Preparation of steamed bricks from a hematite tailings

In the resource utilization of tailings, a lot of research has been carried out at home and abroad, and good results have been achieved. These include comprehensive recovery of valuable elements from tailings, filling of goafs with tailings, preparation of glass-ceramics with tailings, and production of various building materials. Western Hubei a hematite tailings is a composite of mineral raw materials, the fine particles and mixed multi-component characteristics, it is more suitable for the field of construction materials. Based on the preliminary study on the preparation of non-burning and steam-free bricks from the tailings, this study carried out research on the preparation of another new wall material, steamed brick, with higher strength, in order to expand the application of the tailings. Scope, truly realize the efficient use of the bulk of the tailings.

I. Test materials and equipment

(1) Test materials

1. Hematite tailings

Hematite tailings from mineral processing plant Hubei a mining company, is a high phosphorus crushed hematite - tailings recovery after high intensity magnetic separation of the iron ore produced - grinding - centrifugal classification. The results of X-ray diffraction analysis (see Figure 1) indicate that the mineral composition of the tailings is mainly hematite and quartz , followed by chlorite and calcite .

Figure 1 XRD pattern of hematite tailings

1-hematite; 2-quartz; 3-chlorite; 4-calcite

The chemical composition and particle size composition of the tailings are shown in Tables 1 and 2.

Table 1 Chemical composition of iron tailings

Table 2 Iron tailings particle size composition

2, other raw materials

(1) Yellow sand. In order to meet the requirements of raw material chemical composition, mineral activity and particle grading, a certain amount of siliceous material needs to be added instead of part of tailings. In this test, commercially available yellow sand was selected as the aggregate, and its chemical composition and particle size composition are shown in Tables 3 and 4.

Table 3 % chemical composition of yellow sand

Table 4 Yellow sand particle size composition

Production of tailings free-burning brick products, quartz content in tailings should not be less than 30%. It can be seen from Table 1 that the iron content in a hematite tailings in western Hubei is relatively high, while the content of SiO ₂ is low. Low silicon content will reduce the strength of the product and thus affect the quality of the finished product. Therefore, it is conceivable to add other raw materials with low iron content and high silicon content to the ingredients. Table 2 shows that the content of iron in yellow sand is low, and the silicon content reaches 90.60%, which can meet the demand for ingredients.

(2) Cement. Cement is both a cementing agent and an active activator when producing industrial waste bricks. This test used 325 ^# composite Portland cement.

(3) Gypsum . Gypsum acts as a promoter in the brick reaction system, which can improve the strength and stability of the product. Although its dosage is not large, it has a great influence on the activation of the waste residue. The test used commercially available gypsum.

(2) Admixtures

The admixture has the function of improving the workability, water reduction and strength of the mortar, which can save the cement dosage and reduce the construction cost. Admixtures used in this test include the water reducing agent calcium lignosulfonate, the early strength agent calcium chloride, sodium sulfate and triethanolamine.

(3) Main test equipment

Batching equipment: JJ-5 cement mortar mixer, DHG9626A constant temperature blast drying oven.

Molding equipment: YES-100 digital display hydraulic pressure testing machine.

Molding mold: φ50mm×50mm stainless steel mold.

Maintenance equipment: YH-40B standard constant temperature and humidity maintenance box.

Test equipment: YES-100 digital display hydraulic pressure tester, D/Max-IIIA XRD diffractometer, Axios advanced X-ray fluorescence spectrometer, JSM-5610LV scanning electron microscope.

Second, iron tailings steaming brick preparation process

The preparation process of iron tailings steamed bricks is shown in Figure 2. Add 15% water and a certain amount of admixture according to a certain ratio of raw materials, stir in a blender, and mix the materials evenly. The mixed materials were aged at room temperature for 40 min and then press-formed on a hydraulic pressure tester under a pressure of 20 MPa. The press-formed product is placed in a standard constant temperature and humidity curing box and then subjected to moist heat curing according to a certain steaming system, and then naturally maintained for a certain period of time, that is, the tailings steamed brick product is obtained.

Figure 2 Process flow of tailings steaming brick preparation

Through pre-test, it is determined that the ratio of raw materials is m tailings: m aggregate: m cement: m gypsum = 78:0:10:2. This test mainly investigates the type and amount of admixture, steaming system and natural curing cycle. The effect of the compressive strength of the product. The amount of the admixture is the mass ratio of the admixture to the cement.

Third, the test results and discussion

(1) Admixture type and dosage test

Admixtures have a significant effect on the strength and frost resistance of the product. The admixtures used in this test include ordinary water reducing agents and early strength agents. The water reducing agent can enhance the water reduction while maintaining the consistency of the concrete; the main function of the early strengthening agent is to accelerate the hydration speed of the cement and promote the early strength development of the coagulation.

Under the steaming system of 2h→40°C constant temperature for 6h→2h cooling and 28d natural curing cycle, the effects of the type and amount of admixture on the compressive strength of the product were investigated. The test results are shown in Table 5.

Table 5 Test results of types and dosages of admixtures

It can be seen from Table 5 that the addition of different admixtures is beneficial to the improvement of the compressive strength of the product, wherein the effect of a single triethanolamine is the best. The early action of triethanolamine can promote the hydration reaction of tricalcium aluminate and accelerate the formation of ettringite, which not only improves the early strength of the product, but also has a certain post-enhancement effect. According to the test results, a single triethanolamine was used as an admixture in an amount of 0.01%.

(2) Steaming system test

For steamed bricks, the steaming system consisting of steaming temperature and steaming time is a key factor that directly affects product strength development and product energy consumption. In general, the higher the steaming temperature, the faster the strength develops. However, it is not the higher the steaming temperature, the better. When the temperature is too high, the hydration speed of the cement inside the product is too fast, which may lead to uneven distribution of hydration products and excessively rapid formation of hydration products hindering the contact of cement with water, thereby affecting the continued hydration of the cement. In addition, at higher temperatures, ettringite decomposes in a wet environment and causes swelling, resulting in increased porosity and reduced strength.

The steaming time includes three stages of heating, constant temperature and cooling, wherein the constant temperature process is the main stage of steaming brick hardening and strength growth. If the constant temperature time is too short, the compressive strength of steamed bricks is difficult to meet the requirements of MU15 grade products in JC/T422-2007.

The effect of steaming temperature and constant temperature on the compressive strength of the product was investigated under the conditions of triethanolamine dosage of 0.01%, steaming warming and cooling time of 2 h, and natural curing cycle of 28 d. The test results are shown in Table 6.

It can be seen from Table 6 that, when the steaming temperature is constant, the compressive strength of the product increases with the constant temperature, but the constant temperature is too long, and the compressive strength of the product increases slowly or decreases. When the constant temperature time is constant, the compressive strength of the product reaches the highest value when the steaming temperature is 40 °C. According to the test results, taking into account factors such as steaming cycle, operating cost, product strength, etc., the steaming temperature is determined to be 40 ° C, and the constant temperature time is 6 h.

Table 6 Test results of steaming temperature and constant temperature time

(3) Natural maintenance cycle test

The steaming system was heated at a temperature of 2h→40°C for 6h→2h for 2h. Under the condition that the dosage of triethanolamine was 0.01%, the anti-allergic resistance of the products after natural curing at 3, 7, 14, 28, 35d was determined. The compressive strength, the test results are shown in Table 7.

Table 7 Natural maintenance cycle test results

It can be seen from Table 7 that with the extension of the natural curing cycle, the compressive strength of the product is continuously increased; however, the growth of the product after 28 days is small, because the hydration reaction rate of the cement in the late hydration is very low, the reaction process It has basically stabilized. Therefore, the natural maintenance cycle was determined to be 28d.

(4) Comprehensive condition test

In the steaming system, a batch of steamed bricks was prepared under the conditions of heating 2h→40°C constant temperature 6h→cooling 2h, admixture triethanolamine dosage 0.01%, natural curing cycle 28d, refer to JC/T422-2007 “non-sintered garbage” The tailings bricks and GB/T4111-1997 "Test methods for small concrete hollow blocks" are tested for performance. The results are shown in Table 8.

Table 8 Performance indicators of steamed bricks

It can be seen from Table 8 that the performance of the prepared iron tailings steamed bricks meets the requirements of JC/T422-2007 "non-sintered waste tailings bricks" for MU15 grade products.

Fourth, the mechanism analysis

The purpose of steaming is to accelerate the hydration reaction of the brick body under hot and humid conditions, generate more hydration products, and improve the crystallinity of the hydration product, so that the product has higher strength in a short time. In order to understand the microscopic characteristics of steamed bricks, the XRD and SEM analysis of the steamed brick products prepared under the optimal process conditions were carried out. The results are shown in Fig. 3 and Fig. 4.

Figure 3 XRD pattern of steamed brick products

1-hematite; 2-quartz; 3-hydrated calcium silicate; 4-calcite;

5-monosulfide type hydrated calcium sulphoaluminate; 6-calcium gangue

Figure 4 SEM photo of steamed brick products

Comparing the XRD patterns of the hematite tailings in Fig. 3 and Fig. 1, it can be seen that the steamed brick products have new crystalline phase hydrated calcium silicate gel, ettringite and monosulfide hydrated calcium sulphoaluminate, and calcite. The characteristic diffraction peaks are significantly increased. The formation of hydrous silicate gel and ettringite crystal is due to the interfacial reaction between some active tailings particles and alkaline activator; the formation of monosulfide-type hydrated calcium sulphoaluminate is the partial reaction of ettringite The result of this; the increase of calcite is due to the carbonation reaction of the free calcium hydroxide precipitated by the cement during the hydration process with the carbon dioxide in the air under the condition of natural conservation in the later stage. The above process increases the strength of the article.

It can be seen from Fig. 4(a) that a large amount of hydration products inside the product are long rod-shaped, needle-like crystals grow into the pores and interweave to fill the voids, and local rod-like, needle-like crystals and fibrous crystals are gathered together to form. Network gel. As can be seen from Fig. 4(b), the rod crystals and the needle crystals develop in various directions, and the rod-like, needle-like and a small number of sheet crystals are interwoven to form a network-like interlaced structure. This is mainly due to the hydration of tricalcium aluminate into hydrated tetracalcium aluminate, and then accelerates the reaction with gypsum in a hydrothermal atmosphere, and finally forms a needle-like, rod-shaped ettringite structure. The more and more ettringite is produced, the greater the compactness of the product and the higher the strength.

V. Conclusion

(1) Taking iron ore tailings from western Hubei as the main raw material, preparing iron according to the raw material ratio (m tailings: m aggregate: m cement: m gypsum = 78:10:10:2) determined by the previous test. Tailings steamed bricks. Under the condition that the water content is 5%, the mass ratio of admixture triethanolamine to cement is 0.01%, and the molding pressure is 20 MPa, the bricks are naturally maintained at 40 °C for 6 hours and then naturally maintained for 28 days. The performance index of the obtained products reaches JC/ T422-2007 "non-sintered waste tailings bricks" requirements.

(2) Microscopic analysis shows that the main crystalline phase in steamed brick products is hematite, quartz, calcite, hydrated calcium silicate, monosulfide hydrated calcium sulphoaluminate and ettringite, which constitute the mineral skeleton of the product. To give the strength of the product; and the hydration product is mainly composed of ettringite and calcium silicate hydrate gel products. The extremely small crystallite solubility and high strength of these hydration products improve the shrink resistance and strength properties of the product. .

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