Views: 0 Author: Site Editor Publish Time: 2023-04-02 Origin: Site
Abstract: Using iron tailings sand and Portland cement as raw materials and hydroxypropyl methylcellulose (HPMC) as viscosity modifier to prepare cement mortar, the adverse effect of large amount of iron tailings sand on the cohesion of freshly mixed mortar was optimized. , to further study the effect of HPMC on the properties of hardened mortar. The results show that HPMC will reduce the fluidity of iron tailings sand mortar, and the combined use of HPMC and water reducer can improve the bleeding rate of fresh mortar at similar fluidity; the appropriate amount of HPMC will delay the hydration process of cement, but It can improve the homogeneity of hardened mortar and slightly increase the compressive strength; HPMC can improve the pore structure of hardened mortar, reduce porosity, and reduce drying shrinkage of mortar, which is beneficial to long-term durability.
Key words:hydroxypropyl methylcellulose; tailings sand; cement mortar; mechanical properties; porosity
Iron tailings sand is a solid waste produced during the iron ore beneficiation process, which is generally stored in tailings dams. Due to the massive mining of low-grade iron ore in recent years, the unit output of iron tailings sand has gradually increased, resulting in an increase in the height of the existing tailings dam year by year. Using iron tailings as building materials, especially directly as fine aggregate, is a simple, quick and highly efficient tailings waste disposal solution.
The physical and chemical properties and surface characteristics of iron tailings sand lead to serious bleeding and poor cohesion of iron tailings sand concrete. Cai Jiwei et al. used mineral admixtures such as siliceous stone powder to regulate the water-powder ratio in iron tailings sand concrete, ensuring the workability of the mixture and significantly reducing the bleeding rate. Li Meng et al. used recycled aggregates and iron tailings in combination to improve the particle gradation of the aggregates, and its mechanical properties were superior to those of ordinary concrete. The application of the above method is affected by the quality and performance changes of the tailings sand, which is not conducive to large-scale application.
When the workability is similar, the mechanical properties of iron tailings sand concrete are better than ordinary concrete. The negative effect on concrete strength when iron tailings sand is used as aggregate mainly comes from the reduction of the cohesion of the mixture. Therefore, it is a feasible solution to seek an admixture to regulate the performance of freshly mixed iron tailings sand concrete. Hydroxypropyl methylcellulose (HPMC) is a commonly used concrete viscosity modifier, which is widely used in self-compacting concrete and grouting materials, which can improve the cohesion of cement-based materials and the uniformity of hardened structure. In this experiment, iron tailings sand was used as a single fine aggregate to prepare cement mortar, and the influence of HPMC as a viscosity modifier on the fresh performance, mechanical properties and drying shrinkage of mortar was systematically studied, and the effect was analyzed according to the amount of non-evaporating water and pore structure. mechanism, demonstrating the positive effect of HPMC modification in iron tailings sand-cement mortar.
1. Experimental part
1.1 Raw materials
P·O 42.5 Cement (C), Fuyingshan Cement Plant, Fuxin City, Liaoning Province, the specific surface area is 341 m²/kg, and its chemical composition (w/%) is: CaO, 61.9; SiO2, 21.5; Al2O3, 4.03; Fe2O3 , 2.98; SO3, 2.67; MgO, 3.04; other oxides, 3.88. Hydroxypropyl methylcellulose (HPMC), KIMA CHEMICAL CO., LTD; water reducer (SP), SIKA powder high-efficiency water reducer; water is tap water; tailings sand (TS), Tongnai, Fuxin City, Liaoning Province The iron ore tailings pool has a particle size distribution in the range of 0.075-1.18 mm, and its main components are SiO2, Al2O3 and F2O3.
1.2 Test method
1.2.1 Test mix ratio
Iron tailings sand - cement mortar mix ratio.
1.2.2 Fluidity and bleeding rate test
The fluidity of the mortar was tested with a CA mortar spread tester, the height of the cylinder was 150 mm, and the diameters of the upper and lower openings were 50 mm and 100 mm, respectively. The bleeding rate of the mortar was tested according to GB/T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures".
1.2.3 Compressive strength test
Prepare the mortar according to the test mix ratio, mold it in a 40mm×40mm×40mm mold, demould after curing for 24 hours under standard conditions, and then continue curing to the predetermined age under standard conditions, according to GB/T 17671-1999 "Cement mortar strength test method (ISO method)" to determine the compressive strength.
1.2.4 Dry shrinkage test
According to JC/T 603-2004 "Test Method for Drying Shrinkage of Cement Mortar", the mortar specimen was formed, and the change of its length was measured with a mortar ratio meter, and the drying shrinkage rate of 3~56 days was calculated.
1.2.5 Non-evaporative water test
The fragments in the center of the 28-day-old specimen after the compression test were selected, soaked in isopropanol, dried in vacuum for 7 days, crushed and sieved, the tailings sand particles were removed, and the remaining cement slurry was ground. Put an appropriate amount of powder sample into a corundum crucible, place it in a muffle furnace and dry it at 105 °C for 12 h, then raise the temperature to 950 °C and keep it for 3 h, the mass loss of the powder sample in the range of 105-950 °C accounts for When the percentage of sample mass is the non-evaporating water.
1.2.6 Pore characteristic test
A sample of 5 mm × 5 mm × 5 mm was cut from the inside of the hardened mortar, soaked in isopropanol to terminate hydration, dried in a vacuum oven for 7 days, and tested with an AutoPore V 9500 mercury porosimeter from American Mike Company And analyze the pore characteristics of hardened mortar.
2. Results and Discussion
2.1 Fluidity and bleeding rate
From the effect of HPMC on the fluidity and bleeding rate of iron tailings sand mortar, it can be seen that with the increase of HPMC content, the mortar fluidity and bleeding rate decrease. The C0 fluidity of the mortar in the control group is higher than that of the three groups of mortar mixed with HPMC and polycarboxylate superplasticizer. When HPMC is added alone, the viscosity of the mortar increases significantly, which has a very adverse effect on the fluidity. When HPMC is used in combination with an appropriate amount of water reducing agent, the fluidity of the mortar can be controlled in an appropriate range, and the cohesion is also improved. The specific performance is that the bleeding rate over time is significantly lower than that of the control mortar C0, and the decline rate increases with HPMC. increased with the increase of dosage.
When HPMC is mixed into cement-based materials alone, the hydroxyl groups in HPMC combine with ether groups and water molecules to form hydroxyl bonds, forming a flocculation structure that increases the viscosity of the liquid phase, and increases the viscosity of the liquid phase. When HPMC is used in combination with water reducing agent, on the one hand, HPMC will combine with water molecules and calcium ions in the pore solution to form a flocculation structure; on the other hand, HPMC hinders the adsorption of water reducing agent molecules on the surface of cement particles, reducing the Water reduction effect. The combined effect of these two aspects reduces the fluidity of the mortar. By adjusting the amount of HPMC, the fluidity and bleeding rate can be balanced.
2.2 Compressive strength and drying shrinkage
From the effect of HPMC on the 7d and 28d compressive strength of iron tailings sand mortar, it can be seen that the 7d and 28d compressive strength of the mortar specimens showed a trend of first increasing and then decreasing when HPMC was added. The maximum value is reached at 0.3%. When the content of HPMC increased to 0.45%, the 7-day and 28-day compressive strength decreased slightly, but it was still higher than that of the control mortar C0.
From the effect of HPMC on the dry shrinkage rate of iron tailings sand mortar, it can be seen that there is no significant difference in the dry shrinkage rate of the four groups of mortar specimens before the 7-day age. The addition of HPMC had a significant inhibitory effect on the drying shrinkage of the 7-56-day age, and the 56-day drying shrinkage of the HPMC mortar group was lower than that of the control mortar C0. With the increase of HPMC content, the drying shrinkage decreases firstly and then increases. When the content of HPMC is 0.3%, the drying shrinkage is the lowest. When the content of HPMC increases to 0.45%, the drying shrinkage of 56 days increases slightly, which is basically the same as that of 0.15% of HPMC.
2.3 Micro analysis
It can be seen from the effect of HPMC content on the non-evaporating water content of 7-day and 28-day mortars that the non-evaporating water content of cement stone in 7-day and 28-day mortar specimens decreases with the increase of HPMC content. HPMC has a certain inhibitory effect on the hydration of Portland cement, and the HPMC content increases from 0.15% to 0.45%, and the inhibitory effect is enhanced.
From the effect of HPMC content on the pore structure of the 28-day mortar, it can be seen that the pore characteristics of the 28-day mortar specimen show a significant dependence on the HPMC content. The total porosity of the mortar C0 in the control group is the largest, and the volume content of large-diameter pores is higher. When the content of HPMC was 0.15% and 0.3%, the porosity was significantly lower than that of the control group, and the distribution of pore diameters shifted significantly to the left, and the average pore diameter decreased. When the content of HPMC increases to 0.45%, the main distribution peak of pore diameter shifts to the right, and the pore volume content in the range of 100-1000 nm increases, which shows that the increase of average pore diameter leads to the increase of total porosity, but it is still lower than Control group mortar C0.
In summary, the appropriate amount of HPMC has a positive impact on the performance of iron tailings sand mortar. This is because HPMC improves the cohesion of the mortar, improves the uniformity of the physical structure of the hardened mortar, and avoids segregation bleeding to produce a weak mortar layer, so the compressive strength increases and the pore structure is improved; on the other hand, HPMC will Delay the hydration process of cement, reduce the hydration products per unit volume at the same age, and excessive HPMC (0.45%) will form a flocculation structure, reduce the fluidity of the mortar, and increase the probability of internal defects after vibration , resulting in a decrease in compressive strength and an increase in porosity. Therefore, by rationally adjusting the content of HPMC, the problems of low cohesion and high bleeding rate of large-volume iron tailings sand mortar can be improved, and the pore structure of hardened mortar can be improved, which is conducive to improving the compressive strength and drying shrinkage of mortar. rate, which has positive implications for long-term durability.
1. HPMC will significantly increase the viscosity of mortar, and its combined use with water reducing agent can control the fluidity in an appropriate range and reduce the bleeding rate of mortar.
2. With the increase of HPMC content, the 7-day and 28-day compressive strengths of mortar specimens first increased and then decreased, and reached the maximum when the HPMC content was 0.3%.
3. Before 7 days of age, HPMC had no obvious effect on drying shrinkage; within 7-56 days of age, the addition of HPMC had a significant inhibitory effect on drying shrinkage, and the drying shrinkage rate decreased first and then increased with the increase of HPMC content trend, when the content is 0.3%, the drying shrinkage reaches the lowest value.
4. With the increase of HPMC content, the amount of non-evaporating water decreased at 7 days and 28 days, and the pores first decreased and then increased.