Messi Biology stated that moderate-heat cement is mainly applicable to water conservancy and mass concrete projects that require lower heat of hydration. In water conservancy projects, the volume of concrete often reaches millions of cubic meters, which imposes high demands on low temperature rise, crack prevention, and high durability of the concrete. Moderate-heat and low-heat cement only partially alleviate the problem of adiabatic temperature rise in dam concrete, but have no compensation effect on the shrinkage during the concrete cooling process. Therefore, preparing moderate-heat cement with expansion properties is of great significance.
Lead-zinc tailings are powdery or fine-sand waste residues discharged after beneficiation by flotation in lead-zinc mining plants. They possess a certain degree of plasticity and contain calcium oxide, silicon dioxide, iron, aluminum, and a small amount of magnesium oxide. Because the composition of lead-zinc tailings is similar to raw cement meal, they can replace part of the limestone, clay, and iron powder to produce cement clinker. Incorporating lead-zinc tailings into the raw meal to replace partial clay and iron powder, and using them as mineralizers, can yield favorable social and economic effects.
Lead-zinc tailings and their smelting plants are widely distributed in China, and the emission volume of waste residues is huge, requiring a large amount of arable land for storage sites, while the harmful leachable substances in them can also pollute water sources. Therefore, utilizing waste residues as raw materials and fuels for cement production is a major direction for resource and energy conservation. Studying the effect of magnesium oxide on the strength and expansion properties of moderate-heat cement is of great significance for the utilization of waste residues and the preparation of high-performance moderate-heat expansive cement.
1. Experiments
1.1 Raw Materials
The raw materials used in this experiment include limestone, shale, lead-zinc tailings, iron ore, fly ash, and pure chemical reagent-grade magnesium oxide.
1.2 Experimental Methods
According to the national standard GB200-2003 requirements for C3S、C3A、f-CaO、MgO, and alkali content, as well as the industrial analysis of various raw materials and coal, a high-iron, low-aluminum, and high-saturation-ratio proportioning scheme was determined. The rate values were set as: KH=0.94;SM=2.2±0.1;IM=0.96. The blank sample was labeled as M0. Magnesium oxide was added to the blank sample at proportions of 1%, 3%, and 5%, and labeled as M1, M2, and M3, respectively.
All raw materials were ground to pass through a 0.08 mm square-hole sieve with a residue of less than 10%. According to the raw material proportioning table, the raw materials were accurately weighed and thoroughly mixed. An appropriate amount of raw meal powder was then taken, mixed with 20% distilled water, and pressed under a certain pressure into test blocks sizing 60×60×20 mm with a mass of approximately 150±10g. These were dried in an oven at 110°C, placed in a high-temperature silicon-molybdenum rod furnace preheated to 900°C for 30 minutes of heat preservation, and then heated to 1450°C for another 30 minutes before removal and air-quenching to room temperature. The samples were crushed and ground to a specific surface area of 250 m2/kg, packed into self-sealing bags, and stored in a desiccator for later use. The f−CaO content was measured using the glycerol-ethanol method.
Moderate-heat Portland cement was prepared by adding 4% gypsum to the clinker burned from the four proportioning schemes. According to the People’s Republic of China national standard GB2022-80 Test Method for Heat of Hydration of Cement (Direct Method), the heat of hydration at 3 and 7 days was measured. 20×20×20mm six-piece mold sets were used for molding to test the paste strength of the samples, with a water-cement ratio of 0.35. The samples were placed in a water-vapor chamber at 20±3℃ for 24 hours before demolding, and then immersed in a water bath at 20±2℃ to cure for specified ages before strength testing.
10×10×40mm triple molds with stainless steel nail heads at both ends (six bars per group) were used for molding, with a water-cement ratio of 0.35. They were placed in a water-vapor chamber at 20±3℃ for 24 hours before demolding, and then immersed in water baths at 20±2℃ and 80±2℃ respectively to cure for specified ages before testing the expansion rate of the samples.
Compressive strength was measured on an NYL-600 pressure testing machine, and flexural strength was measured on a DKZ-5000 electric flexural testing machine. X-ray diffraction (XRD) was used to determine the phase composition of the samples (operating conditions: CuKα, tube voltage 40 kV, tube current 100 mA). Scanning electron microscopy (SEM, model JSM-5900) was used to observe the morphology of selected samples.
2. Results and Discussion Analysis
2.1 Free Calcium Oxide Test Results and XRD Analysis
The test results of free calcium oxide indicate that magnesium oxide has a certain impact on the burning of clinker. In the reaction, Mg2+ can replace Ca2+ in the C3S lattice, causing Ca2+ to escape, promoting the reaction of C2S, and generating more C3S. In clinkers M1, M2, and M3, merwinite was formed due to the substitution of Mg2+. However, sometimes when the added magnesium oxide increases, the Ca2+ replaced by Mg2+ does not react with C2S, leading to an increase in free calcium oxide. Therefore, the free calcium oxide content in M3 is relatively high, and the peak of free calcium oxide can be clearly observed in the XRD pattern.
2.2 Strength and Heat of Hydration
The heat release of each sample at 3 and 7 days complies with the national standard for moderate-heat 42.5 cement. The heat of hydration of the samples added with magnesium oxide at 3 days is lower than that of the blank sample M0, and with the increase of magnesium oxide dosage, the heat of hydration shows a decreasing trend, though the heat of hydration at 7 days is basically consistent. From this, it can be analyzed that due to the substitution effect of magnesium ions, a certain amount of inactive merwinite is formed, thereby slowing down the hydration rate and weakening the degree of hydration.
After 28 days of water bath curing, the compressive strength of each sample increased rapidly and exceeded the strength requirements of the national standard GB200-2003 for moderate-heat 42.5 cement. Among the three samples M1, M2, and M3, due to the higher amount of magnesium oxide replacing more Ca2+ to react with C2S and generate more C3S, sample M3 exhibited the highest strength.
2.3 Expansion Rate
When hydrated in water at 20°C, the blank sample M0 exhibited continuous shrinkage characteristics of the paste, which slowed down after 28 days. Shrinkage of the paste during cement hardening is the primary cause of cracking in cement-based materials (concrete). The incorporation of magnesium oxide can well compensate for the shrinkage of the cement paste. As shown in the figure, after initial hydration, it first manifested as micro-expansion, reaching a maximum expansion rate at 7 days. The expansion rates of samples M1, M2, and M3 reached 0.02%, 0.025%, and 0.03%, respectively. After 7 days, the paste specimens began to shrink. After 14 days, their shrinkage trend was similar to that of the blank paste specimens without magnesium oxide, but the shrinkage rate was much smaller.
Due to the increase in curing temperature, the hydration reaction accelerated. Sample M0 showed continuous shrinkage after 7 days, whereas the samples doped with magnesium oxide showed continuous expansion, and the expansion rate increased compared to curing at 20°C. Theoretically, this is caused by the hydration of magnesium oxide to form Mg(OH)2(brucite). This reaction belongs to a typical in-situ solid-phase reaction, reaching the reaction interface through the diffusion of the product layer, and its expansion effect stems from the crystallization growth pressure of brucite. The entire hydration process is divided into 4 steps: (1) Physical and chemical adsorption of water molecules on the MgO surface; (2) Diffusion of Mg2+ and OH– ions in the adsorbed water molecule layer; (3) Nucleation of Mg(OH)2 crystals; (4) Growth of Mg(OH)2 crystals. Therefore, the expansion mechanism is: when the Mg(OH)2 crystals are very small, the expansion force of the paste mainly comes from the swelling force of water absorption; as the crystals grow, it shifts to crystallization pressure taking the dominant role. Raising the curing temperature accelerates the conversion rate of magnesium oxide into brucite.
2.4 XRD after Hydration
With the increase of curing age, the cement composition minerals C3S, C2S, C3A, and C4AF decreased significantly, while the contents of hydration products C-S-H gel and CH increased significantly with prolonged age. Samples M1, M2, and M3 doped with magnesium oxide contained a small amount of unhydrated MgO and non-reactive merwinite.
2.5 SEM of Hydration Products
With increased hydration, a large amount of C-S-H gel and acicular ettringite were generated in all samples. As the dosage of magnesium oxide increased, unhydrated flaky magnesium oxide appeared in the 3-day hydration samples. It can be seen that there is a limit to the amount of magnesium oxide added; more is not necessarily better. When added to 5%, it no longer participates in the reaction. Therefore, a 3% addition is sufficient. Comparing figures (1) and (2), (3) and (4), (5) and (6), (7) and (8) in Figure 5, as hydration time progressed, C-S-H gels bonded together due to continued hydration, filling the voids. Ca(OH)2 crystals and ettringite basically disappeared, wrapped by other hydration products, making the cement stone structure denser. Consequently, the 28-day strength was vastly improved compared to the 3-day strength. As hydration progressed, flaky magnesium oxide was basically invisible in the 28-day products, being wrapped among other hydration products. Therefore, as long as the MgO content in the cement is controlled within an appropriate range, its delayed hydration can be utilized to compensate for concrete shrinkage and improve concrete durability.
Hebei Messi Biology Co., Ltd. stated that magnesium oxide exerts a certain compensation effect on the shrinkage of moderate-heat cement produced with lead-zinc tailings raw meal. Under 80°C water bath curing, the expansion effect of magnesium oxide is more pronounced than at 20°C, displaying continuous expansion phenomena. The dosage of magnesium oxide needs to be appropriate. Based on the determination results of free calcium oxide, expansion rate, and scanning electron microscopy analysis, a 5% magnesium oxide addition results in excessive free calcium oxide test results, and electron microscopy results show a relatively large amount of residual magnesium oxide. Thus, a 3% addition is optimal. Due to the burning-promotion effect of magnesium ions during the sintering process, the strength of the cement increases with increasing magnesium oxide content. However, considering the test results of free calcium oxide, the dosage of magnesium oxide should not exceed 3%.