Development and Research of non-alkali FGD Gypsum Products

The set-up of desulphurization units in domestic coal-fired power plants has brought increasing output of FGD gypsum, which results in a wide application of FGD gypsum products. During the application of such products, people found that they are prone to alkalization, i.e., a phenomenon significantly affecting the appearance and performance of the products. In this study, fly ash, another kind of waste from coal-fired power plants, was introduced to inhibit the alkalization of FGD gypsum products under the action of the alkali-activator for better utilization of waste resources.


Introduction
China, as a large coal producer, uses coal as the basic fuel for electricity production.In recent years, the country's rapid development of the power industry has brought a sharp increase in the emissions of FGD gypsum and fly ash, which poses tremendous stress on the national economic construction and ecological environment [1] .FGD gypsum is a product from the reaction of limestone powder with SO2 in the flue gas of coal-fired power plants, and its main component is CaSO4•2H2O.After FGD gypsum is treated with processes such as drying, calcination, and pulverization, semi-hydrated gypsum is obtained, which can be used in gypsum blocks and plastering gypsum [2] .Besides, fly ash has been widely applied in construction, building materials, water conservancy and other fields.
Like dry mortar, FDG gypsum is also subject to alkalization during use.Alkalization, commonly known as whitening or frosting, is a phenomenon often occurring on the surface of the mortar of buildings [3] .The phenomenon is generally manifested as white powders, floccules or flakes, not only affecting the appearance of gypsum products, but even impeding the bonding of gypsum and the veneer layer in serious cases.For example, it may cause the protective paper to be separated from the gypsum core in gypsum plasterboard, or the coating layer surface of gypsum block products is rough or even falls off, resulting in product quality problems [4] .Therefore, an urgent task for us now is to inhibit the alkalization of gypsum products.

Experimental Materials (1) FGD Building Gypsum
In this experiment, the FGD building gypsum produced by a Beijing company was employed.Its chemical composition, phase composition, and physical properties are shown in    2 show that the purity of CaSO4•1/2H2O in the FGD building gypsum was very high, and the content reached 87.09%.In addition, it contains a small amount of chemical components such as Mg, Si, Fe and Al.
(2) Fly Ash The fly ash used in the experiment was the dry ash discharged from a Beijing power plant, and its chemical composition and physical properties are shown in Table 4 and Table 5. (3) Alkali-activator An alkali-activator is used to stimulate the activity of fly ash in order to improve the performance of the fly ash and FGD gypsum cementing material.Alkali-activator A was used in this experiment.

Experimental Tools
(

Sample Molding Method
1000g material was prepared as per the formula, and was poured into a triple mold for molding after being added with water (by the amount for water requirement of normal consistency), and mixed well with the mixer.

Curing Environment in the Experiment
(1) Outdoor curing: a window sill was selected where the environment depending on the weather usually is sunny, ventilated or might be exposed to rain.
(2) Indoor curing: An indoor flat platform about 1m above the ground was selected, with the humidity and temperature depending on the external environment.
(3) Curing in water: The lower part of the experimental sample was immersed in water and the alkalization situation of the upper part was observed.

Analysis of the Alkalization Components on the Surface of FGD Gypsum Products
The study results show that alkalization is a phenomenon in which the soluble salts in the gypsum product evaporate and migrate to the surface of the product to precipitate crystals.The gypsum product is made of a mixture of air-hardening building gypsum and water, and has a number of pores.Such structural feature enables the soluble salts in the FGD gypsum to move to the surface of the product along with water under the action of the external environment.The concentration of the solution keeps increasing with the continuous evaporation of water.After the saturation state is reached, white crystalline substances will continuously precipitate on the surface of gypsum products [5] .Through the analysis on the chemical composition (Table 6) and the XRD test (Figure 1) of alkalization components, it was found that most of the white substances were Mg salts and a small amount of Na and K salts.The XRD test was conducted on the alkalization components.The results are shown in Figure 1: The main phase detected by x-diffraction was MgSO4• 4H2O whose formation process was as follows: the MgSO4, as a soluble substance, precipitated onto the surface of the product from the inside of the same.In case of high humidity in the air, it would absorb moisture to form Mg salt.In addition, when the contents of Na2O and K2O were high, Na2SO4 and K2SO4 would also form, respectively.Different crystallized water-based salts, such as Na2SO4• 10H2O (commonly known as mirabilite) or complex salt, would precipitate on the product's surface to form white crystals.

Inhibiting Effect of Fly Ash on Alkalization of FGD Gypsum Products
The chemical activity of fly ash comes from the active components like soluble SiO2 and Al2O3 in the glassy particles (porous glass bodies and glass beads) formed by quick cooling after melting.The active SiO2 and Al2O3 produce calcium silicate hydrate(C-S-H) and calcium aluminate hydrate (C-A-H) as long as the water is available.These hydrates fill in the pores after the skeleton is formed by FGD gypsum and gather around the fly ash particles, so as to inhibit alkalization.For FGD gypsum products, the inhibition of alkalization by fly ash of various contents in different curing environments is shown in Table 7.As shown in Table 6, it is found that when the fly ash content was greater than 10%, the alkalization of gypsum products was slightly mitigated.However, after an alkali-activator was added to stimulate the activity of fly ash, there was no longer alkalization in gypsum products.Especially when the fly ash content was greater than 10%, no alkalization occurred in the products under the action of the activator, even in the extremely harsh environment such as water immersion.This is because the spherical glass bodies of fly ash were relatively stable, and their surface was quite dense, and not prone to be hydrated.The surface of fly ash particles remained basically unchanged after 7 days of gypsum hydration.Until 28 days, the surface began to be initially hydrated, and a slight gel-like hydrate appeared.After up to 90 days of hydration, a large amount of hydrated calcium silicate colloid began to generate on the surface of fly ash particles.At this time, some alkali metal salts had migrated to the surface of gypsum products, which means alkalization occurs.
The alkali-activator could destroy the Si-O and Al-O bonds in the fly ash glass bodies, greatly improving its activity.Fly ash was continuously hydrated under the stimulation of the activator to form a hydrate colloid, which quickly filled in the pores after the skeleton was formed by FGD gypsum.Due to the large surface area and strong adsorption capacity of the colloid, the dissolution of alkali metal ions was inhibited, thereby solving the alkalization problem of FGD gypsum.

Effect of Fly Ash on the Performance of FGD Gypsum Products
The effect of the fly ash content on the performance of gypsum products is shown in Fly ash contains a large number of spherical particles with a smooth surface.Such spherical particles play a role like "rolling balls" as they are easy to slide, can improve the fluidity of the slurry, and reduce the water requirement of normal consistency.Besides, with the increase of the fly ash content, the flexural and compressive strengths were also reduced.

Effect of Fly Ash on the Strength of FGD Gypsum Products under Action of Alkali-activator
The fixed fly ash content was 15%.The effect of an alkali-activator of different contents on the strength of fly ash-FGD gypsum cementing material was observed and the results are shown in Figure 2. It can be seen in Figure 2 that the strength of the compound cementing material of the fly ash in FGD gypsum increased with the rise of the alkali-activator content.When the content was 3%, the flexural and compressive strengths reached the maximum values larger than those of the pure FGD building gypsum.When the alkali-activator content continued to increase, the strength of the cementing material showed a downward trend.This is because the addition of the alkali-activator when the mixing ratio was low, broke the glass structure of the fly ash, stimulating the activity of the fly ash, and more hydration products generated, which improved the strength of the system.When the content reached about 3%, the alkalinity was large enough to stimulate the activity of the fly ash.When the alkali-activator content continued to increase, the contents of the FGD gypsum and fly ash in the system decreased, so the hydration products generated by their reaction were reduced.As a result, the strength of the entire system was reduced as well.Therefore, a conclusion from this experiment is that there is an optimal mixing ratio between alkali-activator and fly ash, namely 1:5.

Conclusion
(1) The components causing alkalization of FGD building gypsum products are mainly Mg, K and Na salts.
(2) Adding fly ash with a mixing ratio of more than 10% into the FGD building gypsum can mitigate alkalization in the products.
(3) With alkali-activator added, fly ash can inhibit alkalization of FGD building gypsum products.(4) The added fly ash can reduce the initial strength of FGD building gypsum products, and the combined action of alkali-activator and fly ash can increase the initial strength of FGD building gypsum products.The optimal mixing ratio of alkali-activator and fly ash is 1:5.

Figure 1
Figure 1 Diagram for XRD Test of Alkalization Components

Figure 2
Figure 2 Effect of Activator of Different Contents on the Strength of Fly Ash-FGD Gypsum Cementing Material

Table Analysis of
Alkalization Components in Gypsum Products

Table 7
Inhibition of Alkalization by Fly Ash of Various Contents

Table 8 Table 8
Effect of the Fly Ash Content on the Performance of Gypsum Products