Nanostructural evolution of Al(OH)₃ gel formed by the cubic and orthorhombic ye'elimite clinkers of calcium sulfoaluminate cements in an ultra-wide hydration temperature range

Abstract

This paper investigated the influence of hydration temperature on the nanostructure of the AH₃ phase formed in calcium sulfoaluminate cements. Orthorhombic ye'elimite (st-ye'elimite) and cubic ye'elimite (ss-ye'elimite) were hydrated in an ultra-wide temperature range (5–220 °C). Results showed that the AH₃ phase formed by ss-ye'elimite was always microcrystalline in nature at various curing temperatures (5–130 °C), and transformed into the AlOOH phase at 150 °C and above. However, the AH₃ phase formed by st-ye'elimite grew from a microcrystal to a favorable crystal as the temperature increased, and transformed into the AlOOH phase at 170 °C and above. The nanostructure of hydration products was further investigated using TEM/SAED analysis together with FE-SEM images, directly evidencing that the formed AFm-12 phase was single-crystal in nature, and the formed AH₃ phase was microcrystalline at low temperatures and grew into single-crystal hexagonal prisms with the preferred growth direction along [001] direction at high temperatures.

Introduction

Cement production is one of the primary sources of global industrial emissions, contributing to ~26% of the industrial CO₂ emission [1]. The manufacture of Portland cement (PC), as the major constituent of the cement manufacturing, generates a CO₂ emission of 0.8–0.9 ton CO₂/ton PC worldwide [2]. Calcium sulfoaluminate (CSA) cements have been followed with wide interest worldwide as a potential low-CO₂ and economical alternative to PC due to their characteristic manufacture, e.g., (i) low requirement of limestone, (ii) low sinter temperature, and (iii) low energy consumed by grinding [[3], [4], [5], [6]]. Meanwhile, CSA cements have many excellent attributes according to the clinker composition and the type and amount of soluble sulfate, e.g., rapid setting and hardening, high-early strength, shrinkage compensating, self-leveling, low alkali, high sulfate and chloride corrosion resistance, etc. [[7], [8], [9], [10]]. Thus, various CSA-based materials are produced and widely applied in civil or hydraulic engineering, such as leakage and seepage prevention projects, repairing and reinforcing projects, prefabricated concrete, construction in offshore and coastal engineering, etc. [11,12].

The positive properties of CSA cements are closely associated with their clinker composition. Ye'elimite (calcium sulfoaluminate, Klein's salt or C₄A₃S̅) is the main mineral and account for over 50 wt% in CSA cements [13,14]. Cement chemical abbreviations are used here: C = CaO, A = Al₂O₃, S̅ = SO₃, F = Fe₂O₃ and H = H₂O. It is generally agreed that different hydration reactions proceed when C₄A₃S̅ is hydrated with/without calcium sulfate (anhydrite, bassanite, or gypsum) supply, forming monosulfate (AFm-12), ettringite (AFt) and aluminum hydroxide (AH₃), as shown in Eqs. (1), (2), (3), (4) [12,15,16]. Furthermore, the composition of products was influenced by the molar ratio of calcium sulfate to ye'elimite (i.e., the M value), and AFt increases at the cost of AFm with increasing the M value (M ≤ 2.0) [17].

$${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}+18\mathrm{H}\to {\mathrm{C}}_4\mathrm{A}\overline{\mathrm{S}}{\mathrm{H}}_{12}+2{\mathrm{AH}}_3$$

$${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}+2\mathrm{C}\overline{\mathrm{S}}+38\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{\mathrm{S}}}_3{\mathrm{H}}_{32}+2{\mathrm{AH}}_3$$

$${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}+2\mathrm{C}\overline{\mathrm{S}}{\mathrm{H}}_{0.5}+37\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{\mathrm{S}}}_3{\mathrm{H}}_{32}+2{\mathrm{AH}}_3$$

$${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}+2\mathrm{C}\overline{\mathrm{S}}{\mathrm{H}}_2+34\mathrm{H}\to {\mathrm{C}}_6\mathrm{A}{\overline{\mathrm{S}}}_3{\mathrm{H}}_{32}+2{\mathrm{AH}}_3$$

AH₃ phase, as a main gel-like hydration product, plays an important role in the density and porosity, and contributes significantly to the mechanical properties of CSA cements owing to its high indentation modulus and hardness [4,18]. Meanwhile, it is generally accepted that the final AH₃ phase had a microstructure structure in calcium aluminate (CAC) cements (the hydration of CA [19,20]) or CSA cements (the hydration of C₄A₃S̅ [21,22]), taking into consideration (i) the broad X-ray diffraction peaks; (ii) the same Al-O coordination, morphology and dehydration temperature compared with the crystalline AH₃; (iv) the same solubility product compared with the microcrystalline AH₃; (iv) the small particle size; and (v) the crystallite size calculated by Pair Distribution Function analyses. Previous studies also found that amorphous AH₃ phase may be formed and have more bound water at the beginning (before 4 h), which is released to provide an opportunity for further precipitation of ettringite during 6 and 8 h [10]. The amorphous AH₃ phase at an early age would transform into microcrystalline AH₃ as hydration prolongs [23]. In addition, some other elements (e.g., Ca, Li, Fe or Sr) may be found in the AH₃ nanostructure [20,[24], [25], [26], [27]]. All the existing studies evidence that a number of factors have an influence on the microstructure and type of Al(OH)₃, and pH and temperature are seen as the two most important factors [22,[28], [29], [30], [31]]. On the one hand, with increasing pH, the AH₃ phase synthesized by sol-gel method transforms from amorphous phase through AlOOH to Al(OH)₃ [22,30,31], and that formed during the hydration of C₄A₃S̅ would grow from a microcrystal to a favorable crystal [32]. On the other hand, the AH₃ phase shows a various nanostructural evolutions in the chemical engineering as the temperature increases [28,29,33]. However, there have been no studies to date that deeply focused on the nanostructural evolution of the AH₃ phase by C₄A₃S̅ hydrated at various temperatures.

Previous studies have revealed that curing temperature significantly influences the hydration degree, final hydration products, microstructure, and macro performance of PC, CAC, CSA cements, or different cement mixtures [[34], [35], [36], [37], [38], [39], [40]]. With respect to CSA cements, Berger et al. [34] reported that AFt formed in CSA cements is different from that in PC, and decomposes depending on the M value, e.g., AFt has started to decompose at 70 °C when M = 0.6 or 1.0, but it does not occur when M = 1.4 or 3.9. But Zhang and Glasser [37] found that AFt and AH₃ phase can persist in CSA cements at 85 °C and below. Wang et al. [40] also found that AFt is present during the hydration of CSA cements at 80 °C, but disappears during the hydration of CSA clinker at the same temperature. Furthermore, the composition of hydration products remains the same but the amount of hydration products and the hydration process varied with increasing curing temperature from 0 °C to 80 °C. However, there have been no studies that focus on the hydration process of CSA cements in the high-temperature and high-pressure environments (i.e., above 100 °C and 1 atm pressure). Furthermore, the nanostructural evolution of the AH₃ phase in CSA-cement based materials at different temperatures was rarely investigated.

Meanwhile, CSA cements consist of two ye'elimite with different structural modifications (orthorhombic ye'elimite (st-ye'elimite) and cubic ye'elimite (ss-ye'elimite)) [10,13,41,42]. Previous studies reported that st-ye'eimite has an orthorhombic structure at room temperature and a cubic structure at high temperatures, and the cubic ye'elimite increases at the cost of the orthorhombic ye'elimite as the structure of ye'elimite was doped with Fe³⁺, Na⁺, and Si⁴⁺ [[41], [42], [43]]. In addition, ss-ye'elimite shows a different hydration reaction and forms higher amounts of AFt compared with st-ye'elimite [13,27]. Thus, it is also worth investigating the hydration and formed AH₃ phase of these two ye'elimite at different curing temperatures, which would contribute to comprehensively evaluating the microstructure of CSA cements.

Here, an ultra-wide curing temperature range (5–220 °C) was prepared, two ye'elimite clinkers (ss-ye'elimite and st-ye'elimite) were synthesized and hydrated, and then the final nanostructure of AH₃ phase was investigated. The nanostructure and crystal growth of AH₃ phase were studied in further detail using a number of techniques. TEM/SAED analysis directly evidenced the microcrystalline structure of the AH₃ phase at low temperatures and single-crystal structure of the AH₃ phase at high temperatures.