Ternesite as a component of sulfobelitic cements

The cement industry is presently devoting substantial effort to the development of less energy intensive, lower emission cements. Calcium sulfoaluminate (CSA) cements, one of the most promising alternatives, are known as ‘eco-cements’ in light of their lower energy costs and GHG emissions. Whilst their clinker composition may vary, all these cements lie in the CaO-Al2O3-SiO2-Fe2O3-CaSO4 system, with constituent minerals that include C2A, C2S, C12A7, CA, CŜ, and C4AF. Ternesite (C5S2Ŝ), a phase compatible with ye’elemite (C4A3Ŝ) and C2S, can also be found in that system.Recent studies show that it is activated and hydrated in the presence of AH3 and gypsum, yielding ettringite and C-S-H gel, the main hydration products of CSA and OPC, respectively, as well as C2ASH8.This study analyses ternesite reactivity in different aluminate


Introduction
The environmental impact of Portland cement manufacture has prompted the industry to implement a number of strategies geared to lowering energy and natural raw material consumption while reducing greenhouse gas, particularly CO 2 , emissions.The use of secondary cementitious materials (SCMs, often industrial by-products), alternative fuels, grinding additives, along with improvements in combustion, heat exchange and grinding technologies have diminished both consumption and emissions with no detriment to binder properties [1].
Another strategy consists in developing calcium sulfoaluminate cements (CSA) based on ye'elemite (C 4 A 3 Ŝ), a phase initially used to produce expansive cement and subsequently stable high-performance binders.CSA cement manufacture is less energyintensive (at 1250 ºC, the clinkering temperature is 200 ºC lower than in OPC) and, depending on the composition, emits 25 % to 40 % less CO 2 than OPC production [2,3].Whilst not presently used for structural purposes for want of the necessary regulation, these cements are commercialised as components in special mortars and concretes (such as repair or self-levelling concrete, sealing mortar or shotcrete).
The composition of the clinker in these cements may differ, although all lie in the CaO-A l2 O 3 -SiO 2 -Fe 2 O 3 -CaSO 4 system and have lower calcium and silicon and higher aluminate and sulfate phase contents than portland cements.The minerals present in the clinker include C 4 A 3 Ŝ, C 2 S, C 12 A 7 , CA, CŜ and C 4 AF, with a predominance of aluminates [4,5].
Ternesite, C 5 S 2 Ŝ, a phase formed in the reaction between anhydrite and beliteand compatible with ye'elemite (C 4 A 3 Ŝ) and C 2 S, also lies within this system.
Ternesite was found as a natural mineral at Ettringer Bellerberg, near Mayen, Eifel, Germany, in the nineteen nineties.Its synthetic form had been identified earlier, however, as a component of cement [6] or lime [7] kiln crust, the sulphur for which was sourced from the fuel, as well as a minority component in some CSA cements [8,9,10].
Ternesite was chemically, physically, structurally and petrologically characterised between 1960 and 2000 [6,7,[11][12][13].Regarded as a non-hydraulic material [14][15] until it was recently proven to be activated by aluminium hydroxide [16], it has since become a target for research.This paper describes some of the characteristics of ternesite and its hydraulic activation by other mineral compounds.

Physical and structural characteristics
Natural [11] and synthetic ternesite, which are identical, are isostructural with silicocarnotite, Ca 5 (PO 4 ) 2 SiO 4 [12,13].The unit-cell parameters of this orthorhombic mineral,Pnma space group crystal are a= 6.863(1) Å, b=15.387(2)Å, c=10.181(1)Å, Z=4.Its structure consists in isolated SiO 4 and SO 4 tetrahedral units connected by octa-or heptahedrally coordinated Ca ions.Both S and Si tetrahedrally coordinated with oxygen occupy different tetrahedral positions in the structure, in which double layers of silicate tetrahedral alternate with a layer of sulphate tetrahedral.Natural ternesite is optically biaxial and negative, with refractive index n x = 1.630, n y = 1.637 and n z = 1.640 and an optical angle, 2V x = 63.5º.

CMSS-2017
3 Thermal stability of ternesite C 5 S 2 Ŝ thermal stability has been studied by several authors, some of whom position it in a narrow range of temperatures (1100 ºC to 1180 ºC [16] or 1200 ºC to 1298 ºC [15]), above which it would decompose further to the equation: ( The decomposition temperature depends on whether it is determined in an open (≈1180 ºC) or closed (1289 ºC) system at a total pressure of 1 atm.[17,18].Similar findings (≈1290 ºC at 1 atm.) were reported by Hanein et al. [19], who determined the thermodynamic constants of ternesite to lie between 1000 ºC and 1400ºC, based on previously published vapour pressure measurements of several sulphates obtained between those temperatures [22].Those studies showed that the vapour pressure of ternesite, like that of anhydrite, rises with temperature, while ternesite exhibits a pressure lower than anhydrite at 1000 to 1290 ºC and higher above that range.The reaction of ternesite would also depend, then, on the rate of SO 2 loss and for high temperatures could be described by the equation: in which ternesite thermal decomposition would be a function of temperature and the partial pressure of SO 2 and O 2 .
For a given temperature and partial pressure of O 2 (which must always be higher than in SO 2 for SO 3 to form), there is a maximum SO 2 pressure above which ternesite does not form [19].
On the DTA curve (10 ºC/min in CO 2 -free air), ternesite thermal decomposition induces an endothermal peak at around 1290 ºC.As the temperature rises, a second endothermal peak appears due to the fusion of the C 2 S-CaSO 4 eutectic, which accounts for 10 %-20 % of the composition of CaSO 4 [18].Mass loss intensifies on the TG curve at temperatures of over 1200 ºC.During cooling, the DTA curve exhibits two exothermal peaks, respectively denoting the solidification of the C 2 S-CaSO 4 eutectic (1310 ºC) and the α to β polymorphic transformation of CaSO 4 (1175 ºC).No signal whatsoever is detected that would indicate new ternesite formation along cooling.FTIR and XRD identify α'C 2 S and anhydrite (traces of CaO on the XRD pattern), but no ternesite, an indication that at the cooling rate applied (10 ºC/min) the decomposition reaction is not reversible.The XRD findings for ternesite exposed to different thermal treatments (Table 1) suggest that once it decomposes, its recrystallization depends on the cooling rate and the temperature reached.Recrystallization is possible after reheating to 1000 ºC when the temperature reached does not exceed the melting point of the eutectic; otherwise, recrystallization does not appear to occur.
According to the literature, ye'elemite, ternesite and anhydrite clinkers at a ratio (by weight) of 1:1:0.5 can be obtained from traditional raw materials or industrial byproducts at temperatures of around 1200 ºC [14].Twostage clinkerización has also been successfully applied to produce such cements, clinkering the raw meal at 1250 ºC for 1 h, followed by gradual cooling to 1100 ºC and quick cooling below that temperature [10,20,21].Singlestage belite and ternesite-rich calcium sulphoaluminate (BY(F)T) have been obtained at a maximum temperature of 1260 ºC in a pilot plant where the partial pressure of O 2 and SO 2 was controlled in the kiln [19].

Compatibility between ternesite and CaO-SiO 2 -A l2 O 3 -CaSO system phases
The following is a discussion of some of the features of the equilibrium phases of the three subsystems most The equilibrium phases of the system CaO-SiO 2 -A l2 O 3 have long been known in the fields of cement chemistry and ceramics [23].In addition to mullite, the calcium silicates (C 3 S, C 2 S, C 3 S 2 , CS) and calcium aluminates (C 3 A, C 12 A 7 , CA, CA 2 , CA 6 ), the system includes two ternary phases: gehlenite and akermanite.C 2 S is compatible with C 3 A, C 12 A 7 , CA and gehlenite, as well as with C 3 S and C 3 S 2 .
A ternary phase, C 4 A 3 Ŝ, present in system CaO-A l2 O 3 -CaSO 4 , is positioned in between the CaSO 4 and the C 3 A, C 12 A 7 and CA.The phases in the Ca-rich zone of the system that are compatible at temperatures under 1350 ºC are: [24].
Studying system CaO-SiO 2 -A l2 O 3 -CaSO 4 at temperatures between 950 ºC and 1150 ºC, Pliego-Cuervo [9] found ternesite to be incompatible with C 12 A 7 and CA, and both C 2 S and C 3 A to be compatible with ternesite, ye'elemite or both.The compatible phases in the CaOrich zones of the system were identified as: i Strigac [23] broadened the field of study, identifying two new sets of compatible phases in which ye'elemite and ternesite would co-exist: vii Ye'elemite and ternesite can co-exist, then, not only in calcium-rich zones of the system but also in areas with less CaO and more CaSO 4 and SiO 2 , although obviously at temperatures below the ternesite decomposition threshold.

Ternesite activation
Although a number of authors have concluded that ternesite is a non-hydraulic phase [14][15], as early as 1992Berteka et al. [25] synthesised clinkers consisting primarily in ye'elemite and ternesite, with anhydrite as a residual phase.Produced at 1200ºC, they exhibited very high strength after hydration.Later studies [16] showed that ternesite hydration would be moderate and very slow.
According to Ben Haha et al. [26], ternesite can be activated with aluminate hydroxide to yield ettringite, stratlingite and C-S-H, the proportions of which depend on the proportion among reactives and their degree of reaction.Bullerjahn et al. [27] reported that ternesite contributed more to early age cement properties than C 2 S in the CSA cements they synthesised.
Montes et al. [28] showed that ternesite can also be activated by other aluminates, such as C 3 A, C 12 A 7 , CA and C 4 A 3 Ŝ, and established the order of activating effectiveness as: C 12 A 7 > CA> C 3 A>>>>C 4 A 3 Ŝ.
The calorimetric curves for hydrated aluminate pastes contain a first dissolution peak, followed by an induction period and an intense reaction peak in pastes CA, C 12 A 7 and C 4 A 3 Ŝ [ [29][30][31].In 2:1 ternesite: aluminate blends hydrated at 25 ºC for 7 d, the presence of ternesite in the pastes was reported to alter the calorimetric curves of four aluminates, shortening the induction period and bringing the reaction peak forward [28].The presence of ternesite alters the hydration products of the aluminates.Pastes obtained after calorimetric studies of the ternesite/aluminate blends contain C 4 AŜH 12 as a hydration product.C 2 ASH 8 is also found in pastes containing C 12 A 7 or CA.Stratlingite forms neither in ternesite + C 3 A pastes nor in the pastes with C 4 A 3 Ŝ, in which amorphous AH 3 is present [29].