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Effects of sodium gluconate on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder
Construction and Building Materials 157 (2017) 1065–1073
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of sodium gluconate on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder Xiaowei Zhang a,b,⇑, Yan He a, Chunxia Lu c, Zhou Huang a a
School of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China c Suzhou Fangzheng Engineering Technology Development and Testing Co., Ltd, Suzhou 215152, China b
h i g h l i g h t s Sodium gluconate delays AFt formation slightly and then promotes it. Sodium gluconate postpones silicate hydration and reduces its hydration rate slightly. Silicate hydration is delayed less and has higher reaction rate in CAC rich ternary binder at high sodium gluconate dosage. Inhabiting gypsum dissolution and impeding CA or C3A hydration delays the AFt formation. Sodium gluconate results in better defined AFt crystals with larger thickness and length.
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Article history: Received 17 May 2017 Received in revised form 22 September 2017 Accepted 24 September 2017 Available online 6 October 2017 Keywords: Portland cement Calcium aluminate cement Anhydrite Sodium gluconate Early hydration Ettringite
a b s t r a c t Effects of sodium gluconate on early hydration of the Portland cement-calcium aluminate cement ternary binder were investigated through isothermal calorimetric, X-ray diffracanhydrite (PC-CAC-CS) tion analysis (XRD), differential scanning calorimeter analysis (DSC) and scanning electron microscope (SEM). Additionally, mortar performances of the ternary binders, including mechanical strength, fluidity as well as setting time were studied. The results show that sodium gluconate firstly delays ettringite (AFt) formation slightly and then promotes the AFt formation once it begins to form. Sodium gluconate postpones the hydration of silicate and reduces its hydration rate slightly. The hydration of silicate is delayed less in calcium aluminate cement (CAC) rich ternary binder at the same sodium gluconate dosage. High dosage of sodium gluconate will cause significant delay of silicate hydration in CAC poor ternary binder and lower hydration rate. Sodium gluconate impedes CA hydration and inhibits the dissolution of gypsum synchronously, which may lead to the slight delay of AFt formation. Moreover, sodium gluconate influences the growth orientation of AFt, which shows better defined as well as larger shape. Sodium gluconate is not an ideal retarder for fluidity loss control in ternary binder, which increases initial mortar fluidity but cannot control fluidity loss. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, ternary binder, composed of Portland cement (PC), has been calcium aluminate cement (CAC) and calcium sulfate (CS), widely applied in self-leveling mortar as well as grouting mortar [1,2], which is due to its dominant performances, such as shrinkage compensating, rapid hardening [1,3–5]. ternary binHowever, the hydration process of the PC-CAC-CS ders is much more complicated compared with PC, and has drawn ⇑ Corresponding author at: School of Civil Engineering, Suzhou University of Science and Technology, No. 1701 Binhe Road, Hi-Tec District, Suzhou, China. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.conbuildmat.2017.09.153 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
great attention of researchers [6–10]. It has been reported that the composition proportion of the ternary binder significantly influence the performance of the compound, including the setting time, the strength development, the microstructure et al. [9–13]. In PCCAC binary binder, the rapid formation of AFt results in rapid setting and quick strength development [6], but the hydration of PC is ternary binders, postponed [7,8,11]. By comparison, in PC-CAC-CS through XRD analysis and conduction calorimetric studies, it was accelerates the hydration of C3S, which enhances found that CS the late strength [12,14]. With high amount of CAC and low in the PC-CAC-CS ternary binders, AFt forms firstly amount of CS and the formation of monosulfoaluminate (AFm) unlocks the hydration of silicates, while with high content of both CAC and
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AFt forms firstly, then silicates hydrates subsequently, and CS, finally the remaining aluminates reacts. But according to recent advances [15], there is no such AFt inhibiting layer formation during early hydration of PC, so how PC hydrates in ternary binder and how its hydration is postponed may need to study further. Except composition proportions, many other factors may also ternary binders, such affect the hydration process of the PC-CAC-CS as temperature [9], humidity [13], the source of raw materials [12,13,15] as well as chemical admixtures [1,5]. Among these factors, the application of chemical admixtures to regulate the perfor ternary binders is practical, due to the fact mance of the PC-CAC-CS that chemical admixtures not only change the hydration process, but also improve the fluidity as well as setting performance of ternary binders, which is extremely required for the PC-CAC-CS self-leveling or grouting mortar. So, the application of retarders ternary binders is primarily together with SPs in the PC-CAC-CS necessary to improve fluidity retainability as well as setting performance [1,5,16,17]. However, the usage of retarders as well as SPs ternary system makes the hydration process of the PC-CAC-CS more sophisticated. Among the retarders, Sodium gluconate, as a crystalline powder, is widely used together with superplasticizers (SP) in PC due to its good retardation action, aiming to improve the slump loss and fluidity of concrete [18–22]. The retardation action of sodium gluconate has reported to be primarily due to the delay of cement hydration, through the adsorption on particle surfaces or the complexion effect with calcium ions [23,24]. Milestone NB thought that a small amount of sodium gluconate can impede the nucleation as well as the growth of hydrates [25]. However, through XRD as well as DSC analysis of the hydrated samples, Suhua M. et al. [23] found that a low dosage of sodium gluconate would promote the AFt formation, while high amount of sodium gluconate (higher than 1.0%) prolongs the entire hydration process, through hindering the dissolution of gypsum. However, the function mechanism of sodium ternary binders seems to be gluconate applied in the PC-CAC-CS comparatively different from that applied in the PC. The improper application of sodium gluconate can results in serious problems, such as abnormal setting and high fluidity loss [23], which is detrimental to the engineering project. The aim of this research is to preliminarily understand the func ternary tional mechanism of sodium gluconate in the PC-CAC-CS binders. The effect of gluconate on the early hydration process, systems is as well as the mortar performance of the PC-CAC-CS studied.
2. Experimental 2.1. Materials Portland cement typeII 52.5R and calcium aluminate cement CA50 used in this study were obtained from JinYang Cement Co. and Changchen Cement Co. respectively, and a commercial natural anhydrite was from Nanjing. The chemical compositions and mineralogical compositions of the cements provided by the manufacturer are shown in Tables 1 and 2. Industrial grade sodium gluconate was from Xiwang Chemistry Co, and polycarboxylatebased superplasticizer (SP) in dry power type was from XinBang
Co. The nature river sand applied in this research was smaller than 1.25 mm. 2.2. Mix proportions Two types of ternary binder were applied in this research, i.e. mass ratio of 92.5:7.5:3.75 and 85:15:7.5 respectively, PC-CAC-CS which were typical binders of grouting mortar in china for different expansion rate request. All the samples had a constant SP Four differdosage of 0.25% based on total binder (PC + CAC + CS). ent dosages of sodium gluconate, 0.06%, 0.12%, 0.18% and 0.24% were chosen, and the 0% sample was taken as the control to study the effects of sodium gluconate on properties of ternary binder. The mix proportion of the samples and relative measurements are shown in Table 3. 2.3. Methods Fluidity and mechanical strength of cement mortar were tested according to the Chinese Industry Standard GB T 50448-2008, and the setting time was determined according to ASTMC191 [1]. Early hydration of ternary binder was investigated by hydration heat evolution measurement (isothermal calorimetric) [26], X-ray diffraction analysis (XRD) and differential scanning calorimeter analysis (DSC) [1]. Microstructure observation of the hydrated binders was conducted through Quanta 250 FEG scanning electron microscope (SEM). The small pieces of hydrated binders were firstly immersed into alcohol for 24 h, and then dried at 40 °C for 24 h. Before SEM test, the sample was gold coated under 20 mA electrical current for 2 min. For XRD and DSC analysis, the dried samples were further ground to less than 63 lm. XRD analysis was performed on a Bruker D8 system using CuK radiation in the range of 5–60° (2h), with step of 0.04°/s. DSC analysis was carried out on a NETZSCH/STA449F3JUPITER system, sample was heated under N2 over a temperature range of 30–650 °C at a rate of 20 °C/min. For calorimetric analysis, the paste (containing 4 g binder) was premixed for 3 min in the vial then introduced into the sample compartment of conduction calorimeter (TAM Air from Thermometric, Sweden) operating at 20 ± 0.1 °C. Data logging was continued for about 72 h. 3. Results 3.1. Early hydration 3.1.1. Calorimetric analysis Effects of sodium gluconate on the heat flow of the two kinds of binders are exhibited in Figs. 1(a) and (b). Without sodium gluconate, only two exothermic peaks exist for both binders. The first exothermic peak should be ascribed to the contacting of mineral grains with water, the dissolution of various ions and the early formation of AFt that results from the reaction between CAC (or C3A in PC) and anhydrite (or gypsum in PC) [1,27]. The second exothermic peak is attributed to the hydration of silicate phases, and the stage between the first and the second peaks is the so-called induction period [27]. With addition of sodium gluconate, a new exothermic peak closely following the initial peak emerges. The new exothermic peak
Table 1 Chemical composition of cement (wt%).
PC CAC
CaO
SiO2
Al2O3
Fe2O3
MgO
TiO2
SO3
K2O
Na2O
SUM
65.42 34.62
21.57 8.27
5.02 50.71
3.35 1.92
0.77 0.84
0.25 2.76
2.72 –
0.41 –
0.17 –
99.67 99.40
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X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073 Table 2 Mineralogical composition of cement (wt%).
PC CAC
C3S
C2S
C3A
C4AF
R2O
60.30
13.93
7.96
10.00
0.51
CA
CA2
CT
C2F
C2AS
MA
SUM
36.38
14.08
4.69
3.26
37.71
2.97
92.70 99.09
Table 3 Mix proportions of the samples and relative measurements. nder (PC:CAC: CS)
92.5:7.5:3.75 85:15:7.5
SP (%)
Sodium gluconate (%)
Strength
Fluidity
Setting time
DSC
(by the weight of binder)
(mortar)
(paste)
0.25
Water/binder = 0.30 Sand/binder = 1.2
Water/binder = 0.22
0, 0.06, 0.12, 0.18, 0.24
XRD
SEM
Calorimetry
Water/ binder = 0.30
Note: All the samples were dry mixed for an hour in a mortar mixer at low speed before water was added.
Fig. 1a. Heat evolution rate of 92.5/7.5/3.75 binder.
Comparing Fig. 1(a) with Fig. 1(b), it shows that in the CAC rich samples, the new exothermic peak comes out a little earlier and the corresponding heat evolution rate is elevated when the same dosage of sodium gluconate is added, which should be related to more CA in CAC rich binder, but the induction period is less prolonged, especially at high sodium gluconate dosage. For example, with 0.24% sodium gluconate added in binder, an extremely delayed hydration of silicate is observed in Fig. 1(a), while the content of sodium gluconate has relatively less effects on the induction period of CAC rich samples as shown in Fig. 1(b). As seen in Figs. 2(a) and (b), the effects of sodium gluconate on hydration of silicate can be seen more clearly when the hydration peak of silicate is normalized per g of silicate according the mineralogical composition in Table 2 [15]. Without sodium gluconate, the CAC poor ternary binder has higher heat evolution rate, which should be related to less CA content, less CA results in less suppression of silicate hydration [14]. Sodium gluconate delays silicate hydration, and reduces its hydration rate slightly at the same time. For CAC rich binder with higher sodium gluconate content, silicate hydration not only is delayed less, but also has higher reaction rate.
3.1.2. XRD analysis It may be a result from preferred orientation of natural anhydrite crystal, the peak of anhydrite in all the XRD patterns changes irregularly, which makes it difficult to trace the hydration of the ternary system by the change of the anhydrite peak.
Fig. 1b. Heat evolution rate of 85/15/7.5 binder.
cannot be easily distinguished and is due to the formation reaction of AFt [1], which can be further confirmed by XRD analysis. When the dosage of sodium gluconate increases, the new exothermic peak appears slightly delayed and heat evolution rate of the new exothermic peak reduces. Simultaneously, the induction period is prolonged and the reaction of silicate is delayed.
Fig. 2a. Heat evolution rate normalized to silicate (CAC poor binder).
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Fig. 3b. 0.5 h XRD pattern of 85/15/7.5 binder.
Fig. 2b. Heat evolution rate normalized to silicate (CAC rich binder).
The XRD patterns of the 85/15/7.5 binder are exhibited in Figs. 3 (a)–(e). Shown in Fig. 3(a), without sodium gluconate added in binders, the formation of AFt is immediate. Sodium gluconate delays ettringite formation. With higher than 0.18% sodium gluconate added in ternary binders, it remarkably inhibits the dissolution of gypsum. But sodium gluconate has very weak capability of delaying the formation of AFt, as AFt emerges promptly after 0.5 h of hydration, which can be seen in Fig. 3(b). From 0.5 h to 8 h, with sodium gluconate dosage increasing in binders, more AFt forms in comparison with the reference sample (without sodium gluconate added), which is consistent with the appearance of the new peak in Fig. 1(b), so the new exothermic peak in Fig. 1(b) is ascribed to the reaction of CA (or C3A) and sulfates (anhydrite or gypsum in PC) [1]. Comparing the peak of the silicate in Figs. 3(c)–(e), as the dosage of sodium gluconate increases, the silicate hydration is more significantly postponed, which is coincident with the hydration heat analysis (Fig. 1). The peak of Ca(OH)2 (CH) in all the sample is weak even at 24 h (Fig. 3(e)), which means CH is consumed due to the formation of AFt [28]. In addition, the peak of AFm is substantially weak even when the peak of AFt has reduced at
Fig. 3c. 4 h XRD pattern of 85/15/7.5 binder.
Fig. 3d. 8 h XRD pattern of 85/15/7.5 binder.
Fig. 3a. 15 min XRD pattern of 85/15/7.5 binder.
Fig. 3e. 24 h XRD pattern of 85/15/7.5 binder.
X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073
24 h, which is probably due to the poor crystallinity of AFm phase [29]. For the 92.5/7.5/3.75 binders, similar results of XRD patterns are obtained in Figs. 4(a)–(e). But no AFt forms within 15 min, very little AFt forms within 0.5 h, and after then AFt forms in all samples, which is different from that of the 85/15/7.5 binder. The hydration of silicate in the 92.5/7.5/3.75 binders is more remarkably postponed at high sodium gluconate dosage (Fig. 4(e)) in comparison with the 85/15/7.5 binder, which is consistent with hydration heat analysis (Fig. 1(a)). But the peak of CH in the 92.5/7.5/3.75 binders becomes clear and continues to enhance from 4 h to 24 h. Although the peak of CH declines when sodium gluconate dosage increases, much more CH has formed at 24 h even with 0.24% dosage of sodium gluconate (Fig. 4(e)). 3.1.3. DSC analysis The DSC curves of the 85/15/7.5 binder at 15 min, 0.5 h, 4 h, 8 h and 24 h are respectively shown in Fig. 5(a)–(e). The early hydration properties clarified by DSC analysis are consistent with the XRD results. The endothermic peak at 70–110 °C is ascribed to the dehydration of AFt and C-S-H. At 15 min, AFt forms only in reference sample (0% sample) and sodium gluconate accelerates AFt formation significantly from 0.5 h. For samples with 0% and 0.06% of sodium gluconate, the endothermic peaks at 4 h and 8 h include additional dehydration of C-S-H [1]. The endothermic peak at 120–130 °C is due to the dehydration gypsum. In Fig. 5(a), when the dosage of sodium gluconate increases, the endothermic peak at 120–130 °C becomes more evident, and this reveals that sodium gluconate can inhibit the dissolution of gypsum in PC, especially with high dosage of sodium gluconate. The endothermic peak at 435 °C is the decomposition of CH, which reveals the delayed hydration of silicate as sodium gluconate dosage increases just to a certain extent, because more AFt formed consumes more CH. The endothermic peak at 158 °C is due to the dehydration of AFm. Because no clear peak appears in the temperature range of 200–300 °C which can characterize the amorphous gibbsite, AFm phase come from the transformation of AFt. Obvious transformation from AFt to AFm can be seen at 24 h. This trend is not evident in XRD analysis, and this may be due to the low crystallinity of AFm [1,29].
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Fig. 4b. 0.5 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4c. 4 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4d. 8 h XRD pattern of 92.5/7.5/3.75 binder.
3.1.4. SEM analysis Effects of sodium gluconate on the microscopic morphology of ternary binders (85/15/7.5) were investithe hydrated PC-CAC-CS gated through SEM, and the results are shown in Figs. 6 and 7. Fig. 6 shows the effects of sodium gluconate on the microscopic morphology of the 2 h-hydrated binders, and Fig. 7 shows that of
Fig. 4e. 24 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4a. 15 min XRD pattern of 92.5/7.5/3.75 binder.
the 8 h-hydrated binders. At 2 h, when no sodium gluconate is added, a large number of needle-like AFt forms, and the shape of these crystals is relatively small and irregular. When the dosage of sodium gluconate increases, the formed AFt needles get larger and more regular in comparison with the reference sample. At 8 h, the shape of AFt is much better defined, exhibiting typical hexagonal crystals. The thickness as well as the length of the AFt needles is evidently larger than that observed in the reference sam-
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(a) 15min
(b) 0.5h
(c) 4h
(d) 8h
(e) 24h Fig. 5. DSC curves of the 82.5/15/7.5 binder.
(a) without sodium gluconate
(b) with 0.12% sodium gluconate
(c) with 0.24% sodium gluconate
ternary binders (a) without sodium gluconate; (b) with 0.12% sodium gluconate; (c) with 0.24% sodium gluconate. Fig. 6. SEM images of 2 h-hydrated PC-CAC-CS
X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073
(a) without sodium gluconate
(b) with 0.12% sodium gluconate
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(c) with 0.24% sodium gluconate
ternary binders (a) without sodium gluconate; (b) with 0.12% sodium gluconate; (c) with 0.24% sodium gluconate. Fig. 7. SEM images of 8 h-hydrated PC-CAC-CS
ples. The SEM analysis result is in good consistent with the XRD analysis as well as isothermal calorimetric analysis, which approves that the addition of sodium gluconate promotes the formation of AFt once AFt begins to form. In addition, while the effect of sodium gluconate on the mortar strength is investigated, some specimen with higher sodium gluconate dosage appears crack on the surface just before 24 h, which may not only be relative to more AFt formation but also the changes of its microstructure mentioned above. 3.2. Mortar performance 3.2.1. Compressive strength The compressive strengths of mortar for the two kinds of ternary binders are exhibited in Figs. 8(a) and (b). When more than 0.06% sodium gluconate is added into ternary binders, it significantly decreases the mechanical strength of the 92.5/7.5/3.75 binders (Fig. 8(a)), but hardly influences the mechanical strength of the 82.5/15/7.5 binders (Fig. 8(b)). The compressive strength at 3 d and 28 d for both binders fluctuates, when the amount of sodium gluconate increases. Low content of sodium gluconate (0.06%) can improve the compressive strength at 3 d, but it has little influence on compressive strength at 1 d and 28 d on the whole. The ternary binder with higher amount of CAC always has lower 28 d compressive strength with the same dosage of sodium gluconate (including 0% of sodium gluconate).
Fig. 8b. Compressive strength of 85/15/7.5 mortar.
3.2.2. Fluidity As seen in Figs. 9(a) and (b), integrally, the addition of sodium gluconate improves the fluidity of these two ternary binders. A
Fig. 9a. Fluidity of 92.5/7.5/3.75 ternary binder.
Fig. 8a. Compressive strength of 92.5/7.5/3.75 mortar.
small amount of sodium gluconate (0.06%) remarkably improves the initial fluidity of the two binders, especially the binder with high amount of CAC, with no initial fluidity entirely. But the initial fluidity cannot be further improved when the dosage continues to increase. On the contrary, with 0.12% of sodium gluconate, the fluidity of both mortars decreases, which is a strange phenomenon and needs to investigate further. After mixing for 30 min, both mortars have no fluidity at all (bottom inner diameter of the mould is 100 mm) no matter how much sodium gluconate is applied. When used alone, sodium gluconate is not an effective retarder
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Fig. 9b. Fluidity of 85/15/7.5 ternary binder.
ternary system to control fluidity loss, which is difin PC-CAC-CS ferent from tartaric acid [1]. 3.2.3. Setting time Effects of sodium gluconate on the setting of CAC rich binder are shown in Fig. 10. With the increase of the dosage of sodium gluconate, the initial and the final setting are appropriately delayed simultaneously. Even at 0.24% of sodium gluconate, the initial setting time is less than 20 min and the final time is less than 35 min. So sodium gluconate is neither a superactive set retarder nor an effective fluidity loss control retarder for CAC rich ternary binder. 4. Discussions According to the results of hydration study above, sodium gluconate postpones the AFt formation slightly and results in a following significant acceleration once AFt forms. As seen in Fig. 3(a) and Fig. 4(a) or Fig. 5(a), sodium gluconate inhibits the dissolution of gypsum especially at high dosage. Anhydrite has lower dissolution rate than gypsum [12], so it seems reasonable that AFt cannot form because of the deficiency of sulfate ions. Similar results and interpretations can be found in the research about the effects of sodium gluconate on the performance of Portland cement [23]. However, the inhibited dissolution of gypsum is not the only reason in the present ternary binders, due to the fact that the dissolution of gyp-
Fig. 10. Setting time of the 85/15/7.5 ternary binder.
sum is not inhibited completely and no other calcium aluminate hydrates appears even while no AFt forms (Fig. 3(a) and Fig. 4 (a)). So sodium gluconate inhibiting the hydration of CA or C3A temporarily may be another important reason. Further research should be taken on how sodium gluconate affects the formation of AFt especially in the ternary binders, and there are still some unclear aspects. There are mainly two presumptions to explain the mechanisms about the hindrance on the hydration, i.e. nuclear poisoning and surface adsorption. Based on the recent research results, we infer that sodium gluconate can be strongly adsorbed onto the surface of aluminate phases due to its negatively charged surface [17,30,31], which hinders the permeation of sulfate ions and water to react with aluminate phases. However, once the water and sulfate ions contact CA or C3A by diffusion process, the transiently blocked chemical reaction starts immediately [19,30]. A low content of sodium gluconate or sugar may boost AFt formation during hydration of Portland cement, which has also been reported before [23]. Bishop and Barron suggested that the accelerated AFt formation can be ascribed to the complex between sugar and calcium ions, which in turn liberates the sulfate ions to react with C3A [30]. However, in the present research, the dissolution of gypsum is inhibited, so the real mechanism is still unclear. It may be the chemical environment which influences the growth orientation of AFt crystals [32], as shown in Figs. 6 and 7. And more thorough research should be taken through testing the ion concentrations in the pore solutions with time evolution. Effects of sodium gluconate on hydration of silicate in present study may be explained by surface adsorption and the poisoning on the nucleation and growth of C-S-H or portlandite [21,32]. In one respect, sodium gluconate may adsorb directly onto the reactive sites of PC particles, interfering but not locking its dissolution because no amorphous gibbsite can be distinguished when AFt forms, as seen in Fig. 5. Additionally, we infer that sodium gluconate may poison the nucleation and growth of C-S-H, but not the nucleation and growth of CH crystals, possibly due to the fact that CH crystals have formed before silicate begins to hydrate obviously, which is shown in Fig. 1(a) and Fig. 4(d). As shown in Figs. 1 and 2, sodium gluconate delays the hydration of silicates. With the increase of sodium gluconate dosage, the slightly decreased hydration rate of silicates may be ascribed to more reactive sites of PC particles adsorbed and more AFt formation. According to previous report [23], at 1% dosage, sodium gluconate hinders the dissolution of gypsum and the formation of AFt. In the present research, it is shown that less sodium gluconate will inhibit the dissolution of gypsum and delay the formation of AFt. This can be explained that sodium gluconate prefers to adsorb on the surface of CAC or C3A rather than silicate, the residual sodium gluconate will delay the hydration of silicate by poisoning C-S-H nucleation and adsorbing, and prevents the dissolution of gypsum. In this way, the hydration of C3S is less delayed and has higher reaction rate in CAC rich ternary binder, with addition of the same amount of sodium gluconate, as shown in Figs. 2 (a) and 2(b). The performance of the ternary binder mortar consists with its early hydration process. The amount and the time of AFt forming in the ternary binder determine the fluidity and setting time of cement mortar. Sodium gluconate just delays the formation of AFt slightly, so it just slightly prolongs both the initial and the final setting of the ternary binder. AFt may adsorb a large amount of SP and consumes it in an unproductive way [33,34], sodium gluconate may just inhibit AFt formation before 15 min, so the initial fluidity increases while no fluidity can get at 30 min. Sodium gluconate postpones hydration of silicate, which decreases the compressive strength at 1 d for binder, but it has less effect on that for the 82.5/15/7.5 binder, which may be attributed to more sodium gluconate removed and more AFt formed, as described above.
X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073
92.5/7.5/3.75 binder has higher compressive strength at 28 d, which should be attributed to more Portland cement content. The present research only refers to the study on the effects of sodium gluconate on the hydration of the ternary binder, and much of the mechanism is still unclear, such as sodium gluconate strongly adsorbed onto aluminate phases, the permeation of sulfate ions and water to react with aluminate phases, poisoning the nucleation and growth of C-S-H by sodium gluconate, et al. In the near future, a series of research should be taken to thoroughly investigate the mechanism of how sodium gluconate affects the formation of AFt and hence the workability of the ternary system. And research on the chemical environment and the adsorption kinetics is requisite to understand these mechanisms thoroughly. 5. Conclusions 1. Sodium gluconate inhibits the dissolution of gypsum in PC and delays the formation of AFt slightly, but can accelerate the formation of AFt abruptly and results in better defined crystals with larger thickness and length, once AFt begins to form. 2. Sodium gluconate postpones the hydration of silicate, and reduces its hydration rate slightly. At the same dosage, the hydration of silicate in the CAC poor binder is more remarkably postponed in comparison with that of CAC rich binder, and silicate hydration in CAC poor ternary binder is delayed significantly and has lower reaction rate at high dosage of sodium gluconate. 3. The formation of AFt critically determines the fluidity as well as the setting of ternary binders. Sodium gluconate increases initial mortar fluidity but cannot decrease the fluidity loss with time evolution. Sodium gluconate prolongs both initial setting time and final setting time of the CAC rich binder very slightly. Sodium gluconate decreases the mechanical properties at 1 d of the CAC poor binder, but has less effect on that of the CAC rich binder. 4. The slightly delayed formation of AFt in ternary binder caused by the addition of sodium gluconate should be ascribed to the combined action that it impedes CA or C3A hydration and inhibits the dissolution of gypsum simultaneously. Acknowledgements This research was financial support by the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (SYSJJ2014-12), and Suzhou University of Science and Technology. References [1] X.W. Zhang, C.X. Lu, J.Y. Shen, Influence of tartaric acid on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder, Constr. Build. Mater. 112 (2016) 877–884. [2] S. Lamberet, Durability of Ternary Binders Based on Portland Cement, Calcium Aluminate Cement and Calcium Sulphate (Thèse No. 3151), ècole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, 2005. [3] S. Maier, Ternary system: calcium aluminate cement-portland cement gypsum, Calcium aluminate cements, in: Proceedings of the Centenary Conference, Avignon, 2008. [4] K. Onishi, T.A. Bier, Investigation into relations among technological properties, hydration kinetics and early age hydration of self-leveling underlayments, Cem. Concr. Res. 40 (7) (2010) 1034–1040. [5] T. Emoto, T.A. Bier, Rheological behavior as influenced by plasticizers and hydration kinetics, Cem. Concr. Res. 37 (5) (2007) 647–654.
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[6] P. Gu et al., Early strength development and hydration of ordinary Portland cement calcium aluminate cement pastes, Adv. Cem. Based Mater. 6 (2) (1997) 53–58. [7] P. Gu et al., A study of the hydration and setting behavior of opc-hac pastes, Cem. Concr. Res. 24 (4) (1994) 682–694. [8] P. Gu, J.J. Beaudoin, A conduction calorimetric study of early hydration of ordinary Portland cement high alumina cement pastes, J. Mater. Sci. 32 (14) (1997) 3875–3881. [9] L.L. Xu, P.M. Wang, G.F. Zhang, Formation of ettringite in Portland cement/calcium aluminate cement/calcium sulfate ternary system hydrates at lower temperatures, Constr. Build. Mater. 31 (2012) 347–352. [10] L.L. Xu et al., Effect of calcium aluminate cement variety on the hydration of portland cement in blended system, J. Wuhan Univ. Technol.-Mater. Sci. Edit. 29 (4) (2014) 751–756. [11] M. Gawlicki, W. Nocun-Wczelik, L. Bak, Calorimetry in the studies of cement hydration, J. Therm. Anal. Calorim. 100 (2) (2010) 571–576. [12] L.L. Xu, P.M. Wang, G.F. Zhang, Calorimetric study on the influence of calcium sulfate on the hydration of Portland cement-calcium aluminate cement mixtures, J. Therm. Anal. Calorim. 110 (2) (2012) 725–731. [13] D. Torrens-Martin, L. Fernandez-Carrasco, Effect of sulfate content on cement mixtures, Constr. Build. Mater. 48 (2013) 144–150. [14] D. Torrens-Martin, L. Fernandez-Carrasco, M.T. Blanco-Varela, Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate, J. Therm. Anal. Calorim. 114 (2) (2013) 799–807. [15] C. Evju, S. Hansen, The kinetics of ettringite formation and dilatation in a blended cement with beta-hemihydrate and anhydrite as calcium sulfate, Cem. Concr. Res. 35 (12) (2005) 2310–2321. [16] G. Zhang, G.X. Li, Y.C. Li, Effects of superplasticizers and retarders on the fluidity and strength of sulphoaluminate cement, Constr. Build. Mater. 126 (2016) 44–54. [17] G.X. Li et al., Effects of two retarders on the fluidity of pastes plasticized with aminosulfonic acid-based superplasticizers, Constr. Build. Mater. 26 (1) (2012) 72–78. [18] F.Z. Hongbo Tan, Baoguo Ma, Yulin Guo, Xiangguo Li, Junpeng Mei, Effect of competitive adsorption between sodium gluconate and polycarboxylate superplasticizer on rheology of cement paste, Constr. Build. Mater. 144 (2017) 338–346. [19] M. Zajac et al., Effect of retarders on the early hydration of calcium-sulphoaluminate (CSA) type cements, Cem. Concr. Res. 84 (2016) 62–75. [20] A. Salhan, J. Billingham, A.C. King, The effect of a retarder on the early stages of the hydration of tricalcium silicate, J. Eng. Math. 45 (3–4) (2003) 367–377. [21] J. Cheung et al., Impact of admixtures on the hydration kinetics of Portland cement, Cem. Concr. Res. 41 (12) (2011) 1289–1309. [22] K.L. Scrivener, P. Juilland, P.J.M. Monteiro, Advances in understanding hydration of Portland cement, Cem. Concr. Res. 78 (2015) 38–56. [23] S.H. Ma et al., Influence of sodium gluconate on the performance and hydration of Portland cement, Constr. Build. Mater. 91 (2015) 138–144. [24] Tenoutasse, N. S.N., Effect of glucose and calcium gluconate on the hydration of Portland cement, Ind. J. Technol. 16 (5) (1978) 184–189. [25] M. Nb, Hydration of tricalium silicate in the presence of lignosulfates glucose and sodium gluconate, J. Am. Ceram. Soc. 62 (1979) 321–324. [26] Y. He, X. Zhang, R.D. Hooton, Effects of organosilane-modified polycarboxylate superplasticizer on the fluidity and hydration properties of cement paste, Constr. Build. Mater. 132 (2017) 112–123. [27] H.F.W. Taylor, Cement Chemistry. London: Thomas Telford Services Ltd, Thomas Telford Services Ltd, London, 1997. [28] T. Opravil et al., The synthesis and characterization of an expansive admixture for M-type cements I. The influence of free CaO to the formation of ettringite, J. Therm. Anal. Calorim. 111 (1) (2013) 517–526. [29] L. Pelletier-Chaignat et al., Influence of the calcium sulphate source on the hydration mechanism of Portland cement-calcium sulphoaluminate clinkercalcium sulphate binders, Cem. Concr. Compos. 33 (5) (2011) 551–561. [30] N.B. Singh, Effect of gluconates on the hydration of cement, Cem. Concr. Res. 6 (4) (1976) 455–460. [31] M. Bishop, A.R. Barron, Cement hydration inhibition with sucrose, tartaric acid, and lignosulfonate: analytical and spectroscopic study, Ind. Eng. Chem. Res. 45 (21) (2006) 7042–7049. [32] N.B. Milestone, Hydration of tricalcium silicate in the presence of lignosulfonates, glucose and sodium gluconate, J. Am. Ceram. Soc. 62 (1979) 321–324. [33] J. Plank, C. Hirsch, Impact of zeta potential of early cement hydration phases on superplasticizer adsorption, Cem. Concr. Res. 37 (4) (2007) 537–542. [34] R.J. Flatt, Y.F. Houst, A simplified view on chemical effects perturbing the action of superplasticizers, Cem. Concr. Res. 31 (8) (2001) 1169–1176.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of sodium gluconate on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder Xiaowei Zhang a,b,⇑, Yan He a, Chunxia Lu c, Zhou Huang a a
School of Civil Engineering, Suzhou University of Science and Technology, Suzhou 215011, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China c Suzhou Fangzheng Engineering Technology Development and Testing Co., Ltd, Suzhou 215152, China b
h i g h l i g h t s Sodium gluconate delays AFt formation slightly and then promotes it. Sodium gluconate postpones silicate hydration and reduces its hydration rate slightly. Silicate hydration is delayed less and has higher reaction rate in CAC rich ternary binder at high sodium gluconate dosage. Inhabiting gypsum dissolution and impeding CA or C3A hydration delays the AFt formation. Sodium gluconate results in better defined AFt crystals with larger thickness and length.
a r t i c l e
i n f o
Article history: Received 17 May 2017 Received in revised form 22 September 2017 Accepted 24 September 2017 Available online 6 October 2017 Keywords: Portland cement Calcium aluminate cement Anhydrite Sodium gluconate Early hydration Ettringite
a b s t r a c t Effects of sodium gluconate on early hydration of the Portland cement-calcium aluminate cement ternary binder were investigated through isothermal calorimetric, X-ray diffracanhydrite (PC-CAC-CS) tion analysis (XRD), differential scanning calorimeter analysis (DSC) and scanning electron microscope (SEM). Additionally, mortar performances of the ternary binders, including mechanical strength, fluidity as well as setting time were studied. The results show that sodium gluconate firstly delays ettringite (AFt) formation slightly and then promotes the AFt formation once it begins to form. Sodium gluconate postpones the hydration of silicate and reduces its hydration rate slightly. The hydration of silicate is delayed less in calcium aluminate cement (CAC) rich ternary binder at the same sodium gluconate dosage. High dosage of sodium gluconate will cause significant delay of silicate hydration in CAC poor ternary binder and lower hydration rate. Sodium gluconate impedes CA hydration and inhibits the dissolution of gypsum synchronously, which may lead to the slight delay of AFt formation. Moreover, sodium gluconate influences the growth orientation of AFt, which shows better defined as well as larger shape. Sodium gluconate is not an ideal retarder for fluidity loss control in ternary binder, which increases initial mortar fluidity but cannot control fluidity loss. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, ternary binder, composed of Portland cement (PC), has been calcium aluminate cement (CAC) and calcium sulfate (CS), widely applied in self-leveling mortar as well as grouting mortar [1,2], which is due to its dominant performances, such as shrinkage compensating, rapid hardening [1,3–5]. ternary binHowever, the hydration process of the PC-CAC-CS ders is much more complicated compared with PC, and has drawn ⇑ Corresponding author at: School of Civil Engineering, Suzhou University of Science and Technology, No. 1701 Binhe Road, Hi-Tec District, Suzhou, China. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.conbuildmat.2017.09.153 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
great attention of researchers [6–10]. It has been reported that the composition proportion of the ternary binder significantly influence the performance of the compound, including the setting time, the strength development, the microstructure et al. [9–13]. In PCCAC binary binder, the rapid formation of AFt results in rapid setting and quick strength development [6], but the hydration of PC is ternary binders, postponed [7,8,11]. By comparison, in PC-CAC-CS through XRD analysis and conduction calorimetric studies, it was accelerates the hydration of C3S, which enhances found that CS the late strength [12,14]. With high amount of CAC and low in the PC-CAC-CS ternary binders, AFt forms firstly amount of CS and the formation of monosulfoaluminate (AFm) unlocks the hydration of silicates, while with high content of both CAC and
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AFt forms firstly, then silicates hydrates subsequently, and CS, finally the remaining aluminates reacts. But according to recent advances [15], there is no such AFt inhibiting layer formation during early hydration of PC, so how PC hydrates in ternary binder and how its hydration is postponed may need to study further. Except composition proportions, many other factors may also ternary binders, such affect the hydration process of the PC-CAC-CS as temperature [9], humidity [13], the source of raw materials [12,13,15] as well as chemical admixtures [1,5]. Among these factors, the application of chemical admixtures to regulate the perfor ternary binders is practical, due to the fact mance of the PC-CAC-CS that chemical admixtures not only change the hydration process, but also improve the fluidity as well as setting performance of ternary binders, which is extremely required for the PC-CAC-CS self-leveling or grouting mortar. So, the application of retarders ternary binders is primarily together with SPs in the PC-CAC-CS necessary to improve fluidity retainability as well as setting performance [1,5,16,17]. However, the usage of retarders as well as SPs ternary system makes the hydration process of the PC-CAC-CS more sophisticated. Among the retarders, Sodium gluconate, as a crystalline powder, is widely used together with superplasticizers (SP) in PC due to its good retardation action, aiming to improve the slump loss and fluidity of concrete [18–22]. The retardation action of sodium gluconate has reported to be primarily due to the delay of cement hydration, through the adsorption on particle surfaces or the complexion effect with calcium ions [23,24]. Milestone NB thought that a small amount of sodium gluconate can impede the nucleation as well as the growth of hydrates [25]. However, through XRD as well as DSC analysis of the hydrated samples, Suhua M. et al. [23] found that a low dosage of sodium gluconate would promote the AFt formation, while high amount of sodium gluconate (higher than 1.0%) prolongs the entire hydration process, through hindering the dissolution of gypsum. However, the function mechanism of sodium ternary binders seems to be gluconate applied in the PC-CAC-CS comparatively different from that applied in the PC. The improper application of sodium gluconate can results in serious problems, such as abnormal setting and high fluidity loss [23], which is detrimental to the engineering project. The aim of this research is to preliminarily understand the func ternary tional mechanism of sodium gluconate in the PC-CAC-CS binders. The effect of gluconate on the early hydration process, systems is as well as the mortar performance of the PC-CAC-CS studied.
2. Experimental 2.1. Materials Portland cement typeII 52.5R and calcium aluminate cement CA50 used in this study were obtained from JinYang Cement Co. and Changchen Cement Co. respectively, and a commercial natural anhydrite was from Nanjing. The chemical compositions and mineralogical compositions of the cements provided by the manufacturer are shown in Tables 1 and 2. Industrial grade sodium gluconate was from Xiwang Chemistry Co, and polycarboxylatebased superplasticizer (SP) in dry power type was from XinBang
Co. The nature river sand applied in this research was smaller than 1.25 mm. 2.2. Mix proportions Two types of ternary binder were applied in this research, i.e. mass ratio of 92.5:7.5:3.75 and 85:15:7.5 respectively, PC-CAC-CS which were typical binders of grouting mortar in china for different expansion rate request. All the samples had a constant SP Four differdosage of 0.25% based on total binder (PC + CAC + CS). ent dosages of sodium gluconate, 0.06%, 0.12%, 0.18% and 0.24% were chosen, and the 0% sample was taken as the control to study the effects of sodium gluconate on properties of ternary binder. The mix proportion of the samples and relative measurements are shown in Table 3. 2.3. Methods Fluidity and mechanical strength of cement mortar were tested according to the Chinese Industry Standard GB T 50448-2008, and the setting time was determined according to ASTMC191 [1]. Early hydration of ternary binder was investigated by hydration heat evolution measurement (isothermal calorimetric) [26], X-ray diffraction analysis (XRD) and differential scanning calorimeter analysis (DSC) [1]. Microstructure observation of the hydrated binders was conducted through Quanta 250 FEG scanning electron microscope (SEM). The small pieces of hydrated binders were firstly immersed into alcohol for 24 h, and then dried at 40 °C for 24 h. Before SEM test, the sample was gold coated under 20 mA electrical current for 2 min. For XRD and DSC analysis, the dried samples were further ground to less than 63 lm. XRD analysis was performed on a Bruker D8 system using CuK radiation in the range of 5–60° (2h), with step of 0.04°/s. DSC analysis was carried out on a NETZSCH/STA449F3JUPITER system, sample was heated under N2 over a temperature range of 30–650 °C at a rate of 20 °C/min. For calorimetric analysis, the paste (containing 4 g binder) was premixed for 3 min in the vial then introduced into the sample compartment of conduction calorimeter (TAM Air from Thermometric, Sweden) operating at 20 ± 0.1 °C. Data logging was continued for about 72 h. 3. Results 3.1. Early hydration 3.1.1. Calorimetric analysis Effects of sodium gluconate on the heat flow of the two kinds of binders are exhibited in Figs. 1(a) and (b). Without sodium gluconate, only two exothermic peaks exist for both binders. The first exothermic peak should be ascribed to the contacting of mineral grains with water, the dissolution of various ions and the early formation of AFt that results from the reaction between CAC (or C3A in PC) and anhydrite (or gypsum in PC) [1,27]. The second exothermic peak is attributed to the hydration of silicate phases, and the stage between the first and the second peaks is the so-called induction period [27]. With addition of sodium gluconate, a new exothermic peak closely following the initial peak emerges. The new exothermic peak
Table 1 Chemical composition of cement (wt%).
PC CAC
CaO
SiO2
Al2O3
Fe2O3
MgO
TiO2
SO3
K2O
Na2O
SUM
65.42 34.62
21.57 8.27
5.02 50.71
3.35 1.92
0.77 0.84
0.25 2.76
2.72 –
0.41 –
0.17 –
99.67 99.40
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PC CAC
C3S
C2S
C3A
C4AF
R2O
60.30
13.93
7.96
10.00
0.51
CA
CA2
CT
C2F
C2AS
MA
SUM
36.38
14.08
4.69
3.26
37.71
2.97
92.70 99.09
Table 3 Mix proportions of the samples and relative measurements. nder (PC:CAC: CS)
92.5:7.5:3.75 85:15:7.5
SP (%)
Sodium gluconate (%)
Strength
Fluidity
Setting time
DSC
(by the weight of binder)
(mortar)
(paste)
0.25
Water/binder = 0.30 Sand/binder = 1.2
Water/binder = 0.22
0, 0.06, 0.12, 0.18, 0.24
XRD
SEM
Calorimetry
Water/ binder = 0.30
Note: All the samples were dry mixed for an hour in a mortar mixer at low speed before water was added.
Fig. 1a. Heat evolution rate of 92.5/7.5/3.75 binder.
Comparing Fig. 1(a) with Fig. 1(b), it shows that in the CAC rich samples, the new exothermic peak comes out a little earlier and the corresponding heat evolution rate is elevated when the same dosage of sodium gluconate is added, which should be related to more CA in CAC rich binder, but the induction period is less prolonged, especially at high sodium gluconate dosage. For example, with 0.24% sodium gluconate added in binder, an extremely delayed hydration of silicate is observed in Fig. 1(a), while the content of sodium gluconate has relatively less effects on the induction period of CAC rich samples as shown in Fig. 1(b). As seen in Figs. 2(a) and (b), the effects of sodium gluconate on hydration of silicate can be seen more clearly when the hydration peak of silicate is normalized per g of silicate according the mineralogical composition in Table 2 [15]. Without sodium gluconate, the CAC poor ternary binder has higher heat evolution rate, which should be related to less CA content, less CA results in less suppression of silicate hydration [14]. Sodium gluconate delays silicate hydration, and reduces its hydration rate slightly at the same time. For CAC rich binder with higher sodium gluconate content, silicate hydration not only is delayed less, but also has higher reaction rate.
3.1.2. XRD analysis It may be a result from preferred orientation of natural anhydrite crystal, the peak of anhydrite in all the XRD patterns changes irregularly, which makes it difficult to trace the hydration of the ternary system by the change of the anhydrite peak.
Fig. 1b. Heat evolution rate of 85/15/7.5 binder.
cannot be easily distinguished and is due to the formation reaction of AFt [1], which can be further confirmed by XRD analysis. When the dosage of sodium gluconate increases, the new exothermic peak appears slightly delayed and heat evolution rate of the new exothermic peak reduces. Simultaneously, the induction period is prolonged and the reaction of silicate is delayed.
Fig. 2a. Heat evolution rate normalized to silicate (CAC poor binder).
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Fig. 3b. 0.5 h XRD pattern of 85/15/7.5 binder.
Fig. 2b. Heat evolution rate normalized to silicate (CAC rich binder).
The XRD patterns of the 85/15/7.5 binder are exhibited in Figs. 3 (a)–(e). Shown in Fig. 3(a), without sodium gluconate added in binders, the formation of AFt is immediate. Sodium gluconate delays ettringite formation. With higher than 0.18% sodium gluconate added in ternary binders, it remarkably inhibits the dissolution of gypsum. But sodium gluconate has very weak capability of delaying the formation of AFt, as AFt emerges promptly after 0.5 h of hydration, which can be seen in Fig. 3(b). From 0.5 h to 8 h, with sodium gluconate dosage increasing in binders, more AFt forms in comparison with the reference sample (without sodium gluconate added), which is consistent with the appearance of the new peak in Fig. 1(b), so the new exothermic peak in Fig. 1(b) is ascribed to the reaction of CA (or C3A) and sulfates (anhydrite or gypsum in PC) [1]. Comparing the peak of the silicate in Figs. 3(c)–(e), as the dosage of sodium gluconate increases, the silicate hydration is more significantly postponed, which is coincident with the hydration heat analysis (Fig. 1). The peak of Ca(OH)2 (CH) in all the sample is weak even at 24 h (Fig. 3(e)), which means CH is consumed due to the formation of AFt [28]. In addition, the peak of AFm is substantially weak even when the peak of AFt has reduced at
Fig. 3c. 4 h XRD pattern of 85/15/7.5 binder.
Fig. 3d. 8 h XRD pattern of 85/15/7.5 binder.
Fig. 3a. 15 min XRD pattern of 85/15/7.5 binder.
Fig. 3e. 24 h XRD pattern of 85/15/7.5 binder.
X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073
24 h, which is probably due to the poor crystallinity of AFm phase [29]. For the 92.5/7.5/3.75 binders, similar results of XRD patterns are obtained in Figs. 4(a)–(e). But no AFt forms within 15 min, very little AFt forms within 0.5 h, and after then AFt forms in all samples, which is different from that of the 85/15/7.5 binder. The hydration of silicate in the 92.5/7.5/3.75 binders is more remarkably postponed at high sodium gluconate dosage (Fig. 4(e)) in comparison with the 85/15/7.5 binder, which is consistent with hydration heat analysis (Fig. 1(a)). But the peak of CH in the 92.5/7.5/3.75 binders becomes clear and continues to enhance from 4 h to 24 h. Although the peak of CH declines when sodium gluconate dosage increases, much more CH has formed at 24 h even with 0.24% dosage of sodium gluconate (Fig. 4(e)). 3.1.3. DSC analysis The DSC curves of the 85/15/7.5 binder at 15 min, 0.5 h, 4 h, 8 h and 24 h are respectively shown in Fig. 5(a)–(e). The early hydration properties clarified by DSC analysis are consistent with the XRD results. The endothermic peak at 70–110 °C is ascribed to the dehydration of AFt and C-S-H. At 15 min, AFt forms only in reference sample (0% sample) and sodium gluconate accelerates AFt formation significantly from 0.5 h. For samples with 0% and 0.06% of sodium gluconate, the endothermic peaks at 4 h and 8 h include additional dehydration of C-S-H [1]. The endothermic peak at 120–130 °C is due to the dehydration gypsum. In Fig. 5(a), when the dosage of sodium gluconate increases, the endothermic peak at 120–130 °C becomes more evident, and this reveals that sodium gluconate can inhibit the dissolution of gypsum in PC, especially with high dosage of sodium gluconate. The endothermic peak at 435 °C is the decomposition of CH, which reveals the delayed hydration of silicate as sodium gluconate dosage increases just to a certain extent, because more AFt formed consumes more CH. The endothermic peak at 158 °C is due to the dehydration of AFm. Because no clear peak appears in the temperature range of 200–300 °C which can characterize the amorphous gibbsite, AFm phase come from the transformation of AFt. Obvious transformation from AFt to AFm can be seen at 24 h. This trend is not evident in XRD analysis, and this may be due to the low crystallinity of AFm [1,29].
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Fig. 4b. 0.5 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4c. 4 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4d. 8 h XRD pattern of 92.5/7.5/3.75 binder.
3.1.4. SEM analysis Effects of sodium gluconate on the microscopic morphology of ternary binders (85/15/7.5) were investithe hydrated PC-CAC-CS gated through SEM, and the results are shown in Figs. 6 and 7. Fig. 6 shows the effects of sodium gluconate on the microscopic morphology of the 2 h-hydrated binders, and Fig. 7 shows that of
Fig. 4e. 24 h XRD pattern of 92.5/7.5/3.75 binder.
Fig. 4a. 15 min XRD pattern of 92.5/7.5/3.75 binder.
the 8 h-hydrated binders. At 2 h, when no sodium gluconate is added, a large number of needle-like AFt forms, and the shape of these crystals is relatively small and irregular. When the dosage of sodium gluconate increases, the formed AFt needles get larger and more regular in comparison with the reference sample. At 8 h, the shape of AFt is much better defined, exhibiting typical hexagonal crystals. The thickness as well as the length of the AFt needles is evidently larger than that observed in the reference sam-
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(a) 15min
(b) 0.5h
(c) 4h
(d) 8h
(e) 24h Fig. 5. DSC curves of the 82.5/15/7.5 binder.
(a) without sodium gluconate
(b) with 0.12% sodium gluconate
(c) with 0.24% sodium gluconate
ternary binders (a) without sodium gluconate; (b) with 0.12% sodium gluconate; (c) with 0.24% sodium gluconate. Fig. 6. SEM images of 2 h-hydrated PC-CAC-CS
X. Zhang et al. / Construction and Building Materials 157 (2017) 1065–1073
(a) without sodium gluconate
(b) with 0.12% sodium gluconate
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(c) with 0.24% sodium gluconate
ternary binders (a) without sodium gluconate; (b) with 0.12% sodium gluconate; (c) with 0.24% sodium gluconate. Fig. 7. SEM images of 8 h-hydrated PC-CAC-CS
ples. The SEM analysis result is in good consistent with the XRD analysis as well as isothermal calorimetric analysis, which approves that the addition of sodium gluconate promotes the formation of AFt once AFt begins to form. In addition, while the effect of sodium gluconate on the mortar strength is investigated, some specimen with higher sodium gluconate dosage appears crack on the surface just before 24 h, which may not only be relative to more AFt formation but also the changes of its microstructure mentioned above. 3.2. Mortar performance 3.2.1. Compressive strength The compressive strengths of mortar for the two kinds of ternary binders are exhibited in Figs. 8(a) and (b). When more than 0.06% sodium gluconate is added into ternary binders, it significantly decreases the mechanical strength of the 92.5/7.5/3.75 binders (Fig. 8(a)), but hardly influences the mechanical strength of the 82.5/15/7.5 binders (Fig. 8(b)). The compressive strength at 3 d and 28 d for both binders fluctuates, when the amount of sodium gluconate increases. Low content of sodium gluconate (0.06%) can improve the compressive strength at 3 d, but it has little influence on compressive strength at 1 d and 28 d on the whole. The ternary binder with higher amount of CAC always has lower 28 d compressive strength with the same dosage of sodium gluconate (including 0% of sodium gluconate).
Fig. 8b. Compressive strength of 85/15/7.5 mortar.
3.2.2. Fluidity As seen in Figs. 9(a) and (b), integrally, the addition of sodium gluconate improves the fluidity of these two ternary binders. A
Fig. 9a. Fluidity of 92.5/7.5/3.75 ternary binder.
Fig. 8a. Compressive strength of 92.5/7.5/3.75 mortar.
small amount of sodium gluconate (0.06%) remarkably improves the initial fluidity of the two binders, especially the binder with high amount of CAC, with no initial fluidity entirely. But the initial fluidity cannot be further improved when the dosage continues to increase. On the contrary, with 0.12% of sodium gluconate, the fluidity of both mortars decreases, which is a strange phenomenon and needs to investigate further. After mixing for 30 min, both mortars have no fluidity at all (bottom inner diameter of the mould is 100 mm) no matter how much sodium gluconate is applied. When used alone, sodium gluconate is not an effective retarder
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Fig. 9b. Fluidity of 85/15/7.5 ternary binder.
ternary system to control fluidity loss, which is difin PC-CAC-CS ferent from tartaric acid [1]. 3.2.3. Setting time Effects of sodium gluconate on the setting of CAC rich binder are shown in Fig. 10. With the increase of the dosage of sodium gluconate, the initial and the final setting are appropriately delayed simultaneously. Even at 0.24% of sodium gluconate, the initial setting time is less than 20 min and the final time is less than 35 min. So sodium gluconate is neither a superactive set retarder nor an effective fluidity loss control retarder for CAC rich ternary binder. 4. Discussions According to the results of hydration study above, sodium gluconate postpones the AFt formation slightly and results in a following significant acceleration once AFt forms. As seen in Fig. 3(a) and Fig. 4(a) or Fig. 5(a), sodium gluconate inhibits the dissolution of gypsum especially at high dosage. Anhydrite has lower dissolution rate than gypsum [12], so it seems reasonable that AFt cannot form because of the deficiency of sulfate ions. Similar results and interpretations can be found in the research about the effects of sodium gluconate on the performance of Portland cement [23]. However, the inhibited dissolution of gypsum is not the only reason in the present ternary binders, due to the fact that the dissolution of gyp-
Fig. 10. Setting time of the 85/15/7.5 ternary binder.
sum is not inhibited completely and no other calcium aluminate hydrates appears even while no AFt forms (Fig. 3(a) and Fig. 4 (a)). So sodium gluconate inhibiting the hydration of CA or C3A temporarily may be another important reason. Further research should be taken on how sodium gluconate affects the formation of AFt especially in the ternary binders, and there are still some unclear aspects. There are mainly two presumptions to explain the mechanisms about the hindrance on the hydration, i.e. nuclear poisoning and surface adsorption. Based on the recent research results, we infer that sodium gluconate can be strongly adsorbed onto the surface of aluminate phases due to its negatively charged surface [17,30,31], which hinders the permeation of sulfate ions and water to react with aluminate phases. However, once the water and sulfate ions contact CA or C3A by diffusion process, the transiently blocked chemical reaction starts immediately [19,30]. A low content of sodium gluconate or sugar may boost AFt formation during hydration of Portland cement, which has also been reported before [23]. Bishop and Barron suggested that the accelerated AFt formation can be ascribed to the complex between sugar and calcium ions, which in turn liberates the sulfate ions to react with C3A [30]. However, in the present research, the dissolution of gypsum is inhibited, so the real mechanism is still unclear. It may be the chemical environment which influences the growth orientation of AFt crystals [32], as shown in Figs. 6 and 7. And more thorough research should be taken through testing the ion concentrations in the pore solutions with time evolution. Effects of sodium gluconate on hydration of silicate in present study may be explained by surface adsorption and the poisoning on the nucleation and growth of C-S-H or portlandite [21,32]. In one respect, sodium gluconate may adsorb directly onto the reactive sites of PC particles, interfering but not locking its dissolution because no amorphous gibbsite can be distinguished when AFt forms, as seen in Fig. 5. Additionally, we infer that sodium gluconate may poison the nucleation and growth of C-S-H, but not the nucleation and growth of CH crystals, possibly due to the fact that CH crystals have formed before silicate begins to hydrate obviously, which is shown in Fig. 1(a) and Fig. 4(d). As shown in Figs. 1 and 2, sodium gluconate delays the hydration of silicates. With the increase of sodium gluconate dosage, the slightly decreased hydration rate of silicates may be ascribed to more reactive sites of PC particles adsorbed and more AFt formation. According to previous report [23], at 1% dosage, sodium gluconate hinders the dissolution of gypsum and the formation of AFt. In the present research, it is shown that less sodium gluconate will inhibit the dissolution of gypsum and delay the formation of AFt. This can be explained that sodium gluconate prefers to adsorb on the surface of CAC or C3A rather than silicate, the residual sodium gluconate will delay the hydration of silicate by poisoning C-S-H nucleation and adsorbing, and prevents the dissolution of gypsum. In this way, the hydration of C3S is less delayed and has higher reaction rate in CAC rich ternary binder, with addition of the same amount of sodium gluconate, as shown in Figs. 2 (a) and 2(b). The performance of the ternary binder mortar consists with its early hydration process. The amount and the time of AFt forming in the ternary binder determine the fluidity and setting time of cement mortar. Sodium gluconate just delays the formation of AFt slightly, so it just slightly prolongs both the initial and the final setting of the ternary binder. AFt may adsorb a large amount of SP and consumes it in an unproductive way [33,34], sodium gluconate may just inhibit AFt formation before 15 min, so the initial fluidity increases while no fluidity can get at 30 min. Sodium gluconate postpones hydration of silicate, which decreases the compressive strength at 1 d for binder, but it has less effect on that for the 82.5/15/7.5 binder, which may be attributed to more sodium gluconate removed and more AFt formed, as described above.
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92.5/7.5/3.75 binder has higher compressive strength at 28 d, which should be attributed to more Portland cement content. The present research only refers to the study on the effects of sodium gluconate on the hydration of the ternary binder, and much of the mechanism is still unclear, such as sodium gluconate strongly adsorbed onto aluminate phases, the permeation of sulfate ions and water to react with aluminate phases, poisoning the nucleation and growth of C-S-H by sodium gluconate, et al. In the near future, a series of research should be taken to thoroughly investigate the mechanism of how sodium gluconate affects the formation of AFt and hence the workability of the ternary system. And research on the chemical environment and the adsorption kinetics is requisite to understand these mechanisms thoroughly. 5. Conclusions 1. Sodium gluconate inhibits the dissolution of gypsum in PC and delays the formation of AFt slightly, but can accelerate the formation of AFt abruptly and results in better defined crystals with larger thickness and length, once AFt begins to form. 2. Sodium gluconate postpones the hydration of silicate, and reduces its hydration rate slightly. At the same dosage, the hydration of silicate in the CAC poor binder is more remarkably postponed in comparison with that of CAC rich binder, and silicate hydration in CAC poor ternary binder is delayed significantly and has lower reaction rate at high dosage of sodium gluconate. 3. The formation of AFt critically determines the fluidity as well as the setting of ternary binders. Sodium gluconate increases initial mortar fluidity but cannot decrease the fluidity loss with time evolution. Sodium gluconate prolongs both initial setting time and final setting time of the CAC rich binder very slightly. Sodium gluconate decreases the mechanical properties at 1 d of the CAC poor binder, but has less effect on that of the CAC rich binder. 4. The slightly delayed formation of AFt in ternary binder caused by the addition of sodium gluconate should be ascribed to the combined action that it impedes CA or C3A hydration and inhibits the dissolution of gypsum simultaneously. Acknowledgements This research was financial support by the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (SYSJJ2014-12), and Suzhou University of Science and Technology. References [1] X.W. Zhang, C.X. Lu, J.Y. Shen, Influence of tartaric acid on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder, Constr. Build. Mater. 112 (2016) 877–884. [2] S. Lamberet, Durability of Ternary Binders Based on Portland Cement, Calcium Aluminate Cement and Calcium Sulphate (Thèse No. 3151), ècole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, 2005. [3] S. Maier, Ternary system: calcium aluminate cement-portland cement gypsum, Calcium aluminate cements, in: Proceedings of the Centenary Conference, Avignon, 2008. [4] K. Onishi, T.A. Bier, Investigation into relations among technological properties, hydration kinetics and early age hydration of self-leveling underlayments, Cem. Concr. Res. 40 (7) (2010) 1034–1040. [5] T. Emoto, T.A. Bier, Rheological behavior as influenced by plasticizers and hydration kinetics, Cem. Concr. Res. 37 (5) (2007) 647–654.
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