Effect of ambient temperature on the workability of cellulose ether modified gypsum

The performance of cellulose ether modified gypsum at different ambient temperatures is very different, but its mechanism is not clear. The effects of cellulose ether on the rheological parameters and water retention of gypsum slurry at different ambient temperatures were studied. The hydrodynamic diameter of cellulose ether in liquid phase was measured by dynamic light scattering method, and the influence mechanism was explored. The results show that cellulose ether has a good water-retaining and thickening effect on gypsum. With the increase of cellulose ether content, the viscosity of slurry increases and the water retaining capacity increases. However, with the increase of temperature, the water retaining capacity of modified gypsum slurry decreases to a certain extent, and the rheological parameters also change. Considering that the cellulose ether colloid association can achieve water retention by blocking the water transport channel, the temperature rise may lead to the disintegration of the large volume association produced by cellulose ether, thus reducing the water retention and working performance of the modified gypsum.

Key words: gypsum; Cellulose ether; Temperature; Water retention; rheology

0. Introduction

Gypsum, as a kind of environmentally friendly material with good construction and physical properties, is widely used in decoration projects. In the application of gypsum based materials, water retaining agent is usually added to modify slurry to prevent water loss in the process of hydration and hardening. Cellulose ether is the most common water retaining agent at present. Because ionic CE will react with Ca2+, often use non-ionic CE, such as: hydroxypropyl methyl cellulose ether, hydroxyethyl methyl cellulose ether and methyl cellulose ether. It is important to study the properties of cellulose ether modified gypsum for better application of gypsum in decoration engineering.

Cellulose ether is a high molecular compound produced by the reaction of alkali cellulose and etherifying agent under certain conditions. The nonionic cellulose ether used in construction engineering has good dispersion, water retention, bonding and thickening effect. The addition of cellulose ether has a very obvious effect on the water retention of gypsum, but the bending and compressive strength of gypsum hardened body also decreases slightly with the increase of the addition amount. This is because cellulose ether has a certain air entraining effect, which will introduce bubbles in the process of slurry mixing, thus reducing the mechanical properties of the hardened body. At the same time, too much cellulose ether will make gypsum mix too sticky, resulting in its construction performance.

The hydration process of gypsum can be divided into four steps: dissolution of calcium sulfate hemihydrate, crystallization nucleation of calcium sulfate dihydrate, growth of crystalline nucleus and formation of crystalline structure. In the hydration process of gypsum, the hydrophilic functional group of cellulose ether adsorbing on the surface of gypsum particles will fix a part of water molecules, thus delaying the nucleation process of gypsum hydration and extending the setting time of gypsum. Through SEM observation, Mroz found that although the presence of cellulose ether delayed the growth of crystals, but increased the overlap and aggregation of crystals.

Cellulose ether contains hydrophilic groups so that it has a certain hydrophilicity, polymer long chain interconnecting with each other so that it has a high viscosity, the interaction of the two makes cellulose has a good water-retaining thickening effect on gypsum mix. Bulichen explained the water retention mechanism of cellulose ether in cement. At low mixing, cellulose ether adsorb on cement for intramolecular water absorption and accompanied by swelling to achieve water retention. At this time, water retention is poor. High dosage, cellulose ether will form hundreds of nanometers to a few microns of colloidal polymer, effectively blocking the gel system in the hole, to achieve efficient water retention. The action mechanism of cellulose ether in gypsum is the same as that in cement, but the higher SO42- concentration in the fluid phase of gypsum slurry will weaken the water-retaining effect of cellulose.

Based on the above content, it can be found that the current research on cellulose ether modified gypsum mostly focuses on the hydration process of cellulose ether on gypsum mix, water retention properties, mechanical properties and microstructure of hardened body, and the mechanism of cellulose ether water retention. However, the study on the interaction between cellulose ether and gypsum slurry at high temperature is still insufficient. Cellulose ether aqueous solution will gelatinize at a specific temperature. As the temperature increases, the viscosity of cellulose ether aqueous solution will gradually decrease. When the gelatinization temperature is reached, cellulose ether will be precipitated into white gel. For example, in the summer construction, the ambient temperature is high, the thermal gel properties of cellulose ether is bound to lead to changes in the workability of modified gypsum slurry. This work explores the effect of temperature rise on the workability of cellulose ether modified gypsum material through systematic experiments, and provides guidance for the practical application of cellulose ether modified gypsum.

1. Experiment

1.1 Raw Materials

Gypsum is the β-type natural building gypsum provided by Beijing Ecological Home Group.

Cellulose ether selected from Shandong Yiteng Group hydroxypropyl methyl cellulose ether, product specifications for 75,000 mPa·s, 100,000 mPa·s and 200000mPa·s, gelation temperature above 60 ℃. Citric acid was selected as gypsum retarder.

1.2 Rheology Test

The rheological test instrument used was RST⁃CC rheometer produced by BROOKFIELD USA. Rheological parameters such as plastic viscosity and yield shear stress of gypsum slurry were determined by MBT⁃40F⁃0046 sample container and CC3⁃40 rotor, and the data was processed by RHE3000 software.

The characteristics of gypsum mix conform to the rheological behavior of Bingham fluid, which is usually studied using Bingham model. However, due to the pseudoplasticity of cellulose ether added to polymer-modified gypsum, the slurry mixture usually presents a certain shear thinning property. In this case, modified Bingham (M⁃B) model can better describe the rheological curve of gypsum. In order to study the shear deformation of gypsum, this work also uses the Herschel⁃Bulkley (H⁃B) model.

1.3 Water retention test

Test procedure refer to GB/T28627⁃2012 Plastering Plaster. During the experiment with temperature as the variable, the gypsum was preheated 1h in advance at the corresponding temperature in the oven, and the mixed water used in the experiment was preheated 1h in the corresponding temperature in the constant temperature water bath, and the instrument used was preheated.

1.4 Hydrodynamic diameter test

The hydrodynamic diameter (D50) of HPMC polymer association in liquid phase was measured using a dynamic light scattering particle size analyzer (Malvern Zetasizer NanoZS90).

2. Results and discussion

2.1 Rheological properties of HPMC modified gypsum

Apparent viscosity is the ratio of shear stress to shear rate acting on a fluid and is a parameter to characterize the flow of non-Newtonian fluids. The apparent viscosity of the modified gypsum slurry changed with the content of cellulose ether under three different specifications (75000mPa·s, 100,000mpa ·s and 200000mPa·s). The test temperature was 20 ℃. When the shear rate of rheometer is 14min-1, it can be found that the viscosity of gypsum slurry increases with the increase of HPMC incorporation, and the higher the HPMC viscosity is, the higher the viscosity of modified gypsum slurry will be. This indicates that HPMC has obvious thickening and viscosification effect on gypsum slurry. Gypsum slurry and cellulose ether are substances with a certain viscosity. In the modified gypsum mix, cellulose ether is adsorbed on the surface of gypsum hydration products, and the network formed by cellulose ether and the network formed by the gypsum mix are interwoven, resulting in “superposition effect”, which significantly improves the overall viscosity of the modified gypsum based material.

The shear ⁃ stress curves of pure gypsum (G⁃H) and modified gypsum (G⁃H) paste doped with 75000mPa· s-HPMC, as inferred from the revised Bingham (M⁃B) model. It can be found that with the increase of shear rate, the shear stress of the mixture also increases. The plastic viscosity (ηp) and yield shear stress (τ0) values of pure gypsum and HPMC modified gypsum at different temperatures are obtained.

From the plastic viscosity (ηp) and yield shear stress (τ0) values of pure gypsum and HPMC modified gypsum at different temperatures, it can be seen that the yield stress of HPMC modified gypsum will decrease continuously with the increase of temperature, and the yield stress will decrease 33% at 60 ℃ compared with 20℃. By observing the plastic viscosity curve, it can be found that the plastic viscosity of modified gypsum slurry also decreases with the increase of temperature. However, the yield stress and plastic viscosity of pure gypsum slurry increase slightly with the increase of temperature, which indicates that the change of rheological parameters of HPMC modified gypsum slurry in the process of temperature increase is caused by the change of HPMC properties.

The yield stress value of gypsum slurry reflects the maximum shear stress value when the slurry resists shear deformation. The greater the yield stress value, the more stable the gypsum slurry can be. The plastic viscosity reflects the deformation rate of gypsum slurry. The larger the plastic viscosity is, the longer the shear deformation time of slurry will be. In conclusion, the two rheological parameters of HPMC modified gypsum slurry decrease obviously with the increase of temperature, and the thickening effect of HPMC on gypsum slurry is weakened.

The shear deformation of slurry refers to the shear thickening or shear thinning effect reflected by the slurry when subjected to shear force. The shear deformation effect of slurry can be judged by the pseudoplastic index n obtained from the fitting curve. When n < 1, the gypsum slurry shows shear thinning, and the shear thinning degree of gypsum slurry becomes higher with the decrease of n. When n > 1, the gypsum slurry showed shear thickening, and the shear thickening degree of gypsum slurry increased with the increase of n. Rheological curves of HPMC modified gypsum slurry at different temperatures based on Herschel⁃Bulkley (H⁃B) model fitting, thus obtain the pseudoplastic index n of HPMC modified gypsum slurry.

According to the pseudoplastic index n of HPMC modified gypsum slurry, the shear deformation of the gypsum slurry mixed with HPMC is shear thinning, and the n value gradually increases with the increase of temperature, which indicates that the shear thinning behavior of HPMC modified gypsum will be weakened to a certain extent when affected by temperature.

Based on the apparent viscosity changes of the modified gypsum slurry with shear rate calculated from shear stress data of 75000 mPa· HPMC at different temperatures, it can be found that the plastic viscosity of the modified gypsum slurry decreases rapidly with the increase of shear rate, which verifies the fitting result of the H⁃B model. The modified gypsum slurry showed shear thinning characteristics. With the increase of temperature, the apparent viscosity of the mixture decreases to a certain extent at low shear rate, which indicates that the shear thinning effect of the modified gypsum slurry is weakened.

In the actual use of gypsum putty, gypsum slurry is required to be easy to deform in the rubbing process and to remain stable at rest, which requires gypsum slurry to have good shear thinning characteristics, and the shear change of HPMC modified gypsum is rare to a certain extent, which is not conducive to the construction of gypsum materials. The viscosity of HPMC is one of the important parameters, and also the main reason that it plays the role of thickening to improve the variable characteristics of mixing flow. Cellulose ether itself has the properties of hot gel, the viscosity of its aqueous solution decreases gradually as the temperature increases, and white gel precipitates when reaching the gelation temperature. The change of rheological parameters of cellulose ether modified gypsum with temperature is closely related to the change of viscosity, because the thickening effect is the result of the superposition of cellulose ether and mixed slurry. In practical engineering, the impact of environmental temperature on HPMC performance should be considered. For example, the temperature of raw materials should be controlled in high temperature in summer to avoid the poor working performance of modified gypsum caused by high temperature.

2.2 Water retention of HPMC modified gypsum

The water retention of gypsum slurry modified with three different specifications of cellulose ether is changed with the dosage curve. With the increase of HPMC dosage, the water retention rate of gypsum slurry is significantly improved, and the increase trend becomes stable when the HPMC dosage reaches 0.3%. Finally, the water retention rate of gypsum slurry is stable at 90% ~ 95%. This indicates that HPMC has obvious water-retaining effect on stone paste paste, but the water-retaining effect is not significantly improved as the dosage continues to increase. Three specifications of HPMC water retention rate difference is not large, for example, when the content is 0.3%, water retention rate range is 5%, the standard deviation is 2.2. The HPMC with the highest viscosity is not the highest water retention rate, and the HPMC with the lowest viscosity is not the lowest water retention rate. However, compared with pure gypsum, the water retention rate of the three HPMC for gypsum slurry is significantly improved, and the water retention rate of the modified gypsum in the 0.3% content is increased by 95%, 106%, 97% compared with the blank control group. Cellulose ether can obviously improve the water retention of gypsum slurry. With the increase of HPMC content, the water retention rate of HPMC modified gypsum slurry with different viscosity gradually reaches the saturation point. 10000mPa·sHPMC reached the saturation point at 0.3%, 75000mPa·s and 20000mPa·s HPMC reached the saturation point at 0.2%. The results show that the water retention of 75000mPa·s HPMC modified gypsum changes with temperature under different dosage. With the decrease of temperature, the water retention rate of HPMC modified gypsum gradually decreases, while the water retention rate of pure gypsum basically remains unchanged, indicating that the increase of temperature weakens the water retention effect of HPMC on gypsum. The water retention rate of HPMC decreased by 31.5% when the temperature increased from 20 ℃ to 40℃. When the temperature rises from 40℃ to 60℃, the water retention rate of HPMC modified gypsum is basically the same as that of pure gypsum, indicating that HPMC has lost the effect of improving the water retention of gypsum at this time. Jian Jian and Wang Peiming proposed that cellulose ether itself has a thermal gel phenomenon, temperature change will lead to changes in the viscosity, morphology and adsorption of cellulose ether, which is bound to lead to changes in the performance of slurry mix. Bulichen also found that the dynamic viscosity of cement solutions containing HPMC decreased with increasing temperature.

The change of water retention of the mixture caused by the increase of temperature should be combined with the mechanism of cellulose ether. Bulichen explained the mechanism by which cellulose ether can retain water in cement. In cement-based systems,HPMC improves the water retention rate of slurry by reducing the permeability of the “filter cake” formed by the cementing system. A certain concentration of HPMC in the liquid phase will form several hundred nanometers to a few microns of colloidal association, this has a certain volume of polymer structure can effectively plug the water transmission channel in the mix, reduce the permeability of “filter cake”, to achieve efficient water retention. Bulichen also showed that HPMCS in gypsum exhibit the same mechanism. Therefore, the study of the hydromechanical diameter of the association formed by HPMC in the liquid phase can explain the effect of HPMC on the water retention of gypsum.

2.3 Hydrodynamic diameter of HPMC colloid association

Particle distribution curves of different concentrations of 75000mPa·s HPMC in the liquid phase, and particle distribution curves of three specifications of HPMC in the liquid phase at the concentration of 0.6%. It can be seen from the particle distribution curve of HPMC of three specifications in the liquid phase when the concentration is 0.6% that, with the increase of HPMC concentration, the particle size of the associated compounds formed in the liquid phase also increases. When the concentration is low, the particles formed by HPMC aggregation are small, and only a small part of HPMC aggregate into particles of about 100nm. When HPMC concentration is 1%, there are a large number of colloidal associations with a hydrodynamic diameter of about 300nm, which is an important sign of molecular overlap. This “large volume” polymerization structure can effectively block the water transmission channel in the mix, reduce the “permeability of cake”, and the corresponding water retention of gypsum mix at this concentration is also greater than 90%. The hydromechanical diameters of HPMC with different viscosities in liquid phase are basically the same, which explains the similar water retention rate of HPMC modified gypsum slurry with different viscosities.

Particle size distribution curves of 75000mPa·s HPMC with 1% concentration at different temperatures. With the increase of temperature, the decomposition of HPMC colloidal association can be obviously found. At 40℃, the large volume of 300nm association completely disappeared and decomposed into small volume particles of 15nm. With the further increase of temperature, HPMC becomes smaller particles, and the water retention of gypsum slurry is completely lost.

The phenomenon of HPMC properties changing with the rise of temperature is also known as hot gel properties, the existing common view is that at a low temperature, HPMC macromolecules first dispersed in water to dissolve solution, HPMC molecules in high concentration will form large particle association. When the temperature rises, the hydration of HPMC is weakened, the water between chains is gradually discharged, the large association compounds are gradually dispersed into small particles, the viscosity of the solution decreases, and the three-dimensional network structure is formed when the gelation temperature is reached, and the white gel is precipitated.

Bodvik found that the microstructure and adsorption properties of HPMC in liquid phase were changed. Combined with Bulichen’s theory of HPMC colloidal association blocking slurry water transport channel, it was concluded that the increase of temperature led to the disintegration of HPMC colloidal association, resulting in the decrease of water retention of modified gypsum.

3. Conclusion

(1) Cellulose ether itself has high viscosity and “superimposed” effect with gypsum slurry, playing an obvious thickening effect. At room temperature, the thickening effect becomes more obvious with the increase of viscosity and dosage of cellulose ether. However, with the increase of temperature, the viscosity of cellulose ether decreases, its thickening effect weakens, the yield shear stress and plastic viscosity of gypsum mix decrease, the pseudoplasticity weakens, and the construction property becomes worse.

(2) Cellulose ether improved the water retention of gypsum, but with the increase of temperature, the water retention of modified gypsum also significantly decreased, even at 60℃ will completely lose the effect of water retention. The water retention rate of gypsum slurry was significantly improved by cellulose ether, and the water retention rate of HPMC modified gypsum slurry with different viscosity gradually reached saturation point with the increase of dosage. Gypsum water retention is generally proportional to the viscosity of cellulose ether, at high viscosity has little effect.

(3) The internal factors that change the water retention of cellulose ether with temperature are closely related to the microscopic morphology of cellulose ether in liquid phase. At a certain concentration, cellulose ether tends to aggregate to form large colloidal associations, blocking the water transport channel of gypsum mixture to achieve high water retention. However, with the increase of temperature, due to the thermal gelation property of cellulose ether itself, the previously formed large colloid association redisperses, leading to the decline of water retention performance.