Ferrochrome Ash Based Geopolymer Concrete

Jyotirmoy Mishra, Ph.D. Scholar, Department of Civil Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, presents his research on the feasibility and compressive strength performance of geopolymer concrete - an emerging green building material.

In response to the 21st-century challenges such as global warming and climate change due to carbon dioxide gas emissions, the scientific community is progressing towards the pursuit of low carbon building materials. The traditional building materials and construction practices are now recognized to be unsustainable. Cement-based concrete, particularly, the most widely used building material, is responsible for nearly 8-9% of global carbon dioxide gas emissions.

Scientists have come up with innovative green materials to reduce dependency on cement. Geopolymer concrete, invented by Prof. Joseph Davidovits is one such innovative green building material, that does not use cement as a binder.

Geopolymer concrete utilizes industrial waste containing oxides of aluminium and silicon. It is manufactured using ferrochrome ash (FCA) waste from the ferro-alloy industry, along with ground granulated blast furnace slag (GGBFS). The best compressive strength (32.6 MPa) was attained by the mix containing 80% FCA and 20% GGBFS, and hence could be implemented for normal construction works as per Indian standards.

The binder is produced from the reaction between alkaline solution and aluminosilicate wastes. The alkaline solution is usually the combination of sodium hydroxides and/or silicates, while the wastes, known as ‘source materials’, are the industrial wastes rich in aluminium and silicon such as fly ash, GGBFS, rice husk ash, mine tailings, etc.

The geopolymer concrete exhibits many advantages over cement-based concrete such as better strength and higher durability, and the absence of cement helps in bringing down the level of carbon emissions.

India is the third-largest producer of ferrochrome in the world. Its production results in the production of waste such as ferrochrome ash (FCA) - a duct collected from the gas cleaning plants. It contains significant amounts of oxides of aluminium and silicon that make it a potential candidate to be used as a source material for manufacturing geopolymer concrete. Likewise, GGBFS, a waste from steel industries, contains significant amounts of oxides of aluminium, silicon, and calcium, and is widely used as a source material in making geopolymer concrete.

This article presents the investigation into the compressive strength properties of the resultant FCA-GGBFS based geopolymer concrete and aims to establish FCA as effective source material for geopolymerization. The results presented give a general understanding of the material and are cited from the author’s own work.

Materials and methods
To manufacture geopolymer concrete using FCA and GGBFS as source materials in this study. FCA was obtained from Balasore Alloys Limited, Balasore, Odisha, while GGBFS was obtained from Neelanchal Ispat Nigam Limited, Jajpur, Odisha. The details of chemical compositions of FCA and GGBFS determined by X-ray fluorescence (XRF) analysis are presented in table 1. A mixture of commercially available sodium hydroxide (NaOH) pellets and sodium silicate (Na2SiO3) solution were used as the alkaline solution in this study. The details of all the materials used to manufacture geopolymer concrete are presented in Table 2.

Table 1:  Chemical compositions of FCA and GGBFS (% by weight)[8]
Materials	SiO2	Al2O3	CaO	MgO	Fe2O3	K2O	Na2O	P2O5	TiO2	Cr2O3	Cl
FCA	19.10	10.91	3.14	23.60	7.84	11.42	2.46	0.07	0.13	9.89	6.13
GGBFS	36.30	20.40	24.12	8.08	6.64	1.02	0.38	0.05	0.73	0.08	0.05

Table 2: Materials used to manufacture geopolymer concrete
Sl. No.	Materials	Details
	Source materials	FCA and GGBFS
	Alkaline solution	NaOH pellets (98% purity) Na2SiO3 solution – SiO2 (32.15%), Na2O (15.85%) H2O (52%)
	Aggregates	Natural river sand Crushed gravel (20 mm downsize)
	Water	Tap water

To date, no code of practice is available for the manufacturing of geopolymer concrete. Therefore, a mix proportion of 1:1:3 (representing source materials: fine aggregates; coarse aggregates) was adopted for this study. The molarity of NaOH was taken to be 12 M while a ratio of 1:2 for NaOH/ Na2SiO3 was considered by weight. The details of the mix design are presented in Table 3. The dry mixing of the source materials and aggregates was done for 11 minutes, followed by the addition of alkaline solution and water. Three different concrete mix samples containing 60-80% of FCA: F60G40, F70G30, F80G20, were cast in cube having 150 mm size and cured at open-air conditions for 28 days. The compressive strength of each cube sample was tested at the age of 7, 14, and 28 days as per IS:516.

Table 3: Mix design for 1m3 geopolymer concrete
Mix name	FCA (Kg/m3)	GGBFS (Kg/m3)	Fine Aggregate (Kg/m3)	Coarse Aggregate (Kg/m3)	Sodium Hydroxide Liquid (Kg/m3)	Sodium Silicate (Kg/m3)	Water (Kg/m3)
F60G40	294.6	196.4	491	1473	73.08	154.07	14.81
F70G30	343.7	147.3	491	1473	73.08	154.07	14.81
F80G20	392.8	98.2	491	1473	73.08	154.07	14.81

Results and discussion
Fig. 2 presents the 7, 14, and 28-day compressive strength of the resulted geopolymer concrete made with FCA and GGBFS. The highest compressive strength of 32.6 MPa at the age of 28 days was attained by the mix containing 80% FCA and 20% GGBFS (F80G20) while the lowest compressive strength of 19 MPa was attained by the mix containing 60% FCA and 40% GGBFS (F60G40). This indicates that an increasing amount of FCA in the mix leads to improved strength gain. It is also observed that maximum strength gain occurred between the age of 1 and 7 days of curing. It has been reported that strength development in the case of geopolymer concrete depends upon the formation of geopolymer gels such as sodium aluminium silicate hydrate (N-A-S-H), calcium aluminium silicate hydrate (C-A-S-H). Sometimes these gels co-exist in order to form a solid three-dimensional geopolymer matrix. In this study, the strength development occurred due to the generation of N-A-S-H gel formed from the participation of sodium, aluminium, silicon from the alkaline solution as well as the source materials. The presence of calcium from GGBFS resulted in the formation of C-A-S-H gel. Further, FCA has an abundance of magnesium oxide (23.60%) that also contributed to the strength development of the samples by the formation of Mg-bearing geopolymer gels. The extensive investigation regarding microstructure and strength development is provided in the author’s own work.


Conclusion
In this new age of global construction concerning low carbon building materials, the short-term and long-term dominance of geopolymer concrete is evident. It is found from the results that the FCA can be suitably used for making geopolymer concrete, which could further aid effective waste management, leading to a sustainable future.

Acknowledgement
The author expresses gratitude to his supervisors: Dr. Bharadwaj Nanda and Dr. Sanjaya Ku. Patro, Veer Surendra Sai University of Technology, Burla, and the motivation and support from Dr. Syed M. Mustakim (CSIR-IMMT, Bhubaneswar), Mr. Shaswat Ku. Das (CSIR-IMMT, Bhubaneswar), and Mr. R. S. Krishna (Veer Surendra Sai University of Technology, Burla).

The author’s Ph.D. dissertation is focused on the utilization of ferrochrome ash in manufacturing geopolymer concrete. He holds B. Tech and M. Tech (Structural Engineering) degrees from KIIT University, Bhubaneswar, and has published over 30 research articles in the field of geopolymers.

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