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Enhanced mechanical behavior of compacted clayey silts stabilized by reusing steel slag
Construction and Building Materials 239 (2020) 117901
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Enhanced mechanical behavior of compacted clayey silts stabilized by reusing steel slag Clara A. Mozejko a,b, Franco M. Francisca a,b,⇑ a
Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina Instituto de Estudios Avanzados en Ingeniería y Tecnología (IDIT), CONICET and Universidad Nacional de Córdoba, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina b
h i g h l i g h t s Clayey soils mechanical behavior is enhanced by pozzolanic reactions. Steel slag promotes changes in soil microstructure, pores and particle distribution. Strength and stiffness of compacted clays increases due to natural cementation. Pozzolanic reactions favors natural cementation of clayey and slag particles.
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Article history: Received 20 June 2019 Received in revised form 6 November 2019 Accepted 17 December 2019 Available online xxxx Keywords: Compacted clayey silt Steel slag Pozzolanic reactions Strength Natural cementation Soil fabric
a b s t r a c t Finding new uses and applications of byproducts has significant importance due to the present need of minimizing the consumption of natural resources, reducing the amount of the disposal of waste materials and finding novel sustainable applications for these materials. This paper evaluates the behavior of compacted clayey loessical soils stabilized by the addition of steel slag (SS). Used soil is a loessical material made by quarts, feldspar and volcanic glass in different proportions. Different physical, microscopical and mechanical tests were performed in compacted specimens of clayey soil-SS mixtures. The influence of SS content, compaction moisture content and curing time on strength, deformation modulus, particle size, and pore structure were evaluated. The results showed increases in soil resistance and stiffness with the curing time, which can be attributed to the natural cementation that takes place due to interaction between the vitreous fraction of clayey silty particles and SS through pozzolanic reactions. This type of reaction is also confirmed by means of a direct measurement of a pozzolanicity index, evolution of pore sizes with a formation of more homogeneous microstructures and associated reduction in water content. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The Argentinian clayey loess covers approximately 35% of the national territory, and is mainly distributed in the central zone of Argentina. This aeolian sediment can be classified as collapsible in the unstable group of soils according to its main physical and mechanical properties. Loess presents a metastable soil structure, presence of macro-pores, collapsible behavior due to increases in moisture content and/or external loads, and stiffness and shear strength highly dependent on the degree of saturation [1]. Collapsible silts and clays can be found in many places around the world ⇑ Corresponding author at: Facultad de Ciencias Exactas, Físicas y Naturales, Departamento de Construcciones Civiles, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina. E-mail address: [email protected] (F.M. Francisca). https://doi.org/10.1016/j.conbuildmat.2019.117901 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and are, therefore, frequently used as construction material for earth structures (e.g. levees, landfill liners, embankments, base of pavements, etc.) or for the foundation of civil infrastructure (e.g. pipes, tunnels, civil constructions) [2–4]. Clayey loess is also distinguished by a poorly accommodated structure with larger particles separated by open spaces. These particles are joined together by the accumulation or bridges of clayey material and precipitated salts. These joints reduce when moisture content increases, clay particles expand and precipitated salts dissolve, inducing sliding between grains and a significant decrease in voids volume [5,6]. In order to improve mechanical soil properties, densification of the material can be achieved by compaction. This soil stabilization technique produces a decrease in soil voids and compressibility and increase in strength and stiffness [7,8]. It has been proved that several properties of compacted clays, such as density, permeabil-
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ity, diffusion and collapse, depend on compaction moisture content among other things, indicating that soil structure is affected by the compaction procedure. This affects soil fabric and therefore has a fundamental influence on soil behavior. On a microscopic scale, compaction disturbs the original soil fabric, destroying shear planes and eliminating large pores, resulting in a more homogeneous soil fabric generating an alignment of particles. Other techniques are also available to improve mechanical properties of soils, as soil reinforcement methods and chemical stabilization. Different soil stabilizing materials can also be used to improve soil behavior including lime, cement and fly ash [9– 13]. However, these methods have significant environmental impacts associated in terms of energy or non-renewable resources consumption, so, in this way, the use of industrial by-products and metallic slags have been lately encouraged [14–16]. Steel and stainless-steel production generate a solid waste known as Steel Slag (SS) that can be classified according to the type of steel as carbon steel slag or stainless-steel slag. Also, it can be classified according to the manufacturing process as retreat slag, electric arc furnace, refining furnace, blast furnace or foundry residue. During manufacturing basic oxygen furnace steel (BOF) combines coke, produced from coal, with pulverized coal, which by a process of explosion at high temperatures forms hydrogen and carbon monoxide which acts as basic reducing agent for iron oxides. The furnace is loaded from the top with alternative layers of these reducing agents combined with the melting mineral; once mixed, oxygen is injected under pressure causing chemical reactions responsible of separating impurities from steel; these impurities flocculate, and when cooled, form the SS [17]. Using SS in different processes would help in alleviating the disposal problem and helping preservation of natural resources [18]. To date, studies have been focused on determining the viability of using SS as aggregates used in road construction or pavements [19–21] as aggregates in concrete [22–24], or as raw material in other productive processes [25–27]. However, the development of knowledge about SS behavior as stabilizing agent in soils is recent and scare. The use of grand granulated blast furnace slag for stabilized lime-clay soils have beneficial effects on strength and also in reducing the swelling potential of gypsum-bearing kaolinite clay when SS is added with an adequate amount of lime [28]. Also, the addition of SS to soils causes a decrease in plasticity index, increases the maximum dry density of clayey soils, and reduces the swelling potential of expansive clay minerals [29]. Other studies show that partial substitution of lime with ground granulated blast furnace slag is effective to reduce volumetric shrinkage [30]. In this way, some authors have begun to develop different approaches in which SS can be used for remediation of metal contaminated sites as components of permeable reactive barriers, as materials for landfill closures or as reactive material [31–36]. The objective of this research is to propose a sustainable way to reutilize SS to stabilize clayey loessical soils in an environmentally friendly way by studying the mechanical behavior of compacted steel slag-clayed silt mixtures. The main purpose includes to better understand the physicochemical mechanisms that are responsible for changes that take place at particle contacts, and the emergent behavior at macro-scale, such as changes in soil gradation, pore structure and mechanical behavior.
2. Pozzolanic reactions Pozzolans are siliceous and aluminous materials that have little or non-cementitious value but, under the presence of water and finely divided are able to chemically react with calcium hydroxide forming cementitious compounds (ASTM C618) [37].
The definition of pozzolan embraces a wild variety of materials with different properties, compositions and/or origins. They can be classified as natural or artificial materials; the first type is composed mainly by volcanic glass while the second type is a byproduct from high temperature industrial processes (e.g. fly ashes). The reactivity of pozzolans is attributed to the active silica forming silicate minerals. Pozzolanic activity refers to the maximum amount of calcium hydroxide with which pozzolan can combine and the rate at which this reaction takes places. These reactions consist of the solubilization of amorphous silica and alumina compounds in a highly alkaline medium. The feasibility of silica compounds dissolution depends on several factors, the most significant being crystallization degree, alkalinity of the solution and size and fineness of the material. The most common practice for soil stabilization, besides compaction, includes the addition of lime or cement into clay soil systems. In presence of water this process results in cation exchange, hydration and pozzolanic reactions [38]. Pozzolanic reaction is a slow process dominant in lime soil stabilization. Hydrated lime dissolves into pore water increasing calcium concentration and hydroxyl ions, which react with aluminum and silica minerals present in clay particles, generating hydration products. These chemical reactions can be expressed as follows [39–40]:
Ca2þ þ 2ðOHÞ1 þ Al2 O3 Ca2þ þ 2ðOHÞ1 þ SiO2
Calcium aluminate hydrate ðCAHÞ Calcium silicate hydrate ðCSHÞ
ð1Þ ð2Þ
Ca2þ þ 2ðOHÞ1 þ Al2 O3 Calciumaluminumsilicate hydrate ðCASHÞ ð3Þ Researches began to incorporate or replace lime and cement for soil stabilization by blast furnace slag or fly ash in order to reduce natural resources consumption and achieve environmental conservation [41–44]. This trend enhances sustainability in soil stabilization process considering that SS utilization involves a reduction in greenhouse gas emissions [45]. Pozzolanic activity between clay and the stabilizer agent determines the chemical, physical and mechanical properties of the mixture. Increasing pH by the presence of hydroxyls (OH–) promotes the development of pozzolanic reactions. Formed compounds (CSH, CAH, CASH) are responsible for improving the mixture properties by cementing action [44–46]. 3. Materials and methods 3.1. Steel slag (SS) SS used in this study was supplied by the ‘‘Ternium Siderar” group in Argentina, which produces approximately 230,000 tons of hot rolled steel per month, generating 45,000 tons of slag which is neither reused within the production process nor sold, in part, to cement industries. Physical characterization tests were performed by following the ASTM Standard methods in order to characterize the main properties of SS, including granulometry (ASTM D 422-63) [37] and specific gravity tests (ASTM D 854-02) [37]. The gradation curve is shown in Fig. 1, from which coefficients of curvature (Cc) and uniformity (Cu) were obtained. From the particle size analysis, SS is classified as poorly graded sand with low fine particle content (SP). Hence SS used in this work can also be referred as foundry sand. The main physical properties of SS are shown in Table 1. Scanning electronic microscopy (SEM) assays with complementary semi-quantitative chemical analysis (EDAX) were carried out on SS samples. The slag particles have various forms, from sub-angular to sub-rounded. As shown in Fig. 2; larger particles
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Fig. 1. Particle size analysis of tested materials.
3.2. Clayey silt
Table 1 Materials properties. Property
Clayey silt
Steel slag
Specific Gravity Particles < 0.075 mm [%] Particles < 0.002 mm [%] Liquid limit. LL [%] Plastic limit. PL [%] Coefficient of uniformity. Cu Coefficient of curvature. Cc SUCS Pozzolanic Index. PzI pH
2.64 83.43 9.00 24.70 19.02 20.00 2.06 CL-ML 1.2 8.74
3.16 2.88 0 – – 10.50 0.34 SP 0 12.63
possess asperities and are characterized by a porous, rough and heterogeneous surface. The main constituents of the SS are calcium, iron and oxygen, with small proportions of carbon, magnesium, aluminum, phosphorous and manganese.
Typical loess from Córdoba, Argentina was used in this study. This sediment was sampled 2 m depth in an exploratory excavation in the campus of Universidad Nacional de Córdoba. Once sampled, a composite specimen was prepared by quartering and sieving leaving a homogeneous portion. The obtained sample was tested in order to determine the main physical, chemical and mechanical properties of this sediment. All tests were performed by following the ASTM Standard Methods, including granulometry and specific gravity tests ASTM D 854, liquid limit and plasticity index ASTM D 4318, unit weight ASTM D 4254 and unified soil classification ASTM D 2487-00 [37]. The obtained results are shown in Table 1. Complimentary SEM-EDAX tests were also performed in the clayey silt in order to identify main features at particle level and its chemical composition. From the chemical composition, tested soil can be classified as a natural pozzolanic material due to the presence of high percentages of SiO2 and Al2O3 mainly associated
Fig. 2. Scanning electronic microscopy images of tested materials, a) steel slag, b) clayey silt.
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to the fractions of quartz, feldspars, mica and volcanic ash identified in this material. 3.3. Specimen preparation The loessical soil and steel slag were oven-dried at a temperature of ±105 °C during 24 h until obtaining constant weight. Then, mixtures of soil with different contents of SS were prepared (0, 12, 24% by weight). The raw sample of SS was used for the mixtures including all the particle fractions shown in Fig. 1. Compaction tests were performed for each mixture. SS-soil mixtures were compacted in a 50 mm in diameter and 100 mm in height cell, filling it in three layers by using the standard Proctor energy by following the ASTM D 4254 [37]. The obtained results are shown in Fig. 3. The maximum dry density of SS-soil mixtures is in the range of 16 kN/m3 to 17.5 kN/m3, and the optimum water content is between 18 and 22%. The maximum dry density increased with the slag content in agreement with previously reported results [29,44,47]. The increase observed in unit weight can be associated with the higher bulk specific gravity of SS, which is greater than the one of soil, and the amount of SS added to the mixture. 3.4. Test procedures Several identical specimens of compacted SS-soil mixtures were prepared and stored in a humid chamber under controlled temperature and humidity conditions for different curing periods. Stored samples were wrapped in film paper, in order to avoid contact with external oxygen and to prevent moisture loss. Different tests were performed in these specimens in order to determine the influence of time on the physical and mechanical properties of mixtures. The identical specimens were tested at different curing times: 0, 7, 28 and 56 days. The ones with curing time equal to 0 days were tested within one hour after compaction. The weight of each specimen was monitored during curing and no significant variations were detected. Compacted SS-soil mixtures, with 12% of slag content, and curing period equal to 0 and 56 days were carefully segmented and observed under a laser confocal microscope. These observations were made by using an Olympus OLS 4000 LEXT combining the laser acquired images and the color images to produce a highresolution 3D image of the porous media under test. This test
Fig. 3. Effect of steel slag on compaction test.
was combined with SEM-EDAX analysis in order to obtain the main composition of the samples identifying changes at particle level produced during the curing period. Thin sections of selected specimens having different curing time were carefully prepared. These thin sections were used to identify changes in soil particles and pores by means of optical observations by using the Leica DM EP polarization optical microscope and also to perform SEM-EDAX analysis. From the last, the main chemical features and compositional maps were obtained. Measurements of pH and electrical conductivity were carried out for the SS and a compacted mixture of clayey silt with 12% of SS and curing periods equal to 0, 7 and 28 days. The pH was measured in a distilled water solution with 5 g of solid dry material under constant agitation at 20 ± 1 °C at 1-minute intervals during 15 min in order to verify the existence of appropriate conditions that may favor pozzolanic reactions (pH > 12.63). Luxan’s method was carried out for electrical conductivity measurements. It consists of determining the drop in electrical conductivity that occurs after 10 min in a 200 mL calcium hydroxide saturated solution to which 5 g of solid dry material are added (soil, slag or mixtures from both materials), keeping this solution in agitation during the measurement time [48]. This is an indirect method that aims to achieve a characterization of different materials based on pozzolanic activity. Following Luxan’s method, pozzolanicity index (PzI) is determined by the drop of electrical conductivity measured at the beginning of the test (EC0) and the one after 120 s (EC120):
PzI ¼ EC 0 EC 120
ð4Þ
According to this, materials can be classified as non-pozzolanic when PzI < 0.4 mS/cm, with intermediate pozzolanicity if 0.4 < Pz I < 1.2 mS/cm, and as high pozzolanic material for PzI > 1.2. Unconfined compressive strength (UCS) tests were conducted on the compacted SS-soil specimens (ASTM D2166) [37] aiming to get mechanical characteristics of these mixtures. Specimens having different curing periods were tested in order to evaluate the influence of time on changes at particle level that arise at macro-scale as modifications in the UCS and stiffness.
4. Results and analysis 4.1. Clayey silt-SS mixture properties Fig. 4 shows high resolution 3D images obtained by means of laser confocal microscope in the specimens with 12% of SS prepared with optimal humidity conditions and standard energy compaction. Two identical specimens were tested; one the day it was prepared (Fig. 4a) and other after 56 day of curing (Fig. 4b). These 3D images allow reconstructing the pore structure of the specimen under test. Obtained 3D images show a remarkable difference between surfaces at 0 and 56 days of curing. Larger pores with depths of 550 lm were measured the day of compaction meanwhile after 56 days of curing they reduced to 180 lm in depth. This effect is responsible for the more uniform pore size distribution observed in Fig. 4b in comparison with Fig. 4a. Similar trends were obtained for other curing times and slag content. The evolution of pores, particle arrangements and surficial features were also identified by means of SEM images (Fig. 5). Similar trends to those observed with laser confocal microscope were obtained. The specimen with 56 days of curing (Fig. 5b) reveals a homogeneous microstructure with smaller pores and a more regular particle arrangement than the one without curing time (Fig. 5a). Changes observed in Figs. 4 and 5 are attributed here to the development of pozzolanic reactions during curing. The mechanisms of gel formation due to hydration products reduce the porosity of specimens as reported by several authors [44,49]. The pH of
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Fig. 4. 3D confocal laser-scanning microscopy image for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 58 days of curing.
Fig. 5. SEM image for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 56 days of curing.
the solution in contact with the SS grew from 7 (reference level of distilled water) to 12.7 after 15 min and still keeping a slightly increasing tendency. In case of SS-soil mixtures measured pH were higher than 9 in all cases. These results allowed verifying that SS develops an alkaline environment that favors the pozzolanic type reactions, stimulated by: a) presence of iron and calcium hydroxide contributed by the slag (activator), b) the presence of amorphous silica and clay minerals of the clayey silt (siliceous compounds), and c) the presence of water added during the compaction test. Hence, necessary conditions to develop pozzolanic reactions are fulfilled. In order to prove and quantify these reactions within the matrix of compacted clayey silt-SS mixtures the pozzolanicity index (PzI) was determined (Eq. (4)). The evolution of EC with time
for the clayey silt with 12% of SS was measured in mixtures with 0, 7 and 28 days of curing (Electronic Supplementary Material). The EC of pure SS is also shown as reference. From these results, SS can be classified as non-pozzolanic material as it maintains constant electrical conductivity over time. In contrast, the EC of soilSS mixtures changes over time indicating some pozzolanic activity. Obtained pozzolanicity index are PzI = 1.2, 0.9 and 0.7 for curing times equal to 0, 7 and 28 days respectively. The decrease of PzI with the curing period clearly shows that pozzolanic reaction arises and consumes in part the mixture reaction capacity. The formation of these hydration products was verified on the thin sections after microscope observations and from SEM images. Siliceous compounds present on the loessical soil form CAH, CSH or
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CASH compounds after the pozzolanic reaction, which are responsible for the more homogeneous surface roughness and pore sizes shown in Figs. 4 and 5. Fig. 6 shows microscope images were a coarse SS particle is surrounded by the fine soil, at 0 and 56 days of curing. A clear light grey halo can be observed around the SS particles when the curing time was 56 days (Fig. 6b). This trend was observed in all specimens and can be related to the formation of the hydration products (CAH, CSH or CASH compounds).
UCSs were obtained for the specimens compacted with lower moisture content while less important changes in stiffness or deformation modulus were observed. The maximum resistance achieved in the dry-side of the compaction curve can be related to the expected soil microstructure. Particle arrangements in case of clayey soils compacted in dry-side of the compaction curve tends to be flocculated with a more random orientation of platy particles allowing capillary forces to play an important role on UCS and stiffness [7,52,53].
4.2. Mechanical behavior Fig. 7 shows the UCS of compacted clayey with 0, 12 and 24% of SS, prepared at the maximum dry density and optimum water content, measured at 0, 7, 28 and 56 days of curing. All mixtures tested immediately after preparation with 0 days of curing showed no influence of the amount of SS and develop UCS 15 kPa and negligible variations in stiffness (Fig. 7a). After 7 days of curing the specimens containing SS started to develop greater UCS and higher stiffness (Fig. 7b). This trend becomes more significant for 28 and 56 days of curing (Fig. 7c and 7d, respectively). The increase in UCS and stiffness with the curing time observed in all compacted clayey soil-SS mixtures is attributed here to the natural cementation that occurs due the interaction between the steel slag and the vitreous fraction of the clayey silt through pozzolanic reactions. This trend is also in good agreement with the results shown in Figs. 4–6. However, few differences can be observed between the specimens compacted with 12 and 24% of SS, which is associated to the higher number of coarse particles: as the SS content increase, a higher number of coarse particles are present on the mixture so the volume ratio between soil fraction and SS decrease. Higher amounts of coarse particles results on a worse accommodation of particles, which is counterproductive to specimen’s mechanical behavior as reported by several authors [44,50,51]. Moisture content during compaction affects the obtained dry unit weight of the specimens when compacted under controlled energy, as shown in Fig. 3. The UCS of compacted specimens with 12% of SS and different moisture content were measured after 0, 7 and 28 days of curing. Obtained results for 7 and 28 days of curing are shown in Fig. 8. All specimens compacted in the dry-side of the compaction curve (moisture content lower than the optimum) showed a fragile behavior meanwhile those compacted on the wet-side developed ductile behavior. Also, all specimens reached similar UCS at 7 days of curing but remarkable differences were observed after 28 days of curing General trend shows that higher
5. Discussion The main function of SS consists in generating the alkaline environment to favor the solubilization of the siliceous compounds of the fine fraction in presence of water. This produces the proper environment for the pozzolanic reactions to develop and therefore a natural cementation between particles is expected. This phenomenon arises as a macroscale process promoting an increase in UCS as shown in Figs. 7 and 8. Given that negligible variations in the samples weight were observed during curing, the increases in UCS are attributed here to the development of pozzolanic activity, as previously shown in literature [30,51] (See also Electronic Supplementary Material). Even though no significant variations of weight during curing were observed, the final moisture content of identical specimens cured during different periods varies. Fig. 9 shows the moisture content of clayey silt samples mixed with 12% and 24% of SS after 0, 7 and 28 days of curing. Decrease in moisture content becomes more important as curing period increases while negligible changes in mass are observed. These results indicate that some free water molecules become no longer available after the pozzolanic reactions between the fine fraction and the SS takes place. These small changes of humidity prove that the volumetric content of water is consumed throughout the curing time favoring the formation of calcium aluminate hydrate, calcium silicate hydrate, and calcium aluminum silicate hydrate as a result of the cementing process [51,54]. These hydrated compounds can also be responsible for the more homogeneous features observed in the 3D confocal laser-scanning microscopy and SEM images due to the cementing products that pozzolanic reaction generate. All these macroscopic changes along with the natural cementation of clayey particles emerge at macroscale as increases in the UCS. Pozzolanic reactions involve the sol-
Fig. 6. Leica DM EP polarization optical images for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 56 days of curing.
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Fig. 7. Influence of curing time on the UCS of compacted clayey silt specimens, a) curing time = 0 days, b) curing time = 7 days, c) curing time = 28 days, d) curing time = 56 days.
Fig. 8. Mechanical behavior of 12% SS-loessical silt samples with different water content: a) 7 days curing time, b) 28 days curing time.
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material recovered after the compression test. This allowed capture particle size variations associated to the curing time. In the case of clayey silt mixed with 12% of slag the amount of particles passing the sieve #200 ratio reduces from 73% to 59% in weight (particle size lower than 75 lm). This result indicates that the coarse fraction increases which confirms that after the curing time fine particles are aggregated and cemented between themself and with slag particles. This change also emerges as differences in the granulometric curve shape captured as an increase in the uniformity coefficient from 16.40 to 31.5 after 56 days of curing. All these changes are attributed here to the pozzolanic reaction that generates hydration products as CAH, CSH and CASH (Eqs. (1)–(3)) explaining the trends observed for UCS results.
6. Conclusions
Fig. 9. Influence of curing time on compacted specimens weight and moisture content.
ubilization of silica in the alkaline environment imposed by the addition of SS on the specimens. Then silica consumes during reaction and is no longer available. Fig. 10 shows silica compositional maps for the specimen with 12% of SS after 0 and 56 days of curing. This images show that the specimen with 0 days of curing has a well-defined interface between the coarse SS particle and the soil, while in the case of the specimen with 56 days of curing the boundary between the SS and soil fraction becomes fuzzy and undefined. The compositional map presented in Fig. 10a show that uniform silica content is observed in all silt matrix until the SS edge. Conversely, Fig. 10b shows that after 56 days of curing the silica content is uniform far from the SS particle while surrounding the particle there is a region where there is no more available silica. This region where there is no available silica coincides with the halo observed around SS particles in Fig. 6b, and is attributed here to the development of pozzolanic reactions. In order to generate additional data related to particle cementation, complimentary granulometric analysis were performed in the
This paper analyzes the potential of steel slag reutilization aiming to stabilize compacted clayey silty soils. The effect of the slag content on the stiffness and strength of stabilized soils was studied. The main conclusions obtained are summarized as follows: - The addition of steel slag in the compaction of clayed loessical soils produces an increase in strength. It was observed that 12% addition of SS by weight of dry soil increases 200% the UCS. - The pH of water increases when are in contact with BOF slags. This increase in pH and the acid neutralization capacity of SS affects physical and mechanical properties of compacted soilSS mixtures. The high pH promotes chemical reactions with amorphous silica and clay particles present in the tested soil favoring environmental conditions to develop pozzolanic reactions. Under this condition, iron and calcium oxides present in the SS reacts with the amorphous silica and clay particles of the soil. The pozzolanic activity of this material could be demonstrated by means of the PzI and macroscopically emerges as higher expected UCS for the soil-slag mixture over time. The consumption of available silica surrounding SS particles is a direct evidence of the pozzolanic reaction, which is responsible for the natural cementation and the observed increase in the UCS.
Fig. 10. Compositional maps of silica, obtained from SEM analysis of 12% SS-loessical silt samples after: a) 0 days curing time, b) 56 days curing time.
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- The addition of SS into the clayey silty soil promotes evolves in soil structure, obtained from the changes in the microporous soil fabric due to the formation of hydration products (e.g. CAH, CSH and CASH) and the bonding of fine particles between themselves and to the SS particles. This phenomenon produces changes on the granulometric properties of the compacted soilSS mixture over time, which can be attributed mainly to natural cementation produced by pozzolanic reactions. These results confirm that SS can be used as industrial pozzolans for soil stabilization representing a significant contribution to minimize the amount of slag that is disposed as waste and at the same time reuse them in geotechnical or geo-environmental constructions.
[14] [15]
[16]
[17] [18]
[19]
[20]
7. Declarations of interest None.
[21]
Acknowledgements This research was partially financed by Consejo Nacional de Investigaciones Científicas y Tecnicas (CONICET), Argentina (grant number 11220100100390CO), Agencia Nacional de Promoción Científica y Tecnológica, Argentina (grant number PICT 20143101) and Secretaría de Ciencia y Tecnología, Universidad Nacional de Cordoba, Argentina (SECyT-UNC) (grant number 05/M265). C.A. M. thanks CONICET for the doctorate fellowships.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117901.
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[24]
[27]
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Enhanced mechanical behavior of compacted clayey silts stabilized by reusing steel slag Clara A. Mozejko a,b, Franco M. Francisca a,b,⇑ a
Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina Instituto de Estudios Avanzados en Ingeniería y Tecnología (IDIT), CONICET and Universidad Nacional de Córdoba, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina b
h i g h l i g h t s Clayey soils mechanical behavior is enhanced by pozzolanic reactions. Steel slag promotes changes in soil microstructure, pores and particle distribution. Strength and stiffness of compacted clays increases due to natural cementation. Pozzolanic reactions favors natural cementation of clayey and slag particles.
a r t i c l e
i n f o
Article history: Received 20 June 2019 Received in revised form 6 November 2019 Accepted 17 December 2019 Available online xxxx Keywords: Compacted clayey silt Steel slag Pozzolanic reactions Strength Natural cementation Soil fabric
a b s t r a c t Finding new uses and applications of byproducts has significant importance due to the present need of minimizing the consumption of natural resources, reducing the amount of the disposal of waste materials and finding novel sustainable applications for these materials. This paper evaluates the behavior of compacted clayey loessical soils stabilized by the addition of steel slag (SS). Used soil is a loessical material made by quarts, feldspar and volcanic glass in different proportions. Different physical, microscopical and mechanical tests were performed in compacted specimens of clayey soil-SS mixtures. The influence of SS content, compaction moisture content and curing time on strength, deformation modulus, particle size, and pore structure were evaluated. The results showed increases in soil resistance and stiffness with the curing time, which can be attributed to the natural cementation that takes place due to interaction between the vitreous fraction of clayey silty particles and SS through pozzolanic reactions. This type of reaction is also confirmed by means of a direct measurement of a pozzolanicity index, evolution of pore sizes with a formation of more homogeneous microstructures and associated reduction in water content. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The Argentinian clayey loess covers approximately 35% of the national territory, and is mainly distributed in the central zone of Argentina. This aeolian sediment can be classified as collapsible in the unstable group of soils according to its main physical and mechanical properties. Loess presents a metastable soil structure, presence of macro-pores, collapsible behavior due to increases in moisture content and/or external loads, and stiffness and shear strength highly dependent on the degree of saturation [1]. Collapsible silts and clays can be found in many places around the world ⇑ Corresponding author at: Facultad de Ciencias Exactas, Físicas y Naturales, Departamento de Construcciones Civiles, Vélez Sarsfield 1611, Ciudad Universitaria, X5016 CGA Córdoba, Argentina. E-mail address: [email protected] (F.M. Francisca). https://doi.org/10.1016/j.conbuildmat.2019.117901 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and are, therefore, frequently used as construction material for earth structures (e.g. levees, landfill liners, embankments, base of pavements, etc.) or for the foundation of civil infrastructure (e.g. pipes, tunnels, civil constructions) [2–4]. Clayey loess is also distinguished by a poorly accommodated structure with larger particles separated by open spaces. These particles are joined together by the accumulation or bridges of clayey material and precipitated salts. These joints reduce when moisture content increases, clay particles expand and precipitated salts dissolve, inducing sliding between grains and a significant decrease in voids volume [5,6]. In order to improve mechanical soil properties, densification of the material can be achieved by compaction. This soil stabilization technique produces a decrease in soil voids and compressibility and increase in strength and stiffness [7,8]. It has been proved that several properties of compacted clays, such as density, permeabil-
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ity, diffusion and collapse, depend on compaction moisture content among other things, indicating that soil structure is affected by the compaction procedure. This affects soil fabric and therefore has a fundamental influence on soil behavior. On a microscopic scale, compaction disturbs the original soil fabric, destroying shear planes and eliminating large pores, resulting in a more homogeneous soil fabric generating an alignment of particles. Other techniques are also available to improve mechanical properties of soils, as soil reinforcement methods and chemical stabilization. Different soil stabilizing materials can also be used to improve soil behavior including lime, cement and fly ash [9– 13]. However, these methods have significant environmental impacts associated in terms of energy or non-renewable resources consumption, so, in this way, the use of industrial by-products and metallic slags have been lately encouraged [14–16]. Steel and stainless-steel production generate a solid waste known as Steel Slag (SS) that can be classified according to the type of steel as carbon steel slag or stainless-steel slag. Also, it can be classified according to the manufacturing process as retreat slag, electric arc furnace, refining furnace, blast furnace or foundry residue. During manufacturing basic oxygen furnace steel (BOF) combines coke, produced from coal, with pulverized coal, which by a process of explosion at high temperatures forms hydrogen and carbon monoxide which acts as basic reducing agent for iron oxides. The furnace is loaded from the top with alternative layers of these reducing agents combined with the melting mineral; once mixed, oxygen is injected under pressure causing chemical reactions responsible of separating impurities from steel; these impurities flocculate, and when cooled, form the SS [17]. Using SS in different processes would help in alleviating the disposal problem and helping preservation of natural resources [18]. To date, studies have been focused on determining the viability of using SS as aggregates used in road construction or pavements [19–21] as aggregates in concrete [22–24], or as raw material in other productive processes [25–27]. However, the development of knowledge about SS behavior as stabilizing agent in soils is recent and scare. The use of grand granulated blast furnace slag for stabilized lime-clay soils have beneficial effects on strength and also in reducing the swelling potential of gypsum-bearing kaolinite clay when SS is added with an adequate amount of lime [28]. Also, the addition of SS to soils causes a decrease in plasticity index, increases the maximum dry density of clayey soils, and reduces the swelling potential of expansive clay minerals [29]. Other studies show that partial substitution of lime with ground granulated blast furnace slag is effective to reduce volumetric shrinkage [30]. In this way, some authors have begun to develop different approaches in which SS can be used for remediation of metal contaminated sites as components of permeable reactive barriers, as materials for landfill closures or as reactive material [31–36]. The objective of this research is to propose a sustainable way to reutilize SS to stabilize clayey loessical soils in an environmentally friendly way by studying the mechanical behavior of compacted steel slag-clayed silt mixtures. The main purpose includes to better understand the physicochemical mechanisms that are responsible for changes that take place at particle contacts, and the emergent behavior at macro-scale, such as changes in soil gradation, pore structure and mechanical behavior.
2. Pozzolanic reactions Pozzolans are siliceous and aluminous materials that have little or non-cementitious value but, under the presence of water and finely divided are able to chemically react with calcium hydroxide forming cementitious compounds (ASTM C618) [37].
The definition of pozzolan embraces a wild variety of materials with different properties, compositions and/or origins. They can be classified as natural or artificial materials; the first type is composed mainly by volcanic glass while the second type is a byproduct from high temperature industrial processes (e.g. fly ashes). The reactivity of pozzolans is attributed to the active silica forming silicate minerals. Pozzolanic activity refers to the maximum amount of calcium hydroxide with which pozzolan can combine and the rate at which this reaction takes places. These reactions consist of the solubilization of amorphous silica and alumina compounds in a highly alkaline medium. The feasibility of silica compounds dissolution depends on several factors, the most significant being crystallization degree, alkalinity of the solution and size and fineness of the material. The most common practice for soil stabilization, besides compaction, includes the addition of lime or cement into clay soil systems. In presence of water this process results in cation exchange, hydration and pozzolanic reactions [38]. Pozzolanic reaction is a slow process dominant in lime soil stabilization. Hydrated lime dissolves into pore water increasing calcium concentration and hydroxyl ions, which react with aluminum and silica minerals present in clay particles, generating hydration products. These chemical reactions can be expressed as follows [39–40]:
Ca2þ þ 2ðOHÞ1 þ Al2 O3 Ca2þ þ 2ðOHÞ1 þ SiO2
Calcium aluminate hydrate ðCAHÞ Calcium silicate hydrate ðCSHÞ
ð1Þ ð2Þ
Ca2þ þ 2ðOHÞ1 þ Al2 O3 Calciumaluminumsilicate hydrate ðCASHÞ ð3Þ Researches began to incorporate or replace lime and cement for soil stabilization by blast furnace slag or fly ash in order to reduce natural resources consumption and achieve environmental conservation [41–44]. This trend enhances sustainability in soil stabilization process considering that SS utilization involves a reduction in greenhouse gas emissions [45]. Pozzolanic activity between clay and the stabilizer agent determines the chemical, physical and mechanical properties of the mixture. Increasing pH by the presence of hydroxyls (OH–) promotes the development of pozzolanic reactions. Formed compounds (CSH, CAH, CASH) are responsible for improving the mixture properties by cementing action [44–46]. 3. Materials and methods 3.1. Steel slag (SS) SS used in this study was supplied by the ‘‘Ternium Siderar” group in Argentina, which produces approximately 230,000 tons of hot rolled steel per month, generating 45,000 tons of slag which is neither reused within the production process nor sold, in part, to cement industries. Physical characterization tests were performed by following the ASTM Standard methods in order to characterize the main properties of SS, including granulometry (ASTM D 422-63) [37] and specific gravity tests (ASTM D 854-02) [37]. The gradation curve is shown in Fig. 1, from which coefficients of curvature (Cc) and uniformity (Cu) were obtained. From the particle size analysis, SS is classified as poorly graded sand with low fine particle content (SP). Hence SS used in this work can also be referred as foundry sand. The main physical properties of SS are shown in Table 1. Scanning electronic microscopy (SEM) assays with complementary semi-quantitative chemical analysis (EDAX) were carried out on SS samples. The slag particles have various forms, from sub-angular to sub-rounded. As shown in Fig. 2; larger particles
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Fig. 1. Particle size analysis of tested materials.
3.2. Clayey silt
Table 1 Materials properties. Property
Clayey silt
Steel slag
Specific Gravity Particles < 0.075 mm [%] Particles < 0.002 mm [%] Liquid limit. LL [%] Plastic limit. PL [%] Coefficient of uniformity. Cu Coefficient of curvature. Cc SUCS Pozzolanic Index. PzI pH
2.64 83.43 9.00 24.70 19.02 20.00 2.06 CL-ML 1.2 8.74
3.16 2.88 0 – – 10.50 0.34 SP 0 12.63
possess asperities and are characterized by a porous, rough and heterogeneous surface. The main constituents of the SS are calcium, iron and oxygen, with small proportions of carbon, magnesium, aluminum, phosphorous and manganese.
Typical loess from Córdoba, Argentina was used in this study. This sediment was sampled 2 m depth in an exploratory excavation in the campus of Universidad Nacional de Córdoba. Once sampled, a composite specimen was prepared by quartering and sieving leaving a homogeneous portion. The obtained sample was tested in order to determine the main physical, chemical and mechanical properties of this sediment. All tests were performed by following the ASTM Standard Methods, including granulometry and specific gravity tests ASTM D 854, liquid limit and plasticity index ASTM D 4318, unit weight ASTM D 4254 and unified soil classification ASTM D 2487-00 [37]. The obtained results are shown in Table 1. Complimentary SEM-EDAX tests were also performed in the clayey silt in order to identify main features at particle level and its chemical composition. From the chemical composition, tested soil can be classified as a natural pozzolanic material due to the presence of high percentages of SiO2 and Al2O3 mainly associated
Fig. 2. Scanning electronic microscopy images of tested materials, a) steel slag, b) clayey silt.
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to the fractions of quartz, feldspars, mica and volcanic ash identified in this material. 3.3. Specimen preparation The loessical soil and steel slag were oven-dried at a temperature of ±105 °C during 24 h until obtaining constant weight. Then, mixtures of soil with different contents of SS were prepared (0, 12, 24% by weight). The raw sample of SS was used for the mixtures including all the particle fractions shown in Fig. 1. Compaction tests were performed for each mixture. SS-soil mixtures were compacted in a 50 mm in diameter and 100 mm in height cell, filling it in three layers by using the standard Proctor energy by following the ASTM D 4254 [37]. The obtained results are shown in Fig. 3. The maximum dry density of SS-soil mixtures is in the range of 16 kN/m3 to 17.5 kN/m3, and the optimum water content is between 18 and 22%. The maximum dry density increased with the slag content in agreement with previously reported results [29,44,47]. The increase observed in unit weight can be associated with the higher bulk specific gravity of SS, which is greater than the one of soil, and the amount of SS added to the mixture. 3.4. Test procedures Several identical specimens of compacted SS-soil mixtures were prepared and stored in a humid chamber under controlled temperature and humidity conditions for different curing periods. Stored samples were wrapped in film paper, in order to avoid contact with external oxygen and to prevent moisture loss. Different tests were performed in these specimens in order to determine the influence of time on the physical and mechanical properties of mixtures. The identical specimens were tested at different curing times: 0, 7, 28 and 56 days. The ones with curing time equal to 0 days were tested within one hour after compaction. The weight of each specimen was monitored during curing and no significant variations were detected. Compacted SS-soil mixtures, with 12% of slag content, and curing period equal to 0 and 56 days were carefully segmented and observed under a laser confocal microscope. These observations were made by using an Olympus OLS 4000 LEXT combining the laser acquired images and the color images to produce a highresolution 3D image of the porous media under test. This test
Fig. 3. Effect of steel slag on compaction test.
was combined with SEM-EDAX analysis in order to obtain the main composition of the samples identifying changes at particle level produced during the curing period. Thin sections of selected specimens having different curing time were carefully prepared. These thin sections were used to identify changes in soil particles and pores by means of optical observations by using the Leica DM EP polarization optical microscope and also to perform SEM-EDAX analysis. From the last, the main chemical features and compositional maps were obtained. Measurements of pH and electrical conductivity were carried out for the SS and a compacted mixture of clayey silt with 12% of SS and curing periods equal to 0, 7 and 28 days. The pH was measured in a distilled water solution with 5 g of solid dry material under constant agitation at 20 ± 1 °C at 1-minute intervals during 15 min in order to verify the existence of appropriate conditions that may favor pozzolanic reactions (pH > 12.63). Luxan’s method was carried out for electrical conductivity measurements. It consists of determining the drop in electrical conductivity that occurs after 10 min in a 200 mL calcium hydroxide saturated solution to which 5 g of solid dry material are added (soil, slag or mixtures from both materials), keeping this solution in agitation during the measurement time [48]. This is an indirect method that aims to achieve a characterization of different materials based on pozzolanic activity. Following Luxan’s method, pozzolanicity index (PzI) is determined by the drop of electrical conductivity measured at the beginning of the test (EC0) and the one after 120 s (EC120):
PzI ¼ EC 0 EC 120
ð4Þ
According to this, materials can be classified as non-pozzolanic when PzI < 0.4 mS/cm, with intermediate pozzolanicity if 0.4 < Pz I < 1.2 mS/cm, and as high pozzolanic material for PzI > 1.2. Unconfined compressive strength (UCS) tests were conducted on the compacted SS-soil specimens (ASTM D2166) [37] aiming to get mechanical characteristics of these mixtures. Specimens having different curing periods were tested in order to evaluate the influence of time on changes at particle level that arise at macro-scale as modifications in the UCS and stiffness.
4. Results and analysis 4.1. Clayey silt-SS mixture properties Fig. 4 shows high resolution 3D images obtained by means of laser confocal microscope in the specimens with 12% of SS prepared with optimal humidity conditions and standard energy compaction. Two identical specimens were tested; one the day it was prepared (Fig. 4a) and other after 56 day of curing (Fig. 4b). These 3D images allow reconstructing the pore structure of the specimen under test. Obtained 3D images show a remarkable difference between surfaces at 0 and 56 days of curing. Larger pores with depths of 550 lm were measured the day of compaction meanwhile after 56 days of curing they reduced to 180 lm in depth. This effect is responsible for the more uniform pore size distribution observed in Fig. 4b in comparison with Fig. 4a. Similar trends were obtained for other curing times and slag content. The evolution of pores, particle arrangements and surficial features were also identified by means of SEM images (Fig. 5). Similar trends to those observed with laser confocal microscope were obtained. The specimen with 56 days of curing (Fig. 5b) reveals a homogeneous microstructure with smaller pores and a more regular particle arrangement than the one without curing time (Fig. 5a). Changes observed in Figs. 4 and 5 are attributed here to the development of pozzolanic reactions during curing. The mechanisms of gel formation due to hydration products reduce the porosity of specimens as reported by several authors [44,49]. The pH of
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Fig. 4. 3D confocal laser-scanning microscopy image for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 58 days of curing.
Fig. 5. SEM image for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 56 days of curing.
the solution in contact with the SS grew from 7 (reference level of distilled water) to 12.7 after 15 min and still keeping a slightly increasing tendency. In case of SS-soil mixtures measured pH were higher than 9 in all cases. These results allowed verifying that SS develops an alkaline environment that favors the pozzolanic type reactions, stimulated by: a) presence of iron and calcium hydroxide contributed by the slag (activator), b) the presence of amorphous silica and clay minerals of the clayey silt (siliceous compounds), and c) the presence of water added during the compaction test. Hence, necessary conditions to develop pozzolanic reactions are fulfilled. In order to prove and quantify these reactions within the matrix of compacted clayey silt-SS mixtures the pozzolanicity index (PzI) was determined (Eq. (4)). The evolution of EC with time
for the clayey silt with 12% of SS was measured in mixtures with 0, 7 and 28 days of curing (Electronic Supplementary Material). The EC of pure SS is also shown as reference. From these results, SS can be classified as non-pozzolanic material as it maintains constant electrical conductivity over time. In contrast, the EC of soilSS mixtures changes over time indicating some pozzolanic activity. Obtained pozzolanicity index are PzI = 1.2, 0.9 and 0.7 for curing times equal to 0, 7 and 28 days respectively. The decrease of PzI with the curing period clearly shows that pozzolanic reaction arises and consumes in part the mixture reaction capacity. The formation of these hydration products was verified on the thin sections after microscope observations and from SEM images. Siliceous compounds present on the loessical soil form CAH, CSH or
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CASH compounds after the pozzolanic reaction, which are responsible for the more homogeneous surface roughness and pore sizes shown in Figs. 4 and 5. Fig. 6 shows microscope images were a coarse SS particle is surrounded by the fine soil, at 0 and 56 days of curing. A clear light grey halo can be observed around the SS particles when the curing time was 56 days (Fig. 6b). This trend was observed in all specimens and can be related to the formation of the hydration products (CAH, CSH or CASH compounds).
UCSs were obtained for the specimens compacted with lower moisture content while less important changes in stiffness or deformation modulus were observed. The maximum resistance achieved in the dry-side of the compaction curve can be related to the expected soil microstructure. Particle arrangements in case of clayey soils compacted in dry-side of the compaction curve tends to be flocculated with a more random orientation of platy particles allowing capillary forces to play an important role on UCS and stiffness [7,52,53].
4.2. Mechanical behavior Fig. 7 shows the UCS of compacted clayey with 0, 12 and 24% of SS, prepared at the maximum dry density and optimum water content, measured at 0, 7, 28 and 56 days of curing. All mixtures tested immediately after preparation with 0 days of curing showed no influence of the amount of SS and develop UCS 15 kPa and negligible variations in stiffness (Fig. 7a). After 7 days of curing the specimens containing SS started to develop greater UCS and higher stiffness (Fig. 7b). This trend becomes more significant for 28 and 56 days of curing (Fig. 7c and 7d, respectively). The increase in UCS and stiffness with the curing time observed in all compacted clayey soil-SS mixtures is attributed here to the natural cementation that occurs due the interaction between the steel slag and the vitreous fraction of the clayey silt through pozzolanic reactions. This trend is also in good agreement with the results shown in Figs. 4–6. However, few differences can be observed between the specimens compacted with 12 and 24% of SS, which is associated to the higher number of coarse particles: as the SS content increase, a higher number of coarse particles are present on the mixture so the volume ratio between soil fraction and SS decrease. Higher amounts of coarse particles results on a worse accommodation of particles, which is counterproductive to specimen’s mechanical behavior as reported by several authors [44,50,51]. Moisture content during compaction affects the obtained dry unit weight of the specimens when compacted under controlled energy, as shown in Fig. 3. The UCS of compacted specimens with 12% of SS and different moisture content were measured after 0, 7 and 28 days of curing. Obtained results for 7 and 28 days of curing are shown in Fig. 8. All specimens compacted in the dry-side of the compaction curve (moisture content lower than the optimum) showed a fragile behavior meanwhile those compacted on the wet-side developed ductile behavior. Also, all specimens reached similar UCS at 7 days of curing but remarkable differences were observed after 28 days of curing General trend shows that higher
5. Discussion The main function of SS consists in generating the alkaline environment to favor the solubilization of the siliceous compounds of the fine fraction in presence of water. This produces the proper environment for the pozzolanic reactions to develop and therefore a natural cementation between particles is expected. This phenomenon arises as a macroscale process promoting an increase in UCS as shown in Figs. 7 and 8. Given that negligible variations in the samples weight were observed during curing, the increases in UCS are attributed here to the development of pozzolanic activity, as previously shown in literature [30,51] (See also Electronic Supplementary Material). Even though no significant variations of weight during curing were observed, the final moisture content of identical specimens cured during different periods varies. Fig. 9 shows the moisture content of clayey silt samples mixed with 12% and 24% of SS after 0, 7 and 28 days of curing. Decrease in moisture content becomes more important as curing period increases while negligible changes in mass are observed. These results indicate that some free water molecules become no longer available after the pozzolanic reactions between the fine fraction and the SS takes place. These small changes of humidity prove that the volumetric content of water is consumed throughout the curing time favoring the formation of calcium aluminate hydrate, calcium silicate hydrate, and calcium aluminum silicate hydrate as a result of the cementing process [51,54]. These hydrated compounds can also be responsible for the more homogeneous features observed in the 3D confocal laser-scanning microscopy and SEM images due to the cementing products that pozzolanic reaction generate. All these macroscopic changes along with the natural cementation of clayey particles emerge at macroscale as increases in the UCS. Pozzolanic reactions involve the sol-
Fig. 6. Leica DM EP polarization optical images for compacted clayey silt with 12% of SS, a) with 0 days of curing, b) after 56 days of curing.
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Fig. 7. Influence of curing time on the UCS of compacted clayey silt specimens, a) curing time = 0 days, b) curing time = 7 days, c) curing time = 28 days, d) curing time = 56 days.
Fig. 8. Mechanical behavior of 12% SS-loessical silt samples with different water content: a) 7 days curing time, b) 28 days curing time.
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material recovered after the compression test. This allowed capture particle size variations associated to the curing time. In the case of clayey silt mixed with 12% of slag the amount of particles passing the sieve #200 ratio reduces from 73% to 59% in weight (particle size lower than 75 lm). This result indicates that the coarse fraction increases which confirms that after the curing time fine particles are aggregated and cemented between themself and with slag particles. This change also emerges as differences in the granulometric curve shape captured as an increase in the uniformity coefficient from 16.40 to 31.5 after 56 days of curing. All these changes are attributed here to the pozzolanic reaction that generates hydration products as CAH, CSH and CASH (Eqs. (1)–(3)) explaining the trends observed for UCS results.
6. Conclusions
Fig. 9. Influence of curing time on compacted specimens weight and moisture content.
ubilization of silica in the alkaline environment imposed by the addition of SS on the specimens. Then silica consumes during reaction and is no longer available. Fig. 10 shows silica compositional maps for the specimen with 12% of SS after 0 and 56 days of curing. This images show that the specimen with 0 days of curing has a well-defined interface between the coarse SS particle and the soil, while in the case of the specimen with 56 days of curing the boundary between the SS and soil fraction becomes fuzzy and undefined. The compositional map presented in Fig. 10a show that uniform silica content is observed in all silt matrix until the SS edge. Conversely, Fig. 10b shows that after 56 days of curing the silica content is uniform far from the SS particle while surrounding the particle there is a region where there is no more available silica. This region where there is no available silica coincides with the halo observed around SS particles in Fig. 6b, and is attributed here to the development of pozzolanic reactions. In order to generate additional data related to particle cementation, complimentary granulometric analysis were performed in the
This paper analyzes the potential of steel slag reutilization aiming to stabilize compacted clayey silty soils. The effect of the slag content on the stiffness and strength of stabilized soils was studied. The main conclusions obtained are summarized as follows: - The addition of steel slag in the compaction of clayed loessical soils produces an increase in strength. It was observed that 12% addition of SS by weight of dry soil increases 200% the UCS. - The pH of water increases when are in contact with BOF slags. This increase in pH and the acid neutralization capacity of SS affects physical and mechanical properties of compacted soilSS mixtures. The high pH promotes chemical reactions with amorphous silica and clay particles present in the tested soil favoring environmental conditions to develop pozzolanic reactions. Under this condition, iron and calcium oxides present in the SS reacts with the amorphous silica and clay particles of the soil. The pozzolanic activity of this material could be demonstrated by means of the PzI and macroscopically emerges as higher expected UCS for the soil-slag mixture over time. The consumption of available silica surrounding SS particles is a direct evidence of the pozzolanic reaction, which is responsible for the natural cementation and the observed increase in the UCS.
Fig. 10. Compositional maps of silica, obtained from SEM analysis of 12% SS-loessical silt samples after: a) 0 days curing time, b) 56 days curing time.
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- The addition of SS into the clayey silty soil promotes evolves in soil structure, obtained from the changes in the microporous soil fabric due to the formation of hydration products (e.g. CAH, CSH and CASH) and the bonding of fine particles between themselves and to the SS particles. This phenomenon produces changes on the granulometric properties of the compacted soilSS mixture over time, which can be attributed mainly to natural cementation produced by pozzolanic reactions. These results confirm that SS can be used as industrial pozzolans for soil stabilization representing a significant contribution to minimize the amount of slag that is disposed as waste and at the same time reuse them in geotechnical or geo-environmental constructions.
[14] [15]
[16]
[17] [18]
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7. Declarations of interest None.
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Acknowledgements This research was partially financed by Consejo Nacional de Investigaciones Científicas y Tecnicas (CONICET), Argentina (grant number 11220100100390CO), Agencia Nacional de Promoción Científica y Tecnológica, Argentina (grant number PICT 20143101) and Secretaría de Ciencia y Tecnología, Universidad Nacional de Cordoba, Argentina (SECyT-UNC) (grant number 05/M265). C.A. M. thanks CONICET for the doctorate fellowships.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117901.
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