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Time Dependency of CaO-Treated Expansive Soils by Research Professor, Postgrado de Ingeniería; Universidad Autónoma de Querétaro; Cerro de las Campanas SN, CP 76010, Santiago de Querétaro, Querétaro; Mexico e-mail: lolte@sunserver.uaq.mx V. M. Castaño Researcher, Instituto de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México; AP 1-1010; Querétaro, Querétaro 76000, Mexico e-mail: meneses@servidor.unam.mx |
ABSTRACT
In some practical cases of CaO-stabilized soils, a reversibility phenomenon seems to appear. This implies that, after some time, a CaO-treated expansive clay soil recovers its initial expansive properties. In the present work, a method for soil aging is proposed and applied for observing the mineralogical composition behavior of the treated soil through time, from material addition. With this method, the existence of such reversibility could be confirmed or rejected. X-Ray Diffraction and another characterization technique were also used.
KEYWORDS: Lime-treated soils, treated soil, stabilized expansive soils, expansive soils, soil aging, soils stabilization.
INTRODUCTION
The problem of expansive soils has become almost a worldwide problem. The growth of cities has triggered the need for doing research in this area. As a matter of fact, the expansive clay soil, when hydrated, recovers its expansive properties producing cracks in most of light structures, (Clearly, we cannot neglect also some other factors that could influence foundations) [9,15]. The use of CaO-stabilization techniques on clay soils has been used for some time. This has been done in order to inhibit its expansion-contraction properties. Nevertheless, there is still the uncertainty about the effectiveness of this mixture as time goes by. In general, it has been widely shown that for designing a soil stabilization technique, it is necessary to know a priori the soils mineralogy and the likely reaction that these could have with the additives. Unfortunately, many times this is overlooked.
Soil aging is a known phenomenon in Soil Mechanics. It is used to explain the secondary compression phenomenon that occurs in fine soils due to the viscosity. It has also been explained as the thixotropic hardening without either volumetric changes or soil chemical modification in pore water [13]. Recently, soil aging has been interpreted as a chemical change [14]. The present work is based on the last interpretation.
One of the techniques that allows to know semiquantitative information about crystallinity, and therefore about mineralogical composition, is the X-Ray Diffraction (XRD) by the powders method. This allows to follow the changes due to the interaction between the soil and the stabilizing additive. Moreover, via XRD, one can obtain all the mineralogical information with a small sample (approximately 1 g) in relatively short times (approximately 30 min).
The temperature of a soil sample under aging must guarantee either the total or partial minerals-soils undestructibility and or reaction. (Of course, this won’t occur in a natural aging process). Such temperature can be found either via Differential Thermal Analysis (DTA) or Gravimetric Thermal Analysis (GTA). The first technique gives the exact temperature at which some physical or chemical change occurs in the sample like can be the water evaporation, organic matter loss, some structural transformation or some minerals formation or decomposition [7]. The second one gives in addition to DTA the amount of mass being lost. Results obtained jointly with calorimetric analysis and X-Ray diffraction for powders give a valuable and reliable information about the soil stability and composition under a given temperature.
The expansive clay identification is not an easy task due to the structural differences among clays of the same group, like montmorrinollite, that show small composition differences, but the same crystalline structure. Nevertheless, by infrared spectroscopy one can determine the atoms-bond vibration and thus to identify different clays.
METHOD OF ANALYSIS
Materials Used
In this work, a commercial CaO was used for the soil stabilization. This material was chosen to avoid the volumetric changes that expansive clays undergo when there is humidity variations.
Sample Preparation
The natural soils used were from the Jacarandas Zone, located at Queretaro, Mexico. This soil has been classified as CH (Inorganic clay of high plasticity), according to USCS (Unified Soil Classification System) with a high expansive degree [2,5,9]. The optimum percentage of the required CaO for stabilization was obtained via the lowest plasticity index of several CaO-soil mixtures with different percentages of CaO (2, 4, 6, 8, and 10) in relation to soil dry weight. To simulate the aging process of expansive soils treated with CaO, several soil mixtures with natural soil from Jacarandas, Qro., with the optimum percentage of CaO in a agatha mortar were done. To complete the mixture distilled water was added to the samples for getting a paste. Then, these samples were heated up to 200 °C into a electric muffle (Lindberg). Every 24 h were taken out from the oven for adding distilled water, heat them up to 80° C up to full evaporation. From this, they were subjected to a 200 °C into the muffle. This treatment was done to simulate and accelerate the aging process to which the soil undergoes at atmospherical conditions. Then, the characterization of the natural soil and the mixtures CaO-soil was done at different times, the techniques used are described next.
X-Ray Diffraction, XRD
To identify the mineralogical composition of the different crystalline phases present in the soil, X-Ray diffraction studies by powders method were performed. This was done in a Siemens diffractometer D5000 with Cu radiation, Ka = 1.5405 Å in the 5 < 2q < 65 range. For this, the samples were milled inside an agatha mortar, to avoid possible contamination with other materials, and passed through a 200 sieve.
Infrared Spectroscopy, IRS
Both samples, i.e., natural and CaO-treated soil, were analyzed by IRS with a Nicolet 510, FT-IR Spectrometer, in the range from 4000 to 400 cm-1, using KBr as support.
Thermal Analysis
For observing and quantifying the volatile phases, and to determine the phase transformations as well, the samples were analyzed via DTA (Differential Thermal Analysis, DTA-910), and GTA, (Gravimetric Thermal Analysis, GTA-2950), at a rate of 10°C/min, in air.
RESULTS AND DISCUSSION
Determination of the Optimum CaO Percentage
The results about the index properties determined for the treated soil with different amounts of CaO, the USCS (Unified Soil Classification System) sample classification and the effects of different percentages of CaO-addition on the natural soil plastic index are shown in Table 1. Based on the above, it can be seen that from 8 %, the reduction of the plastic index remains constant for greater values of CaO. Thus, the 8 % wt of CaO is the optimum percentage for stabilizing this soil.
Table 1. Soil classification treated with different percentages of CaO
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% of CaO added | Liquid Limit | Plastic Limit | Plasticity Index | Classification USCS |
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2 | 83.52 | 36.09 | 47.43 | MH |
4 | 75.22 | 53.37 | 21.85 | MH |
6 | 72.56 | 56.00 | 16.55 | MH |
8 | 74.17 | 60.03 | 14.14 | MH |
10 | 73.20 | 59.35 | 13.85 | MH |
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X-Ray Diffraction
The mineralogical analysis of the different crystalline phases of a laboratory-aged soil was determined via XRD by the powders method. Diffractograms for a natural soil are shown in Figures 1 and 2. From these figures it can be seen that such material has four main phases, namely, calcite, albite, anorthite, and montmorrillonite [8]. Moreover, from these figures it can be seen that the last mineral is the expansive phase of the analyzed soil.
Figure 1. Phases identification in a natural soil sample by X-ray diffraction.
Figure 2. Phases identification in a natural soil sample by X-ray diffraction.
From the diffractograms shown in Figures 1 and 2, it can be seen that the diffraction lines are not well defined. This suggests that the sample contains amorphous substances like organic matter or silicon. To eliminate those phases and achieve a better identification of the soil constituents, some samples were treated at 70 °C for 3 hours, with a 0.5 normal sodium hydroxide solution with continuous stirring [10]. After this treatment, the soils were washed with distilled water and dried at 80 °C for 24 hours in an electrical stove. Then, the samples were analyzed via XRD and the observed patterns were better defined. This allowed to identify clearly each of the crystalline phases, as shown in Figures 3 and 4.
The aged-laboratory treated-soil samples were also analyzed via XRD showing the same crystalline phases as the untreated natural soil, except that such samples showed an increase in the calcite diffraction lines, as shown in Figure 5
M= Montmorillonite: Na0.3(Al,Mg)2Si4O10(OH)28H2O (JCPDS No. 29-1499).
A= Ordered sodic anorthite: (Ca, Na)(Al,Si)2Si2O8 (JCPDS No. 20-528)
B= Disordered albite: NaAlSi3O8 (JCPDS No. 20-572).
I= Natural Soil.
II= Treated Soil with NaOH0.5N.
M=Montmorillonite: Na0.3(Al,Mg)2Si4O10(OH)2nH2O (JCPDS No. 29-1498 y 29-1499).
C= Calcite CaCO3 (JCPDS No. 24-27)
Figure 3. Phase identification in a NaOH-0.5-treated natural soil sample
via XRD for to eliminate the organic matter.
I = Natural Soil.
II = Treated soil with NaOH0.5N.
M = Montmorillonite:
Na0.3(Al,Mg)2Si4O10(OH)28H2O (JCPDS No. 29-1499).
A= Ordered sodic anorthite : (Ca, Na)(Al,Si)2Si2O8 (JCPDS No. 20-528)
B= Disordered albite: NaAlSi3O8 (JCPDS No. 20-572).
Figure 4. Phases identification in a NaOH-0.5-treated natural soil sample
via XRD to eliminate the organic matter.
I= Natural Soil.
II= Trated Soil with 8% in lime weight.
C= Calcite CaCO3 (JCPSD No. 24-27).
Figure 5. Phases identification in a 8% CaO-treated soil sample via XRD in dry weight.
The study of the aging of a CaO-treated natural soil at laboratory scale was obtained by heating the samples at 200 °C in an electrical muffle (Lindberg) during 10 continuous weeks. The choice of this temperature was done based on the results of the DTA and GTA together with XRD. These techniques guarantee that the soil minerals crystalline structure remains unchanged. For this elapsed time, the samples were treated with distilled water up to saturation, forming a paste. After this treatment, the samples were dried in an electrical stove (Lab-Line Instruments) at 80 °C up to the total water evaporation. Then were heated at 200 °C in a muffle. These treatments were performed to simulate, in shorter times, the aging process that the CaO-treated soil undergoes at normal atmospherical conditions.
The treated samples were analyzed via XRD at different times up to 10 weeks (1680 hours). The results are shown in Table 2. From that Table it can be seen that a greater amount of calcite is being formed up to 1176 hours. After this, the amount of calcite remains constant. This can be observed more clearly from the diffractograms shown in Figures 6a and 6b and the plot shown in Figure 7.
Table 2. Relative amount of calcite being formed with aging, at laboratory scale.
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Sample number | Submit hours in the treatment | Relative Percentage of CaCO3* |
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1 | 168 | 3.78 |
2 | 336 | 4.52 |
3 | 604 | 5.66 |
4 | 672 | 5.98 |
5 | 840 | 6.39 |
6 | 1008 | 8.17 |
7 | 1176 | 8.05 |
8 | 1344 | 7.37 |
9 | 1572 | 7.09 |
10 | 1680 | 7.06 |
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* Relative amount calculated measuring the area
under the diffraction sharp point to 2 q
= 29. 45o for CaCO3
I = Natural Soil washed with NH4Cl1N
II = Natural Soil.
III = Treated Soil with 8% in lime weight and aged one week
IV = Treated Soil with 8% in lime weight and aged two weeks
V = Treated Soil with 8% in lime weight and aged four weeks
Figure 6a. X-ray Diffraction for the soil treated with calcium oxide at 8% in weight and aged at different times.
VI = Treated Soil with 8% in lime weight and aged six weeks
VII = Treated Soil with 8% in lime weight and aged eight weeks
VIII = Treated Soil with 8% in lime weight and aged night weeks
IX = Treated Soil with 8% in lime weight and aged ten weeks
Figure 6b. X-ray Diffraction for the soil treated with calcium oxide at 8% in weight and aged at different times.
Figure 7. Percentage behavior of CaCO3 vs Treatment hours.
(c) Infrared Analysis
To assess and complement the information obtained via XRD, the samples were analyzed via infrared spectroscopy in the 4000 to 400 cm-1 range. The infrared spectra are shown in Figures 8 and 9 and correspond to natural and CaO-treated soils, respectively. From those it can be observed the presence of different bands due to the bonds vibration H-O, Si-O, O-H-Al, Si-O-Al, and Si-O-H, all of them being characteristics of a montmorillonite. This band allocation, as well as the different vibration frequencies are shown in Table 3 [10,4,3]. The case of samples CaO-treated is shown in Figure 9. From this figure it can be observed an increase in bands intensity due to the calcite [4], in agreement with the results obtained via XRD.
Figure 8. Infrared spectrum of a natural soil
Figure 9. Infrared spectrum for a natural soil with lime
Table 3. Vibration frequencies from a stabilized soil with lime [10, 8]
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Frequency (cm -1) |
Assignment of band |
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Montmorillonite | |
3560 | H-O |
3300 | O-H |
1625 | H-O |
1080 | Si-O |
1030 | Si-O |
910 | O-H-Al |
735 | Si-O-Al |
621 | Si-O |
520 | Si-O-H |
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Calcite (CaCO3) | |
2920 | |
2858 | |
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(d) Gravimetric Thermal Analysis
The volatile phases identification was done via GTA in natural and CaO-treated soil samples, and are shown in Figures 10 and 11, and in Tables 4 and 5. From the thermograms it can be observed 5 mass losses. The first one, at 90 °C is due to the water being absorbed by the soil. The second one, at 290 °C is due to the loss of crystallization water (coming from the OH groups) of the montmorinollite. The third and fourth mass loss, at 460°C and 595°C are due to the organic matter decomposition as CO and CO2, respectively. Finally, the mass loss at 850 °C is due to the calcium carbonate decomposition in CO2 [11,6,12,1].
Figure 10. GTA in air for a natural soil
Table 4. GTA results for a natural soil sample
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Loss | Temperature (ºC) |
% of lost matter |
Assignment |
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1 | 90 | 11.19 | H2O |
2 | 290 | 1.41 |
H2O of crystall- ization, OH |
3 | 460 | 2.32 | CO, organic matter |
4 | 595 | 1.05 | CO2, organic matter |
5 | 850 | 0.55 |
CO2, decomposition of CaCO3 |
TOTAL | 16.42 | ||
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Figure 11. GTA for a stabilized soil with calcium oxide
Table 5. GTA results for a stabilized soil sample with lime
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Loss | Temperature (ºC) |
% of lost matter |
Assignment |
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1 | 90 | 12.00 | H2O |
2 | 300 | 1.80 | H2O of cristallization, OH |
3 | 455 | 2.10 | CO, organic matter |
4 | 615 | 3.00 | CO2, organic matter |
5 | 850 | 0.99 | CO2, decomposition of CaCO3 |
TOTAL | 19. 89 | ||
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(e) Differential Thermal Analysis
The characteristic thermogram of natural and CaO-treated soils is shown in Figure 12. From these figures it can be seen four endothermical signals at 93, 306, 435, and 882 °C. The first two signals at 93 °C and 306 °C are due to the adsorbed water and crystallization, respectively. The peak at 435 °C is due to the CO2 loss, due in turn, to the organic matter decomposition. The last signal, close to 882 °C, is due to the calcium carbonate decomposition in CaO and CO2 [11, 6, 12, 1]. The phases transformations and its temperatures are shown in Table 6.
Figure 12. DTA thermogram for a natural soil sample
Table 6. DTA results for a natural soil in air
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Temperature change | Temperature (ºC) | Assignment |
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1 | 82.59 | H2O |
2 | 305.80 | H2O of cristallization, OH |
3 | 435.27 | CO organic matter |
4 | 881.70 | CO2 decomposition of CaCO3span class=CellData |
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CONCLUSIONS
With the proposed method for the soil aging study at laboratory scale, it is possible to determine, in a reasonably short time, the soil behavior when it is stabilized with CaO. The clay soil used in the study is from the Jacarandas zone, Queretaro, Mexico is composed mainly of montmorinollite, albite, and anorthite. The soil is classified as a high plasticity clay or CH. It was found that during the CaO-stabilization process calcium carbonate was formed which increased with aging. This formation seems to be exponential reaching its maximum value after 1008 hours of treatment, and then, remains constant. Thus, it can be concluded that the soil-CaO reaction develops during this time. It ends when the calcium carbonate formation is concluded and then remains constant. Moreover, if the calcium carbonate formation remains constant, it can be concluded that the stabilization process is not reversible.
REFERENCES