Preparation Technology of Lightweight High-strength Desulphurized Gypsum Block

In this paper, the relationship between the air entraining agent, retarder, lightweight aggregate, water-cement ratio and the performance of gypsum block is studied. Study results showed that the apparent density of gypsum block can be significantly reduced and its strength can be increased by using double mixing technology of air entraining agent and perlite to optimize production process and reduce water-cement ratio and energy consumption.


Introduction
As the fire coal used in thermal power plants contains 1.5%~2% combustible sulfur and the flue gas after coal combustion contains a large amount of SO2 and SO3, it will cause serious pollution to the ecological environment if these gases are directly discharged into the atmosphere without recycling.Rainfall will be acidified, which will endanger people's health, corrode metals and building materials, and acidify soil and water body.Therefore, it is required that the flue gas discharged from coal-fired power plants must be desulfurized.At present, among a large number of flue gas desulfurization processes, wet lime (stone)-gypsum method is the most mature and widely used desulfurization process all over the world.The by-product of this process is desulfurized gypsum.In China, wet limestone-gypsum method is the main desulfurization method, accounting for about 70% of the installed desulfurization unit capacity.In this method, limestone is used as the desulfurizing agent.By spraying absorbent slurry into the absorption tower, limestone is fully contacted and mixed with the flue gas to wash the flue gas, so that the SO2 in the flue gas reacts with CaCO3 in the slurry and the blew strongly oxidized air to form desulfurized gypsum.The main component of desulfurized gypsum is calcium sulfate (CaSO4•2H2O) containing two crystal waters.The desulfurized gypsum is processed through drying, calcination, crushing and other processes to produce hemihydrate gypsum.As the cementing material of gypsum based building materials, most of them are hemihydrate gypsum (CaSO4•1/2H2O) [1] .Therefore, hemihydrate gypsum is often called building gypsum.
At present, the application scope of desulphurized building gypsum in China is limited and the utilization amount is small.Therefore, exploring the application of desulfurized gypsum in the field of building dry powder mortar and meeting the demand of building materials market is the development direction of desulfurized gypsum in the future.In order to protect China's natural resources and reduce environmental pollution, it has been clearly pointed out in China's Tenth Five-Year Plan for Building Materials Industry that it is necessary to actively use industrial waste residues such as phosphogypsum, fluorogypsum and flue gas desulphurized gypsum, so as to save energy and utilize waste, and vigorously develop "green building materials" [3] .Therefore, it is of great significance to study the application technology of flue gas desulphurized building gypsum in gypsum block.Materials: Two types of natural building gypsum and two types of desulphurized building gypsum were used for comparative study in the experiment.

Effect of retarder on setting time and strength of desulfurized gypsum
Characteristic of desulfurized building gypsum like fast setting and hardening is beneficial to shortening the production cycle and improving the utilization rate of equipment.However, it is extremely disadvantageous to the controllability required by achieving industrialized production.Due to short setting time of desulfurized building gypsum and hard to control the industrialized production, the setting time regulator must be added when preparing the gypsum block of desulfurized gypsum to adjust its setting time to meet the production process requirements [2] .Generally, an appropriate amount of retarder is added to the materials.At present, the commonly used gypsum retarders are generally citric acid, trisodium citrate, tartaric acid, gypsum retarder (Academy of Forestry), and self-made powder retarder, etc. Retarder can be used to control/prolong the fluidity retention time of gypsum slurry, so as to meet the operation time requirements in the production process.Comparative tests were carried out for different kinds of retarders, and the test results were shown in Table 1.The test results showed that different types of retarders had different degrees of retarding effect, but some retarders had an unsatisfactory retarding effect, and some retarders had an impact on the strength of gypsum block.Therefore, it is necessary to select the appropriate type and dosage of retarder based on the demands.The test confirmed that B35 was selected as the retarder, and its dosage was 0.2% -0.3%.Under the same conditions, the effect of different dosages of retarders on the performance of gypsum block was shown in Fig. 1.

Fig.1 Effect of different dosages of retarder on the performance of gypsum block
It can be seen from Fig. 1 that the setting time of gypsum block slurry can be significantly improved with the increase of retarder content, but the strength of the block is greatly reduced.Therefore, the dosage of retarder should be as low as possible provided that the production operation time is guaranteed.

Relationship between air entraining agent and gypsum block performance
Air entraining agent is a kind of surface active substance with a hydrophobic effect, and it is an additive that can introduce a large number of evenly distributed, stable and closed tiny bubbles during gypsum mixing.It is a mostly anionic and non-ionic surfactant that can significantly reduce the surface tension of gypsum mixing water.After mixing, a large number of closed, stable and evenly distributed tiny bubbles can be produced in the mixture.As the foam is an unstable system, it is difficult for pure liquids to form stable and durable foam.During the mixing, the bubbles will move, merge and increase, and even destroy and disappea r [4] .According to the Laplace principle, the pressure in the tiny bubble is larger than that in the large bubble.After a certain time, the air moves to the large bubble through the diaphragm and finally forms a large bubble.Therefore, in order to stabilize the pouring slurry and ensure the formation of a fine and uniform porous structure, a certain amount of foam stabilizer should be added.Adding foam stabilizer aims to change the produced foam from an unstable system to a stable system.After adding the slurry, the foam stabilizer can be adsorbed on the surface of the bubble to form a bimolecular film and make the outer surface of the bubble film become a hydrophobic layer, thereby stabilizing and dispersing the bubbles.In addition, the directional distribution of the foam stabilizer on the bubble water film reduces the surface tension of the water film, so as to increases the stability of the water film.The commonly used bubble stabilizer is chlorinated paraffin, with a dosage of 0.01%-0.02%.At present, common types of air entraining agents mainly include sodium rosin acid, sodium alkyl sulfonate, sodium alkyl benzene sulfonate and so on.In this experiment, imported 6031, 8051, RN-31HDH (Hercules company) and domestic PC-2 (rosin pyrolytic polymer) were used for experimental study.See Table 2 for the relationship between the addition amount and the block properties (such as apparent density, flexural strength, compressive strength and dimensional stability).   2 that when the dosage of air entraining agent PC-2 and 6031 is 0.02-0.04%, the apparent density of blocks does not decrease significantly and the dimensional stability is basically not affected.However, the strength of gypsum block decreases obviously with the increase of dosage, the dimensional stability becomes worse, and the 2h strength is low.Air entraining agent 8051 and RN-31HDH can effectively decrease the apparent density of gypsum block, but the use of air entraining agent alone cannot meet the project requirements in terms of apparent density and dimensional stability of blocks.In addition, it was found in the experiment that when the amount of air entraining agent was 0.06%, the bleeding phenomenon on the surface of gypsum block was serious.

Relationship between lightweight aggregate and performance of gypsum block
The mechanical performance of gypsum block is an important quality index.Although gypsum block is only used as infilled wall of frame structure without bearing the core weight of structure, it still has certain requirements for mechanical performance.The mechanical performance of gypsum block is related to its material composition, proportion and production process conditions, etc.According to the test statistics, the strength calculation formula of gypsum block is: Where, R-Gypsum block strength Rs -strength of building gypsum (Mpa) (strength of building gypsum after mixing with active mineral admixture and additive); C/W -water-cement ratio; f -Influence coefficient of lightweight aggregate (it is related to the cylinder compressive strength of lightweight aggregate and the percentage of mixing amount).The formula is based on the fact that the strength of lightweight aggregate is lower than that of gypsum block, and its water-cement ratio should meet the pouring characteristics of slurry.The formula shows that the strength of gypsum block is closely related to the strength of building gypsum and lightweight aggregate and the water-cement ratio.It is a stacking structure formed by the cementation of gypsum with fine micropores and lightweight porous aggregate.In the structure, there are two kinds of microporous microtubule systems, namely, the microporous microtubule system in gypsum and the microporous microtubule system in lightweight aggregate.This structural feature makes the gypsum block have many superior architectural physical properties [5] .In addition, the pore structure of gypsum block is composed of dihydrate gypsum crystallizing into needle-like crystal, so there is a lot of free space in the needle-like crystal structure, that is, the porosity is very high.Therefore, when the humidity in the air is too high, the gypsum block can absorb the water in the air through the capillary pore structure.When the external environment becomes increasingly dry, the stored water can be gradually evaporated into the indoor air along with the capillary pore structure, so as to adjust the indoor air humidity.That is to say, the gypsum block has a certain "breathing" function.Lightweight aggregate is mainly added to reduce the unit weight of blocks, and the commonly used lightweight aggregate are expanded perlite, fine ceramsite and vitrified beads.The relationship between their dosage and block performance is shown in Table 3.  3 that when the addition amount of expanded perlite and vitrified beads is 3% and the water-cement ratio is 82%-85%, the apparent density of the gypsum block can meet the standard requirements (no more than 700kg/m3), while the effect of fine ceramsite is poor.However, because the amount of water added exceeds the standard thickening by more than 20%, the strength of the block is low, the dimensional stability is poor, and the energy consumption increases while drying, so the water-cement ratio should be reduced.It can be seen that only adding lightweight aggregate such as expanded perlite to produce light gypsum blocks is not the best process.

Relationship between the water-cement ratio and the apparent density and strength of gypsum block
When the composition ratio of gypsum block is fixed, the finished water-cement ratio becomes the vital process parameter.It directly affects the mixing quality and pouring quality of slurry, and thus reflecting the defects of internal structure from the level of mechanical strength.In principle, the strength of gypsum block increases with the decrease of water-cement ratio.Under the condition of a low water-cement ratio, the gypsum slurry has a small fluidity, the gypsum hardened body has high compactness and strength, and the finished product has a large unit weight [6] .Under the condition of a high water-cement ratio, the gypsum slurry has a large fluidity, the gypsum hardened body has low compactness and strength, and the finished product has a small unit weight, with the possibility of bleeding.
From the microscopic analysis, under the condition of appropriate water-cement ratio, the high liquid phase supersaturation of gypsum slurry leads to the formation of many crystal nuclei with small grains, occurrence of many crystal contact points and formation of crystal structure network, so the strength of the block will be high after hardening; when the water-cement ratio is lower than the limit value, the fluid phase supersaturation of gypsum slurry is high.After the initial structure is formed, the hydration products continue to form, which further compacts the structural network and generates crystallization stress on the formed structural network.When the crystallization stress is greater than the structural strength at that time, the structural strength of hardened blocks is damaged.When the water-cement ratio is greater than the limit value, there are fewer crystal nuclei and crystal contact points due to the increase of the water filling space of the slurry before hardening and the decrease of the saturation of the slurry, which is difficult to form the crystal structure network.Moreover, the internal porosity of the hardened slurry increases, resulting in the low strength of the hardened block [7] .Under the condition of different water-cement ratios, the relationship between air entraining agent RN-31HDH (addition amount of 0.02%) and lightweight aggregate perlite (addition amount of 1%) and block performance is shown in Table 4.  4 that the apparent density of the block gradually decreases with the increase of water-cement ratio, and the strength of the gypsum block also decreases.When the water-cement ratio is 0.85, the unit weight of the block can also meet the design requirements (less than 700kg/m 3 ), but its flexural strength is low and cannot meet the requirements; considering the unit weight and strength of the block, the water-cement ratio should be about 0.80 according to the experiment.It can be seen from Fig. 2 that the volume of the block expands slightly with the increase of the water-cement ratio.

Relationship between moisture content and strength
After gypsum block is cast and formed, there is an opposite relationship between structure formation and structure damage in its own hardening process [8] .The structural strength of gypsum blocks increases rapidly from casting and formation to final setting of gypsum block, and then continues to increase until the maximum value.After the strength reaches the maximum value, the block shall be dried quickly, so that the maximum value of the block can be relatively stable.A block that has risen to its maximum strength will gradually decline in strength if it is not dried in time, as shown in Fig. 3. Fig. 3 Relationship between drainage speed and strength of gypsum block It can be seen from Fig. 3 that after the gypsum block reaches the maximum strength, the free water contained in the structure cannot be discharged in time, which will dissolve the crystal contact points in the crystal network formed in the block and distort the crystal network structure.The occurrence and development of this process will decrease irreversibly with the extension of time (such in curve 2).If the block can be dried in time, the crystal contact points formed in the internal structure of the gypsum block can be well developed and interlinked, so that the crystallization network system is more complete and the maximum strength of the gypsum blocks can be stabilized (curve 1 in figure 1).Therefore, the molded gypsum block must be dried in time after a certain period of static rest when the structural strength reaches the maximum value.

Conclusion
1) The apparent density and cost of the block can be effectively reduced by adopting the double mixing technology of air entraining agent and lightweight aggregate perlite.
2) The water-cement ratio has a great influence on the performance of the block, and the water-cement ratio is generally 0.80.
3) The grade of gypsum powder is well controlled, the water consumption for standard consistency of gypsum powder is accurately measured, and the water-cement ratio is fine-tuned according to the Compressive strength(MPa)

Fig. 2
Fig. 2 Relationship between water-cement ratio and block volume change

Table 1
Effect of different retarders on setting time of desulfurized gypsum block with large dosage

Table 2
Relationship between the type and dosage of air entraining agent and the block properties (under standard consistency conditions)

Table 3
Relationship between type and dosage of lightweight aggregate and gypsum block property

Table 4
Relationship between lightweight aggregate and block properties under different water-cement ratios