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Potassium Nitrate as an Accelerator in Plain Concrete

Amaziah Walter Otunyo*, Ekiti Godknows

Faculty of Engineering, Department of Civil Engineering, Rivers State University, Nkpolu, Port, Harcourt, Nigeria

Abstract: The effect of potassium nitrate as an accelerator in plain concrete was investigated. Fifteen concrete cubes were cast with 0, 5, 10, 15 and 20 % by weight of potassium nitrate. The compressive strength was measured after 7, 14 and 28 curing days Other parameters including the workability, initial and final setting times were tested. Compressive strength of the plain concrete decreased as the content of the potassium nitrate increased. The workability and setting times decreased as the content of the potassium nitrate increased.

Keywords: concrete, compressive strength, potassium nitrate, setting time and workability

Citation: Amaziah Walter Otunyo, Ekiti Godknows, Potassium Nitrate as an Accelerator in Plain Concrete. American Journal of Civil Engineering and Architecture, 6 (3). 119-122. January 2018. doi:10.12691/ajcea-6-3-4

1. Introduction

Potassium nitrate has a chemical formula of KNO₃. It is an ionic salt of potassium ions K+ and nitrate ions NO³⁻, and is therefore an alkali metal nitrate. It is a natural source of nitrate and has been used as a constituent for several and different purposes, including food preservatives, fertilizers, tree stump removal, rocket propellants and fireworks. Potassium nitrate is a common active ingredient in dentrifices where it exerts an antisensitivity action. It provides increasing protection against painful sensitivity of the teeth to cold, heat, acids, sweets or contact [1].

Potassium nitrate is the oxidizing agent (oxygen-supplying) component of black powder. Prior to the large-scale industrial fixation of nitrogen through the Haber process, the major sources of potassium nitrate were the deposits crystallizing from cave walls or the draining of decomposing organic materials. Dung heaps were a particularly common source. Ammonia from the decomposition of urea and other nitrogenous materials would undergo bacteria oxidation to produce nitrate [2].

Historically, nitrate beds were prepared by mixing manure with either mortar or wood ashes, common earth and organic materials such as straw to give porosity to a compost pile typically 1.5 m high by 2 m wide by 5 m long. The heap was usually under a cover from the rain, kept moist with urine often to accelerate the decomposition and leached with water after approximately one year. The liquid containing various nitrates was then converted with wood ashes to potassium nitrates [3].

Today most potassium nitrates come from the vast deposits of sodium nitrate in the Chilean deserts.

One of the most useful applications of potassium nitrate is in the production of nitric acid, by the addition of concentrated sulfuric acid to an aqueous solution of potassium nitrate, yielding nitric acid and potassium sulphate which are separated through fractional distillation [4].

Potassium nitrate is a salt. Salts in deicing solutions decrease the freezing point of concrete pore solution, leading to significant hydraulic pressure [5].

Potassium nitrate as a salt can give rise to chemical deterioration of concrete from leaching and decomposition of cement hydration products. [6,7,8].

Potassium nitrate has a chemical formula of KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−, and is therefore an alkali metal nitrate. It is a natural source of nitrate and has been used as a constituent for several different purposes, including food preservatives, fertilizers, tree stump removal, rocket propellants and fireworks.

Salt does not damage concrete, but the effects of salt can. Salt does not chemically react with hardened concrete. Salt does however lower the freezing point of water, attract moisture, and increase pressure of frozen water. Salt can also increase the freeze-thaw cycles if the temperature fluctuates between 15°F and 25°F. Concrete scaling can occur in the absence of salts too if there were problems at installation [9].

Potassium nitrate as a salt is a water reducer in the concrete mix. It therefore minimizes water in the mix resulting in stronger concrete concrete. It however maintain flowability, that is keeping it easy and cheap to place. Excessive water will lower the strength of the concrete and if that water becomes trapped it could also have additional weakening effects on the surface. The moment pressure of trapped ice exceeds the tensile strength of the concrete [9].

2. Materials and Methods

2.1. Materials

2.1.1. Cement

Portland Limestone Cement (PLC) manufactured by DANGOTE INDUSTRIES Plc was used. The cement used in the study was (Grade 42.5). It conformed to [10].

2.1.2. Fine Aggregates

Naturally occurring river sand obtained from Imo River in Oyigbo Local Government Area of Rivers State was used. The maximum size was 4.75mm. Impurities were removed and it conformed to the requirements of [11].

2.1.3. Coarse Aggregates

Coarse aggregates used are crushed angular and rough textured granite obtained from CRUSHED ROCK INDUSTRIES Plc at Isghiagu io Ebonyi State, South Eastern Nigeria. Maximum size was 20 mm. It conformed to [11].

2.1.4. Potassium Nitrate

Potassium Nitrate was obtained in from a chemical supplier Mile 3 area of Port Harcourt.

2.1.5. Water

Potable water used was obtained from the Civil Engineering Laboratory of the Rivers State University, Port Harcourt. The water conformed to [12].

2.2. Methods

2.2.1. Mix Design

The concrete mix ratio was 1: 2 : 4. by weight (cement,: fine aggregate: coarse aggregate) with water/cement ratio of 0.5.

2.2.2. Slump Test

The workability of all concrete mixtures was determined through slump test utilizing a metallic slump mould. The difference in level between the height of mould and hat of the highest point of the subsidized concrete was measured and reported as slump. The slump tests were performed according to [13].

2.2.3. Setting Time of Concrete

The setting time of the concrete was measured by extracting some portion of concrete (mortar portion) by subjecting to wet sieving through 4.75 mm sieve. The test was performed in accordance with [14].

2.2.4. Compressive Strength

The compressive strength of the concrete with (0, 5, 10, 15 and 20 %) Potassium Nitrate were determined after 7, 14 and 28 days curing period. The 0 % Potassium Nitrate was the control experiment. The test was performed in accordance with [15].

3. Results and Discussion

3.1. Slump Test

Figure 1 shows the plot of the slump values for the concrete with various percentages of Potassium Nitrate. The workability decreases as the potassium nitrate content increases. This is due to the fact that the potassium nitrate as a salt is a water reducer in the concrete mix. It therefore minimizes water in the mix resulting in stronger concrete It however maintain flowability that is keeping it easy and cheap to place [9].

3.2. Setting Time

Figure 2 is the graph of the setting time versus the percentage of potassium nitrate in the concrete. The initial and final setting times decreased as the percentage of the potassium nitrate is increased. The reason for this phenomenon is that the potassium nitrate is a water reducer in the concrete. It therefore minimizes water in the mix resulting in stronger concrete (fast setting). [16]

3.3. Compressive Strength

Figure 3 shows the plot of the compressive strength after 7, 14 and 28 days of curing for the (0 %, 5 %, 10 %, 15 % and 20 %) Potassium Nitrate. At all ages of curing the compressive strength of the concrete decreased as the percentage of the potassium nitrate increased. Potassium nitrate as a salt can give rise to chemical deterioration of concrete from leaching and decomposition of cement hydration products [6,7,8].

Figure 1. Slump values against percentage concentration of potassium nitrate

Figure 2. Setting Time against percentage concentration of potassium nitrate

Figure 3. Plot of Compressive strength versus percentage potassium nitrate

Figure 4. Plot of Flexural strength versus percentage potassium nitrate

Figure 5. Plot of Split Tensile strength versus percentage potassium nitrate

4. Conclusion

(1) Potassium nitrate reduces the setting time of concrete. This is attributed to the fact that potassium nitrate as a salt is a water reducer, therefore it gives rise to quick setting without necessarily resulting in increased strength as a result of the reduced hydration process arising from water reduction.

(2) Workability of the concrete decreased as the potassium nitrate content increased.

(3) Compressive strength at all ages of curing decreased as the percentage of potassium nitrate in the concrete increased.

(4) Potassium nitrate can be used as an accelerator in foundation of buildings for the blinding of the foundation in cases that require quick setting without consideration for strength.

Abbreviations

BS – British Standard

OPC- Ordinary Portland Cement

PC – Plain Concrete

PLC – Portland Limestone Cement

References

[1]   | https://pubchem.ncbi.nim.nih/gov/compound/10909504. 01/02/2018.
[2]   | https://en.wipedia.org/wiki/Haber_process. 01/02/2018.
[3]   | www.instructables.com/topics/How-do-you-make-potassium-nitrate-from-fertilizer/ 01/02/2018.
[4]   | www.cs.mcgill.ca/rwest/Wikipedia/wpcd/wp/p/Potassiumnirate.htm, 01/02/2018.
[5]   | Setzer, M.J. 1997. “Action of frost and deicing chemicals. Basic phenomena and testing. In: Macrchand J. Pigen M. Zetzer M. editing. Freeze thaw durability of concrete”. London E & FN Spon:, pp. 3-21.
[6]   | Hoffmann, D.W. 1984. “Changes in structure and chemistry of cement mortars stressed by a sodium chloride solution”. Cem. Conc Res, Vol 14, No.1, pp. 49-56.
[7]   | Kurdowski, W., Duszak, S.and Trybalaska, B. Corrosion of tobemorite in strong chloride solution. In: Scrivener, K.LYoung, J.F, editors Mechanisms of chemical degradation of cement-based systems. London efn Spon : 1997pp. 114-21, 1997.
[8]   | Heukamp, F.H., Ulm, F.J. and Germaine, J.T. Mechanical properties of calcium -leached cement pastes: triaxial stress states and the influence of pore pressures. Cem Concr Res, Vol. 30, No. 3, pp. 767-74, 2001.
[9]   | https://www.chaneyenterprises.com/article/Effect-of-Salt-on-concrete, 13/02/2018.
[10]   | BS EN 197- Cement composition, Specifications and conformity criteria for cements, British Standards Institute, London, United Kingdom, 2011.
[11]   | BS 882- Specifications for aggregates from natural sources for concrete. British Standards Institute, London, United Kingdom, 1992.
[12]   | BS 3148- Methods of test for water for making concrete. British Standards Institute, London, United Kingdom, 1980.
[13]   | BS 12350-2 – Testing of concrete – Method for determination of slump, British Standards Institute, London, United Kingdom, 2009.
[14]   | BS EN 196-3 – Method of testing cement. Determination of setting time and soundness. British Standards Institute, London, United Kingdom, 1995.
[15]   | BS EN 12390-3- Testing of hardened concrete. Compressive Strength Test of Specimens. British Standards Institute, London, United Kingdom, 2009.
[16]   | www.engr.Psu.edu/ce/courses/ces584/concrete/library/materials/admixture, 13/02/2018.