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anhydrite ratio on PC-CSA hybrid cements
Journal Pre-proof Influence of the ye’elimite/anhydrite ratio on PC-CSA hybrid cements ˜ ´ ´ A. Lecomte I. Bolanos-V asquez, R. Trauchessec, J.I. Tobon,
PII:
S2352-4928(19)30176-X
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100778
Reference:
MTCOMM 100778
To appear in:
Materials Today Communications
Received Date:
12 April 2019
Revised Date:
8 November 2019
Accepted Date:
17 November 2019
˜ ´ ´ JI, Lecomte A, Influence Please cite this article as: Bolanos-V asquez I, Trauchessec R, Tobon of the ye’elimite/anhydrite ratio on PC-CSA hybrid cements, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100778
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Manuscript submitted to Materials Today Communications
Influence of the ye’elimite/anhydrite ratio on PC-CSA hybrid cements Bolaños-Vásquez, I.1), Trauchessec, R.2), Tobón, J.I.1), Lecomte, A.2) 1. Grupo del Cemento y Materiales de Construcción (CEMATCO), Calle 75 # 79A-51, Bloque M17-101 Departamento de Materiales y Minerales, Facultad de Minas, Universidad Nacional de Colombia, Medellín, Colombia 2. Institut Jean Lamour, UMR 7198, CP2S, Equipe Matériaux pour le Génie Civil, Université de Lorraine, IUTNB, 54600 Villers-lès-Nancy, France
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corresponding author: [email protected]
Abstract
Portland cement (PC) and Calcium sulfoaluminate cement (CSA) blends are used to produce binders with properties
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as rapid hardening. However, characteristics of calcium sulfoaluminate cements such as chemical and mineralogical composition, particle size distribution, among others, can have an enormous variation and affect the performance of
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the PC-CSA cement mixture. Therefore, current research focus on the mechanical properties and hydration of PCCSA (75 wt%-25wt%) blend cement using three different CSA cements in order to better understand the influence of
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property variations like ye’elimite quantity and ye’elimite/anhydrite ratio in CSA cements on PC-CSA systems. Compressive strength was determined on mortars and hydration was studied on paste using isothermal calorimetry, X-ray diffraction and thermal analysis. Results showed that ye’elimite quantity in CSA used into PC-CSA hybrid
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cements have a large influence in early hardening and hydration. But, beyond the quantity of ye’elimite, findings showed that there is a greater influence of the ye’elimite/anhydrite ratio on the heat release, mineralogy and
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compressive strength of PC-CSA blends. For example, it was found that it is necessary to adjust the sulfate content to avoid compressive strength stagnation. Higher ye’elimite/anhydrite yields to higher mechanical strengths independent
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on the type of CSA.
Keywords: Calcium sulfoaluminate, blended cements, cement hydration, PC-CSA system, PC-CSA blends hybrid cements.
1. Introduction Cement production generates between 5-8 % of global CO2 emissions [1], [2]. Looking for new and less polluting binder options, investigations on alternative cements have been increasing in recent years [3], [4]. Calcium
sulfoaluminate (CSA) cements offer a low-CO2 alternative to Portland cement, since they are produced at lower temperatures and with lower limestone content in their raw materials. CSA cement can be produced at temperatures about 1250 °C, that is to say, 200 ºC less than Portland cement temperatures production [5]. This represents a 25-35 % reduction of carbon dioxide net emissions in comparison to Portland cement, contributing thus toward reducing energy demand and carbon footprint [6]. CSA cements bear a compound named ye’elimite, aka “Klein’s salt” (C4A3S̄̅ ), as the main phase. It has been used as cementitious material since 1960s when it was patented by Alexander Klein as an expansive or shrinkagecompensating addition to PC. These cements are known as type k cements or expansive cements [7], [8]. In China, CSA cements are known as the “third cement series” and were developed in 1970s [9]. Besides ye’elimite, CSA clinker also contains several phases, depending on the initial composition of raw materials, such as belite (C 2S),
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calcium aluminate (CA), mayenite (C12A7), brownmillerite (C4AF), anhydrite (CS̄̅ ), perovskite (CT), and others [10]– [12].
The main raw materials of CSA cement are calcite, clay or bauxite and gypsum as sources of calcium, silica /
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aluminum and sulfur respectively. There are other sources, for example waste from the production of bauxite, socalled "red mud", and plaster, but those alternative sources are not widely used. It is important to note that the high
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cost of bauxite, one of the raw materials of CSA, allows a limited application compared with low-priced Portland cements, and this why CSA cements are being used for special applications and without being intended to replace PC
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[11], [13], [14]. Therefore, hybrid or blended cements come into focus. The Business Council on Sustainable Development (WBCSD, 2009) has shown that the use of blended cement has considerably increased in relation to
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Portland cement in most countries [6].
In CSA cement, ye'elimite hydration can induce the formation of monosulfoaluminate (C4AS̄̅ H12) and gibbsite (AH3),
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ettringite (C6AS̄̅ 3H32) and gibbsite or only ettringite (reaction I, II and III) depending on the calcium sulfate and calcium hydroxide proportions.
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I. C4A3𝑆̄̅ + 18H → C4A𝑆̄̅ H12 + 2AH3
II. C4A3𝑆̄̅ + 2C𝑆̄̅ + 38H → C6A𝑆̄̅ 3H32 + 2AH3 III. C4A3𝑆̄̅ + 8C𝑆̄̅ + 6CH + 90H → 3C6A𝑆̄̅ 3H32 The fundamental behavior of PC-CSA cement blends is based on reaction III involving one mole of ye’elimite being combined with gypsum or anhydrite plus portlandite to produce three moles of ettringite, instead of one, resulting in greater potential for expansion. Additionally, the ettringite crystals in reaction III are smaller than those produced in reaction II [11], [14]. Alite or belite hydration can lead to the formation of various hydrates (C-S-H, strälingite,
portlandite, etc.) depending on the PC and CSA proportions [15], [16]. These type of blends can be used to obtain particular properties (early strength, dimensional stability, etc.) that could be valuable for 3D printing [17] or radioactive waste encapsulation [18]. PC-CSA cement blends (PC-CSA) may change properties, with regard to each individual cement [15], [19]. PC-CSA have a higher mechanical strength than PC at relatively short curing times, e.g. 1 day, and a very low drying shrinkage [20]. In PC-CSA with a very low percentage of CSA (10 %) the hydration products are the same as for the PC, but with a greater amount of ettringite [21]. When CSA cement proportion is modified in the PC-CSA, the hardening rate and mechanisms of hydration (amount and nature of hydrates) changed, and also the higher the amount of CSA the higher the ettringite content and greater compressive strengths at early ages [22]. The increase in CSA cement content in PC-CSA also increased the extent of expansion, even when CSA has low values of ye’elimite [23]. The variation of
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anhydrite content at fixed PC/(CSA+CS̄̅ ) ratio affects the compressive strength and do not strongly influence the hydrate assemblage of the blend [16].
These different studies seem to show that everything in PC-CSA cement blends research is covered, however, it is
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worth mentioning that each of the cited investigations used CSA cements with huge differences is its composition, fineness and other properties. In addition, to the variation of C𝑆̄̅ percentages in the PC-CSA system which also counts,
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this is why it is difficult to compare and define conclusive results. The current research studies the mechanical properties and hydration of PC-CSA blend cement using three different CSA cements into PC-CSA (75%-25%) hybrid
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cement to better understand the possible influence of property variations like ye’elimite quantity and ye’elimite/anhydrite ratio in CSA cements on PC-CSA systems.
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2. Materials and Methods
The raw materials used in this study were one CSA cement and two commercial CSA clinkers produced by Tangshan
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Polar Bear Building Materials in China; a commercial calcium sulfate dihydrate (CS̄̅ H2), synthesized by Bell Chem International S.A. and distributed by Proquimes S.A. in Colombia and a Portland Cement, with limestone addition,
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produced by Argos S.A. in Colombia. Anhydrite was prepared by heating the CS̄̅ H2 in an oven at 700 °C for two hours [24], [25]. X-ray diffraction (XRD) of the resulting product after burning confirmed that anhydrite was obtained. Quantitative X-ray diffraction (QXRD) analysis was employed for determining the mineralogical composition of CSA cement and PC. The elemental by X-ray fluorescence and mineralogical composition by Rietveld method of the raw materials can be found in Table 1. XRD analysis was carried out in a Bruker equipment, in an interval 2𝜃 between 4⁰ and 70⁰ , with a step of 0.02⁰ and an accumulation time of 30 s. TOPAS software was utilized for the Rietveld refinement. The Blaine surface and density are presented in Table 2. The density of the materials was determined by an AccuPyc II 1340 gas pycnometer and Blaine surface test was made complying with standard NF EN 196-6 (201204-01). Anhydrite addition in clinkers CSA2 and CSA3 was 13.5 wt%, to obtain CSA2 and CSA3 cements. It is
important to highlight that CSA1, CSA2 and CSA3 cements correspond to the CSA cement containing an increasing amount of ye’elimite but different ye’elimite/anhydrite ratios. Three mixtures were studied containing 75 wt% PC with 25 wt% of each one of the CSA cements. PC was also studied as reference. Mixtures proportions and mineralogical composition are shown in Table 3. 2.1. Mortars Water/cement (w/c) ratio for the mortars was determined according to ASTM C 1437 [26] by keeping constant the flow at 110±5 %, according to ASTM C 348 (section 3) and ASTM C 349 [27], [28] recommendation related to work with cements other than Portland, thus ensuring good workability and avoiding the use of additives in the elaboration of mortars for subsequent flexion and compression test. Water/cement ratios for all the mixtures and for the reference
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are listed in Table 4. Those w/c ratios provide a workability leading to mortars molding without the need of chemical admixtures. ASTM C 348 and C 349 standards [27], [28] were followed for compression and flexural tests. Prismatic samples of 40x40x160 mm3 were molded, prepared with 450 g of cement, 1350 g of standard sand (ASTM C778 [29]) and water according to Table 4. Compressive and flexural strength measurements were made after 6 hours and 1, 3,
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7, and 28 days. Mortar prisms mixture samples were cured in water at 25 °C, the PC sample in saturated lime water. In order to evaluate the incidence of the w/c on each of the mixtures, compressive strength tests were also performed
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at a constant water to cement ratio (0.5) at ages 1 and 28 days.
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2.2. Pastes
For the different hybrid cements, setting time and plastic consistency tests were carried out according to ASTM C 191 and ASTM C 305, respectively. Conductive calorimetry tests were made using a thermal analysis (TA) instruments
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calorimeter Tam Air Thermostat 90/3116-in accordance with ASTM C1679 and ASTM C1702 [30], [31], and were performed on cement pastes prepared with 3 g of anhydrous cement and 1.8 g of water. An automatic admix device
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was used to mix the powder samples with water inside the calorimeter for 2 min in order to avoid any data loss at the beginning of the reaction. Heat flows were registered for 3 days at 25 ºC. 3 g of sand were used as a reference.
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Mineralogical properties were studied in cement pastes with 0.45 water/cement ratio. The pastes were prepared by hand by mixing the cement blend with the water in a glass beaker container using a glass stir rod for one minute. After curing in hermetic plastic containers, specimens were immersed in acetone for two hours to stop hydration, and dried in an oven at 60 ⁰ C for 24 hours to remove any residual water. Then the samples were crushed and sieved at 75 µm for mineralogical evaluation. The mineralogical composition of the hydrated blends was determined with XRD after 6 hours and 1, 3, 7 and 28 days of hydration. Thermogravimetric analysis tests (TGA) were performed at the same XRD ages in a SETARAM model: TG/ATD 92-16.18 equipment. During TGA and differential thermal analysis (DTG), the samples were heated in an air atmosphere from 20 °C to 1000 °C at the rate of 10 °C/min using a platinum crucible. The weight of the samples was approximately 45 mg.
3. Results and discussion 3.2. Water demand and setting time The water demand (Table 5) for the three PC-CSA hybrid cements increases compared to PC alone. This higher w/c ratio could be linked to the very fast hardening and the higher Blaine surface of the CSA cement and anhydrite (Table 2). Indeed, Table 5 also shows a dramatic decrease in the final setting time, considering that the differences among the PC and PC-CSA cement blends is more than 130 minutes. The CSA1-25 sample has the longer setting time and CSA2-25 and CSA3-25 have quite similar values. 3.2. Isothermal calorimetric analysis Heat flow and cumulative heat for the three blends and for the reference cement are shown in Figure 1. Figure 1a
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depicts the heat flow for durations over 150 hours for all samples, Figure 1b shows a zoom of the first two hours of hydration and Figure 1c presents the cumulative heat. During the first minutes, the PC curve exhibits a first short and intense peak corresponding to the energy released during the dissolution stage by the mixing and the cement wetting processes. After that a “dormant” period of slow reaction (about two hours), an acceleration period and a deceleration
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period are observed. The maximum hydration heat flow is reached after almost 9 hours. The presence of 25 wt% of CSA cement completely modifies the heat flow released during the first hours. During the first 20 minutes, if compared
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with PC, the hydration heat flow for CSA1-25 is more than double and more than quadruple than those for CSA2-25 and CSA3-25, respectively. CSA2-25 and CSA3-25 heat flows are similar and exhibit a second peak after 40 minutes
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of hydration. The first peak (10 minutes) presumably corresponds to ye’elimite and calcium sulfate dissolution with ettringite formation (reaction II) and the second peak for CSA2-25 and CSA3-25 or shoulder (30 minutes) in Figure 1b
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are assumed to result from the formation of AFm phases (reaction I) when sulfate in solution is depleted [11]. This presumption will be analyzed in section 4. This dramatic heat flow release in the first hours is consistent with the amount of ye’elimite in each one of the hybrid cements (Table 1). It can also be noted that the higher the early heat
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flow the lower the setting time (Table 5). Cumulative heat (Figure 1c) for samples CSA2-25 and CSA3-25 is very discontinuous and is lower than PC and CSA1-25 samples until approximately 125 hours, then the cumulative heat is
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higher for the latter two samples. At this time duration, it is also possible to see another peak in the heat flow curve, and at a duration time of 150 hours CSA2-25 and CSA3-25 samples there is a continuing increase of their cumulative heat while CSA1-25 and PC seem to have reached their maximum heat release. This suggests that the compressive strength CSA2-25 and CSA3-25 could continue increasing. XRD and DTG analyses make it possible to further explain the origin of this difference in reactivity. 3.3. X-ray diffraction Figure 2a-d shows the X-Ray diffraction for PC-CSA cement blends after 6 hours, 1, 3, 7, and 28 days of curing. In all the blends, a ye’elimite peak is observed only after six hours, in a lesser intensity in the CSA1-25, as expected, since
this mixture contain less ye’elimite. Moreover, CSA2-25 and CSA3-25, when compared with PC and with CSA1-25, exhibit a lower hydration rate of alite and belite especially after one and three days of hydration. This is consistent with Gastaldi et al and Londono-Zuluaga et al [20], [32]. The main hydration products are ettringite, portlandite and AFm phases. The peaks linked to ettringite are clearly visible even after 6h of hydration and lightly decrease until 28 days. This hydrate is formed by the rapid hydration of ye’elimite and its reaction (reactions II or III) with calcium sulfate (anhydrite and gypsum). After ettringite formation (during the first day), AFm phases (monosulfoaluminate, hemicarboaluminate) are formed and clearly appear after 7 days of hydration. The addition of 25 % CSA cement to the PC causes a marked decrease in the intensity of the portlandite peaks. This decrease can be associated to the lower PC proportion, lower hydration rate of alite in blended cements and reaction III. In blends containing CSA2 or CSA3, portlandite formation occurs only after 7 days of
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hydration whereas it’s formed after 3 days in blends with CSA1, it is assumed that this behavior is related to ye’elimite contents. 3.4. Thermal Analysis
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Derivative thermogravimetric signals (DTG) are presented in Figure 3a-d and Table 6 shows the mass of portlandite and the loss of water calculated from TGA data, which correspond to chemically bound water in hydrated phases. The
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range of temperatures taken for calculations were 400-500 and 50-550 °C, respectively. The presence of C-S-H (≈120 °C), ettringite (≈135 °C), AFm phases (180-220 °C), gibbsite (220-280 °C), portlandite (400-500 °C) and calcite (600-
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700 °C) is revealed by the signals (Figure 3a-d). For PC or CSA1-25 (Figure 3a-b), portlandite begins to be detected by a weight loss at 450 ºC signals after six hours. For CSA2-25 and CSA3-25 portlandite appears later (after 7d), this
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may confirm the presence of III-type reactions or/and slower PC hydration kinetics. Portlandite proportions for CSA225 and CSA3-25 samples are below 3.5 % before 7 days (Table 6), thus confirming XRD results. These analyses also confirmed that the portlandite amount decreases with an increase in ye’elimite content as can be seen in comparing
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Figure 3b for CSA1-25 and Figure 3c-d for CSA2-25 and CSA3-25. The observations made in the XRD analysis regarding AFm phases can be confirmed in the DTG signals and become apparent especially in the blends with a
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higher ye'elimite amount (CSA2-25 and CSA3-25). DTG also reveals the presence of a small amount of amorphous gibbsite after 6 hours, 1 and 3 days in CSA2-25 and CSA3-25 samples but not in CSA1-25. This result indicates that the hydration mechanism of ye’elimite in CSA1 may preferentially go through reaction III, while those of CSA2 and CSA3 follow reactions I and II. This may be due to the higher reactivity of PC in CSA1-25 blend. 3.5. Mechanical properties Compressive strength results for PC, CSA1-25, CSA2-25 and CSA3-25, prepared with w/c indicated in Table 4, after 6 hours as well as after 1, 3, 7 and 28 days are shown in Figure 4. It is possible to note that there are only compressive strength data for samples CSA2-25 and CSA3-25 after 6 hours and that both results are comparable. For PC and
CSA1-25 samples were impossible to measure a trustable value. After one day, mechanical strength in CSA2-25 and CSA3-25 samples remains practically equal as after six hours, and only a slight increase appears after three days. This behavior is atypical given that mechanical strength at early ages strongly increases with higher amounts of C 4A3𝑆̄̅ [33], [34], however, the rapid setting time could be the cause [35]. After one day, PC shows a higher strength data than all the blends. Compared with PC, the lower strength of PC-CSA blends (decrease of 7-11 MPa) after 28 days of hydration can be partially explained by the higher w/c ratio used for keeping constant flow (Table 4). CSA1-25 shows higher strength data after 7 and 28 days when comparing it with the other two blends which have a similar behavior. In spite of having less amount of ye’elimite, this could be due to i) lower w/c ratio and/or ii) slightly higher initial setting time [35]. This atypical behavior could be explained also by a deficit in ye’elimite/anhydrite ratio, this hypothesis will be evaluated in section 4. Flexural strength data in Figure 5 shows the same behavior than compressive strength.
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Compressive strength results for mortars with constant water-cement ratio 0.5 (Figure 6) show that after one day of curing PC and CSA1-25 sample had a very similar result, while samples CSA2-25 and CSA3-25 had a decrease close to 50 %, it is assumed that this decrease is due to lack in anhydrite amount in CSA2 and CSA3 cements. However, at
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28 curing days both in PC and in the three mixtures give statistically equal results.
4. Influence of the calcium sulfate proportion
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The first part of this paper showed that an addition of 25 % of CSA cement in PC-CSA blends can accelerate the setting time and strength after few hours. However, even if the ye’elimite amount is higher as in the case of samples
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CSA2-25 and CSA3-25 in comparison to CSA1-25, compressive strength after one and three curing days can be lower and remain stagnant. This is the opposite to what is usually expected. It has been confirmed by isothermal
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calorimetry and DTG/XRD that this phenomenon can be linked to the low reactivity of PC (absence of portlandite after one and three days in CSA2-25 and CSA3-25). It is assumed that the low compressive strength and the low PC reactivity is due to low anhydrite amounts in CSA2 and CSA3. The presence of gibbsite observed in DTG signals
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(reaction I or II) after six hours, one and three curing days in CSA2-25 and CSA3-25, could slow the hydration of alite and belite which depends on the quantity and nature of the hydrates formed by ye’elimite hydration (gibbsite,
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ettringite, etc.). The importance of the calcium sulfate amount in belite hydration kinetics has already been studied in CSA cements containing ye’elimite, belite and alumino-ferrite [36]. Those low strengths and this lack of PC reactivity after one and three days in the CSA2-25 and CSA3-25 samples lead to the second part of the experimentation which is to probe a constant ye’elimite/anhydrite ratio in the three CSA cements. CSA1 cement was used as a reference, since the CSA1-25 sample did not present a compressive strength stagnation at early ages. Therefore, ye’elimite/anhydrite weight ratio (Y/CS̄̅ ) in CSA2 and CSA3 cements was adjusted to the same weight ratio as that of CSA1 cement (Y/CS̄̅ = 2.3). Anhydrite amount was increased to 20.4 % and 21.9 % for CSA2 and CSA3 cements, respectively. Compressive strength tests were repeated for the CSA2-25 and CSA3-25
samples using the same w/c than before. Results are shown in Figure 7. After six hours there are no data for any of the samples, there is no accelerated hardening in the first hours for the CSA2-25 and CSA3-25 samples, but an upward compressive strength behavior occurs which thus confirms the influence of the Y/CS̄̅ ratio of CSA cements on the compressive strength of mixtures with Portland cement. These results also show that, with the same Y/𝑆̄̅ ratio, the CSA cement type slightly affects the mortar compressive strength. With the cement containing the highest ye’elimite proportion (13.3 % in CSA3-25 (>CS̄̅ )), the compressive strength is 17 % higher on average than CSA1 (containing 8.9 % of ye’elimite). Isothermal calorimetric tests were carried out in one of the samples with the two different Y/CS̄̅ ratios to evaluate the heat flow released by both samples. Figure 8 shows the influence of a higher amount of anhydrite addition on the heat
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flow development of the CSA3-25 sample. Figure 8a shows a zoom-in of the first two hours where it can be seen that with the smallest amount of anhydrite the heat flow shows a shoulder in the curve before one hour. The CSA3-25 with a higher amount of anhydrite does not show any shoulder and both CSA3-25 curves reached almost the same hydration heat flow levels at the same time. This confirmed that the shoulder was related to the anhydrite
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consumption, leading probably to reaction I. Cumulative heat flow increases with the anhydrite proportion in the evaluated sample (Figure 8b) which may explain the low initial strengths. If the Y/CS̄̅ ratio are similar, CSA3-25 (>CS̄̅ )
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and CSA1-25 have similar isothermal calorimetric curves.
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5. Conclusions
This paper has shown that strength stagnation during the first days of hardening associated with a low heat released can be observed for 75%PC-25%CSA cement blends containing the highest proportion of ye’elimite (CSA2 and
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CSA3). Complementary tests with higher anhydrite proportion reveal that the particular strength stagnation between one and three days of these blends was linked to the high ye’elimite/anhydrite ratio. Therefore, conclusions lead to the
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need to adjust the calcium sulfate amount appropriately for each CSA type to avoid mechanical strength stagnation, a modification of hydrate formation at early ages (gibbsite, ettringite, AFm proportions), and a modification of the
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Portland cement reactivity. With an adjusted calcium sulfate and ye’elimite ratio, the CSA cement properties (Blaine, ye’elimite percentage, etc.) seem to affect only the blends properties to a lesser extent. The mechanical behavior of PC at constant w/c ratio is slightly affected by the 25 % addition of any of the three CSA cements, an increase in the compressive strength results was not observed after one day and was not significant after 28 days of curing. There was a dramatic setting time decrease for PC-CSA blends but admixtures could be used to adjust setting and hardening. Additional studies are necessary to investigate the dimensional stability and durability of these PC-CSA blends.
Declaration of interests 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.
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blends of sulfoaluminate-belite cement with Portland cement, vol. 33, no. 2003. 2003, pp. 489–497. D. Gastaldi et al., “Hydraulic Behaviour of Calcium Sulfoaluminate Cement alone and in Mixture with Portland
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Cement,” 13th Int. Congr. Chem. Cem., no. January, pp. 1–7, 2011. G. Le Saoût, B. Lothenbach, A. Hori, T. Higuchi, and F. Winnefeld, “Hydration of Portland cement with additions of calcium sulfoaluminates,” Cem. Concr. Res., vol. 43, no. 1, pp. 81–94, 2013. R. Trauchessec, J. M. Mechling, A. Lecomte, A. Roux, and B. Le Rolland, “Hydration of ordinary Portland
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[22]
cement and calcium sulfoaluminate cement blends,” Cem. Concr. Compos., vol. 56, pp. 106–114, 2015. P. Chaunsali and P. Mondal, “Influence of Calcium Sulfoaluminate (CSA) Cement Content on Expansion and
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[23]
Hydration Behavior of Various Ordinary Portland Cement-CSA Blends,” J. Am. Ceram. Soc., vol. 98, no. 8, pp.
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2617–2624, 2015.
F. Winnefeld and S. Barlag, “Influence of calcium sulfate and calcium hydroxide on the hydration of calcium sulfoaluminate clinker,” ZKG Int., vol. 62, no. 12, pp. 42–53, 2009.
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M. García-Maté, A. G. De La Torre, L. León-Reina, E. R. Losilla, M. A. G. Aranda, and I. Santacruz, “Effect of calcium sulfate source on the hydration of calcium sulfoaluminate eco-cement,” Cem. Concr. Compos., vol. 55, pp. 53–61, 2015.
[26]
ASTM C1437, “Standard Test Method for Flow of Hydraulic Cement Mortar: C1437-01,” ASTM lnternational,
pp. 7–8, 2001. [27]
ASTM C348, “Standard Test Method for Flexural Strength of Hydraulic-Cement Mortars,” ASTM Int. West Conshohocken, PA, vol. 04, pp. 2–7, 1998.
[28]
ASTM C349, “C349-97. Standard Test Method for Compressive Strength of Hydraulic-Cement Mortars (Using Portions of Prisms Broken in Flexure),” Annu. B. ASTM Stand., pp. 1–4, 1997.
[29]
ASTM C778, “Standard Specification for standard sand,” 65.198.187.10, pp. 1–3, 2014.
[30]
ASTM C 1679, “Standard Practice for Measuring Hydration Kinetics of Hydraulic Cementitious Mixtures Using Isothermal Calorimetry 1,” Am. Soc. Test. Mater., vol. 13, p. 15, 2014. ASTM C 1702, “Standard Test Method for Measurement of Heat of Hydration of Hydraulic Cementitious
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[31]
Materials Using Isothermal Conduction Calorimetry,” Annual Book of ASTM Standards, vol. 4, no. 1. p. 9, 2017. [32]
D. Londono-Zuluaga, J. I. Tobón, M. A. G. Aranda, I. Santacruz, and A. G. De la Torre, “Influence of fly ash blending on hydration and physical behavior of belite–alite–ye’elimite cements,” Mater. Struct. Constr., vol. 51,
J. Beretka, M. Marroccoli, N. Sherman, and G. L. Valenti, “The influence of C4A3$ content and ratio on the
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performance of calcium sulfoaluminate-based cements,” Cem. Concr. Res., vol. 26, no. 11, pp. 1673–1681,
[34]
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1996.
L. Senff, A. Castela, W. Hajjaji, D. Hotza, and J. A. Labrincha, “Formulations of sulfobelite cement through
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design of experiments,” Constr. Build. Mater., vol. 25, no. 8, pp. 3410–3416, 2011. F. Bullerjahn, M. Zajac, J. Skocek, and M. Ben Haha, “The role of boron during the early hydration of belite
J. Wang, “Hydration mechanism of cements based on low-CO2 clinkers containing belite , ye ’ elimite and calcium par Hydration mechanism of cements based on low-CO 2 clinkers containing belite , ye ’ elimite and
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[36]
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ye’elimite ferrite cements,” Constr. Build. Mater., vol. 215, pp. 252–263, 2019.
calcium,” Université de Lille, 2011.
Figures Captions Figure 1. Heat flow development: (a) Total hydration time, (b) peak one (first two hours of hydration), (c) cumulative
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heat
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Figure 2. X-Ray diffraction: (a) PC, (b) CSA1-25, (c) CSA2-25, (d) CSA3-25. E: Ettringite/P: Portlandite/Y:
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̅ : Calcite/A: Alite/B: Belite/G: Gibbsite/S: Anhydrite. Ye’elimite/Afm: monosulfoaluminate/Q: Quartz/CC
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Figure 3. DTG curves: (a) PC (b) CSA1-25 (c) CSA2-25 (d) CSA3-25.
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Figure 5. Flexural strength
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Figure 4. Compressive strength
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Figure 6. Compressive strength at constant w/c
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Figure 7. Compressive strength at constant Y/C𝑆̄
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Figure 8. Influence of a higher amount of anhydrite addition on the heat flow development: (a) Heat Flow, first two
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hours of hydration, (b) Cumulative heat
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Table 1. Mineralogical and chemical composition (wt. %) Elemental composition by X-ray fluorescence
Mineralogical phase composition Clinker CSA 2
Clinker CSA 3
26.7
21.6
2.0
2.0
37.9
63.7
69.2
Mayenite
4.0
5.1
4.7
PC
Cement CSA 1
PC
CSA 1
CSA 2
CSA 3
CaO
62.5
43
44
42.7
Alite
58.6
SiO2
18.6
11.7
8.9
6.9
Belite
11.2 22.5
Al2O3
3.8
20.3
31.8
34.1
Ferrite
11.8
Fe2O3
3.5
1.8
2
2
Aluminate
2.8
SO3
2.9
13.4
8.7
9.5
Periclase
0.5
1.7
MgO
2.1
2.9
1.9
2
Calcite
9.9
9.3
TiO2
0.4
0.9
1.4
1.5
Quartz
1.3
1.9
Mn3O4
0.1
0
0
0
Gypsum
3.9
Na2O
0.3
0.2
0.1
0.1
Ye'elimite
K2O
0.2
0.4
0.3
0.2
0.1
0.1
0.1
0.1
Anhydrite
17.1
1.6
1.2
SrO
0.1
0.1
0.1
0.1
CT
0.9
1.0
1.2
ZrO2
0
0
0.1
0.1
Dolomite
4.7
LOI
5.5
5.1
0.7
0.6
Rwp /%
9.6
Table 2. Blaine surface and density Clinker CSA 2
Blaine (cm2/g)
4575
5003
5081
Density (g/cm3)
3.11
2.91
2.89
9.9
9.9
Clinker CSA 3
ANHYDRITE
5395
6622
10.9
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Cement CSA 1
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2.86
2.93
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Table 3. Blend proportions and mineralogical composition (wt. %) Cement CSA 1
Clinker CSA 2
Clinker CSA 3
Anhydrite
C4A3𝐒̄̅
C𝐒̄̅
C3S
C2S
C4AF
CC̄
C3A
C𝐒̄̅ H2
CSA1-25
75
25
-
-
-
9.4
4.2
43.5
13.8
8.8
9.7
2.1
2.9
CSA2-25
75
-
21.6
-
3.4
13.7
3.7
43.5
14
8.8
7.3
2.1
2.9
CSA3-25
75
-
-
21.6
3.4
14.9
3.6
43.5
12.9
8.8
7.3
2.1
2.9
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PC
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Table 4. Water/cement to achieve the same initial flow Sample
w/c
PC
0.52
CSA1-25
0.55
CSA2-25
0.61
CSA3-25
0.57
Table 5. Water demand for plastic consistency and setting time Sample
w/c Plastic Consistency
Setting Time (Min)
PC 100
0.28
155
CSA1-25
0.31
17
CSA2-25
0.35
9
CSA3-25
0.33
11
Table 6. Mass calculated from TGA: (left) portlandite, (right) bound water related to hydrates Portlandite (g/100g anhydrous cement)
Bound water related to hydrates (g/100g anhydrous cement)
1 day
3 days
7 days
28 days
6 hours
1 day
3 days
7 days
28 days
3.6
12.4
17.9
19.8
20.8
3.6
10.0
15.9
17.5
21.1
CSA1-25
1.6
2.1
7.6
9.1
8.7
11.4
11.8
16.9
21.6
23.9
CSA2-25
1.7
2.7
3.3
7.4
11.0
13.8
15.8
17.0
23.0
25.8
CSA3-25
1.7
2.1
3.1
7.2
10.2
13.1
14.6
16.4
23.8
25.4
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6 hours PC
PII:
S2352-4928(19)30176-X
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100778
Reference:
MTCOMM 100778
To appear in:
Materials Today Communications
Received Date:
12 April 2019
Revised Date:
8 November 2019
Accepted Date:
17 November 2019
˜ ´ ´ JI, Lecomte A, Influence Please cite this article as: Bolanos-V asquez I, Trauchessec R, Tobon of the ye’elimite/anhydrite ratio on PC-CSA hybrid cements, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100778
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Manuscript submitted to Materials Today Communications
Influence of the ye’elimite/anhydrite ratio on PC-CSA hybrid cements Bolaños-Vásquez, I.1), Trauchessec, R.2), Tobón, J.I.1), Lecomte, A.2) 1. Grupo del Cemento y Materiales de Construcción (CEMATCO), Calle 75 # 79A-51, Bloque M17-101 Departamento de Materiales y Minerales, Facultad de Minas, Universidad Nacional de Colombia, Medellín, Colombia 2. Institut Jean Lamour, UMR 7198, CP2S, Equipe Matériaux pour le Génie Civil, Université de Lorraine, IUTNB, 54600 Villers-lès-Nancy, France
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corresponding author: [email protected]
Abstract
Portland cement (PC) and Calcium sulfoaluminate cement (CSA) blends are used to produce binders with properties
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as rapid hardening. However, characteristics of calcium sulfoaluminate cements such as chemical and mineralogical composition, particle size distribution, among others, can have an enormous variation and affect the performance of
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the PC-CSA cement mixture. Therefore, current research focus on the mechanical properties and hydration of PCCSA (75 wt%-25wt%) blend cement using three different CSA cements in order to better understand the influence of
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property variations like ye’elimite quantity and ye’elimite/anhydrite ratio in CSA cements on PC-CSA systems. Compressive strength was determined on mortars and hydration was studied on paste using isothermal calorimetry, X-ray diffraction and thermal analysis. Results showed that ye’elimite quantity in CSA used into PC-CSA hybrid
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cements have a large influence in early hardening and hydration. But, beyond the quantity of ye’elimite, findings showed that there is a greater influence of the ye’elimite/anhydrite ratio on the heat release, mineralogy and
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compressive strength of PC-CSA blends. For example, it was found that it is necessary to adjust the sulfate content to avoid compressive strength stagnation. Higher ye’elimite/anhydrite yields to higher mechanical strengths independent
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on the type of CSA.
Keywords: Calcium sulfoaluminate, blended cements, cement hydration, PC-CSA system, PC-CSA blends hybrid cements.
1. Introduction Cement production generates between 5-8 % of global CO2 emissions [1], [2]. Looking for new and less polluting binder options, investigations on alternative cements have been increasing in recent years [3], [4]. Calcium
sulfoaluminate (CSA) cements offer a low-CO2 alternative to Portland cement, since they are produced at lower temperatures and with lower limestone content in their raw materials. CSA cement can be produced at temperatures about 1250 °C, that is to say, 200 ºC less than Portland cement temperatures production [5]. This represents a 25-35 % reduction of carbon dioxide net emissions in comparison to Portland cement, contributing thus toward reducing energy demand and carbon footprint [6]. CSA cements bear a compound named ye’elimite, aka “Klein’s salt” (C4A3S̄̅ ), as the main phase. It has been used as cementitious material since 1960s when it was patented by Alexander Klein as an expansive or shrinkagecompensating addition to PC. These cements are known as type k cements or expansive cements [7], [8]. In China, CSA cements are known as the “third cement series” and were developed in 1970s [9]. Besides ye’elimite, CSA clinker also contains several phases, depending on the initial composition of raw materials, such as belite (C 2S),
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calcium aluminate (CA), mayenite (C12A7), brownmillerite (C4AF), anhydrite (CS̄̅ ), perovskite (CT), and others [10]– [12].
The main raw materials of CSA cement are calcite, clay or bauxite and gypsum as sources of calcium, silica /
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aluminum and sulfur respectively. There are other sources, for example waste from the production of bauxite, socalled "red mud", and plaster, but those alternative sources are not widely used. It is important to note that the high
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cost of bauxite, one of the raw materials of CSA, allows a limited application compared with low-priced Portland cements, and this why CSA cements are being used for special applications and without being intended to replace PC
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[11], [13], [14]. Therefore, hybrid or blended cements come into focus. The Business Council on Sustainable Development (WBCSD, 2009) has shown that the use of blended cement has considerably increased in relation to
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Portland cement in most countries [6].
In CSA cement, ye'elimite hydration can induce the formation of monosulfoaluminate (C4AS̄̅ H12) and gibbsite (AH3),
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ettringite (C6AS̄̅ 3H32) and gibbsite or only ettringite (reaction I, II and III) depending on the calcium sulfate and calcium hydroxide proportions.
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I. C4A3𝑆̄̅ + 18H → C4A𝑆̄̅ H12 + 2AH3
II. C4A3𝑆̄̅ + 2C𝑆̄̅ + 38H → C6A𝑆̄̅ 3H32 + 2AH3 III. C4A3𝑆̄̅ + 8C𝑆̄̅ + 6CH + 90H → 3C6A𝑆̄̅ 3H32 The fundamental behavior of PC-CSA cement blends is based on reaction III involving one mole of ye’elimite being combined with gypsum or anhydrite plus portlandite to produce three moles of ettringite, instead of one, resulting in greater potential for expansion. Additionally, the ettringite crystals in reaction III are smaller than those produced in reaction II [11], [14]. Alite or belite hydration can lead to the formation of various hydrates (C-S-H, strälingite,
portlandite, etc.) depending on the PC and CSA proportions [15], [16]. These type of blends can be used to obtain particular properties (early strength, dimensional stability, etc.) that could be valuable for 3D printing [17] or radioactive waste encapsulation [18]. PC-CSA cement blends (PC-CSA) may change properties, with regard to each individual cement [15], [19]. PC-CSA have a higher mechanical strength than PC at relatively short curing times, e.g. 1 day, and a very low drying shrinkage [20]. In PC-CSA with a very low percentage of CSA (10 %) the hydration products are the same as for the PC, but with a greater amount of ettringite [21]. When CSA cement proportion is modified in the PC-CSA, the hardening rate and mechanisms of hydration (amount and nature of hydrates) changed, and also the higher the amount of CSA the higher the ettringite content and greater compressive strengths at early ages [22]. The increase in CSA cement content in PC-CSA also increased the extent of expansion, even when CSA has low values of ye’elimite [23]. The variation of
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anhydrite content at fixed PC/(CSA+CS̄̅ ) ratio affects the compressive strength and do not strongly influence the hydrate assemblage of the blend [16].
These different studies seem to show that everything in PC-CSA cement blends research is covered, however, it is
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worth mentioning that each of the cited investigations used CSA cements with huge differences is its composition, fineness and other properties. In addition, to the variation of C𝑆̄̅ percentages in the PC-CSA system which also counts,
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this is why it is difficult to compare and define conclusive results. The current research studies the mechanical properties and hydration of PC-CSA blend cement using three different CSA cements into PC-CSA (75%-25%) hybrid
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cement to better understand the possible influence of property variations like ye’elimite quantity and ye’elimite/anhydrite ratio in CSA cements on PC-CSA systems.
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2. Materials and Methods
The raw materials used in this study were one CSA cement and two commercial CSA clinkers produced by Tangshan
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Polar Bear Building Materials in China; a commercial calcium sulfate dihydrate (CS̄̅ H2), synthesized by Bell Chem International S.A. and distributed by Proquimes S.A. in Colombia and a Portland Cement, with limestone addition,
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produced by Argos S.A. in Colombia. Anhydrite was prepared by heating the CS̄̅ H2 in an oven at 700 °C for two hours [24], [25]. X-ray diffraction (XRD) of the resulting product after burning confirmed that anhydrite was obtained. Quantitative X-ray diffraction (QXRD) analysis was employed for determining the mineralogical composition of CSA cement and PC. The elemental by X-ray fluorescence and mineralogical composition by Rietveld method of the raw materials can be found in Table 1. XRD analysis was carried out in a Bruker equipment, in an interval 2𝜃 between 4⁰ and 70⁰ , with a step of 0.02⁰ and an accumulation time of 30 s. TOPAS software was utilized for the Rietveld refinement. The Blaine surface and density are presented in Table 2. The density of the materials was determined by an AccuPyc II 1340 gas pycnometer and Blaine surface test was made complying with standard NF EN 196-6 (201204-01). Anhydrite addition in clinkers CSA2 and CSA3 was 13.5 wt%, to obtain CSA2 and CSA3 cements. It is
important to highlight that CSA1, CSA2 and CSA3 cements correspond to the CSA cement containing an increasing amount of ye’elimite but different ye’elimite/anhydrite ratios. Three mixtures were studied containing 75 wt% PC with 25 wt% of each one of the CSA cements. PC was also studied as reference. Mixtures proportions and mineralogical composition are shown in Table 3. 2.1. Mortars Water/cement (w/c) ratio for the mortars was determined according to ASTM C 1437 [26] by keeping constant the flow at 110±5 %, according to ASTM C 348 (section 3) and ASTM C 349 [27], [28] recommendation related to work with cements other than Portland, thus ensuring good workability and avoiding the use of additives in the elaboration of mortars for subsequent flexion and compression test. Water/cement ratios for all the mixtures and for the reference
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are listed in Table 4. Those w/c ratios provide a workability leading to mortars molding without the need of chemical admixtures. ASTM C 348 and C 349 standards [27], [28] were followed for compression and flexural tests. Prismatic samples of 40x40x160 mm3 were molded, prepared with 450 g of cement, 1350 g of standard sand (ASTM C778 [29]) and water according to Table 4. Compressive and flexural strength measurements were made after 6 hours and 1, 3,
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7, and 28 days. Mortar prisms mixture samples were cured in water at 25 °C, the PC sample in saturated lime water. In order to evaluate the incidence of the w/c on each of the mixtures, compressive strength tests were also performed
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at a constant water to cement ratio (0.5) at ages 1 and 28 days.
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2.2. Pastes
For the different hybrid cements, setting time and plastic consistency tests were carried out according to ASTM C 191 and ASTM C 305, respectively. Conductive calorimetry tests were made using a thermal analysis (TA) instruments
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calorimeter Tam Air Thermostat 90/3116-in accordance with ASTM C1679 and ASTM C1702 [30], [31], and were performed on cement pastes prepared with 3 g of anhydrous cement and 1.8 g of water. An automatic admix device
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was used to mix the powder samples with water inside the calorimeter for 2 min in order to avoid any data loss at the beginning of the reaction. Heat flows were registered for 3 days at 25 ºC. 3 g of sand were used as a reference.
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Mineralogical properties were studied in cement pastes with 0.45 water/cement ratio. The pastes were prepared by hand by mixing the cement blend with the water in a glass beaker container using a glass stir rod for one minute. After curing in hermetic plastic containers, specimens were immersed in acetone for two hours to stop hydration, and dried in an oven at 60 ⁰ C for 24 hours to remove any residual water. Then the samples were crushed and sieved at 75 µm for mineralogical evaluation. The mineralogical composition of the hydrated blends was determined with XRD after 6 hours and 1, 3, 7 and 28 days of hydration. Thermogravimetric analysis tests (TGA) were performed at the same XRD ages in a SETARAM model: TG/ATD 92-16.18 equipment. During TGA and differential thermal analysis (DTG), the samples were heated in an air atmosphere from 20 °C to 1000 °C at the rate of 10 °C/min using a platinum crucible. The weight of the samples was approximately 45 mg.
3. Results and discussion 3.2. Water demand and setting time The water demand (Table 5) for the three PC-CSA hybrid cements increases compared to PC alone. This higher w/c ratio could be linked to the very fast hardening and the higher Blaine surface of the CSA cement and anhydrite (Table 2). Indeed, Table 5 also shows a dramatic decrease in the final setting time, considering that the differences among the PC and PC-CSA cement blends is more than 130 minutes. The CSA1-25 sample has the longer setting time and CSA2-25 and CSA3-25 have quite similar values. 3.2. Isothermal calorimetric analysis Heat flow and cumulative heat for the three blends and for the reference cement are shown in Figure 1. Figure 1a
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depicts the heat flow for durations over 150 hours for all samples, Figure 1b shows a zoom of the first two hours of hydration and Figure 1c presents the cumulative heat. During the first minutes, the PC curve exhibits a first short and intense peak corresponding to the energy released during the dissolution stage by the mixing and the cement wetting processes. After that a “dormant” period of slow reaction (about two hours), an acceleration period and a deceleration
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period are observed. The maximum hydration heat flow is reached after almost 9 hours. The presence of 25 wt% of CSA cement completely modifies the heat flow released during the first hours. During the first 20 minutes, if compared
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with PC, the hydration heat flow for CSA1-25 is more than double and more than quadruple than those for CSA2-25 and CSA3-25, respectively. CSA2-25 and CSA3-25 heat flows are similar and exhibit a second peak after 40 minutes
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of hydration. The first peak (10 minutes) presumably corresponds to ye’elimite and calcium sulfate dissolution with ettringite formation (reaction II) and the second peak for CSA2-25 and CSA3-25 or shoulder (30 minutes) in Figure 1b
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are assumed to result from the formation of AFm phases (reaction I) when sulfate in solution is depleted [11]. This presumption will be analyzed in section 4. This dramatic heat flow release in the first hours is consistent with the amount of ye’elimite in each one of the hybrid cements (Table 1). It can also be noted that the higher the early heat
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flow the lower the setting time (Table 5). Cumulative heat (Figure 1c) for samples CSA2-25 and CSA3-25 is very discontinuous and is lower than PC and CSA1-25 samples until approximately 125 hours, then the cumulative heat is
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higher for the latter two samples. At this time duration, it is also possible to see another peak in the heat flow curve, and at a duration time of 150 hours CSA2-25 and CSA3-25 samples there is a continuing increase of their cumulative heat while CSA1-25 and PC seem to have reached their maximum heat release. This suggests that the compressive strength CSA2-25 and CSA3-25 could continue increasing. XRD and DTG analyses make it possible to further explain the origin of this difference in reactivity. 3.3. X-ray diffraction Figure 2a-d shows the X-Ray diffraction for PC-CSA cement blends after 6 hours, 1, 3, 7, and 28 days of curing. In all the blends, a ye’elimite peak is observed only after six hours, in a lesser intensity in the CSA1-25, as expected, since
this mixture contain less ye’elimite. Moreover, CSA2-25 and CSA3-25, when compared with PC and with CSA1-25, exhibit a lower hydration rate of alite and belite especially after one and three days of hydration. This is consistent with Gastaldi et al and Londono-Zuluaga et al [20], [32]. The main hydration products are ettringite, portlandite and AFm phases. The peaks linked to ettringite are clearly visible even after 6h of hydration and lightly decrease until 28 days. This hydrate is formed by the rapid hydration of ye’elimite and its reaction (reactions II or III) with calcium sulfate (anhydrite and gypsum). After ettringite formation (during the first day), AFm phases (monosulfoaluminate, hemicarboaluminate) are formed and clearly appear after 7 days of hydration. The addition of 25 % CSA cement to the PC causes a marked decrease in the intensity of the portlandite peaks. This decrease can be associated to the lower PC proportion, lower hydration rate of alite in blended cements and reaction III. In blends containing CSA2 or CSA3, portlandite formation occurs only after 7 days of
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hydration whereas it’s formed after 3 days in blends with CSA1, it is assumed that this behavior is related to ye’elimite contents. 3.4. Thermal Analysis
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Derivative thermogravimetric signals (DTG) are presented in Figure 3a-d and Table 6 shows the mass of portlandite and the loss of water calculated from TGA data, which correspond to chemically bound water in hydrated phases. The
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range of temperatures taken for calculations were 400-500 and 50-550 °C, respectively. The presence of C-S-H (≈120 °C), ettringite (≈135 °C), AFm phases (180-220 °C), gibbsite (220-280 °C), portlandite (400-500 °C) and calcite (600-
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700 °C) is revealed by the signals (Figure 3a-d). For PC or CSA1-25 (Figure 3a-b), portlandite begins to be detected by a weight loss at 450 ºC signals after six hours. For CSA2-25 and CSA3-25 portlandite appears later (after 7d), this
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may confirm the presence of III-type reactions or/and slower PC hydration kinetics. Portlandite proportions for CSA225 and CSA3-25 samples are below 3.5 % before 7 days (Table 6), thus confirming XRD results. These analyses also confirmed that the portlandite amount decreases with an increase in ye’elimite content as can be seen in comparing
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Figure 3b for CSA1-25 and Figure 3c-d for CSA2-25 and CSA3-25. The observations made in the XRD analysis regarding AFm phases can be confirmed in the DTG signals and become apparent especially in the blends with a
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higher ye'elimite amount (CSA2-25 and CSA3-25). DTG also reveals the presence of a small amount of amorphous gibbsite after 6 hours, 1 and 3 days in CSA2-25 and CSA3-25 samples but not in CSA1-25. This result indicates that the hydration mechanism of ye’elimite in CSA1 may preferentially go through reaction III, while those of CSA2 and CSA3 follow reactions I and II. This may be due to the higher reactivity of PC in CSA1-25 blend. 3.5. Mechanical properties Compressive strength results for PC, CSA1-25, CSA2-25 and CSA3-25, prepared with w/c indicated in Table 4, after 6 hours as well as after 1, 3, 7 and 28 days are shown in Figure 4. It is possible to note that there are only compressive strength data for samples CSA2-25 and CSA3-25 after 6 hours and that both results are comparable. For PC and
CSA1-25 samples were impossible to measure a trustable value. After one day, mechanical strength in CSA2-25 and CSA3-25 samples remains practically equal as after six hours, and only a slight increase appears after three days. This behavior is atypical given that mechanical strength at early ages strongly increases with higher amounts of C 4A3𝑆̄̅ [33], [34], however, the rapid setting time could be the cause [35]. After one day, PC shows a higher strength data than all the blends. Compared with PC, the lower strength of PC-CSA blends (decrease of 7-11 MPa) after 28 days of hydration can be partially explained by the higher w/c ratio used for keeping constant flow (Table 4). CSA1-25 shows higher strength data after 7 and 28 days when comparing it with the other two blends which have a similar behavior. In spite of having less amount of ye’elimite, this could be due to i) lower w/c ratio and/or ii) slightly higher initial setting time [35]. This atypical behavior could be explained also by a deficit in ye’elimite/anhydrite ratio, this hypothesis will be evaluated in section 4. Flexural strength data in Figure 5 shows the same behavior than compressive strength.
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Compressive strength results for mortars with constant water-cement ratio 0.5 (Figure 6) show that after one day of curing PC and CSA1-25 sample had a very similar result, while samples CSA2-25 and CSA3-25 had a decrease close to 50 %, it is assumed that this decrease is due to lack in anhydrite amount in CSA2 and CSA3 cements. However, at
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28 curing days both in PC and in the three mixtures give statistically equal results.
4. Influence of the calcium sulfate proportion
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The first part of this paper showed that an addition of 25 % of CSA cement in PC-CSA blends can accelerate the setting time and strength after few hours. However, even if the ye’elimite amount is higher as in the case of samples
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CSA2-25 and CSA3-25 in comparison to CSA1-25, compressive strength after one and three curing days can be lower and remain stagnant. This is the opposite to what is usually expected. It has been confirmed by isothermal
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calorimetry and DTG/XRD that this phenomenon can be linked to the low reactivity of PC (absence of portlandite after one and three days in CSA2-25 and CSA3-25). It is assumed that the low compressive strength and the low PC reactivity is due to low anhydrite amounts in CSA2 and CSA3. The presence of gibbsite observed in DTG signals
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(reaction I or II) after six hours, one and three curing days in CSA2-25 and CSA3-25, could slow the hydration of alite and belite which depends on the quantity and nature of the hydrates formed by ye’elimite hydration (gibbsite,
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ettringite, etc.). The importance of the calcium sulfate amount in belite hydration kinetics has already been studied in CSA cements containing ye’elimite, belite and alumino-ferrite [36]. Those low strengths and this lack of PC reactivity after one and three days in the CSA2-25 and CSA3-25 samples lead to the second part of the experimentation which is to probe a constant ye’elimite/anhydrite ratio in the three CSA cements. CSA1 cement was used as a reference, since the CSA1-25 sample did not present a compressive strength stagnation at early ages. Therefore, ye’elimite/anhydrite weight ratio (Y/CS̄̅ ) in CSA2 and CSA3 cements was adjusted to the same weight ratio as that of CSA1 cement (Y/CS̄̅ = 2.3). Anhydrite amount was increased to 20.4 % and 21.9 % for CSA2 and CSA3 cements, respectively. Compressive strength tests were repeated for the CSA2-25 and CSA3-25
samples using the same w/c than before. Results are shown in Figure 7. After six hours there are no data for any of the samples, there is no accelerated hardening in the first hours for the CSA2-25 and CSA3-25 samples, but an upward compressive strength behavior occurs which thus confirms the influence of the Y/CS̄̅ ratio of CSA cements on the compressive strength of mixtures with Portland cement. These results also show that, with the same Y/𝑆̄̅ ratio, the CSA cement type slightly affects the mortar compressive strength. With the cement containing the highest ye’elimite proportion (13.3 % in CSA3-25 (>CS̄̅ )), the compressive strength is 17 % higher on average than CSA1 (containing 8.9 % of ye’elimite). Isothermal calorimetric tests were carried out in one of the samples with the two different Y/CS̄̅ ratios to evaluate the heat flow released by both samples. Figure 8 shows the influence of a higher amount of anhydrite addition on the heat
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flow development of the CSA3-25 sample. Figure 8a shows a zoom-in of the first two hours where it can be seen that with the smallest amount of anhydrite the heat flow shows a shoulder in the curve before one hour. The CSA3-25 with a higher amount of anhydrite does not show any shoulder and both CSA3-25 curves reached almost the same hydration heat flow levels at the same time. This confirmed that the shoulder was related to the anhydrite
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consumption, leading probably to reaction I. Cumulative heat flow increases with the anhydrite proportion in the evaluated sample (Figure 8b) which may explain the low initial strengths. If the Y/CS̄̅ ratio are similar, CSA3-25 (>CS̄̅ )
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and CSA1-25 have similar isothermal calorimetric curves.
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5. Conclusions
This paper has shown that strength stagnation during the first days of hardening associated with a low heat released can be observed for 75%PC-25%CSA cement blends containing the highest proportion of ye’elimite (CSA2 and
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CSA3). Complementary tests with higher anhydrite proportion reveal that the particular strength stagnation between one and three days of these blends was linked to the high ye’elimite/anhydrite ratio. Therefore, conclusions lead to the
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need to adjust the calcium sulfate amount appropriately for each CSA type to avoid mechanical strength stagnation, a modification of hydrate formation at early ages (gibbsite, ettringite, AFm proportions), and a modification of the
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Portland cement reactivity. With an adjusted calcium sulfate and ye’elimite ratio, the CSA cement properties (Blaine, ye’elimite percentage, etc.) seem to affect only the blends properties to a lesser extent. The mechanical behavior of PC at constant w/c ratio is slightly affected by the 25 % addition of any of the three CSA cements, an increase in the compressive strength results was not observed after one day and was not significant after 28 days of curing. There was a dramatic setting time decrease for PC-CSA blends but admixtures could be used to adjust setting and hardening. Additional studies are necessary to investigate the dimensional stability and durability of these PC-CSA blends.
Declaration of interests 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.
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J. Wang, “Hydration mechanism of cements based on low-CO2 clinkers containing belite , ye ’ elimite and calcium par Hydration mechanism of cements based on low-CO 2 clinkers containing belite , ye ’ elimite and
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Figures Captions Figure 1. Heat flow development: (a) Total hydration time, (b) peak one (first two hours of hydration), (c) cumulative
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Figure 2. X-Ray diffraction: (a) PC, (b) CSA1-25, (c) CSA2-25, (d) CSA3-25. E: Ettringite/P: Portlandite/Y:
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̅ : Calcite/A: Alite/B: Belite/G: Gibbsite/S: Anhydrite. Ye’elimite/Afm: monosulfoaluminate/Q: Quartz/CC
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Figure 3. DTG curves: (a) PC (b) CSA1-25 (c) CSA2-25 (d) CSA3-25.
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Figure 5. Flexural strength
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Figure 4. Compressive strength
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Figure 6. Compressive strength at constant w/c
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Figure 7. Compressive strength at constant Y/C𝑆̄
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Figure 8. Influence of a higher amount of anhydrite addition on the heat flow development: (a) Heat Flow, first two
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hours of hydration, (b) Cumulative heat
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Table 1. Mineralogical and chemical composition (wt. %) Elemental composition by X-ray fluorescence
Mineralogical phase composition Clinker CSA 2
Clinker CSA 3
26.7
21.6
2.0
2.0
37.9
63.7
69.2
Mayenite
4.0
5.1
4.7
PC
Cement CSA 1
PC
CSA 1
CSA 2
CSA 3
CaO
62.5
43
44
42.7
Alite
58.6
SiO2
18.6
11.7
8.9
6.9
Belite
11.2 22.5
Al2O3
3.8
20.3
31.8
34.1
Ferrite
11.8
Fe2O3
3.5
1.8
2
2
Aluminate
2.8
SO3
2.9
13.4
8.7
9.5
Periclase
0.5
1.7
MgO
2.1
2.9
1.9
2
Calcite
9.9
9.3
TiO2
0.4
0.9
1.4
1.5
Quartz
1.3
1.9
Mn3O4
0.1
0
0
0
Gypsum
3.9
Na2O
0.3
0.2
0.1
0.1
Ye'elimite
K2O
0.2
0.4
0.3
0.2
0.1
0.1
0.1
0.1
Anhydrite
17.1
1.6
1.2
SrO
0.1
0.1
0.1
0.1
CT
0.9
1.0
1.2
ZrO2
0
0
0.1
0.1
Dolomite
4.7
LOI
5.5
5.1
0.7
0.6
Rwp /%
9.6
Table 2. Blaine surface and density Clinker CSA 2
Blaine (cm2/g)
4575
5003
5081
Density (g/cm3)
3.11
2.91
2.89
9.9
9.9
Clinker CSA 3
ANHYDRITE
5395
6622
10.9
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Cement CSA 1
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P2O5
2.86
2.93
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Table 3. Blend proportions and mineralogical composition (wt. %) Cement CSA 1
Clinker CSA 2
Clinker CSA 3
Anhydrite
C4A3𝐒̄̅
C𝐒̄̅
C3S
C2S
C4AF
CC̄
C3A
C𝐒̄̅ H2
CSA1-25
75
25
-
-
-
9.4
4.2
43.5
13.8
8.8
9.7
2.1
2.9
CSA2-25
75
-
21.6
-
3.4
13.7
3.7
43.5
14
8.8
7.3
2.1
2.9
CSA3-25
75
-
-
21.6
3.4
14.9
3.6
43.5
12.9
8.8
7.3
2.1
2.9
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Table 4. Water/cement to achieve the same initial flow Sample
w/c
PC
0.52
CSA1-25
0.55
CSA2-25
0.61
CSA3-25
0.57
Table 5. Water demand for plastic consistency and setting time Sample
w/c Plastic Consistency
Setting Time (Min)
PC 100
0.28
155
CSA1-25
0.31
17
CSA2-25
0.35
9
CSA3-25
0.33
11
Table 6. Mass calculated from TGA: (left) portlandite, (right) bound water related to hydrates Portlandite (g/100g anhydrous cement)
Bound water related to hydrates (g/100g anhydrous cement)
1 day
3 days
7 days
28 days
6 hours
1 day
3 days
7 days
28 days
3.6
12.4
17.9
19.8
20.8
3.6
10.0
15.9
17.5
21.1
CSA1-25
1.6
2.1
7.6
9.1
8.7
11.4
11.8
16.9
21.6
23.9
CSA2-25
1.7
2.7
3.3
7.4
11.0
13.8
15.8
17.0
23.0
25.8
CSA3-25
1.7
2.1
3.1
7.2
10.2
13.1
14.6
16.4
23.8
25.4
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6 hours PC