Construction gypsum: The construction gypsum produced by Shandong is tested with a standard consistency of 51% water consumption. The main chemical components are shown in Table I
Table I The main chemical components of construction gypsum %
Water-reducing agents: 3 categories and four types of water-reducing agents were used for comparative research. Calcium lignosulfonate water reducer (code M), light yellow powder, produced by a Beijing admixture Co., Ltd.. Naphthalene water reducer (code N), brown powder, produced by a Beijing admixture Co., Ltd.. Polycarboxylic acid water reducing agent (code J1), light brown liquid, 30% solid content, produced by a Beijing Admixture Co., Ltd.. Polycarboxylic acid water reducing agent (code J2), light brown liquid, 22% solid content, produced by a building material company in Shanxi.
The fluidity, standard consistency and strength of building gypsum are tested according to the methods specified in GB 9776-88 “Building Gypsum”. The microstructure analysis test was completed in Harbin Institute of Technology. During the test, the gypsum test block that had been hydrated for 2h, terminated in absolute ethanol when broken, then dried at 60°C. The flat surface was observed under a scanning electron microscope.
2. Results and discussion
2.1. Water reducing rate test of different types of water-reducing agents
In the production process of gypsum board, to meet the needs of slurry fluidity and molding, the additional water that needs to be added accounts for 40% of the wet board mass, and the corresponding wet board drying energy consumption is also large. Therefore, water reduction is of great significance to reducing production energy consumption. The experiment tested the water reducing rate of 4 kinds of water-reducing agents at different dosages, and the results are shown in Figure 1.
Figure 1 Water reducing rate of different types of water-reducing agents
It can be seen from Figure 1 that, on the one hand, the water reducing rate of the four water-reducing agents gradually increases with the increase in the amount of water reducing agent. In the case of the same dosage, the naphthalene-based water-reducing agent (N) has the best water-reducing effect, and the water-reducing rate is the highest. When the dosage is 0.6%, the water-reducing rate has exceeded 20%. The water-reducing rate of polycarboxylic acid type water reducing agent (J1) and lignin type water reducing agent (M) can reach 20% when the blending amount is 0.8%. During the test, it was also found that when the polycarboxylic acid and lignin water reducers are mixed in a large amount (≥1.0%), the gypsum slurry will bleed during the hardening process, and it is mixed with lignin The hardened gypsum body of water-reducing agent also has serious staining. On the other hand, by comparing the water reduction rate of the two polycarboxylic acid water reducers at different dosages, it is found that although the main components of the two are the same, the specific elements are various and the solid content is additional, so the water reduction rate is quite different. Among them, J2 has the lowest water reduction rate under the same content due to its low solid content: while Jl has a solid content of 30%, so the water reduction rate is much higher than that of J2. However, considering the solid content of the liquid, the amount of polycarboxylic acid water-reducing agent is minimal. The dosage of 0.8% J1 and J2 liquids converted into solid dosages are 0.24% and 0.18% respectively, so the water reduction efficiency is not much different compared with 0.2% of naphthalene-based water reducing agent (N).
2.2. Influence of different kinds of water-reducing agents on gypsum setting time
The experiment tested the influence of different types and different dosages of water reducing agent on the setting time of gypsum when the slurry reached the standard consistency (the slurry expansion diameter is 180mm). The results are shown in Figure 2 and Figure 3, respectively.
Figure 2 The influence of water content on the initial setting time of gypsum
Figure 3 The influence of water content on the initial setting time of gypsum
It can be seen from Figure 2 and Figure 3 that the four water-reducing agents have very different effects on the setting time of building gypsum. After adding naphthalene-based water reducing agent (N), the initial setting and final setting time of gypsum are shortened, which plays a role of accelerating coagulation, and with the increase of the content, the coagulation accelerating effect gradually increases, the content of which is 0.6% When the initial setting time is reduced to 10 min, the final setting time is reduced to 13 min. The lignin-based water reducing agent (M) shows obvious retarding effect. When the content is 0.2%, the final setting time is more than 30 minutes, which is very unfavorable for the production of gypsum board. Polycarboxylic acid water reducers (J1, J2) also show the effect of retarding, the larger the content, the more obvious the retarding, but because the solid content of J1 is larger, so the retarding effect is much more obvious than J2.
2.3. The influence of different types of water-reducing agents on the strength of gypsum
In the production process of gypsum board, the use of water reducing agent can also reduce the amount of mixing water and increase its strength. When the slurry reaches the standard consistency, the flexural strength and compressive strength of gypsum vary with the type and amount of water reducing agent. The results are shown in Figure 4 and Figure 5, respectively.
Figure 4 The influence of water content on the flexural strength of gypsum
Figure 5 The influence of water content on the flexural strength of gypsum
It can be seen from Figure 4 that the four water-reducing agents have different effects on the flexural strength of gypsum. After adding water reducer to the blank sample, the flexural strength of gypsum is improved. Compared with polycarboxylic acid series and lignin series water reducers, naphthalene series water reducers have better reinforcing effect and contribute the most to the improvement of the flexural strength of gypsum. When its content is small, the flexural strength increases faster, when the content is 0.4%, the flexural strength reaches 3.6 MPa, and when the content is greater than 0.4%, the increase gradually becomes flat. Polycarboxylic acid water-reducing agents (J1, J2) also have a certain enhancement effect, but the increase in flexural strength does not change much with the dosage, and it reaches the maximum when the dosage is 0.6%. The enhancement effect of lignin-based water-reducing agent is more obvious when the dosage is small (0.2%), but the enhancement effect is not obvious when the dosage is larger.
Figure 5 shows that among the four types of water-reducing agents, naphthalene-based water-reducing agents can significantly increase the compressive strength of building gypsum. The larger the amount, the higher the compressive strength. The incorporation of polycarboxylic acid-based water-reducing agents (J1, J2) is not as effective as the naphthalene system in enhancing gypsum. The compressive strength is greater at 4%, and the change of the compressive strength tends to be gentle when the content is increased. The reason is analyzed because the polycarboxylic acid-based water-reducing agent has a certain air-entraining effect, which causes the pores in the hardened gypsum to increase. When the ratio of water to paste is reduced, the increase in compressive strength is not obvious. However, the lignin-based water-reducing agent appears to have a slight decrease in compressive strength as the amount of water-reducing agent increases. This may be due to the fact that the lignin-based water-reducing agent is not fully dissolved in the mixing water, resulting in dispersion in the gypsum slurry Very uneven, so the compressive strength drops instead.
2.4. Test of gypsum hydration temperature rise curve
The hydration of hemihydrate gypsum is an exothermic process. The heat release is large and concentrated. By testing the change of hydration temperature with time, the hydration process of gypsum can be reflected. Weigh 3 parts of 100g gypsum and 51ML of water, keep 1 part, and add 0.6% of naphthalene-based water reducing agent (N) and O. 6% polycarboxylic acid water-reducing agent (J1). Mix the 3 samples uniformly and put them into the heat preservation device. Use a thermometer to record the temperature change of the gypsum slurry at intervals until the temperature stabilizes. The test results are shown in Figure 6.
Figure 6 Hydration temperature rise curve of paste
It can be seen from Figure 6 that the addition of water-reducing agent to the blank sample significantly changed its hydration process. The hydration curve of the blank sample shows that the hemihydrate gypsum immediately dissolves and hydration reaction occurs after adding water. After tens of minutes, the reaction speeds up, the heat release increases, and the exothermic peak appears. The hydration reaction basically ends after about 2 h. The exothermic process of the sample doped with a naphthalene-based water-reducing agent is similar to that of the blank sample, but the exothermic peak is advanced by 25 min and the exothermic heat is greater. The hydration reaction basically ends at 70 min, indicating that the naphthalene-based water-reducing agent is mixed It can accelerate the hydration process of gypsum. The hydration reaction of the sample mixed with polycarboxylic acid-based water-reducing agent almost started slowly after adding water for 60 minutes. The exotherm was small and its exothermic peak appeared at about 120 minutes. The hydration reaction basically ended after 150 minutes. This shows that the polycarboxylic acid-based water-reducing agent greatly delayed the hydration process of gypsum, resulting in the slow setting of the sample mixed with the poly-carboxylic acid-based water-reducing agent and the low strength of 2 h.
2.5. Gypsum microstructure analysis
The addition of water-reducing agent to blank gypsum will affect the crystal morphology and crystal structure of the hardened body. Figure 7 shows the crystalline morphology of gypsum with a blank sample, 0.6% naphthalene-based water-reducing agent (N) and 0.6% polycarboxylic acid-based water-reducing agent (J1) (slurry expansion diameter is 180mm). Comparing Fig. 7(a) and Fig. 7(b), it can be found that the addition of naphthalene-based water-reducing agent reduces the plate-shaped and sheet-shaped dihydrate gypsum crystals, increases the needle-shaped crystals, and makes the crystals refined, and the aspect ratio As it becomes larger, the density of the overlap between the crystals increases significantly, so the flexural and compressive strength of gypsum is higher macroscopically. It can be seen from Figure 7(c) that most of the gypsum crystals are still needle-like crystals and criss-crossed intertwined after the polycarboxylic acid-based water reducing agent is incorporated. The effective contact points of the crystal are reduced, and the degree of lap compaction of the crystal is reduced. Therefore, macroscopically, the strengthening effect of polycarboxylic acid-based water-reducing agents on gypsum is not as obvious as that of naphthalene-based water-reducing agents.
Figure 7 Blank sample and crystal morphology of gypsum after adding water reducing agent
(1) Among the 4 kinds of water reducing agents tested, the naphthalene-based water reducing agent has the best adaptability to the gypsum system, which has the best water reducing effect, and the mixing amount of 0.6% can make the water reducing rate reach 20%. the above. In addition, adding a naphthalene-based water reducing agent to gypsum can play a significant role in accelerating coagulation and meet the production requirements of gypsum board; on the other hand, due to the reduction of mixing water, it can also significantly improve the mechanical properties of gypsum . This is beneficial for improving the performance of gypsum board and reducing production energy consumption. The least adaptable to gypsum system is the lignin-based water reducer, which not only has serious retardation, but also has obvious bleeding and staining with the increase of the content, which is not suitable for the production of gypsum board.
(2) The adaptability of polycarboxylic acid-based water-reducing agents to gypsum systems is worse than that of naphthalene-based water-reducing agents, and polycarboxylic acid-based water-reducing agents produced by different manufacturers have different effects on gypsum performance due to their different specific components. Its poor adaptation is mainly manifested in the retardation effect, which leads to less obvious enhancement effect than naphthalene after the same hydration time. The main reason is that the molecular structure of the naphthalene series and polycarboxylic acid series water reducers is different. The polycarboxylic acid series water reducers disperse the gypsum particles through the synergistic effect of electrostatic repulsion and steric hindrance, which also makes the gypsum slurry stable Increase and prolong the setting time. This is consistent with the results of the polycarboxylic acid-based water reducing agent delaying the hydration process of gypsum and the local crystal orientation arrangement in the SEM photos.
(3) As a new type of high-efficiency water-reducing agent, polycarboxylic acid water-reducing agent has great development potential. Through the water reduction test of polycarboxylic acid water reducing agent in gypsum system, it is found that the water reduction rate is very high based on the solid content. Therefore, the molecular structure design of the polycarboxylic acid-based water-reducing agent can be used to improve its adaptability to building gypsum and achieve the purpose of being suitable for the production of gypsum board. For example, mixing a liquid polycarboxylic acid water-reducing agent with a suitable solid content and a strong coagulation effect of raw materials can achieve the purpose of high-water reduction rate, low content, and little effect on the setting time.