 |
 |
Mineralogical Characterization of Stabilized Soils
Postgrado de Ingeniería, Universidad Autónoma de Querétaro
J. B. Hernández Zaragoza, C. López-Cajun Postgrado de Ingeniería, Universidad Autónoma de Querétaro |
Abstract
Currently, existing soil classifications in Geotechnics and affine areas correspond to the general behavior of different soil groups. These classifications sometimes are superficial and could lead to ambiguity due, for example, to the way for obtaining the parameters being used to classify such soils, which are, frequently, empirical. Indeed, one classification can be attributed to soils with similar characteristics, but different mineralogical contents. This, in turn, lead to behavior differences, difficult to detect with the soils classification. Because of the above, in this work, soil index properties evaluation, both, expansive and CaO treated are reported. In both cases mineralogical characterization were done via, among others, X-Ray Diffraction, Scanning Electronic Microscopy, and Thermal Analysis.
Keywords: Mineralogical characterization, stabilized soils, expansive soil, lime, lime characterization, treated soils, characterization techniques, calcite.
Introduction
Research on expansive soils, in Mexico, is quite recent. In 1983, the Universidad Autónoma de Querétaro started a research line to try to understand and to develop knowledge leading to assist a demand due to the problems they represent in different regions of the country and, particularly, at Santiago de Querétaro, Mexico. Since there exist uncertainties in soil-clay stabilization, it was decided to start from a physicochemical soil characterization and the material (CaO) to be used. This will help understand and answer questions on its application.
characterization techniques
Techniques Used
Characterization techniques used in this research were X-Ray Diffraction, Scanning Electron Microscopy, Infrared Spectroscopy and Thermal Analysis.
X-Ray Diffraction
For mineralogical identification of the different soil crystalline phases, X-Ray diffraction via powders technique was performed. A Siemens D5000 diffractometer with Cu radiation, Ka = 1.5405 Å, in the range 5<2q<65 was used. For this, all samples were milled in an agate mortar, to avoid possible contamination of other materials, and sifted by mesh 200.
Infrared Spectroscopy
Both soil samples, natural and CaO-treated were analyzed via infrared spectroscopy (Nicolet 510, FT-IR Spectrometer) in the range from 4000 to 400 cm-1, using KBr as support.
Calorimetric Analysis
For observing and quantifying the possible lost of volatile phases, as well as for determining phase transformations, samples were analyzed via differential thermal analysis, DTA-910, and gravimetric thermal analysis, TGA-2950 TA, at a warming speed of de 10 ºC /min on air.
Scanning Electron Microscopy
The microestructural and microanalysis of both soils, samples were analyzed via scanning electron microscopy with a JEOL 5200.
Preparation of Materials and Mixtures
To determine the best stabilization conditions, some soil mechanics laboratory tests were performed on both soils. The soil used was from Jacarandas, Santiago de Querétaro, Mexico. Soil mixtures were prepared at 2, 4, 6 8 y 10% of CaO of the soil dry weight. Once the liquid, plastic, contraction limits as well as the plastic index were determined, the optimum CaO percentage was selected. This was done in such a way to produce the maximum decrement in the plastic index (see Table 1). The CaO used were from “Los Arcos”, “ Santa Cruz ” y “Del Valle” (results are shown in Table 1).
Table 1. Soil index properties of natural and treated soils.
| |||||||
| Identification | % material | Liquid Limit (%) |
Plastic Limit (%) | Plastic Index (%) |
Contraction Limit (%) |
USSC(*) Classification |
Expansion degree Chen, 1975 |
| |||||||
| Natural soil | - | 92.7 | 36.0 | 56.7 | 12.5 | CH | VERY HIGH |
| CaO Santa Cruz | 8 | 79.6 | 68.1 | 11.5 | 41.7 | MH | LOW |
| CaO del Valle | 8 | 78.2 | 63.8 | 14.4 | 40.8 | MH | LOW |
| CaO Los Arcos | 8 | 74.5 | 62.5 | 12.0 | 40.3 | MH | LOW |
| |||||||
| (*) Unified System of Soil Classification | |||||||
RESULTS AND DISCUSSION
Plots of plastic index vs. % of soil stabilizer material are shown in Fig. 1. In agreement to USSC, these soils classification is shown in Fig. 2.
Figure 1. Plastic Index vs. % of stabilizer material.
Figure 2. Natural and treated soils classification, according to USSC.
X-Ray Diffraction
Natural Soil Diffractograms
Some Montmorilonite-Ilite and Anordite soil phases are shown in Fig. 3. Other diffractograms show an additional soil characterization which can be identified as other phases such as Anortita, Albita and silicates aluminates.
Figure 3. Diffractogram of a natural soil (Montmorilonite).
Diffractrograms of Natural Soil Treated with 8% of CaO
An analysis of a CaO soil treated is shown in Fig. 4. In this figure it can be seen a calcite increase respect to the soil natural state (line corresponding to 2q =29.5º).
Figure 4. X-Ray Difraction of natural soil-CaO.
On the other hand, of a study of Aging of soils treated with lime, the quantity of calcite increases with time, staying other phases of complete form, Figures 5 and 6.
Figures 5 and 6. Difractogram that shows the increase of calcite with the time departing from a natural soil
CaO Analysis
Santa Cruz CaO
From the X-Ray diffraction and microsonde analysis, one has that the material is composed of calcium hidroxide (Portlandita) and calcium carbonate (calcite). The elements detected by microsonde were O (41.2%), Mg (0.57%) Si (1.52%) and Ca (56.7%).
Del Valle CaO
From the X-Ray diffraction and microsonde analysis, one has that the material is composed of calcium hidroxide (Portlandita) and calcium carbonate (calcite). The elements detected by microsonde were O (39.69%), Mg (2.31%) Si (0.66%), S (0.43%), K (0.27%) and Ca (56.65%).
Los Arcos CaO
From the X-Ray diffraction and microsonde analysis, one has that the material is composed of calcium hidroxide (Portlandita) and calcium carbonate (calcite), calcium sulphate with two water molecules (Gypsum), calcium sulphate (Anhidrita) and calcium sulphate with 0.5 water molecules (Basanita). The elements detected by microsonde were C (9.13%), O (34.85%), Mg (3.49%), Si (1.20%), S (1.05%) and Ca (50.28%).
Infrared Spectroscopy
To verify and complement the information obtained via X-Ray diffraction, soils were analyzed by infrared spectroscopy in the range from 4000 to 400 cm -1. Infrared spectra are shown in Figs. 7 and 8, corresponding to samples of natural soil and treated soil, respectively.
Figure 7. Infrared spectrum for natural soil.
Figure 8. Infrared spectrum for CaO treated soil.
From the spectra it can be observed different bands which can be assigned to bonding vibrations H-O, Si-O, O-H-Al, Si-O-Al y Si-O-H, characteristics of a montmorillonita. Bands allocation, as well as the different vibration frequencies are shown in Table 2. For the case of CaO-treated samples, shown in Fig. 8, it can be observed an intensity increase of bands corresponding to calcium carbonate vibrations. These results are in agreement with the ones obtained via X-Ray diffraction.
Table 2. Soil vibration frequencies
| |
| Frequency (cm -1) |
Band Allocation |
| |
| 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 |
| 470 | Si-O |
| |
Calorimetric Analysis (GTA and DTA)
Gravimetric Thermal Analysis (GTA).
Volatile phases quantification was done via GTA in natural and CaO-stabilized samples and are shown in Figs. 9 and 10. Results of this analysis are included in Tables III and IV.
Figure 9. Air GTA for natural soil.
Figure 10. GTA for a CaO stabilized natural soil.
Table 3. GTA Results for a natural soil sample.
| |||
| Lost | T (°C) |
% of lost matter | Allocation |
| |||
| 1 | 90 | 11.19 | H2O |
| 2 | 290 | 1.41 | H2O crystallization, OH |
| 3 | 460 | 2.32 | CO organic matter |
| 4 | 595 | 1.05 | CO2 organic matter |
| 5 | 850 | 0.55 | CO2, CaCO3 decomposition |
| TOTAL | 16.42 | ||
| |||
Table 4. GTA Results for a CaO-stabilized soil sample.
| |||
| Lost | T (ºC) |
% of lost matter | Allocation |
| |||
| 1 | 90 | 12.00 | H2O |
| 2 | 300 | 1.80 | H2O cristallization, OH |
| 3 | 455 | 2.10 | CO organic matter |
| 4 | 615 | 3.00 | CO2 organic matter |
| 5 | 850 | 0.99 | CO2, CaCO3 decomposition |
| TOTAL | 19. 89 | ||
| |||
From the thermograms, it can be observed five losses of matter. The first one, at 90 o C, is due to water lost absorbed by soil, while the second one, at 290 C, is due to crystallization water lost. The third and fourth losses, at 460 y 595 o C, are due to organic matter decomposition such as CO y CO2, respectively. Finally, the mass lost detection at 850oC is due to the calcium carbonate in CO2.
Differential Thermal Analysis (DTA)
The characteristic thermogram, via DTA, of natural soil samples and CaO-stabilized are shown in Fig. 11. From that figure 11, endothermic signals can be seen at 83, 306, 453 y 882 o C. Only one thermogram is presented because it was the same for both, natural and treated soil.
Figure 11. DTA thermogram for a natural soil sample and CaO-stabilized.
The first signals, around 83 to 306oC, are due to the water lost adsorbed and crystallization, respectively. The peak, around 453oC is due to the lost of CO2, result of organic matter decomposition. The last signal, close to 882oC, is due to the calcium carbonate decomposition in CaO and CO2. A summary of temperatures of the different phases transformation is shown in Table 5.
Table 5. DTA Results for natural soil sample in air.
| ||
| Change in T | T (ºC) | Allocation |
| ||
| 1 | 82.59 | H2O |
| 2 | 305.80 | H2O cristallization, OH |
| 3 | 435.27 | CO organic material |
| 4 | 881.70 | CO2, CaCO3 decomposition |
| ||
Scanning Electron Microscopy (SEM)
SEM is a useful method to identify textures and shapes of mineral aggregates. It provides information about morphology, topology, particle size and other important characteristics on a soil sample.
Natural and stabilized soil samples were studied via SEM and photographs are shown in Figs. 12 and 13. From figure 12, it can be observed that natural soil is composed of small agglomerates formed by flaked particles, whereas from Fig. 13, which shows an stabilized natural soil, immediately after 8 % CaO addition, the flaked particles disappear and one can see a uniform particle formation.
Figure 12. Natural soil.
Figure 13. Soil-CaO mixture.
CONCLUSIONS
By analyzing the soil index properties of a CaO-treated soil, one has that all the CaO behaved similarly. The optimum amount for this material is between 6-8% of soil dry weight. According to its soil classification, the natural soil is CH, whereas a treated a MH. With the previous natural soil characterization, one had that soil was composed mainly by Montmorilonite, Anortite and Albite, whose organic matter content was 3.37%. From the X-Ray diffraction, the natural soil crystalline phases are not affected with CaO addition, which is confirmed by infrared analysis. From the CaO performed analysis (X-Ray diffraction and microsonde analysis), it was determined that the 3 types had calcium hydroxide (Portlandite) and calcium carbonate (calcite), which are the two compounds that only should have. Lefts are contaminants or material impurities. These could be responsible for the difference in the obtained values of the index properties of treated soil. A fact that can be stated is that the CaO.
“Los Arcos” was the most contaminated, whereas CaO “Santa Cruz” showed very little impurities. The soil treated with this CaO showed better results of the index properties. Thus, this work will continue using this CaO. Regarding SEM, one can conclude that CaO addition contributes agglomerates of almost mirror surface. Elementary analysis showed an increase of calcium in CaO treated soil.
It is important to mention that the use of these skills assures a better characterization, besides which it helps us to identify the predominant and constituent minerals of the soil in question, the content of organic matter and the morphology.
ACKNOWLEDGMENT
The authors wish to thank Eng. Eduardo Calderón-Rivas for his help in the production of the document.
REFERENCES
 | |
| © 2004 ejge | |