Technical Field

The present invention relates to a chromium reducing agent composition, preferably a chromium reducing agent composition used in a cement.

Background

The water-soluble hexavalent chromium in a cement is harmful to human body and the environment. Due to the complexity and variety of raw materials used in the cement production, hexavalent chromium could be carried into the cement by the raw materials, the high temperature refractory of the rotary cement kiln, and the crushing and grinding equipment. Many cement plants are all facing this problem and when the water-soluble hexavalent chromium in a cement does not meet the standard requirements, it is not allowed to be sold and used. Therefore, many cement plants need to use chromium reducing agents to meet the standard requirements. The present invention aims at cement plants and grinding sections, and when the water- soluble hexavalent chromium exceeds the national standards, the chromium reducing agent must be used.

At present, almost all cement manufacturers use reducing agents as chromium reducing agents. The water-soluble hexavalent chromium in a cement is reduced into harmless trivalent chromium by a reducing agent, and the purpose of reducing the chromium is achieved by using this method. Ferrous sulfate is a commonly used reducing agent, which is widely used as a chromium reducing agent due to its low price and strong reducibility. However, since ferrous sulfate has strong reducibility, its chromium reduction effect will be decreased due to its own oxidation when stored in air. In addition, cement will be subjected to high temperature during the production process, and ferrous sulfate is oxidized acceleratedly at high temperatures, leading to the decrease of the chromium reduction effect thereof, namely the ferrous sulfate has worse high-temperature resistance. Stannous and antimony salts may also be used as chromium reduction agents. Their chromium reduction effect and high-temperature resistance are remarkable, but their price is too expensive to be accepted in practical applications.

In view of the above problems, it is designed a new idea for using the chelating agent. Hexavalent chromium in a cement is chelated and captured by using a chelating agent with various chelating groups. By using this method, the effect of reducing the water-soluble hexavalent chromium is remarkable, and this effect is not influenced by high temperature, namely the chelating agent has good high-temperature resistance.

CN101282917A provides a new method and composition for maintaining the effectiveness of chromium reducing agents in a cement over a period of time. Before adding a chromium reducing agent to a hydrable cement, chromium (VI) reducing agent, e.g. stannous sulfate (tin II) is combined with a non-lignin sulfonate salt based complexing agent, e.g. sodium gluconate, forming molecular association or coordination compounds, thereby stabilizing the chromium (VI) reducing agent in the hydrable cement during storage, so that the chromium (VI) reducing agent remains active in reducing the water-soluble chromium VI to chromium III when the cement is eventually mixed with water to cause the hydration thereof.

W02009073026A1 discloses a composition for reducing hexavalent chromium levels, the composition comprising: (A) at least one transition metal carbonyl compound or a derivative thereof; and (B) at least one other additive comprising (i) a cement additive; (ii) an antioxidant; (iii) at least one other chromium reducing agent (e.g., ferrous sulfate, stannous sulfate); (iv) a chelating agent; or (v) the mixture thereof.

JP2002029805A discloses a method for preparing a cement mortar, the method including kneading cement with water, a reducing agent, and a dithiocarbamic acidbased chelating agent and optional water reducer. The reducing agent is preferably an iron compound, particularly is ferrous sulfate, ferrous nitrate and ferrous chloride. The dithiocarbamic acid-based chelating agent, among others, includes dimethyl dithiocarbamic acid and a salt thereof and diethyl dithiocarbamic acid and a salt thereof and the like.

JP2003306365A discloses a cement additive, the additive comprises a metal salt of a chelating agent, wherein the reduced metal is incorporated to at least a portion of the functional group with metal coordination capability, with said reduced metal being iron (II). The example of the chelating agent, among others, includes a compound having a dithiocarbamic acid group.

JP2002060751 A discloses a hexavalent chromium elution reducing agent comprising a chelating compound and a hexavalent chromium elution reducing agent comprising a chelating compound and a sulfate salt. The chelating compound is selected from one or more of a thiourea compound, a thiazole compound, a dithiocarbamate salt compound and a triazine compound, or a chelating resin having sulfur and nitrogen atoms. The sulfate salt is selected from one or more of ferrous sulfate, sodium sulfite, sodium bicarbonate, potassium sulfite and potassium bicarbonate.

Most of the chelating/complexing agents reported in the prior art are used in combination with reducing agents, mainly aiming at increasing the stability of the reducing agents. Although JP2002060751A discloses the use of a chelating agent alone as a chromium reducing agent, it does not investigate the durability of the chromium reducing agent.

Therefore, there is still a need for a chromium reducing agent used in a cement, which can overcome the disadvantages of the chromium reducing agents in the prior art, has a remarkable chromium reduction effect, excellent high-temperature resistance, durability and storage stability, and is low in price.

Summary of Invention

The object of present invention is to provide a novel chromium reducing agent used in a cement, which has a remarkable effect of reducing the water-soluble hexavalent chromium in a cement, and this effect is not influenced by high temperature and is not impaired over time, i.e. having excellent high-temperature resistance and durability, and the chromium reducing agent itself is storage stable and low in price.

It is surprisingly found that this object can be achieved by the chromium reducing agent composition defined in claim 1 , which comprises a chelating agent and optionally a stabilizer. Unlike the reduction of hexavalent chromium into trivalent chromium via a strong reducing agent mostly used in the prior art, the present invention uses a chelating agent with various chelating groups as the major component functioning chromium reduction. The chelating agent has strong chelating capability, can be chelated with the water-soluble hexavalent chromium in a cement for the capture thereof, and has a remarkable chromium reduction effect. The chromium reducing agent composition of present invention has good high- temperature resistance, and the chromium reduction effect thereof is not influenced by high temperature. The chromium reducing agent composition of present invention is stable in air and does not need to worry about being oxidized, so it has good storage stability. In particular, by introducing the stabilizer, it is possible to release the slow oxidation of the chromium reducing agent in a cement, prolonging the durability of the product.

Other aspects of the present invention are subject of other independent claims. Preferred embodiments of the present invention are subject of the dependent claims.

In a first aspect, the present invention provides a chromium reducing agent composition, preferably a chromium reducing agent composition used in a cement, comprising: a chelating agent, preferably in an amount of 30-60 wt%, preferably 35-55 wt%, preferably 40-50 wt%, based on the total weight of the chromium reducing agent composition; and optionally a stabilizer, preferably in an amount of 0.1-1 wt%, preferably 0.3-0.7 wt%, preferably 0.4-0.6 wt%, based on the total weight of the chromium reducing agent composition.

In a second aspect, the present invention provides a method for reducing the water- soluble hexavalent chromium in a cement, comprising adding the chromium reducing agent composition as described herein, preferably in a form of liquid, to a cementitious binder, preferably the chromium reducing agent composition is added in an amount such that proportion of the chelating agent is 0.01 -0.5 wt%, preferably 0.03-0.125 wt%, preferably 0.05-0.1 wt%, relative to the cement on a dry weight basis.

In a third aspect, the present invention provides a cementitious composition, comprising: at least one cementitious binder; and the chromium reducing agent composition as described herein, preferably the chromium reducing agent composition is present in an amount such that proportion of the chelating agent is 0.01 -0.5 wt%, preferably 0.03-0.125 wt%, preferably 0.05-0.1 wt%, relative to the cementitious binder on a dry weight basis.

Description of Invention

Definitions

The term “hydraulic binder” in particular stands for substances that harden because of hydration chemical reactions and produces hydrates that are not water-soluble. In particular, the hydration chemical reactions of the hydraulic binder take essentially place independently of the water content. This means that hydraulic binder can harden and retain their strength even when exposed to water, e.g. underwater or under high humidity conditions. In contrast, the “non-hydraulic binder” (e.g. air-slaked lime or and gypsum) is at least partially water soluble and must be kept dry in order to retain their strength. Especially, within the present context, a “hydraulic binder” is capable to harden directly when mixed with water without the need of any additional activation.

The hydraulic binder comprises or consists of a cement. Preferably, the cement is Portland cement, and/or alumina cement, and/or sulphoaluminate cement, and/or ternary adhesive. It can be mixtures of Portland cement and alumina cement. The ternary adhesive can comprise or consist of alumina cement, calcium sulfate and optional Portland cement.

Preferred Portland cement is according to standard EN 197, in particular of type CEM I. The term "alumina cement" stands in particular for a cement with an aluminum content, measured as AI2O3, of at least 30 wt%, especially at least 35 wt%, in particular 35-58 wt%. Preferably, the alumina cement is alumina cement according to standard EN 14647. Preferably, the sulphoaluminate cement is calcium sulphoaluminate cement.

The term “gypsum” designates any known form of gypsum, in particular calcium sulfate dehydrate, calcium sulfate a-hemihydrate, calcium sulfate - hemihydrate, or calcium sulfate anhydrite or mixtures thereof.

The term “latent hydraulic and/or pozzolanic binder materials” stands in particular for type II concrete additives with latent hydraulic and/or pozzolanic character according to EN 206-1. In particular, the latent hydraulic or pozzolanic binder material comprises or consists of slag, fly ash, silica fume and/or natural pozzolanes. In particular, “latent hydraulic and/or pozzolanic binder materials” are not able to harden directly when mixed with water. These materials usually need an additional activation.

The term “slag” has its usual meaning and denotes in particular a byproduct of iron and steel making commonly used for blending cements, especially according to EN 15167. Preferably, the slag is a ground granulated slag, in particular a ground granulated blast furnace slag. In the present context, favorable slag is ground to a Blaine fineness of 3000 - 5000 cm ² /g. Slags with Blaine finenesses higher than 5000 cm ² /g can in principle also be used but they are more expensive to produce.

“Fly ash” denotes a residue generated in coal combustion mainly consisting of silicon dioxide, aluminum oxide and calcium oxide. Preferable fly ash is the one according to norm EN 450-1 .

“Silica fume” is a byproduct of silicon production and is mainly consisting of amorphous silicon dioxide. Preferable silica fume is the one according to norm EN 13263-1.

In a first aspect, the present invention provides a chromium reducing agent composition. The chromium reducing agent composition according to present invention comprises a chelating agent as the main chromium reduction component, preferably the only chromium reduction component. This means that the chromium reducing agent composition according to present invention is distinguished from the redox-mechanism in the prior art, captures the water-soluble hexavalent chromium via the chelating effect of the chelating agent on it only.

In an embodiment, the chromium reducing agent composition according to present invention comprises a chelating agent, preferably in an amount of 30-60 wt%, preferably 35-55 wt%, preferably 40-50 wt%, based on the total weight of the chromium reducing agent composition.

The chelating agent is a compound having a chelating group containing metal coordinated atoms, such as oxygen, nitrogen, sulfur, etc., which can form metal complex compound with metal cations, wherein the chelating agent forms bidentate or multidentate ligands in a singly or multiply deprotonated form. The example of the chelating group includes, but not limited to, a dithiocarbamic acid group, a phosphoric acid group, a carboxylic acid group, an amino carboxylic acid group, a dithiocarboxylic acid group, a phosphoramidic acid group, a thiol group and the like.

In an embodiment, the chelating agent used in the chromium reducing agent composition according to present invention is preferably an organosulfur chelating agent. The organosulfur chelating agent generally refers to a chelating agent containing sulfur as a coordinating group, and the example thereof includes a dithiocarbamic chelating agent, an xanthic acid chelating agent, trimercapto-s-triazine (TMT) chelating agent and trithiocarbonic acid (STC) chelating agent and the like.

Preferably, the organosulfur chelating agent used in the chromium reducing agent composition according to present invention is a dithiocarbamic chelating agent. The dithiocarbamic chelating agent is generally prepared by the reaction of primary amine or secondary amine with carbon disulphide.

Preferably, the dithiocarbamic chelating agent used in the chromium reducing agent composition according to present invention is selected from dimethyl dithiocarbamic acid or a salt thereof, diethyl dithiocarbamic acid or a salt thereof, and/or dibutyl dithiocarbamic acid or a salt thereof, preferably from dimethyl dithiocarbamate salt, diethyl dithiocarbamate salt, and/or dibutyl dithiocarbamate salt, most preferably dimethyl dithiocarbamate salt.

In an embodiment, the dimethyl dithiocarbamate salt used in the chromium reducing agent composition according to present invention is selected from sodium dimethyl dithiocarbamate, potassium dimethyl dithiocarbamate, magnesium dimethyl dithiocarbamate, calcium dimethyl dithiocarbamate, barium dimethyl dithiocarbamate, and/or zinc dimethyl dithiocarbamate, preferably from sodium dimethyl dithiocarbamate and/or potassium dimethyl dithiocarbamate, most preferably sodium dimethyl dithiocarbamate.

Commercially available dithiocarbamic chelating agents include SDD from Shandong Boshan Dongfang Chemical Engineering Co., LTD, the heavy metal capture agent from Zhonglong Environment Protection Technology Co., LTD, and sodium dimethyl dithiocarbamate from Hunan Fortune Environmental Protection Technology Co., LTD.

As described above, in the chromium reducing agent composition of present invention, the chelating agent is the main chromium reduction component. The chelating agent is sensitive to metal, so it should be avoided to introduce metal ions during the production, storage, transportation and use of the chromium reducing agent composition so as to keep the chromium reduction capability of the chelating agent.

In an embodiment, the chromium reducing agent composition according to present invention is free of metal ions. By the phrase “is free of”, it means that the chromium reducing agent composition comprises metal ions in an amount by weight of less than 1 wt%, preferably less than 0.5 wt%, more preferably less than 0.1 wt%, most preferably 0 wt%, based on the total weight of the chromium reducing agent composition.

The inventors of present invention have surprisingly found that by introducing the stabilizer, it is possible to mitigate the slow oxidation of the chromium reducing agent in a cement, prolonging the durability of the product.

In an embodiment, the chromium reducing agent according to present invention further comprises a stabilizer, preferably in an amount of 0.1 -1 wt%, preferably 0.3- 0.7 wt%, preferably 0.4-0.6 wt%, based on the total weight of the chromium reducing agent composition. In an embodiment, the stabilizer is a cellulose ether. Cellulose ether is a derivative of cellulose. Specifically, for the cellulose ethers, hydrogen atoms of hydroxyl groups are partially or fully substituted, typically with alkyl groups, hydroxyalkyl groups, carboxy groups and/or carboxy alkyl groups. In particular, the alkyl groups comprise methyl, ethyl and/or propyl groups.

Especially, the cellulose ether is a water-soluble cellulose ether.

In an embodiment, the cellulose ether is selected from hydroxypropyl methylcellulose, hydroxylethyl methylcellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, and/or carboxymethyl cellulose, preferably from hydroxypropyl methylcellulose and/or hydroxylethyl methylcellulose, most preferably hydroxypropyl methylcellulose.

Commercially available cellulose ether includes 60HD20 from Taian Ruite New Material Co., LTD.

The chromium reducing agent composition according to present invention is preferably produced and used in a form of liquid. Accordingly, in order to keep the flowability of the chromium reducing agent composition during use, the content of stabilizer, if present, should not be too high, otherwise, it will cause the viscosity of the system to be too high for application. The inventors of present invention have found that in the chromium reducing agent composition according to present invention, the content of cellulose ether in excess of 1 wt% will result in a too high viscosity, so the upper limit of the content thereof is 1 wt%.

The inventors of present invention have surprisingly found that by further introducing molasses, e.g. by introducing the stabilizer as described above and molasses simultaneously, it is possible to prevent the slow oxidation of the chromium reducing agent in a cement, significantly prolonging the durability of the product.

In an embodiment, the chromium reducing agent composition according to present invention further comprises molasses, preferably in an amount of 10-40 wt%, preferably 15-35 wt%, preferably 18-30 wt%, based on the total weight of the chromium reducing agent composition.

Molasses is a sucrose-containing composition that is produced in large quantities as a by-product in the sugar-making process and is available in low price worldwide. In addition to the main component sucrose, molasses especially contains dextrins, lactic acid, nitrogen compounds and inorganic salts. Molasses mainly comprises beet molasses, cane molasses, glucose molasses, corn molasses, and additionally converted molasses and refined molasses. W020051 10941 A1 discloses the use of chemically modified molasses as a plasticizer for cementitious compositions.

CN103732556A discloses the use of proteolytic converted molasses as a dispersing agent for a mineral binder. There is no report in the prior art on the use of molasses, in particular, in combination with cellulose ethers to improve the durability of the chromium reducing agent composition.

Without being limited to any theory, it is believed that in the chromium reducing agent composition according to present invention, molasses mainly functions as an antioxidant, so it can retard the oxidation of the chromium reducing agent composition in a cement, for example in connection with the cellulose ethers as a stabilizer, to significantly improve the durability of the chromium reducing agent composition. Additionally, in the chromium reducing agent composition according to present invention, molasses can also function as a pH regulator, so the use thereof can omit the use of pH regulator.

Like cellulose ethers, when the content of molasses is too high, it also makes the viscosity of the chromium reducing agent composition too high to be used in a form of liquid. Accordingly, if used, the content of molasses in the chromium reducing agent composition according to present invention is preferably below 40 wt%, preferably below 35 wt%.

Commercially available molasses includes the molasses from Changshu Huadong Chemical Auxiliary Sales Co. Ltd., and the molasses from Shanghai Kangbaizhi Technology Co. Ltd.

The chelating agent has a request on the pH of the medium, so as to exert the chelating capability thereof. As for the preferred chelating agent according to present invention, dimethyl dithiocarbamate salt, it requires an alkaline medium, in particular of a pH about 10. Accordingly, in order to meet the pH requirement of the chelating agent, sometimes it is necessary to add a pH regulator to the chromium reducing agent composition.

In an embodiment, the chromium reducing agent composition according to present invention further comprises a pH regulator, preferably in an amount of 1 -10 wt%, preferably 1 .5-5 wt%, preferably 2-3 wt%, based on the total weight of the chromium reducing agent composition.

In an embodiment, the pH regulator is selected from a polymeric polyol and/or sodium lignosulphonate.

Commercially available pH regulators include the polymeric polyol from Nanjing Changjiang Jiangyu Energy Technology Co. Ltd, and sodium lignosulphonate from Foshan Junda Import and Export Co, Ltd.

In an embodiment, the chromium reducing agent composition according to present invention further comprises a solvent, preferably water, preferably in an amount of 10- 58 wt%, preferably 15-48 wt%, preferably 20-39 wt%, based on the total weight of the chromium reducing agent composition.

In a preferred embodiment, the chromium reducing agent composition according to present invention comprises or consists of:

- a chelating agent, preferably in an amount of 30-60 wt%, preferably 35-55 wt%, preferably 40-50 wt%, based on the total weight of the chromium reducing agent composition;

- optionally a stabilizer, preferably in an amount of 0.1-1 wt%, preferably 0.3-0.7 wt%, preferably 0.4-0.6 wt%, based on the total weight of the chromium reducing agent composition;

- optionally molasses, preferably in an amount of 10-40 wt%, preferably 15-35 wt%, preferably 18-30 wt%, based on the total weight of the chromium reducing agent composition;

- optionally a pH regulator, preferably in an amount of 1-10 wt%, preferably 1.5-5 wt%, preferably 2-3 wt%, based on the total weight of the chromium reducing agent composition; and

- a solvent, preferably water, preferably in an amount of 10-58 wt%, preferably 15-48 wt%, preferably 20-39 wt%, based on the total weight of the chromium reducing agent composition, wherein the sum of the aforementioned components is 100 wt%.

In a second aspect, the present invention provides a method for reducing the water- soluble hexavalent chromium in a cement, comprising adding the chromium reducing agent composition as described herein, preferably in a form of liquid, to a cementitious binder, preferably the chromium reducing agent composition is added in an amount such that proportion of the chelating agent is 0.01 -0.5 wt%, preferably 0.03-0.125 wt%, preferably 0.05-0.1 wt%, relative to the cementitious binder on a dry weight basis.

The chromium reducing agent can be added into the chromium reducing agent composition in a form of powder or in a form of liquid. When adding in a form of powder, it is necessary to add additional charging equipment, thereby increasing additional cost. In contrast, adding in a form of liquid makes use of existing equipment and therefore can save costs. Accordingly, the chromium reducing agent composition according to present invention is preferably added to a cement in a form of liquid.

The chromium reducing agent composition according to present invention can be added into the cementitious binder before, during or after hydration caused by adding water to the cementitious binder.

In a third aspect, the present invention provides a cementitious composition, comprising: at least one cementitious binder; and the chromium reducing agent composition as described herein, preferably the chromium reducing agent composition is present in an amount such that proportion of the chelating agent is 0.01 -0.5 wt%, preferably 0.03-0.125 wt%, preferably 0.05-0.1 wt%, relative to the cementitious binder on a dry weight basis.

The chromium reducing agent composition according to present invention is suitable for almost all cements.

In an embodiment, in above method and the cementitious composition, the cementitious binder includes a hydraulic binder, a non-hydraulic binder and/or a latent hydraulic binder, preferably includes Portland cement, alumina cement, sulphoaluminate cement, gypsum, lime, slag, fly ash, silica fume, and/or natural pozzolanes.

The chromium reducing agent composition according to present invention has following advantages:

1. The chromium reducing agent composition according to present invention has a remarkable effect of reducing water-soluble hexavalent chromium, and has good high- temperature resistance, that is to say, the chromium reduction effect is not influenced by high temperature.

2. The chromium reducing agent composition according to present invention is stable in air and does not need to worry about being oxidized, so it has good storage stability. 3. By introducing the stabilizer, especially by introducing the molasses and the stabilizer together, it can prevent the slow oxidation of the chromium reducing agent composition according to present invention in a cement, thereby achieving excellent durability. 4. Since the chelating agent is used at a low content to achieve a remarkable chromium reduction effect, the chromium reducing agent composition according to present invention has a commercially acceptable low price.

Detailed Description The invention is illustrated further by examples below, which are not intended to limiting the invention.

EXAMPLES

The raw materials used in the examples are shown in table 1 below.

Table 1. The feedstocks used in the examples and description thereof Example 1. chromium reduction effect testing in a high temperature environment High temperature process is simulated: 0.1 wt% of ferrous sulfate, 0.05 wt% of chelating agent (powder) and 0.1 wt% of chelating agent (powder) are added to a cement, respectively, relative to the cement on a weight basis, the mixture is placed in an oven at 130 °C and dried for 5 hours, and then after cooling the mixture, the hexavalent chromium content is tested. Likewise, the cement without any chromium reducing agent added is placed in an oven at 130 °C and dried for 5 hours, and then after cooling the cement, the hexavalent chromium content is tested. The chromium content testing is carried out according to GB31893-2015. The results are shown in table 2 below.

Table 2. chromium reduction effect in a high temperature environment

As shown by the results in table 2, when adding 0.1 wt% of ferrous sulfate, the amount of water-soluble hexavalent chromium is decreased to 7.5 ppm. In contrast, when only adding 0.05 wt% of chelating agent, the amount of water-soluble hexavalent chromium is decreased to 1.6. While upon adding 0.1 wt% of chelating agent, the amount of water-soluble hexavalent chromium is decreased to 0. The results above indicate that in a high temperature environment, compared to ferrous sulfate, the chromium reduction effect of the chelating agent is more remarkable. Example 2. chromium reduction effect testing in a grinding environment

The grinding process is stimulated: 0.05 wt% of chelating agent (powder) and 0.125 wt% of chelating agent (liquid, the chelating agent accounts for 40 wt%) are added to a mix of the clinker and gypsum (95 wt% clinker and 5 wt% gypsum), respectively, relative to the total weight of the clinker and gypsum (95 wt% clinker and 5 wt% gypsum), and then the mixture is placed in a lab ball mill and ground for 20 min, the water-soluble hexavalent chromium content is tested. Likewise, the cement without any chelating agent added is placed in a lab ball mill and ground for 20 min, the water-soluble hexavalent chromium content is tested. The chromium content testing is carried out according to GB31893-2015. The results are shown in table 3 below.

Table 3. chromium reduction effect in a grinding environment

As shown by the results in table 3, when adding 0.05 wt% of chelating agent in powder, the amount of water-soluble hexavalent chromium is decreased to 0.3 ppm. When adding 0.125 wt% of chelating agent in liquid (the chelating agent compound is also 0.05 wt%), the amount of water-soluble hexavalent chromium is decreased to 1.3 ppm. The above results indicate that the chelating agent, whether in a form of powder or liquid, behaves remarkable chromium reduction effect in a grinding environment.

Example 3. durability test

The durability of the chromium reducing agent composition is studied: chromium reducing agent compositions A, B and C have been prepared according to the compositions shown in table 4 below. The preparation method is as follows: a chelating agent dosed and prepared is added into molasses for stirring and dissolving, then adding water and continuing stirring until completely homogeneous, HPMC is then added for stirring to completely dissolved. Finally, a pH regulator is added for stirring until homogeneous, if needed. The chromium reducing agent compositions A, B and C are added into a cement, respectively, in amount of 0.05 wt% chelating agent relative to the cement on a weight basis, then the mixture is placed in a lab ball mill and ground, the water-soluble hexavalent chromium content over grinding time (in ppm) is tested. Likewise, the cement without any chromium reducing agent added is placed in a lab ball mill and ground, the water-soluble hexavalent chromium content over grinding time (in ppm) is tested. The chromium content testing is carried out according to GB31893-2015. The results are shown in table 5 below.

Table 4. the composition of chromium reducing agent compositions Table 5. durability test results

As shown by the results in table 5, three chromium reducing agent compositions all can achieve remarkable chromium reduction effect at the beginning of the test, but the chromium reduction effect of the chromium reducing agent composition A (only with pH regulator but without HPMC and molasses) deteriorates gradually over time, as demonstrated by the increase in hexavalent chromium content, it is believed that this may be the result of the slow oxidation of chelating agent in a cement. In contrast, when using the chromium reducing agent composition B containing HPMC, hexavalent chromium content slowly increases over time, indicating that introducing

HPMC can mitigate the slow oxidation of the chromium reducing agent composition in a cement, prolonging the durability of the product. When using the chromium reducing agent composition C containing both HPMC and molasses, the increasing trend of hexavalent chromium content over time is further slowed down, indicating that introducing HPMC and molasses simultaneously can essentially prevent the slow oxidation of the chromium reducing agent composition in a cement, significantly prolonging the durability of the product.