Use of Phosphoric Acid and Rice Hulk Ash as Lateritic Soil Stabilizers for Paving Applications

Abstract

Phosphoric acid (H₃PO₄) is a product that can be used as a stabilizing additive for tropical soils in an exploratory manner by the construction industry. For the drying process of this grain, its husks are used as fuel for ovens, generating rice husk ash (RHA), which is considered an environmental liability if not reused. In this sense, this paper aimed to evaluate the resilient behavior and the simple compressive strength, at different curing ages, of the use of rice husk ash and phosphoric acid in a simple and combined mixture for the stabilization of lateritic soil. The lateritic soil was mixed with different contents of RHA, H₃PO₄, and water and compacted in intermediate and modified Proctor energies. Fractured soil samples in the mechanical compressive strength tests were analysed by scanning electron microscopy and X-ray diffraction. The results show the potential for stabilization of the lateritic soil in question in terms of resilience and simple compressive strength through the addition of RHA and H₃PO₄ at different curing ages. The insertion of only H₃PO₄ produced the most satisfying resilient behavior. By adding RHA, the strength properties were improved, and good mixtures were obtained for use in paving.

1. Introduction

The use of soils in pavement structures designed according to the Brazilian empirical-mechanistic methodology is conditioned to the performance of cyclic tests to assess the resilient modulus (RM), which measures its stiffness and which conditions, together with the thickness of the layers, the states of the stresses, strains, and displacements of the structure. Resilient behavior is dependent on several factors, such as the physical state, the nature and composition of the materials, the degree of saturation, the compaction density and moisture, and the history and state of stresses, among other things [1,2,3,4,5,6,7].

Soils of tropical origin used in paving works can be made unfeasible due to the support capacity of the material in its natural state and when compacted. An alternative used to enable the use of soils with low bearing capacity is stabilization, whether chemical, mechanical and/or granulometric [8,9,10]. Traditional stabilizers such as cement, lime, fly ash and bituminous materials are widely used and studied; however, non-traditional stabilizers need more precise analysis to verify their effectiveness in treating soils [11,12,13].

A wide variety of materials can be used to stabilize the soils used in pavements, aiming to improve the properties of strength, compressibility, permeability, plasticity, and durability [14,15]. Therefore, it is necessary to know the physical, chemical, and mechanical properties of the soil and the additive to be used [16]. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests have been useful in evaluating the microstructure of soils with different stabilizing agents, as each type of additive causes different influences and mechanisms in soil stabilization [17,18].

In this alignment, additives that are unconventional but with local availability, as by-products of the industry and previously discarded as waste, have gained notoriety aiming at sustainability [13,15,19,20,21,22,23,24,25,26,27,28,29,30,31].

Rice husk ash (RHA) is waste generated globally, and studies on its use are important for sustainable reasons. Brazil is one of the largest rice producers in the world, with 11.7 million tons produced in 2018, and the southern region is responsible for around 70% of national production [32]. RHA is used as fuel for ovens in the rice-drying process, generating a large amount of ash. In this process, in many cases, there is no control of temperature, burning, or cooling time, and RHA’s pozzolanic properties are compromised, requiring industrial landfills to be deposited, generating, in addition to environmental liabilities, transport and disposal costs.

The use of RHA has shown positive results in the field of paving [33,34,35,36,37,38,39]. RHA is a light, bulky material with high porosity, consisting of more than 90% silica [40]. Its physical/chemical characteristics are directly related to aspects of the burning process, including the temperature, burning time, and cooling method.

H₃PO₄ is a product used in the pharmaceutical and food industries, among others, and in the field of paving, it has been the object of studies, showing good performance in the stabilization of tropical soils [11,13,41]. Given the fact that stabilization takes place by the chemical action of the additive on the soil minerals, thin soils are the most suitable for this stabilization. The stabilization of soils with phosphoric acid is understood because of the chemical reactions of the acid with the minerals present in the soil based on Fe₂O₃ and Al₂O₃. According to [13,42], H₃PO₄ reacts with those minerals exothermically and may contribute, in a promising way, to the stabilization of soils rich in iron and aluminium oxides.

The treatment of fine clayey soils with phosphoric acid results in the formation of compounds of phosphorus and iron or aluminium, which are hard, insoluble, and crystalline. Free Fe₂O₃ promotes the formation of these compounds, which makes the bearing capacity of soil thus treated greatly increased. This stabilization process generates strong chemical bonds, and in case of leaching, there will be no environmental damage by the generated compounds [41]. The isolated addition of CCA does not result in gains in strength and resilience in mixtures with lateritic soil; however, the combined addition of H₃PO₄ makes the use of this waste viable because, besides the environmental bias, it facilitates workability and improves mechanical performance.

The increase in H₃PO₄ in tropical soil demonstrates the reduction in the intensity of the crystalline peaks, showing that the acid reacts with the crystalline phases of the tropical soil, possibly forming amorphous or crystalline products, which justifies the better mechanical performance of that material [11].

In this sense, this work aimed to evaluate the use of rice husk ash and phosphoric acid, in a simple and combined way, to stabilize lateritic soil and to use it in base and subbase layers of pavements. For this purpose, resilient modulus and simple compressive strength tests were conducted at different curing ages and under varied compaction energies. To understand the behavior of the mixtures under the influence of compaction, scanning electron microscopy tests were conducted, and X-ray diffraction tests were conducted to analyse the mixtures’ compositions.

2. Materials and Methods

2.1. Materials

The lateritic clayey soil came from the city of Cruz Alta, located in the state of Rio Grande do Sul (Brazil). Its collection was conducted on pedological horizon B, under coordinates 28°37′39.40″ S and 53°37′30.50″ W. The soil in question, shown in Figure 1A, originates from Botucatu sandstone or the junction of Botucatu sandstone with basalt. The soil has a reddish colour and medium texture, with approximately 35% clay in its composition. It consists of a typical dystrophic red latosol, with the presence of A, B, and C horizons, with an indistinct transition between them. It presents a slight podzolization, with the migration of aluminium and organic matter, with or without the presence of iron, up to the B horizon.

The authors [43] performed the physical, chemical, and soil classification, shown in

Table 1. Physical and chemical characterization and classification of the lateritic soil used in this study [43].

Physical Parameters		Chemical Analysis
% coarse sand (0.6–2.0 mm)	0	Cation exchange capacity	1.8
% medium sand (0.2–0.6 mm)	8	Basic cations Ca/K/Mg (Cmolcdm3)	0.3/0.02/ 0.4
% fine sand (0.06–0.2 mm)	25
% silt (2 µm–0.6 mm)	26	Saturation-Al/Bases (%)	55.6/9.2
% clay (% 2 µm)	41	Organic matter (%)	0.20
Specific weight (kN/m3)	27.8	pH	5.8
Liquidity limit (%)	55	X-ray Fluorescence—ED XRF
Plasticity limit (%)	44	Fe2O3	36.88
Plasticity index (%)	11	SiO2	32.45
Classification		Al2O3	22.38
AASHTO classification	A-7-6	TiO2	4.72
USCS classification	MH	P2O5	0.99
MCT classification	LG’	Others	2.58

. They verified the influence of compaction energy on the resilient behavior of the three horizons of the deposit. They showed the good resilient behavior of the compressed horizons in the three Proctor energies, particularly the intermediate and modified energies. There is an increase in stiffness as the compaction energy increases. This can directly impact the distribution of the internal forces of a pavement, which is correlated with the parameters of the compaction curve and the physical indices of the soil condition.

By analyzing the granulometric distribution of the soil, it appears that it is composed of 67% fine grains and 33% sandy material. According to the Atterberg limits, the soil presents high plasticity, corroborating the particle size analysis and the predominance of clay particles. Regarding the AASHTO and USCS classifications, the soil in question falls, respectively, in the A-7-6 group, corresponding to plastic clays with the presence of organic matter and high compressibility, and in the MH group, being considered an inorganic elastic silt. However, in the MCT classification [44,45], which was developed for tropical soils, the soil is considered a clay with lateritic behavior (LG’), having low mass loss by immersion, high bearing capacity, low expansion, low permeability, and medium to high plasticity.

Regarding chemical characterization, the presence of silicon dioxide, iron oxide, and aluminium oxide prevailed, with results consistent with the MCT classification and the physical particularities of the deposit. The presence of iron and aluminium hydroxides causes the natural cementation of the material, enhancing its application. The organic matter portion is related to the cation exchange capacity (CEC), characterizing the B horizon as clay with low activity and little or no presence of organic matter.

The rice husk ash used (Figure 1B) was generated in an industry located in the city of Alegrete in the State of Rio Grande do Sul (Brazil). The ash came from the burning of this husk in a rice drying oven without temperature control and with unknown burning and cooling time. The RHA presents a dark color due to the presence of high carbon content and due to the burning being performed at temperatures ranging from 400 to 700 °C. According to the authors [46], burning in this temperature range results in an RHA with a predominance of amorphous structures, responsible for pozzolanic reactions in the presence of calcium hydroxides.

An attempt was made to verify the size of its particles; however, the ash presented with a brittle behavior, and its particles broke as it was sieved. The longer it was sieved, the finer its granulometry. A good portion of the mixture of soil and RHA passed through a sieve of size 0.075 mm, being necessary to perform a sedimentation test. However, when the two materials were stirred, the ash remained in suspension in inside the test cylinder, making it impossible to analyze it.

2.2. Methods

Table 2. Mixing conditions, compaction energy, and nomenclatures used.

Condition	Compaction Energy	Nomenclature Adopted	MDD (kg/m3)	OMC (%)
Reference soil	Intermediate	Soil-IE	1625	25.60
	Modified	Soil-ME	1652	22.60
Soil + 3% H3PO4	Intermediate	S3-IE	1683	22.49
	Modified	S3-ME	1729	21.67
Soil + 1% H3PO4 + 5% RHA	Intermediate	S15-IE	1581	27.15
	Modified	S15-ME	1657	24.19
Soil + 3% H3PO4 + 5% RHA	Intermediate	S35-IE	1585	27.22
	Modified	S35-ME	1644	24.72
Soil + 3% H3PO4 + 10% RHA	Intermediate	S310-IE	1504	27.17
	Modified	S310-ME	1605	25.17

IE—Intermediate Proctor compaction energy (13 kgf.cm/cm³), ME—Modified Proctor compaction energy (27.3 kgf.cm/cm³).

summarizes the mixtures conducted, the nomenclatures adopted, the compaction energy used, and the ideal parameters for the compaction of the samples. The definitions of the mixing conditions analyzed in the present research were based on the conclusions of some researchers [41,47] who evaluated the use of RHA and H₃PO₄ with lateritic soils, concluding that the ideal RHA content varies between 3 and 5%, as well as that the acid is limited to 3% insertion.

The compaction parameters were obtained for each of the ten conditions in a tripartite mould of 100 mm in diameter and 200 mm in height, following the precepts of [48], to obtain the maximum apparent dry mass of the soil (MDD) and optimum moisture (OMC). Ten conditions were analyzed, including two as reference (without chemical stabilizer) and eight mixtures with different energies. Figure 1C illustrates the mixture of lateritic soil with RHA and H₃PO₄ before material compaction, and Figure 1D shows a compacted specimen about to be tested in the repeated load triaxial equipment (Figure 1E).

The increase in compaction energy results in an increase in the maximum apparent dry mass and a decrease in the optimum reference soil moisture. This behavior is repeated in soil mixtures with RHA and H₃PO₄ as energy increases. On the other hand, as more RHA is added to the mixtures, the maximum apparent dry mass is increasingly smaller while the moisture content increases. The low density of ash grains justifies this phenomenon of reduced density with the inclusion of RHA, a behavior also evidenced by some research [47,49,50].

2.2.1. Mechanical Tests

The resilient modulus tests were conducted in the repeated load triaxial (RLT) equipment, which is capable of reproducing the cyclic loading conditions that occur in a pavement structure in the labratory. The precepts of the DNIT standard [4] were followed, which has procedures similar to those in AASHTO [51].

For molding the specimens in a mechanical impact compactor, the maximum variations of ±0.5% about the optimum moisture and ±1.0% about the maximum density were respected. Different curing ages of the samples were evaluated with additions of RHA and/or H₃PO₄ at IE and ME energies, with tests being conducted at the ages of 7, 28, 56, and 91 days. The reference soil was tested with 0 days in the two Proctor compaction energies.

To determine the resilient parameters, two consolidated models, presented in

Table 3. Models used to obtain resilient parameters.

Model	Equation
Pezo et al. [59]	$M_R=k_1{·{\sigma }_3}^{k_2}{·{\sigma }_d}^{k_3}$
NCHRP 1-37A [60]	$M_R=k_1{·{\rho }_a\left(\frac{\theta }{\rho a}\right)}^{k_2}·{\left(\frac{{\tau }_{oct}}{\rho a}+1\right)}^{k_3}$

Where: $M_R$: resilient modulus; ${\sigma }_3$: confining stress; ${\sigma }_d$: deviator stress; $\theta$: main stress; ${\tau }_{oct}$: octahedral stress; ${\rho }_a$: atmospheric pressure; $k_1,k_2$, $k_3$: experimentally determined resilience parameters.

, were used [43,52,53,54,55,56,57,58], which correlate the applied stresses with experimentally determined constants. Non-linear multiple regressions were performed using the Statistica 10.0 software to verify the model that best represents the behavior of each condition.

Furthermore, to identify the contribution of additives to soil strength, simple compressive strength (UCS) tests were performed according to [61] in molded specimens measuring 50 mm in diameter and 100 mm in height following the optimum compaction conditions presented in Table 2. These were tested at the curing ages of 7 days for the two reference soil conditions and 7 and 91 days for the other mixtures. The choices of these curing ages were strategic, aiming to conduct the tests at the initial and older ages at which the resilient parameters of the proposed mixtures were obtained.

2.2.2. Scanning Electron Microscopy

Scanning electron microscopy was performed to observe the modifications that occurred in different sample conditions. From scanning electron microscopy, it is possible to perform physical visualization of the structural arrangement of the particles, as well as the shape and particle size, while also enabling a perception of the voids present in the material, which tends to vary with the compaction energy and with different compositions. Thus, samples of fractured soils were analysed by scanning electron microscopy, including the reference soil (aged 7 days) and soils containing RHA and H₃PO₄ (aged 7, 28, and 56 days), at 1000× magnification. The samples were dried in an oven at a temperature of 60 °C for 24 h and then metalized with gold. A JEOL JSM-6360 (Austin, TX, USA) scanning electron microscope was used.

2.2.3. X-ray Diffraction

The mineralogical characterization of the reference soil, RHA, and soil samples containing different levels of RHA, and H₃PO₄ was performed with the Rigaku diffractometer, model Miniflex^® 300, which presents Cu Kα radiation (λ = 1.54051 Å) and energy source of 30 kV, and 10 mA. The Step Mode has a Scan speed of 0.5 s and Scan step of 0.03°, in angles from 5° to 60°. To conduct the XRD analyses, the reference soil and the RHA passing through a 0.149 mm mesh (mesh 100) and 75 µm mesh (mesh 200) sieve were analysed, respectively. Furthermore, the samples for the four mixtures presented in Table 2 were analysed from the crushing and sieving (100 mesh opening) of the specimens compacted in the modified energy at the curing ages of 7, 28, and 56 days. In order to identify the crystalline phases of the studied samples, crystallographic files from the American Mineralogist Crystal Structure Database (AMCSD) were consulted.

3. Results and Discussion

3.1. Resilient Modulus and Simple Compressive Strength

For each condition of addition of phosphoric acid and/or RHA, three specimens were compacted and submitted to the resilient modulus test at the four curing ages, except for dosage S3, which had its samples tested at 7 and 28 days. After repeated load triaxial testing, using non-linear multiple regression, the compound and universal models were evaluated, which have their parameters k1, k2, and k3, shown in

Table 4. Resilient parameters and average resilient modulus for compound and universal models.

Condition	Curing Age	Model	k1	k2	k3	R2	Average RM (MPa)
Soil-IE	0	Compound	739.2	0.34	0.00	0.90	287
		Universal	688.64	0.52	−4.71	0.93	271
Soil-ME	0	Compound	1640.16	0.43	0.08	0.93	340
		Universal	1185.26	0.65	−0.55	0.94	349
S3-IE	7	Compound	2323.70	0.50	0.12	0.96	465
		Universal	1141.95	0.72	−1.43	0.97	459
	28	Compound	2494.14	0.53	0.10	0.96	475
		Universal	1204.53	0.74	−1.57	0.95	470
S3-ME	7	Compound	2407.38	0.51	0.09	0.99	493
		Universal	1158.79	0.67	−1.33	0.97	494
	28	Compound	2574.28	0.53	0.08	0.99	523
		Universal	1285.39	0.69	−1.63	0.97	525
S15-IE	7	Compound	608.35	0.42	−0.05	0.86	209
		Universal	479.78	0.54	−2.92	0.84	209
	28	Compound	957.30	0.48	0.00	0.91	260
		Universal	751.09	0.70	−3.69	0.94	260
	56	Compound	1431.56	0.54	0.05	0.93	297
		Universal	897.20	0.79	−3.10	0.95	296
	91	Compound	1948.12	0.60	0.08	0.96	324
		Universal	1025.78	0.86	−2.77	0.96	323
S15-ME	7	Compound	1633.51	0.53	0.11	0.98	305
		Universal	762.06	0.73	−1.42	0.97	304
	28	Compound	1755.25	0.49	0.17	0.92	326
		Universal	664.17	0.63	0.20	0.92	333
	56	Compound	2652.11	0.62	0.17	0.95	352
		Universal	983.21	0.86	−1.21	0.94	352
	91	Compound	2943.89	0.65	0.15	0.94	376
		Universal	1153.21	0.91	−1.76	0.95	376
S35-IE	7	Compound	1506.25	0.55	0.11	0.93	265
		Universal	655.78	0.73	−1.23	0.91	265
	28	Compound	1679.36	0.51	0.14	0.95	313
		Universal	768.59	0.72	−1.24	0.95	314
	56	Compound	2339.51	0.62	0.14	0.96	327
		Universal	970.83	0.87	−1.77	0.95	329
	91	Compound	2785.62	0.63	0.16	0.94	362
		Universal	1089.52	0.93	−1.65	0.93	352
S35-ME	7	Compound	2056.53	0.62	0.07	0.96	333
		Universal	1129.76	0.90	−3.18	0.96	332
	28	Compound	3215.45	0.72	0.10	0.97	381
		Universal	1563.81	1.07	−3.58	0.98	378
	56	Compound	3771.49	0.73	0.11	0.97	417
		Universal	1715.25	1.09	−3.39	0.97	414
	91	Compound	3968.75	0.73	0.14	0.98	423
		Universal	1576.53	1.08	−2.93	0.98	399
S310-IE	7	Compound	1261.34	0.49	0.12	0.85	258
		Universal	571.73	0.67	−0.97	0.89	251
	28	Compound	1187.18	0.45	0.15	0.97	256
		Universal	534.73	0.62	−0.52	0.97	255
	56	Compound	1312.62	0.45	0.12	0.95	298
		Universal	602.36	0.58	−0.52	0.94	300
	91	Compound	1658.49	0.51	0.12	0.97	319
		Universal	710.73	0.66	−0.74	0.95	321
S310-ME	7	Compound	1788.00	0.58	0.09	0.97	303
		Universal	915.27	0.84	−2.48	0.96	301
	28	Compound	2150.45	0.60	0.15	0.99	312
		Universal	890.87	0.85	−1.58	0.98	312
	56	Compound	2584.05	0.66	0.16	0.99	320
		Universal	1005.63	0.93	−1.83	0.98	320
	91	Compound	2719.99	0.64	0.17	0.98	339
		Universal	1055.12	0.93	−1.74	0.98	338

, as well as the average resilient modulus obtained after regression for the joint analysis of the three samples tested for each condition.

The compound and universal models showed good correlations, thus representing the behavior of the conditions and mixtures in question well. When comparing the curing ages within the same mixture condition in both models, it is noted that there is an increase in k1 values in all tested mixtures, regardless of the compaction energy. Therefore, there is a gain in resilience with the addition of the curing age for the same dosage. When comparing the two compaction energies, it is evident that the value of the resilient modulus increases as the compaction energy increases from intermediate to modified.

To facilitate the visualization of the mechanical behavior of the evaluated conditions, Figure 2 demonstrates the average resilient modulus for the compound model at different curing ages and the simple compressive strength obtained at 7 and 91 days. The soil compacted in the intermediate energy presented an average RM of 287 MPa and 340 MPa in the modified energy. Regarding the simple compressive strength, the variation of the curing age did not interfere with the behavior, presenting 0.73 MPa and 1.53 MPa for the intermediate and modified energies, respectively, in the two evaluated ages.

The S3 mixture, which involves soil and phosphoric acid, showed the best resilient behavior, reaching 475 MPa and 523 MPa at 28 days in intermediate and modified energies, respectively. There was no considerable gain in compressive strength as the curing ages increased, ranging from 1.91 to 2.10 MPa for intermediate energy in 7 and 91 days, respectively, and 2.11 to 2.22 MPa for modified energy at the same ages.

The addition of RHA in the soil and H₃PO₄ mixtures resulted in different behavior from the S3 condition. The increase in RHA interfered negatively with the resilient modulus of the S15, S35, and S310 mixtures compared to the reference soil and the S3 mixture. At an early age, the resilient modulus value of these three mixtures was lower than the reference soil in the two compaction energies. Among the mixtures involving RHA, S35 was the one that stood out both in terms of resilient behavior and simple compressive strength when compared to the two energies of the reference soil. S310 had the worst resilience gain among the samples involving RHA as the time interval was varied, while the worst behavior and strength gain was for S15.

Thus, when analyzing all test conditions, it can be concluded that S3 has the best mechanical behavior in the performed tests. Among the samples involving RHA, S35 showed good behaviors. However, the insertion of 10% RHA was not beneficial in the S310 mixture.

In Figure 3, it is possible to observe the behavior of the samples analyzed with the composite model; in Figure 3A, the behavior of Soil-IE is shown, and in Figure 3B,C, the comportment of S3-IE and S35-IE can be analyzed, respectively. In these figures, it is possible to verify the behavior as the stresses are elevated as well as the average resilient modulus.

The behavior of Soil-IE (Figure 3A) indicates that as the deviating stress increases, there is a decrease in the resilient modulus, whereas as the confining stress increases, there is an increase in the RM. The same behavior regarding stresses can be observed for the S3-IE (Figure 3B) and S35-IE conditions at 28 days (Figure 3C). The RM rises when adding H₃PO₄ alone and when adding RHA and H₃PO₄ in combination with fine soil.

It is important to note that the workability when using a mixture with phosphoric addition was difficult compared to the reference soil. However, when introducing RHA into the mixture, it was noticed that RHA reduced the impact of H₃PO₄ on this factor, and the higher the content of ash on the acid, the better the workability of the mixture. Thus, the use of RHAA, besides contributing to a correct destination aiming at sustainability, tends to improve the mechanical behavior of this lateritic soil.

3.2. Electron Microscopy

Figure 4A,B shows micrographs of the fracture surface of samples of compacted reference soils at intermediate and modified energies, respectively, aged 7 days, with a magnitude of 1000×. It is observed that in both samples, the surface presents with a dense aspect, with no noticeable difference in morphology regardless of the compaction energy.

Although the simple compressive strength is greater for the modified energy sample (1.53 MPa) than for the intermediate energy sample (0.73 MPa), no morphological change is noticed. It is inferred that the reference soil sample compacted at intermediate energy (Figure 4A) presents higher added moisture content, providing this sample with a morphological aspect with greater compactness, similar to samples compacted at modified energy (Figure 4B).

Figure 5 shows several micrographs of the fracture surface of soil samples containing different mixtures with H₃PO₄ and/or RHA compacted in intermediate and modified energies, aged 28 days. All micrographs presented have a magnitude of 1000× to enable comparison between samples.

It is observed that there was greater compactness of soil samples compacted in modified energy (Figure 5B,D,F,H) when compared to samples compacted in intermediate energy (Figure 5A,C,E,G), with emphasis on the S3 sample compressed in the modified energy. This corroborates with the greater dry apparent specific weight reached by the S3-ME sample (see Table 1), as well as the greater compressive strength at the ages of 7 days and 91 days, as shown in Figure 2. On the other hand, the morphology of the S15-IE soil sample presents the lowest compactness compared to the other morphologies. This lower compactness corroborates the lower compressive strength results of the S15-IE sample at the ages of 7 days and 28 days when compared to the strengths reached by the other samples, except for the reference sample.

3.3. X-ray Diffractograms

Based on the X-ray diffractogram of the natural soil, shown in Figure 6A, characteristic crystalline peaks of quartz (SiO₂), kaolinite (Al₂Si₂O₅(OH)₄), and hematite (Fe₂O₃) were identified. It was possible to identify the presence of quartz (SiO2) in regions close to 2Ɵ = 20.87°, 26.65°, and 50.17° (#AMCSD 0000790), kaolinite (Al₂Si₂O₅(OH)4)) in regions close to 2Ɵ = 12.41°, 20.38°, and 24.97° (#AMCSD 0012237), and hematite (Fe₂O₃) in regions close to 2Ɵ = 24.16°, 33.21°, 35.62°, 49.49°, and 54.14° (#AMCSD 0002226).

An analysis of the RHA X-ray diffractogram, Figure 6B, shows an amorphous halo between 10° and 20°. This diffractogram confirms that there is no control in the rice husk ash burning process. It is also observed that characteristic crystalline peaks are identified as cristobalite phases in regions close to 2Ɵ = 22.07°, 28.57°, 31.53°, 36.50°, 42.89°, 45.02°, 47.16°, 48.83°, and 57.41° (#AMCSD 0018342). The identification of characteristic peaks of this mineral in the RHA indicates that there was not an adequate burning process, which resulted in the absence of a predominantly amorphous material. This behavior is the characteristic of the RHA studied in the present work, which had its burning conducted without controlling temperature, time, and the form of cooling. The authors of [62] studied different temperatures and firing times of rice husks and observed that at 600 °C, the rice husk ash showed an amorphous halo between 15° and 30° (2Ɵ). However, at 700 °C, they observed the formation of crystalline cristobalite phases near 22° (2Ɵ), and at 800 °C they were able to identify new crystalline cristobalite peaks, similar to the peaks identified in Figure 6B.

Figure 7 demonstrates the X-ray diffractograms of the four mixtures with two different curing ages compared to the reference soil. It is observed that the characteristic peaks of the quartz (2Ɵ = 20.87°, 26.65°, and 50.17°), kaolinite (2Ɵ = 12.41°, 20.38°, and 24.97°), and hematite (2Ɵ = 24.16°, 33.21°, 35.62°, 49.49°, and 54.14°) phases identified in the natural soil sample remain constant in the samples containing RHA and H₃PO₄, regardless of age (28 and 56 days). This shows that these phases do not change in the presence of H₃PO₄.

In the samples containing RHA, the main peak of the cristobalite phases (2Ɵ = 22.07°) may be superimposed on peaks characteristic of the quartz phase (2Ɵ 20.87°) identified in the natural soil sample. The other characteristic peaks of cristobalite (28.57°, 31.53°, and 36.50°), due to the low intensity in the original RHA (Figure 6B), were not identified in the samples containing the natural soil, H₃PO₄, and RHA.

A new peak in regions close to 42.22° was identified only in the samples S35 (28 days) and S310 (28 days). It is noted that these samples were obtained by mixing the natural soil with 3% H₃PO₄ and 5% and 10% RHA, respectively. It is inferred that there was the formation of a crystalline aluminium phosphate (AlPO4) phase from the reaction of H₃PO₄ with natural soil compounds. Characteristic peaks of berlinite (AlPO4) were identified at 2Ɵ = 26.43°, 20.75°, 49.70°, and 42.22° (#AMCSD 0007986). However, the most intense peaks of the berlinite (AlPO4) phase may be superimposed on peaks characteristic of quartz, kaolinite, and hematite. However, in samples S35 (56 days) and S310 (56 days), the peak was 42.26° (2Ɵ), which indicates that the AlPO4 phase is unstable and, from reactions in the hardened material between the ages of 28 and 56 days, underwent a chemical transformation. This could justify the 27% gain in compressive strength for the S15-ME sample between the ages of 7 and 91 days.

4. Conclusions

Physical, chemical, and mechanical characterizations are essential to choosing stabilized soils and mixtures for use in viable pavement structures. The isolated analysis of soil compaction tests demonstrates that as the compaction energy is modified and increased, the maximum apparent dry mass increases, and the optimum moisture content decreases. By inserting phosphoric acid into the soil in isolation, the MDD increased, and the moisture content was reduced. The insertion of RHA together with H₃PO₄ in the lateritic soil results in an increase in optimum moisture and a reduction in MDD. The higher the RHA content, the lower the MDD and the higher the optimum moisture content.

Regarding the resilient modulus tests, the additions proved to be positive in the analysis of the resilient behavior of the soil, especially at more advanced curing ages. Even with the decrease in the values noted when rice husk ash was used, all the levels investigated showed an improvement in the results over time, indicating that the RHA, even with the indications that its burning did not occur with temperature control, resulted in a material with pozzolanic activity. As for phosphoric acid, when used alone, the improvement in the resilient modulus is indisputable. However, with the inconvenience noted in the workability of the mixture, the addition of RHA to the mixtures is justified. The gain in compressive strength was more significant under conditions with RHA in their composition. The best strengths occurred in the S3 and S35 mixtures. In general, considering the two mechanical tests, the additions of RHA and H₃PO₄ were beneficial, mainly in the S3 and S35 mixtures, which showed great potential for use.

With the help of SEM, it was found that there was a greater compactness of fractured soil samples compacted in modified energy when compared to samples compacted in intermediate energy, with an emphasis on sample S3-ME. In the mineralogical analysis, no substantial changes were identified between the diffractograms of the different samples, regardless of the content of H₃PO₄ and RHA, and the diffractogram of the reference soil. However, it was verified that the characteristic peak of cristobalite reduced the intensity between 28 and 56 days for S15. It is understood that H₃PO₄ reacts with this crystalline phase over time, which could justify the gain in compressive strength.

Therefore, the additions of H₃PO₄ and RHA were positive in analysing the resilient behavior and the simple compressive strength of lateritic clayey soil. The increase in compaction energy and the insertion of these materials can be a beneficial resource in the search for better behavior of pavement subbases and bases.