Effect of hydroxyethyl cellulose ether on early hydration of CSA cement

The effects of hydroxyethyl cellulose (HEC) and high or low substitution hydroxyethyl methyl cellulose (H HMEC, L HEMC) on the early hydration process and hydration products of sulfoaluminate (CSA) cement were studied. The results showed that different contents of L‑HEMC could promote the hydration of CSA cement in 45.0 min~10.0 h. All the three cellulose ethers delayed the hydration of cement dissolution and transformation stage of CSA first, and then promoted the hydration within 2.0~10.0 h. The introduction of methyl group enhanced the promoting effect of hydroxyethyl cellulose ether on the hydration of CSA cement, and L HEMC had the strongest promoting effect; The effect of cellulose ether with different substituents and degrees of substitution on the hydration products within 12.0 h before hydration is significantly different. HEMC has a stronger promotion effect on the hydration products than HEC. L HEMC modified CSA cement slurry produces the most calcium-vanadite and aluminum gum at 2.0 and 4.0 h of hydration.
Key words: sulfoaluminate cement; Cellulose ether; Substituent; Degree of substitution; Hydration process; Hydration product

Sulfoaluminate (CSA) cement with anhydrous calcium sulfoaluminate (C4A3) and boheme (C2S) as the main clinker mineral is with the advantages of fast hardening and early strength, anti-freezing and anti-permeability, low alkalinity, and low heat consumption in the production process, with easy grinding of clinker. It is widely used in rush repair, anti-permeability and other projects. Cellulose ether (CE) is widely used in mortar modification because of its water-retaining and thickening properties. CSA cement hydration reaction is complex, the induction period is very short, the acceleration period is multi-stage, and its hydration is susceptible to the influence of admixture and curing temperature. Zhang et al. found that HEMC can prolong the induction period of hydration of CSA cement and make the main peak of hydration heat release lag. Sun Zhenping et al. found that HEMC’s water absorption effect affected the early hydration of cement slurry. Wu Kai et al. believed that the weak adsorption of HEMC on the surface of CSA cement was not enough to affect the heat release rate of cement hydration. The research results on the effect of HEMC on CSA cement hydration were not uniform, which may be caused by different components of cement clinker used. Wan et al. found that the water retention of HEMC was better than that of hydroxyethyl cellulose (HEC), and the dynamic viscosity and surface tension of the hole solution of HEMC-modified CSA cement slurry with high substitution degree were greater. Li Jian et al. monitored the early internal temperature changes of HEMC-modified CSA cement mortars under fixed fluidity and found that the influence of HEMC with different degrees of substitution was different.
However, the comparative study on the effects of CE with different substituents and degrees of substitution on the early hydration of CSA cement is not sufficient. In this paper, the effects of hydroxyethyl cellulose ether with different contents, substituent groups and degrees of substitution on the early hydration of CSA cement were studied. The hydration heat release law of 12h modified CSA cement with hydroxyethyl cellulose ether was emphatically analyzed, and the hydration products were quantitatively analyzed.

1. Test
1.1 Raw Materials
Cement is 42.5 grade fast hardening CSA cement, the initial and final setting time is 28 min and 50 min, respectively. Its chemical composition and mineral composition (mass fraction, the dosage and water-cement ratio mentioned in this paper are mass fraction or mass ratio) modifier CE includes 3 hydroxyethyl cellulose ethers with similar viscosity: Hydroxyethyl cellulose (HEC), high degree of substitution hydroxyethyl methyl cellulose (H HEMC), low degree of substitution hydroxyethyl methyl fibrin (L HEMC), the viscosity of 32, 37, 36 Pa·s, the degree of substitution of 2.5, 1.9, 1.6 mixing water for deionized water.
1.2 Mix ratio
Fixed water-cement ratio of 0.54, the content of L HEMC (the content of this article is calculated by the quality of water mud) wL=0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, HEC and H HEMC content of 0.5%. In this paper: L HEMC 0.1 wL=0.1% L HEMC change CSA cement, and so on; CSA is pure CSA cement; HEC modified CSA cement, L HEMC modified CSA cement, H HEMC modified CSA cement are respectively referred to as HCSA, LHCSA, HHCSA.
1.3 Test method
An eight-channel isothermal micrometer with a measuring range of 600 mW was used to test the heat of hydration. Before the test, the instrument was stabilized at (20±2) ℃ and relative humidity RH= (60±5) % for 6.0~8.0 h. CSA cement, CE and mixing water were mixed according to the mix ratio and electric mixing was performed for 1min at the speed of 600 r/min. Immediately weigh (10.0±0.1) g slurry into the ampoule, put the ampoule into the instrument and start the timing test. The hydration temperature was 20 ℃, and the data was recorded every 1min, and the test lasted until 12.0h.
Thermogravimetric (TG) analysis: Cement slurry was prepared according to ISO 9597-2008 Cement — Test methods — Determination of setting time and soundness. The mixed cement slurry was put into the test mold of 20 mm×20 mm×20 mm, and after artificial vibration for 10 times, it was placed under (20±2) ℃ and RH= (60±5) % for curing. The samples were taken out at the age of t=2.0, 4.0 and 12.0 h, respectively. After removing the surface layer of the sample (≥1 mm), it was broken into small pieces and soaked in isopropyl alcohol. Isopropyl alcohol was replaced every 1d for consecutive 7 days to ensure the complete suspension of hydration reaction, and dried at 40 ℃ to constant weight. Weigh (75±2) mg samples into the crucible, heat the samples from 30℃ to 1000℃ at the temperature rate of 20 ℃/min in the nitrogen atmosphere under adiabatic condition. The thermal decomposition of CSA cement hydration products mainly occurs at 50~550℃, and the content of chemically bound water can be obtained by calculating the mass loss rate of the samples within this range. AFt lost 20 crystalline waters and AH3 lost 3 crystalline waters during thermal decomposition at 50-180 ℃. The contents of each hydration product could be calculated according to TG curve.

2. Results and discussion
2.1 Analysis of hydration process
2.1.1 Influence of CE content on hydration process
According to the hydration and exothermic curves of different content L HEMC modified CSA cement slurry, there are 4 exothermic peaks on the hydration and exothermic curves of pure CSA cement slurry (wL=0%). The hydration process can be divided into dissolution stage (0~15.0min), transformation stage (15.0~45.0min) and acceleration stage (45.0min) ~54.0min), deceleration stage (54.0min~2.0h), dynamic equilibrium stage (2.0~4.0h), reacceleration stage (4.0~5.0h), redeceleration stage (5.0~10.0h) and stabilization stage (10.0h~). In 15.0min before hydration, the cement mineral dissolved rapidly, and the first and second hydration exothermic peaks in this stage and 15.0-45.0 min corresponded to the formation of metastable phase AFt and its transformation to monosulfide calcium aluminate hydrate (AFm), respectively. The third exothermal peak at 54.0min of hydration was used to divide the hydration acceleration and deceleration stages, and the generation rates of AFt and AH3 took this as the inflection point, from boom to decline, and then entered the dynamic equilibrium stage lasting 2.0 h. When the hydration was 4.0h, hydration again entered the stage of acceleration, C4A3 is a rapid dissolution and generation of hydration products, and at 5.0h, a peak of hydration exothermic heat appeared, and then entered the stage of deceleration again. Hydration stabilized after about 10.0h.
The influence of L HEMC content on CSA cement hydration dissolution and conversion stage is different: when L HEMC content is low, L HEMC modified CSA cement paste the second hydration heat release peak appeared slightly earlier, the heat release rate and heat release peak value is significantly higher than the pure CSA cement paste; With the increase of L HEMC content, the heat release rate of L HEMC modified CSA cement slurry gradually decreased, and lower than pure CSA cement slurry. The number of exothermic peaks in the hydration exothermic curve of L HEMC 0.1 is the same as that of pure CSA cement paste, but the 3rd and 4th hydration exothermic peaks are advanced to 42.0min and 2.3h, respectively, and compared with 33.5 and 9.0 mW/g of pure CSA cement paste, their exothermic peaks are increased to 36.9 and 10.5 mW/g, respectively. This indicates that 0.1% L HEMC accelerates and enhances hydration of L HEMC modified CSA cement at the corresponding stage. And L HEMC content is 0.2%~0.5%, L HEMC modified CSA cement acceleration and deceleration stage gradually combined, that is, the fourth exothermic peak in advance and combined with the third exothermic peak, the middle of the dynamic balance stage no longer appear, L HEMC on CSA cement hydration promotion effect is more significant.
L HEMC significantly promoted the hydration of CSA cement in 45.0 min~10.0 h. In 45.0min ~ 5.0h, 0.1%L HEMC has little effect on the hydration of CSA cement, but when the content of L HEMC increases to 0.2%~0.5%, the effect is not significant. This is completely different from the effect of CE on hydration of Portland cement. Literature studies have shown that CE containing a large number of hydroxyl groups in the molecule will be adsorbed on the surface of cement particles and hydration products due to acid-base interaction, thus delaying the early hydration of Portland cement, and the stronger the adsorption, the more obvious the delay. However, it was found in the literature that the adsorption capacity of CE on AFt surface was weaker than that on calcium silicate hydrate (C‑S‑H) gel, Ca (OH) 2 and calcium aluminate hydrate surface, while the adsorption capacity of HEMC on CSA cement particles was also weaker than that on Portland cement particles. In addition, the oxygen atom on the CE molecule can fix the free water in the form of hydrogen bond as adsorbed water, change the state of evaporable water in the cement slurry, and then affect the cement hydration. However, the weak adsorption and water absorption of CE will gradually weaken with the extension of hydration time. After a certain time, the adsorbed water will be released and further react with the unhydrated cement particles. Moreover, the enventing effect of CE can also provide long space for hydration products. This may be the reason why L HEMC promotes CSA cement hydration after 45.0 min hydration.
2.1.2 Influence of CE substituent and its degree on hydration process
It can be seen from the hydration heat release curves of three CE modified CSA slurries. Compared with L HEMC, the hydration heat release rate curves of HEC and H HEMC modified CSA slurries also have four hydration heat release peaks. All of the three CE have delayed effects on the dissolution and conversion stages of CSA cement hydration, and HEC and H HEMC have stronger delayed effects, delaying the emergence of the accelerated hydration stage. The addition of HEC and H‑HEMC slightly delayed the 3rd hydration exothermic peak, significantly advanced the 4th hydration exothermic peak, and increased the peak of the 4th hydration exothermic peak. In conclusion, the hydration heat release of the three CE modified CSA slurries is greater than that of the pure CSA slurries in the hydration period of 2.0~10.0 h, indicating that the three CE’s all promote the hydration of CSA cement at this stage. In the hydration period of 2.0~5.0 h, the hydration heat release of L HEMC modified CSA cement is the largest, and H HEMC and HEC are the second, indicating that the promotion effect of low substitution HEMC on the hydration of CSA cement is stronger. The catalytic effect of HEMC was stronger than that of HEC, indicating that the introduction of methyl group enhanced the catalytic effect of CE on the hydration of CSA cement. The chemical structure of CE has a great influence on its adsorption on the surface of cement particles, especially the degree of substitution and the type of substituent.
The steric hindrance of CE is different with different substituents. HEC has only hydroxyethyl in the side chain, which is smaller than HEMC containing methyl group. Therefore, HEC has the strongest adsorption effect on CSA cement particles and the greatest influence on the contact reaction between cement particles and water, so it has the most obvious delay effect on the third hydration exothermic peak. The water absorption of HEMC with high substitution is significantly stronger than that of HEMC with low substitution. As a result, the free water involved in hydration reaction between flocculated structures is reduced, which has a great influence on the initial hydration of modified CSA cement. Because of this, the third hydrothermal peak is delayed. Low substitution HEMCs have weak water absorption and short action time, resulting in early release of adsorbent water and further hydration of a large number of unhydrated cement particles. The weak adsorption and water absorption have different delayed effects on the hydration dissolution and transformation stage of CSA cement, resulting in the difference in the promotion of cement hydration in the later stage of CE.
2.2 Analysis of hydration products
2.2.1 Influence of CE content on hydration products
Change the TG DTG curve of CSA water slurry by different content of L HEMC; The contents of chemically bound water ww and hydration products AFt and AH3 wAFt and wAH3 were calculated according to TG curves. The calculated results showed that the DTG curves of pure CSA cement paste showed three peaks at 50~180 ℃, 230~300 ℃ and 642~975 ℃. Corresponding to AFt, AH3 and dolomite decomposition, respectively. At hydration 2.0 h, TG curves of L HEMC modified CSA slurry are different. When hydration reaction reaches 12.0 h, there is no significant difference in the curves. At 2.0h hydration, the chemical binding water content of wL=0%, 0.1%, 0.5% L HEMC modified CSA cement paste was 14.9%, 16.2%, 17.0%, and AFt content was 32.8%, 35.2%, 36.7%, respectively. The content of AH3 was 3.1%, 3.5% and 3.7%, respectively, indicating that the incorporation of L HEMC improved the hydration degree of cement slurry hydration for 2.0 h, and increased the production of hydration products AFt and AH3, that is, promoted the hydration of CSA cement. This may be because HEMC contains both hydrophobic group methyl and hydrophilic group hydroxyethyl, which has high surface activity and can significantly reduce the surface tension of liquid phase in cement slurry. At the same time, it has the effect of entraining air to facilitate the generation of cement hydration products. At 12.0 h of hydration, AFt and AH3 contents in L HEMC modified CSA cement slurry and pure CSA cement slurry had no significant difference.
2.2.2 Influence of CE substituents and their degrees of substitution on hydration products
The TG DTG curve of CSA cement slurry modified by three CE (the content of CE is 0.5%); The corresponding calculation results of ww, wAFt and wAH3 are as follows: at hydration 2.0 and 4.0 h, TG curves of different cement slurries are significantly different. When the hydration reaches 12.0 h, TG curves of different cement slurries have no significant difference. At 2.0 h hydration, the chemically bound water content of pure CSA cement slurry and HEC, L HEMC, H HEMC modified CSA cement slurry are 14.9%, 15.2%, 17.0%, 14.1%, respectively. At 4.0 h of hydration, the TG curve of pure CSA cement slurry decreased the least. The hydration degree of the three CE modified CSA slurries was greater than that of pure CSA slurries, and the content of chemically bound water of HEMC modified CSA slurries was greater than that of HEC modified CSA slurries. L HEMC modified CSA cement slurry chemical binding water content is the largest. In conclusion, CE with different substituents and degrees of substitution has significant differences on the initial hydration products of CSA cement, and L‑HEMC has the greatest promotion effect on the formation of hydration products. At 12.0 h hydration, there was no significant difference between the mass loss rate of the three CE modified CSA cement slurps and that of pure CSA cement slurps, which was consistent with the cumulative heat release results, indicating that CE only significantly affected the hydration of CSA cement within 12.0 h.
It can also be seen that AFt and AH3 characteristic peak strength of L HEMC modified CSA slurry are the largest at hydration 2.0 and 4.0 h. AFt content of pure CSA slurry and HEC, L HEMC, H HEMC modified CSA slurry were 32.8%, 33.3%, 36.7% and 31.0%, respectively, at 2.0h hydration. AH3 content was 3.1%, 3.0%, 3.6% and 2.7%, respectively. At 4.0 h of hydration, AFt content was 34.9%, 37.1%, 41.5% and 39.4%, and AH3 content was 3.3%, 3.5%, 4.1% and 3.6%, respectively. It can be seen that L HEMC has the strongest promoting effect on the formation of hydration products of CSA cement, and the promoting effect of HEMC is stronger than that of HEC. Compared with L‑HEMC, H‑HEMC improved the dynamic viscosity of pore solution more significantly, thus affecting the water transport, resulting in a decrease in slurry penetration rate, and affecting the hydration product production at this time. Compared with HEMCs, the hydrogen bonding effect in HEC molecules is more obvious, and the water absorption effect is stronger and longer lasting. At this time, the water absorption effect of both high-substitution HEMCs and low-substitution HEMCs is no longer obvious. In addition, CE forms a “closed loop” of water transport in the micro-zone inside the cement slurry, and the water released slowly by CE can further react directly with the surrounding cement particles. At 12.0 h of hydration, the effects of CE on AFt and AH3 production of CSA cement slurry were no longer significant.

3. Conclusion
(1) The hydration of sulfoaluminate (CSA) sludge in 45.0 min~10.0 h can be promoted with different dosage of low hydroxyethyl methyl fibrin (L HEMC).
(2) Hydroxyethyl cellulose (HEC), high substitution hydroxyethyl methyl cellulose (H HEMC), L HEMC HEMC, these three hydroxyethyl cellulose ether (CE) have delayed the dissolution and conversion stage of CSA cement hydration, and promoted the hydration of 2.0~10.0 h.
(3) The introduction of methyl in hydroxyethyl CE can significantly enhance its promotion effect on the hydration of CSA cement in 2.0~5.0 h, and the promotion effect of L HEMC on the hydration of CSA cement is stronger than H HEMC.
(4) When the content of CE is 0.5%, the amount of AFt and AH3 generated by L HEMC modified CSA slurry at hydration 2.0 and 4.0 h is the highest, and the effect of promoting hydration is the most significant; H HEMC and HEC modified CSA slurries produced higher AFt and AH3 content than pure CSA slurries only at 4.0 h of hydration. At 12.0 h of hydration, the effects of 3 CE on the hydration products of CSA cement were no longer significant.