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

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., C2AH7.5 and C4AHx, 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 m and mean particle sizes of 11.0 m and 14.0 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 m and a mean particle size of 19.0 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, CaCO32H2O).

Figure 1

X-ray diffractograms for the un-reacted materials. (A) Cub-C3A; (B) Orth-C3A; and (C) Gypsum.

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; Ca4Al2O719H2O; powder diffraction file (PDF#) 00-014-0628), calcium hemicarboaluminate hydrate (C0.5-AFm; Ca4Al2O6(CO3)0.511.5H2O; PDF# 00-041-0221), calcium monocarboaluminate hydrate (-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.

Figure 2

In situ X-ray diffraction (XRD) results for (A) cub-C3A and (B) orth-C3A hydration without gypsum. OH-AFm19 = C4AH19; C0.5-AFm = calcium hemicarboaluminate hydrate; AH3 = gibbsite; k = katoite (C3AH6); c-C = cub-C3A; and o-C = orth-C3A.

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 C0.5-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 (-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 (-AFm, Ca4Al2O6(SO4)14H2O; PDF# 00-042-0062) at the expense of ettringite (E, Ca6Al2(SO4)3(OH)1226H2O, C6A3H32; PDF# 00-041-1451)), as expected. Peaks attributed to ettringite exhibited high intensities for the cub-C3A systems with higher contents of gypsum (CaSO4H2O; G; PDF# 00-033-0311). -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.

Figure 3

In situ XRD results of cub-C3A and gypsum hydration at gypsum/cub-C3A ratios of (A) 0.20; (B) 0.60; and (C) 1.90. c-C = cub-C3A, E = ettringite, G = gypsum, OH-AFm = C4AH19; C0.5-AFm = hemicarboaluminate; -AFm = monosulfoaluminate.

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 -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 -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, -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 4

In situ XRD results of orth-C3A and gypsum hydration at gypsum/orth-C3A ratios of (A) 0.20; (B) 0.60; and (C) 1.90. o-C = orth-C3A, E = ettringite, G = gypsum, OH-AFm = C4AH19; k = katoite (C3AH6), C0.5-AFm = calcium hemicarboaluminate hydrate; -AFm = monosulfoaluminate.

Figure 5 presented the RIR analysis using the major diffraction peaks for gypsum, orth-C3A, cub-C3A, ettringite, and -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.

Figure 5

Reference intensity ratio (RIR) analysis of cub-C3A hydration with gypsum/C3A ratios of (A) 0.20; (B) 0.60; and (C) 1.9, and orth-C3A hydration with gypsum/C3A ratios of (D) 0.20; (E) 0.60; and (F) 1.9.

Figure 6

Rates of heat evolution (top) and cumulative heat (bottom) for (A) cub- and (B) orth-C3A hydration in the presence and absence of gypsum.

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 -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 -AFm can be seen after 15 h. The low intensity of the -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 -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.