Study on the performance of gravel stabilization by red mud-based cementitious materials

2.1. Raw materials

The main constituent materials of red mud-based cementitious materials used in this test are Bayer red mud, slag micro powder, desulfurization gypsum, and the chemical composition is shown in Table 1. The red mud is Bayer red mud, which is taken from Shandong WeiqiaoChuangye Group Co. Its pH value is 11.74, specific surface area is 443.6 m²/kg, density is 2.86 g/cm^3, Al₂O₃, Fe₂O₃, SiO₂, Na₂O are the main components of red mud. The slag is S₉₅ blast furnace slag micro powder, taken from Shandong Luxin Building Materials Co., Ltd, with a specific surface area of 410.3 m² /kg, density of 2.84 g/cm³, and the main components of Al₂O₃, SiO₂ and CaO. Desulfurization gypsum is taken from Shandong WeiqiaoChuangye Group Co., Ltd, and the main component of CaSO₄-H₂O. The excitant is sodium silicate, purchased from Sinopharm Chemical Reagent Co. Ltd, the main component is Na₂SO₄-9H₂O, modulus 1.03. All raw materials according to the red mud: slag: gypsum: sodium silicate = 50:43:6:1 preparation of red mud-based cementitious materials, cementitious materials of the physical properties of the indicators shown in Table 2, can be found that the red mud-based cementitious materials have a good performance, and its cementitious strength indicators higher than the ordinary silicate 32.5 cement. The aggregate used in the test is limestone, which is divided into four grades of 20-30 mm, 10-20 mm, 5-10 mm and 0-5 mm according to the particle size, and the sieving results of the aggregate meet the requirements of the specification, and the physical indexes of each grade of aggregate are shown in Table 3.

2.2. Mixing ratio design

According to the sieving results of the four grades of aggregates, the skeleton dense cement stabilized gravel material was prepared. The external mixing method is used to add cementitious materials with mass fractions of 4 %, 5 %, 6 %, 7 % and 8 %, respectively, and the compaction test is carried out according to the C method in the Test Procedure for Stabilized Materials of Inorganic Binding Materials for Highway Engineering (JTG E51-2009), to obtain the maximum dry density and the best water content.

2.3. Test protocol

The preparation of specimens and the determination of mechanical properties were carried out according to the Test Procedure for Stabilized Materials of Inorganic Binding Materials for Highway Engineering (JTG E51-2009). Firstly, the homogeneous mixture was prepared by hydrostatic molding method to prepare the specimens of corresponding sizes, and then the specimens were sealed in plastic bags and put into the standard curing room for curing, and finally the specimens were put into the water to soak for 24 h for the test of the mechanical properties one day before reaching the specified age. Scanning electron microscope (SEM) was used to analyze the micro-mechanisms of the cementitious materials tested at different ages.

3.1. Analysis of compaction test results

The optimum moisture content and maximum dry density of stabilized gravel with cementitious material can be obtained from the compaction test, and the results of the optimum moisture content and maximum dry density of stabilized gravel with the dosage of red mud-based cementitious material are shown in Table 4. It can be seen that the optimum moisture content and maximum dry density of stabilized gravel material increase gradually with the increase of the dosage of red mud-based cementitious material. In the range of cementitious material dosage, the optimum moisture content is between 4.5 % and 5.4 %, and the maximum dry density is between 2.403 g/cm³ and 2.448 g/cm³.

3.2. Compressive strength test

The 7-day unconfined compressive strength test was carried out on different types of cementitious materials stabilized gravel materials. The specimen size is a cylindrical specimen of $\phi$150 mm×150 mm, and the results of unconfined compressive strength are shown in Fig. 1. The results in Fig. 1. show that the representative value of 7-day unconfined compressive strength increases with the increase of cementitious material dosage, and the representative value of the strength of the specimen when the dosage of cementitious material is 5 % meets the strength requirement of the base layer of the highway, and the representative value of the strength of the specimen when the dosage is 4 % meets the strength requirement of the subbase layer. In addition, the strength of the specimen gradually tends to a certain value with the increase of age. The strength value of stabilized gravel material mainly depends on the following three aspects: one is the skeleton embedding effect between coarse aggregates, the second is the filling effect of fine aggregates, and the third is from the bonding effect between cement and aggregates. Under the same condition of grading, the increase of binder dosage highlights the bonding effect between binder and aggregate, which further increases the 7-day unconfined compressive strength value of specimen.

Fig. 1Variation of compressive strength with a) cementitious material dosage and b) different ages

a)

b)

3.3. Splitting strength test

The split strength test is used to evaluate the strength of the semi-rigid base layer material and reflect the tensile property of the material through the size of its split strength. Cylindrical specimens of $\phi$150 mm×150 mm was prepared for the splitting test with different dosages. Fig. 2. shows the splitting strength results of stabilized aggregates under different maintenance age conditions. It can be seen that as the dosage of cementitious material increases, the corresponding indirect splitting strength shows a gradual increase. This is attributed to the fact that as the dosage of cementitious material increases, the hydration is more significant, generating more hydrated calcium silicate (C-S-H) gels, which in turn leads to an increase in the splitting strength.

3.4. Fatigue performance test

According to the specification using hydrostatic pressure method to form the specimen of 100 mm×100 mm×400 mm center beam, using the method of three-point loading of different dosage and type of cementitious material stabilized gravel mixture for 28 d bending and tensile strength test, the test results are shown in Fig. 3. As shown in Fig. 3, under the condition of the same age, the fatigue life of stabilized gravel material increases with the increase of the dosage of red mud-based cementitious material, and the fatigue life of the mixture is improved by about 30 %-40 % for every 1 % increase of cementitious material dosage. A large amount of AFt in the system can play the role of fiber reinforcement in the system, bear part of the tensile stress in the stress process, reduce the cracking phenomenon of stabilized gravel material, thus improving the splitting strength and resistance to bending and tensile deformation of the mixture.

Fig. 2Relationship between splitting strength and cementitious material type and content

Fig. 3Fatigue life at different stress ratios for different red mud dosages

3.5. Microanalysis

SEM was used to analyze the microscopic morphology and physical phase composition of the hydration products of the red mud-based cementitious materials, and Fig. 4 shows the SEM photographs of the specimens at different ages, respectively.

Fig. 4SEM of samples of a) 7 d and b) 28 d

a)

b)

As can be seen from Fig. 4, the surface of the specimen hydrated for 7 days has hexagonal plates of calcium hydroxide and needle-and-rod AFt, while more unreacted materials are embedded in the slurry, with more obvious pits on the surface and loose structure. With the prolongation of hydration age, the hydration degree increased, a large amount of gypsum was consumed to generate AFt, and a large number of needle-and-rod AFt intertwined to improve the flexural strength of the material, and the surface of the specimen hydrated to the age of 28 days was very dense, and there were a lot of AFt and C-S-H gels stacked and cemented together with each other.