Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate

Abstract

The tricalcium aluminate (C₃A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C₃A and Na-doped orthorhombic (orth-)C₃A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C₃A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C₃A and the subsequent precipitation of ettringite, which is contrary to the cub-C₃A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C₃A hydration and corroborate the reported ex situ findings of these systems.

1. Introduction

Tricalcium aluminate (Ca₃Al₂O₆ or also known in cement chemistry notation as C₃A)—the most reactive phase in Portland cement (PC)—begins reacting essentially instantaneously once in contact with water to produce a hydroxy-AFm-type meta-stable product, which is subsequently converted to katoite (Ca₃Al₂(OH)₁₂ or C₃AH₆) and heat. Flash setting can occur if this reaction proceeds unhindered [1,2,3], which is undesirable because it reduces the workability and final strength of the cement paste and, consequently, the mortar and concrete. Other calcium aluminate hydrates (4CaO·Al₂O₃·nH₂O or hydroxyl-AFm), e.g., C₂AH_7.5 and C₄AH_x, are also produced from the hydration of PC clinker in the absence of added calcium sulfate. Calcium sulfate (typically ~5 wt %) is normally milled with PC clinker to retard the C₃A hydration rate [4], which leads to longer setting times [5,6,7] and a longer time window in which acceptable workability is achieved. The most common type of calcium sulfate added to PC clinker is gypsum, but anhydrite and hemihydrate can also be used.

Previous research regarding the C₃A-CaSO₄·H₂O system showed that the C₃A hydration rate is related to its crystal structure and specific surface area, the temperature, water/solid ratio [8], type and amount of CaSO₄ (gypsum, hemihydrate, anhydrite) [6,9,10,11,12,13,14], the presence of other mineral or chemical admixtures [15,16,17,18,19], and the solution chemistry [20,21].

Minor chemical constituents such as MgO, K₂O, Na₂O, or SO₃ that are present in the raw materials or fuels used in clinker production can significantly modify the types and amounts of the clinker phases produced [1]. Na₂O is typically incorporated into the C₃A phase, which modifies its crystal structure from a cubic (cub) to an orthorhombic (orth) type through a Ca for Na substitution [22,23,24,25,26,27]. Although the hydration of cub-C₃A has been extensively studied over the past several decades [2,3,5,6,8,10,12], research regarding the hydration of orth-C₃A is much scarcer [28,29,30,31,32,33] with additional work needed to more reliably understand the chemistry of this latter system. It is, however, known that C₃A hydration is significantly influenced by the presence of Na in its C₃A structure and thus its crystal polymorph type [29]. Recently, it was reported that the dissolution of Al₆O₁₈¹⁸⁻ ring structures is strongly dependent on the C₃A crystal structure. The Ca/Al_[aq] ratio measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) during the first few hours of the reaction showed a lower solubility of cub-C₃A within a calcium sulfate solution when compared to the orth-C₃A-based systems. This relatively low solubility plays an important role in the formation of an Al-rich layer at the partially dissolved cub-C₃A solution interphase and therefore affects the dissolution rate [34].

To consolidate the understanding of the effects that gypsum has on the hydration of cub- and orth-C₃A, this paper presents an integrated in situ assessment of the chemistry of the (cub- and orth-) C₃A-CaSO₄-H₂O system using isothermal conduction calorimetry (IC) and in situ X-ray diffraction (XRD) over a range of CaSO₄ concentrations relevant to hydrated PC.

2. Materials and Methods

Powder samples of cub- and orth-C₃A were obtained from CTL, Inc., Skokie, IL, USA. Both compounds were synthesized in a laboratory by heating a stoichiometric blend of reagent grade CaCO₃ and alumina (Al₂O₃). Orth-C₃A was prepared from reagent grade CaCO₃, Al₂O₃, and Na₂CO₃ in stoichiometric proportions, similar to the process reported by Regourd et al. [35]. The blends were fired at 1400 °C for 1 h and then quenched using forced air convection. The powders were then produced by ball milling to obtain particle size distributions with d₉₀ < 30 $\mathsf{\mu }$m and mean particle sizes of 11.0 $\mathsf{\mu }$m and 14.0 $\mathsf{\mu }$m for orth- and cub-C₃A, respectively. Pure gypsum (CaSO₄·2H₂O) was used in this study (Fisher Scientific, Hampton, NH, USA), which had a particle size distribution with d₉₀ < 39 $\mathsf{\mu }$m and a mean particle size of 19.0 $\mathsf{\mu }$m.

The crystal structures of the samples were verified by XRD using a PANalytical X’Pert Pro (PANalytical, Almelo, The Netherlands) (Cu Kα radiation with a step size of 0.02° 2θ). The major diffraction signals of orthorhombic C₃A International Crystal Structure Database (ICSD) code # 1880 at d-spacings of 2.692 Å and 2.714 Å were clearly identified in the diffractogram of the orth-C₃A powder, as is shown in Figure 1. The single major reflection of cubic C₃A (ICSD# 1841) at 2.6987 Å was observed in the diffractogram of the cubic-C₃A powder. Rietveld analyses using HighScore Plus™ software (Version 4.6, PANalytical, Almelo, The Netherlands) (estimated uncertainty = ±5 wt %) showed that the cub-C₃A powder contained ≥99 wt % cubic C₃A (ICSD# 1841) and ~1 wt % C₃AH₆ (ICSD# 202316), whereas the orth-C₃A powder contained ≥92 wt % orthorhombic C₃A (ICSD# 1880), ~7 wt % C₃AH₆ (ICSD# 202316) and ~1 wt % monohydrocalcite (ICSD# 100846, CaCO₃$·$2H₂O).

Samples were produced from mixtures of gypsum and cub- or orth-C₃A at respective mass ratios of 0, 0.20, 0.60, and 1.90. The gypsum/C₃A ratio of 1.90 corresponds to the stoichiometry of ettringite formation at a 100% reaction of C₃A. A water/solid (w/s) mass ratio of 1.2 was used.

In situ XRD: Samples were analyzed using a PANalytical Empyrean diffractometer equipped with an RTMS (real-time multiple strip) detector (X’Celerator) (PANalytical, Almelo, The Netherlands). Materials were weighed and hand mixed inside a plastic bag for 1 min before filling the sample holder, and then immediately covered with a Kapton film to avoid water evaporation and minimize superficial carbonation. A semi-quantitative analysis of the X-ray diffractograms was performed using the reference intensity ratio (RIR) method based on the multi peaks approach as implemented in the High Score Plus™ software. The method involves comparing the intensity of one or more peaks of a phase with the intensity of a peak of a standard material to yield an approximation of the solid phase assemblage in a sample as was discussed by [36]. It was used as an alternative method to get information and a brief comparison based in overall peak intensities from the multiple identified phases. Some of the solid phases produced have a preferred orientation, e.g., ettringite, which can modify the relative intensities of the diffraction signals and the quantitative results. The displacements in the X-ray diffractogram peaks caused by the changing sample volume, a result of the hydration reaction, were corrected by assigning the major ettringite peak to 9.1° 2θ from the observed 2θ value and by shifting the other phases by the same amount using HighScore Plus™ software.

Isothermal calorimetry (IC): The hydration of both cub- and orth-C₃A was followed using a high-sensitivity (20 μW) isothermal calorimeter (TA Instruments, New Castle, DE, USA) with an integrated stirrer for internal mixing. Samples (3–6 g) of the dry material (gypsum and C₃A) were first introduced into the cell. Water was added when thermal equilibrium was achieved, and the sample was stirred for 2 min. The evolution of the heat flow produced from the hydration reaction proceeded for 24 h, which began when water was introduced into the cell. The results were normalized to the total mass of solids added.

3. Results and Discussion

3.1. Hydration of C₃A without Gypsum—In Situ XRD

Figure 2 shows the crystalline hydration products formed between 2 min and 165 min after initiating hydration for cub- and orth-C₃A without gypsum. The main solid phases produced in both samples are C₄AH₁₉ (OH-AFm₁₉; Ca₄Al₂O₇$·$19H₂O; powder diffraction file (PDF#) 00-014-0628), calcium hemicarboaluminate hydrate (C_0.5-AFm; Ca₄Al₂O₆(CO₃)_0.5$·$11.5H₂O; PDF# 00-041-0221), calcium monocarboaluminate hydrate ($\overline{\mathrm{C}}$-AFm, 3CaO·Al₂O₃·CaCO₃·11H₂O; PDF#01-087-0493), gibbsite (AH₃; PDF#01-070-2038) and katoite (k; C₃AH₆; PDF# 00-024-0217). Cubic C₃A (c-C; PDF# 00-038-1429) and orthorhombic C₃A (o-C; PDF# 00-026-0958) were also identified in the samples.

The peaks related to C₃AH₆ exhibited high intensities and appeared prior to two minutes of the reaction, which are attributed to the precipitation of this phase as a major hydration product in both cub- and orth-C₃A systems (Figure 2A,B). The diffraction signals for C_0.5-AFm occur after 60 min of hydration in the cub-C₃A system and after two minutes in the orth-C₃A system. This phase is formed through superficial carbonation of the calcium aluminum hydrate (such as C₄AH₁₉). The presence of calcium monocarboaluminate hydrate ($\overline{\mathrm{C}}$-AFm, 3CaO·Al₂O₃·CaCO₃·11H₂O) and the earlier formation of hemicarboaluminate in the orth-C₃A system is consistent with the results reported by Dubina et al. [37], showing that orth-C₃A exhibits a higher susceptibility to superficial carbonation. The results are consistent with previous reports [10], although OH-AFm₁₉ is sometimes identified as a major solid hydration product [10,38], but the intensity of the peaks related to its presence were lower.

3.2. Hydration of C₃A with Gypsum—In Situ XRD

Figure 3 presents the in situ XRD results of cub-C₃A and gypsum hydration at gypsum/cub-C₃A ratios of 0.20, 0.60, and 1.90. A fast depletion of gypsum was identified for the cub-C₃A system with 20% of gypsum after approximately two hours (Figure 3A), and after five hours some unreacted cub-C₃A remained. The total consumption of gypsum for this sample led to the formation of calcium monosulfoaluminate hydrate ($\overline{\mathrm{S}}$-AFm, Ca₄Al₂O₆(SO₄)$·$14H₂O; PDF# 00-042-0062) at the expense of ettringite (E, Ca₆Al₂(SO₄)₃(OH)₁₂$·$26H₂O, C₆A$\overline{\mathrm{S}}$₃H₃₂; PDF# 00-041-1451)), as expected. Peaks attributed to ettringite exhibited high intensities for the cub-C₃A systems with higher contents of gypsum (CaSO₄$·$H₂O; G; PDF# 00-033-0311). $\overline{\mathrm{S}}$-AFm was identified mainly for gypsum/cub-C₃A ratios of 0.20, which is consistent with previous reports [5], where the dissolution of ettringite and the remnant cub-C₃A formed monosulfoaluminate and/or hydroxy-AFm phases. This reaction occurs after the depletion of gypsum, which is also coherent with the heat released, as is shown below. After eight hours of hydration time, OH-AFm₁₉ was detected. Cub-C₃A was not fully consumed even after 15 h of hydration regardless of the gypsum content.

For the systems, gypsum/cub-C₃A = 0.60 and 1.9, ettringite is the first crystalline phase identified after two minutes of hydration. Unhydrated gypsum and cub-C₃A are still present at the end of the measurements (15 h) but exhibit peaks with smaller intensities in the gypsum/cub-C₃A = 0.6 sample. For the system gypsum/cub-C₃A = 0.6, the precipitation of $\overline{\mathrm{S}}$-AFm began after eight hours.

Similar results are found for the orth-C₃A and gypsum hydration systems (Figure 4) with ettringite formed over the full range of gypsum/orth-C₃A ratios and $\overline{\mathrm{S}}$-AFm precipitation favored at lower gypsum content. The consumption of the C₃A during the hydration is higher for the orthorhombic-type structure regardless the gypsum content (except for the sample with no gypsum added), and the intensities of their characteristic peaks are lower compared to the corresponding systems based on cub-C₃A. Furthermore, $\overline{\mathrm{S}}$-AFm is more stable in the orth-C₃A system, as it is also identified at a gypsum/orth-C₃A ratio = 0.20 and 0.60. This phase is again produced as the gypsum peaks decrease from major to minor intensities, which occurs after five hours in the gypsum/orth-C₃A = 0.20 sample and after 30 min in the gypsum/orth-C₃A = 0.60 sample. The gypsum/orth-C₃A = 0.60 system (Figure 4B) exhibits the highest reactivity degree due to a more pronounced decrease in the gypsum peaks. Unhydrated C₃A appears before 20 min of hydration with almost complete hydration after roughly eight hours, and a higher and progressive formation of ettringite is clearly identified. Total gypsum consumption was not observed in the gypsum/orth-C₃A = 1.90 sample, and this was the only sample in which the orth-C₃A fully reacted within the first few minutes of hydration. In the presence of sulfates, the reactivity of C₃A increases at a higher content of Na⁺ in its structure, and therefore, the orthorhombic phase is a more reactive polymorph when dissolved sulfate ions are present. The diffraction peaks for ettringite appear after two minutes of hydration in the orth-C₃A samples at gypsum/orth-C₃A ratios of 0.20, 0.60, and 1.90. The ettringite peaks in the gypsum/orth-C₃A = 0.60 and 1.90 samples are more prominent than in the X-ray diffractograms for the cub-C₃A samples. Unlike the sample, gypsum promotes the accelerated consumption of orth-C₃A during the reaction due to the higher solubility of their Al₆O₁₈¹⁸⁻ ring structures in the presence of a sulfate source compared to that of the cub-C₃A samples [34]. This finding is also corroborated with the slower cub-C₃A consumption regardless of the smaller gypsum content during 15 h of analysis.

Figure 5 presented the RIR analysis using the major diffraction peaks for gypsum, orth-C₃A, cub-C₃A, ettringite, and $\overline{\mathrm{S}}$-AFm. Figure 5A shows coherence between the minimum value for C₃A and maximum of monosulfate, in 300 min (five hours), its formation was favored by the very low gypsum content. The levels of C₃A and monosulfate remain constant, with no increasing in the reaction. There is also consistency in the increase and then decrease in the ettringite content, and a steady decrease in the gypsum content. This can be correlated to the cumulative heat of this sample (Figure 6A) in the IC test, where the image shows an abrupt change in the amount of the cumulative heat after five hours. Figure 5E indicates that the hydration of orth-C₃A is fastest in the gypsum/orth-C₃A = 0.60 system. The diffraction peaks for orth-C₃A are not identified after 15 min of hydration in the gypsum/orth-C₃A = 1.9 sample (Figure 5F), indicating that it is consumed faster than cub-C₃A in its counterpart system at the same gypsum content (Figure 5C). The presence of amorphous calcium aluminate hydrates and some AFm-type structures cannot be reliably quantified by conventional XRD due their short-range ordering and is identified only through the deviation in the baseline between 15 and 25 degrees.

In summary, the XRD measurements showed that gypsum accelerates the consumption of orth-C₃A more than cub-C₃A during hydration. Stephan and Wistuba [11] studied the hydration behaviour of C₃A solid solutions with MgO, SiO₂, Fe₂O₃, Na₂O and K₂O. The authors found out that the hydration of these materials in the presence of CaSO₄ was accelerated when C₃A was doped with K₂O or Na₂O, whereas Fe₂O₃ strongly retarded the hydration.

3.3. Isothermal Conduction Calorimetry

The rate of heat evolution measured by IC depends greatly on the C₃A crystal structure and gypsum content (Figure 6) with up to three exothermic peaks identified in each curve. For both cub- and orth-C₃A samples, the first peak with maximum at roughly five minutes corresponds to the initial wetting and dissolution including the precipitation of OH-AFm and C₃AH₆ (at gypsum/C₃A content of 0 and 0.20) and ettringite (at gypsum/C₃A content of 0.60 and 1.9). The formation of these solid phases have been reported for the more dilute cub-C₃A samples (w/s = 4, 10, and 25) [5,6,28,32] and pastes (w/s = 1) [8] and agrees with the previous results.

The addition of gypsum to the cub-C₃A sample decreases the intensity of the main heat evolution peak (376.8 mW/g) to an approximately constant peak value (135.9 mW/g), irrespective of the gypsum content. This result is consistent with previous research [5], where the time of the highest heat release identified is unaffected by the gypsum content once present. However, the occurrence of the secondary peak, attributed to $\overline{\mathrm{S}}$-AFm formation and consumption of initially precipitated ettringite, depends on the gypsum content. This peak occurs in the results for the gypsum/cub-C₃A = 0.20 sample only at roughly six hours of hydration (Figure 6). This peak originates from the renewed hydration of cub-C₃A that occurs once gypsum has completely dissolved. Recent research suggests that this renewed hydration of cub-C₃A coincides with the desorption or consumption of Ca-S complexes, which adsorb onto the cub-C₃A surface and inhibit the dissolution of this phase at earlier hydration times [39]. In the XRD result for the gypsum/cub-C₃A = 0.60 sample (Figure 3B), peaks with very low intensities attributed to $\overline{\mathrm{S}}$-AFm can be seen after 15 h. The low intensity of the $\overline{\mathrm{S}}$-AFm peaks in the XRD results for this sample suggest the presence of a very low content, and the quantity of this phase formed (at this time) is quite low, and insufficient heat is measured by the equipment. In contrast to the cub-C₃A system, adding gypsum to the orth-C₃A hydration does not decrease the intensity of the initial exothermic peak (~220–250 mW/g), except for the gypsum/orth-C₃A = 0.60 sample. The intensity of the initial heat release peak for the gypsum free orth-C₃A sample is ~40% lower than that identified for the corresponding cub-C₃A sample, indicating that orth-C₃A is less reactive than cub-C₃A in water, which agrees with [11,40] and the XRD results.

The addition of gypsum to the orth-C₃A system does not greatly change the heat evolution rate at a gypsum/orth-C₃A ratio = 0.20 with its main exothermic peak ~51% higher than that of the corresponding cub-C₃A system. A prominent second exothermic peak is identified at ~23 min in the orth-C₃A samples synthesized at gypsum/orth-C₃A ratios of 0.60 and 1.90. This second peak coincides with the consumption of gypsum and precipitation of ettringite in the gypsum/orth-C₃A = 0.60 sample. However, in the gypsum/orth-C₃A = 1.90 sample, the amount of gypsum stabilized after the first few minutes of the reaction; therefore, this second peak can be attributed to the consumption of orth-C₃A and the precipitation of ettringite. The higher heat released before 8 h in the gypsum/orth-C₃A = 0.60 sample (Figure 5B) can be assigned to $\overline{\mathrm{S}}$-AFm formation and mainly to the continuous formation of ettringite at the expense of gypsum; as is shown in the XRD results in Figure 3B. This result indicates that the hydration of orth-C₃A occurs faster than cub-C₃A in the presence of gypsum, which is consistent with existing research on the cub- and orth-C₃A pastes in the presence and absence of lime [28,30,32].

4. Conclusions

This paper presented IC and in situ XRD analyses to follow the hydration of cub- and orth-C₃A hydration in the absence and presence of gypsum. The results showed that orth-C₃A reacts faster than cub-C₃A in the presence of gypsum with early ettringite formation and gypsum and C₃A consumption occurring in the former system. However, the hydration rates of cub- and orth-C₃A still depend on the gypsum content. According to the XRD results, gypsum is consumed faster in the orth-C₃A systems, mainly for an orth-C₃A/gypsum ratio of 0.60. In the absence of gypsum, cub-C₃A was found to dissolve faster than orth-C₃A, releasing higher quantities of heat and producing calcium aluminate hydrate phases earlier. The different effects of gypsum on the hydration of both cub- and orth-C₃A have important practical implications, indicating that the crystal structure type and quantity are key parameters to consider in optimizing calcium sulfate addition to PC clinker to ensure good workability throughout placement.