The results from this research, as well as the analyses and discussions, are presented in this section.
Unconfined Compression
When preparing the specimens for the simple compression tests, it was difficult to mold compacted specimens for Samples 5 (91HH-9L) and 7 (91HH-9C) in the lathe, because soon after being compacted, they already presented great stiffness. This behavior is due to the rapid reaction of the HH PG with binder and water, potentialized by the application of the compaction energy. Samples 6 (11HH-80S-9L) and 8 (11HH-80S-9C) were relatively easy to mold. However, during the curing time, these mixtures stiffened considerably. Note that molding a specimen of the test size without using a lathe may provide higher strength values.
Fig.
3 presents the maximum strength values obtained in the tests as a function of the curing time for each sample. For the mixtures containing HH PG, mainly Samples 5 (91HH-9L) and 7 (91HH-9C), high strength values were observed. For Sample 5, the value obtained was 20.9 times higher than the one in Sample 1 (91DH-9L) after 28 days of curing. The presence of HH PG influences the increase in strength, even in mixtures containing soil because at 28 days, Sample 8 (11HH-80S-9C) reached twice the value obtained by Sample 4 (11DH-80S-9C). This shows the extent to which HH PG has better mechanical behavior than DH. The greatest variation in strength over time was verified in Sample 6 (11HH-80S-9L).
Ahmed et al. (
2011) explain that the PG HH mechanical behavior is better than that of DH because the former absorbs water from the soil, reducing the voids and strengthening the soil. This mechanism can also explain the lower expansion observed in HH PG.
When PG is added to the soil stabilization process, an ettringite precipitation may appear and strengthen the soil by filling up pores. However, according James and Pandian (
2014), for higher PG levels, more large clusters of ettringite can be formed and this can cause a decrease in strength. On the other hand, Ahmed (
2015) studied the microstructure and mineralogical compositions of soft clay soil stabilized with recycled bassanite, produced from gypsum waste plasterboards by heating. The results showed various cementation compounds in the soil matrix when recycled bassanite was added, which improved the compressive strength. The author also noted that, as the admixture increased, the ettringite formation would also increase. It is also notable that the admixture ratio would influence ettringite formation differently in each bassanite mixture: in bassanite-cement, it would reduce ettringite formation, while in bassinte-lime, there would be no influence. Thus, the impact of ettringite in the mechanical behavior of mixtures with PG depends on the soil type and solidification agents. The discussion about this issue should to be based on other test results like X-ray and SEM.
The fine tropical soil (Sample 9) exhibited 0.37 MPa compressive strength. When stabilized with 9% lime (Sample 10), the initial strength increased to 0.53 MPa. At 21 days, the strength reached 1.30 MPa, and its behavior was similar to that of Sample 2 (11DH-80S-9L) and was better than that of Sample 1 (91DH-9L), as shown in Fig.
3. This finding may induce smaller gains in strength in mixes of DH PG and lime. This smaller gain should be evaluated to determine whether it is sufficient for the good performance of this material as a pavement layer.
The results for PG
cement presented by Parreira et al. (
2003) are coherent with the values herein (PG
cement). However, for mixtures with lime (
Oliveira 2005), the strength was lower, but the compacted energy was different (
lime).
The value of unconfined compression strength obtained by James and Pandian (
2014) for
lime was higher than the value obtained for
PG
lime. This result probably was influenced by the soil type, the level of PG, and the compaction type. Unconfined compression strength values of HH gypsum with cement statically compacted, obtained by Ahmed and Issa (
2014) and Ahmed et al. (
2011), were lower than the values obtained in this study for HH PG. These results can be explained by the type of cement used (furnace cement type-B versus portland cement CPII Z-32 RS) or by the cement content (3% versus more than 9%). For samples with HH and lime (
,
Ahmed and Issa 2014), the results were similar (
PG
lime).
Studies conducted in the United States presented unconfined compressive strength values varying between 0.60 and 5.50 MPa for PG from different sources (
Chang et al. 1989). In this study, most samples had values that agreed with this range without considering the curing time. After 7 days of curing, Samples 5 (91HH-9L) and 7 (91HH-9C) had much higher values. Aiming to use the materials in a pavement base, North American specifications recommend compressive strength values greater than 1.70 MPa after 7 days of curing (
Gutti et al. 1996). In this sense, Samples 4 (11DH-80S-9C), 5 (91HH-9L), 7 (91HH-9C), and 8 (11HH-80S-9C) could be used for this application.
Dynamic Triaxial
To optimize costs and time of the dynamic triaxial equipment used, not all of the samples were tested. Thus, based on the simple compressive strength results, the samples containing HH PG were selected for this assessment because they had the best mechanical results. Table
7 presents a summary of the results from the triaxial dynamic tests. Except for Sample 8 (11HH-80S-9C) without curing, the mathematical model as a function of the confining stress (
) was verified to represent the data obtained from the tests more accurately.
The values of the resilient modulus (RM) presented in Fig.
4 were obtained for 0.371 MPa confining stress (
) and 0.126 MPa deviator stress (
). These stresses were determined considering the load of a tire 10.8 cm in diameter and with 0.55 MPa contact pressure on a structure commonly used in urban roads of the region composed of three homogeneous, isotropic, and elastic layers with the following characteristics:
•
3-cm-thick asphalt coating, Poisson coefficient () equal to 0.5, and RM equal to 2,000 MPa;
•
15-cm-thick base, equal to 0.5, and RM equal to 300 MPa; and
•
Subbase with equal to 0.5 and RM equal to 50 MPa.
The RM values obtained for the mixes containing HH PG are observed to be higher than those for the granular materials conventionally used in the region. This is the case of the lateritic gravel with RM values approximately equal to 430 MPa (
Cunha 2011), which demonstrates the technical viability of using these materials and the possibility of constructing durable roadways. In the cases of DH PG and cement mixtures, Pericleos and Metcalf (
1996) obtained RM values of 300 MPa after 28 days of curing. Thus, also considering this parameter, it is possible to verify the best mechanical behavior of HH PG.
The lateritic clayey soil (Sample 9) features an RM equal to 50 MPa, which is considered very low and does not show the possibility of being used as pavement base and subbase. When stabilized with 9% lime (Sample 10), the RM value rises to 360 MPa without evaluating the curing period, which shows the viability of using this stabilized soil. However, the RM values obtained from the incorporation of HH PG are much higher than the others, which shows that these mixtures have a great potential to use in regions with abundant fine lateritic soils.
In Samples 6 (11HH-80S-9L) and 8 (11HH-80S-9C), the values obtained were smaller than for the others, but they were sufficient to achieve good performance as pavement layers. In this case, depending on the type of asphalt mix used as a surface course, the base may still exhibit the structural behavior and stress distribution usually found in a flexible pavement.
In Samples 5 (91HH-9L) and 7 (91HH-9C), the RM values obtained were very high. If these mixtures are used as a base layer, depending on the type of surface course, they may even exceed the RM values of the asphalt mix, altering the structural behavior. Therefore, this should be considered when designing the pavement. These results agree with those obtained in the unconfined compression tests. In Sample 5 (91HH-9L), it is also verified that the RM value obtained in the test without curing was higher than in those with curing. This behavior was not expected, and a repetition of this test with better control of the mix homogenization process is recommended. In the international literature, there were no published data that presented RM results for mixtures with HH PG. Thus, for this parameter it was not possible to do comparisons.
X-Ray Diffraction and Scanning Electron Microscopy
Fig.
5(a) presents the results from the X-ray diffraction test conducted on the soil used in this study, where different peaks for the composition are visible, which includes quartz (
), kaolinite
, gibbsite
, and hematite (
). Fig.
5(b) also shows the SEM image for the natural soil (
Alves 2015), where it is possible to verify the presence of microaggregations with macropores between them. In the compaction process, microporosity is preserved, but the pore distribution is altered (
Camapum de Carvalho et al. 2015). These results are typical of fine lateritic soils.
Fig.
6 presents the results from the X-ray diffraction tests of the DH PG and of the thermally treated PG. The major phase of the DH PG was observed to correspond to
and that of the thermally treated PG corresponds to
. That is, the thermal treatment process at 130°C causes the formation of HH PG. The process for transforming DH PG into HH through oven-drying was also verified by Yang et al. (
2009). It is worth noting that the PG minor phases occur in low content, with relative peak intensities of 1%, which hinders identification.
While conducting this research, the mechanical results showed the need to perform X-ray diffractions on samples with DH containing lime or cement (Samples 1 and 3) and HH with lime or cement (Samples 5 and 7) (Figs.
7 and
8). During the DH transformation process into HH, not only bassanite but anhydrite can also be formed, as shown in Fig.
8. When HH is used on admixtures and compacted, the water addition caused its rehydration, which led to the presence of gypsum in Fig.
8.
For DH samples, the presence of ettringite [Figs.
7(a and b)] was observed, which can explain some mechanical characteristics that will be discussed next. For HH samples, there is no ettringite formation in lime mixtures [Fig.
8(a)], but this compound appeared in cement mixtures [Fig.
8(b)]. Ettringite is an expansive mineral that develops in the presence of sulfate, calcium, and aluminum compounds of a fine grain fraction at high pH levels. In the DH and HH samples, ettringite was a result of the reaction between aluminate, calcium in cement, lime, PG, sulfate in PG, and water in the tested sample, as discussed by Ahmed (
2015). In some cases, the aluminate amount is insufficient to react and form ettringite, which in this study it happened with the HH and lime mixture. Moreover, in the HH-lime mixture calcium carbonate was formed due the reaction between calcium in PG or lime with water in the tested sample, thus producing calcium oxide that reacts with carbon dioxide in atmosphere (carbonation). Ahmed (
2015) showed that the carbonation can limit the ettringite formation. Both substances, ettringite and calcium carbonate, will interfere on the mixture’s mechanical behavior.
In the SEM HH PG mixtures shown in images [Figs.
10(b and d)], it was not possible to identify ettringite formation. Thus, to complement the SEM tests, energy-dispersive X-ray spectroscopy (EDS) tests were conducted. As a result, Fig.
11(a) shows that there was aluminum in the DH sample, and Fig.
11(b) shows that this element was not identified on the HH sample, which could be due to the removal of gypsum impurities, such as aluminum, when subjected to thermal treatment. These results could indicate that ettringite cannot be formed in HH mixtures. However, in the X-ray results, ettringite was identified in the HH and cement mixture. This difference can be explained by the size and shape analysis between the DH and HH plates. Figs.
9 and
10 show that the DH plates are larger than the ones found in HH, as a result of the porous microstructure and less interactions between grains. In the DH mixtures, ettringite is placed around the plates because in this region there are porous and large surfaces that facilitate its formation [Figs.
10(a and c)]. On the other hand, in the HH mixtures there are less porous and plate surfaces [Figs.
10(b and d)] that can contribute to the ettringite effect.
According to Fig.
2, by adding cement or lime to HH PG, a reduction in swelling compared to the mixes for which DH PG will occur. This behavior can be justified by the microstructural difference between the HH and DH mixtures on ettringite formation as discussed. The ettringite formation in the DH PG and cement mixture and its relation with expansion are also discussed by Gutti et al. (
1996) and Roy et al. (
1996).
By analyzing Figs.
10(a and c), a larger amount of ettringite is observed in the mixture with DH cement, which can justify the greater swelling of this mixture compared to the DH lime, as verified in Fig.
2. The X-ray test results also confirmed the presence of ettringite in these samples (Fig.
7). In addition, the morphology of the ettringite observed in the mixture with cement [Fig.
10(a)] is different from that observed in the mixture with lime [Fig.
10(c)]. In the former, the elements have greater lengths; in the latter, the elements are smaller and form radial crystal spheres. Thus, the ettringite morphology in DH cement mixes appears to be more harmful in terms of swelling than in DH lime mixes.
As depicted in Fig.
3, the HH mixtures that were stabilized with cement exhibited greater strength than the mixtures stabilized with lime when considering the same content of materials and the same type of PG with samples similarly prepared. This behavior can be explained by Fig.
12 in which it can be observed that the cement and HH PG mix [Fig.
12(a)] is less porous and more homogeneous than the lime and HH PG mix [Fig.
12(b)]. These results illustrate the matrix bounding that occurs with the stabilization process. Thus, HH PG interacts better with cement than with lime, which causes an increase in strength. Hence, owing to the shape of the grains, HH PG interacts better with the components of the mixtures, resulting in materials with greater strength compared with those for which DH PG was used, as verified in Fig.
3.
For mixtures with the fine lateritic soil, DH or HH, cement or lime, the SEM images (Fig.
13) show that the DH soil mixtures show aggregations from the reaction of the cement or the lime with the soil, along with some pores and ettringite from the reaction of the DH and the lime or cement [Figs.
13(a and b)]. One can note that ettringite was attached only in the PG plate. For the HH soil mixtures [Figs.
13(c and d)], the images showed a more homogeneous mass, with microaggregations resulting from reactions between solidification agents and soil, but it was not possible to observe ettringite clusters. As verified from the mechanical properties presented in this paper, these mixtures had an intermediate behavior when compared with the others. However, the advantage to use the latertic soil in PG cement or lime mixtures is to reduce the swelling, which is explained by the soil’s mineralogical composition as shown in Fig.
5(a). The timely appearance of ettringite in DH mixtures is a likely result of the PG and does not generate negative consequences for the mixtures’ performance.