Fly ash geopolymer concrete durability to sulphate, acid and peat attack

. The durability of concrete has a profound impact on the service life of structural elements. Indonesia has extensive peat soils, which provide a highly aggressive environment for concrete structures. Geopolymer concrete has demonstrated good durability when exposed to acid /sulphate conditions similar to those encountered in peat soils. This paper investigates the performance of geopolymer concretes produced using Indonesian type F fly ash under sulphate and acid chemical attack. Geopolymer concrete specimens have been exposed for 12-months in a range of solutions: 5% sodium sulphate, 5% magnesium sulphate, 1% and 3% sulphuric acid, and simulated peat solution. The mechanical and durability properties of specimens together with a control concrete have been monitored for compressive strength, change in mass, water absorption and volume of permeable voids, ultra pulse velocity, air and water permeability, pH profile, and microstructural analysis (XRD, SEM/EDS). The control immersed in water achieved 56.93 MPa at 12-months of age. Magnesium sulphate exposure had a significant deterioration impact on the compressive strength of geopolymer concrete, demonstrating an 11% reduction in strength, while those exposed to sodium sulphate had an 8.9% increase in strength. Specimens exposed to peat solution displayed a slightly increased strength and those in acid conditions a 1.2% and 4.5% decrease in 1% acid and 3% acid, respectively. In general, the geopolymer concrete displayed a high level of resistance against sodium sulphate, 1% sulphuric acid and simulated peat attack.


Introduction
The industrial sector produces large quantities of waste products, creating disposal problems and contributing to global warming, one of the most pressing environmental hazards.This problem is exacerbated by cement manufacture, which releases greenhouse gases such as carbon dioxide.Accordingly, various studies are investigating the use of industrial waste products as a substitute for ordinary Portland cement (OPC).Fly ashbased geopolymer material is one of the most popular alternatives to concrete due to such factors as availability [1], superior mechanical properties [2,3], low environmental costs [4], and better durability [5] than conventional Portland concrete.
The demand for construction materials to have a long service life and low-cost maintenance requires durable concrete, particularly under aggressive conditions.A significant chemical deterioration reaction in concrete is sulphate attack due to expansive chemical reactions.Studies have shown fly ash geopolymer concrete (FAGC) have excellent resistance to sulphate attack compared to cement-based materials [6][7][8].S. Wallah et al. [9] confirmed that the high alkali content in geopolymer improves resistance to sulphate attack.From a short-term study (up to 5 months exposure), Bakharev [10] indicated the deterioration of FAGC is more significant in sodium sulphate than magnesium sulphate solution.It also noted very different durability of FAGC specimens when interacting with sulphate solutions, where FAGC prepared using sodium hydroxide as an activator had better performance than sodium silicate or a mix of sodium hydroxide and potassium hydroxide.This observation is in agreement with the results reported by Albitar et al. [5], where FAGC with a combination of sodium hydroxide and sodium silicate as activator exhibited a decrease in strength in sodium sulphate media, and that the detrimental impact caused by sodium sulphate is due to leaching of sodium hydroxide from the geopolymer specimens when exposed to sodium sulphate.In contrast, Cho et al. [11] concluded that fly ash geopolymer mortar mass and compressive strength were not affected by 10% sodium sulphate and 10% magnesium sulphate solution after 1-year exposure, while Elyamany et al. [12] reported a 72.7 to 80.6% residual compressive strength of fly ash geopolymer mortars in 10% magnesium sulphate solution after 48 weeks.
An acidic environment is another aggressive mechanism that deteriorates concrete.Past studies generally concluded that sulphuric acid has a negative impact on compressive strength development and causes mass loss in FAGC, but compared to OPC, geopolymeric materials have superior performance in an acid environment [13].Song et al. [14] investigated low calcium FAGC, an approx.61-67% residual compressive strength was reported after 56-days of exposure to 10% sulphuric acid and a further decrease in the range of 72 to 90% after 1-year exposure to 1% sulphuric acid.Mehta et al. [1] reported FAGC resistance in 2% sulfuric acid solution; and observed a mass loss of 4.28% at 3-months and 12.97% at 1-year.Moreover, only few studies have been conducted to observe the resistance of geopolymer material to a peat acid environment.Peat is the accumulated organic remains of dead plants [15] containing humic and fulvic acid [16].Peat can have a very low pH due to pyrite oxidation [15].A study on the early strength performance of various FAGC, including FAGC in peat environment, conducted by Olivia et al. [17] reported that geopolymer shows a slow gain of earlyage strength until 28 days compared to OPC, high volume fly ash, and blended cement concrete subjected to peat water.Felix Wijaya et al. [18] noted that a geopolymer hybrid (mix of low-quality fly ash containing >15% carbon and Portland cement) shows an increase in strength and a decrease in porosity.Conversely, Satya et al. [19] studied the performance of blended geopolymer mortars (90% of fly ash and 10% of palm oil fuel ash, mass ratio) in peat water (pH 4-5) for 120 days.They concluded that peat water generally decreases the strength and enhance the porosity and sorptivity of the blended geopolymer mortars.

Research Significance
Past studies have investigated the durability of FAGC in sulphate and acid solutions.However, there are conflicting reports in these studies on the performance of fly ash geopolymer, particularly under sulphate and peat acid attack.This study addresses this gap providing comprehensive data regarding the deterioration of FAGC when exposed to sodium and magnesium sulphate, sulphuric acid, and a simulated peat solution for up to 12months.The findings of this study are significant for the utilization of fly ash in Indonesia and the durability of FAGC in the native Indonesian environment.Indonesia has a significant area of peat and acid sulphate soils where concrete is subject to an aggressive environment and extensive durability issues due acid and sulphate attack.This work reports compressive strength, change in mass, water absorption and volume of permeable voids, ultra pulse velocity, air and water permeability of FAGC in these aggressive environments.The study employs various analytical and chemistical methods for the analysis of the FAGC, including XRD, SEM and EDS.
3 Experimental Procedure

Materials
Type F fly ash obtained from Paiton Power Station, East Java Province, Indonesia, is used with a specific gravity of 2.21, a specific surface area of 1041 m 2 /kg and loss on ignition of 0.32 (ASTM D7348-13).X-ray fluorescence (XRF) is used to evaluate the chemical composition.Table 1 gives the XRF result, and according to ASTM C618-19, the fly ash is classified as class F (low calcium).Fig. 1 shows the XRD patterns, identifying 17.5% of crystalline phases (quartz, hematite, maghemite, and mullite).Fig. 2 observes the spherical particles of microspheres, irregular mineral fragments, and large enclosed-plerosphere containing microspheres from Scanning Electron Microscopy (SEM) analysis of fly ash.Uncrushed river sand is used as fine aggregate with a fineness modulus of 2.65 (ASTM C33/C33M-18) and specific gravity of 2.60 (ASTM C128-15).The sand is dried at 110 °C for 24 h in the oven, then cooling to room temperature.The 7 and 10 mm sizes of crushed coarse aggregate have a SSD specific gravity of 2.585 and 2.641, and water absorption of 0.80% and 0.64%, respectively.A mix of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) is used as the alkaline reagent in liquid form.Na2SiO3 is supplied by PQ Chemicals Australia (14.7% Na2O and 29.4% SiO2), with specific gravity of 1.53.The 15M NaOH is manufactured by Australian Chemical Reagents with specific gravity of 1.45.

Concrete synthesis
The FAGC is made with Na2O dosages of 10% and alkali modulus (AM) of 1.375, where AM is mass ratio of SiO2 to Na2O in the alkaline solution.The ratio of fly ash to aggregate and the ratio of water to solid (fly ash, solid in alkaline solution) is fixed at 0.808 and 0.35, respectively.The aggregate volume is a combination of 40% of max.size of 10mm, 19% of max.size of 7 mm and 41% of sand.FAGC specimens are prepared by mixing the coarse aggregates using a 120-litre mixer for 3 minutes.Fine aggregate is then added and mixed for another 3 min, followed by the addition of fly ash and mixing for 5 min.The liquids, Na2SiO3, NaOH and 80% of additional water, are added, stirred to homogenize and left for 5 min.The remaining water is added followed by mixing for another 5 min.The fresh mix mortar is poured into teflon moulds in 2 layers and vibrated for 20 sec using a vibration table for each layer.The concrete specimens are stored for 24 hours at room temperature (21±1 o C) followed by heat curing at 80ºC for 24 hours in the oven.After the heat-curing, the specimens are demoulded and stored in a humidity chamber (22 o C, 70%) for 1-month prior to placement in solutions for chemical exposure.

Sulphate and acid exposure
Once reaching 28-days, the 100x100x100 mm 3 cube specimens are immersed in water for 24 hours to obtain water-saturated specimens before being exposed to chemical solutions, and the initial saturated weights of the specimens are measured.The specimens are exposed to 5% sodium sulphate (Na2SO4), 5% magnesium sulphate (MgSO4) solutions, 1 and 3% sulfuric acid (H2SO4), and peat acid (0.49% humic acid, 0.49 fulvic acid, and 0.03% sulfuric acid, pH 2.5) solutions.The immersed specimens are then for 12-months in a humidity-controlled room at a temperature of 22 o C, and relative humidity of 70%.The specimens are immersed in a 52L sealed container with 20L chemical solutions.The sulphate solutions are renewed every 3 months, the peat solution is refreshed monthly, while the sulphuric acid solution is refreshed every 6 months, and the pH is periodically monitored every month.A set of control specimens immersed in water is prepared for the same duration.

Testing method
The compressive strength test is performed using Technotest concrete testing equipment at a loading rate of 0.33 MPa/s (ASTM Standard C109/C109M).The reported 1-month and 12-months compressive strength values are the average of three cube specimens (100x100x100 mm 3 ).The specimens are taken from the solutions and stored at room temperature 24-hours prior to compressive strength testing and tested in semisaturated conditions.The density, absorption and volumetric proportion of void test is undertaken following ASTM C642-13.The air and water permeability tests are performed using the Autoclam Permeability System.Both permeability tests are conducted at 1-month and 12months after casting.Three prism specimens of 100x300x300 mm 3 are tested.The average of three cube specimens of 100×100×100 mm 3 is measured monthly to evaluate the mass change of concrete, measured in saturated surface dry conditions.Ultrasonic measurement is conducted on 100x200x200 mm 3 cylinder specimens using a portable ultrasonic non-destructive digital indicating tester (Proceq Pundit PL-200PE) with a 54 kHz transducer.All tests are conducted at 1-month and 12months after casting.Three samples are used in each test and the results presented are the average of the data obtained.The suspension method was applied to obtained the pH profiles by mixing the concrete powder with deionised water with a powder-to-suspension ratio of 2:1, and stirring for 15 mins immediately after grinding each depth.The pH of the solution is measured using a calibrated pH probe.X-ray fluorescence (XRF) is carried out using a Bruker S4 Pioneer, while X-ray diffraction (XRD) data is acquired using a Bruker AXS D4 Endeavor wide-angle X-ray diffractometer with copper anode at 40kV and 35 mA.The powder samples for XRD and pH profiles are taken at an interval of 3-mm by using a profile grinder from Germann Instruments.The microstructure morphology of fly ash and FAGC is observed using FEI Quanta 200 SEM employing secondary electron.Energydispersive X-ray spectroscopy (EDS) detector is used to observe the element and further analyzed by Aztec-4.3 software.3 depicts the compressive strength of the control and geopolymer concrete specimens subjected to sulphate and acid solutions at 12-months.The notation for the data is control specimens in water (W), 5% sodium sulphate (Na), 5% magnesium sulphate (Mg), 1% sulfuric acid (1SA), 3% sulfuric acid (3SA), peat acid (P).The compressive strength of the control concrete at 28-days age is 46.54 MPa, corresponding to an increase of more than 22% over the 1-year duration in the control specimen.In sulphate solution exposure, an 8.9% strength increase is measured in the sodium sulphate solution, but approx.11% strength decline in magnesium sulphate.In the case of immersion in acid solutions, FAGC exhibits a decline compared to control specimens, corresponding to approx.1.2% and 4.5% for 1SA and 3SA, respectively, at 12-months, but an increase approx.1.25% for peat specimens.The percentage mass loss of the geopolymer concrete specimens is presented in Fig. 4. A minor mass loss is observed in the water specimen of 0.1% over the 12months period.When exposed to sulphate solutions, the Na and Mg specimens presented a slight gain in mass over the exposure time, 0.24% and 0.25%, respectively.However, a significant decrease in mass is noted in both specimens subjected to sulphuric acid attack.The 1SA specimen had a mass loss of approaching 2% and 3SA more than 3% at the end of 12 months.Specimens submerged in peat solution gave a slight mass loss of approx.0.27%.

UPV, water and air permeability, water absorption and volume of permeable voids
Table 3 displays the UPV, water and air permeability, water absorption and volume of permeable voids of room specimens.The result shows the air permeability index is between 0.1 and 0.5 Ln(mbar)/min at 1-month and 12months, conforming to good quality concrete [20].The control concrete is classified as low water permeable concrete at 1-month and 12-months, as the water permeability index did not exceed 1.3x10 −7 m 3 /√min.The concrete displays an enhanced UPV with age.The values are between 3000 and 3500 m/s, which is identified as medium quality concrete [21].This implies that the concrete contains defects, such as voids or cracks, which may adversely affect long term performance.A higher UPV value would indicate a higher solid density and lower porosity.The water absorption of FAGC is more than 5% in the first month and decreases to less than 5% at the end of 12 months.Water absorption is greater than 5% and is classified as high permeable concrete in conventional concrete [22].Thus, the geopolymer concrete indicates a highly porous external surface at an early age.The volume of permeable voids shows a similar trend to water absorption, decreasing with time.In PC concrete, a volume of permeable voids less than 13% is classified as good quality concrete, while greater than 18% is classified as poor quality concrete [23].Thus, the geopolymer concrete, which has a volume of permeable voids less than 13% indicates good concrete, with limited pore interconnectivity between the capillary pores, gel pores and air voids within the structure.

Visual observation
Fig. 5 shows the appearance of geopolymer concrete immersed in sulphate and acid solution after a period of 12-months and the control specimens.The results illustrate that the visual degradation is minimal, and the specimens generally remain structurally intact in the control and test solutions.Rough surfaces, a more porous and less dense structure are observed in the sulphuric acid specimens, with greater delamination in the 3% sulphuric acid.

XRD
The XRD analysis results of the first 1 mm from the surface of the geopolymer concrete specimens are presented in Fig. 6.The quartz, microline, muscovite and albite observed are attributed as minerals derived from the aggregate used in the mixtures.A broad hump is visible between 25° and 35° 2θ for all XRD spectra, confirming the formation of amorphous N-A-S-H gel reaction products.Specimens immersed in sulphuric acid solutions indicate mineralogical character changes due to the formation of gypsum.There is no evidence of ettringite in the acid and sulphate samples.

EDS
Table 4 shows the summary of elements' atomic and atomic ratios from EDS analysis from an average of three locations.The atomic ratio of Si/Al of room sample displays a ratio of 2.66, while specimen in water is slightly higher, 3.15.For both sulphate samples and the peat samples, the ratio of Si/Al on the surface is within the range 3.0 ≤ Si/Al ≤ 4.After being exposed to sulphuric acid for 1 year, the top layer of 1SA and 3SA samples exhibit significantly increased Si/Al ratios of 6.79 and 10.98, respectively.EDS spectra detect the presence of sulphur in Mg, 1SA, and 3SA specimens but the Na specimen is free from sulphur, as is the control specimen.

Durability properties
FAGC stored in water shows good strength development over time, an increase approx.24% between 1-month and 12-months, which indicates ongoing geopolymerization.The increasing strength is coupled with a decrease in the permeable voids ratio, as demonstrated by an increase in UPV and a decrease in water absorption.It is also supported by the decrease in air and water permeability index during the same period.SEM images exhibit pores with a range of sizes at 1-month.The larger pores may be caused by air bubbles introduced in the mixing process.At 12-months, the number of pores is reduced in number and relative size.This is attributed to continuing chemical reaction between fly ash and the alkaline solution, forming N-A-S-H gel, which fills the pores and densifies the matrix [24].Moreover, there is no discernible difference between 1-month and 12-months specimens based on visual observations, and there is no significant change in mass over the 12-month period, with less than 0.3% mass gain being observed.

Sulphate resistance
The FAGC concrete shows superior resistance to sodium sulphate compared to magnesium sulphate.The study notes an increase of nearly 9% in compressive strength in the sodium sulphate solution to the control specimen at 12-months of age.The increase in strength is likely due to continuing of the geopolymerization reaction in the sulphate solution [25].Cho et al. [11] reported an enhanced strength of FA geopolymer mortar after 1-year exposed to 10% sodium sulphate media.However, there is a significant reduction in compressive strength in the magnesium sulphate specimens (residual strength of 89%) compared to the control specimens.This suggests the formation of magnesium aluminium silicate hydrate (M-A-S-H) gel in the geopolymer matrix.Long et al. [26] stated that the M-A-S-H gel has lower strength than N-A-S-H gel; and is produced by the reaction of magnesium sulphate with N-A-S-H gel.The EDS analysis in the Mg specimen depicts high Mg and S concentrations in the matrix of geopolymer concrete, evidence of magnesium and sulphur ion migration into the concrete, while Na ions are not detected, likely caused by migration of Na ions into the solution [10].Similar to this study, Thokchom et al. [27] reported up to 56% compressive strength loss of fly ash geopolymer mortar after 24 weeks of exposure to 10% magnesium sulphate, while Elyamany et al. [12] observed up to 20% compressive strength reduction after 48 weeks with similar concentration magnesium sulphate solution.
According to Ismail et al. [28], the cation accompanying the sulphate anions plays a key role in controlling the deterioration mechanism for geopolymers.It appears sodium sulphate favors the structural maturity of the binding phases, while magnesium, as the cation accompanying the sulphate anions, promotes decalcification and destruction of the main binding phase.
The data demonstrates negligible changes in the mass with exposure to sulphates, an increase of less than 0.24% for sodium sulphate and 0.25% for magnesium sulphate over the 12-months.Elyamany et al. [12] reported a weight loss in FA geopolymer mortars exposed to 10% magnesium sulphate solution at 48 weeks was in the range of 1.13-1.49%.Meanwhile, Bakharev [10] stated that FA geopolymer of specimens gained mass of between 3.1% and 4.7% over 5 months exposure to 5% sodium sulphate solution, and 1.4% to 5.3% of weight gain when exposed to 5% magnesium sulphate solution.
X-ray and SEM investigation identified no new crystalline phases, such as ettringite or gypsum, in the specimens exposed to sodium and magnesium sulphates.The absence of the expansion products ettringite and gypsum could account for the relatively minor increase in mass observed.This finding is consistent with the previous studies [25,29], but contrary to Bakharev [10], where his study observed the formation of ettringite in the 5-months exposed specimens to sulphate solution using a sodium silicate activator.Elyamany et al. [12] reported the formations of gypsum crystals in fly ash geopolymer mortars after 48 weeks in 10% magnesium sulphate exposure.Sata et al. [30] suggested the formation of ettringite and gypsum may depend on the calcium content from precursors and aggregates, where a higher calcium content led to the formation of expansion products when attacked by sulphates.

Acid resistance
A reduction in compressive strength in 1% and 3% sulphuric acid of approx.1.2% and 4.5% is noted in comparison to the control specimen after 1-year.Valencia-Saavedra et al. [31] reported a compressive strength loss of almost 68% after 1-year submerged in sulphuric acid with pH 0, while Albitar et al. [5] noted a 12% loss over 9 months in 3% sulphuric acid.According to Mehta & Siddique, 2017 [1], the decrease in mechanical performance is influenced by the stability of the N-A-S-H network when under sulfuric acid attack.Song [14] explained the deterioration mechanism of sulphuric acid was initiated by the penetration of hydrogen and sulphate ions from the acid solution into the concrete matrix.This is verified in this study by the low pH profile and EDS spectra which identify the S ions on the surface of the specimen.Proton exchange occurs and depletes cations such as Na ions from the geopolymer matrix, as supported by EDS analysis, where the surface of the specimens is free from Na ions.Acid exposure also results in the partial removal aluminium ions in the geopolymer gel which results in the destabilization of Si-O-Al bonds and formation of Si-OH and Al-OH groups of bonds, such that a silicon-rich amorphous layers remain.The major increase in the Si/Al ratio for both sulphuric acid specimens supports this proposed mechanism indicating the dealumination of the gel network.Meanwhile, more than 3% mass reduction is measured over 12-months of exposure in 3% sulphuric acid, while approx.2% mass loss is observed in 1% sulphuric acid.The sulphuric acid specimens retain their shape structurally, with minor delamination on the surface and at the edges of the specimens.This may be due to a concentration of sulphate ions combined with different cations in voids near the surface [14], such that specimens did not experience severe visual deterioration as is commonly seen in OPC concrete specimens attacked by sulfuric acid.The results are consistent with previous experiments, which reported mass losses in the range from 0.51% to 5% and reductions in compressive strength between 30 and 66% in fly ash geopolymer concrete [14,31] On the other hand, specimens in the peat solution generally show good performance, with a 1.25% increase in strength noted at the end of 12-months compared to control samples and a 0.26% loss in mass.No noticeable deterioration is observed on the surface of specimens.The surface of the peat specimen is darker than the control specimen, attributed to the dark colour of humic acid.Unlike in sulphuric acid specimens, EDS spectra indicate specimens free from S ions migration, and Na ions still exist in the geopolymer matrix.Nonetheless, compared to control specimens, Na ions concentration in peat specimens is lower, corresponding to the higher value of Si/Na and lower value of Na/Al ratio, indicating the possibility of proton exchange noted in the sulphuric acid specimens, though this is significantly less than that observed in the acid specimens.

Conclusions
Based on the laboratory analysis of the deterioration of FAGC subjected to sulphate and acid attack, the main findings could be concluded as follows: 1. FAGC demonstrates very different resistance to sulphate attacks.In sodium sulphate solution, compressive strength develops a higher value (9%), while in magnesium sulphate media, FAGC experiences strength loss (11%) when compared to control specimens.However, both types of media do not contribute to a mass change (0.25% increase), supported by the absence of expansion product in the FAGC specimens.2. Based on the microstructural analysis, migration of ions in the matrix of FAGC when exposed to sulphate solution demonstrate that sodium cations in sodium solution improve the structural maturity of the binding phase, while magnesium cations in magnesium sulphate solution decay the binding phase.3. FAGC decreases in compressive strength in 1% and 3% sulphuric acid solutions by 1.2% and 4.5%, respectively, with mass loss of up to 3%.Meanwhile, FAGC in peat media demonstrates good performance, with a 1.25% increase in strength and a 0.26% loss in mass.4. Based on microstructural data, when exposed to the sulphuric acid solution, FACG indicates a proton exchange mechanism, leading to the instability of the N-A-S-H network, while the peat media appears to have little or no effect on the structure of the matrix.

Fig. 3 .
Fig. 3. Compressive strength FAGC specimens exposed to various sulphate and acid solutions at 12-months Fig.3depicts the compressive strength of the control and geopolymer concrete specimens subjected to sulphate and acid solutions at 12-months.The notation for the data is control specimens in water (W), 5% sodium sulphate (Na), 5% magnesium sulphate (Mg), 1% sulfuric acid (1SA), 3% sulfuric acid (3SA), peat acid (P).The compressive strength of the control concrete at 28-days age is 46.54 MPa, corresponding to an increase of more than 22% over the 1-year duration in the control specimen.In sulphate solution exposure, an 8.9% strength increase is measured in the sodium sulphate solution, but approx.11% strength decline in magnesium sulphate.In the case of immersion in acid solutions, FAGC exhibits a decline compared to control specimens, corresponding to approx.1.2% and 4.5% for 1SA and 3SA, respectively, at

Fig. 4 .
Fig. 4. Measured mass in change for the 12-months duration

Fig. 7 .
Fig. 7. SEM secondary electron images at 12 month This project is funded by ARC-ITRH (Australian Research Council-Industrial Transformation Research Hub) research grant (IH200100010) allocated for Transformation of Reclaimed Waste Resources to Engineered Materials and Solutions for a Circular Economy (TREMS).We acknowledge PT.Lestari Anugrah Tritunggal, Probolinggo, Indonesia for providing fly ash from PT. PJB Paiton.RMIT University provided the microscopy, Xray facility and microanalysis facility for this study also acknowledged.

Table 1 .
Chemical composition of raw fly ash

Tabel 2 .
Measured pH of concretes in different depthThe pH values are measured in intervals of 3 mm from the top surface of the 100-mm cube concrete specimens.For concrete in water, the pH is 11.72.The pH of the first 3 mm from the top of specimens in sodium and magnesium sulphate solutions decreased to 10.92 and 10.29, respectively.A significantly lower pH is detected in specimens exposed to sulphuric acid.The surface pH of specimens in 3% sulphuric acid is less than 5, but increases to 10.24 at 12 mm depth, while in 1% sulphuric acid, the pH is 5.5 at the surface and 11.17 at 12 mm.In peat solution, the pH changes are similar to those immersed in sulphate, where pH in the initial 3 mm is approx.10.50 and increases to pH 11.46 at a depth of 12 mm.

Table 4 .
Atomic elements and atomic ratios data from EDS analysis after 12-months exposure