3.1 Effect of temperature on gradation of South Iran Marl
The gradation curve and changes in the percentage of clay-sized particles in the marl soil are illustrated in Fig. 2. According to the results, the gradation curve does not change significantly as the temperature increased to 110°C. As the temperature increased to 300°C, the diameter of particles finer than 0.01 mm decreased by about 8% and those finer than 0.002 mm by 6%. Raising the temperature to 500°C resulted in a significant increase in the particle size as the percentage of clay-sized particles (0.002 mm) decreased from 34 to 9%. The start of dehydroxylation and structural destruction of the clay soil may lead to an increase in the particle size. The crystal structures in the clay minerals are broken down and connected through chemical bonds to form larger particles, reducing the percentage of clay-sized particles by 15%.
On the other hand, due to cementation caused by the formation of some salts and oxides, heating results in the combination and cohesion of particles, thus altering soil characteristics (Zhang et al., 2018). This process can affect soil gradation and its engineering properties. Elevating the temperature beyond 500°C did not significantly change the soil gradation so that no significant difference was found in the percentage of clay-sized particles at 900 and 700°C.
3.2 Effect of temperature on atterberg limits of South Iran Marl
The studied natural marl had a liquid limit of 28 and a plasticity limit of 18. According to the results in Fig. 3, elevating the temperature to 110, 200, and 300°C reduces the liquid limit by 0.5, 3, and 4 units, respectively.
On the other hand, the plasticity index decreased by only one unit as the temperature increased to 300°C. By further increasing the temperature to 500°C, the liquid limit decreased by another seven units, showing a transition in the soil behavior (classification) from plastic to granular behavior. It must be noted that the specimens immediately set when exposed to moisture at 700°C. This can be related to the formation of cement compounds in the soil.
The change in the Atterberg limits at different temperatures is a function of the type of clay mineral and the temperature. Dehydroxylation causes destruction of the structure of clay minerals in the marl soil, affecting the Atterberg limits of the soil. The plasticity limit also declined to 15 as the temperature increased to 300°C where dehydration took place. It should be noted that further heating the soil made it non-plastic, making it impossible to conduct the PL test. The decline of Atterberg limits can be related to the structural changes in the soil due to heating and the shift in the soil behavior (Abu-Zreig et al., 2001; Rondón-Quintana et al., 2020).
3.3 XRD analysis of heat-treated marl
Based on the results in Fig. 4, the intensity of the palygorskite characteristic peak (3.17 Å) was not significantly affected by raising the temperature to 110°C and was reduced by nearly 73 CpS due to the removal of physical water in the soil. Even further increasing the temperature to 500°C did not noticeably change the palygorskite characteristic peak. However, at 700°C, the intensity of the palygorskite peak declined by 37% from 1082 to 670 CpS. Raising the temperature helps to expel the interlayer water leading to a decrease in the intensity of clay mineral peaks (Dellisanti and Valdré, 2005; Vidonish et al., 2016). At 900°C, the palygorskite peak intensity was reduced to 301 CpS. Similar observations were made for other palygorskite peaks, such as the one at a 7.09 Å spacing, which decreased from 608 to 230 CpS. Palygorskite remains stable up to 550°C beyond which it becomes unstable (Wang et al., 2020).
Accordingly, it appears that dehydroxylation initiates at above 500°C, destructing the clay minerals in the marl soil. It must be noted that new peaks appear in the XRD patterns of the samples heated at 700 and 900°C.
The intensity of the main calcite peak at d = 3.03 Å did not change notably from 25 to 500°C, and the variations remained between 5 and 8%. By raising the temperature to 700°C, the calcite peak intensity was reduced by 820 CpS. This declining trend continued up to 900°C, where the peak completely disappeared, indicating the total breakdown of calcite. According to the literature, calcium carbonate is decomposed completely at 800–870°C (Cultrone et al., 2001).
(Cultrone et al., 2001) claim that calcium carbonate is decomposed by heating and transformed to quicklime, releasing carbon dioxide in the process according to reaction 1. The calcium oxide released at 700–900°C reacts with water according to reaction 2 and produces calcium hydroxide aka hydrated lime.
CaCO3 + Heat → CO2 + CaO (1)
With silica, aluminosilicate, and sufficient moisture available, the calcium hydroxide produced in reaction 2 may form calcium silicate hydrate (C-S-H) and calcium alumina hydrate (C-A-H) nanostructures through pozzolanic reactions (Singh and Garg, 2006; Trindade et al., 2009).
CaO + H2O→Ca(OH)2 (2)
Above 500°C, after the calcium carbonate is decomposed in the marl soil with a high carbonate content (38.5%), the carbonate oxide reacts with phases produced by destruction of phyllosilicates, forming carbonate silicates, such as gehlenite in accordance with reaction 3.
2CaO + SiO2 + Al2O3→ Ca2Al2SiO7 Gehlenite (3)
Moreover, dolomite is decomposed at a lower temperature than calcite at around 350°C, transforming into magnesium oxide and calcium oxide by releasing carbon dioxide according to reaction 4 (Cultrone and Rosua, 2020).
CaMg(CO3 )2→CaO + MgO + 2CO2 (4)
The gehlenite peaks at 2.85 Å and 1.75 Å appear in the XRD pattern of the marl soil at 900°C. Gehlenite is classified as a calcium aluminosilicate formed in a reaction between calcium oxide—produced by the decomposition of calcite—and amorphous phases such as silica and alumina (Cultrone and Rosua, 2020). When the temperature reaches 800°C, calcium carbonate is decomposed into calcium oxide, forming a semi-stable gehlenite (Trindade et al., 2009). Gehlenite continues to disappear as the temperature rises. The presence of gehlenite can be attributed to the higher calcite content than other minerals (Jordán et al., 1999).
At 700°C, weak peaks in the XRD pattern appear at d = 3.25 Å, 3.2 Å, and 2.69 Å corresponding to alite (C3S) and belite (C2S) cement compounds. The formation of these compounds can be the primary reason for the studied samples to set at 700°C, making it virtually impossible to conduct the Atterberg limit test at this temperature.
Note that further increasing the temperature to 900°C weakens the quartz peaks but does not eliminate them. During heating, quartz is transformed into other silica polymorphs. According to (Weiss et al., 1970) alpha-quartz (low quartz or low-temperature quartz) is transformed into beta-quartz (high quartz or high-temperature quartz) at 573°C. Structurally, beta-quartz is similar to alpha-quartz, but with some distortion (Ringdalen, 2015). The process is reversible up to 800°C, and quartz grains are transformed into alpha-quartz by releasing heat under tensile stress, creating voids in the soil structure. In this process, quartz grains swell and microcracks appear, affecting the mechanical properties of the soil (Pabst and Gregorová, 2013; Štubňa et al., 2018).
3.4 Effects of temperature on weight loss
Dehydroxylation refers to the ejection of hydroxyl ions from the structure of clay minerals in the form of water, resulting in the sudden weight loss of the soil. Therefore, sudden weight loss is a proper criterion for determining the dehydroxylation temperature of clay minerals. Based on the XRD analysis results, dehydroxylation was expected to take place above 500°C in the marl soil.
According to Fig. 5, the weight loss is insignificant at 200–300°C. By raising the temperature to 500°C, the weight loss reached nearly 2.2%. At 700°C, the marl sample displayed a sudden weight loss of 14%. This considerable loss can be attributed to the start of dehydroxylation and destruction of clay minerals in the marl soil (D'Elia et al., 2018). Therefore, one can conclude that the dehydroxylation of clay minerals in the soil marl begins in the temperature range of 500–700°C, which is confirmed by the XRD analysis. Dehydroxylation of palygorskite, as the main clay mineral in the marl soil decomposed above 500°C, is similar to what happens to kaolinite. Another reason for the weight loss can be the transformation of alpha-quartz to beta-quartz (Štubňa et al., 2018). The weight loss continues up to 900°C and reaches 20.5%.
In fact, the weight loss takes place in both clay and carbonate constituents of the marl soil. A weight loss of 14% at 700°C can be attributed to dehydroxylation of clay minerals in the marl soil, whereas the 6.5% loss at 700–900°C is due to the release of carbon dioxide (Trindade et al., 2009).
3.5 UCS Variations of the heat-treated marl soil
Figure 6 depicts the UCS curves of the heat-treated marl soil in two different cases. In the first case, the soil was first heated, the UCS specimens were prepared and then cured for seven days. In the second case, the UCS specimens were first prepared and then cured for three days before being heated to 25–900°C.
First, the effect of temperature on the compressive strength of specimens made of heat-treated soil is discussed. According to the results, the strength of the natural marl is 0.7 kg.cm− 2. Raising the temperature to 200°C increased the soil strength to 0.92 kg.cm− 2 by a factor of 1.31. At 300 and 500°C, the compressive strength reached 1.05 and 1.84 kg.cm− 2, respectively. In this temperature range, the soil is dehydrated leading to an increase in the compressive strength. At 700°C, the soil compressive strength increased considerably, reaching 69.1 kg.cm− 2, 37.5 times higher than that at 500°C and nearly 100 times higher than the compressive strength of natural soil. This can be attributed to dehydroxylation taking place in this temperature range, as well as the structural changes in the soil and the formation of cement compounds, alite (C3S) and belite (C2S) (Joshi et al., 1994).
In fact, the applied heat causes the formation of cement compounds, and introducing moisture to the soil for preparing the uniaxial specimens causes the soil to set and harden at this temperature. Pozzolanic reactions and the formation of C-S-H and C-A-H nanostructures can be the main reasons for the improved compressive strength.
C-S-H and C-A-H compounds with complex and strong hydrogen bonds improve the compressive strength of the soil specimens treated at 700°C. By increasing the temperature to 900°C, the soil strength declines considerably (9 kg.cm− 2) compared to the specimen treated at 700°C. The studied marl contains 38.5% carbonate. Calcium carbonate is decomposed at 700–870°C, realizing gaseous carbon dioxide leading to a porous soil structure. On the other hand, glassy and highly-porous gehlenite compounds reduce the compressive strength of the soil (Cultrone et al., 2001). Structural transformations of quartz and formation of voids and microcracks also cause a decrease in the compressive strength.
In the second case, the UCS specimens were heat-treated after preparation. According to the results in Fig. 6, the marl soil has an UCS of 12.90 kg.cm− 2 at 110°C, which is 18 times higher than natural marl. This enhanced strength can be attributed to the loss of moisture. The compressive strength gradually increases from 110 to 300°C and reaches about 13.9 kg.cm− 2 at 300°C, and the interlayer water decreases with heating due to dehydration. The lower electrostatic repulsion between the clay particles also causes an increase in the compressive strength (Afrin, 2017).
The compressive strength reaches around 27.7 kg.cm− 2 at 700°C, which is 39.5 times higher than that of natural marl. Given that dehydroxylation occurs above 550°C, the strength improvement can be attributed to dehydroxylation. In other words, the atomic configuration following dehydroxylation improves the compressive strength of the marl soil specimens (Joshi et al., 1994). Clay minerals in the marl soil are dehydroxylated, and the wrinkled clay particles and flakes are replaced by cohesive and homogeneous particles, improving the comprehensive strength.
The compressive strength reaches 14.73 kg.cm− 2 at 900°C, suggesting a 46.8% decline. Decomposing at 830–870°C, calcium carbonate is transformed into calcium oxide (quicklime) and carbon dioxide is released (Cultrone et al., 2001). It is, therefore, safe to attribute the loss of strength to the structural transition of the soil above the dehydroxylation temperature and the increased soil porosity.
The XRD pattern of the soil sample at 900°C reveals the appearance of the new gehlenite peaks at d = 2.85 Å and 1.75 Å. The glassy and porous structure of gehlenite causes a decrease in the compressive strength of the specimens at this temperature. Accordingly, the strength of the marl soil can be said to be reduced by the poor strength of gehlenite compounds. Nonetheless, it must be noted that the strength of the specimen treated at 900°C is nearly 21 times that of natural soil.
Overall, the high-carbonate clays (marl soil), especially those containing calcite, have a positive effect on the compressive strength, as carbonates form pores and crystalline phases during curing, improving the compressive strength (Gualtieri et al., 2010). On the other hand, due to poor strength, gehlenite compounds reduced the strength of the soil treated at 900°C.
3.6 Permeability coefficient of the heat-treated marl soil
Figure 7 illustrates the permeability coefficient of the studied heat-treated marl soil. Natural marl has a permeability coefficient of about 1.1 × 10− 7 cm.s− 1, which is acceptable for geo-environmental projects (Daniel and Benson, 1990; Kalkan and Akbulut, 2004). According to the results, the permeability was slightly improved by 5% by raising the temperature to 110°C.
Further increasing the temperature to 200°C promoted the nucleation of new microcracks and the growth of existing ones, naturally, affecting the physical properties and improving the permeability coefficient (Sun et al., 2016). The permeability coefficient was improved 26-fold at 200°C, reaching nearly 2.87 × 10− 6 cm.s− 1. Heating up to 200°C ejects interlayer water, causing irreversible changes in the soil structure. The ejection of interlayer water increases the salt concentration around clays, reducing the thickness of the electric double layer and, consequently, the electrostatic repulsion. These conditions promoted microstructural changes, consequently, increasing the microscopic voids in the soil structure. Accordingly, more paths open up for the flow of water, increasing the permeability coefficient of the soil (Ouhadi et al., 2010).
Increasing the temperature to 300°C promoted the nucleation of microcracks, increasing the permeability coefficient to nearly 3.6 × 10− 6 cm.s− 1. In the temperature range of 300–500°C, the clay soils underwent a major structural transformation as a result of changes in their structural water.
The permeability was increased 246-fold compared to natural soil at 500°C, reaching 2.71 × 10− 5 cm.s− 1. According to (Chen et al., 2017), elevated temperatures cause a fundamental structural transition in the soil, develop larger interparticle voids, change the effective flow cross-section, and release absorbed water.
At 700°C, the soil behavior changed entirely and the permeability coefficient drastically reduced to 1.63 × 10− 8 cm.s− 1, indicating a 6-fold reduction compared to natural soil. According to the XRD analysis, the formation of cement compounds (C2S and C3S) at 700°C, and the resulting pozzolanic reaction and setting upon saturation and the formation of C-S-H and C-A-H nanostructures reduce the permeability coefficient. The permeability decreased as the cement compounds formed at 700°C begin to set. The formation of the highly-porous gehlenite at 900°C increased the permeability coefficient nearly 672-fold to 7.39 × 10− 5 cm.s− 1.
3.7 Morphology of heat-treated marl
The arrangement of soil particles and the microscopic texture of marl specimens were analyzed by SEM imaging to investigate textural changes in the soil samples treated at different temperatures.
Figure 8a illustrates the texture of natural marl and the fibrous morphology of palygorskite minerals. Moreover, the white masses are suggestive of the high concentration of calcium carbonate in the studied soil.
At 500°C (Fig. 8b), palygorskite rarely appears in the soil due to the start of dehydroxylation causing destruction of clay minerals and formation of larger particles, thus promoting pores and the growth of microcracks. The changes in the particle arrangement, destruction of clay minerals, and the formation of coarser compounds at this temperature can be the main reasons for the 246-fold increase of the permeability coefficient.
According to Figs. 8c and d, palygorskite is entirely disappeared in the temperature range of 700–900°C, as the fibrous and needle-like grains cannot be found in the SEM micrographs. With the initial melting of the soil at 700°C, calcium carbonate was reduced drastically by decomposition. The figure also confirms the limited formation of cement compounds, alite and belite.
A new gehlenite structure was formed following the heat treatment at 900°C (Fig. 8d). At this temperature, a porous structure (gehlenite) develops in the soil sample leading to the formation of multiple microscopic voids and pores. This can also be attributed to the decomposition of calcium carbonate and the release of carbon dioxide from the soil. Accordingly, the compressive strength of the soil treated at 900°C (Fig. 6) was reduced. Further, the glassy texture of the soil at this temperature is indicative of a phase transition and the formation of new compounds. This structure increased the permeability coefficient by a factor of 672 (Fig. 7) compared to the natural soil.