Elsevier

Thermochimica Acta

Volume 696, February 2021, 178821
Thermochimica Acta

The application of thermal analysis to study the hydration behavior of tricalcium aluminate-gypsum in the presence of polycarboxylate-based superplasticizers

https://doi.org/10.1016/j.tca.2020.178821Get rights and content

Highlights

  • The hydration of C3A and formation of monosulphate were evidently delayed by PCE.

  • The precipitate rate of C3AH6 in the presence of PCE was accelerated.

  • The morphology and size of ettringite crystals are changed by adsorption of PCE.

  • PCE play an important role on composition and microstructure of cement hydrates.

Abstract

The effect of polycarboxylate-based superplasticizers (PCE) on hydration behavior of tricalcium aluminate-gypsum system, which is a partial system of ordinary Portland cement, is investigated by means of isothermal calorimeter, thermal analysis and X-ray diffraction. The adsorption characteristics of PCE and morphology change of ettringite crystals upon PCE introduction are also examined by gel permeation chromatography and scanning electron microscope, respectively. Our results show that the initial hydration process and formation of crystalline monosulphate are significantly slowed down due to the presence of PCE. It is concluded that this retardation phenomenon is due to a stabilization effect induced by the adsorption of the PCE on ettringite precursor. Moreover, the morphology and size of ettringite crystals are found to be changed by the adsorption of PCE.

Introduction

The newly developed polycarboxylate-based superplasticizers (PCE) are very efficient in the formulation of modern concrete [1]. The PCE are comb-like copolymers which consist of a negatively charged backbone with carboxylic groups and grafted side chains which are mainly composed of polyethylene oxide units. Their dispersing effect is due to the adsorption of polymers on particle surfaces and then induce electrostatic and/or steric repulsive forces [2]. Through this unique interactions, PCE exhibit superior dispersing force compared to polycondensates [3]. As a side effect, PCE may also lead a undesired retardation phenomena on the setting of the cement paste [4].

Tricalcium aluminate (Ca3Al2O6, C3A) is one of the most reactive components of Portland clinker. The hydration of this mineral presenting in Portland cement leads to the formation of ettringite and monosulphate phase, which plays a vital role in the early hydration and setting behavior of cement paste [5]. Generally, many compatibility problems occurred between PCE and cement, such as early slump loss, poor flow behavior, higher PCE dosage or severe segregation [6], could be attributed to the hydration behavior of C3A. A number of studies on PCE and C3A in cement paste are available. Some studies indicated that the addition of PCE in cement paste would slow down the dissolution rate of the constituents [7], meanwhile the formation of ettringite is retarded [8]. On the contrary, some researchers suggested that early growth rate of ettringite was accelerated in cementitious system. For example, Roncero et al. [9] noted that at the age of 15 min, no ettringite was detected by the XRD analysis of the reference paste whereas it was observed in the pastes with superplasticizer, especially in the PCE paste. Yu et al. [10] studied the impact of PCE on the hydration of C3A-gypsum suspension, and found that the addition of PCE accelerated the depletion of sulfate as well as the C3A dissolution, and led to the precipitation of monosulphate phase. So far, there are still controversies on the effects of PCE on the hydration of clinkers [11].

The aim of the present work is to investigate the detailed influence of PCE on the hydration of the C3A-gypsum system, which is a partial system of ordinary Portland cement, and illustrate the mechanism for these effects. For this purpose, pure C3A was synthesized and the interactions between C3A and PCE were carefully studied. Isothermal calorimeter, complex thermal analysis (TG-DSC) and X-ray diffraction (XRD) are applied to investigate the hydration processes of C3A in the presence of PCE. Gel permeation chromatography (GPC) and scanning electron microscopy (SEM) have been performed to clarify possible morphology changes of the ettringite phase upon PCE addition.

Section snippets

Materials

The pure C3A phase was prepared by a high-temperature calcination of analytically pure CaCO3 and Al2O3 with molar ratio 3:1 at 1350 ℃ for 3 h. Prior to the second calcination, the prepared specimens were crushed and ground in a laboratory mill. The obtained product was re-calcined until X-ray diffraction indicates that no other phases were present. The XRD pattern of synthesized C3A samples is shown in Fig. 1. The obtained C3A powder had a nitrogen BET surface area of 220 ± 10 m2/g.

The PCE used

Isothermal calorimetry of C3A-gypsum

The hydration process of C3A-gypsum paste was monitored by isothermal calorimeter. Fig. 3 shows the comparison of the calorimetric curves of paste samples containing 0, 0.1 and 0.2 wt.% PCE hydrated for 30 h. When anhydrous C3A-gypsum began to contact with water, there was a rapid rate of heat development lasting for a few minutes. Subsequent, the hydration rate increased to a maximum vale and then declined. The initial heat peak could be attributed to rapid dissolution and formation of

Discussion

The addition of PCE to C3A-gypsum system influences the hydration rate and amount of formed hydration products. Additionally, the morphology and size of hydrate crystals were changed.

Conclusions

The initial hydration of C3A-gypsum system and formation of crystalline monosulphate were evidently slowed down by the introduction of PCE. It was found that the retardation is due to adsorption of the PCE on surface of anhydrous mineral as well as hydrated compounds. The PCE adsorbed on the ettringite precursor and stabilized it, thus extending the period of ettringite nucleation, and delaying the formation of monosulphate. Moreover, the precipitate rate of C3AH6 phase in the presence of PCE

CRediT authorship contribution statement

Ming Liu: Methodology, Investigation, Software, Writing - original draft. Yuxin Gao: Conceptualization, Project administration. Lei Zhang: Methodology, Writing - review & editing. Guangming Jiang: Conceptualization, Writing - review & editing. Chao Zeng: Methodology. Peng Wang: Software.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors wish to thank the financial support of China West Construction Group Co., Ltd., science and technology research and development foundation (ZJXJ-2019-19). The authors would like to acknowledge the great help from Xiaoqin Liu and Xiaorun Chen.

References (23)

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