Effect of cellulose ether on heat of hydration of different cement and single ore

the effects of cellulose ether on the hydration heat of Portland cement, sulfoaluminate cement, tricalcium silicate and tricalcium aluminate in 72h were compared by isothermal calorimetry test. The results show that cellulose ether can significantly reduce the hydration and heat release rate of Portland cement and tricalcium silicate, and the decrease effect on the hydration and heat release rate of tricalcium silicate is more significant. The effect of cellulose ether on reducing the heat release rate of the hydration of sulfoaluminate cement is very weak, but it has a weak effect on improving the heat release rate of the hydration of tricalcium aluminate. Cellulose ether will be adsorbed by some hydration products, thus delaying the crystallization of hydration products, and then affect the hydration heat release rate of cement and single ore.

Key words: cellulose ether; Cement; Single ore; Heat of hydration; adsorption

1. Introduction

Cellulose ether is an important thickening agent and water retaining agent in dry mixed mortar, self-compacting concrete and other new cement-based materials. However, cellulose ether will also delay cement hydration, which is conducive to improve the operational time of cement-based materials, improve mortar consistency and concrete slump time loss, but also may delay the construction progress. In particular, it will have adverse effects on mortar and concrete used in low temperature environment conditions. Therefore, it is very important to understand the law of cellulose ether on cement hydration kinetics.

OU and Pourchez systematically studied the effects of molecular parameters such as molecular weight of cellulose ether, type of substituent or degree of substitution on cement hydration kinetics, and drew many important conclusions: The ability of hydroxyethyl cellulose ether (HEC) to delay the hydration of cement is usually stronger than that of methyl cellulose ether (HPMC), hydroxymethyl ethyl cellulose ether (HEMC) and methyl cellulose ether (MC). In the cellulose ether containing methyl, the lower the methyl content, the stronger the ability to delay the hydration of cement; The lower the molecular weight of cellulose ether, the stronger the ability to delay the hydration of cement. These conclusions provide scientific basis for selecting cellulose ether correctly.

For different components of cement, the effect of cellulose ether on cement hydration kinetics is also a very concerned problem in engineering applications. However, there is no research on this aspect. In this paper, the influence of cellulose ether on the hydration kinetics of ordinary Portland cement, C3S(tricalcium silicate), C3A(tricalcium aluminate) and sulfoaluminate cement (SAC) was studied through isothermal calorimetry test, so as to further understand the interaction and internal mechanism between cellulose ether and cement hydration products. It provides further scientific basis for rational use of cellulose ether in cement-based materials and also provides research basis for the interaction between other admixtures and cement hydration products.

2. Test

2.1 Raw Materials

(1) ordinary Portland cement (P·0). Manufactured by Wuhan Huaxin Cement Co., LTD., the specification is P· 042.5 (GB 175-2007), determined by wavelength dispersion-type X-ray fluorescence spectrometer (AXIOS advanced, PANalytical Co., LTD.). According to the analysis of JADE 5.0 software, in addition to cement clinker minerals C3S, C2s, C3A, C4AF and gypsum, cement raw materials also include calcium carbonate.

(2) sulfoaluminate cement (SAC). The fast hard sulfoaluminate cement produced by Zhengzhou Wang Lou Cement Industry Co., Ltd. is R.Star 42.5 (GB 20472-2006). Its main groups are calcium sulfoaluminate and dicalcium silicate.

(3) tricalcium silicate (C3S). Press Ca(OH)2, SiO2, Co2O3 and H2O at 3:1:0.08: A mass ratio of 10 was mixed evenly and pressed under a constant pressure of 60MPa to make cylindrical green billet. The billet was calcined at 1400℃ for 1.5 ~ 2 h in a silicon-molybdenum rod high temperature electric furnace, and then moved into a microwave oven for further microwave heating for 40min. After taking out the billet, it was abruptly cooled and repeatedly broken and calcined until the content of free CaO in the finished product was less than 1.0%

(4) tricalcium aluminate (c3A). CaO and A12O3 were mixed evenly, calcined at 1450℃ for 4 h in a silicon-molybdenum rod electric furnace, ground into powder, and repeatedly calcined until the content of free CaO was less than 1.0%, and the peaks of C12A7 and CA were ignored.

(5) cellulose ether. The previous work compared the effects of 16 kinds of cellulose ethers on the hydration and heat release rate of ordinary Portland cement, and found that different kinds of cellulose ethers have significant differences on the hydration and heat release law of cement, and analyzed the internal mechanism of this significant difference. According to the results of previous study, three kinds of cellulose ether which have obvious retarding effect on ordinary Portland cement were selected. These include hydroxyethyl cellulose ether (HEC), hydroxypropyl methyl cellulose ether (HPMC), and hydroxyethyl methyl cellulose ether (HEMC). The viscosity of cellulose ether was measured by a rotary viscometer with a test concentration of 2%, a temperature of 20℃ and a rotation speed of 12 r/min. The viscosity of cellulose ether was measured by a rotary viscometer with a test concentration of 2%, a temperature of 20℃ and a rotation speed of 12 r/min. The molar substitution degree of cellulose ether is provided by the manufacturer.

(6) Water. Use secondary distilled water.

2.2 Test method

Heat of hydration. TAM Air 8-channel isothermal calorimeter produced by TA Instrument Company was adopted. All raw materials were kept constant temperature to test temperature (such as (20± 0.5)℃) before the experiment. Firstly, 3 g cement and 18 mg cellulose ether powder were added into the calorimeter (mass ratio of cellulose ether to cemellative material was 0.6%). After full mixing, mixed water (secondary distilled water) was added according to the specified water-cement ratio and stirred evenly. Then, it was quickly put into the calorimeter for testing. The water-binder ratio of c3A is 1.1, and the water-binder ratio of the other three cementitious materials is 0.45.

3. Results and discussion

3.1 Test results

The effects of HEC, HPMC and HEMC on the hydration heat release rate and cumulative heat release rate of ordinary Portland cement, C3S and C3A within 72 h, and the effects of HEC on the hydration heat release rate and cumulative heat release rate of sulfoaluminate cement within 72 h, HEC is the cellulose ether with the strongest delay effect on hydration of other cement and single ore. Combining the two effects, it can be found that with the change of cementitious material composition, cellulose ether has different effects on hydration heat release rate and cumulative heat release. The selected cellulose ether can significantly reduce the hydration and heat release rate of ordinary Portland cement and C, S, mainly prolongs the induction period time, delays the appearance of hydration and heat release peak, among which the cellulose ether to C, S hydration and heat release rate delay is more obvious than ordinary Portland cement hydration and heat release rate delay; Cellulose ether can also delay the heat release rate of sulfoaluminate cement hydration, but the delay ability is very weak, and mainly delay the hydration after 2 h; For the heat release rate of C3A hydration, cellulose ether has weak accelerating ability.

3.2 Analysis and discussion

The mechanism of cellulosic ether delay cement hydration. Silva et al. hypothesized that cellulosic ether increased the viscosity of pore solution and hindered the rate of ionic movement, thus delaying cement hydration. However, much literature has doubted this assumption, as their experiments have found that cellulose ethers with lower viscosity have a stronger ability to delay cement hydration. In fact, the time of ion movement or migration is so short that it is obviously not comparable to the time of cement hydration delay. The adsorption between cellulose ether and cement hydration products is considered to be the real reason for the delay of cement hydration by cellulose ether. Cellulose ether is easily adsorbed to the surface of hydration products such as calcium hydroxide, C-S-H gel and calcium aluminate hydrate, but it is not easy to be adsorbed by ettringite and unhydrated phase, and the adsorption capacity of cellulose ether on calcium hydroxide is higher than that of C-S-H gel. Therefore, for ordinary Portland cement hydration products, cellulose ether has the strongest delay on calcium hydroxide, the strongest delay on calcium, the second delay on C-S-H gel, and the weakest delay on ettringite.

Previous studies have shown that the adsorption between non-ionic polysaccharide and mineral phase mainly includes hydrogen bonding and chemical complexation, and these two effects occur between the hydroxyl group of polysaccharide and the metal hydroxide on the mineral surface. Liu et al. further classified the adsorption between polysaccharides and metal hydroxides as acid-base interaction, with polysaccharides as acids and metal hydroxides as bases. For a given polysaccharide, the alkalinity of the mineral surface determines the strength of the interaction between polysaccharides and minerals. Among the four gelling components studied in this paper, the main metal or non-metal elements include Ca, Al and Si. According to the order of metal activity, the alkalinity of their hydroxides is Ca(OH)2>Al(OH3>Si(OH)4. In fact, Si(OH)4 solution is acidic and does not adsorb cellulose ether. Therefore, the content of Ca(OH)2 on the surface of cement hydration products determines the adsorption capacity of hydration products and cellulose ether. Because calcium hydroxide, C-S-H gel (3CaO·2SiO2·3H20), ettringite (3CaO·Al2O3·3CaSO4·32H2O) and calcium aluminate hydrate (3CaO·Al2O3·6H2O) in the content of inorganic oxides of CaO is 100%, 58.33%, 49.56% and 62 .2%. Therefore, the order of their adsorption capacity with cellulose ether is calcium hydroxide > calcium aluminate >C-S-H gel > ettringite, which is consistent with the results in the literature.

The hydration products of c3S mainly include Ca(OH) and c-s-H gel, and cellulose ether has a good delay effect on them. Therefore, cellulose ether has a very obvious delay on C3s hydration. Besides c3S, ordinary Portland cement also includes C2s hydration which is slower, which makes the delay effect of cellulose ether not obvious in the early stage. The hydration products of ordinary silicate also include ettringite, and the delay effect of cellulose ether is poor. Therefore, the delay ability of cellulose ether to c3s is stronger than that of ordinary Portland cement observed in the test.

C3A will dissolve and hydrate quickly when it meets water, and the hydration products are usually C2AH8 and c4AH13, and the heat of hydration will be released. When the solution of C2AH8 and c4AH13 reaches saturation, the crystallization of C2AH8 and C4AH13 hexagonal sheet hydrate will be formed, and the reaction rate and heat of hydration will be reduced at the same time. Due to the adsorption of cellulose ether to the surface of calcium aluminate hydrate (CxAHy), the presence of cellulose ether would delay the crystallization of C2AH8 and C4AH13 hexagonal-plate hydrate, resulting in the decrease of reaction rate and hydration heat release rate than that of pure C3A, which shows that cellulose ether has a weak acceleration ability to C3A hydration. It is worth noting that in this test, cellulose ether has a weak accelerating ability to the hydration of pure c3A. However, in ordinary Portland cement, because c3A will react with gypsum to form ettringite, due to the influence of ca2+ balance in slurry solution, cellulose ether will delay the formation of ettringite, thus delaying the hydration of c3A.

From the effects of HEC, HPMC and HEMC on the hydration and heat release rate and cumulative heat release of ordinary Portland cement, C3S and C3A within 72 h, and the effects of HEC on the hydration and heat release rate and cumulative heat release of sulfoaluminate cement within 72 h, it can be seen that among the three cellulose ethers selected, The ability of delayed hydration of c3s and Portland cement was strongest in HEC, followed by HEMC, and weakest in HPMC. As far as C3A is concerned, the three cellulose ethers’ ability to accelerate hydration is also in the same order, that is, HEC is the strongest, HEMC is the second, HPMC is the weakest and strongest. This mutually confirmed that cellulose ether has delayed the formation of hydration products of gelling materials.

The main hydration products of sulfoaluminate cement are ettringite and Al(OH)3 gel. The C2S in sulfoaluminate cement will also hydrate separately to form Ca(OH)2 and c-S-H gel. Because the adsorption of cellulose ether and ettringite can be ignored, and the hydration of sulfoaluminate is too fast, therefore, at the early stage of hydration, cellulose ether has little effect on the hydration heat release rate of sulfoaluminate cement. But to a certain time of hydration, because c2s will separately hydrate to generate Ca(OH)2 and C-S-H gel, these two hydration products will be delayed by cellulose ether. Therefore, it was observed that cellulose ether delayed the hydration of sulfoaluminate cement after 2 h.

4. Conclusion

In this paper, through isothermal calorimetry test, the influence law and formation mechanism of cellulose ether on hydration heat of ordinary Portland cement, c3s, c3A, sulfoaluminate cement and other different components and single ore in 72 h were compared. The main conclusions are as follows:

(1) Cellulose ether can significantly reduce the hydration heat release rate of ordinary Portland cement and tricalcium silicate, and the effect of reducing the hydration heat release rate of tricalcium silicate is more significant; The effect of cellulose ether on reducing the heat release rate of sulfoaluminate cement is very weak, but it has a weak effect on improving the heat release rate of tricalcium aluminate.

(2) cellulose ether will be adsorbed by some hydration products, thus delaying the crystallization of hydration products, affecting the heat release rate of cement hydration. The type and quantity of hydration products are different for different components of cement bill ore, so the effect of cellulose ether on their hydration heat is not the same.