Evaluation of Physical and Mechanical Properties of Coating Mortar with Partial Replacement of Sand by Blast Furnace Flue Dust

Abstract

This study evaluates the potential partial replacement of sand by blast furnace flue dust (BFFD) in coating mortars to reduce the environmental impact caused by solid residues generated by the iron-making industry. Mortars were produced with varying rates of sand replacement by BFFD, and curing was conducted at room temperature and in lime-saturated water. Axial and radial compressive strength were evaluated at ages up to 28 days, and density, water absorption, and apparent porosity were measured after 28 days of curing. The results indicate that partial replacement of sand by BFFD is viable after curing at ambient conditions, leading to a reduction in mortar density without significantly altering compressive strength. However, the compressive strength of BFFD-added mortars decreases when curing is carried out in water-saturated lime.

Keywords:
Mortar; blast furnace flue dust; industrial ecology

INTRODUCTION

Many of the current research efforts aim to reduce the industry’s impact on the environment. The waste management strategies are usually based on reducing, reusing, recycling, and recovering the materials ¹. Promoting these strategies is of great importance, especially in Brazil, where its rich biodiversity may be affected by improper disposal of industrial wastes.

The iron and steel industry generates considerable solid waste from their processes. In the blast furnace operation, lump iron ore, sinter, pellets, fluxes, and coke or charcoal are fed from the top, and hot air is blown through the tuyeres from the bottom ². This process produces molten pig iron, slag, and flue gas ³. The blast furnace flue dust (BFFD) is recovered from the wet cleaning of flue gas coming from the blast furnace. The BFFD is a solid black waste that contains unburnt carbon, iron oxides, and fluxes ³.

In Brazil, 26,8 million tons of pig iron were produced in 2022 ⁴, generating approximately 268,000 tons/year of BFFD ⁵. Currently, these wastes are disposed of in open-air piles without further use. It is therefore important to develop processes or methods that reuse the BFFD, once its large-scale generation may affect the environment, population, and workers’ health.

Since many industrial solid wastes have no further application for commercial use, their incorporation into the mortar’s mixes and other construction materials are interesting approach ^6)-(8. Mortar is an essential material that is used in almost every stage of construction. Besides the use of natural resources in its production, mortars represent a significant share of the costs of construction: ten to thirty percent of the costs are associated with coating mortars ⁹. Thus, this strategy contributes not only to sustainable development but also to cost reduction.

Several researchers have investigated the technical feasibility of the partial replacement of sand and/or cement by different residues in the production of mortars. Coelho et al. ¹⁰ reported that a partial replacement of up to 10% of Portland cement by palm oil empty fruit bunch ashes did not alter the compressive strength of mortars at 28 days. The fine aggregate of mortars (natural sand) was successfully replaced by scheelite mining waste in the study of Medeiros et al. ¹¹ and by steel making slag in the work of Amancio et al. ¹².

The reuse of BFFD has been challenging to the iron and steel industry ^3),(13, and the incorporation of BFFD in mortars appears as an outstanding alternative for the reuse of it. Still, to our knowledge, the incorporation of BFFD as a partial substitute of sand in mortars has not yet been studied. Thus, the main objective of this study is to produce a coating mortar by partially replacing the fine aggregate (sand) with BFFD obtained from a pig iron producer from Corumbá-Brazil. Physical and mechanical characterization was carried out to evaluate its applicability in civil construction using density, water absorption, apparent porosity, and mechanical tests.

EXPERIMENTAL

The materials used in the mixes of this study are shown in Fig. 1. The cement employed in the mortar mixes was Portland CP II-E 32, produced by Votorantim, with a specific gravity of 3.00 g.cm^-3, which was determined by the ABNT-NBR NM 52 standard procedure ¹⁴ (similar to ASTM C128 international standard ¹⁵). It is worth mentioning that a single batch was used in specimen production to maintain homogeneity.

Figure 1:
Materials used in the production of the mortars.

Medium-sized sand from Corumbá, Brazil was used as the fine aggregate of the mortars. Before use, it was dried in a laboratory oven at 100 ºC for 24 hours to ensure better control of water to cement ratio. The sand’s specific gravity of 2.65 g.cm^-3 was determined by the procedure of ABNT-NBR NM 52 standard ¹⁴. The particle size distribution was assessed according to the ABNT-NBR NM 248 standard ¹⁶ (similar to ASTM C136 international standard ¹⁷) and is given in Table I.

Table I
Particle size distribution of sand.

Sieve Opening (mm)	Retained Mass (g)	Individual percent retained (%)	Cumulative percent retained (%)
4.750	0.0	0.0	0.0
2.360	0.0	0.0	0.0
1.180	0.0	0.0	0.0
0.600	19.0	3.79	3.79
0.300	157.5	31.41	35.20
0.150	288.1	57.46	92.66
0.075	35.0	6.98	99.64
Pan	1.8	0.36	100.00
Fineness Modulus	2.31
Maximum diameter (mm)	0.60

The BFFD utilized in this investigation was obtained from a pig iron producer in Corumbá, Brazil, and it was employed as a partial replacement for sand in the mortar mixes. The specific gravity of the BFFD was determined to be 2.00 g.cm^-3 following the procedure of ABNT-NBR NM 52 standard ¹⁴. The lower specific gravity of BFFD compared to sand is attributed to its high carbon content, as evidenced by its loss on ignition (LOI) at 1000 ºC for 1 hour, which was found to be 30.83%. The particle size distribution of the BFFD (determined according to the ABNT-NBR NM 248 standard ¹⁶) is shown in Table II.

Table II
Particle size distribution of blast furnace flue dust.

Sieve Opening (mm)	Retained Mass (g)	Individual percent retained (%)	Cumulative percent retained (%)
4.750	0.00	0.00	0.00
2.360	2.80	0.93	0.93
1.180	9.80	3.27	4.20
0.600	23.30	7.77	11.97
0.300	52.20	17.42	29.39
0.150	77.70	25.93	55.32
0.075	84.70	28.26	83.58
Pan	49.20	16.42	100.00
Fineness Modulus	1.85
Maximum diameter (mm)	1.18

The chemical composition of the raw materials was determined by X-ray fluorescence (XRF) using a Shimadzu EDX-720 spectrometer, and the results are presented in Table III. It is noteworthy that the BFFD’s chemical composition includes 50.8% of iron oxide and 30.8% of loss on ignition (carbon compounds).

Table III:
Chemical composition of the raw materials determined by XRF (in wt%).

Chemical composition (oxides)	Sand	CP II E-32 cement	BFFD
SiO2	92.538	18.947	8.104
Al2O3	-	-	4.290
Fe2O3	1.951	5.074	50.807
CaO	0.612	69.953	2.284
K2O	1.255	1.489	1.074
MnO	0.019	0.420	0.095
TiO2	1.459	0.400	-
SO3	1.408	3.291	1.439
SrO	0.005	0.327	0.059
ZnO	0.011	0.049	0.039
P2O5	-	-	0.926
ZrO2	0.515	0.031	0.030
Others	0.227	0.003	0.022
LOI	-	-	30.83

A comparative study of the properties of BFFD-added mortars to a reference mortar (without the addition of BFFD) was carried out. A reference mortar was prepared using a mass ratio of 1:3:0.7 (cement:sand:water) following ABNT NBR 13281 standard ¹⁸. In addition, four different mixes were prepared by partially replacing sand by BFFD at 5%, 10%, 15% and 20%. The water to cement ratio was maintained at 0.7, as employed for the reference mix. For each mix, 25 cylindrical specimens measuring 50 mm in diameter and 100 mm in height were produced following the ABNT NBR 5738 standard ¹⁹. The curing process was carried out at room temperature (32±5 ºC) and submerged in lime-saturated water (28±2 ºC).

The axial and diametral compression tests were conducted using a universal testing machine EMIC DL 30000 following the ABNT NBR 7215 ²⁰ and ABNT NBR 7222 ²¹ standards, respectively. Axial compression tests were performed in five replicates for each mix at the curing times of 7, 14, 21, and 27 days, while diametral compression tests were carried out in five replicates of each mix at the curing time of 28 days.

The water absorption tests were executed according to the ABNT NBR NM 30 standard ²², which is similar to the procedure of ASTM C128 international standard ¹⁵. The specimens subjected to the water absorption test were cured in lime-saturated water for 28 days. Then, they were removed from the lime-saturated water, the superficial water was removed with a cloth, and immediately the wet mass (M_w) was measured. Finally, the samples were placed in a laboratory oven at 100 ºC until the specimens were completely dried (48 hours) to determine the dried mass (M_d). The weight measurements were carried out in an analytical balance. Finally, the water absorption (A) was obtained by Equation (A):

$A\left(\%\right)=\frac{M_w-M_d}{M_d}·100$ (A)

The apparent porosity was determined based on the Archimedes principle (the immersed mass was established by the fluid displacement caused by the immersed body). The apparent porosity (P_ap) was determined for each mix after 28 days of curing by Equation (B):

$P_{ap}\left(\%\right)=\frac{M_w-M_d}{M_w-M_i}·100$ (B)

Where M_w is the wet mass, M_d is the dried mass, and M_i is the immersed mass.

The results are shown as mean values, and the standard deviations of the means are indicated by error bars. Additionally, a one-way ANOVA was performed, and significant differences were distinguished using Duncan’s test with significance set at p ≤ 0.05.

RESULTS AND DISCUSSION

Figure 2 shows the density of mortars after 28 days of curing under both ambient conditions and submerged in lime-saturated water. Depending upon their density, mortars can be classified as light (density lower than 1.40 g.cm^-3), normal (density between 1.40 and 2.30 g.cm^-3), or heavy (density higher than 2.30 g.cm^-3) ²³. Therefore, all the mortars produced in this research fall into the normal density range. The reference mix exhibits a density of 1.87 ± 0.02 g.cm^-3, and the partial replacement of sand by BFFD decreases the average density of the mortars (excluding the mix with 5% of BFFD).

Figure 2:
Effect of partial replacement of sand by blast furnace flue dust (BFFD) in the density of the mortars at 28 days of curing.

ANOVA indicates a significant difference in the density of mortars between mixes after curing at room temperature (p=4.4x10^-9) and after curing in lime-saturated water (p=7.9x10^-15). Duncan’s test reveals that the mix with 20% BFFD has a significative lower density than the other mixes for both curing methods. The replacement of 20% of sand by BFFD reduced the density from 1.87 ± 0.02 g.cm^-3 (reference mix) to 1.74 ± 0.02 g.cm^-3 after curing at room temperature and to 1.58 ± 0.02 g.cm^-3 after submerged curing in lime-saturated water.

It is important to emphasize that low-density mortars are generally highly desirable for coating slabs and buildings and in civil construction, once the load on the structure is reduced, allowing for smaller structural elements. Moreover, lower-density mortars exhibit greater workability, which reduces physical efforts for workers and enhances the production rate ^12),(24.

However, one might note that the relation between the content of sand replaced by BFFD and the density of the mortars is not straightforward. The density of the mortar does not linearly decrease with the replacement of sand by BFFD, which could be attributed to the difference in specific gravities of sand (2.65 g.cm^-3) and BFFD (2.00 g.cm^-3). Surprisingly, the mix containing 5% BFFD exhibited a significantly higher density than the reference and other mixes. The density of the mortar depends on two main factors: the gain or loss of the porosity with the incorporation of residues and the specific gravity of the residue itself ²⁵. Despite BFFD having a lower specific gravity than sand, the apparent porosity varied according to the partial replacement of sand by BFFD. The BFFD exhibits a finer powder fraction compared to sand, indicating that granulometry may have an impact on physical properties. However, further research and studies are needed to investigate the influence of granulometry on the physical properties of mortars.

The water absorption and the apparent porosity of the mortars are given in Table IV. One might note that a coherent trend between density, porosity, and water absorption is observed; as the density decreases, the porosity and water absorption increase. Indeed, the samples with higher density (e.g., the mix with 5%) exhibited lower porosity and water absorption levels, while samples with lower density (e.g., the mix with 20%) showed higher porosity and water absorption levels. Generally, the partial substitution of sand by BFFD led to an increase in the porosity and water absorption of the mortars, except for the mix with 5%.

Table IV
Water absorption and apparent porosity of the mortars at 28 days of curing.

	Water absorption (%)		Apparent porosity (%)
Mix	Ambient curing	Submerged curing	Ambient curing	Submerged curing
Reference	10.63 ± 0.30	13.52±0.20	20.20 ± 0.62	26.05±0.31
5% BFFD	10.28 ± 0.42	11.06±0.32	19.81 ± 0.83	21.51±0.53
10% BFFD	11.60 ± 0.31	13.42±0.42	20.78 ± 0,57	24.68±0.51
15% BFFD	11.88 ± 0.58	14.55±0.37	20,20 ± 0,62	27.74±0.29
20% BFFD	12.53 ± 0.32	20.05±1.89	22.04 ± 0.57	30.41±2.31

Furthermore, the water absorption is affected by the composition of the mixes in a significant manner after curing at room temperature (p=1.1x10^-7) and in lime-saturated water (p=4.5x10^-9). In general, lower water absorption in mortar indicates better prevention of moisture intrusion in walls, beams, pillars, and slabs. Therefore, aiming for lower water absorption, it is possible to conclude after Duncan’s test that the mix containing 5% BFFD is preferred (after curing at room temperature, the 5% BFFD and the reference mix did not differ significantly). On the other hand, the mix with the lowest density (20% BFFD) exhibits the highest water absorption in both curing methods. However, in roughcast applications, the increase in water absorption does not affect the construction quality, as the cement plaster layer is responsible for the coating’s insulation.

Regarding the effect of the curing method, the mixes containing 15% and 20% of BFFD exhibited a significantly lower density after submerged curing in water-saturated lime, which is coherent with the higher apparent porosity and water absorption of these mortars (Table IV). The high quantity of unburnt carbon in BFFD (LOI of 30.83%, as shown in Table III) could be a contributing factor. The interaction of carbon with lime-saturated water (such as the carbon corrosion in alkaline media ^26),(27) may exacerbate when higher contents of BFFD are used, leading to the observed effects.

The axial compressive strength of the mortars at the ages of 7, 14, and 28 days are shown in Table V. At the age of 7 days, the mix containing 5% BFFD exhibited an average increase of 22.2% in axial compressive strength compared to the reference mortar (0% BFFD) after curing at room temperature. However, the compressive strength decreased by an average of 25.4% and 20.4% when 10 and 20% of BFFD were incorporated into the mix, respectively, compared to the reference mix.

Table V
Axial compressive strength of the mortars at 7, 14, and 28 days of curing.

	Axial compressive strength (MPa)
	7 days		14 days		28 days
Mix	Ambient curing	Submerged curing	Ambient curing	Submerged curing	Ambient curing	Submerged curing
Reference	9.44±1.51	8.26±1.78	9.86±2.13	14.22±1.88	10.26±1.54	16.13±2.88
5% BFFD	11.54±2.03	6.75±0.79	12.53±2.19	10.53±1.76	10.41±2.24	15.12±4.23
10% BFFD	7.04±0.78	6.17±0.74	9.37±2.04	8.43±1.70	9.07±0.90	11.68±0.92
15% BFFD	9.79±1.50	5.39±1.89	12.98±0.57	6.72±1.80	13.46±2.14	10.73±1.85
20% BFFD	7.51±1.28	3.49±1.22	8.21±1.13	4.15±0.78	8.21±1.24	5.87±1.51

After 28 days of curing, the analysis of variance showed that the BFFD content significantly affects the axial compressive strength of the mortars (between at least one pair of means evaluated), as the p-value was 3.6x10^-3 for the groups cured at room temperature and 2.6x10^-5 for those cured submerged in lime-saturated water. Duncan’s test indicated that the mix containing 15% BFFD has a significantly higher strength than the other mixes cured at room temperature. However, no significant difference was observed in the compressive strength among the other levels (including the reference mix). After submerged curing, both the reference and 5% BFFD mixes exhibited significantly higher strength than the other mixes. There was no significant difference between the 10 and 15% BFFD mixes, but they both demonstrated a significantly higher strength than the mix containing 20% BFFD.

The iron oxide and carbon compounds present in the chemical composition of BFFD are inert during the curing reactions with cement. Moreover, BFFD contains a small amount of silicon oxide which promotes the formation of calcium silicate during curing. The mechanical results show a reduction in the compressive strength with increasing BFFD content, possibly due to the decrease of silicon oxide resulting from the replacement of sand by BFFD.

The diametral compressive strength of the mortars at 28 days is presented in Figure 3. One might observe that the compressive strength in the diametral configuration is lower than in the axial configuration, which is primarily due to the indirect tensile loads in the former configuration. Therefore, diametral compressive strength can be used to estimate the tensile strength of the mortars. ANOVA data analysis revealed a p-value of 9.0x10^-5 for ambient curing and 5.7x10^-3 for submerged curing. In both cases, the p-value is lower than 0.05, indicating significant variation between at least one pair of means. According to Duncan’s test, the mix containing 5% BFFD exhibits a significantly higher diametral compressive strength than the other mixes, which do not differ significantly among themselves. Furthermore, Duncan’s test indicates that after submerged curing, the reference, 5% BFFD, and 10% BFFD mortars do not differ significantly among themselves, but they do differ significantly from the 20% BFFD mix.

Figure 3:
Diametral compressive strength of the mortars at 28 days of curing.

Both axial and diametral compressive strength were affected by the curing method. The reference mix, when cured in water-saturated lime, exhibited higher strength compared to ambient curing. Indeed, it has been reported that ambient curing often produces mortars and concretes with lower strength than other curing methods, such as submerged curing ^28)-(31. However, the compressive strength of the mortars decreased as sand was partially replaced by BFFD, which could be attributed to the interaction between the carbon content of BFFD and the alkaline media ^26),(27, as previously discussed. Additionally, compressive strength is typically related to the porosity of the mortar, and indeed, a consistent trend between the porosity of the mortars and their compressive strength was observed.

CONCLUSIONS

The addition of blast furnace flue dust (BFFD) to the composition of mortars was investigated by physical and mechanical tests. The partial replacement of sand by BFFD generally decreased the density of the mortars (although they remained classified as normal density mortars), and a coherent trend was observed among apparent porosity, water absorption, and density. The partial replacement of sand by BFFD at rates up to 20% did not significantly decrease the axial and diametral compressive strength of mortars after ambient curing, highlighting the potential for BFFD reuse as a mortar material component. However, the curing method affected the axial and diametral compressive strength of mortars. While the submerged curing in water-saturated lime increased the strength of the reference mix, a significant decrease was observed as sand was increasingly partially replaced by BFFD.

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  » https://doi.org/10.31026/j.eng.2012.05.02
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Edited by

• (AE: Rafael Salomão)

Publication Dates

• Publication in this collection
  24 Feb 2025
• Date of issue
  2025

History

• Received
  27 Feb 2024
• Reviewed
  09 Apr 2024
• Accepted
  15 Apr 2024