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A study on suitability of iron ore overburden waste rock for partial replacement of coarse aggregates in concrete pavements

Concrete is the most widely used construction material, several sustainable materials are under investigation to reduce the impact of its production processes, by considering partial replacement of natural constituents with the waste materials without compromising the durability and strength of concrete in view to reduce both the demand of raw material and effective waste disposal of millions of tons from different sources. There has been a constant research to substitute the natural aggregates with other waste materials to develop sustainable pavements. In the process, the waste produced from Mining industry is one of such source. The aim of this study is to evaluate the suitability of iron ore waste as partial replacement of coarse aggregates in concrete with reference to the conventional materials. Physico-chemical properties and aggregate tests such as crushing, abrasion, impact, shape test, the specific gravity and water absorption were determined as per Standards and the results were within the limits indicating their applicability as a pavement material. Partial replacement by 20%, 40%, 60%, 80% and 100% with waste rock was tested for its mechanical properties and it was found that, 40% replacement of waste rock yielded highest compressive strength compared to the control mix. Regression analysis was carried out to predict the performance analysis of the compressive strength of the samples with IOT.The performance prediction analysis shows that curing days influencing more compared to % replacement with IOT.


Introduction
Over the period of time the amount and type of waste material generated increased accordingly. Many of the non-decaying waste materials will remain in the environment for hundreds or perhaps thousands of years. The non-decaying waste materials cause a waste disposal crisis, thereby contributing to the environmental problems. Mining Industry is one source of availability of a large quantity of waste material after agriculture in nation development. Waste from mining includes overburden waste, tailings, dumps, and other processed waste. There are two methods of mining, one is open-cast and the other is underground mining. Open-cast mining involves removal of complete waste rock to expose the ore body or coal. One of the common environmental problems associated with these tailings is the formation of acid mine drainage which can be a potential source of surface and ground water pollution [1]. These tailings also consume a lot of land that could be used for other purposes, and compromise the good looks of the environment in these areas [7], [9]. There can be a potential of erosion from these tailings dumps into the environment [10]. Utilization of such waste rock is being investigated by various researchers for different purposes such as in construction materials, pavements, back filling 2 etc. Hence, a partial replacement of the aggregates in concrete by waste rock produced from mines is not only economic in the handling and maintenance of waste dumps but also reduces the cost of construction of roads. In addition, it also reduces the environmental problems at mine site. The feasibility of utilization of steel slag as aggregate in stone mastic asphalt (SMA) mixtures was investigated [8]. Sand is replaced partially with laterite and sandstone respectively, there was not much improvement on strength properties of concrete yet these can be used as a replacement for sand [5], [6]. Iron ore tailings were used as replacement of fine and coarse aggregate and it was found that it is suitable as a construction material [2].

Experimental Program
To assess the suitability of iron ore overburden waste rock (OB-WR) as replacement of coarse aggregates in concrete, laboratory scale investigations were carried out. In the first stage, studies were carried out to understand the properties of materials used. In the second stage, the strength characteristics were determined using OB-WR as partial replacement for coarse aggregates.

Cement
Ordinary Portland cement (Grade 53) with Blaine fineness of 300 m 2 /kg and specific gravity of 3.15 was used. Physical properties of the cement used in the production of concrete are listed in Table 1. Should not exceed 10 mm

River sand (Fine aggregates)
Locally available river sand conforming to zone II of Table 4 in IS 383:1970, passing through 4.75mm sieve was used. The sand was free from organic matter and silt. The properties of sand such as fineness modulus and specific gravity were determined as per IS 2386-1963 (Part I). The physical properties of sand are shown in Table 2, and grading curve is shown in Figure 1.   Table 3.

Overburden Waste Rock (OB-WR)
The iron ore OB-WR was collected from Hospet, Bellary district of Karnataka state in India ( Figure  2). Sampling was done as per IS 2386 (Part III, IV): 1963. The average values of physical properties of the OB-WR are given in Table 3. The grading curve of OB-WR is shown in Figure 3. The aggregate test was performed on OB-WR and it is as per IS code given in Table 4.

Water and Superplasticizer
Tap water was used for concreting. Superplasticizer, is chemical admixtures used to reduce the water content in a mixture. Sulfonated naphthalene formaldehyde polymer admixture ("Conplast SP 430") was used to improve the workability of concrete (Table 5).

Chemical composition of Overburden waste rock
The various metal oxides present in OB-WR are analysed for there concentration (%) by X-Ray Fluorescence (XRF) investigations.

Methodology
The volumetric method was used for the proportion design of the mixture. In this study, M40 grade concrete is designed and cement is used as the binder material. The water-binder ratios were 0.40, 0.45, and 0.5, and overburden waste rock (OWR) as coarse aggregate were added at volume replacement ratios of 0%, 20%, 40%, 60%, 80% and 100%. Workability of concrete was determined  (1999). A 10cm*10cm*10cm cubical concrete specimen was made and solidified. The specimens were placed and cured in water for varying curing days as per IS: 516-1959. The compressive strength was tested at the ages of 3, 7, 14, and 28 days as per IS: 516-1959. Multiple regression analysis is used to analyse the laboratory results with the predicted values.

Physical properties and Chemical composition
In this study, 10-12 mm sizes crushed stone and overburden waste rock (OB-WR) used as the coarse aggregate (Fig. 1). The specific gravity of the crushed stone and the OB-WR were 2.8 and 2.85, respectively. The water absorption of OB-WR was measured to be 0.45. Slake durability test was also conducted with respect to IS10050-1981 to determine the stability of OB-WR against fragmentation and weakening.
 The physical properties of OPC Grade 53 are presented in  (Table 7).

Workability
Workability and consistency of fresh concrete are two closely related properties. Workability is that property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, transported, placed, compacted and finished. Consistency refers to the degree of solidity or fluidity of a material. The concrete slump test is an empirical test that measures workability of fresh concrete. The slump values for the different concrete mix are shown in table 7. Concrete was designed for 25-55 mm range of slump (figure 4). Water cement ratio and superplasticizer content was kept constant for all mixes.

Compressive strength
The specimens were tested at 3, 7, 14, 90 and 28 days of curing ( Figure 5 and 6) and the results are graphical represented in the Figure 7 and 8. From the test results it was observed that, compressive strength increased with increase in OB-WR. The increase was prominent in 28 days strength. It depicts that compressive strength of specimens containing 40% OWR was higher than that of control concrete at 28 days. The strength of concrete increased with 100% replacement with OB-WR compared to the normal concrete.   7 backward elimination method was used as the test procedure. Analysis of variance were performed to observe the essential parameters carried out in statistical models for compressive strength with respect to curing days (3, 7, 14 and 28 days) and 5 replacement levels (0%, 20%, 40%, 60%, 80% and 100%. With significant of 96.47 and 96.5 repectively for 0.4 and 0.45 water-cement ratio. Influenceof parametric level of the compressive strength obtained were compared using (ANOVA) with Minitab 17, where the P-values equal to or smaller than 0.05 were considered to be statistically significant ( Table 8, 9,10,11). The plot is showin in figure 7 and 8 for the predicted and obtained values.

Conclusions
In this paper the effect of use of OB-WR on the properties of concrete were studied and it can be concluded that, i) The physical properties were determined and the specific gravity is 2.85 which is higher than the crushed stone with water absorption of 0.45. ii) The impact test, Los Angeles Abrasion test, crushing test and soundness test revealed that, the results are within the limits with reference to IS: 2386-1963 and OB-WR can be considered as a construction material for road works. iii) The workability of all the concrete mixes increased upto 40% replacement with OB-WR and decreased thereafter with reference to the crushed stone. This is because of the water cement ratio and size and shape of the aggregates. iv) The Compressive strength of the concrete shows an upward trend till 40% OB-WR used as coarse aggregate in concrete. This is mainly because of the favorable chemical composition of OB-WR. v) Based on the multiple regression analysis, R 2 obtained was 96.5 of the total variations in the observed compressive strength test. The test reveals that, curing days > percentage replacement with IOT. The measured property values and those calculated from the regression model are fairly close which indicates that the developed models could be effectively used, with acceptable accuracy, at the preliminary stage of the design to predict the Compressive strength in the construction industry.