INTRODUCTION:

Ordinary Portland cement is by far the most commonly used binder in construction practices. For a long time Ordinary Portland cement concrete was considered to be very durable material requiring a little or no maintenance. Unfortunately, its resistance to chemical attacks such as acids and sulphates is of concern. In the case of acid attack on ordinary Portland cement (OPC) concrete, calcium salts of the attacking acid rapidly form and the concrete loses its strength and deteriorates quickly. Acid attack has not traditionally attracted much attention, even when cement composites are severely damaged by acids wherein calcium hydroxide is dissolved and the hydrated silicate and aluminium phases are decomposed. Geopolymers are a class of binders manufactured by activation of solid aluminosilicate source material with a highly alkaline activating solution and aided by thermal curing. In the past few decades, Geopolymer binders have emerged as one of the possible alternative to OPC binders due to their reported high early strength and resistance against acid and sulphate attack apart from its environmental friendliness. Fly ash based Geopolymers are one branch in the Geopolymer family and these have attracted more attention since the 1990s. As a novel binder, the performance of fly ash based Geopolymers is promising, especially in some aggressive situations where Portland cement concretes are vulnerable [5]. Geopolymer binders might be a suitable alternative in the development of acid resistant concrete [8]. Since Geopolymers are a novel binder that relies on alumina-silicate rather than calcium silicate hydrate bonds for structural integrity, they have been reported as being acid resistant. Davidovits [1] found that metakaoline based Geopolymer has very low mass loss when samples were immersed in 5% sulphuric acid solutions for 4 weeks. Bakharev [2] studied the resistance of geopolymer materials prepared from fly ash against 5% sulphuric acid up to 5 months exposure and concluded that geopolymer materials have better resistance than ordinary cement counterparts. Song X.J, Marosszeky M, Brungs M and Munn R [5] conducted an accelerated test to assess the durability of geopolymer concrete in a 10% sulphuric acid solution for 56 days and reported its good durability. S. E. Wallah and B. V. Rangan[6] have shown that geopolymer composites possesses excellent durability properties in a study conducted to evaluate the long term properties of fly ash based geopolymers. Caijun and Stegemann [11] found the formation of a protective layer around specimen which acted as a barrier for further corrosion. Allahverdi and Skavara [3],[4],[9],[10] studied the mechanism of corrosion of Geopolymer cements in both Sulphuric acid and Nitric acid. The absence of standard methods to evaluate the performance of cements in acid environments has led to research in different exposure conditions and procedures by various authors making it difficult to correlate the results. The present study is aimed at evaluating the response of Fly ash based Geopolymer mortars to Sulphuric and Nitric acid using physico-mechanical indicators as degree of deterioration. The study comprised determination of changes in weight, alkalinity and compressive strength as a measure of its resistance against acid. The findings of the present study shall be useful in determining durability and hence the applicability of Geopolymer materials for use in acid environments.

MATERIALS:

1) Fly ash:

Low calcium, class F dry fly ash obtained from the dry packed ready to use as mineral admixture to concrete is used as the base material. Fly ash has been obtained from National Thermal Power Corporation, Farakka. Chemical composition of fly ash is given in Table1.

Table 1: Chemical composition of fly ash

Serial no.	Element	Amount (%)
1	SiO₂	62.36
2	Al₂O₃	28.82
3	Fe₂O₃	3.86
4	TiO₂	1.64
5	CaO	0.83
6	MgO	0.50
7	Na₂O	0.27
8	K₂O	1.08
9	LOI	0.32
10	Mn₂O₃	0.05
11	SO₃	0.14

2) Fine aggregate:

Zone 3 sand is used in this experiment obtained from local market. Particles passing through 1.18mm sieve were used as fine aggregate.

3) Sodium Silicate:

Sodium Silicate solution having 45% solid content of specific gravity 1.53g/cc is obtained from local market having the composition Na₂O = 14.7%, SiO₂ = 29.4%, Water = 55.9%. The solution is grey in colour and highly viscous in nature.

4) Sodium Hydroxide:

Sodium Hydroxide in pallet form of 97% purity is used, obtained from local market of RANKEM company.

Experiment:

Fly ash and fine aggregates were sent for oven drying in 100⁰C for 24 hours before casting. Total 3 types of specimens have been finalized for further study. At first fly ash and fine aggregates were mixed thoroughly until the mix seemed uniform. Then required amount of alkali activator fluid was introduced and mixing was done for 3 minutes. Alkali activator fluid contained specified concentration of sodium silicate and sodium hydroxide. The solutions were prepared at least one day before mixing. The mixing of sodium hydroxide solution and sodium silicate solution was done just before mixing. After mixing the fly ash and aggregates with the alkaline fluid the paste was poured into 50mm*50mm*50mm moulds and vibrated for 1 minute so that air bubbles appear at the surfaces. The samples were then kept in oven at 60⁰ C for 48 hours. For each type of sample, 3 specimens were tested at a time and average value was taken from the test results. Details of mix proportions are shown in Table 2.

· M in Table 2 stands for molarity

Table 2: Mix proportion of fly ash

Mix designation	fly ash(g)	fly ash to fine aggregate ratio	Strength of alkali activator solution	Alkali activator fluid to fly ash ratio	curing temperature (⁰C)	Curing time (hours)
GPC1	1000	1:1	2M Na₂SiO₃+2M NaOH	0.45	60⁰	48
GPC2	1000	1:1	2M Na₂SiO₃+2M NaOH	0.5	60⁰	48
GPC3	1000	1:1	2M Na₂SiO₃+2M NaOH	0.6	60⁰	48

To study the effects of exposure to acidic environment, specimens were immersed in 10 % solution of Sulphuric acid and 10% Nitric acid after 28 days for a period of 24 weeks, tests being carried out at regular intervals. The volume of acid solution was taken as four times the volume of specimens immersed and stirred every week. The solution was refreshed after 12 weeks. The effects of acid on the specimen were constantly monitored through visual inspection, weight change measurements and strength tests during the exposure period. Residual alkalinity was approximately determined after cutting the specimens into halves using a low speed saw and spraying a 1% Phenolphthalein solution on the freshly cut surface. Weights were measured using a Digital balance in saturated surface dry condition. Samples for weight change measurements were initially primed in water for 3 days and its weight in saturated surface dry condition was taken as initial weight. A Digital compression testing machine was employed to determine the compressive strength of the specimen at regular intervals

RESULTS AND DISCUSSIONS:

Residual Alkalinity:

specimens were examined roughly by spraying a 1% Phenolphthalein solution on the freshly cut surface. On spraying, dealkalized part of specimen showed colourless while remaining part exhibited a magenta colour indicating its residual alkalinity. Figure. 1,2,3 and Figure.4,5,6 shows the residual alkalinity of specimen in sulphuric acid and nitric acid solution respectively. It was noticed that the process of dealkalization progressed inwards with time. Alkalinity were seen to have almost lost in about 18 weeks for GPC1 for Nitric acid and Sulphuric acid specimen while GPC2 and GPC3 specimens still had some portions to be dealkalized . Specimen with lower amount of alkaline fluid had a faster rate of dealkalization than those containing higher amount of alkaline fluid. This might be connected to a better and lesser permeable microstructure developed in specimen containing higher alkali. For the same exposure duration, specimens in Nitric acid solution showed faster dealkalization than its counterparts in Sulphuric acid.

Fig 1,2,3 Residual alkalinity of specimen after 18 weeks in Sulphuric acid

Figure 1: GPC3 Figure 2: GPC2 Figure 3: GPC1

Fig 4,5,6 Residual alkalinity of specimen after 18 weeks in Nitric acid

Figure 4: GPC3 Figure 5: GPC2 Figure 6: GPC1

Weight change:

Results of the weight changes for the Geopolymer mortars are presented in Figure 7 and Figure8. In the specimens immersed in 10% Sulphuric acid, a sudden loss of weight was noticed initially during 2 to 4 weeks. Beyond 4 weeks the weight increased in the specimens and from 12 weeks till the end of experiment. The increase in weight might be due to deposits within the surface pores. Rendell and Jauberthie[12] observed such deposits on cement mortar specimens in sulphate environments which were confirmed as gypsum.GPC3 specimens with highest percent of alkali had the maximum loss of 1.46% and GPC1 specimens exhibited only 0.72% loss after 24 weeks. As the alkali content increased in the samples, weight loss also increased correspondingly in Sulphuric and Nitric acid. Specimens in 10% Nitric acid showed sudden fluctuation in weight changes during the test duration from 2 to 8 weeks. Weight loss at the end of 24 weeks was found 0.61% for GPC1 specimens and 1.76% for GPC3 specimens. Table 3 shows the weight changes of specimens after 24 weeks.

Table 3: Weight change of specimens

Specimen designation	Weight change after 24 weeks exposure in Sulphuric acid (%)	Weight change after 24 weeks exposure in Nitric acid (%)
GPC1	-0.72	-0.61
GPC2	-1.04	-0.78
GPC3	-1.46	-1.76

pH of solution:

The variation in pH of the solutions containing the three different series of geopolymer mortar specimens GPC1, GPC2 and GPC3 is shown in Fig. 9 & 10. The initial value of pH for 10% Sulphuric acid solution prior to immersion of specimens was 1.07. After 10 weeks exposure pH increased considerably to about 5.3 in the solution containing GPC3 specimen. The increase in pH was rapid during first 8 weeks and thereafter it was not appreciable. Though pH of all the solutions containing specimens with varying alkali content showed an increase, maximum increase occurred in solution with GPC3 specimen which had maximum alkali. Specimens of GPC1 and GPC2 which were manufactured with lesser alkali content recorded lower pH values at the end of 24 weeks in sulphuric acid solution. The increase in pH may be attributed to migration of alkalis from specimen into the solution as reported by Bakharev [13]. Rate of migration of alkali appears to be higher within the initial weeks as indicated by the rapid rise in pH value at 8 weeks. Continuous exposure beyond 8 weeks did not result in notable increase which suggests that further migration of alkali from the specimen has diminished or rather stopped. Same trend can be noticed in case of specimens exposed to 10% Nitric acid solution. Initial pH of the solution was 1.11 which grew near to 4 within just 2-3 weeks for GPC3 specimen. Thus rate of increase of pH is more in initial stage for Nitric acid solution. Table 4 shows the pH of different solutions after 24 weeks of exposure.

Figure 9: pH change for Nitric acid Figure 10: pH change for Sulphuric acid

Table 4: Change of pH of acid solutions

Specimen designation	pH of Nitric acid solution after 24 weeks of exposure	pH of Sulphuric acid solution after 24 weeks of exposure
GPC 1	5.44	5.05
GPC 2	5.53	5.18
GPC 3	5.99	5.24

Residual compressive strength:

Figure.11 and Figure. 12 shows the compressive strength evolution of Geopolymer mortars in Sulphuric acid and Nitric acid environment. At regular intervals, the compressive strength was determined using a digital compression testing machine and the residual compressive strength was calculated as percentage of initial compressive strength. Geopolymer mortar specimens of lesser alkali content showed lesser loss in strength initially. Specimens with greatest percentage of alkali exhibited large initial strength loss. Minimum residual compressive strength was observed in specimens of GPC3 ( 46.1%) in sulphuric acid and maximum in GPC1(66.3%) in nitric acid. Exposure to Nitric acid resulted comparatively a little lesser strength loss than those in Sulphuric acid. Even after being fully dealkalized by acids, Geopolymer mortar specimens still possessed substantial residual compressive strength.

Table 5: Residual compressive strength after 24 weeks of exposure in acid solution

Specimen designation	Residual compressive strength (Nitric acid) (MPa)	Residual compressive strength (Sulphuric acid) (MPa)
GPC1	66.3	57.2
GPC2	58.3	52
GPC3	48.2	46.1

CONCLUSIONS:

1. Loss of alkalinity depended on alkali content in the Geopolymer samples. Specimen with lesser NaOH lost its alkalinity faster than those with higher NaOH content in both Sulfuric acid and Nitric acid solutions. However, rate of dealkalization seemed faster in Nitric acid.

2. Exposure to the solutions yielded very low weight losses in the range of 0.39 to 1.76 across the two solutions. In both Nitric & Sulphuric acid, specimens with higher alkali content showed greater weight loss

3. Though specimens were almost fully dealkalized, it still had substantial residual compressive strength confirming its high resistance.

4. Residual compressive strength was highest for specimens containing lowest percentage of alkali as the amount of water content was lowest and lowest for specimens containing highest percentage of alkali as water content was highest for both Nitric and Sulphuric acid solution.

REFERENCE:

[1] Davidovits J, “Properties of Geopolymer cements”, Proceedings of the First International conference on Alkaline Cements and Concretes,vol. 1, SRIBM, Kiev, Ukraine, 1994, pp. -131 – 149.

[2] Bakharev T, “Resistance of geopolymer materials to acid attack”, Cement and Concrete Research.35 (2005) pp.658 – 670

[3] Allahverdi Ali, Skavara, Frantisek, “Sulfuric acid attack on Hardened paste of Geopolymer cements, Part 1. Mechanism of Corrosion at relatively high Concentrations”, Ceramics - Silikáty 49 (4) 225-229(2005)

[4] Allahverdi Ali, Skavara, Frantisek, “Sulfuric acid attack on Hardened paste of Geopolymer cements, Part 2. Corrosion Mechanism at mild and relatively low concentrations”, Ceramics– Silikaty 50 (1) pp.1 – 4 (2006).

[5] Song X.J, Marosszeky M, Brungs M, Munn R, “Durability of fly ash based Geopolymer concrete against sulphuric acid attack”, 10 DBMC International Conferences on Durability of Building Materials and Components, Lyon, France, 17- 20 April 2005

[6] Wallah S.E, Rangan B.V, “Low Calcium Fly ash based Geopolymer Concrete: Long term properties, Research report GC 2”, Curtin University of Technology, Australia (2006)

[7] Thakur Ravindra N, Ghosh Somnath, “Fly ash based Geopolymer composites”, Proceedings of 10th NCB international seminar on Cement and building materials, NEW Delhi, India Nov, 2007,Vol.3, pp.442- 451

[8] Palomo,A . Grutzeck, M.W, Blanco, M.T., “Alkali activated fly Ashes a cement for the future”, Cement and Concrete Research. 29 (1999) pp.1323– 1329.

[9] Allahverdi Ali, Skavara, Frantisek , “Nitric acid attack on hardened Paste of Geopolymeric cements Part 1”. Ceramics - Silikáty 45 (3) 81-88 (2001)

[10] Allahverdi Ali, Skavara, Frantisek, “Nitric acid attack on Hardened paste of Geopolymeric cements, Part 2”, Ceramics – Silikaty 45 (4) 143-149 (2001)

[11] Shia Caijun, Stegemann J.A, “Acid corrosion resistance of different cementing materials”, Cement and Concrete Research 30 (2000),pp. 803-808

[12] F. Rendell and R. Jauberthie, The deterioration of mortar in sulphate environments, Cement and Concrete Research 13 (1999) 321-327. gain in weight; the maximum gain being noticed in the specimen with minimum Na2O content.

[13] T. Bakharev, Durability of geopolymer materials in sodium and magnesium sulphate solutions, Cement and Concrete Research 35 (2005) 1233-1246.

Received on 06.11.2015 Accepted on 26.12.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 164-168

DOI: 10.5958/2231-3915.2015.00014.0