Deterioration Of Ordinary Portland Cement Biology Essay

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Corrosion in concrete sewers and wastewater treatment plants has been a major problem but this issue has not been resolved satisfactorily yet. Generally, deterioration is happening due to sulfuric acid reaction with treatment units and sewer materials. Geo-polymer binders especially fly ash (FA) is an acid resistant and can be used as a substitute binder for sewer construction. This research work highlights the laboratory results of fly ash based geo-polymer concrete and effects on its durability under sulfuric acid exposure. Class 'F' fly ash was used in the preparation of samples that were properly cured for 28 days at room temperature. After curing, specimens were immersed in three types of sulfate containing solutions. These sulfate containing solutions include static sulfuric acid, dynamic wastewater and static wastewater. Samples were tested at 28, 45 and 60 days after immersing in different type of solutions. By visual inspection the corrosion depth and residual compressive strength was observed according to the modified ASTM C267. Reaction products of gypsum remained on the surface of concrete samples absorbed in diluted sulfuric acid, while reaction products of gypsum were not seen on the surfaces of concrete samples absorbed in static as well as dynamic wastewater. Static wastewater also produced corrosion but in a limited fashion, it causes only surface weathering. The obtained results are strongly confirming that geo-polymer concrete samples are showing great resistant to sulfuric acid solution. Moreover, geo-polymer samples were also showing reasonable load carrying capacity after entire section had been neutralized by sulfuric acid.

Keywords: Geo-polymer, Concrete durability, Fly ash, Acid resistance, Wastewater concrete structures, Sewers

INTRODUCTION

The deterioration of wastewater conveyance and treatment infrastructures has long been a cause for concern but the issues remained unknown for many years. Wastewater treatment systems are traditionally designed to resist high levels of sulfate attack but subjected to a considerably more aggressive form of deterioration under sulfuric acid corrosion1. Gypsum and ettringite both are the causes of concrete deterioration. Sulfates in wastewater are converted to hydrogen sulfide (H2S) gas which is then converted to sulfuric acid. Sulfuric acid reacts with calcium hydrates to form gypsum which on further reaction with calcium aluminates produces ettringite2-3. Sulfuric acid is formed by more dissolution of hydrogen sulfide gas, thus more gypsum will be produced and hence more deterioration will occur, therefore, flowing wastewater attacks much severely than the static one3.

Fly ash can be used in order to reduce sulphate attack under different circumstances4. Several research works have been conducted on concrete deterioration due to sulphate attack. Most of research works are also presenting different cement replacement materials in different ratios and prone to different concentrations of acids. Acid resistance tests were also conducted but no universally accepted methods or specifications for such tests are available. Hence it is very difficult to draw conclusions. There is a need to make simulative studies of concrete having fly ash and identification of different parameters to determine resistance of concrete to sulphate attack.

LITERATURE REVIEW

Sulfates and acids are found in different forms in nature, e.g., as humic acid in organic form or these can be found in industrial wastes. Liquids having pH less than 6.5 can attack concrete. Concrete particles are held together by alkaline compounds and it cannot resist the strong acid attack. Hence the ultimate result of sustained attack is disintegration and destruction of the concrete. The mechanism of attack involves the reaction between cement hydrates and acid resulting in the formation of calcium salts of that acid. The dissolution of these salts tends to further exposure of cement hydrates to attack. Damaging rate is controlled by solubility of calcium salts. Therefore flowing wastewater yields more deterioration rather than the static one. Acid rain may cause the surface weathering of concrete as it mainly consists of sulphuric acid and nitric acid. Reduced content of calcium hydroxide incorporating fly ash has been found to be much beneficial in reducing sulphate attack.

High values of Biological Oxygen Demand (BOD), high concentration of sulfate and sulfide, extreme temperatures, lofty H2S gas flumes, and low wastewater pH are the factors that promote sulphate attack. These contributory factors are seriously affecting the concrete structures of wastewater treatment plants (WWTPs). Concrete deprivation has been recorded in a limited fashion in aeration tanks during wastewater treatment, in septic tanks and in primary influent channels5,6,7. The corrosion has been recorded just over the effluent line at WWTPs. It is considerable in the previous investigational studies to understand, the deprivation of sewer lines has also shown the similar trend like WWTPs8. A research work was carried out to determine the depth affect on concrete structures under sulfate attack by collecting different core samples from sewage treatment plant at different components9.

Sulfuric acid is seriously acting as a corrosive agent for sewer lines and as well as WWTPs10. Reaction by the sulfuric acid is presenting a joint sulfate-acid reaction and hydrogen ion concentrations are generating a dissolution effect3. In the first step when the sulfuric acid reacts with a cementations material, a reaction between acid and calcium hydroxide forming calcium sulfate as shown in the following equation 1:

Ca (OH) 2 + H2SO4 → CaSO4 + 2H2O (1)

It is subsequently hydrated to form gypsum (CaSO4·2H2O) and then its presence on the surface of RCC/PCC pipes presents a white soggy powder without any cohesive properties2. During the same reaction, produced gypsum would be able to react with calcium aluminates hydrate (C3A) to form an expansive product like ettringite as shown in the equation 2:

3CaSO4·2H2O+3CaO·Al2O3+26H2O → (CaO)3·(Al2O3)·(CaSO4)3·32 H2O (2)

(Ettringite)

Skalny et al. (2002) stated that ettringite can be observed in lower sections of concrete structures under high pH. Davis et al. (1988), observed that a minor ettringite was deposited on damaged concrete pipes. Findings from the previous researches are clearly indicating that there is a diverse relationship between concrete corrosion and nature of wastewater. Common variables include environmental conditions, the nature of attack and the physical results of the attack on concrete. Mehta and Burrows (2001) explained that how a model-shift is required for a proper concrete design, converting from a traditional approach to performance-based design. Therefore it is important to consider the aggressiveness of wastewater environment to concrete structures during their service11.

Parker (1945) discovered that concrete sewer under sulfuric acid attack are producing white deposits, those are moist, flaky and removable from the concrete surface12. At early stages of concrete corrosion sulfur ions are being released by the special type of bacteria. Further, in wastewater the presence of dissolve oxygen expedite whole process to generate hydrogen sulfide13. Generally, the pH value of normal sewage is 5-6 and presenting weak acidic behavior but under low values of pH, the produced amount H2S can be recovered from the top surface of wastewater. A slim film of dampness survives on the face of the concrete pipe exposed to the atmosphere and the hydrogen sulfide gas gets dissolved. Hydrogen sulfide (H2S) is divided into HS- or S2- ions which further attract more H2S into the damp deposit14. Research illustrates that the amount of the hydrogen sulfide (H2S) under the damp layer enhances as the pH value inside the concrete pipe starts decreasing15. When oxygen is present, the H2S responds to form essential sulfur or moderately oxidized sulfur variety which can be seen in the deterioration products placed on the concrete face8&16. The acid behavior of the wastewater must be controlled to overcome the concrete corrosion problems. Further, the biological attack on wastewater concrete structures has also showing significant impacts on concrete deterioration. Removal of loosely adhering particles may be important to minimize biological activity17.

Materials and Methods

Wastewater corrosion in PCC and RCC sewer has become a major issue. It creates problems in wastewater treatment plants as well. Therefore this research was conducted to describe the comparison of corrosion depth and compressive strength using different water-binder (W/B) ratio with different binder materials and fly ash was selected as mineral admixture. Different exposure mediums were selected which are the most common in Pakistan. 1.0 mol/liter static sulfuric acid, the static wastewater and dynamic wastewater were selected as exposure mediums. Concrete cubes of 150Ã-150Ã-150 mm in size were caste with water-binder ratios of 0.5, 0.57, and 0.65. These b/w ratios were selected while considering the economic aspects in developing countries like Pakistan. The cubes specimens of size 150Ã-150Ã-150 mm were used because they comply with ASTM standards and 30% cement was replaced with FA.

Medium sized graded sand collected from local quarry at Lawerencepur was utilized. The coarse aggregates collected from another local mine from Margallah near Taxila having maximum size of 20 mm and minimum size of 10 mm was used. The physical properties of consumed materials are shown in Table 1.

Table 1: Physical Properties of Materials

Materials

Properties

Cement

Ordinary Portland Cement

Density:3.04 g/cm3

Fly Ash

Density:2.10 g/cm3

Fine Aggregates

Lawrencepur Sand Pit

Specific Gravity:2.27 g/cm3

Fineness Modulus: 2.72

Coarse Aggregates

Margalla Crushed Stone

Specific Gravity:2.69 g/cm3

Fineness Modulus: 5.91

Experimental Methods

The concentrations of sulfuric acid solutions in immersion tests were 1.0 mol/L for concrete specimen. The immersion tests contain three types of solutions. Specimens were immersed in static sulfuric acid solution, in dynamic wastewater and in static wastewater. The initial depth of any cube was considered the linear dimension of any one side e.g. 150 mm. After immersion tests were started, corrosion depth was measured after 28, 45 and 60 days. The corrosion depth is defined as "the distance between the initial surface and the current surface". Before every measurement intentional removal of deteriorated zone on surface was not carried out in order to prevent specimens from any strength loss due to gypsum and ettringite removal collected at the surface of specimens.

RESULTS AND DISCUSSIONS

(a) Corrosion effects due to sullphuric acid and wastewater

The corrosion depth of concrete specimens immersed in 1.0 mol/L of sulfuric acid solution was more. For specimens with each W/B ratio, the corrosion depth of concrete in dynamic wastewater was greater than in static waste water. Reaction products of gypsum remained on the exterior faces of concrete samples placed under sulfuric acid solution, while nothing was seen on the face of concrete samples engrossed under dynamic as well as static wastewater as shown in following Figs. 1, 2 and 3.

Fig. 1: Sample cubes under sulfuric acid attack

Fig. 2: Sample cubes under static wastewater

Fig. 3: Sample cubes under dynamic wastewater

Kurashige [2002] describes that sulfuric acid penetrates into concrete cubes and reacts with calcium hydroxide of cement hydrates to produce gypsum18. It causes the volume of concrete to increase largely which causes the expansions of reaction products resulting in corrosion. Concrete with the high water cement ratio contain larger and more pore than that with the low water cement ratio. These pores are actually the capacity to absorb expansions caused by the production of gypsum. Hence concrete with the high W/B has a larger capacity to absorb the expansions of gypsums. In other words concrete with low W/B ratio corrode earlier than that with the high W/B. Concrete deterioration occurs only in the surface portion of specimens. It is all because the reaction of gypsum in the surface portion is faster than the penetration of sulphate ions into that specimen. Hence surface portion is a main field of reaction of sulfuric acid. Therefore, specimens immersed in static wastewater eroded larger than those immersed in dynamic wastewater. Since the flow of solution resisted the reaction product of gypsum.

(b) Resistance to Corrosion by the addition of Fly Ash

The corrosion depth of concrete samples having fly ash was smaller than the samples without fly ash. Less calcium hydroxide was observed in concrete samples having fly ash as compare to concrete samples without fly ash. As 30% binding material was substituted with fly ash. The corrosion depth in concrete specimens containing fly ash was the smallest one. Hence the production of gypsum is the main cause of concrete deterioration due to sulfuric acid attack. From the obtained results, it is clear that compressive strength of OPC concrete is least in static sulfuric acid when W/B ratio was kept 0.5; it gradually increases for W/B ratios of 0.57 and 0.65. Same is the case with Geopolymer concrete.

From the summarized results in Table 2, it is clear that strength of OPC and Geopolymer concrete is least in static sulfuric acid solution and was high when medium was static wastewater. The strength of cubes being soaked in dynamic wastewater lies in between of the both above mentioned mediums. The corrosion depths of different specimens are shown in Table 3. These corrosion depths are also explained in Figs. 7, 8 and 9.

Table 2: Comparison of compressive strength of concrete with & without FA using different W/B ratios & medium exposed.

Sr. No.

W/B Ratio

Medium Exposed

Compressive Strength (kN)

OPC

Concrete Cubes

OPC+FA Concrete Cubes

28 Days

45 Days

60 Days

28 Days

45 Days

60 Days

01

0.50

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

352

520

576

425

530

590

475

550

610

490

660

707

710

720

820

730

750

845

02

0.57

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

286

402

520

350

415

528

405

430

560

436

595

614

460

620

790

490

675

815

03

0.65

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

212

240

487

250

280

500

275

330

520

400

538

609

430

560

700

460

582

805

Fig. 4: Comparison of 28 days compressive strength of concrete with & without FA using different W/B ratios & medium exposed.

Fig. 5: Comparison of 45 days compressive strength of concrete with & without FA using different W/B ratios & medium exposed.

Fig. 6: Comparison of 60 days compressive strength of concrete with & without FA using different W/B ratios & medium exposed.

Table 3: Corrosion depths of concrete specimens with & without FA using different W/B ratios & medium exposed.

Sr. No.

W/B Ratio

Medium Exposed

Corrosion Depth (mm)

OPC

Concrete Cubes

OPC+FA Concrete Cubes

28 Days

45 Days

60 Days

28 Days

45 Days

60 Days

01

0.50

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

5

0.3

0.1

7

0.5

0.2

10

0.8

0.4

4

0.1

0.08

5

0.2

0.1

7

0.3

0.2

02

0.57

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

8

0.5

0.3

10

0.7

0.5

13

0.9

0.6

5

0.3

0.2

6

0.4

0.3

8

0.5

0.4

03

0.65

Sullphuric Acid (Static)

Wastewater (Dynamic)

Wastewater (Static)

10

1

0.4

12

1.2

0.6

15

1.5

0.8

6

0.5

0.3

7

0.6

0.4

9

0.8

0.6

Fig. 7: Comparison of corrosion depth of 28 days concrete with & without FA using different W/B ratios & medium exposed.

Fig. 8: Comparison of corrosion depth of 45 days concrete with & without FA using different W/B ratios & medium exposed.

Fig. 9: Comparison of corrosion depth of 60 days concrete with & without FA using different W/B ratios & medium exposed.

CONCLUSIONS

Concrete deterioration increases by increasing the water binder ratio due to more expansion occurring while decreases by increasing the amount of fly ash as cement substitute. Concrete deterioration due to sulfuric acid attack in geo-polymer concrete having fly ash contents is lesser as compared to ordinary mix concrete at 28, 45 and 60 days since the content of calcium hydroxide is small. In case of dynamic wastewater concrete containing fly ash causes less deterioration than ordinary concrete at 28, 45 and 60 days. Same is the case with static wastewater. From another point of view more corrosion and strength loss occurs in sulfuric acid exposed specimens rather than the immersed in dynamic wastewater as well as static wastewater. Another conclusion was also drawn that dynamic wastewater causes more corrosion and strength loss than the static wastewater at all ages e.g. 28, 45 and 60 days.

At early stage, the mixing of high percentages of FA in the concrete samples showed low strength but the observed strength for the same samples were high. During the research, under constant water-binder ratio of 0.5 and replacement of some quality of OPC with FA showed decrease in compressive strength at early stage up to 60 days but after that the compressive strength was significantly improved for the same samples. These conclusions would highlight that ordinary concrete containing fly ash results in the best performance regarding strength and sulfate resistance.

RECOMMENDATIONS

The replacement of cement by fly ash has considerable advantages. It not only increase the strength of concrete, but also reduce sulfate attack in wastewater concrete infrastructures like sewers and wastewater treatment systems. Similarly, it makes concrete more economical as the fly ash is not expensive as compared to cement and it minimizes corrosion. Fly ash also resists sulfate attack much well rather than the ordinary concrete for long durations.

This experimental study investigated that some quantity of OPC can be replaced with a reasonable percentage of fly ash to introduce a durable concrete without compromising the strength. In this study, to investigate the reactions of wastewater with concrete the wastewater was collected from the communal source only. Although, the wastewaters from different sources are presenting different properties, thus the behavior ordinary and geo-polymer concretes may be studied under different types of wastewaters characteristics.

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