-S-H gel creates a buffering effect on maintaining the pH above 13 [70].

Xray diffraction test for geopolymer mortar
Figure 6 shows XRD for the mortar samples before acid exposure and after acid exposure Viz, sulfuric, hydrochloric, and nitric acid solutions, at 28 and 90 days for control CM-2.5 (a) and Combined CMB-2.5 sample (b).
Before exposure, samples exhibited minerals in crystalline phases such as quartz, zeolite, albite, and amorphous phase C-A-S-H gel for both CM-2.5 and CMB-2.5 samples are present in the diffraction pattern as shown in figures 6(a), (b).The results of the polymerization reaction were amorphous minerals.The existence of an amorphous phase can be seen as the cause of the modest, hump in the vicinity of 25 and 35°2θ for both samples.The crystalline component among calcium compounds of geopolymer mortar and sulfate from acid was gypsum which was well established after 28 days of acid immersion, as shown by the diffraction pattern at 20.31°, 29.03°for CM-2.5, and 20.7°, 41.90°, and 49.75°for CMB-2.5 mortar sample after immersion in the The sulfuric acid deteriorates the mortar by H + ions released from the pored acid solution which weakens the hydrated C-A-S-H gel.The SO 4 2-ion further initiates the formation of gypsum by reacting with Ca 2+ released from the C-A-S-H and also induces the tensile stress resulting in cracking and spalling due to its expansive nature [72].However, the calcium content of the geopolymer mortar significantly impacts the amount of gypsum formation and its mechanistic function in the sulfuric acid attack.This can be observed in the CMB-2.5 sample showed a better acid resistance than the CM-2.5 at 90 days of exposure attributed to the presence of more silica content than calcium content this phenomenon is well supported by the compressive strength results, wherein CM-2.5 samples significantly lost its strength at 90 days.Having said that gypsum formation also has some advantages by sealing in the porosity [73].
Even after being immersed in a 3% HCl solution, the phases found in the original specimens remain the same except a new mineral calcium chloride is formed by dismantling the C-A-S-H gel.The position and strength of the peaks have undergone a few little adjustments, though.For instance, following exposure to HCl, the quantity of crystalline quartz albite, and zeolite phases reduced after 28 and 90 days of exposure.Additionally, these peaks' intensities are noticeably weaker than those found in the sample before acid exposure this effect can especially be observed for CMB-2.5 at the beginning of acid exposure wherein the rate of acid reaction is very high.After being exposed to HCl acid, C-A-S-H peak shapes changed from broad hump to steep peaks.This suggests that when exposed to an acid solution, the slow disintegration of polymeric structure results in the enhancement of non-crosslinked crystalline minerals in the matrix [74].
The geopolymer mortar CM-2.5 exhibited the formation of calcium chloride at 20.44°, and 39.25°and for CMB-2.5 samples at 20.13°, 39.12°respectively.This can be due to the HCL acid disintegrating crosslinked polymer eliminating calcium ions from C-A-S-H gel by H + ions, Cl -ions act on these free calcium ions to form calcium chloride is well documented in [75].These peaks were observed in both samples but the intensity of the peak differs attributed to their solubility in water.
When alkali activated binders were subjected to nitric acid nitrate from the acid reacted with the calcium compounds from the precursor to form calcium nitrate which exhibits a high solubility of 56% [76].In actuality, at greater nitric acid pH levels, sodium and calcium, which are the soluble constituents of samples, are leached not only to a shorter depth but also to a smaller extent [76].The formation of calcium nitrate is evident from XRD images at 20.44°for CM-2.5 and 20.88°, 29.17°for CMB-2.5.The deterioration may also be due to another phenomenon wherein electrophilic assault act on Si-O-Al bonds thereby eliminating tetrahedral aluminum from the aluminosilicate framework this is well documented in [77].
3.6.Fourier transform infrared Spectroscopy results of mortars Figure 7 illustrates the FTIR spectra from CM-2.5 (a) and CMB-2.5(b) samples acquired from mortar specimens prior and after exposure to the sulfuric, hydrochloric, and nitric acid solution the figure shows identifying bands that appeared in these FTIR spectra.The most significant band indicates the presence of C/N-A-S-H in geopolymer mortar by corresponding to the asymmetric stretching of Si-O-T (T = Al or Si) [78].The CM-2.5 and CMB-2.5 before exposure to acid samples showed this band at a wavelength of 949 and 944 cm −1 which are very broad.When exposed to sulfuric, hydrochloric, and nitric acid solutions respectively, this band migrated somewhat towards higher wavenumbers and becomes narrow at roughly 1080 and 1114 cm −1 for CM-2.5 and CMB-2.5 respectively, whose intensity is diminished after acid exposure especially in CM-2.5 due to intense calcium-based material which is more vulnerable to acid attack, according to a comparison of the spectra.This shifting is a sign that the geopolymer mortar molecular structure has partially depolymerized as a result of some tetrahedral aluminum leaching out during acid treatment.A strong signal showing at 418, 435 cm −1 is corroborated to the asymmetric stretching vibration of Si-O-Si for both samples respectively.
When the mortar samples were exposed to acids new band formed at 870 cm −1 for the CM-2.5, 876, and 873 cm −1 , for the CMB-2.5 sample respectively at 90 days of exposure corresponds to Al-OH attributed to acid attack depolymerization which results in leaching of tetrahedral aluminum from C-A-S-H gel similar observation were made by [78].
However, the 90 days results for HCL and HNO 3 immersed samples show the bands getting narrow witness the strengthening of Si-O-T bands compared to 28 days of immersion due to prolonged exposure this is well supported by the improvement in compressive strength for CM-2.5 and CMB-2.5 at 90 days.
3.7.Macroscopic and microscopic images, depth of penetration, and area of acid attack into the geopolymer mortar at 90 days of acid exposure Figures 8 and 9 show the results of macroscopic, and optical microscopy imaging respectively, including color alterations, depth, and area of deterioration given in table 7 for the samples at 90 days of exposure to acid.The mortars' surfaces have changed noticeably since the acid attack.All of the mixes had shown the growth of acid reactant products.
The mortar samples macroscopic image before acid immersion show dense and compact structure as shown in figure 8.The CM-2.5 mortar exhibited more acid reaction product compared to other mortar samples which can be observed clearly in macroscopic and microscopic levels, especially in sulfuric and nitric acid, this can be explained by the amount of silica and alumina increased, the surface deterioration decreased, which can be attributed to the aluminosilicate structure's stronger stability in the presence of more Si and Al and a relatively low Ca content [55].The MSW-2.5 showed more deterioration due to acid attack because the precursors originated from domestic sewers which are already acidic and again subjected to acid attack.
From table 7 depicts depth of acid penetration macroscopic images were analyzed by Image J software on the top side of the mortar, and depth was measured at 10 different locations and an average of these value was considered for acid penetration.The thickest degraded zone across the CM-2.5 mortars which is 0.1522 Cm given in table 7 indicates that this matrix is unstable against an acidic environment and has poor resistance to acid (sulfuric and nitric acids) however the ternary blended mortar CMB-2.5 showed better resistance against all the three-acid attributed to low ca content and better aluminosilicate bond.The depth of penetration and the area of acid attack by sulfuric acid is more compared to nitric acid.as shown in figure 9.
The microscopic image shows the deterioration of geopolymer mortar.The image exhibits the separation of the acid attack region from the intact region marked by the saffron line which also shows the propagation of cracks represented by the letter 'C' along the separated line and the deposition of the acid product in a few images can be observed which is presented by letter 'P', especially in nitric acid exposed mortars 'WITZ' represents the weak interfacial transition zone due to acid attack between paste and aggregate region.The acid attack of chloride which was not observed at the macroscopic level can be observed in the microscopic image indicating the chloride ingress into the mortar specimen as shown in figure 9.
3.8.Microstructural characterization of geopolymer mortars using FESEMEDX before and after the acid attack Elemental content and microstructure at 90 days of exposure to acid solutions were studied using the cracked surface for Geopolymer mortar samples CM-2.5 and CMB-2.5.The microstructure of the geopolymer mortar before the acid attack is depicted in figures 10(a) and (b) showed a dense or partially dense matrix, especially in CM-2.5.Figures 10(c) and (d) show the mortar samples exposed to sulfuric acid attack on a high magnification depict the chemical attack or acid effects on the polymerized matrix by enhanced porosity disintegration of aluminosilicate bonds formation of fissure corroborated to the elimination of tetrahedral aluminum as a result of depolymerization process.Similar results were also reported in studies carried out by Vafaei et al [55].Along with porosity expansion and microcracking, samples subjected to sulfuric acid also showed gypsum formation due to interaction between ca + ions from C-A-S-H gel and SO 4 2-from acid, as shown in figures 10(c) and (d) and previously validated in XRD results.
The FESEM showed a more porous microstructure, it is quite likely that the HCL acid and the calcium compounds in (equation ( 4)) [79], will react to generate a high dissolving calcium chloride (CaCl 2 ), which accounts for the specimens.The samples that were treated with nitric acid are depicted in figures 9(g), and (h).The HNO 3 acid incursion on the components that is accountable for compressive strength causes some microstructural deficiencies, which are revealed by the FESEM.The exposed mortar matrix resembles the HCl exposed mortar specimens in terms of internal microstructure porosity.The nitric acid attack causes scarcity of Na and Ca, indicating that there had been some sort of reaction between C-A-S-H gel and the NO 3 ion from the exposed acid.In the matrix of CM-2.5 and CMB-2.5 mortar specimens, this reaction caused the soluble nitrates as given in (equation ( 4)) that are melted into the exposed acid to leave behind empty pore spaces.The microstructure of the geopolymer mortar specimen changed noticeably after being exposed to HCl solution for 90 days figures 9(e) and (f).
Due to the sever sulfuric acid attack EDS were taken only on mortar samples exposed to sulfuric acid.Geopolymer mortar matrix before and after exposure to a sulfuric acid solution at 90 days, together with typical micrographs, EDS elemental spectra, and the matching elemental compositions generated from fracture surfaces of mortar samples are shown in figure 11.The 90day chemical attack by 3% H 2 SO 4 acid caused leaching of Si, Al, and the ingress of sulfur (S) from the pore acid solution, as shown in table 8.
From EDS results it can be observed that leaching of Si and Al for CM-2.5 is 84.23 and 85.1% and for CMB-2.5 it is 78.87 and 90.90% respectively showing better resistance against the sulfuric acid due to low calcium content and better crosslinked polymer bonds in the CMB-2.5 mix.The sulfur ingress into the matrix is much more into the CM-2.5 compared to CMB-2.5 a.

Conclusion
The detailed investigation of GBFS based geopolymer mortar and partial replacement of base material with the SCBA and MSWA at different acid exposure (H 2 SO 4 , HCL, and HNO 3 ) for different time regime (28, 56, and 90 days) was carried out.Mass loss, compressive strength, durability properties such as RCPT, and carbonation test  Morphological studies such as XRD, FE-SEM etc, were carried out.Following conclusion can be drawn based on the studies conducted.
• Geopolymer binders exhibited a better resistance against acid incursion, compared to the CM-2.5 mortar sample, the CMB-2.5 showed noticeably better or comparable acid resistance, in all three acids.
• The visual observation showed that even though the mass loss in sulfuric acid and nitric acids were similar but there is a variation in strength loss.
• The incorporation of MSWA into the mix provided weak resistance against acid ingress, but blending it with SCBA has improved the acid resistance.The chloride ingress into the mortar were moderate.The carbonation depth was reduced in the CMB-2.5 mortar.
• The loss in strength after immersion in H 2 SO 4 acid for the CM-2.5 mortar exhibited severe damage and lost more than half of its strength (64%) while the CMB-2.5 mortar flexed its strength and lost only 30% of its strength at 90 days of time regime.
• In nitric acid, CMB-2.5 loss of strength was comparable to the control sample.The HCL immersed sample showed the least strength loss at 90 days of exposure for all samples.
• The macro and optical microscopic image revealed the ingress of H 2 SO 4 acid is more intense in the mortar compared to HCL and HNO 3 acid.The SCB-2.5 sample immersed in H 2 SO 4 acid and MSW-2.5 in H 2 SO 4 and HNO 3 acid showed more acid penetration.
• The XRD and FESEM results revealed the formation of gypsum crystals by ingress of sulfur into the matrix causing much damage in the control mortar compared to ternary blended mortar CMB-2.5.The other two acids just managed to reduce the mass loss and strength which was less significant.
The mortar synthesized in the present research work can be implemented in precast elements subjected to different acid exposure conditions, marine structures, and other non-structural works.Geopolymers can be used in construction, infrastructure repair, and even for making fire resistant materials.However, widespread adoption of geopolymer technology may require further research, standardization, and acceptance in the construction industry.

2. 1 . 3 .
Morphological properties D8 advanced Bruker and X'pert high score software was utilized for x-ray diffraction (XRD) analysis.The XRD results of the precursor revealed that GBFS and SCBA are amorphous indicated by A-H in figure 2(a), whereas MSWA exhibited crystalline peaks.The MSWA mainly exhibits minerals such as quartz, anhydrite, anorthite, and calcite.The GBFS consists of minerals namely gehlenite (Ca 2 Al (AlSiO 7 )), and kyanite.The SCBA contains minerals such as quartz and kyanite.The IRAffinity-1 model was used for the Fourier-transform infrared spectroscopy (FTIR) test.The FTIR analysis for precursor as shown in figure 2(b), around 1024, and 873 cm −1 in the MSWA spectrum corresponding to asymmetric vibrations of Si-O-T bonds, which are also present in the SCBA and GBFS spectrum at 1031, 785, and 899 cm −1 respectively, where Si-O-T (T: tetrahedral Si, Al, or Fe)[46].The bands associated with the 571 cm −1 , and 603 cm −1 for MSWA have ascribed to the Fe 2 O stretching vibrations of the hematite[47].The weak peak at 1424 cm −1 of MSWA is corroborated by the asymmetric stretching O-C-O bond belonging to the carbonate group[48].The stretching band centered in the region of 436 cm −1 to 423 cm −1 represents the asymmetric bending vibration of O-Si-O, which exhibits amorphous nature[49].The weak band at 2240-1900 cm −1 corresponds to the carbon-carbon tetrahedral bond.

Figure 3 .
Figure 3. % Change in mass for the mortar samples after acid exposure.

Figure 8 .
Figure 8. Macroscopic images of Geopolymer mortar at 90 days of acid exposure.

Figure 9 .
Figure 9. Optical microscopic images of geopolymer mortar mixes at 90 days of acid exposure.Note: C-Crack, P-Acid product, W-ITZ Weak interfacial transition zone.

Figure 11 .
Figure 11.EDS of Geopolymer mortar mixes for before and after acid exposure.

Table 1 .
Literature study for Geopolymer subjected to different acid exposure conditions.

Table 2 .
Physical properties of materials used.

Table 5 .
Loss in strength (%) after exposure to different acid.

Table 6 .
RCPT results for the Geopolymer mortar mixes.
sulfuric acid solution as shown in figures 6(a), (b) respectively.Similar observations were made by the authors Allahverdi et al Vafaei et al and Sata et al where crystalline gypsum was formed after reaction between sulfate and calcium ions [55, 63, 71].

Table 8 .
EDS composition (mass %) for before and after acid exposure.