Cellulose ether on the morphology of early ettringite

The effects of hydroxyethyl methyl cellulose ether and methyl cellulose ether on the morphology of ettringite in early cement slurry were studied by scanning electron microscopy (SEM). The results show that the length-diameter ratio of ettringite crystals in hydroxyethyl methyl cellulose ether modified slurry is smaller than that in ordinary slurry, and the morphology of ettringite crystals is short rod-like. The length-diameter ratio of ettringite crystals in methyl cellulose ether modified slurry is larger than that in ordinary slurry, and the morphology of ettringite crystals is needle-rod. The ettringite crystals in ordinary cement slurries have an aspect ratio somewhere in between. Through the above experimental study, it is further clear that the difference of molecular weight of two kinds of cellulose ether is the most important factor affecting the morphology of ettringite.

Key words: ettringite; Length-diameter ratio; Methyl cellulose ether; Hydroxyethyl methyl cellulose ether; morphology

Ettringite, as a slightly expanded hydration product, has a significant effect on the performance of cement concrete, and has always been the research hotspot of cement-based materials. Ettringite is a kind of trisulfide type calcium aluminate hydrate, its chemical formula is [Ca3Al (OH)6·12H2O]2·(SO4)3·2H2O, or can be written as 3CaO·Al2O3·3CaSO4·32H2O, often abbreviated as AFt. In Portland cement system, ettringite is mainly formed by the reaction of gypsum with aluminate or ferric aluminate minerals, which plays the role of delaying hydration and early strength of cement. The formation and morphology of ettringite are affected by many factors such as temperature, pH value and ion concentration. As early as 1976, Metha et al. used scanning electron microscopy to study the morphological characteristics of AFt, and found that the morphology of such slightly expanded hydration products was slightly different when the growth space was large enough and when the space was limited. The former was mostly slender needle-rod-shaped spherules, while the latter was mostly short rod-shaped prism. Yang Wenyan’s research found that AFt forms were different with different curing environments. Wet environments would delay AFt generation in expansion-doped concrete and increase the possibility of concrete swelling and cracking. Different environments affect not only the formation and microstructure of AFt, but also its volume stability. Chen Huxing et al. found that the long-term stability of AFt decreased with the increase of C3A content. Clark and Monteiro et al. found that with the increase of environmental pressure, AFt crystal structure changed from order to disorder. Balonis and Glasser reviewed the density changes of AFm and AFt. Renaudin et al. studied the structural changes of AFt before and after immersion in solution and the structural parameters of AFt in Raman spectrum. Kunther et al. studied the effect of the interaction between C-S-H gel calcium-silicon ratio and sulfate ion on AFt crystallization pressure by NMR. At the same time, based on the application of AFt in cement-based materials, Wenk et al. studied AFt crystal orientation of concrete section through hard synchrotron radiation X-ray diffraction finishing technology. The formation of AFt in mixed cement and the research hotspot of ettringite were explored. Based on delayed ettringite reaction, some scholars have conducted a lot of research on the cause of AFt phase.

The volume expansion caused by the formation of ettringite is sometimes favorable, and it can act as an “expansion” similar to magnesium oxide expansion agent to maintain the volume stability of cement-based materials. The addition of polymer emulsion and redispersible emulsion powder changes the macroscopic properties of cement-based materials due to their significant effects on the microstructure of cement-based materials. However, unlike the redispersible emulsion powder which mainly enhances the bonding property of hardened mortar, the water-soluble polymer cellulose ether (CE) gives the newly mixed mortar good water retention and thickening effect, thus improving the working performance. Non-ionic CE is commonly used, including methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), etc., and CE plays a role in newly mixed mortar but also affects the hydration process of cement slurry. Studies have shown that HEMC changes the amount of AFt produced as a hydration product. However, no studies have systematically compared the effect of CE on the microscopic morphology of AFt, so this paper explores the difference of the effect of HEMC and MC on the microscopic morphology of ettringham in early (1-day) cement slurry through image analysis and comparison.

1. Experiment

1.1 Raw Materials

P·II 52.5R Portland cement produced by Anhui Conch Cement Co., LTD was selected as the cement in the experiment. The two cellulose ethers are hydroxyethyl methylcellulose (HEMC) and methylcellulose (methylcellulose, Shanghai Sinopath Group) respectively. MC); The mixing water is tap water.

1.2 Experimental methods

The water-cement ratio of the cement paste sample was 0.4 (the mass ratio of water to cement), and the content of cellulose ether was 1% of the mass of cement. The preparation of the specimen was carried out according to GB1346-2011 “Testing Method for Water Consumption, Setting Time and Stability of Cement Standard Consistency”. After forming the specimen, plastic film was encapsulated on the surface of the mould to prevent surface water evaporation and carbonization, and the specimen was placed in a curing room with a temperature of (20±2)℃ and relative humidity of (60±5) %. After 1 day, the mold was removed, and the specimen was broken, then a small sample was taken from the middle and soaked in anhydrous ethanol to terminate hydration, and the sample was taken out and dried before testing. The dried samples were glued to the sample table with conductive double-sided adhesive, and a layer of gold film was sprayed on the surface by Cressington 108auto automatic ion sputtering instrument. The sputtering current was 20 mA and the sputtering time was 60 s. FEI QUANTAFEG 650 environmental Scanning electron Microscope (ESEM) was used to observe the morphological characteristics of AFt on the sample section. The high vacuum secondary electron mode was used to observe the AFT. The acceleration voltage was 15 kV, the beam spot diameter was 3.0 nm, and the working distance was controlled at about 10 mm.

2. Results and discussion

SEM images of ettringite in hardened HEMC-modified cement slurry showed that the orientation growth of layered Ca (OH)2(CH) was obvious, and AFt showed irregular accumulation of short rod-like AFt, and some short rod-like AFT was covered with HEMC membrane structure. Zhang Dongfang et al. also found short rod-like AFt when observing the microstructure changes of HEMC modified cement slurry through ESEM. They believed that ordinary cement slurry reacted quickly after encountering water, so AFt crystal was slender, and the extension of hydration age led to the continuous increase of length-diameter ratio. However, HEMC increased the viscosity of the solution, reduced the binding rate of ions in the solution and delayed the arrival of water on the surface of clinker particles, so the length-diameter ratio of AFt increased in a weak trend and its morphological characteristics showed short rod-like shape. Compared with AFt in ordinary cement slurry of the same age, this theory has been partially verified, but it is not applicable to explain the morphological changes of AFt in MC modified cement slurry. SEM images of ettridite in 1-day hardened MC modified cement slurry also showed orientated growth of layered Ca(OH)2, some AFt surfaces were also covered with film structure of MC, and AFt showed morphological characteristics of cluster growth. However, by comparison, AFt crystal in MC modified cement slurry has a larger length-diameter ratio and a more slender morphology, showing a typical acicular morphology.

Both HEMC and MC delayed the early hydration process of cement and increased the viscosity of the solution, but the differences in AFt morphological characteristics caused by them were still significant. The above phenomena can be further elaborated from the perspective of molecular structure of cellulose ether and AFt crystal structure. Renaudin et al. soaked the synthesized AFt in the prepared alkali solution to get “wet AFt”, and partially removed it and dried it on the surface of saturated CaCl2 solution (35% relative humidity) to get “dry AFt”. After the structure refinement study by Raman spectroscopy and X-ray powder diffraction, it was found that there was no difference between the two structures, only the direction of crystal formation of cells changed in the drying process, that is, in the process of environmental change from “wet” to “dry”, AFt crystals formed cells along the normal direction of a gradually increased. The AFt crystals along the c normal direction became less and less. The most basic unit of three-dimensional space is composed of a normal line, b normal line and c normal line which are perpendicular to each other. In the case that b normals were fixed, AFt crystals clustered along a normals, resulting in an enlarged cell cross section in the plane of ab normals. Thus, if the HEMC “stores” more water than the MC, a “dry” environment can occur in a localized area, encouraging lateral aggregation and growth of AFt crystals. Patural et al. found that for CE itself, the higher the degree of polymerization (or the larger the molecular weight), the greater the viscosity of CE and the better the water retention performance. The molecular structure of HEMCs and MCS supports this hypothesis, with the hydroxyethyl group having a much larger molecular weight than the hydrogen group.

Generally, AFt crystals will form and precipitate only when relevant ions reach a certain saturation in the solution system. Therefore, factors such as ion concentration, temperature, pH value and formation space in the reaction solution can significantly affect the morphology of AFt crystals, and changes in artificial synthesis conditions can change the morphology of AFt crystals. Therefore, the ratio of AFt crystals in ordinary cement slurry between the two may be caused by the single factor of water consumption in the early hydration of cement. However, the difference in AFt crystal morphology caused by HEMC and MC should be mainly due to their special water retention mechanism. Hemcs and MCS create a “closed loop” of water transport within the microzone of the fresh cement slurry, allowing for a “short period” in which water is “easy to get in and difficult to get out.” However, during this period, the liquid phase environment in and near the microzone is also changed. Factors such as ion concentration, pH, etc., The change of growth environment is further reflected in the morphological characteristics of AFt crystals. This “closed loop” of water transport is similar to the mechanism of action described by Pourchez et al. HPMC playing a role in water retention.

3. Conclusion

(1) The addition of hydroxyethyl methyl cellulose ether (HEMC) and methyl cellulose ether (MC) can significantly change the morphology of ettringite in early (1 day) ordinary cement slurry.

(2) The length and diameter of ettringite crystal in HEMC modified cement slurry are small and short rod shape; The length and diameter ratio of ettringite crystals in MC modified cement slurry is large, which is needle-rod shape. The ettringite crystals in ordinary cement slurries have an aspect ratio between these two.

(3) The different effects of two cellulose ethers on the morphology of ettringite are essentially due to the difference in molecular weight.