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Effect of curing temperature and relative humidity on the hydrates and porosity of calcium sulfoaluminate cement
Construction and Building Materials 213 (2019) 627–636
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of curing temperature and relative humidity on the hydrates and porosity of calcium sulfoaluminate cement Lin Li a, Ru Wang a,b,⇑, Shaokang Zhang a a b
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai 201804, China
h i g h l i g h t s The formation of ettringite is favor of curing with low temperature and high RH. The generation of AH3 and AFm is promoted with curing temperature and RH rising. The porosity of CSA cement paste is decreased by curing with low temperature and high RH.
a r t i c l e
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a b s t r a c t
Article history: Received 29 November 2018 Received in revised form 16 March 2019 Accepted 8 April 2019
The main objective of this study was to investigate the effect of curing temperature and relatively humidity (RH) on the hydrates and porosity of calcium sulfoaluminate (CSA) cement pastes. A series of experiments including X-ray diffraction (XRD), thermogravimetric (TG) and mercury intrusion porosimetry (MIP) were performed. The results indicate that the formation of hydration products and pore characteristics of CSA cement are dependent on the curing conditions. The formation of ettringite is favor of curing with low temperature and high RH. The rate of formation of aluminum hydroxide (AH3) and monosulfoaluminate (Ms) is well promoted when curing temperature and RH are rising. The total porosity of CSA cement paste is gradually decreased by curing with low temperature and high RH. Ó 2019 Elsevier Ltd. All rights reserved.
Keywords: Calcium sulfoaluminate cement Curing conditions Hydrates Porosity
1. Introduction Calcium sulfoaluminate (CSA) cement has drawn a great attention from academia and industry during the last decades due to its considerably environmental and technical benefits when in comparison with ordinary Portland cement (OPC) [1]. CSA cement is fabricated by calcination of bauxite, limestone and gypsum at round 1250 °C, which is 200 °C lower than that of OPC clinker
[2]. CSA cement mainly consists of ye’elimite (C4A3S, also known
as Klein’s compound), belite (C2S), and calcium sulfate (CS) as main components with other minor phases including calcium aluminates (CA, C12A7), brownmillerite (C4AF) and gehlenite (C2AS) which are mainly dependent on the sources of the raw materials [3–5] (hereafter cement chemistry shorthand notation will be
used: C = CaO, S = SiO2, A = Al2O3, S = SO3, H = H2O, F = Fe2O3, M = MgO and T = TiO2).
The hydration kinetics and phases formed upon hydration of CSA cement are mainly dependent on the amounts as well as the reactivity of the added calcium sulfates (anhydrite, hemihydrate or gypsum), which have been already extensively indicated in previous studies [6–8]. Ye’elimite will react with calcium sulfate
immediately to form ettringite (C6AS3H32) according to Eq. (1), AH3 gel is also generated at this stage [9]. Compared to higher reactive calcium sulfate such as hemihydrate or gypsum, the dissolution kinetics of anhydrite is much slower and resulting in an undersupply of calcium and sulfate ions with respect to the formation of ettringite [2,10,11]. The replacement of a part of the anhydrite by gypsum is able to enhance hydration kinetics and both early and late compressive strength [12]. However, some other literatures reported that anhydrite pastes showed slightly larger ettringite contents at later ages and hence, mortars with slightly higher compressive strengths [13].
C4 A3 S þ2C S þ38H ! C6 AS3 H32 þ 2AH3 ⇑ Corresponding author at: School of Materials Science and Engineering, Tongji University, Shanghai 201804, China. E-mail address: [email protected] (R. Wang). https://doi.org/10.1016/j.conbuildmat.2019.04.044 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ð1Þ
When calcium sulfate is completely consumed or only in the presence of water, ye’elimite hydrates to Ms and AH3 according to Eq. (2) [13].
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C4 A3 S þ18H ! C4 A S H12 þ 2AH3
2. Experimental
ð2Þ
The hydration of CSA cement is accelerated and accordingly more ettringite will be formed in presence of calcium hydroxide (CH) in accordance with Eq. (3) [14].
C4 A3 S þ8C S þ6CH þ 74H ! C6 AS3 H32
2.1. Materials A typical rapid hardening CSA cement was employed for all experiment. Its chemical compositions obtained via X-ray fluorescence spectrometry (XRF) are shown in Table 1. The XRD pattern of CSA is shown in Fig. 1. The mineral phases of the CSA cement were achieved by XRD quantitative analysis through X’Pert HighScorePlus software. The mineralogical compositions are shown in
ð3Þ
Belite starts to react with AH3 formed in reactions (1) and (2) and free water to give rise to strätlingite (C2ASH8) formation according to Eq. (4) after calcium sulfate depletion [15].
C2 S þ AH3 þ 5H ! C2 ASH8
Table 2. The main compounds are ye’elimite (C4A3S), belite
ð4Þ
(b-C2S) and anhydrite (CS), with brownmillerite (C4AF), mayenite (C12A7), perovskite (CT) and dolomite (CaMg(CO3)2) as minor compounds. It has a Blaine fineness of 5350 cm2/g. The particle size distribution is illustrated in Fig. 2. Ultrapure water (resistivity = 18.0 MXcm) was used for all the experiments except particle size distribution measurement for cement which was dispersed by high purity ethanol.
Strätlingite will react with the remaining belite phase to give rise to siliceous-hydrogarnet along with C-S-H hydrates [16]. After strätlingite is completely consumed and in the case of remaining belite phase and free water, the hydrates of C-S-H and portlandite (CH) will be generated [16]. Depending on the composition of the minor phases, other hydration products may also be observed, such as hemicarboaluminate (Hc) and monocarboaluminate (Mc) [17,18]. The hydration evolution as well as properties of CSA cement is significantly influenced by curing conditions. A significantly later initial and final setting, a longer dormant period in the calorimetric curves and much lower strength up to 1 day of CSA cement pastes cured at 5 °C are clearly observed compared to 20 °C [5]. Generally, curing temperatures do not change the type of hydration products but do affect their amounts [19]. With curing temperature rising, the rate of formation of ettringite as well as the hydration degree of clinker minerals within 24 h are well promoted, accordingly the early-age compressive strength increases [19,20]. Meanwhile, the proportion of ettringite versus Ms hydrates is decreased due to elevated curing temperature, indicating that high amount of Ms is formed, especially at low initial gypsum contents. This can be explained by that the increased curing temperature improved the stability of Ms against ettringite which required more gypsum to be stabilized [4]. However, low temperature (<20 °C) is in favor of a denser and more homogeneous matrix due to continuous substantial hydration [19,21]. Despite of the above limit literatures, most of studies are focused on the influence of curing temperature on the early hydration and properties of CSA cement composites, while little is known on the effect of humidity on the hydration of CSA cement. And the further study on the coupling effect between temperature and humidity on the hydration and properties of CSA cement is deficient. However, the applications of cement may undergo various external curing conditions. Thus, it should be meaningful to study the influence of curing regimes on the hydration and properties of CSA cement. In order to better understand the influence of curing regimes on the hydration of CSA cement, the effect of curing temperature and relative humidity on the hydrates and porosity of CSA cement pastes was investigated. One commercial CSA cement with water to cement ratio (w/c) of 0.4 was taken to prepare CSA cement paste. Three temperatures (5 °C, 20 °C and 40 °C) and four RH varying from 30%, 60% and 95% controlled with saturated salt solution and water were considered for cement specimens curing. XRD and TGA were employed to identify the hydration products. MIP was used to determine the pore size distribution.
2.2. Preparation and curing of samples The CSA cement and water used for cement paste preparation as well as casting moulds were pre-cured at temperatures of 5 °C, 20 °C or 40 °C for 24 h. The CSA cement pastes were prepared at a constant water to cement ratio (w/c) of 0.4. CSA cement was weighted into the corresponding water within 10 s and mixed for 30 s at low speed, then the mixtures were blended at high speed for another 90 s. Afterwards, the CSA cement paste was immediately casted into the 20 mm 20 mm 20 mm steel moulds after mixing. These moulds with cement pastes were quickly transferred into sealed glass desiccators which contained saturated salt solution to achieve constant RH, then these glass desiccators were conditioned in environmental chambers for constant temperatures. The specimens were demoulded after 24 h and then stored back to the same curing conditions to the age of 28 days. Afterwards, the samples were taken out for following measurement. The constant RH was achieved in a glass desiccator according to ASTM E104-02 [22]. The saturated salt solutions were prepared under three curing temperatures by using reagent grade chemicals including magnesium chloride (MgCl2), sodium bromide (NaBr) and potassium sulfate (K2SO4) to achieve three different levels of
Fig. 1. XRD pattern of CSA cement.
Table 1 Chemical composition of CSA cement (w %). Al2O3
CaO
SO3
SiO2
Fe2O3
K2O
MgO
Mn2O3
Na2O
P2O5
SrO
TiO2
LOI
23.84
44.20
15.04
9.82
1.95
0.26
2.46
0.02
0.08
0.11
0.10
0.95
1.17
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RH, as shown in Table 3. In this study, the three different levels of RH were labeled as 30%RH, 60%RH and 95%RH. At the same time, water curing was also considered for further comparison analysis. In this case, the cement paste for water curing was firstly cured at 95%RH then demoulded to store in water.
3. Results and discussion 3.1. XRD analysis
2.3. Characterization The slices taken from the inner of small cube specimens were immediately broken slightly and immersed into ethanol to stop the hydration. Subsequently, the materials were rinsed with diethyl ether and shortly dried at 40 °C. Then some of the samples were ground to completely pass a 100 lm sieve for XRD and TG analysis while some others without grinding were taken to conduct MIP measurement. 2.3.1. XRD analysis XRD measurement was performed by Rigaku equipment (model D/max2550VB3+). The powdered samples were scanned between 5° and 50° with a 2h increment 0.02° per step, and a dwell time of 4 s and a Cu Ka radiation were applied. 2.3.2. TG analysis TG analysis was carried out on 20 ± 2 mg of the powdered samples using a Netzsch TGA 209F1 Instrument under nitrogen atmosphere. The samples were heated from ambient temperature to 1000 °C with a ramp rate of 10 °C/min. The DTG curve was obtained after derivation to demonstrate various peaks corresponding to the dehydration of different phases. TG analysis was employed to determine the main hydrates such as ettringite and AH3 semi-quantitatively. Ettringite content was calculated by the weight loss between 50 °C and 150 °C corresponding to 20 molecules of crystal water per molecule of ettringite. AH3 content was achieved by the weight loss between 230 °C and 300 °C (corresponding to 3 molecules of crystal water per molecule aluminum hydroxide). The amounts of ettringite, AH3 were roughly estimated due to the overlaps among these hydration products in the range of temperature described above. Ms was not calculated based on TG data due to strongly overlaps from Ms, Hc, CAH10 and strätlingite. 2.3.3. MIP analysis Mercury intrusion porosimeter was used to measure the porosity characteristics of CSA cement pastes. The pore diameter was calculated using Washburn equation [23] according to Eq. (7).
d¼
accordance with EN 196-1 although the specimen size is different [24].
4cCOSh P
ð7Þ
where d is the pore diameter, c is the surface tension of mercury (0.480 N/m), h is the contact angle between mercury and cement (140°), and P is the applied pressure (MPa). MIP was carried out on Quantachrome Poremaster GT-60 with maximum and minimum applied pressures as 420 MPa and 1.5 kPa, corresponding to approximately minimum pore size of 3.5 nm and maximum pore size of 400 lm. 2.3.4. Compressive strength At the curing age of 28 days, the small cubic specimens (20 mm 20 mm 20 mm) were taken out from curing container for compressive strength measurement. The testing process is in
XRD patterns of CSA cement pastes cured at different conditions after 28 days are shown in Fig. 3. It can be seen from Fig. 3(a) that when curing temperature is 20 °C, the dominant hydration product of CSA cement is ettringite while other two hydrates Ms and Hc are also generated. Different RH for CSA cement curing leads to different hydration degree, which can be reflected by the depletion speed of ye’elimite as well as the formation of various hydration products. It can be found that when RH increases from 30% to water curing, the main hydration product ettringite seems to keep the same level. However, the formation of Ms and Hc is quite different. Ms favors to be formed when CSA cement is cured under high RH. The trend of Hc formation differs from Ms. Hc tends to be generated at RH of 30% and 60%. The effect of RH on the hydration products of CSA cement curing at 5 °C is different from that at 20 °C. CSA cements cured under 95%RH and water show the quicker depletion rate of ye’elimite, and finally generate higher ettringite content when in comparison with other curing RH. The lowest ettringite is generated when CSA cement is cured at 30%RH. It is interesting to find from Fig. 3(b) that the peak at 2h of 12.4° appears when the CSA cement is cured under 95%RH and water. This is attributed to the presence of CAH10 which is formed according to Eq. (8) [25–27]. Such phase favors to be presence at low temperature and tends to be transformed to other hydrates when subjected to higher temperature [4]. In literatures [12,28], traces of CAH10 occur as intermediate hydration product after 28 days and disappear again at longer hydration times.
3C4 A3 S þ98H ! C6 AS3 H32 þ 6CAH10 þ 2AH3
ð8Þ
Similar as curing temperature at 5 °C, the higher amount of ettringite tends to be generated when CSA cement is cured under higher RH at 40 °C, which can be seen from Fig. 3(c). High RH is also benefit to the generation of Ms. The formation of Hc can be detected for all cases while the biggest amount is happened for curing at 60%RH, which is the same as that at 20 °C. There are another two peaks shown at 2h around 7.0° and 21.3° appear in the CSA cement cured in water. This is mainly attributed to the presence of strätlingite, which is ascribed to the reaction between belite, aluminum hydroxide and water (see Eq. (4)). Generally, strätlingite favors to be generated at higher w/c (>0.6) or high curing RH (e.g. water curing) [2,12]. The effect of temperature on the hydrates of CSA cement subjected to various RH is also significant. It can be observed from Fig. 3(a)–(c) that the curing conditions with high curing temperature and low or middle RH are benefit to the formation of Hc. The highest intensity of the peak corresponding to Hc is from 40 °C and 60%RH, as seen from Fig. 3(c), indicating the biggest formation of Hc at this condition. When curing RH is more than 30%, the highest amount of ettringite tends to be generated at 5 °C and the lowest happens at 40 °C according to the XRD peak intensity. In this case, the proportion of ettringite versus Ms hydrates is
Table 2 Mineral composition of CSA cement (w %).
C4A3S
C2S
CS
C4AF
C12A7
CT
CaMg(CO3)2
MgO
40.1
29.9
16.2
5.2
3.5
1.1
1.7
2.2
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Fig. 2. Particle size distribution of CSA cement.
decreased due to elevated curing temperature [4], indicating that higher amount of ettringite is transformed to Ms. As previous introduced, CAH10 only happens at the conditions with low temperature (5 °C) and high RH (95%RH and water curing). Contrary to CAH10, the highest amount of Ms is generated at high temperature (40 °C) and high RH (e.g. 95%RH and water curing). Ye’elimite is the dominant mineral of the CSA cement used in our study, the above hydration products of CSA cement are mainly formed via the reaction between ye’elimite and other phases. Accordingly, the hydration degree of CSA cement can be estimated by the depletion rate of ye’elimite [29]. Fig. 4 illustrates the main peak intensity of ye’elimite at various curing conditions. It can be seen that curing with higher temperature and higher RH leads to lower intensity of ye’elimite. The lowest intensity of ye’elimite from CSA cement paste is from 40 °C and water curing while the highest is from 5 °C and 30%RH. Thus this curing condition (40 °C and water curing) can be estimated to reach the highest degree of hydration for CSA cement. 3.2. TG analysis Thermogravimetry (TG) analysis is a powerful technique that has been extensively used to analyze CSA cements and their hydrates. The main hydrates ettringite, Ms and even amorphous phase AH3 could be detected and quantified from their structure water loss at specific temperature ranges [2]. The TG/DTG curves of CSA cement pastes cured at different conditions are plotted in Fig. 5. It can be seen that there are several weight loss peaks appearing in the DTG curves indicating different hydrates of CSA cement. The main derivative peak in the range of 100–130 °C corresponds to the water removal from ettringite, which is significantly influenced by various curing conditions. The second peak happens in the 170–200 °C temperature range and can be ascribed to AFm phases including Ms as well as Hc. The DTG peaks of Ms and Hc are strongly overlapped in this range, thus it is hardly to quantify Ms and Hc separately based on TG results. The third peak is from AH3 phase, which loses its bound water at around 250–280 °C. However, there is no visible peak at
450–550 °C, which would be the indicator for the decomposition of calcium hydroxide [30]. Thus, it can be inferred that the hydration products of CSA cement at 28 days in this study do not contain calcium hydroxide or calcium hydroxide has been already consumed completely according to reaction in Eq. (3). Another two peaks found in the temperature range of 700–900 °C can be ascribed to the decomposition of dolomite from original cement. It can be observed from Fig. 5(a) that the intensity of DTG peak displayed at around 130 °C from water curing is the highest, meaning the biggest amount of ettringite generated from the CSA cement with water curing at 20 °C. The rest DTG peaks at this temperature have similar intensities indicating the same level content of ettringite. The DTG peaks in the range of 170–200 °C from CSA cement cured under 95%RH and water are more significant than those cured under low RH, corresponding to the formation of AFm phases. The results here are well consistent with that from XRD analysis. When temperature decreases to 5 °C (see Fig. 5(b)), the trend of DTG peak intensity is quite different from those cured at 20 °C, as above described. It can be found that the intensity of DTG peaks at 100–150 °C increases with curing RH increasing. In this case, the higher curing RH is benefit to the formation of ettringite. The lowest ettringite content is from CSA cement curing at 30%RH which is also consistent with XRD result. The DTG peaks in the range of 170–200 °C are small indicating only little AFm phases have formed under such curing conditions. It is found from Fig. 5(c) for curing temperature of 40 °C that the strongest intensity of DTG peak for ettringite is from 95%RH. The results are well consistent with that from XRD analysis. It could be also seen that the intensity of AH3 peaks in the range of 250– 280 °C from various curing RH at 40 °C (Fig. 5(c)) is stronger than that at 20 °C (Fig. 5(a)) and 5 °C (Fig. 5(b)), which means that AH3 favors to be generated at high temperatures. The hydration product of ettringite from CSA cement pastes cured under various conditions were calculated based on TG data and the results are shown in Fig. 6(a). Generally, the effect from curing temperature on the formation of ettringite is very significant. The maximum amount of ettringite is from 20 °C while the lowest is from 40 °C when CSA cement paste was cured with low RH (30%). When the RH reaches 60% or more, the formation of ettringite is decreased with curing temperature increasing. Compared to that from CSA cement pasted curing at 20 °C, much more ettringite is formed from CSA cement paste when curing temperature is 5 °C. The effect of RH on the formation of ettringite is different when curing temperature is different. When curing temperature is 5 °C, elevated RH leads to sharply generation of ettringite. When curing temperature reaches 20 °C and 40 °C, the ettringite content is quite similar for those CSA cement pastes cured at 30%RH and 60%RH. The formation of ettringite is increased with curing RH increasing when the RH is more than 60%. Generally, the effect from curing RH on the formation of ettringite is very significant when curing temperature is 5 °C A positive correlation between the formation of AH3 and curing temperature as well as RH can be seen in Fig. 6(b). Generally, high curing temperature and RH are beneficial for the generation of AH3. The biggest amount of AH3 is generated from CSA cement paste
Table 3 Relative humidity of saturated salt solution for specimens cured under various temperatures [22]. Code
30%RH
60%RH
95%RH
Water
Salt solution 5 °C 20 °C 40 °C
MgCl2 33.6 ± 0.3% 33.1 ± 0.2% 31.6 ± 0.2%
NaBr 63.5 ± 0.8% 59.1 ± 0.5% 53.2 ± 0.5%
K2SO4 98.5 ± 1.0% 97.6 ± 0.6% 96.4 ± 0.4%
– – – –
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Fig. 3. XRD profiles of CSA cement pastes cured under various relative humidities until 28 days: (a) 20 °C, (b) 5 °C and (c) 40 °C (E: ettringite, Ms: monosulfoaluminate, Hc: hemicarboaluminate, C: CAH10, S: strätlingite, Y: ye’elimite, A: anhydrite).
not significant. Besides of ettringite, some other hydrates eg. AFm phases and CAH10 are also formed by the reaction of ye’elimite. However, a systematic law can be seen that much more ye’elimite is consumed and less ettringite is generated with curing temperature increasing. The relation between AH3 and the depletion of ye’elimite is illustrated in Fig. 7(b). As the hydration proceeds, ye’elimite is gradually depleted. Accordingly, more and more AH3 is formed according to Eqs. (1) and (2). Curing with higher temperature and RH tends to promote hydration kinetics by consuming more ye’elimite and forming higher amount of AH3. 3.3. MIP analysis
Fig. 4. XRD peak intensity of ye’elimite at 2h value around 23.6° in XRD patterns from CSA cement pastes cured with various conditions.
cured at 40 °C and water, while the lowest is from that cured at 5 °C and 30%RH or 60%RH. Fig. 7(a) shows the relation between generated ettringite and the depletion of ye’elimite. It can be found their correlation is
The cumulative pore size distribution (CPSD) of CSA cement pastes cured at various temperatures and RH investigated in this work are plotted in Fig. 8. Generally, the main pore size of CSA cement pastes is within 1 lm. With curing RH increasing to 95% and water curing, CSA cement pastes tend to gain lower main pore size (<0.1 lm), the effect seems to be much more significant for those CSA cement pastes cured at 20 °C. Meanwhile, CSA cement pastes cured under high RH have systematic lower CSPD than those under low RH, and this trend is much more obvious when RH is more than 60%. It can be also found from Fig. 8 that higher curing temperature would induce higher CPSD to CSA cement pastes.
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Fig. 5. TG profiles of CSA cement pastes cured under various relative humidities until 28 days: (a) 20 °C, (b) 5 °C and (c) 40 °C (E: ettringite, Ms: monosulfoaluminate, Hc: hemicarboaluminate, S: strätlingite, AH3: aluminum hydroxide).
Fig. 6. The generation of (a) ettringite and (b) AH3 in CSA cement pastes cured at different conditions until 28 days. The estimated error of the measurement was ± 2.0%.
The total porosity of CSA cement pastes cured at different conditions determined by MIP are illustrated in Fig. 9. It can be seen that with curing RH rising, the total porosity tends to be stable and then to decline to reach the lowest value at water curing,
the decline point is mainly dependent on the curing temperature. The highest porosity is from CSA cement paste cured at 30%RH while the lowest is from that cured at water curing. Curing temperature also demonstrates a significant effect on the total porosity of
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Fig. 7. The relation between (a) ettringite and the depletion of ye’elimite and (b) AH3 and the depletion of ye’elimite. The estimated error of the measurement was ± 2.0%.
Fig. 8. Cumulative pore size distribution of CSA cement pastes measured by MIP at different curing conditions: (a) 20 °C, (b) 5 °C and (c) 40 °C.
CSA cement paste especially when RH is above 60%. Generally, the higher the curing temperature, the larger the total porosity of CSA cement pastes. The lowest porosity happens in those CSA cement pastes cured at 5 °C while the highest porosity is from CSA cement
pastes cured at 40 °C. The result is well consistent with literatures [19,21] that low temperature is in favor of a denser and more homogeneous matrix due to continuous substantial hydration. Therefore, it can be inferred that curing condition with low
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Fig. 9. Porosity characteristics of CSA cement pastes measured by MIP.
temperature (5 °C) and high RH (water curing) can bring the lowest porosity (3.2%) to CSA cement paste while curing condition with high temperature (40 °C) and low RH (30%) would cause the highest total porosity (27.4%). 3.4. Compressive strength The curing conditions show big impact on the compressive strength of CSA cement paste. As shown in Fig. 10, CSA cement pastes cured at 5 °C own the highest compressive strength compared to that at 20 °C and 40 °C. The strength tends to decrease with curing temperature increasing. This result is well consistent with literature [21] in which CSA clinker pastes cured at 5 °C shows higher compressive strength when in comparison with that cured at 20 °C and 40 °C. The effect of RH on the compressive strength is also significant. As curing RH goes up, the compressive strength increases and water curing is benefit to gain the highest strength. The explanation can be given based on above analysis that curing with low temperature and high RH is benefit to generate more ettringite and decrease porosity, which are very helpful to gain high strength. 3.5. Discussion It is seen from Fig. 6(a) that the largest amount of ettringite is formed from CSA cement pastes cured under low temperature (5 °C) while the lowest is from that under 40 °C when RH is at 60% and more. Xu et al. [21] also reported that the formation of
Fig. 10. Compressive strength of CSA cement paste cured under various conditions.
ettringite from CSA clinker paste cured at 40 °C is less than half of that at 5 °C. It means that curing with high temperature is detrimental to the formation of ettringite, but it greatly favors the formation of Ms (Fig. 3(c)). The same result can be also found in the reference [19]. The generated ettringite would decompose or convert to other phases at elevated temperatures. It starts to form dehydrate at quite low temperatures as some water molecules are removed from its structure to form metaettringite [28]. According to the thermodynamic model in previous studies [31,32], ettringite is predicted to decompose to Ms and anhydrite at a relative lower RH when it is exposed to higher temperature, indicating lower amount of ettringite is formed at higher curing temperature in comparison with that at lower curing temperature. With low curing RH (30%), the biggest amount of ettringite is formed at normal curing temperature (20 °C) while the lowest is still from 40 °C. This result is not totally agreement with initial thermodynamic model in literatures [31,32], and it requires further research to better understand its mechanism. The curing of CSA cement paste with high temperature or high RH is benefit to the formation of Ms. It should be noted that ettringite is more likely to convert to Ms at elevated temperatures [19,21,33–36]. This transformation is also affected by water activities: at very low RHs, the nucleation of Ms is hindered due to the absence of an aqueous phase, and ettringite may decompose to metaettringite, which is amorphous with a water content of 9 to 13 H2O [31]. Accordingly, less Ms is formed when cured at low RH. CAH10 is only present at 5 °C when RH is high (95%RH and water curing). In this study, the used CSA cement contains calcium aluminate phase (C12A7), it would dissolute at high RH to form CAH10, which is stable phase at low temperature [37]. As reported in [38], at early hydration times, CAH10 can be stable initially at 30 °C and above, as the formation of amorphous AH3 stabilizes CAH10 with respect to C3AH6. The solubility of AH3 decreases with hydration evolution due to the formation of microcrystalline AH3, CAH10 becomes unstable at 20 °C and above [38]. It tends to be transferred to C2AH8 and AH3. The reaction between C2AH8 and soluble silica, which could be from belite phase, leads to the additional formation of strätlingite (C2ASH8), according to Eq. (9). Jeong et al. [39] compared the hydration products of CSA cement with three different w/c (0.4, 0.6 and 0.8) at 28 days and found that strätlingite formation was identified only in high w/c (0.8) samples, likely due to easier diffusion of ions at higher w/c. Some other studies [2,12] also indicated that strätlingite favors to be generated at higher w/c (>0.6) or high curing RH (e.g. water curing), which all confirmed our results.
C2 AH8 þ S ! C2 ASH8
ð9Þ
The formation of AH3 is closely accompanied with the reactions according to Eqs. (1) and (2). Curing with increasing temperature and RH leads to the accelerated formation of ettringite and Ms, finally contributes to higher formation of AH3 (Fig. 6(b)). It is interesting to find from Fig. 11 that the proportion of ettringite and AH3 is mainly influenced by curing temperature. The lower curing temperature leads to higher proportion of ettringite and AH3. The effect from curing RH is not significant in comparison with curing temperature although the formation of ettringite at low temperature is significantly affected by curing RH (Fig. 6(a)). It can be also seen from Fig. 11 that the proportion of ettringite and AH3 from CSA cement cured at 20 °C is slight decreased with RH increasing, indicating the effect from RH cannot be underestimated for normal curing. In addition to hydrates, pore characteristics also change at exposure to different curing temperatures and RHs. It is found from Fig. 9 that elevated curing temperature increases the total porosity of CSA cement paste. Contrary to temperature, the elevated RH benefits to decrease the total porosity. This could be linked with
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ettringite/AH3 ratio while 40 °C demonstrates the lowest value. (4) High temperature is benefit to the formation of AFm phases including Ms and Hc. Hc tends to be generated at low and middle RH (30% and 60%) while the formation of Ms is favor of high RH (95% and water). CAH10 only happens at those curing conditions with low temperature (5 °C) and high RH (95%RH and water curing). (5) The lower total porosity can be achieved when the higher content of ettringite is formed. Generally, the curing of CSA cement pastes with lower temperature and higher RH is benefit to the formation of ettringite, thus to contribute to the lower total porosity and higher compressive strength. Conflict of interest Fig. 11. The proportion of ettringite and AH3 at different conditions. The estimated error of the measurement was ± 2.0%.
There is no conflicts of interest to disclose. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 51572196 and 51872203) and Sino-German Center for Research Promotion (Grant No. GZ 1290). The authors would appreciate the efforts of the anonymous reviewer to improve the quality of this study. References
Fig. 12. Relationship between total porosity and ettringite content. The estimated error of the measurement was ± 2.0%.
the formation of ettringite, as shown in Fig. 12, that the higher content of ettringite is formed, the lower total porosity is achieved. Curing of CSA cement paste with higher temperature causes lower formation of ettringite, therefore has higher total porosity. Contrary to temperature, the curing with higher humidity makes higher formation of ettringite, accordingly the lower porosity is achieved.
4. Conclusions In this study the effect of curing temperature and RH on the hydrates and porosity of CSA cement pastes was investigated. Three temperatures (5 °C, 20 °C and 40 °C) and four RHs (30%, 60%, 95% and water) were considered for cement pastes curing. The main conclusions are summarized in below: (1) The higher hydration degree can be reflected by the higher depletion of ye’elimite, which is found with increasing temperature and RH. (2) Curing temperature and RH show significant effect on the formation of ettringite. Generally, curing with low temperature and high RH is benefit to the formation of ettringite. AH3 favors to be generated at high curing temperature and high RH. (3) The proportion of ettringite and AH3 is mainly influenced by curing temperature. Curing with 5 °C owns the highest
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of curing temperature and relative humidity on the hydrates and porosity of calcium sulfoaluminate cement Lin Li a, Ru Wang a,b,⇑, Shaokang Zhang a a b
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai 201804, China
h i g h l i g h t s The formation of ettringite is favor of curing with low temperature and high RH. The generation of AH3 and AFm is promoted with curing temperature and RH rising. The porosity of CSA cement paste is decreased by curing with low temperature and high RH.
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 29 November 2018 Received in revised form 16 March 2019 Accepted 8 April 2019
The main objective of this study was to investigate the effect of curing temperature and relatively humidity (RH) on the hydrates and porosity of calcium sulfoaluminate (CSA) cement pastes. A series of experiments including X-ray diffraction (XRD), thermogravimetric (TG) and mercury intrusion porosimetry (MIP) were performed. The results indicate that the formation of hydration products and pore characteristics of CSA cement are dependent on the curing conditions. The formation of ettringite is favor of curing with low temperature and high RH. The rate of formation of aluminum hydroxide (AH3) and monosulfoaluminate (Ms) is well promoted when curing temperature and RH are rising. The total porosity of CSA cement paste is gradually decreased by curing with low temperature and high RH. Ó 2019 Elsevier Ltd. All rights reserved.
Keywords: Calcium sulfoaluminate cement Curing conditions Hydrates Porosity
1. Introduction Calcium sulfoaluminate (CSA) cement has drawn a great attention from academia and industry during the last decades due to its considerably environmental and technical benefits when in comparison with ordinary Portland cement (OPC) [1]. CSA cement is fabricated by calcination of bauxite, limestone and gypsum at round 1250 °C, which is 200 °C lower than that of OPC clinker
[2]. CSA cement mainly consists of ye’elimite (C4A3S, also known
as Klein’s compound), belite (C2S), and calcium sulfate (CS) as main components with other minor phases including calcium aluminates (CA, C12A7), brownmillerite (C4AF) and gehlenite (C2AS) which are mainly dependent on the sources of the raw materials [3–5] (hereafter cement chemistry shorthand notation will be
used: C = CaO, S = SiO2, A = Al2O3, S = SO3, H = H2O, F = Fe2O3, M = MgO and T = TiO2).
The hydration kinetics and phases formed upon hydration of CSA cement are mainly dependent on the amounts as well as the reactivity of the added calcium sulfates (anhydrite, hemihydrate or gypsum), which have been already extensively indicated in previous studies [6–8]. Ye’elimite will react with calcium sulfate
immediately to form ettringite (C6AS3H32) according to Eq. (1), AH3 gel is also generated at this stage [9]. Compared to higher reactive calcium sulfate such as hemihydrate or gypsum, the dissolution kinetics of anhydrite is much slower and resulting in an undersupply of calcium and sulfate ions with respect to the formation of ettringite [2,10,11]. The replacement of a part of the anhydrite by gypsum is able to enhance hydration kinetics and both early and late compressive strength [12]. However, some other literatures reported that anhydrite pastes showed slightly larger ettringite contents at later ages and hence, mortars with slightly higher compressive strengths [13].
C4 A3 S þ2C S þ38H ! C6 AS3 H32 þ 2AH3 ⇑ Corresponding author at: School of Materials Science and Engineering, Tongji University, Shanghai 201804, China. E-mail address: [email protected] (R. Wang). https://doi.org/10.1016/j.conbuildmat.2019.04.044 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ð1Þ
When calcium sulfate is completely consumed or only in the presence of water, ye’elimite hydrates to Ms and AH3 according to Eq. (2) [13].
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C4 A3 S þ18H ! C4 A S H12 þ 2AH3
2. Experimental
ð2Þ
The hydration of CSA cement is accelerated and accordingly more ettringite will be formed in presence of calcium hydroxide (CH) in accordance with Eq. (3) [14].
C4 A3 S þ8C S þ6CH þ 74H ! C6 AS3 H32
2.1. Materials A typical rapid hardening CSA cement was employed for all experiment. Its chemical compositions obtained via X-ray fluorescence spectrometry (XRF) are shown in Table 1. The XRD pattern of CSA is shown in Fig. 1. The mineral phases of the CSA cement were achieved by XRD quantitative analysis through X’Pert HighScorePlus software. The mineralogical compositions are shown in
ð3Þ
Belite starts to react with AH3 formed in reactions (1) and (2) and free water to give rise to strätlingite (C2ASH8) formation according to Eq. (4) after calcium sulfate depletion [15].
C2 S þ AH3 þ 5H ! C2 ASH8
Table 2. The main compounds are ye’elimite (C4A3S), belite
ð4Þ
(b-C2S) and anhydrite (CS), with brownmillerite (C4AF), mayenite (C12A7), perovskite (CT) and dolomite (CaMg(CO3)2) as minor compounds. It has a Blaine fineness of 5350 cm2/g. The particle size distribution is illustrated in Fig. 2. Ultrapure water (resistivity = 18.0 MXcm) was used for all the experiments except particle size distribution measurement for cement which was dispersed by high purity ethanol.
Strätlingite will react with the remaining belite phase to give rise to siliceous-hydrogarnet along with C-S-H hydrates [16]. After strätlingite is completely consumed and in the case of remaining belite phase and free water, the hydrates of C-S-H and portlandite (CH) will be generated [16]. Depending on the composition of the minor phases, other hydration products may also be observed, such as hemicarboaluminate (Hc) and monocarboaluminate (Mc) [17,18]. The hydration evolution as well as properties of CSA cement is significantly influenced by curing conditions. A significantly later initial and final setting, a longer dormant period in the calorimetric curves and much lower strength up to 1 day of CSA cement pastes cured at 5 °C are clearly observed compared to 20 °C [5]. Generally, curing temperatures do not change the type of hydration products but do affect their amounts [19]. With curing temperature rising, the rate of formation of ettringite as well as the hydration degree of clinker minerals within 24 h are well promoted, accordingly the early-age compressive strength increases [19,20]. Meanwhile, the proportion of ettringite versus Ms hydrates is decreased due to elevated curing temperature, indicating that high amount of Ms is formed, especially at low initial gypsum contents. This can be explained by that the increased curing temperature improved the stability of Ms against ettringite which required more gypsum to be stabilized [4]. However, low temperature (<20 °C) is in favor of a denser and more homogeneous matrix due to continuous substantial hydration [19,21]. Despite of the above limit literatures, most of studies are focused on the influence of curing temperature on the early hydration and properties of CSA cement composites, while little is known on the effect of humidity on the hydration of CSA cement. And the further study on the coupling effect between temperature and humidity on the hydration and properties of CSA cement is deficient. However, the applications of cement may undergo various external curing conditions. Thus, it should be meaningful to study the influence of curing regimes on the hydration and properties of CSA cement. In order to better understand the influence of curing regimes on the hydration of CSA cement, the effect of curing temperature and relative humidity on the hydrates and porosity of CSA cement pastes was investigated. One commercial CSA cement with water to cement ratio (w/c) of 0.4 was taken to prepare CSA cement paste. Three temperatures (5 °C, 20 °C and 40 °C) and four RH varying from 30%, 60% and 95% controlled with saturated salt solution and water were considered for cement specimens curing. XRD and TGA were employed to identify the hydration products. MIP was used to determine the pore size distribution.
2.2. Preparation and curing of samples The CSA cement and water used for cement paste preparation as well as casting moulds were pre-cured at temperatures of 5 °C, 20 °C or 40 °C for 24 h. The CSA cement pastes were prepared at a constant water to cement ratio (w/c) of 0.4. CSA cement was weighted into the corresponding water within 10 s and mixed for 30 s at low speed, then the mixtures were blended at high speed for another 90 s. Afterwards, the CSA cement paste was immediately casted into the 20 mm 20 mm 20 mm steel moulds after mixing. These moulds with cement pastes were quickly transferred into sealed glass desiccators which contained saturated salt solution to achieve constant RH, then these glass desiccators were conditioned in environmental chambers for constant temperatures. The specimens were demoulded after 24 h and then stored back to the same curing conditions to the age of 28 days. Afterwards, the samples were taken out for following measurement. The constant RH was achieved in a glass desiccator according to ASTM E104-02 [22]. The saturated salt solutions were prepared under three curing temperatures by using reagent grade chemicals including magnesium chloride (MgCl2), sodium bromide (NaBr) and potassium sulfate (K2SO4) to achieve three different levels of
Fig. 1. XRD pattern of CSA cement.
Table 1 Chemical composition of CSA cement (w %). Al2O3
CaO
SO3
SiO2
Fe2O3
K2O
MgO
Mn2O3
Na2O
P2O5
SrO
TiO2
LOI
23.84
44.20
15.04
9.82
1.95
0.26
2.46
0.02
0.08
0.11
0.10
0.95
1.17
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RH, as shown in Table 3. In this study, the three different levels of RH were labeled as 30%RH, 60%RH and 95%RH. At the same time, water curing was also considered for further comparison analysis. In this case, the cement paste for water curing was firstly cured at 95%RH then demoulded to store in water.
3. Results and discussion 3.1. XRD analysis
2.3. Characterization The slices taken from the inner of small cube specimens were immediately broken slightly and immersed into ethanol to stop the hydration. Subsequently, the materials were rinsed with diethyl ether and shortly dried at 40 °C. Then some of the samples were ground to completely pass a 100 lm sieve for XRD and TG analysis while some others without grinding were taken to conduct MIP measurement. 2.3.1. XRD analysis XRD measurement was performed by Rigaku equipment (model D/max2550VB3+). The powdered samples were scanned between 5° and 50° with a 2h increment 0.02° per step, and a dwell time of 4 s and a Cu Ka radiation were applied. 2.3.2. TG analysis TG analysis was carried out on 20 ± 2 mg of the powdered samples using a Netzsch TGA 209F1 Instrument under nitrogen atmosphere. The samples were heated from ambient temperature to 1000 °C with a ramp rate of 10 °C/min. The DTG curve was obtained after derivation to demonstrate various peaks corresponding to the dehydration of different phases. TG analysis was employed to determine the main hydrates such as ettringite and AH3 semi-quantitatively. Ettringite content was calculated by the weight loss between 50 °C and 150 °C corresponding to 20 molecules of crystal water per molecule of ettringite. AH3 content was achieved by the weight loss between 230 °C and 300 °C (corresponding to 3 molecules of crystal water per molecule aluminum hydroxide). The amounts of ettringite, AH3 were roughly estimated due to the overlaps among these hydration products in the range of temperature described above. Ms was not calculated based on TG data due to strongly overlaps from Ms, Hc, CAH10 and strätlingite. 2.3.3. MIP analysis Mercury intrusion porosimeter was used to measure the porosity characteristics of CSA cement pastes. The pore diameter was calculated using Washburn equation [23] according to Eq. (7).
d¼
accordance with EN 196-1 although the specimen size is different [24].
4cCOSh P
ð7Þ
where d is the pore diameter, c is the surface tension of mercury (0.480 N/m), h is the contact angle between mercury and cement (140°), and P is the applied pressure (MPa). MIP was carried out on Quantachrome Poremaster GT-60 with maximum and minimum applied pressures as 420 MPa and 1.5 kPa, corresponding to approximately minimum pore size of 3.5 nm and maximum pore size of 400 lm. 2.3.4. Compressive strength At the curing age of 28 days, the small cubic specimens (20 mm 20 mm 20 mm) were taken out from curing container for compressive strength measurement. The testing process is in
XRD patterns of CSA cement pastes cured at different conditions after 28 days are shown in Fig. 3. It can be seen from Fig. 3(a) that when curing temperature is 20 °C, the dominant hydration product of CSA cement is ettringite while other two hydrates Ms and Hc are also generated. Different RH for CSA cement curing leads to different hydration degree, which can be reflected by the depletion speed of ye’elimite as well as the formation of various hydration products. It can be found that when RH increases from 30% to water curing, the main hydration product ettringite seems to keep the same level. However, the formation of Ms and Hc is quite different. Ms favors to be formed when CSA cement is cured under high RH. The trend of Hc formation differs from Ms. Hc tends to be generated at RH of 30% and 60%. The effect of RH on the hydration products of CSA cement curing at 5 °C is different from that at 20 °C. CSA cements cured under 95%RH and water show the quicker depletion rate of ye’elimite, and finally generate higher ettringite content when in comparison with other curing RH. The lowest ettringite is generated when CSA cement is cured at 30%RH. It is interesting to find from Fig. 3(b) that the peak at 2h of 12.4° appears when the CSA cement is cured under 95%RH and water. This is attributed to the presence of CAH10 which is formed according to Eq. (8) [25–27]. Such phase favors to be presence at low temperature and tends to be transformed to other hydrates when subjected to higher temperature [4]. In literatures [12,28], traces of CAH10 occur as intermediate hydration product after 28 days and disappear again at longer hydration times.
3C4 A3 S þ98H ! C6 AS3 H32 þ 6CAH10 þ 2AH3
ð8Þ
Similar as curing temperature at 5 °C, the higher amount of ettringite tends to be generated when CSA cement is cured under higher RH at 40 °C, which can be seen from Fig. 3(c). High RH is also benefit to the generation of Ms. The formation of Hc can be detected for all cases while the biggest amount is happened for curing at 60%RH, which is the same as that at 20 °C. There are another two peaks shown at 2h around 7.0° and 21.3° appear in the CSA cement cured in water. This is mainly attributed to the presence of strätlingite, which is ascribed to the reaction between belite, aluminum hydroxide and water (see Eq. (4)). Generally, strätlingite favors to be generated at higher w/c (>0.6) or high curing RH (e.g. water curing) [2,12]. The effect of temperature on the hydrates of CSA cement subjected to various RH is also significant. It can be observed from Fig. 3(a)–(c) that the curing conditions with high curing temperature and low or middle RH are benefit to the formation of Hc. The highest intensity of the peak corresponding to Hc is from 40 °C and 60%RH, as seen from Fig. 3(c), indicating the biggest formation of Hc at this condition. When curing RH is more than 30%, the highest amount of ettringite tends to be generated at 5 °C and the lowest happens at 40 °C according to the XRD peak intensity. In this case, the proportion of ettringite versus Ms hydrates is
Table 2 Mineral composition of CSA cement (w %).
C4A3S
C2S
CS
C4AF
C12A7
CT
CaMg(CO3)2
MgO
40.1
29.9
16.2
5.2
3.5
1.1
1.7
2.2
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Fig. 2. Particle size distribution of CSA cement.
decreased due to elevated curing temperature [4], indicating that higher amount of ettringite is transformed to Ms. As previous introduced, CAH10 only happens at the conditions with low temperature (5 °C) and high RH (95%RH and water curing). Contrary to CAH10, the highest amount of Ms is generated at high temperature (40 °C) and high RH (e.g. 95%RH and water curing). Ye’elimite is the dominant mineral of the CSA cement used in our study, the above hydration products of CSA cement are mainly formed via the reaction between ye’elimite and other phases. Accordingly, the hydration degree of CSA cement can be estimated by the depletion rate of ye’elimite [29]. Fig. 4 illustrates the main peak intensity of ye’elimite at various curing conditions. It can be seen that curing with higher temperature and higher RH leads to lower intensity of ye’elimite. The lowest intensity of ye’elimite from CSA cement paste is from 40 °C and water curing while the highest is from 5 °C and 30%RH. Thus this curing condition (40 °C and water curing) can be estimated to reach the highest degree of hydration for CSA cement. 3.2. TG analysis Thermogravimetry (TG) analysis is a powerful technique that has been extensively used to analyze CSA cements and their hydrates. The main hydrates ettringite, Ms and even amorphous phase AH3 could be detected and quantified from their structure water loss at specific temperature ranges [2]. The TG/DTG curves of CSA cement pastes cured at different conditions are plotted in Fig. 5. It can be seen that there are several weight loss peaks appearing in the DTG curves indicating different hydrates of CSA cement. The main derivative peak in the range of 100–130 °C corresponds to the water removal from ettringite, which is significantly influenced by various curing conditions. The second peak happens in the 170–200 °C temperature range and can be ascribed to AFm phases including Ms as well as Hc. The DTG peaks of Ms and Hc are strongly overlapped in this range, thus it is hardly to quantify Ms and Hc separately based on TG results. The third peak is from AH3 phase, which loses its bound water at around 250–280 °C. However, there is no visible peak at
450–550 °C, which would be the indicator for the decomposition of calcium hydroxide [30]. Thus, it can be inferred that the hydration products of CSA cement at 28 days in this study do not contain calcium hydroxide or calcium hydroxide has been already consumed completely according to reaction in Eq. (3). Another two peaks found in the temperature range of 700–900 °C can be ascribed to the decomposition of dolomite from original cement. It can be observed from Fig. 5(a) that the intensity of DTG peak displayed at around 130 °C from water curing is the highest, meaning the biggest amount of ettringite generated from the CSA cement with water curing at 20 °C. The rest DTG peaks at this temperature have similar intensities indicating the same level content of ettringite. The DTG peaks in the range of 170–200 °C from CSA cement cured under 95%RH and water are more significant than those cured under low RH, corresponding to the formation of AFm phases. The results here are well consistent with that from XRD analysis. When temperature decreases to 5 °C (see Fig. 5(b)), the trend of DTG peak intensity is quite different from those cured at 20 °C, as above described. It can be found that the intensity of DTG peaks at 100–150 °C increases with curing RH increasing. In this case, the higher curing RH is benefit to the formation of ettringite. The lowest ettringite content is from CSA cement curing at 30%RH which is also consistent with XRD result. The DTG peaks in the range of 170–200 °C are small indicating only little AFm phases have formed under such curing conditions. It is found from Fig. 5(c) for curing temperature of 40 °C that the strongest intensity of DTG peak for ettringite is from 95%RH. The results are well consistent with that from XRD analysis. It could be also seen that the intensity of AH3 peaks in the range of 250– 280 °C from various curing RH at 40 °C (Fig. 5(c)) is stronger than that at 20 °C (Fig. 5(a)) and 5 °C (Fig. 5(b)), which means that AH3 favors to be generated at high temperatures. The hydration product of ettringite from CSA cement pastes cured under various conditions were calculated based on TG data and the results are shown in Fig. 6(a). Generally, the effect from curing temperature on the formation of ettringite is very significant. The maximum amount of ettringite is from 20 °C while the lowest is from 40 °C when CSA cement paste was cured with low RH (30%). When the RH reaches 60% or more, the formation of ettringite is decreased with curing temperature increasing. Compared to that from CSA cement pasted curing at 20 °C, much more ettringite is formed from CSA cement paste when curing temperature is 5 °C. The effect of RH on the formation of ettringite is different when curing temperature is different. When curing temperature is 5 °C, elevated RH leads to sharply generation of ettringite. When curing temperature reaches 20 °C and 40 °C, the ettringite content is quite similar for those CSA cement pastes cured at 30%RH and 60%RH. The formation of ettringite is increased with curing RH increasing when the RH is more than 60%. Generally, the effect from curing RH on the formation of ettringite is very significant when curing temperature is 5 °C A positive correlation between the formation of AH3 and curing temperature as well as RH can be seen in Fig. 6(b). Generally, high curing temperature and RH are beneficial for the generation of AH3. The biggest amount of AH3 is generated from CSA cement paste
Table 3 Relative humidity of saturated salt solution for specimens cured under various temperatures [22]. Code
30%RH
60%RH
95%RH
Water
Salt solution 5 °C 20 °C 40 °C
MgCl2 33.6 ± 0.3% 33.1 ± 0.2% 31.6 ± 0.2%
NaBr 63.5 ± 0.8% 59.1 ± 0.5% 53.2 ± 0.5%
K2SO4 98.5 ± 1.0% 97.6 ± 0.6% 96.4 ± 0.4%
– – – –
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Fig. 3. XRD profiles of CSA cement pastes cured under various relative humidities until 28 days: (a) 20 °C, (b) 5 °C and (c) 40 °C (E: ettringite, Ms: monosulfoaluminate, Hc: hemicarboaluminate, C: CAH10, S: strätlingite, Y: ye’elimite, A: anhydrite).
not significant. Besides of ettringite, some other hydrates eg. AFm phases and CAH10 are also formed by the reaction of ye’elimite. However, a systematic law can be seen that much more ye’elimite is consumed and less ettringite is generated with curing temperature increasing. The relation between AH3 and the depletion of ye’elimite is illustrated in Fig. 7(b). As the hydration proceeds, ye’elimite is gradually depleted. Accordingly, more and more AH3 is formed according to Eqs. (1) and (2). Curing with higher temperature and RH tends to promote hydration kinetics by consuming more ye’elimite and forming higher amount of AH3. 3.3. MIP analysis
Fig. 4. XRD peak intensity of ye’elimite at 2h value around 23.6° in XRD patterns from CSA cement pastes cured with various conditions.
cured at 40 °C and water, while the lowest is from that cured at 5 °C and 30%RH or 60%RH. Fig. 7(a) shows the relation between generated ettringite and the depletion of ye’elimite. It can be found their correlation is
The cumulative pore size distribution (CPSD) of CSA cement pastes cured at various temperatures and RH investigated in this work are plotted in Fig. 8. Generally, the main pore size of CSA cement pastes is within 1 lm. With curing RH increasing to 95% and water curing, CSA cement pastes tend to gain lower main pore size (<0.1 lm), the effect seems to be much more significant for those CSA cement pastes cured at 20 °C. Meanwhile, CSA cement pastes cured under high RH have systematic lower CSPD than those under low RH, and this trend is much more obvious when RH is more than 60%. It can be also found from Fig. 8 that higher curing temperature would induce higher CPSD to CSA cement pastes.
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Fig. 5. TG profiles of CSA cement pastes cured under various relative humidities until 28 days: (a) 20 °C, (b) 5 °C and (c) 40 °C (E: ettringite, Ms: monosulfoaluminate, Hc: hemicarboaluminate, S: strätlingite, AH3: aluminum hydroxide).
Fig. 6. The generation of (a) ettringite and (b) AH3 in CSA cement pastes cured at different conditions until 28 days. The estimated error of the measurement was ± 2.0%.
The total porosity of CSA cement pastes cured at different conditions determined by MIP are illustrated in Fig. 9. It can be seen that with curing RH rising, the total porosity tends to be stable and then to decline to reach the lowest value at water curing,
the decline point is mainly dependent on the curing temperature. The highest porosity is from CSA cement paste cured at 30%RH while the lowest is from that cured at water curing. Curing temperature also demonstrates a significant effect on the total porosity of
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Fig. 7. The relation between (a) ettringite and the depletion of ye’elimite and (b) AH3 and the depletion of ye’elimite. The estimated error of the measurement was ± 2.0%.
Fig. 8. Cumulative pore size distribution of CSA cement pastes measured by MIP at different curing conditions: (a) 20 °C, (b) 5 °C and (c) 40 °C.
CSA cement paste especially when RH is above 60%. Generally, the higher the curing temperature, the larger the total porosity of CSA cement pastes. The lowest porosity happens in those CSA cement pastes cured at 5 °C while the highest porosity is from CSA cement
pastes cured at 40 °C. The result is well consistent with literatures [19,21] that low temperature is in favor of a denser and more homogeneous matrix due to continuous substantial hydration. Therefore, it can be inferred that curing condition with low
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Fig. 9. Porosity characteristics of CSA cement pastes measured by MIP.
temperature (5 °C) and high RH (water curing) can bring the lowest porosity (3.2%) to CSA cement paste while curing condition with high temperature (40 °C) and low RH (30%) would cause the highest total porosity (27.4%). 3.4. Compressive strength The curing conditions show big impact on the compressive strength of CSA cement paste. As shown in Fig. 10, CSA cement pastes cured at 5 °C own the highest compressive strength compared to that at 20 °C and 40 °C. The strength tends to decrease with curing temperature increasing. This result is well consistent with literature [21] in which CSA clinker pastes cured at 5 °C shows higher compressive strength when in comparison with that cured at 20 °C and 40 °C. The effect of RH on the compressive strength is also significant. As curing RH goes up, the compressive strength increases and water curing is benefit to gain the highest strength. The explanation can be given based on above analysis that curing with low temperature and high RH is benefit to generate more ettringite and decrease porosity, which are very helpful to gain high strength. 3.5. Discussion It is seen from Fig. 6(a) that the largest amount of ettringite is formed from CSA cement pastes cured under low temperature (5 °C) while the lowest is from that under 40 °C when RH is at 60% and more. Xu et al. [21] also reported that the formation of
Fig. 10. Compressive strength of CSA cement paste cured under various conditions.
ettringite from CSA clinker paste cured at 40 °C is less than half of that at 5 °C. It means that curing with high temperature is detrimental to the formation of ettringite, but it greatly favors the formation of Ms (Fig. 3(c)). The same result can be also found in the reference [19]. The generated ettringite would decompose or convert to other phases at elevated temperatures. It starts to form dehydrate at quite low temperatures as some water molecules are removed from its structure to form metaettringite [28]. According to the thermodynamic model in previous studies [31,32], ettringite is predicted to decompose to Ms and anhydrite at a relative lower RH when it is exposed to higher temperature, indicating lower amount of ettringite is formed at higher curing temperature in comparison with that at lower curing temperature. With low curing RH (30%), the biggest amount of ettringite is formed at normal curing temperature (20 °C) while the lowest is still from 40 °C. This result is not totally agreement with initial thermodynamic model in literatures [31,32], and it requires further research to better understand its mechanism. The curing of CSA cement paste with high temperature or high RH is benefit to the formation of Ms. It should be noted that ettringite is more likely to convert to Ms at elevated temperatures [19,21,33–36]. This transformation is also affected by water activities: at very low RHs, the nucleation of Ms is hindered due to the absence of an aqueous phase, and ettringite may decompose to metaettringite, which is amorphous with a water content of 9 to 13 H2O [31]. Accordingly, less Ms is formed when cured at low RH. CAH10 is only present at 5 °C when RH is high (95%RH and water curing). In this study, the used CSA cement contains calcium aluminate phase (C12A7), it would dissolute at high RH to form CAH10, which is stable phase at low temperature [37]. As reported in [38], at early hydration times, CAH10 can be stable initially at 30 °C and above, as the formation of amorphous AH3 stabilizes CAH10 with respect to C3AH6. The solubility of AH3 decreases with hydration evolution due to the formation of microcrystalline AH3, CAH10 becomes unstable at 20 °C and above [38]. It tends to be transferred to C2AH8 and AH3. The reaction between C2AH8 and soluble silica, which could be from belite phase, leads to the additional formation of strätlingite (C2ASH8), according to Eq. (9). Jeong et al. [39] compared the hydration products of CSA cement with three different w/c (0.4, 0.6 and 0.8) at 28 days and found that strätlingite formation was identified only in high w/c (0.8) samples, likely due to easier diffusion of ions at higher w/c. Some other studies [2,12] also indicated that strätlingite favors to be generated at higher w/c (>0.6) or high curing RH (e.g. water curing), which all confirmed our results.
C2 AH8 þ S ! C2 ASH8
ð9Þ
The formation of AH3 is closely accompanied with the reactions according to Eqs. (1) and (2). Curing with increasing temperature and RH leads to the accelerated formation of ettringite and Ms, finally contributes to higher formation of AH3 (Fig. 6(b)). It is interesting to find from Fig. 11 that the proportion of ettringite and AH3 is mainly influenced by curing temperature. The lower curing temperature leads to higher proportion of ettringite and AH3. The effect from curing RH is not significant in comparison with curing temperature although the formation of ettringite at low temperature is significantly affected by curing RH (Fig. 6(a)). It can be also seen from Fig. 11 that the proportion of ettringite and AH3 from CSA cement cured at 20 °C is slight decreased with RH increasing, indicating the effect from RH cannot be underestimated for normal curing. In addition to hydrates, pore characteristics also change at exposure to different curing temperatures and RHs. It is found from Fig. 9 that elevated curing temperature increases the total porosity of CSA cement paste. Contrary to temperature, the elevated RH benefits to decrease the total porosity. This could be linked with
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ettringite/AH3 ratio while 40 °C demonstrates the lowest value. (4) High temperature is benefit to the formation of AFm phases including Ms and Hc. Hc tends to be generated at low and middle RH (30% and 60%) while the formation of Ms is favor of high RH (95% and water). CAH10 only happens at those curing conditions with low temperature (5 °C) and high RH (95%RH and water curing). (5) The lower total porosity can be achieved when the higher content of ettringite is formed. Generally, the curing of CSA cement pastes with lower temperature and higher RH is benefit to the formation of ettringite, thus to contribute to the lower total porosity and higher compressive strength. Conflict of interest Fig. 11. The proportion of ettringite and AH3 at different conditions. The estimated error of the measurement was ± 2.0%.
There is no conflicts of interest to disclose. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 51572196 and 51872203) and Sino-German Center for Research Promotion (Grant No. GZ 1290). The authors would appreciate the efforts of the anonymous reviewer to improve the quality of this study. References
Fig. 12. Relationship between total porosity and ettringite content. The estimated error of the measurement was ± 2.0%.
the formation of ettringite, as shown in Fig. 12, that the higher content of ettringite is formed, the lower total porosity is achieved. Curing of CSA cement paste with higher temperature causes lower formation of ettringite, therefore has higher total porosity. Contrary to temperature, the curing with higher humidity makes higher formation of ettringite, accordingly the lower porosity is achieved.
4. Conclusions In this study the effect of curing temperature and RH on the hydrates and porosity of CSA cement pastes was investigated. Three temperatures (5 °C, 20 °C and 40 °C) and four RHs (30%, 60%, 95% and water) were considered for cement pastes curing. The main conclusions are summarized in below: (1) The higher hydration degree can be reflected by the higher depletion of ye’elimite, which is found with increasing temperature and RH. (2) Curing temperature and RH show significant effect on the formation of ettringite. Generally, curing with low temperature and high RH is benefit to the formation of ettringite. AH3 favors to be generated at high curing temperature and high RH. (3) The proportion of ettringite and AH3 is mainly influenced by curing temperature. Curing with 5 °C owns the highest
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