Review of characteristics and mechanism of hydration of cellulose ether cement

Abstract: Cellulose ether can significantly delay the setting and hardening of cement. Generally, the release rate and peak value of cement hydration heat are reduced, the formation of cement hydration products is delayed, and the morphology of hydration products and the pore structure of cement slurry are affected. The main mechanisms of cellulose ether affecting cement hydration include adsorption, hindering the dissolution of cement particles, and delaying the nucleation and growth of hydration products. The delay of cement hydration by cellulose ether is closely related to the chemical composition of cement and the chemical structure of cellulose ether. The high viscosity of cellulose ether solution hinders ion diffusion is also the reason for the delay of cement hydration. Cellulose ethers are alkali stable.

Key words: cellulose ether; cement; hydration; review

0.Preface

At present, the annual output of ready-mixed mortar in the world exceeds 200 million tons, and almost all ready-mixed mortars need to add a certain amount of cellulose ether (Cellulose Ether, CE for short), because cellulose ether has excellent water retention performance, It can prevent the moisture in the wet mortar from evaporating prematurely or being absorbed by the base layer, ensure that the cement is fully hydrated, and improve the mechanical properties such as the final tensile bond strength of the mortar. The water retention effect of cellulose ether on mortar is particularly important in thin-layer mortar, high water-absorbing base, and under high temperature and dry conditions. In addition, cellulose ether also has obvious thickening effect, which increases the anti-bleeding, anti-scouring and anti-dispersion capabilities of freshly mixed cement-based materials, and can be used for pumping concrete, underwater concrete and self-compacting concrete.

Cellulose ether not only improves the water retention and thickening effect of cement-based materials, but also significantly delays the setting and hardening of cement, which is beneficial to improve the operability time of mortar, but sometimes it also delays the construction progress and brings inconvenience to the construction.

1. Molecular structure of cellulose ether

The raw material of cellulose ether is natural cellulose. Cellulose is a linear polysaccharide composed of D-glucose monomers linked by B-1,4-glycosidic bonds. Glucose monomer is a ring structure composed of 5 carbon atoms and 1 oxygen atom. Each glucosyl ring contains three hydroxyl groups, namely C-2 hydroxyl, C-3 hydroxyl and C-6 hydroxyl. The existence of hydroxyl groups makes cellulose have a large number of intramolecular hydrogen bonds and intermolecular hydrogen bonds, so pure cellulose is insoluble in water.

Natural cellulose can be synthesized into cellulose ether through the etherification process, that is, the C-2, C-3 and C-6 hydroxyl groups (-OH) in the cellulose molecule are replaced by methoxy (-OCH3), hydroxyethoxy [ -OCH2CH2OH] or hydroxypropoxy [-OCH2CH2CH2OH] and other groups are partially or even completely substituted. Products substituted by only one group are called monoethers, such as methyl cellulose ether (MC) and hydroxyethyl cellulose ether (HEC); products substituted by two or more groups are called mixed ethers, For example, hydroxyethyl methyl cellulose ether (HEMC) and hydroxypropyl methyl cellulose ether (HPMC) are often used in construction mortar.

Since the hydroxyl group is replaced, the hydrogen bonds between or within the original cellulose molecules are weakened, so the cellulose ether can be dissolved in water. The average number of hydroxyl groups substituted by etherified groups per glucose unit is described by the degree of substitution (DS). Obviously DS=0~3; the average amount of etherifying agent reactant added to each glucose unit is described by molar substitution (MS). For hydroxyalkyl, the substitution reaction will start from the new free hydroxyl The etherification starts again, so there is theoretically no upper limit to the molar substitution of hydroxyalkyl groups.

2. Hydration characteristics of vitamin ether-cement

2.1 Heat of hydration

According to the release curve of the heat of hydration over time, the hydration process of cement is usually divided into five stages, namely, the initial hydration period (0-15min), the induction period (15min-4h), the acceleration and setting period (4-8h ), deceleration and hardening period (8~24h), curing period (1~28d).

The addition of cellulose ether will not change the typical profile of the cement hydration curve, but will increase the time of the induction phase and reduce the heat release rate and peak value of the acceleration phase. Singhad et al. studied the heat of hydration of HEC-cement mixtures at 3, 7, 28d, and 91d and pointed out that with the increase of HEC content, the heat of hydration first gradually increased, and then gradually decreased. The amount of HEMC varies with the curing age; the experiments of Zhang Guofang and Wang Peiming showed that in the early stage of induction (that is, the initial hydration period), when the amount of HEMC is 0.1% relative to the blank cement slurry, the first An exothermic peak advances, and the peak increases significantly when the HEMC content increases above 0.3%. The first exothermic peak of the slurry is delayed, and the peak gradually decreases with the increase of HEMC content; HEMC will obviously delay the induction period and acceleration period of cement slurry, and the greater the content, the longer the induction period , the more backward the acceleration period, the smaller the heat release peak; the change of cellulose ether content has no obvious effect on the length of the deceleration period and stable period of cement slurry; cellulose ether can also reduce the cement slurry for 72 h However, when the hydration exotherm time is above 36 h, the change of cellulose ether content has little effect on the hydration exotherm of cement paste.

2.2 Morphology of hydration products and microstructure of cement slurry

Zhang Guofang and Wang Peiming analyzed the 3d and 28d hydration products of HEMC-cement with XRD and DSC, and found no new phase formation; Singh et al. analyzed the 91-day hydration products of HEC-cement with infrared spectroscopy and XRD, and the results showed that compared to pure cement. The HEC-cement system has a new phase formation; Kmpen et al. studied the influence of water-soluble polymers on the hydration of cement, and found that the cement slurry mixed with MC or HEC appeared on the DTG diagram of the initial hydration (<24 h). peaks, which then disappeared, while no new phases were detected in XRD and IR spectra. Neither singh nor Knapen could determine the chemical composition of the new phase.

Polymers tend to change the morphology of Ca(OH)2 crystals. Knapen et al. showed through secondary electron images that cellulose ether caused Ca(OH)2-crystal layered deposition, stacked arrangement, but no deformation; cellulose ether was distributed in the interlayer gap of layered Ca(OH)2-crystal , by "bridging" the layers of crystals together, thereby increasing the cohesion of the cement paste and improving its overall strength, since the Ca(OH)2- crystals represent weak points in the cement paste. BSE images also show that the number of micro-cracks in the cement mortar without cellulose ether is more than that with cellulose ether.

Cellulose ether will change the morphology of cement hydration products. Zhang Guofang and Wang Peiming showed by Si nuclear magnetic resonance spectrum that: HEMC can make the polymerization state of silicon-oxygen tetrahedron in C-S-H gel change from one polymerization state to one polymerization state and two polymerization state coexistence. HEMC also makes the size of C-S-H gel smaller and becomes fine wool, while the size of ettringite is shorter and thicker. Studies by Silva et al. showed that HPMC promoted the formation of internal products rather than external products in C, S particles, but did not change the morphology of C3A hydration products. According to Pourchez et al., cellulose ether makes the continuous C-S-H shell thicker, more permeable and more porous.

Cellulose ethers can affect the microstructure of cement paste. Pourchez et al. used light microscopy and fast x-ray tomography in 2D and 3D. The effect of cellulose ether on the pore structure of cement slurry was analyzed, and the results showed that cellulose ether led to the increase of pores with a diameter of 50-250 μm. Research by Zhang Guofang, Wang Peiming, etc. also showed that HEMC significantly increases the total pore volume and average pore diameter of cement paste, reduces the pore specific surface area, and leads to a significant increase in large capillary pores. However, the pores introduced are mainly closed pores.

Cellulose ether also affects cement hydrate production. HEMC makes C-S-H, ettringite and Ca(OH)2 formed later, Zhang Weifang, Wang Peiming, through the quantitative calculation of TG curve, showed that the addition of cellulose ether had no effect on Ca(OH)2 at the age of 28 days. The amount of formation has a significant impact; however, Knapen pointed out through thermal analysis that the cellulose ether makes the Ca(OH)2 production slightly lower, and thinks that this may be because the cellulose ether is adsorbed on the Ca(OH)2 nucleation site, which hinders the growth of the crystal , it is also possible that the content of Ca(OH)2 is underestimated.

3. Hydration kinetic mechanism of cellulose ether-cement

3.1 Adsorption

Most organic additives added to cement have a tendency to be adsorbed to the surface of cement particles or hydration products. This adsorption may hinder the dissolution of cement particles and the crystallization of hydration products, thereby affecting the rate of cement hydration and setting.

The adsorption mechanism of polysaccharides and mineral phase usually includes: hydrogen bonding, hydrophobic interaction, chemical complexation and electrostatic attraction (organic molecules have charged groups). Among them, hydrogen bonding and chemical complexation occur between the hydroxyl groups of polysaccharides and the hydroxylated metals on the mineral surface. Liu et al. classified the interaction between polysaccharides and hydroxylated metals as acid-base interactions, with polysaccharides as acids and metal oxides as bases, because the type of hydroxylated metals on the mineral surface has a great influence on the mineral-adsorbed polysaccharides. Capacity plays an important role, some polysaccharides are strongly absorbed by some basic oxides (or hydroxides), while acidic oxides are less able to adsorb polysaccharides. for a given polysaccharide. The alkalinity of the mineral surface determines the strength of the interaction between polysaccharides and minerals. If the interaction is strong, it will be a chemical complexation, if the interaction is weak, it will be a hydrogen bond. At the same time, it is believed that the hydrophobic effect is not the main factor of adsorption, but it has a synergistic effect on the acid-base effect.

The C-2 and C-3 hydroxyl groups of the glucosyl ring can participate in the complexation reaction. For example, Liu et al. believe that the effect of dextrin and lead hydroxide can be that the hydroxyl groups on c-2 and c-3 of the sugar unit form a five-membered complex with lead. Weissenbom et al. are studying the adsorption of starch and hematite During the function, it is assumed that the hydroxyl groups on C-2 and C-3 of two adjacent glucose units form an eight-membered complex with iron. When Raju et al. studied the effect of dextrin with calcium hydroxide and lead hydroxide, it was assumed that the three hydroxyl groups of C-2, C-3 and C-6 were all involved in the bond formation. Regarding the role of sugar and cement, it is generally believed that the complexation between sugar and calcium is the main cause of cement retardation.

The degree of substitution of cellulose ethers is generally low. Generally less than 1. Since the reactivity of C-2, C-3 and C-6 hydroxyl groups in glucose monomers is C-6>C-2>C-3, the substituents are generally at C-6, while C-2 and C-3 The hydroxyl group is usually a complete mock, so the cellulose ether can participate in the complexation reaction. However, the research results of Pourchez et al. show that the complexation effect of sugar on calcium ions is obvious, but the complexation ability of cellulose ether on calcium ions is negligible. Young believes that organic additives can also form complexes with aluminum and silicon ions.

Cement is selective for the adsorption of cellulose ether. That is, it is closely related to the mineral composition of cement, the type of hydration products and the chemical structure of cellulose ether. In cement minerals, due to the rapid reaction of C3A with water to form calcium aluminate hydrate, the adsorption of cellulose ether on the surface of C3A is impossible, and the adsorption capacity of C3S to cellulose ether is weak; among the hydration products, aluminum hydrate Calcium acid, C-S-H and Ca(OH)2 have obvious adsorption on cellulose ether. However, the adsorption capacity of ettringite is poor; the chemical structure of cellulose ether has a great influence on the adsorption capacity, compared with the molecular weight. The type of substituent and the degree of substitution have a greater impact on the adsorption. The adsorption of calcium aluminate hydrate to HEC is much greater than that of HPMC, and the adsorption of C-S-H and Ca(OH)2 to HEC is also higher than that of HPMC. And the lower the content of hydroxypropoxy in HPMC or hydroxyethoxy in HEC, the stronger the adsorption.

3.2 The dissolution process of cement minerals and the formation process of hydration products

Pourchez et al pointed out through experiments that cellulose ether can delay the dissolution of C3A. In the presence of gypsum, this retardation effect is weak; cellulose ether also makes the dissolution rate of C3S very slow, but this is not directly hindered by cellulose ether. The result of the dissolution of C3S, but the cellulose ether affects the composition of ions in the liquid phase, thereby limiting the dissolution of C3S.

In the absence of gypsum, cellulose ether can reduce the crystallization of calcium aluminate hydrate, which is mainly caused by the adsorption of cellulose ether, because the higher the amount of cellulose ether adsorbed by calcium aluminate hydrate, the more cellulose ether The crystallization of calcium aluminate hydrate is reduced the stronger. In the case of adding gypsum, cellulose ether has little effect on the precipitation of ettringite, and still has a strong retarding effect on the crystallization of calcium aluminate hydrate, which is mainly because cellulose ether increases the precipitation of calcium aluminate hydrate critical ion saturation concentration.

Cellulose ether would reduce the nucleation and growth rate of C-S-H on the surface of C3S particles. Cellulose ether preferentially reduces the growth rate of C-S-H parallel to the C3S surface, increases the time and quantity of C-S-H free growth, with the increase of cellulose ether content. This retardation effect is more obvious. It has no or little effect on the growth of C-S-H perpendicular to the C3S surface, that is, the diffusion growth of C-S-H.

Pourchez et al. also found that cellulose ether led to the opposite crystallization effect of Ca(oH)2 while modifying the porous structure of the C-S-H layer and the process of C-S-H nucleation-growth. Under the condition of high lime concentration, the retarding effect of cellulose ether on C-S-H nucleation is more important than the change of C-S-H shell, resulting in delayed Ca(OH)2 crystallization; The change in shell permeability is more important than the retarding effect on C-S-H nucleation, resulting in accelerated Ca(OH)2 crystallization.

The retarding effect of cellulose ether on C-S-H and Ca(OH)2 is attributed to the latter’s adsorption to cellulose ether, and the stronger the adsorption capacity of C-S-H and Ca(OH)2 on cellulose ether. The cellulose ethers have a greater influence on their crystallization kinetics.

3.3 Influence of cement minerals

Cellulose ethers affect the hydration kinetics of tricalcium silicate (C3S) and tricalcium aluminate (C3A), the most important clinker mineral phases in cement, in different ways. Cellulose ether mainly reduces the reaction rate of C3S acceleration phase, while for C3A/Ca2SO4 system it is because cellulose ether prolongs the induction period. The hindrance to the hydration of C3S in the above way will delay the hardening process of the mortar, and the prolongation of the induction period of the C3A/Ca2SO4 system will delay the setting of the mortar.

Peschard et al. studied the retarding effect of several polysaccharides, including cellulose ether, on cement, and found that the retarding effect was related to cement components, and believed that the content of GA was a key parameter. The lower the content of C3A, The higher the retarding effect.

3.4 Effect of cellulose ether chemical structure

The chemical structure of cellulose ether has an important influence on cement hydration. The substituent group is an important factor for delaying cement hydration, and the delaying effect of HEC is usually higher than that of HPMC and HEMC. Further research also shows that the content of methoxyl group is the main parameter that affects the delay of cement hydration in HPMC and HEMC. With the decrease of methoxyl content, the delay of cement hydration is more obvious, while the molecular weight and hydroxypropoxyl content have a significant effect on cement water. The cement hydration process has a lower influence; the molecular parameters of HEC also have an important influence on the cement hydration. The degree of substitution DS is the main parameter affecting the delay of cement hydration. The effect of molecular weight can be ignored. The research of Weyer et al. through synchrotron XRD also showed that the degree of substitution DS is the main factor affecting the dissolution of gypsum and the hydration of C3S and C3A. Usually, the smaller the DS, the more obvious the delay of cement hydration.

Pourchez et al. also found that some low-molecular-weight HECs had a very obvious delaying effect on cement hydration, and believed that molecular weight and DS were not independent in the mechanism of delaying cement hydration.

3.5 Ion diffusion rate

Silva et al. analyzed the effect of water-soluble polymers with different viscosities on cement hydration, and believed that the delay of cellulose ether on cement hydration was due to the fact that cellulose ether increased the viscosity of the pore solution and restricted the movement of ions, thereby reducing the unhydration. Phase dissolution rate and crystallization of hydration products. However, when Pourchez et al. summarized the hydration of a relatively dilute cellulose ether-cement aqueous solution, they pointed out that the viscosity of the cellulose ether-cement slurry had no obvious relationship with the delay, and pointed out that the delay caused by cellulose ether could not be attributed to ion Movement is hindered. The author believes that in the thicker cellulose ether-cement slurry system, the obstruction of ion movement will delay the hydration of cement.

3.6 Alkali stability of cellulose ethers

Polysaccharides are usually easily degraded under alkaline conditions to form hydroxycarboxylic acids, which are well-known chelating agents and retarders, and the degradation products of sugars in alkaline environments have a more effective retarding effect than native sugars. Some studies have also shown that the degradation products of cellulose under alkaline conditions (such as d-isosaccharinic acid) have a large complexing effect on calcium.

Pourchez et al. studied the possible degradation products of several HPMC and HEC in saturated calcium hydroxide solution by chromatography. The results showed that the filtrate of HPMC included three kinds of hydroxycarboxylic acids (lactic acid, glycolic acid and oxalic acid). and 2 alcohols (ethylene glycol and glycerol). The filtrate of HEC includes the remaining four kinds of the above products except oxalic acid. These products were all present in small amounts (≤2 mg/g) and were likely derived from degradation products of cellulose, hemicellulose, and monosaccharides, as well as from the production process. Studies have also shown that they have a negligible retarding effect on cement hydration. Therefore, these cellulose ethers are considered to be alkali stable. Muller also came to the same conclusion.

4. Conclusion

Current research shows that cellulose ether can significantly delay the setting and hardening of cement. Cellulose ether usually delays the release rate of cement hydration heat and reduces the peak heat of hydration; compared with the hydration of pure cement, cellulose ether-cement hydration products may produce new phases; (OH)2 has a bridging effect, thereby increasing the cohesion of the cement slurry; cellulose ether affects the morphology of Ca(OH)2 and C-S-H, and the microstructure of the cement slurry, resulting in a significant increase in the large capillary pores of the cement slurry structure; fiber Plain ether delays the formation of hydration products such as C-S-H gel, ettringite and Ca(OH)2, making the yield of Ca(OH)2 slightly lower.

The main mechanisms of cellulose ether affecting cement hydration include adsorption, hindering the dissolution of cement particles, and delaying the nucleation and growth of hydration products. Adsorption can be attributed to acid-base interaction. The hydrophobic effect also has a synergistic effect; the adsorption of unhydrated cement to cellulose ether is relatively weak, and the hydration products such as calcium aluminate hydrate, C-S-H and Ca(OH)2 have obvious adsorption on cellulose ether, and the adsorption of ettringite The adsorption capacity is poor; the cement unhydrated phase and hydration products have selective adsorption to cellulose ether. Cellulose ether can delay the dissolution of C3A and C3S, the nucleation of C-S-H and the growth of calcium aluminate hydrate, C-S-H and Ca(OH)2. The effect of cellulose ether on cement hydration is related to the chemical composition of cement and the chemical structure of cellulose ether. In the higher viscosity cellulose ether-cement slurry. The high viscosity of cellulose ether solution hinders ion diffusion and is also the cause of delayed cement hydration. Cellulose ether has alkali stability and will not delay cement hydration due to alkali degradation.

The current research has greatly increased the understanding of the characteristics and mechanism of cellulose ether-cement hydration. However, Portland cement is a multi-component complex phase. The varieties and chemical structures of cellulose ethers are ever-changing. Many ready-mixed cellulose ethers Modification has also been carried out, the interaction between cellulose ether and cement must be very complicated, and there are many deficiencies in the current research, such as the change of heat of hydration, the formation of new phases, the change of the amount of hydration products and the water content. There are still controversies about the chemical kinetics mechanism, and there are few cellulose ethers used for research, so it is impossible to systematically establish the relationship between the chemical structure of cellulose ethers and cement hydration. Therefore, it is also necessary to combine engineering applications to continuously broaden and deepen the research on cellulose ether-cement hydration.