INTRODUCTION:

Fly ash based geopolymer concrete has emerged as a new eco-friendly technology in the world of construction. The addition of fly ash reduces the OPC contribution to CO₂ emissions during concrete production. The production of 1 tonne of OPC cement releases 1 tonne of CO₂ gas. Combining OPC cement with slag could produce 248kg/m³ of CO₂ while the geopolymer only produce 78kg/m³[1]. The geopolymer can be manufactured from a reaction of the alkaline solution with the Silica and Alumina in the fly ash to produce a compact cementing material. This material possesses good mechanical properties and durability in aggressive environment [2-4]. Past experiments and researches have showed that the properties of geopolymer concrete are affected by the concentration of alkaline solution, water content ratio of fly ash to alkaline solution [5-7].

The performance of a mortar is usually determined by its strength and durability. The strength and durability are also influenced by the amount, size and type of pores, while durability is affected by the volume, size and continuity of the pores. Water penetrability can be defined as the degree to which a material permits the transport of gases, liquids or ionic species through it. Those properties are linked to the performance of porosity. Since the harmful ions penetrate into the mortar through the pores. Water permeability measurement is a method to determine the water penetrability of geopolymer mortar. The Ultrasonic Pulse Velocity is a Non Destructive Test method to determine the homogeneity, presence of cracks, voids, quality grade and strength of concrete. The aim of this study is to draw a relation of water permeability with the homogeneity, density and uniformity of geopolymer mortar.

DIN:1048, PART 5-1991 [9]:

A cylindrical test specimen 150mm diameter and 160mm high shall be prepared. After 28 days of curing the test will be conducted between 28 and 35 days. The test specimen shall be fitted in a machine such that specimen subjected to a water up to 7 bars. The specimen shall be subjected to a water pressure of 0.5MPa from the top for a period of 3 days. The pressure shall be maintained constant throughout the test period. If the water penetrates through the underside of the specimen, the test may be terminated and the specimen rejected as failed. After 3 days the pressure shall be released and the sample shall be taken out. The specimen shall be split in the middle by compression applied on two round bars on opposite sides above and below. When the split faces show signs of drying (after 5-10 minutes), the maximum depth of penetration in the direction of height shall be measured with the scale and extent of water penetration established. The mean maximum depth of penetration obtained from three specimens thus tested, shall be taken as the test result and it shall not exceed 25mm.

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 color and highly viscous in nature.

4) Sodium Hydroxide:

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

Experiment:

Fly ash and fine aggregates were sent for oven drying in 100⁰C for 24 hours before casting. Total 12 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 150mm*150mm*150mm moulds and vibrated for 1 minute so that air bubbles appear at the surfaces. The samples were then left at room temperature until the tests were done on them. 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(days)
GPC1	1000	1:1	2M Na₂SiO₃+2M NaOH	0.6	Room temperature	28
GPC2	1000	1:1	2M Na₂SiO₃+2M NaOH	0.5	Room temperature	28
GPC3	1000	1:1	2M Na₂SiO₃+2M NaOH	0.45	Room temperature	28
GPC4	1000	1.1	2M Na₂SiO₃+4M NaOH	0.6	Room temperature	28
GPC5	1000	1.1	2M Na₂SiO₃+4M MaOH	0.5	Room temperature	28
GPC6	1000	1.1	2M Na₂SiO₃+4M NaOH	0.45	Room temperature	28
GPC7	1000	1.1	2M Na₂SiO₃+6M NaOH	0.6	Room temperature	28
GPC8	1000	1.1	2M Na₂SiO₃+6M NaOH	0.5	Room temperature	28
GPC9	1000	1.1	2M Na₂SiO₃+6M NaOH	0.45	Room temperature	28
GPC10	1000	1.1	2M Na₂SiO₃+8M NaOH	0.6	Room temperature	28
GPC11	1000	1.1	2M Na₂SiO₃+8M NaOH	0.5	Room temperature	28
GPC12	1000	1.1	2M Na₂SiO₃+8M NaOH	0.45	Room temperature	28

RESULTS AND DISCUSSIONS:

Table 3 shows the UPV and Water Permeability Test results

Table 3: UPV and permeability test results

Mix designation	GPC1	GPC2	GPC3	GPC4	GPC5	GPC6	GPC7	GPC8	GPC9	GPC10	GPC11	GPC12
UPV test results (Km/s)	3.53	3.79	3.92	4.14	4.17	4.21	4.24	4.31	4.35	4.38	4.42	4.46
Permeability test results (mm)	12.4	12.1	11.8	11.4	11.1	10.6	10.2	9.5	9.3	8.8	8.4	7.9

The quality of concrete in terms of uniformity, incidence of internal flaws, cracks and segregations, etc, indicative of the level of workmanship employed, can thus be assessed using the guidelines given in table 4 as per IS-13311 part 1, 1992

Table 4

Ultrasonic pulse velocity (Km/s)	Concrete quality grading
>4.5	Excellent
3.5-4.5	Good
3.0-3.5	Medium
<3.0	Doubtful

It can be easily observed that both the UPV and Permeability test show best results for GPC12 specimen and worst for GM1 specimen comparing with others. Thus decrease in alkali activator fluid to fly ash ratio results higher UPV values and lower water permeability values. UPV results depend on a number of parameters, thus the results of UPV gives the outline of grade of concrete in general [8]. If a specimen shows good UPV results, it cannot be firmly said that the specimen is good in homogeneity, uniformity, density and will show good compressive strength. In general pulse velocity increases with increase in moisture content [8] and the influence is more for low strength concrete. Here is the need of DIN permeability test results. Higher permeability means more cracks and voids in the specimens. A lower permeability and higher UPV value means the concrete is good in quality and the UPV test result is not affected much by moisture content. If the moisture content was more than the permeability value would have been more. Here all the specimens have shown lower permeability (<25mm) and GM12 specimen has shown an excellent result in permeability getting penetrated only by 7.9mm. This gives support to the UPV test results which indicate all the specimens are good in homogeneity, density and uniformity for showing UPV value greater than 3.5km/s.

Figure 1: SEM image of GPC2 Figure 2: SEM image of GPC 5

Figure 3: SEM image of GPC 8 Figure 4: SEM image of GPC 11

Figure 1 to 4 shows the scanning electron microscopy images of the above mentioned specimens. It is observed that permeability of geopolymer paste increases with increase in concentration of NaOH and decrease in alkali activator fluid to fly ash ratio (when fluid to fly ash ratio is constant). In general it may be concluded that an important factor of UPV result and permeability result is the porosity and water content of specimens. In figure 1 a large amount of unreached fly ash can be observed because of insufficient amount of alkali and thus the matrix formation is incomplete resulting greater porosity. A gradual increase in completeness in matrix formation can be observed from figure 1 to figure 4 and figure 4 shows the least amount of unreached fly ash as the concentration of alkali was greatest in that specimen compared to other 3. So the permeability or the porosity also depends concentration of alkali. The amount of micro pores and micro cracks are gradually decreasing from figure 1 to figure 2 as the amount of water decreases.

CONCLUSIONS:

1) Water is an important factor in UPV and permeability test results of geopolymer paste

2) The permeability of geopolymer paste increases with increase in alkali activator fluid to fly ash ratio

3) The permeability of geopolymer paste decreases with increase in concentration of alkali as less unreached fly ash, pores are there in the specimens.

4) Increase in alkaline activator fluid to fly ash ratio results in decrease in Ultrasonic Pulse Velocity which indicates, higher the amount of water, greater the porosity.

5) Increase in alkaline fluid to fly ash ratio results in greater UPV results because greater the alkali lesser the unreached fly ash and lesser the discontinuities in the specimens.

6) Geopolymer mortars are less permeable and can show good performance for specified concentration of alkali and fluid to fly ash ratio as permeability and UPV results of all specimens do not exceed the threshold limit given in standard guidelines.

REFERENCES:

1. Wimpenny D. 2009. Low carbon concrete-options for the next generation of infrastructure, In: Proceedings of concrete solutions. Ian Gilbert(Ed.). Concrete Institute of Australia, Sydney, Australia.

2. Davidovits J. 1994, high alkali cements for 21^st century concretes. In: Concrete Technology, Past, Present and Future. V. Mohan Malotra Symposium., Mehta, K.(Ed.). ACI Special Publication

3. Sofi., van Deventer J.S.J., Mendis P.A. and Lukey G.C. 2007, Engineering properties of inorganic polymer concretes (IPCs). Cement and Concrete Research. 37(2):251-257

4. Wallah S.E., Harditjo D., Sumajouw D.M.J. and Rangan B.V. 2003. Sulfate resistance of fly ash based geopolymer concrete. In: Proceedings of Concrete in the 3^rd Millennium. Then 21^st Century Biennial of the Concrete Institute of Australia.

5. Fernandez-Jimenez A., Palomo J.G. and Puertas F. 1999. Alkali activated slag mortars: mechanical strength behavior. Cement and Concrete Research. 29(8):1313-1321.

6. Hardjito D., Wallah S.E., Sumajouw B.M.J. and Rangan B.V. 2004. On the development of fly ash based geopolymer concrete. ACI Materials Journal. 101(6) 467-472

7. Van Jaarsveld J.G.S., van Deventer J.S.J. and Lukey G.C. 2002. The effect of composition and temperature on the properties of fly ash and kaolinite based geopolymers. Chemical Engineering Journal. 89(1-3): 63-73.

8. Indian Standard 13311 (part 1) 1992: Nondestructive testing of concrete-methods of tests.

9. DIN 1048-5 1991: Testing concrete, testing of hardened concrete.

Received on 15.11.2015 Accepted on 16.12.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 177-180

DOI: 10.5958/2231-3915.2015.00017.6