Properties of Marl-Cement-EAFD Mixtures
The compaction tests were conducted on marl mixtures containing 0-, 5-, 10-, 20- or 30%-EAFD to determine the maximum dry density and optimum moisture content. As seen in Fig. 2, the density increased marginally with increasing EAFD content since the specific gravity of EAFD (2.76) is more than that of marl (2.69). The maximum dry density was in the range of 1.90 to 2.23 g/cm3, while the optimum moisture content was in the range of 7.6 to 9.2%.
The UCS of the 0-, 5-, 10-, 20- or 30%-EAFD marl mixtures was in the range of 644 to 2,430 kPa, as shown in Fig. 3a. A significant increase in the UCS was noted at higher EAFD content. There was almost four times increase in the UCS due to the addition of 30% EAFD.
It must be noted that according to the ACI Committee 230 guidelines, the minimum strength requirement for soils to be used in sub-base course in rigid pavements is 1,380 kPa19. This requirement is fulfilled by the marl mixture with 20% EAFD. The corresponding requirement for flexible pavement is 1,725 kPa19, which is satisfied by the marl mixture with 30% EAFD.
The soaked CBR of marl increased linearly with the quantity of EAFD, as shown in Fig. 3b. The minimum soaked CBR requirement for a soil to be suitable for application as base-course in both flexible and rigid pavements is 50%20,21, which is satisfied by all the examined marl-EAFD-cement mixtures.
Moisture and temperature changes can produce wet and dry or freeze and thaw cycles. Hence, stabilized soils should be sufficiently durable to maintain their dimensional stability under these conditions. Consequently, a durability evaluation was conducted on the mixtures that satisfied the UCS requirements (with 20 and 30% EAFD).
The weight loss (indicative of durability) in the marl-cement mixtures with 20 and 30% EAFD was 9 and 8%, respectively. Both of these values are less than the maximum allowable value of 14% specified by the Portland Cement Association for soils classified as SP and 11% required by the US Corps of Engineers for soils with a Plasticity Index (PI) of less than ten19.
The concentration of cadmium and lead in the marl-cement mixture with 20% EAFD was 0.58 and 0.12 mg/l, respectively, and the corresponding values for the 30%-EAFD mixture were 0.67 and 0.17 mg/l, respectively. These values are well below the corresponding allowable values of 1 and 5 mg/l22. Therefore, the examined EAFD-stabilized marl mixtures fulfilled the leachability requirements.
Properties of Sand-Cement-EAFD Mixtures
The maximum dry density for sand-cement-EAFD (0, 5, 10, 20 and 30%) was in the range of 1.78 to 2.19 g/cm3, as shown in Fig. 4. The corresponding optimum moisture content was in the range of 10.0 to 7.8%. The density increased with increasing quantity of EAFD. However, there was not much increase in the optimum moisture content.
The UCS of sand-cement-EAFD mixtures was in the range of 369 to 2,419 kPa (Fig. 5a). It may be noted that the mixtures with 20- and 30%-EAFD fulfilled the minimum strength requirements of 1,380 and 1,725 kPa for the sub-base course in rigid and flexible pavements, respectively19.
The soaked CBR of sand-cement-EAFD mixtures was in the range of 171 to 750%, as shown in Fig. 5b. These values are much more than the required value of 50%.
The weight loss of the mixtures with 20- and 30%-EAFD was 9.1 and 7.2%, respectively. Since these two mixtures satisfied the durability requirements, they could be used in the sub-base in both rigid and flexible pavements.
The concentration of cadmium and lead in the sand-cement mixtures with 20 and 30% EAFD was 0.246 and 0.186 and 0.819 and 0.969 mg/l, respectively. These values are well below the US Environmental Protection Authority (USEPA) restrictions on these metals22.
Mechanisms of Stabilization
The morphology and mineralogical composition of the stabilized marl and sand are mixtures utilized to study the role of EAFD in improving their UCS and soaked CBR of the investigated sand and marl.
Stabilization of Sand-Cement-EAFD Mixtures
The SEM micrograph of sand-cement plus 20% EAFD, shown in Fig. 6a, indicates the formation of a porous cementing gel, i.e., C-S-H, which develops as a result of the addition of 2% cement to the sand. In contrast, the SEM micrograph of the sand-cement mixture with 30% EAFD, shown in Fig. 6b, shows a dense microstructure.
Compared with the XRD spectra of sand without admixtures (Fig. 6a), which shows mainly quartz, the spectra of the sand-cement-30% EAFD mixture (Fig. 6b) indicates the formation of quartz (SiO2, 47%), calcite (CaCO3, 2.6%), goethite (FeO(OH), 19.6%) and wustite (FeO, 30.9%). These minerals are formed due to the reaction between sand, cement and EAFD. It may be noted that wustite, in the presence of 2% cement, has good cementing property17,23,24,25 and could, therefore, be the main factor causing the observed dense microstructure in the sand-cement-EAFD (30%) mixture, as shown in Fig. 6b.
Since the electro-negativities of Fe (1.8, Pauling) and Si (1.9 Pauling) are almost the same26, the interaction between them is weak. Therefore, in the absence of cement (that is, insufficient Ca), the interaction is retarded and there is no improvement in the UCS. However, improved UCS was measured in sand-cement mixtures with varying quantities of EAFD. This may be attributed to the fact that the low-electronegativity (1.0 Pauling) of Ca in cement facilitates the formation of stronger bonds with both Fe and Si. Under this scenario, the Ca atom is oriented between Fe and Si, thereby generating a binding reaction between silica (quartz from sand), calcium hydroxide (from cement) and iron oxide (wustite and goethite from EAFD), as shown in the XRD pattern in Fig. 7. Therefore, the authors propose the binding sequence, shown in Fig. 8, for the stabilization of sand mixture with 2% cement plus 30% EAFD.
Stabilization of Marl-Cement-EAFD Mixtures
The micrograph of marl stabilized with 2% cement (Fig. 9a) shows a porous microstructure with isolated voids and lack of sufficient cementing gel, suggesting the need for additional stabilizer to create a dense matrix. Comparatively, the SEM of marl stabilized with 2% cement plus 30% EAFD (Fig. 9b) shows a dense morphology, thereby confirming the development of extensive cementitious matrix due to the addition of EAFD.
The XRD spectra (Fig. 10) of marl stabilized with 2% cement and 30% EAFD indicate the formation of ankerite ((CaMg0.3(CO3)2, 56.6%), wustite (FeO; 20.4%), quartz (SiO2; 16.1%) and calcite (CaCO3; 6.9%) while these compounds are not noted in marl with cement only. The profuse formation of ankerite and wustite, in the presence of 30% EAFD along with the 2% cement, contributes to the significant increase in the UCS and soaked CBR owing to the excellent cementing properties of these two minerals17,23,24,25. These improved properties will certainly lead to stable soil mixtures in highway pavements.
The XRD data explain the improvement in the mechanical properties of the marl mixture with cement and EAFD. Quartz and dolomite, in addition to a marginal quantity of calcite, were observed in marl with cement only (Fig. 10a), whereas quartz, wustite and ankerite were present in the XRD spectra of the mixture containing marl, 2% cement plus 30% EAFD (Fig. 10b). As a result of the presence of wustite, the proposed interaction of Si-Ca-Fe discussed earlier may also be partly applicable to the marl mixtures. Based on this assumption, a further interaction between dolomite [CaMg(CO3)2] (present in the marl), calcium hydroxide (from the cement) and iron oxide (from EAFD) is proposed. This mechanism is based on the electronegativity differences of Mg (1.3 Pauling), Ca (1.0 Pauling) and Fe (1.8 Pauling). Because of its high electronegativity, Fe can form a strong bond with Ca which can then make another bond with Mg to achieve a complete outermost shell with eight electrons. This scenario orients the Ca atom between Mg and Fe, thereby generating a reaction between dolomite, calcium hydroxide and iron oxide. Thus, the interaction illustrated in Fig. 11 is proposed for the formation of ankerite in marl with 2% cement plus 30% EAFD. The improvement in the strength of this mixture can, therefore, be attributed to the binding effect of EAFD through the formation of both wustite and ankerite.