How to prepare steam-cured bricks from a certain hematite tailings

In terms of resource utilization of tailings, a lot of research has been conducted at home and abroad, and good results have been achieved. These include comprehensive recovery of valuable elements from tailings, filling goafs with tailings, preparing glass-ceramics from tailings, and producing various building materials. A certain hematite tailing in western Hubei is a composite mineral raw material. Its fine particles and multi-component mixing characteristics make it more suitable for application in the field of building materials. Based on the previous research on using the tailings to prepare no-burning and no-steaming bricks, this experiment carried out research on using the tailings to prepare another new wall material with higher strength – steam-cured bricks, in order to expand the application of the tailings. scope, truly realizing the efficient utilization of large quantities of the tailings.
1. Test materials and equipment
(1) Test raw materials
1. Hematite tailings
​ Hematite tailings are taken from the mineral processing plant of a mining company in Hubei. They are tailings produced after crushing, grinding, centrifugal classification and strong magnetic separation of high-phosphorus hematite ore to recover iron concentrate. X-ray diffraction analysis results (see Figure 1) show that the mineral components of the tailings are mainly hematite and quartz, followed by chlorite, calcite, etc.

2. Other raw materials
(1) Yellow sand. In order to meet the requirements for raw material chemical composition, mineral activity and particle gradation, a certain amount of siliceous material needs to be added to replace part of the tailings. This test selects commercially available yellow sand as aggregate. Its chemical composition and particle size composition
When producing tailings-free brick products, the quartz content in the 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 SiO2 content is low. Low silicon content will reduce the strength of the product and thus affect the quality of the finished product. Therefore, you can consider adding other raw materials with low iron content and high silicon content into the ingredients. Table 2 shows that the iron content in yellow sand is low and the silicon content reaches 90.60%, which can meet the ingredients requirements.
(2) Cement. When producing industrial waste bricks, cement is both a binding agent and an active activator. This test uses 325# composite Portland cement.
(3) Gypsum. Gypsum acts as an accelerator in the brick reaction system. It can improve the strength and stability of the product. Although its dosage is small, it has a great impact on the activation of waste residue. Commercially available gypsum was used in the test.
(2) Admixtures
​Admixtures have the function of improving the workability, water reduction and portfolio high strength glass ceramic structural parts of mortar, which can save cement consumption and reduce construction costs. The admixtures used in this test include water-reducing agent calcium lignosulfonate, early-strengthening agents 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.
Forming mold: φ50mm×50mm stainless steel mold.
Maintenance equipment: YH-40B standard constant temperature and humidity curing box.
Test equipment: YES-100 digital display hydraulic pressure testing machine, D/Max-IIIA XRD diffractometer, Axios advanced X-ray fluorescence spectrometer, JSM-5610LV scanning electron microscope.
2. Preparation process of iron ore steam-cured bricks
The preparation process of iron tailings steam-cured bricks is shown in Figure 2. Add 15% water and a certain amount of additives to the raw materials prepared according to a certain ratio, and stir them in a mixer to mix the materials evenly. The mixed materials were aged at room temperature for 40 minutes and then pressed into shape on a hydraulic pressure testing ceramic machine shop under a pressure of 20 MPa. The pressed products are placed in a standard constant temperature and humidity curing box for moist heat curing according to a certain steam curing system, and then naturally cured for a certain period of time to obtain the tailings steam cured brick products.

After preliminary testing, it was determined that the ratio of raw materials is m tailings: m aggregate: m cement: m gypsum = 78:0:10:2. This test mainly examines the impact of the type and dosage of admixtures, steam curing system and natural curing cycle. Impact on product compressive strength. The dosage of admixture is the mass ratio of admixture to cement.
3. Test results and discussion
(1) Test on type and dosage of admixtures
​Admixtures have a significant impact on the strength and frost resistance of products. The admixtures used in this test include ordinary water-reducing admixtures and early strength admixtures. The water-reducing agent can enhance the water reduction while maintaining the consistency of the concrete; the main function of the early-strength agent is to accelerate the hydration rate of cement and promote the development of early strength in concrete.
Under the steam curing system of 2 hours of heating → 6 hours of constant temperature at 40°C → 2 hours of cooling and a 28-day natural curing cycle, the effects of the type and amount of admixtures on the compressive strength of the product were investigated
It can be seen from Table 5 that adding different admixtures is beneficial to improving the compressive strength of the product, among which triethanolamine alone has the best effect. The early effect of triethanolamine can promote the hydration reaction of tricalcium aluminate and accelerate the formation of ettringite. It can not only improve the early strength of the product, but also have a certain later strengthening effect. According to the test results, a single triethanolamine was selected as the admixture, and its dosage was 0.01%.
(2) Steam curing system test
For steam-cured bricks, the steam-curing system consisting of steam-curing temperature and steam-curing time is a key factor that directly affects the strength development of the product and the energy consumption of the product. Generally speaking, the higher the steam curing temperature, the faster the strength development. However, it is not that the higher the steaming temperature, the better. When the temperature is too high, the hydration rate of the cement inside the product is too fast, which may lead to uneven distribution of hydration products and excessive and rapid formation of hydration products that hinder the contact between cement and water, thereby affecting the continued hydration of the cement. In addition, at higher temperatures, ettringite will dehydrate and decompose in a wet environment, causing expansion, resulting in increased porosity and decreased strength of the product.
The steam-curing time includes three stages: heating, constant temperature and cooling. The constant temperature process is the main stage for the hardening and strength growth of steam-cured bricks. If the constant temperature time is too short, the compressive strength of steam-cured bricks will be difficult to meet the requirements of MU15 products in JC/T422-2007.
Under the conditions that the dosage of triethanolamine is 0.01%, the steam-curing heating and cooling times are both 2 hours, and the natural curing cycle is 28 d, the effects of steam-curing temperature and constant temperature time on the compressive strength of the product are investigated. The test results are shown in Table 6.
It can be seen from Table 6: Generally speaking, when the steam curing temperature is constant, the compressive strength of the product shows an upward trend with the extension of the constant temperature time. However, if the constant temperature time is too long, the compressive strength of the product increases slowly or decreases instead. When the constant temperature time is constant, the compressive strength of the product reaches its highest value when the steam curing temperature is 40°C. According to the test results, taking into account factors such as the steam-curing cycle, operating costs, product strength, etc., the steam-curing temperature is determined to be 40°C and the constant temperature time is 6 hours.

(3) Natural maintenance cycle test
Using a steam curing system of 2 hours of temperature rise → 6 hours of constant temperature at 40°C → 2 hours of cooling, and with the dosage of the additive triethanolamine being 0.01%, the resistance of the products after steam curing was measured for 3, 7, 14, 28 and 35 days. Pressure strength,.

It can be seen that with the extension of the natural curing cycle, the compressive strength of the product continues to increase; however, the increase in the strength of the product after 28 days is very small. This is because the hydration reaction rate of cement in the late hydration stage is very low, and the reaction process has basically tended to Stablize. Therefore, the natural maintenance cycle is determined to be 28 days.
(4) Comprehensive condition test
Prepare a batch of steam-cured bricks under the condition that the steam-curing system is 2 hours of heating → 6 hours of constant temperature at 40°C → 2 hours of cooling, the dosage of the admixture triethanolamine is 0.01%, and the natural curing cycle is 28 days. Refer to JC/T422-2007 “Non-sintered garbage” Tailings Bricks” and GB/T4111-1997 “Test Methods for Concrete Small Hollow Blocks” for performance testing
It can be seen from Table 8 that the various properties of the prepared iron tailings steam-cured bricks meet the requirements of JC/T422-2007 “Non-sintered garbage tailings bricks” for MU15 grade products.
4. Mechanism analysis
The purpose of steam curing is to speed up the hydration reaction of the bricks under hot and humid conditions, generate more hydration products, and improve the crystallinity of the hydration products, so that the products have higher strength in a short time. In order to understand the microscopic characteristics of steam-cured bricks, XRD and SEM analyzes were conducted on steam-cured brick products produced under optimal process conditions.
1-Hematite; 2-Quartz; 3-Calcium silicate hydrate; 4-Calcite;
5-monosulfide calcium sulfoaluminate hydrate; 6-ettringite

Comparing the XRD patterns of hematite tailings, it can be seen that the steam-cured brick products have new crystal phases of calcium silicate hydrate gel, ettringite and monosulfide calcium sulfoaluminate hydrate, and the characteristic diffraction peaks of calcite are significantly increased. The formation of hydrated silicate gel and ettringite crystals is due to the interfacial reaction between part of the active tailings particles and the alkaline activator; the formation of monosulfide calcium sulfoaluminate hydrate is due to the continued reaction of part of the ettringite The result; and the increase in calcite is due to the carbonation reaction of the free calcium hydroxide precipitated during the hydration process of the cement with carbon dioxide in the air under the conditions of later natural curing. The above process improves the strength of the product.
(a) It can be seen that there are a large number of hydration products inside the product in the form of long rod-shaped and needle-shaped crystals growing into the pores and intertwining with each other to fill the voids. Locally, rod-shaped, needle-shaped crystals and fibrous crystals gather together to form a network-like condensation. glue. As can be seen from Figure 4(b), rod-shaped crystals and needle-shaped crystals develop in all directions, and rod-shaped, needle-shaped and a small amount of flake crystals interweave and grow to form a network-like intertwined structure. This is mainly due to the hydration of tricalcium aluminate into tetracalcium aluminate hydrate, which then accelerates the reaction with gypsum in a hydrothermal atmosphere, and finally generates a needle-like and rod-like ettringite structure. The more and faster the ettringite is produced, the greater the density and strength of the product.
5. Conclusion
(1) Using a certain hematite tailings in western Hubei as the main raw material, prepare iron according to the raw material ratio determined in the previous test (m tailings: m aggregate: m cement: m gypsum = 78:10:10:2) Tailings steam curing bricks. Under the condition that the water content is 5%, the mass ratio of the additive triethanolamine to cement is 0.01%, and the molding pressure is 20MPa, the bricks are steamed and cured at a constant temperature of 40°C for 6 hours and then naturally cured for 28 days. The performance index of the obtained product reaches JC/ T422-2007 “Non-sintered garbage tailings bricks” requirements.
(2) Microscopic analysis shows that the main crystal phases in steam-cured brick products are hematite, quartz, calcite, hydrated calcium silicate, monosulfide-type calcium sulfoaluminate hydrate and ettringite, which constitute the mineral skeleton of the product. , giving the product strength; and the hydration products are mainly ettringite and hydrated calcium silicate gel products. These hydration products have extremely small microcrystalline solubility and high strength, which improve the anti-shrinkage and strength properties of the product. .