- Email: [email protected]
Effect of retarders on the early hydration and mechanical properties of reactivated cementitious material
Construction and Building Materials 212 (2019) 192–201
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of retarders on the early hydration and mechanical properties of reactivated cementitious material Linglei Zhang a,b,c, Yongsheng Ji a,b,c,⇑, Jun Li c, Furong Gao c, Guodong Huang c a
Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics & Civil Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China c Jiangsu Collaborative Innovation Center for Building Energy Saving and Construction Technology, Xuzhou, Jiangsu 221116, China b
h i g h l i g h t s The retarding effects of different retarders on DCRCM are variable. Borax can effectively reduce initial hydration rate and increase setting time. The best mechanics properties can be obtained when borax amount is 2%. The mechanism for retarding effect of borax on DCRCM paste is analyzed.
a r t i c l e
i n f o
Article history: Received 8 September 2018 Received in revised form 28 March 2019 Accepted 29 March 2019
Keywords: Dehydrated cement paste Retarder Borax Setting time Early hydration
a b s t r a c t Effects of different retarders (gypsum, sodium citrate, sodium gluconate and borax) on setting and hardening behaviors of dehydrated cement paste (DCP) composite reactivated cementitious materials (DCRCM) are studied through determinations of setting time and mechanical properties. Mechanism of Early hydration and hardening is also studied based on hydration kinetics and SEM analysis. The results indicate that sodium citrate and gypsum are of poor retarding effect on DCRCM. Proper amount of borax will effectively retard the setting process. Sodium gluconate is of best retarding effect, whereas it will result in low early strength of DCRCM. When used as a retarder, borax is of obvious retarding effect. It can not only effectively restrain initial hydration process of DCRCM, but also accelerate early hardening process of DCRCM, promoting the establishment and development of mechanics strength. When the mixing amount of borax is 2%, both early and late strengths of DCRCM can reach to the maximum. SEM analysis shows that the hardening paste of DCRCM with borax is characterized by dense microstructure and less defects, greatly improving the mechanical properties of DCRCM. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Recycled aggregate concrete can be used as a building material. Therefore, it is of great environmental value, and lots of researches have been done on it by researchers at home and abroad during recent years, obtaining plenty of research achievements [1–4]. However, Fine powders less than 0.15 mm, mainly consisting of hardened cement paste (HCP), are produced during the preparation process of recycled aggregate concrete by using waste concrete. An effective way of reusing waste concrete is gelling property recy-
⇑ Corresponding author at: Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China. E-mail address: [email protected] (Y. Ji). https://doi.org/10.1016/j.conbuildmat.2019.03.323 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
cling of these fine powders, which has been a major concern of waste concrete reutilization [5–7]. Researches show that dehydrated cement paste (DCP), obtained from HCP with high temperature calcination, has partially recovered cementitious capacity [8–11]. In our previous research [12], DCP can be modified with GGBFS based on micro-ball milling effect and micro-aggregate effect, producing DCP composite cementitious material (DCRCM) with excellent properties. But when the DCRCM is used as a cementitious material, there are still some inevitable problems such as short setting time and quick fluidity loss, which will not meet construction requirements in practical engineering. Therefore, the retarder research of DCRCM is the key to cementitious property recycling of waste concrete. Suitable amount of gypsum is added to solve fast setting problem of Portland cement. The dissolution rate of gypsum is higher
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
than hydration rate of C3A in cement clinker, and thus the hydration product of C3A, i.e. C4AH13, will immediately react with dissolved gypsum to form ettringite, which prevents fast setting due to the formation of network structure from the lamellar crystal of C4AH13 [13]. And it is the basic reason that gypsum is of retarding ability when added to Portland cement. Compared with Portland cement, the microstructure of DCP powders with high specific surface area is loose and porous [14,15]. DCP mainly contains dehydrated phases of C-S-H, which is in metastable state and hydrates fast [16]. Free calcium oxide, produced from dehydration of calcium hydroxide in HCP, will react with water instantaneously to produce large amounts of heat, thus further quickening setting process of DCP. However, the dissolution rate of gypsum, with low dissolubility, is obviously lower than the hydration reaction rate of DCP, which is much higher than Ordinary Portland cement (OPC) [7]. Therefore, gypsum can neither promptly react with aluminate hydration product of DCP nor delay hydrations of dehydrated phases of C-S-H and free calcium oxide. As a result, gypsum cannot work as a retarder to solve the problems of fast setting when DCRCM is used in practice. Due to the retarding effect of gypsum on C3A hydration, other common retarders added to Portland cement, such as sodium gluconate [17,18] and sodium citrate [19], mainly aim at crystallization ratios of both C3S [20]and Ca(OH)2. Hence, whether these retarders will work for DCRCM, which mainly consists of dehydrated phases of calcium silicate hydrate, aluminate and Ca(OH)2, needs further study. Borax, a weakly alkaline salt, is a kind of inorganic retarder [21], and can easily dissolve in water. When dissolved in water, borax can react with Ca2+ to form a calcium-base borate layer in alkaline solution [22]. DCRCM paste is strong alkaline, of which PH value can reach 12. When mixed with DCRCM, borax may be capable of retarding effect due to the fast-produced complex, which may retard hydration and crystal separation of the cementitious system. Moreover, the solubility of borax is far larger than that of gypsum, which makes it more advantageous in terms of retarding effect. The setting process of DCRCM may be delayed in this way. In this paper, the influences of gypsum, sodium citrate, sodium gluconate and borax on setting time and compressive strength of DCRCM are investigated to determine the optimum retarder with well retarding effect and mechanical properties. Heat evolution rate and hydration heat of DCRCM with different mixing amounts of the optimum retarder are also studied. By hydration kinetics and SEM analysis, mechanisms of early setting and hardening process of DCRCM are analyzed.
2. Materials and methodology 2.1. Materials Ordinary Portland cement (OPC) was provided by China United Cement Co., Ltd, with the characteristics presented in Table 1. The Blaine surface areas was 350 m2/ kg.
193
In accordance with the results of a previous study, DCRCM can be obtained by grinding the mixtures of DCP and ground granulated blast-furnace slag (GGBFS) at the mass ratios of 2:1. The detailed procedures of DCRCM preparation are shown in Reference [12]. The chemical compositions of GGBFS and DCP were presented in Tables 1. Chemical grade powders of gypsum (G), sodium citrate (SC), sodium gluconate (SG) and borax (B) with the purity higher than 99% were adopted in the paste formulation. The superplasticizer used here was polycarboxylic acid type with a solid mass fraction of 25%. 2.2. Methodology 2.2.1. Water requirement of normal consistency and setting time Referring to the State Standard of China (GB/T1346-2011), measurement of water requirement of normal consistency and setting time of DCRCM with different retards (sodium citrate, borax, sodium gluconate and gypsum) were determined to investigate the influence of different retarders on water requirement of normal consistency and setting time of DCRCM. Sodium citrate, borax, and sodium gluconate were respectively tested with both amounts of 2% and 4% by mass of DCRCM, and gypsum was tested with both amounts of 4% and 8%. The paste of pure DCRCM without any retarders was used as the control sample. All retarders should be predissolved in the mixing water to assure homogeneous distribution of the retarders in the samples, and then mixed with DCRCM. The dosages were chosen to clearly distinguish the effects of retarders on setting time and compressive strength. However, the effects of retarders might change with dosage variation, which were not investigated in this paper. 2.2.2. Compressive strength All DCRCM mortars were mixed with water-to-binder ratio of 0.5 (the mixture proportions of binders are same to the above 1.2.1 section), and binder-to-sand ratio of 1:3. Then DCRCM mortars were casted into 40 mm 40 mm 160 mm molds to determine the compressive strength in accordance with the specifications of the State Standard of China (GB/T17671-1999). The OPC mortar was also prepared with the same conditions of DCRCM mortars. Specimens were consolidated using a vibrating table and trowel finished. Cast specimens were stored in a moist curing room with humidity of more than 95% RH and temperature of 20 ± 2 °C. Compressive strength was determined using a hydraulic compression tester at 3 days and 28 days. 2.2.3. Hydration kinetics Referring to the State Standard of China (GB/T12959-2008), measurement of heat evolution rate and hydration heat of DCRCM were determined by a semiadiabatic calorimeter to investigate the influence of borax amounts (0%, 1%, 1.5%, 2%, 4%) on the hydration kinetics of DCRCM. All measurements lasted for 3 days. All samples were mixed with water-to-binder ratio of 0.5 and binder-to-sand ratio of 1:3, separately. Each sample contained 450 g DCRCM, 1350 g standard sand (ISO) and 225 g water (with superplasticizer and borax). After fully mixed, the mixer was quickly taken down, and the mortar was mixed by a spoon for several times. Then two mortar samples of (800 ± 1) g were weighted by electronic balance and packed into the measuring container, respectively. 2.2.4. Scanning electron microscopy SEM analysis was performed on the samples of both pure DCRCM and DCRCM with 2% borax (water-binder-ratio 0.5) at hydration time of 3 days and 28 days. The fresh surfaces from the representative samples, without polishing to keep the fractured surface uncontaminated, were used for SEM analysis to investigate the microstructure development of the specimens. The SEM analysis was performed in low vacuum mode using the FEI Quanta 250 with an accelerating voltage of 15 kV.
3. Results and discussions 3.1. Water requirement of normal consistency
Table 1 Chemical composition of OPC, GGBFS and DCP. Materials
OPC
GGBFS
DCP
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 L.O. I
24.55 7.77 3.62 54.59 2.68 0.31 1.5 2.24 1.2
36.51 15.65 1.08 32.93 8.02 0.81 1.11 0.07 1.33
30.26 6.64 2.54 46.47 2.42 0.7 1.63 2.25 4.97
Water requirements of normal consistency of DCRCM with different admixtures are shown in Fig. 1. As shown in Fig. 1, water requirement of normal consistency of pure DCRCM is 0.6. And water requirements of normal consistency of DCRCM, respectively with 4% gypsum, 1% sodium citrate, 1% sodium gluconate and 1% borax, are 0.55, 0.57, 0.53 and 0.42. Compared with pure DCRCM, water requirements of normal consistency of DCRCM with different admixtures are with varying decrease. The more retarders are added, the more water requirement of normal consistency decreases. Among the retarders, sodium citrate is of the poorest effect. Even if the amount of sodium citrate increases to 2%, there
194
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Borax, as a retarder, shows better retarding effect. When borax amount is 1%, initial setting time and final setting time of DCRCM are 45 min and 78 min respectively. And when borax amount increases to 2%, initial setting time and final setting time of DCRCM prolong to 85 min and 125 min respectively, much higher than those of pure DCRCM. Therefore, the setting time of DCRCM with borax is greatly prolonged. When the amount of sodium gluconate is 1%, the initial setting time and final setting time of DCRCM are 90 min and 155 min respectively. And the initial setting time and final setting time respectively reach to 125 min and 205 min in the paste of DCRCM with 2% sodium gluconate. The setting time of DCRCM with sodium gluconate greatly increases and its retarding performance is significantly better than any other retarders. In conclusion, the retarding effects of sodium gluconate, borax, sodium citrate and gypsum tend to be worse gradually. Whereas, all of the retarders can prolong setting time of DCRCM to some degree, which provides basic conditions for the investigation of hydration and hardening properties of DCRCM. Fig. 1. Water requirement of normal consistency.
is no obvious decrease in water requirement of normal consistency of DCRCM. When the borax amount is 2%, water requirement of normal consistency of DCRCM decreases to 0.45. For sodium gluconate, when its mixing amount is 2%, water requirement of normal consistency of DCRCM is 0.35, reducing 41.7% compared with that of DCRCM without any retarders. In conclusion, the influences of sodium citrate, gypsum, borax, and sodium gluconate on water requirement of normal consistency of DCRCM tend to be gradually greater.
3.3. Compressive strength Compressive strengths of DCRCM with different retarders are shown in Fig. 3. It can be obtained from Fig. 3(a), the 3 days compressive strengths of OPC and pure DCRCM are 27.6 MPa and
3.2. Setting time Setting times of DCRCM with different retarders based on the water requirement are shown in Fig. 2. As is shown in Fig. 2, the initial setting time and final setting time of pure DCRCM are 23 min and 45 min respectively. The initial setting time and final setting time of DCRCM with 4% gypsum are 25 min and 48 min respectively. When gypsum amount reaches to 8%, initial setting time only increases to 42 min. Therefore, gypsum has little influence on setting time of DCRCM, which indicates that gypsum is barely capable of retarding when added to DCRCM. When sodium citrate amount is 2%, the initial setting time and final setting time of DCRCM are 40 min and 73 min respectively. Setting time of DCRCM with sodium citrate is remarkably increased compared with that of pure DCRCM, but its setting rate is still quite fast.
Fig. 2. Effect of retarders on the setting time of DCRCM.
Fig. 3. Effect of retarders on the compressive strength of DCRCM: (a) 3 days; (b) 28 days.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
29.5 MPa respectively. The compressive strength of DCRCM with 4% gypsum is equivalent to that of pure DCRCM. When gypsum amount reaches to 8%, the compressive strength decreases slightly (25.6 MPa). While the compressive strength of DCRCM with 2% sodium citrate significantly decreases to 5.6 MPa. Therefore, sodium citrate shows negative effect on early strength development of DCRCM. When the amount of borax is 1%, the compressive strength of DCRCM is 31.4 MPa, slightly higher than that of pure DCRCM. When the borax amount reaches to 2%, the compressive strength of DCRCM reaches the maximum value of 37.4 MPa, 26.7% higher than that of pure DCRCM. Therefore, borax can greatly promote early strength development of DCRCM. When sodium gluconate amount is 2%, the compressive strength of DCRCM is only 10.5 MPa, which results from strong retarding effect of sodium gluconate. Thus, the early hydration rate of DCRCM is quite low and the setting process is excessively prolonged. As a result, the amounts of hydration products decrease, which affects the early strength development of DCRCM. The 28 days compressive strength variations of DCRCM with different retarders are similar to 3 days compressive strength (see Fig. 3(b)). The 28 days compressive strengths of DCRCM, respectively mixed with gypsum and sodium citrate, are lower than that of pure DCRCM. Moreover, the compressive strength tends to decease more with increasing amounts of the two retarders. When sodium gluconate amount is 1%, the compressive strength (42.3 MPa) is slightly higher than that of DCRCM without any retarders (40.5 MPa). The compressive strength of DCRCM with 2% sodium gluconate is equivalent to that of pure DCRCM. Compared with 3-day compressive strength of DCRCM with sodium gluconate, it can be obtained that although sodium gluconate has strong retarding effect on the setting process of DCRCM, leading to lower early strength, it has no negative influence on the development of latter strength. The highest 28 days compressive strength can be obtained when the borax amount is 2% (53.2 MPa), which is 34.3% higher than that of pure DCRCM and 8.8% higher than that of OPC (48.9 MPa). Therefore, borax has good promoting effect on later strength of DCRCM. In conclusion, the retarding effects of gypsum and sodium citrate are quite poor. Moreover, the compressive strengths of DCRCM, respectively mixed with gypsum and sodium citrate, decease to some degree. Sodium gluconate shows the best retarding ability, while it has negative influence on the early strength development. Although the latter strength of DCRCM with sodium gluconate is improved compared with its early strength, the strength increasement is not obvious compared with that of pure DCRCM. Borax exhibits obvious retarding effect and the mechanical property of DCRCM can be significantly improved at both early and later stage. Therefore, borax is used as the retarder for DCRCM, and the mechanisms of its influence on setting and hardening process are studied. 3.4. Hydration kinetics 3.4.1. Exothermic rate Exothermic rate curves of OPC and DCRCM are illustrated in Fig. 4. As shown in Fig. 4, OPC strongly reacts with water when they are mixed together, and the exothermic rate is quite fast due to dissolution of cement minerals and quick hydration of C3A, corresponding to the pre-induction period. Then the hydration progress of OPC enters the induction period, during which period the exothermic rate quickly decreases to a low value for 1–2 h. Whereafter, exothermic rate speeds up due to formation of large amounts of C-S-H. This period is referred to as the acerbation period. Then exothermic rate gradually decreases and enters the steady period. The hydration reaction of pure DCRCM is much violent than OPC, the initial exothermic rate is almost 5 times than OPC and
195
Fig. 4. Effect of borax amounts on the hydration exothermic rate of DCRCM.
lasts for approximately 30 min. Hydration exothermic rate declines obviously with increasing borax amounts during this period, which indicates that initial hydration process (hydration time 30 min) can be effectively retarded by borax. After the violent reaction, the exothermic rate of pure DCRCM becomes lower. This period is known as the induction period. However, time for induction period of pure DCRCM is delayed compared with that of OPC. The induction period lasts for 5–6 h, and this period appears to be shorter with increasing borax amounts. When the borax amount is 2%, there is nearly no induction period. The above results illustrate that apart from obvious setting retarding effect of initial setting, borax can also promote harden process of DCRCM, and the promoting effect is evident with increasing borax amounts. However, when borax amount is 4%, the induction period trend to be longer, which indicates that excessive amounts of borax will result in over setting of DCRCM. After induction period, the exothermic rate of DCRCM speeds up again and the main exothermic rate peak occurs, known as the acceleration period. The main exothermic rate peak of DCRCM will gradually increase with increasing borax amounts, which further indicates that borax can promote hardening process of DCRCM after initial setting. However, the exothermic peak of DCRCM with 4% borax is obviously lower than that of DCRCM with other borax amounts in acceleration period, which is due to the influence of excessive borax amount on hydration process of DCRCM. From the above results, it can be obtained that with increasing borax amounts, the exothermic rate significantly decreases in the preinduction period, and that the induction period will be significantly shortened, which greatly promotes the hydration process of DCRCM after initial setting [23]. However, excessive borax amount will result in longer induction period and lower exothermic rate, and thus the hydration process of cementitious system is severely restrained, which is disadvantageous for the establishment and development of early strength.
3.4.2. Hydration heat Hydration heat curves of OPC and DCRCM are shown in Fig. 5. As shown in Fig. 5, in early hydration stage (usually hydrated for 3 days), hydration heat of pure DCRCM and OPC is 276.0 J/g and 163.4 J/g, respectively. When the amount of borax increases from 1% to 2%, hydration heat of DCRCM increases from 303.1 J/g to 385.5 J/g, which increases 39.67% compared with that of pure DCRCM. This further demonstrates that borax can promote early
196
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Fig. 5. Effect of borax amounts on the hydration heat of DCRCM.
hydration process of DCRCM. When the borax amount is 4%, the setting time of DCRCM will be prolonged, and hydration exothermic rate will be lower, thus resulting in decreasing hydration heat. In conclusion, borax is capable of restraining initial hydration (hydration time 30 min) and promoting early hydration (usually hydrated for 3 days). Therefore, initial hydration rate can be declined and the setting time of DCRCM can be prolonged, and thus the objective of retarding is achieved. Moreover, borax can also improve the early strength due to promoting effect of early hydration of DCRCM. Therefore, borax is a suitable retarder for DCRCM.
At the hydration of 28 days, the hydration products amount of DCRCM without borax will increase with the increase of hydration degree (see Fig. 7(a)), and the bonding between DCP and GGBFS is enhanced. Moreover, good bonding has been formed at the interfacial zones. Large porous structures will be smaller and smaller due to filling of hydration products and thus density is improved (see Fig. 7(b)). However, there still exist some defects inside the hardened paste. Therefore, whole performances of DCRCM tend to be poor. With the amount of hydration products gradually increasing, compactness of hardened paste of DCRCM with 2% borax has been further improved (see Fig. 7(d)). Therefore, the bonding becomes much tighter among the hydration products of DCP and GGBFS particles, and barely no single GGBFS particles can be found (see Fig. 7 (e)), which have been encapsulated by the hydration products of DCP. Thus, the bond strength among the interfacial zones are firmer, resulting in good whole performances. The chemical compositions of the hydration products are measured by an EDS test based on the obtained SEM images. The results of element mapping of Fig. 6(b), (d) and Fig. 7(c), (e) are shown in Fig. 6(c), (f) and Fig. 7(c), (f), which corresponds to pure DCRCM and DCRCM-2%B paste at 3 days and 28 days of age. The results of EDS elemental mapping show that the major elements of hydration products of the DCRCM pastes are calcium, silicon, aluminum and magnesium, which mainly attributes to the formed C-S-H gel, Ca(OH)2 and unhydrated particles of DCP and GGBFS in the pastes. 4. Mechanism analysis 4.1. Quick setting analysis of DCRCM
3.5. Scanning electron microscopy Fig. 6 shows the microstructure of DCRCM pastes with different amounts of borax at 3 days. As is shown in Fig. 6 (a), bonding among hydration products of DCRCM without borax is not tight, and the bonding strength among the hydration products of DCP and GGBFS particles is quite low. Obvious cracks and pores exist at the interfacial zones among hydration products of DCP and GGBFS particles (GGBFS particles are regarded as microaggregate due to relatively weak activity), resulting in relatively loose microstructures and further low mechanical properties. Hydration action of DCP without any retarders can take place quickly to form a large amount of gels in a short time, but the hydration products mainly consist of coarse and complete crystallization like Ca(OH)2, resulting in loose microstructure and poor bonding strength. Therefore, lots of perfect crystalline hydration products and quite a lot of defects exist in the hydration products of DCP, which contributes little to the bonding strength and mechanical property (see Fig. 6 (b)). Fig. 6 (d) and Fig. 6 (e) show the microstructure of DCRCM with 2% borax at 3 days. As is shown in the Fig. 6 (d), interwoven gel particles form the network structure of network gel, of which the flocculation gel accounts for a considerable amount. The gel then interconnects with each other to form integrated structures. Hydration products of DCP can bond well with GGBFS particles, and therefore compact structures and reliably tight bonding can be formed at the interfacial zone (Fig. 6 (e)). Moreover, there is no large porous structures inside the hardened paste of DCRCM, and no perfect crystalline hydration products are found, meaning that the whole structure is compact. The above analyses illustrate that DCP can fully react with GGBFS particles. Meanwhile, apart from good retarding ability, borax can significantly promote establishment and development of mechanics properties for the whole cementitious system, which greatly increases hydration degree of cementitious system.
Cementitious properties of DCRCM are mainly dependent on activities of DCP, and GGBFS is more used as supplementary cementitious materials [24,25]. Therefore, physical, chemical properties and cementitious properties of DCRCM are largely dependent on properties of DCP. The essence of the preparation and usage process of cementitious materials lies in the change process of SiO4 tetrahedron from the polymerization state to the isolated state and then to the polymerization state. While material activation is the depolymerization process of polymeric SiO4 tetrahedron. The preparation process of DCP is a thermal activation process. During this process, the main hydration productions of HCP will dehydrate: ettringite, low sulphated calcium sulfate and calcium hydroxide will decompose, losing combined water to produce dehydrated phases of aluminate, C12A7, and calcium oxide. For C-S-H gel, it will not only dehydrate but also depolymerize, changing into a low polymerization state [26–28]. Generally, the lower polymerization degree of the network structure is, the higher hydraulic activity will be. Therefore, DCP is of high activity. Both dehydrated phases of C12A7 and C-S-H exist in the metastable state; and calcium hydroxide will dehydrate to produce large amounts of f-CaO. As a result, DCP will quickly rehydrate when mixed with water. Moreover, DCP is loose and porous [14,15], resulting in hydration reaction taking place at both sides around the original boundaries of the particles at the same time (see Fig. 8) [12], which greatly increases actual reaction area of hydration. Hence, problem of fast setting exists for DCRCM application, which in return increases the difficulty in fully utilizing it. 4.2. Retarding analysis 4.2.1. Hydration process of DCRCM without retarder Based on the results obtained from the above analysis, the mechanism of borax effect on the setting process of the DCRCM
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
197
Fig. 6. SEM micrographs comparison of the samples at 3 days of age: (a), (b) pure DCRCM, (c) EDS result of pure DCRCM; (d), (e) DCRCM + 2%B, (f) EDS result of DCRCM + 2%B.
paste is illustrated in Fig. 9. DCRCM without any retarders reacts with water quickly (see Fig. 9(a)). The main rehydration reaction contains three parts:
(I) Hydration of C-S-H in oligomer state. The hydration of C-S-H in oligomer state mainly includes two parts. The first one is water molecule and Ca2+ easily entering the original position
198
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Fig. 7. SEM micrographs comparison of the samples at 28 days of age: (a), (b) pure DCRCM, (c) EDS result of pure DCRCM; (d), (e) DCRCM + 2%B, (f) EDS result of DCRCM + 2% B.
of the C-S-H dehydration phase to restore the C-S-H gel structure and the dehydration phase polymerizing with each other, which keeps the structure of C-S-H gel unchanged. The other one is the hydration reaction of b-C2S produced by C-S-H dehydration. Moreover, the C-S-H dehydrated
phase in the metastable state reduces the nucleation barrier of C-S-H gel in the new cementitious system and accelerates the formation of hydration products. Therefore, the hydration rate of C-S-H dehydrated phase is very fast.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
199
Fig. 8. Hydration process of DCP particle.
Fig. 9. Control mechanism of borax on the setting process of DCRCM.
(II) Hydration of calcium oxide. The f-CaO reacts with water quickly to reproduce calcium hydroxide, together releasing large amounts of hydration heat, which can increase heat exothermic rate of the whole cementitious system and make the fresh paste lose its fluidity and set rapidly. (III) Hydration of dehydrated Aluminate phases. The dehydrated Aluminate phases are mainly C12A7. The coordination of Ca and Al in the crystals is extremely irregular, and thus lots
of cavities exist in the structure. Water can easily get into these cavities, which results in quick hydration and setting. In the alkaline condition, C2AH8 with hexagonal structures is quickly formed due to DCRCM hydration [29]. C2AH8 can form network lapping, leading to fast setting of the fresh paste. On the other hand, C12A7 are in metastable state and DCP particles are loose and porous, thus leading to high heat exothermic rate and large hydration area. Therefore, even if
200
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
gypsum is added as a retarder, it cannot promptly react with C2AH8 produced by hydration of C12A7 due to its low dissolubility and dissolution rate. As a result, gypsum has limit retarding ability for the cementitious system. With the hydration reaction continuing, migration and diffusion of the internal hydration products in the system are blocked, and the internal hydration products accumulates in the inner of particles or among a few particles, causing the formation of flocculated structure (see Fig. 9 (b)). The increasing of flocculated structure will result in crystallization of hydration products, and the hydration product crystal is coarse (see Fig. 6 (b).). At the same time, the amount of external hydration products is relatively reduced. Therefore, no effective connections can be formed between the external hydration products and particles outside, resulting in more pores and poor integrity in the cementing system (see Fig. 6 (a).). 4.2.2. Borax retarding analysist 4.2.2.1. Restrain formation of hydration production. As a retarder, borax hydrolyzes to generate more borate ions in water before mixed with DCRCM. Borate ions can react with Ca2+, hydrolyzed production of DCRCM, and aluminate in dehydrated phases to form a complex. The complex is similar to ettringite and its chemical formula is 3CaO.Al2O33Ca(BO2)231H2O [30]. The complex will form a thick film layer around the surfaces of DCRCM particles (see Fig. 9 (c)) [22,31–33], and it can effectively prevent water from entering inside the DCRCM particles compared with common hydration productions. So, water can barely penetrate inside the DCRCM particles, thus significantly restraining the hydration and crystallization of DCRCM. Moreover, the formation of the film layer can effectively consume C12A7 in dehydrated phases, preventing it from hydrating to produce C2AH8. Meanwhile, hydration reaction of DCRCM will only take place outside the original boundaries of the DCRCM particles. Therefore, hydration area and heat exothermic rate are great reduced. Apart from the hydration reaction of generating the complex, borate ions can also react with Ca2+ to form wrapping layers of calcium borate (see Fig. 9(c)) [34]. Wrapping layer of calcium borate is similar to gel. It has the property of semipermeable membrane, so water molecules and smaller ions can slowly penetrate inside the DCRCM particles through the wrapping layers. The wrapping layers will further wrap DCRCM particles to restrain DCRCM hydration and prolong setting time. Furthermore, if the amount of borax is excessive, the retarding effect is significantly improved and hydration rate is greatly reduced. However, the setting time of the cementitious system is too long, resulting in decreasing of hydration products, which makes the establishment and development of early strength greatly restrained. 4.2.2.2. Restrain crystallization of Ca(OH)2. Ion composition and concentration of fresh DCRCM paste are changed when borax is added as a retarder. According to Joisel electrolyte dissolubility theory, when there are strong cations in the solution, such as Na+ and K+, they will reduce dissolubility of other weaker cations, such as Ca2+ [27]. Therefore, Na+, introduced by borax, will reduce Ca2+ dissolubility of DCRCM. Moreover, borate ions can react with Ca2+ in the solution to produce complex and calcium borate. These all help to restrain crystallization of Ca(OH)2. As a result, the amounts of Ca (OH)2 produced by the hydration of free CaO is effectively reduced at the initial hydration period of fresh DCRCM paste. Meanwhile, hydration heat due to the formation of Ca(OH)2 is also reduced, which effectively lowers the hydration temperature and heat exothermic rate of the cementitious system. Therefore, with the increase of borax amount, electrolyte concentration increases at
the initial hydration period of fresh DCRCM paste, and thus Ca2+ dissolubility and Ca(OH)2 crystallization will be further restrained. In conclusion, borax can reduce formation rate of hydration products and restrain the crystallization of Ca(OH)2 in the cementitious system. Thus, the setting process is prolonged. With hydration reaction continuing, unstable complex and borate wrapping layer will be destroyed, resulting in reacceleration of hydration reaction (see Fig. 9 (d)). Based on hydration heat analysis and mechanics strength results, borax can effectively promote the establishment and development of early strength of the cementitious system. Besides, it can greatly improve late strength as well. Therefore, borax can be used as an efficient retarder for DCRCM. 5. Conclusions A systematically experimental study has been designed in order to investigate the effects of different retarders (gypsum, sodium citrate, sodium gluconate and borax) on setting and hardening behaviors of DCRCM. Conclusions can be drawn as follows: 1) The retarding effects of different retarder on DCRCM are variable. Gypsum and sodium citrate have poor retarding effect. Borax can effectively prolong setting process of DCRCM. Sodium gluconate has the best retarding ability but leads to lower early strength. 2) Retarding effect of borax is quite obvious. Borax can effectively reduce initial hydration rate of DCRCM and increase setting time. Meanwhile, borax can quicken the hardening process of DCRCM, thus promoting establishment and development of early strength. Besides, borax can contribute to the increase of the late strength. The best mechanics properties of DCRCM can be obtained when borax amount is 2%. 3) Microstructure analysis suggests that the hydration products of DCP has bonded well with GGBFS particles. Compact structures and reliably tight bonding have been formed at the interfacial zone. With the increase of hydration ages, the microstructure and bonding at the interfacial zone are further enhanced. Moreover, the whole structure of hardened DCRCM paste is extremely compact and with little defects. 4) The mechanism for retarding effect of borax mainly lies in two aspects. On the one hand, borate, from the borax hydrolyzation, can react with Ca2+and aluminate in dehydrated phase, and generate complex film layer and borate wrapping layer, which can prevent water from penetrating inside the particles of the cementitious materials. Thus, the hydration rate of DCRCM will decrease and the generation of hydration products will be restrained. On the other hand, the crystallization of Ca(OH)2 can be effectively restrained due to variations of the initial ion concentration and formation of calcium borate, and thus the setting process of DCRCM is prolonged.
Acknowledgements The authors would like to express their appreciations to the Project supported by ‘‘the Fundamental Research Funds for the Central Universities (2018XKQYMS02). The research works belong to one part of the projects which are financially supported by FRFCU. Conflict of interest The authors declare that they have no conflict of interest to this work.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, and there is no commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] M. Amario, C. Rangel, M. Pepe, R.D.T. Filho, Optimization of normal and high strength recycled aggregate concrete mixtures by using packing model, Cem. Concr. Compos. 84 (2017) 83–92. [2] J.Z. Xiao, C.H. Wang, T. Ding, A. Akbarnezhad, A recycled aggregate concrete high-rise building: Structural performance and embodied carbon footprint, J. Cleaner Prod. 199 (2018) 868–881. [3] S. Omary, E. Ghorbel, G. Wardeh, Relationships between recycled concrete aggregates characteristics and recycled aggregates concretes properties, Constr. Build. Mater. 108 (2016) 163–174. [4] C.J. Shi, Y.K. Li, J.K. Zhang, W.G. Li, L.L. Chong, Z.B. Xie, Performance enhancement of recycled concrete aggregate - A review, J. Cleaner Prod. 112 (2016) 466–472. [5] L.N. Lv, Y.J. He, S.G. Hu, Binding materials of dehydrated phases of waste hardened cement paste and pozzolanic admixture, J. Wuhan Univ. Technol. 24 (2009) 140–144. [6] A. Bordy, A. Younsi, S. Aggoun, B. Fiorio, Cement substitution by a recycled cement paste fine: Role of the residual anhydrous clinker, Constr. Build. Mater. 132 (2017) 1–8. [7] R. Yu, Z.H. Shui, Efficient reuse of the recycled construction waste cementitious materials, J. Cleaner Prod. 78 (2014) 202–207. [8] S.K. Handoo, S. Agarwal, S.K. Agarwal, Physical–chemical, mineralogical, and morphological characteristics of concrete exposed to elevated temperatures, Cem. Concr. Res. 32 (2002) 1009–1018. [9] C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (2004) 3015–3024. [10] G.M. Wang, C. Zhang, B. Zhang, Q. Li, Z.H. Shui, Study on the high-temperature behavior and rehydration characteristics of hardened cement paste, Fire Mater. 39 (2015) 741–750. [11] R. Serpell, M. Lopez, Reactivated cementitious materials from hydrated cement paste wastes, Cem. Concr. Compos. 39 (2013) 104–114. [12] L.L. Zhang, Y.S. Ji, G.D. Huang, J. Li, Y.J. Hu, Modification and enhancement of mechanical properties of dehydrated cement paste using ground granulated blast-furnace slag, Constr. Build. Mater. 164 (2018) 525–534. [13] H.F.W. Taylor, Cement chemistry, 2nd ed., Thomas Telford, London, UK, 1997, pp. 218–221. [14] Q. Zhang, G. Ye, E. Koender, Investigation of the structure of heated Portland cement paste by using various techniques, Constr. Build. Mater. 38 (2013) 1040–1050. [15] G.F. Peng, Z.S. Huang, Change in microstructure of hardened cement paste subjected to elevated temperatures, Constr. Build. Mater. 22 (2008) 593–599. [16] M. Vyšvarˇil, P. Bayer, M. Chromá, P. Rovnaníková, Physico-mechanical and microstructural properties of rehydrated blended cement pastes, Constr. Build. Mater. 54 (2014) 413–420.
201
[17] G. Zhang, G. Li, Y. Li, Effects of superplasticizers and retarders on the fluidity and strength of sulphoaluminate cement, Constr. Build. Mater. 126 (2016) 44– 54. [18] F.B. Zou, H.B. Tan, Y.L. Guo, Effect of sodium gluconate on dispersion of polycarboxylate superplasticizer with different grafting density in side chain, J. Ind. Eng. Chem. 55 (2017) 91–100. [19] I.R.D. Oliveira, A.R. Studart, F.A.O. Valenzuela, V.C. Pandolfelli, Setting behavior of ultra-low cement refractory castables in the presence of citrate and polymethacrylate salts, J. Eur. Ceram. Soc. 23 (2003) 2225–2235. [20] S.H. Ma, W.F. Li, S.B. Zhang, D.S. Ge, J. Yu, X.D. Shen, Influence of sodium gluconate on the performance and hydration of Portland cement, Constr. Build. Mater. 91 (2015) 138–144. [21] Q. Sun, J. Li, J. Wang, Effect of borate concentration on solidification of radioactive wastes by different cements, Nucl. Eng. Design 241 (2011) 4341– 4345. [22] H.B. Tan, Y.L. Guo, F.B. Zou, S.W. Jian, B.G. Ma, Z.Z. Zhi, Effect of borax on rheology of calcium sulphoaluminate cement paste in the presence of polycarboxylate superplasticizer, Constr. Build. Mater. 139 (2017) 277–285. [23] G.X. Li, J.J. Zhang, Z.P. Song, C. Shi, A. Zhang, Improvement of workability and early strength of calcium sulphoaluminate cement at various temperature by chemical admixtures, Constr. Build. Mater. 160 (2018) 427–439. [24] E. Deir, B.S. Gebreziabiher, S. Peethamparan, Influence of starting material on the early age hydration kinetics and the composition of binding gel in alkali activated binder systems, Cem. Concr. Compos. 48 (2014) 108–117. [25] E. Altan, S.T. Erdogan, Alkali activation of a slag at ambient and elevated temperatures, Cem. Concr. Compos. 34 (2012) 131–139. [26] G.Y. Pan, R.Q. Mao, H.L. Zhang, Research of dehydrated calcium sulphoaluminate hydrates (AFm) and its hydration, J. Wuhan Univ. Technol. 19 (1997) 28–30 (In Chinese). [27] G.F. Peng, S.Y.N. Chan, M. Anson, Chemical kinetics of C-S-H decomposition in hardened cement paste subjected to elevated temperatures up to 800 °C, Adv. Cem. Res. 13 (2001) 47–52. [28] L.N. Lv, Y.J. He, S.G. Hu, Structural characteristics of dehydrated phase of hardened cement paste and its rehydrating ability, J. Chin. Ceram. Soc. 36 (2008) (2008) 1343–1347. [29] W. Kurdowski, Cement and Concrete Chemistry, Springer, Netherlands, 2014, pp. 607–611. [30] M. Tong, X.G. Li, Concrete Admixture and its Application, China Architecture and Building Press, Beijing, 2014, pp. 110–112 (In Chinese). [31] S.L. Guo, H.J. Liu, Y. Shan, Y.H. Bu, The application of a combination of treated AMPS/IA copolymer and borax as the retarder of calcium aluminate phosphate cement, Constr. Build. Mater. 142 (2017) 51–58. [32] D.A. Hall, R. Stevens, B. El-Jazairi, The effect of retarders on the microstructure and mechanical properties of magnesia–phosphate cement mortar, Cem. Concr. Res. 31 (2001) 455–465. [33] H.B. Tan, X.F. Deng, B.Q. Gu, Effect of borax and sodium tripolyphosphate on fluidity of gypsum paste plasticized by polycarboxylate superplasticizer, Constr. Build. Mater. 176 (2018) 394–402. [34] M. Zajac, J. Skocek, F. Bullerjahn, Effect of retarders on the early hydration of calcium-sulphoaluminate (CSA) type cements, Cem. Concr. Res. 84 (2016) 62– 75.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of retarders on the early hydration and mechanical properties of reactivated cementitious material Linglei Zhang a,b,c, Yongsheng Ji a,b,c,⇑, Jun Li c, Furong Gao c, Guodong Huang c a
Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics & Civil Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China c Jiangsu Collaborative Innovation Center for Building Energy Saving and Construction Technology, Xuzhou, Jiangsu 221116, China b
h i g h l i g h t s The retarding effects of different retarders on DCRCM are variable. Borax can effectively reduce initial hydration rate and increase setting time. The best mechanics properties can be obtained when borax amount is 2%. The mechanism for retarding effect of borax on DCRCM paste is analyzed.
a r t i c l e
i n f o
Article history: Received 8 September 2018 Received in revised form 28 March 2019 Accepted 29 March 2019
Keywords: Dehydrated cement paste Retarder Borax Setting time Early hydration
a b s t r a c t Effects of different retarders (gypsum, sodium citrate, sodium gluconate and borax) on setting and hardening behaviors of dehydrated cement paste (DCP) composite reactivated cementitious materials (DCRCM) are studied through determinations of setting time and mechanical properties. Mechanism of Early hydration and hardening is also studied based on hydration kinetics and SEM analysis. The results indicate that sodium citrate and gypsum are of poor retarding effect on DCRCM. Proper amount of borax will effectively retard the setting process. Sodium gluconate is of best retarding effect, whereas it will result in low early strength of DCRCM. When used as a retarder, borax is of obvious retarding effect. It can not only effectively restrain initial hydration process of DCRCM, but also accelerate early hardening process of DCRCM, promoting the establishment and development of mechanics strength. When the mixing amount of borax is 2%, both early and late strengths of DCRCM can reach to the maximum. SEM analysis shows that the hardening paste of DCRCM with borax is characterized by dense microstructure and less defects, greatly improving the mechanical properties of DCRCM. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Recycled aggregate concrete can be used as a building material. Therefore, it is of great environmental value, and lots of researches have been done on it by researchers at home and abroad during recent years, obtaining plenty of research achievements [1–4]. However, Fine powders less than 0.15 mm, mainly consisting of hardened cement paste (HCP), are produced during the preparation process of recycled aggregate concrete by using waste concrete. An effective way of reusing waste concrete is gelling property recy-
⇑ Corresponding author at: Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China. E-mail address: [email protected] (Y. Ji). https://doi.org/10.1016/j.conbuildmat.2019.03.323 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
cling of these fine powders, which has been a major concern of waste concrete reutilization [5–7]. Researches show that dehydrated cement paste (DCP), obtained from HCP with high temperature calcination, has partially recovered cementitious capacity [8–11]. In our previous research [12], DCP can be modified with GGBFS based on micro-ball milling effect and micro-aggregate effect, producing DCP composite cementitious material (DCRCM) with excellent properties. But when the DCRCM is used as a cementitious material, there are still some inevitable problems such as short setting time and quick fluidity loss, which will not meet construction requirements in practical engineering. Therefore, the retarder research of DCRCM is the key to cementitious property recycling of waste concrete. Suitable amount of gypsum is added to solve fast setting problem of Portland cement. The dissolution rate of gypsum is higher
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
than hydration rate of C3A in cement clinker, and thus the hydration product of C3A, i.e. C4AH13, will immediately react with dissolved gypsum to form ettringite, which prevents fast setting due to the formation of network structure from the lamellar crystal of C4AH13 [13]. And it is the basic reason that gypsum is of retarding ability when added to Portland cement. Compared with Portland cement, the microstructure of DCP powders with high specific surface area is loose and porous [14,15]. DCP mainly contains dehydrated phases of C-S-H, which is in metastable state and hydrates fast [16]. Free calcium oxide, produced from dehydration of calcium hydroxide in HCP, will react with water instantaneously to produce large amounts of heat, thus further quickening setting process of DCP. However, the dissolution rate of gypsum, with low dissolubility, is obviously lower than the hydration reaction rate of DCP, which is much higher than Ordinary Portland cement (OPC) [7]. Therefore, gypsum can neither promptly react with aluminate hydration product of DCP nor delay hydrations of dehydrated phases of C-S-H and free calcium oxide. As a result, gypsum cannot work as a retarder to solve the problems of fast setting when DCRCM is used in practice. Due to the retarding effect of gypsum on C3A hydration, other common retarders added to Portland cement, such as sodium gluconate [17,18] and sodium citrate [19], mainly aim at crystallization ratios of both C3S [20]and Ca(OH)2. Hence, whether these retarders will work for DCRCM, which mainly consists of dehydrated phases of calcium silicate hydrate, aluminate and Ca(OH)2, needs further study. Borax, a weakly alkaline salt, is a kind of inorganic retarder [21], and can easily dissolve in water. When dissolved in water, borax can react with Ca2+ to form a calcium-base borate layer in alkaline solution [22]. DCRCM paste is strong alkaline, of which PH value can reach 12. When mixed with DCRCM, borax may be capable of retarding effect due to the fast-produced complex, which may retard hydration and crystal separation of the cementitious system. Moreover, the solubility of borax is far larger than that of gypsum, which makes it more advantageous in terms of retarding effect. The setting process of DCRCM may be delayed in this way. In this paper, the influences of gypsum, sodium citrate, sodium gluconate and borax on setting time and compressive strength of DCRCM are investigated to determine the optimum retarder with well retarding effect and mechanical properties. Heat evolution rate and hydration heat of DCRCM with different mixing amounts of the optimum retarder are also studied. By hydration kinetics and SEM analysis, mechanisms of early setting and hardening process of DCRCM are analyzed.
2. Materials and methodology 2.1. Materials Ordinary Portland cement (OPC) was provided by China United Cement Co., Ltd, with the characteristics presented in Table 1. The Blaine surface areas was 350 m2/ kg.
193
In accordance with the results of a previous study, DCRCM can be obtained by grinding the mixtures of DCP and ground granulated blast-furnace slag (GGBFS) at the mass ratios of 2:1. The detailed procedures of DCRCM preparation are shown in Reference [12]. The chemical compositions of GGBFS and DCP were presented in Tables 1. Chemical grade powders of gypsum (G), sodium citrate (SC), sodium gluconate (SG) and borax (B) with the purity higher than 99% were adopted in the paste formulation. The superplasticizer used here was polycarboxylic acid type with a solid mass fraction of 25%. 2.2. Methodology 2.2.1. Water requirement of normal consistency and setting time Referring to the State Standard of China (GB/T1346-2011), measurement of water requirement of normal consistency and setting time of DCRCM with different retards (sodium citrate, borax, sodium gluconate and gypsum) were determined to investigate the influence of different retarders on water requirement of normal consistency and setting time of DCRCM. Sodium citrate, borax, and sodium gluconate were respectively tested with both amounts of 2% and 4% by mass of DCRCM, and gypsum was tested with both amounts of 4% and 8%. The paste of pure DCRCM without any retarders was used as the control sample. All retarders should be predissolved in the mixing water to assure homogeneous distribution of the retarders in the samples, and then mixed with DCRCM. The dosages were chosen to clearly distinguish the effects of retarders on setting time and compressive strength. However, the effects of retarders might change with dosage variation, which were not investigated in this paper. 2.2.2. Compressive strength All DCRCM mortars were mixed with water-to-binder ratio of 0.5 (the mixture proportions of binders are same to the above 1.2.1 section), and binder-to-sand ratio of 1:3. Then DCRCM mortars were casted into 40 mm 40 mm 160 mm molds to determine the compressive strength in accordance with the specifications of the State Standard of China (GB/T17671-1999). The OPC mortar was also prepared with the same conditions of DCRCM mortars. Specimens were consolidated using a vibrating table and trowel finished. Cast specimens were stored in a moist curing room with humidity of more than 95% RH and temperature of 20 ± 2 °C. Compressive strength was determined using a hydraulic compression tester at 3 days and 28 days. 2.2.3. Hydration kinetics Referring to the State Standard of China (GB/T12959-2008), measurement of heat evolution rate and hydration heat of DCRCM were determined by a semiadiabatic calorimeter to investigate the influence of borax amounts (0%, 1%, 1.5%, 2%, 4%) on the hydration kinetics of DCRCM. All measurements lasted for 3 days. All samples were mixed with water-to-binder ratio of 0.5 and binder-to-sand ratio of 1:3, separately. Each sample contained 450 g DCRCM, 1350 g standard sand (ISO) and 225 g water (with superplasticizer and borax). After fully mixed, the mixer was quickly taken down, and the mortar was mixed by a spoon for several times. Then two mortar samples of (800 ± 1) g were weighted by electronic balance and packed into the measuring container, respectively. 2.2.4. Scanning electron microscopy SEM analysis was performed on the samples of both pure DCRCM and DCRCM with 2% borax (water-binder-ratio 0.5) at hydration time of 3 days and 28 days. The fresh surfaces from the representative samples, without polishing to keep the fractured surface uncontaminated, were used for SEM analysis to investigate the microstructure development of the specimens. The SEM analysis was performed in low vacuum mode using the FEI Quanta 250 with an accelerating voltage of 15 kV.
3. Results and discussions 3.1. Water requirement of normal consistency
Table 1 Chemical composition of OPC, GGBFS and DCP. Materials
OPC
GGBFS
DCP
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 L.O. I
24.55 7.77 3.62 54.59 2.68 0.31 1.5 2.24 1.2
36.51 15.65 1.08 32.93 8.02 0.81 1.11 0.07 1.33
30.26 6.64 2.54 46.47 2.42 0.7 1.63 2.25 4.97
Water requirements of normal consistency of DCRCM with different admixtures are shown in Fig. 1. As shown in Fig. 1, water requirement of normal consistency of pure DCRCM is 0.6. And water requirements of normal consistency of DCRCM, respectively with 4% gypsum, 1% sodium citrate, 1% sodium gluconate and 1% borax, are 0.55, 0.57, 0.53 and 0.42. Compared with pure DCRCM, water requirements of normal consistency of DCRCM with different admixtures are with varying decrease. The more retarders are added, the more water requirement of normal consistency decreases. Among the retarders, sodium citrate is of the poorest effect. Even if the amount of sodium citrate increases to 2%, there
194
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Borax, as a retarder, shows better retarding effect. When borax amount is 1%, initial setting time and final setting time of DCRCM are 45 min and 78 min respectively. And when borax amount increases to 2%, initial setting time and final setting time of DCRCM prolong to 85 min and 125 min respectively, much higher than those of pure DCRCM. Therefore, the setting time of DCRCM with borax is greatly prolonged. When the amount of sodium gluconate is 1%, the initial setting time and final setting time of DCRCM are 90 min and 155 min respectively. And the initial setting time and final setting time respectively reach to 125 min and 205 min in the paste of DCRCM with 2% sodium gluconate. The setting time of DCRCM with sodium gluconate greatly increases and its retarding performance is significantly better than any other retarders. In conclusion, the retarding effects of sodium gluconate, borax, sodium citrate and gypsum tend to be worse gradually. Whereas, all of the retarders can prolong setting time of DCRCM to some degree, which provides basic conditions for the investigation of hydration and hardening properties of DCRCM. Fig. 1. Water requirement of normal consistency.
is no obvious decrease in water requirement of normal consistency of DCRCM. When the borax amount is 2%, water requirement of normal consistency of DCRCM decreases to 0.45. For sodium gluconate, when its mixing amount is 2%, water requirement of normal consistency of DCRCM is 0.35, reducing 41.7% compared with that of DCRCM without any retarders. In conclusion, the influences of sodium citrate, gypsum, borax, and sodium gluconate on water requirement of normal consistency of DCRCM tend to be gradually greater.
3.3. Compressive strength Compressive strengths of DCRCM with different retarders are shown in Fig. 3. It can be obtained from Fig. 3(a), the 3 days compressive strengths of OPC and pure DCRCM are 27.6 MPa and
3.2. Setting time Setting times of DCRCM with different retarders based on the water requirement are shown in Fig. 2. As is shown in Fig. 2, the initial setting time and final setting time of pure DCRCM are 23 min and 45 min respectively. The initial setting time and final setting time of DCRCM with 4% gypsum are 25 min and 48 min respectively. When gypsum amount reaches to 8%, initial setting time only increases to 42 min. Therefore, gypsum has little influence on setting time of DCRCM, which indicates that gypsum is barely capable of retarding when added to DCRCM. When sodium citrate amount is 2%, the initial setting time and final setting time of DCRCM are 40 min and 73 min respectively. Setting time of DCRCM with sodium citrate is remarkably increased compared with that of pure DCRCM, but its setting rate is still quite fast.
Fig. 2. Effect of retarders on the setting time of DCRCM.
Fig. 3. Effect of retarders on the compressive strength of DCRCM: (a) 3 days; (b) 28 days.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
29.5 MPa respectively. The compressive strength of DCRCM with 4% gypsum is equivalent to that of pure DCRCM. When gypsum amount reaches to 8%, the compressive strength decreases slightly (25.6 MPa). While the compressive strength of DCRCM with 2% sodium citrate significantly decreases to 5.6 MPa. Therefore, sodium citrate shows negative effect on early strength development of DCRCM. When the amount of borax is 1%, the compressive strength of DCRCM is 31.4 MPa, slightly higher than that of pure DCRCM. When the borax amount reaches to 2%, the compressive strength of DCRCM reaches the maximum value of 37.4 MPa, 26.7% higher than that of pure DCRCM. Therefore, borax can greatly promote early strength development of DCRCM. When sodium gluconate amount is 2%, the compressive strength of DCRCM is only 10.5 MPa, which results from strong retarding effect of sodium gluconate. Thus, the early hydration rate of DCRCM is quite low and the setting process is excessively prolonged. As a result, the amounts of hydration products decrease, which affects the early strength development of DCRCM. The 28 days compressive strength variations of DCRCM with different retarders are similar to 3 days compressive strength (see Fig. 3(b)). The 28 days compressive strengths of DCRCM, respectively mixed with gypsum and sodium citrate, are lower than that of pure DCRCM. Moreover, the compressive strength tends to decease more with increasing amounts of the two retarders. When sodium gluconate amount is 1%, the compressive strength (42.3 MPa) is slightly higher than that of DCRCM without any retarders (40.5 MPa). The compressive strength of DCRCM with 2% sodium gluconate is equivalent to that of pure DCRCM. Compared with 3-day compressive strength of DCRCM with sodium gluconate, it can be obtained that although sodium gluconate has strong retarding effect on the setting process of DCRCM, leading to lower early strength, it has no negative influence on the development of latter strength. The highest 28 days compressive strength can be obtained when the borax amount is 2% (53.2 MPa), which is 34.3% higher than that of pure DCRCM and 8.8% higher than that of OPC (48.9 MPa). Therefore, borax has good promoting effect on later strength of DCRCM. In conclusion, the retarding effects of gypsum and sodium citrate are quite poor. Moreover, the compressive strengths of DCRCM, respectively mixed with gypsum and sodium citrate, decease to some degree. Sodium gluconate shows the best retarding ability, while it has negative influence on the early strength development. Although the latter strength of DCRCM with sodium gluconate is improved compared with its early strength, the strength increasement is not obvious compared with that of pure DCRCM. Borax exhibits obvious retarding effect and the mechanical property of DCRCM can be significantly improved at both early and later stage. Therefore, borax is used as the retarder for DCRCM, and the mechanisms of its influence on setting and hardening process are studied. 3.4. Hydration kinetics 3.4.1. Exothermic rate Exothermic rate curves of OPC and DCRCM are illustrated in Fig. 4. As shown in Fig. 4, OPC strongly reacts with water when they are mixed together, and the exothermic rate is quite fast due to dissolution of cement minerals and quick hydration of C3A, corresponding to the pre-induction period. Then the hydration progress of OPC enters the induction period, during which period the exothermic rate quickly decreases to a low value for 1–2 h. Whereafter, exothermic rate speeds up due to formation of large amounts of C-S-H. This period is referred to as the acerbation period. Then exothermic rate gradually decreases and enters the steady period. The hydration reaction of pure DCRCM is much violent than OPC, the initial exothermic rate is almost 5 times than OPC and
195
Fig. 4. Effect of borax amounts on the hydration exothermic rate of DCRCM.
lasts for approximately 30 min. Hydration exothermic rate declines obviously with increasing borax amounts during this period, which indicates that initial hydration process (hydration time 30 min) can be effectively retarded by borax. After the violent reaction, the exothermic rate of pure DCRCM becomes lower. This period is known as the induction period. However, time for induction period of pure DCRCM is delayed compared with that of OPC. The induction period lasts for 5–6 h, and this period appears to be shorter with increasing borax amounts. When the borax amount is 2%, there is nearly no induction period. The above results illustrate that apart from obvious setting retarding effect of initial setting, borax can also promote harden process of DCRCM, and the promoting effect is evident with increasing borax amounts. However, when borax amount is 4%, the induction period trend to be longer, which indicates that excessive amounts of borax will result in over setting of DCRCM. After induction period, the exothermic rate of DCRCM speeds up again and the main exothermic rate peak occurs, known as the acceleration period. The main exothermic rate peak of DCRCM will gradually increase with increasing borax amounts, which further indicates that borax can promote hardening process of DCRCM after initial setting. However, the exothermic peak of DCRCM with 4% borax is obviously lower than that of DCRCM with other borax amounts in acceleration period, which is due to the influence of excessive borax amount on hydration process of DCRCM. From the above results, it can be obtained that with increasing borax amounts, the exothermic rate significantly decreases in the preinduction period, and that the induction period will be significantly shortened, which greatly promotes the hydration process of DCRCM after initial setting [23]. However, excessive borax amount will result in longer induction period and lower exothermic rate, and thus the hydration process of cementitious system is severely restrained, which is disadvantageous for the establishment and development of early strength.
3.4.2. Hydration heat Hydration heat curves of OPC and DCRCM are shown in Fig. 5. As shown in Fig. 5, in early hydration stage (usually hydrated for 3 days), hydration heat of pure DCRCM and OPC is 276.0 J/g and 163.4 J/g, respectively. When the amount of borax increases from 1% to 2%, hydration heat of DCRCM increases from 303.1 J/g to 385.5 J/g, which increases 39.67% compared with that of pure DCRCM. This further demonstrates that borax can promote early
196
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Fig. 5. Effect of borax amounts on the hydration heat of DCRCM.
hydration process of DCRCM. When the borax amount is 4%, the setting time of DCRCM will be prolonged, and hydration exothermic rate will be lower, thus resulting in decreasing hydration heat. In conclusion, borax is capable of restraining initial hydration (hydration time 30 min) and promoting early hydration (usually hydrated for 3 days). Therefore, initial hydration rate can be declined and the setting time of DCRCM can be prolonged, and thus the objective of retarding is achieved. Moreover, borax can also improve the early strength due to promoting effect of early hydration of DCRCM. Therefore, borax is a suitable retarder for DCRCM.
At the hydration of 28 days, the hydration products amount of DCRCM without borax will increase with the increase of hydration degree (see Fig. 7(a)), and the bonding between DCP and GGBFS is enhanced. Moreover, good bonding has been formed at the interfacial zones. Large porous structures will be smaller and smaller due to filling of hydration products and thus density is improved (see Fig. 7(b)). However, there still exist some defects inside the hardened paste. Therefore, whole performances of DCRCM tend to be poor. With the amount of hydration products gradually increasing, compactness of hardened paste of DCRCM with 2% borax has been further improved (see Fig. 7(d)). Therefore, the bonding becomes much tighter among the hydration products of DCP and GGBFS particles, and barely no single GGBFS particles can be found (see Fig. 7 (e)), which have been encapsulated by the hydration products of DCP. Thus, the bond strength among the interfacial zones are firmer, resulting in good whole performances. The chemical compositions of the hydration products are measured by an EDS test based on the obtained SEM images. The results of element mapping of Fig. 6(b), (d) and Fig. 7(c), (e) are shown in Fig. 6(c), (f) and Fig. 7(c), (f), which corresponds to pure DCRCM and DCRCM-2%B paste at 3 days and 28 days of age. The results of EDS elemental mapping show that the major elements of hydration products of the DCRCM pastes are calcium, silicon, aluminum and magnesium, which mainly attributes to the formed C-S-H gel, Ca(OH)2 and unhydrated particles of DCP and GGBFS in the pastes. 4. Mechanism analysis 4.1. Quick setting analysis of DCRCM
3.5. Scanning electron microscopy Fig. 6 shows the microstructure of DCRCM pastes with different amounts of borax at 3 days. As is shown in Fig. 6 (a), bonding among hydration products of DCRCM without borax is not tight, and the bonding strength among the hydration products of DCP and GGBFS particles is quite low. Obvious cracks and pores exist at the interfacial zones among hydration products of DCP and GGBFS particles (GGBFS particles are regarded as microaggregate due to relatively weak activity), resulting in relatively loose microstructures and further low mechanical properties. Hydration action of DCP without any retarders can take place quickly to form a large amount of gels in a short time, but the hydration products mainly consist of coarse and complete crystallization like Ca(OH)2, resulting in loose microstructure and poor bonding strength. Therefore, lots of perfect crystalline hydration products and quite a lot of defects exist in the hydration products of DCP, which contributes little to the bonding strength and mechanical property (see Fig. 6 (b)). Fig. 6 (d) and Fig. 6 (e) show the microstructure of DCRCM with 2% borax at 3 days. As is shown in the Fig. 6 (d), interwoven gel particles form the network structure of network gel, of which the flocculation gel accounts for a considerable amount. The gel then interconnects with each other to form integrated structures. Hydration products of DCP can bond well with GGBFS particles, and therefore compact structures and reliably tight bonding can be formed at the interfacial zone (Fig. 6 (e)). Moreover, there is no large porous structures inside the hardened paste of DCRCM, and no perfect crystalline hydration products are found, meaning that the whole structure is compact. The above analyses illustrate that DCP can fully react with GGBFS particles. Meanwhile, apart from good retarding ability, borax can significantly promote establishment and development of mechanics properties for the whole cementitious system, which greatly increases hydration degree of cementitious system.
Cementitious properties of DCRCM are mainly dependent on activities of DCP, and GGBFS is more used as supplementary cementitious materials [24,25]. Therefore, physical, chemical properties and cementitious properties of DCRCM are largely dependent on properties of DCP. The essence of the preparation and usage process of cementitious materials lies in the change process of SiO4 tetrahedron from the polymerization state to the isolated state and then to the polymerization state. While material activation is the depolymerization process of polymeric SiO4 tetrahedron. The preparation process of DCP is a thermal activation process. During this process, the main hydration productions of HCP will dehydrate: ettringite, low sulphated calcium sulfate and calcium hydroxide will decompose, losing combined water to produce dehydrated phases of aluminate, C12A7, and calcium oxide. For C-S-H gel, it will not only dehydrate but also depolymerize, changing into a low polymerization state [26–28]. Generally, the lower polymerization degree of the network structure is, the higher hydraulic activity will be. Therefore, DCP is of high activity. Both dehydrated phases of C12A7 and C-S-H exist in the metastable state; and calcium hydroxide will dehydrate to produce large amounts of f-CaO. As a result, DCP will quickly rehydrate when mixed with water. Moreover, DCP is loose and porous [14,15], resulting in hydration reaction taking place at both sides around the original boundaries of the particles at the same time (see Fig. 8) [12], which greatly increases actual reaction area of hydration. Hence, problem of fast setting exists for DCRCM application, which in return increases the difficulty in fully utilizing it. 4.2. Retarding analysis 4.2.1. Hydration process of DCRCM without retarder Based on the results obtained from the above analysis, the mechanism of borax effect on the setting process of the DCRCM
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
197
Fig. 6. SEM micrographs comparison of the samples at 3 days of age: (a), (b) pure DCRCM, (c) EDS result of pure DCRCM; (d), (e) DCRCM + 2%B, (f) EDS result of DCRCM + 2%B.
paste is illustrated in Fig. 9. DCRCM without any retarders reacts with water quickly (see Fig. 9(a)). The main rehydration reaction contains three parts:
(I) Hydration of C-S-H in oligomer state. The hydration of C-S-H in oligomer state mainly includes two parts. The first one is water molecule and Ca2+ easily entering the original position
198
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
Fig. 7. SEM micrographs comparison of the samples at 28 days of age: (a), (b) pure DCRCM, (c) EDS result of pure DCRCM; (d), (e) DCRCM + 2%B, (f) EDS result of DCRCM + 2% B.
of the C-S-H dehydration phase to restore the C-S-H gel structure and the dehydration phase polymerizing with each other, which keeps the structure of C-S-H gel unchanged. The other one is the hydration reaction of b-C2S produced by C-S-H dehydration. Moreover, the C-S-H dehydrated
phase in the metastable state reduces the nucleation barrier of C-S-H gel in the new cementitious system and accelerates the formation of hydration products. Therefore, the hydration rate of C-S-H dehydrated phase is very fast.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
199
Fig. 8. Hydration process of DCP particle.
Fig. 9. Control mechanism of borax on the setting process of DCRCM.
(II) Hydration of calcium oxide. The f-CaO reacts with water quickly to reproduce calcium hydroxide, together releasing large amounts of hydration heat, which can increase heat exothermic rate of the whole cementitious system and make the fresh paste lose its fluidity and set rapidly. (III) Hydration of dehydrated Aluminate phases. The dehydrated Aluminate phases are mainly C12A7. The coordination of Ca and Al in the crystals is extremely irregular, and thus lots
of cavities exist in the structure. Water can easily get into these cavities, which results in quick hydration and setting. In the alkaline condition, C2AH8 with hexagonal structures is quickly formed due to DCRCM hydration [29]. C2AH8 can form network lapping, leading to fast setting of the fresh paste. On the other hand, C12A7 are in metastable state and DCP particles are loose and porous, thus leading to high heat exothermic rate and large hydration area. Therefore, even if
200
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
gypsum is added as a retarder, it cannot promptly react with C2AH8 produced by hydration of C12A7 due to its low dissolubility and dissolution rate. As a result, gypsum has limit retarding ability for the cementitious system. With the hydration reaction continuing, migration and diffusion of the internal hydration products in the system are blocked, and the internal hydration products accumulates in the inner of particles or among a few particles, causing the formation of flocculated structure (see Fig. 9 (b)). The increasing of flocculated structure will result in crystallization of hydration products, and the hydration product crystal is coarse (see Fig. 6 (b).). At the same time, the amount of external hydration products is relatively reduced. Therefore, no effective connections can be formed between the external hydration products and particles outside, resulting in more pores and poor integrity in the cementing system (see Fig. 6 (a).). 4.2.2. Borax retarding analysist 4.2.2.1. Restrain formation of hydration production. As a retarder, borax hydrolyzes to generate more borate ions in water before mixed with DCRCM. Borate ions can react with Ca2+, hydrolyzed production of DCRCM, and aluminate in dehydrated phases to form a complex. The complex is similar to ettringite and its chemical formula is 3CaO.Al2O33Ca(BO2)231H2O [30]. The complex will form a thick film layer around the surfaces of DCRCM particles (see Fig. 9 (c)) [22,31–33], and it can effectively prevent water from entering inside the DCRCM particles compared with common hydration productions. So, water can barely penetrate inside the DCRCM particles, thus significantly restraining the hydration and crystallization of DCRCM. Moreover, the formation of the film layer can effectively consume C12A7 in dehydrated phases, preventing it from hydrating to produce C2AH8. Meanwhile, hydration reaction of DCRCM will only take place outside the original boundaries of the DCRCM particles. Therefore, hydration area and heat exothermic rate are great reduced. Apart from the hydration reaction of generating the complex, borate ions can also react with Ca2+ to form wrapping layers of calcium borate (see Fig. 9(c)) [34]. Wrapping layer of calcium borate is similar to gel. It has the property of semipermeable membrane, so water molecules and smaller ions can slowly penetrate inside the DCRCM particles through the wrapping layers. The wrapping layers will further wrap DCRCM particles to restrain DCRCM hydration and prolong setting time. Furthermore, if the amount of borax is excessive, the retarding effect is significantly improved and hydration rate is greatly reduced. However, the setting time of the cementitious system is too long, resulting in decreasing of hydration products, which makes the establishment and development of early strength greatly restrained. 4.2.2.2. Restrain crystallization of Ca(OH)2. Ion composition and concentration of fresh DCRCM paste are changed when borax is added as a retarder. According to Joisel electrolyte dissolubility theory, when there are strong cations in the solution, such as Na+ and K+, they will reduce dissolubility of other weaker cations, such as Ca2+ [27]. Therefore, Na+, introduced by borax, will reduce Ca2+ dissolubility of DCRCM. Moreover, borate ions can react with Ca2+ in the solution to produce complex and calcium borate. These all help to restrain crystallization of Ca(OH)2. As a result, the amounts of Ca (OH)2 produced by the hydration of free CaO is effectively reduced at the initial hydration period of fresh DCRCM paste. Meanwhile, hydration heat due to the formation of Ca(OH)2 is also reduced, which effectively lowers the hydration temperature and heat exothermic rate of the cementitious system. Therefore, with the increase of borax amount, electrolyte concentration increases at
the initial hydration period of fresh DCRCM paste, and thus Ca2+ dissolubility and Ca(OH)2 crystallization will be further restrained. In conclusion, borax can reduce formation rate of hydration products and restrain the crystallization of Ca(OH)2 in the cementitious system. Thus, the setting process is prolonged. With hydration reaction continuing, unstable complex and borate wrapping layer will be destroyed, resulting in reacceleration of hydration reaction (see Fig. 9 (d)). Based on hydration heat analysis and mechanics strength results, borax can effectively promote the establishment and development of early strength of the cementitious system. Besides, it can greatly improve late strength as well. Therefore, borax can be used as an efficient retarder for DCRCM. 5. Conclusions A systematically experimental study has been designed in order to investigate the effects of different retarders (gypsum, sodium citrate, sodium gluconate and borax) on setting and hardening behaviors of DCRCM. Conclusions can be drawn as follows: 1) The retarding effects of different retarder on DCRCM are variable. Gypsum and sodium citrate have poor retarding effect. Borax can effectively prolong setting process of DCRCM. Sodium gluconate has the best retarding ability but leads to lower early strength. 2) Retarding effect of borax is quite obvious. Borax can effectively reduce initial hydration rate of DCRCM and increase setting time. Meanwhile, borax can quicken the hardening process of DCRCM, thus promoting establishment and development of early strength. Besides, borax can contribute to the increase of the late strength. The best mechanics properties of DCRCM can be obtained when borax amount is 2%. 3) Microstructure analysis suggests that the hydration products of DCP has bonded well with GGBFS particles. Compact structures and reliably tight bonding have been formed at the interfacial zone. With the increase of hydration ages, the microstructure and bonding at the interfacial zone are further enhanced. Moreover, the whole structure of hardened DCRCM paste is extremely compact and with little defects. 4) The mechanism for retarding effect of borax mainly lies in two aspects. On the one hand, borate, from the borax hydrolyzation, can react with Ca2+and aluminate in dehydrated phase, and generate complex film layer and borate wrapping layer, which can prevent water from penetrating inside the particles of the cementitious materials. Thus, the hydration rate of DCRCM will decrease and the generation of hydration products will be restrained. On the other hand, the crystallization of Ca(OH)2 can be effectively restrained due to variations of the initial ion concentration and formation of calcium borate, and thus the setting process of DCRCM is prolonged.
Acknowledgements The authors would like to express their appreciations to the Project supported by ‘‘the Fundamental Research Funds for the Central Universities (2018XKQYMS02). The research works belong to one part of the projects which are financially supported by FRFCU. Conflict of interest The authors declare that they have no conflict of interest to this work.
L. Zhang et al. / Construction and Building Materials 212 (2019) 192–201
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, and there is no commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] M. Amario, C. Rangel, M. Pepe, R.D.T. Filho, Optimization of normal and high strength recycled aggregate concrete mixtures by using packing model, Cem. Concr. Compos. 84 (2017) 83–92. [2] J.Z. Xiao, C.H. Wang, T. Ding, A. Akbarnezhad, A recycled aggregate concrete high-rise building: Structural performance and embodied carbon footprint, J. Cleaner Prod. 199 (2018) 868–881. [3] S. Omary, E. Ghorbel, G. Wardeh, Relationships between recycled concrete aggregates characteristics and recycled aggregates concretes properties, Constr. Build. Mater. 108 (2016) 163–174. [4] C.J. Shi, Y.K. Li, J.K. Zhang, W.G. Li, L.L. Chong, Z.B. Xie, Performance enhancement of recycled concrete aggregate - A review, J. Cleaner Prod. 112 (2016) 466–472. [5] L.N. Lv, Y.J. He, S.G. Hu, Binding materials of dehydrated phases of waste hardened cement paste and pozzolanic admixture, J. Wuhan Univ. Technol. 24 (2009) 140–144. [6] A. Bordy, A. Younsi, S. Aggoun, B. Fiorio, Cement substitution by a recycled cement paste fine: Role of the residual anhydrous clinker, Constr. Build. Mater. 132 (2017) 1–8. [7] R. Yu, Z.H. Shui, Efficient reuse of the recycled construction waste cementitious materials, J. Cleaner Prod. 78 (2014) 202–207. [8] S.K. Handoo, S. Agarwal, S.K. Agarwal, Physical–chemical, mineralogical, and morphological characteristics of concrete exposed to elevated temperatures, Cem. Concr. Res. 32 (2002) 1009–1018. [9] C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (2004) 3015–3024. [10] G.M. Wang, C. Zhang, B. Zhang, Q. Li, Z.H. Shui, Study on the high-temperature behavior and rehydration characteristics of hardened cement paste, Fire Mater. 39 (2015) 741–750. [11] R. Serpell, M. Lopez, Reactivated cementitious materials from hydrated cement paste wastes, Cem. Concr. Compos. 39 (2013) 104–114. [12] L.L. Zhang, Y.S. Ji, G.D. Huang, J. Li, Y.J. Hu, Modification and enhancement of mechanical properties of dehydrated cement paste using ground granulated blast-furnace slag, Constr. Build. Mater. 164 (2018) 525–534. [13] H.F.W. Taylor, Cement chemistry, 2nd ed., Thomas Telford, London, UK, 1997, pp. 218–221. [14] Q. Zhang, G. Ye, E. Koender, Investigation of the structure of heated Portland cement paste by using various techniques, Constr. Build. Mater. 38 (2013) 1040–1050. [15] G.F. Peng, Z.S. Huang, Change in microstructure of hardened cement paste subjected to elevated temperatures, Constr. Build. Mater. 22 (2008) 593–599. [16] M. Vyšvarˇil, P. Bayer, M. Chromá, P. Rovnaníková, Physico-mechanical and microstructural properties of rehydrated blended cement pastes, Constr. Build. Mater. 54 (2014) 413–420.
201
[17] G. Zhang, G. Li, Y. Li, Effects of superplasticizers and retarders on the fluidity and strength of sulphoaluminate cement, Constr. Build. Mater. 126 (2016) 44– 54. [18] F.B. Zou, H.B. Tan, Y.L. Guo, Effect of sodium gluconate on dispersion of polycarboxylate superplasticizer with different grafting density in side chain, J. Ind. Eng. Chem. 55 (2017) 91–100. [19] I.R.D. Oliveira, A.R. Studart, F.A.O. Valenzuela, V.C. Pandolfelli, Setting behavior of ultra-low cement refractory castables in the presence of citrate and polymethacrylate salts, J. Eur. Ceram. Soc. 23 (2003) 2225–2235. [20] S.H. Ma, W.F. Li, S.B. Zhang, D.S. Ge, J. Yu, X.D. Shen, Influence of sodium gluconate on the performance and hydration of Portland cement, Constr. Build. Mater. 91 (2015) 138–144. [21] Q. Sun, J. Li, J. Wang, Effect of borate concentration on solidification of radioactive wastes by different cements, Nucl. Eng. Design 241 (2011) 4341– 4345. [22] H.B. Tan, Y.L. Guo, F.B. Zou, S.W. Jian, B.G. Ma, Z.Z. Zhi, Effect of borax on rheology of calcium sulphoaluminate cement paste in the presence of polycarboxylate superplasticizer, Constr. Build. Mater. 139 (2017) 277–285. [23] G.X. Li, J.J. Zhang, Z.P. Song, C. Shi, A. Zhang, Improvement of workability and early strength of calcium sulphoaluminate cement at various temperature by chemical admixtures, Constr. Build. Mater. 160 (2018) 427–439. [24] E. Deir, B.S. Gebreziabiher, S. Peethamparan, Influence of starting material on the early age hydration kinetics and the composition of binding gel in alkali activated binder systems, Cem. Concr. Compos. 48 (2014) 108–117. [25] E. Altan, S.T. Erdogan, Alkali activation of a slag at ambient and elevated temperatures, Cem. Concr. Compos. 34 (2012) 131–139. [26] G.Y. Pan, R.Q. Mao, H.L. Zhang, Research of dehydrated calcium sulphoaluminate hydrates (AFm) and its hydration, J. Wuhan Univ. Technol. 19 (1997) 28–30 (In Chinese). [27] G.F. Peng, S.Y.N. Chan, M. Anson, Chemical kinetics of C-S-H decomposition in hardened cement paste subjected to elevated temperatures up to 800 °C, Adv. Cem. Res. 13 (2001) 47–52. [28] L.N. Lv, Y.J. He, S.G. Hu, Structural characteristics of dehydrated phase of hardened cement paste and its rehydrating ability, J. Chin. Ceram. Soc. 36 (2008) (2008) 1343–1347. [29] W. Kurdowski, Cement and Concrete Chemistry, Springer, Netherlands, 2014, pp. 607–611. [30] M. Tong, X.G. Li, Concrete Admixture and its Application, China Architecture and Building Press, Beijing, 2014, pp. 110–112 (In Chinese). [31] S.L. Guo, H.J. Liu, Y. Shan, Y.H. Bu, The application of a combination of treated AMPS/IA copolymer and borax as the retarder of calcium aluminate phosphate cement, Constr. Build. Mater. 142 (2017) 51–58. [32] D.A. Hall, R. Stevens, B. El-Jazairi, The effect of retarders on the microstructure and mechanical properties of magnesia–phosphate cement mortar, Cem. Concr. Res. 31 (2001) 455–465. [33] H.B. Tan, X.F. Deng, B.Q. Gu, Effect of borax and sodium tripolyphosphate on fluidity of gypsum paste plasticized by polycarboxylate superplasticizer, Constr. Build. Mater. 176 (2018) 394–402. [34] M. Zajac, J. Skocek, F. Bullerjahn, Effect of retarders on the early hydration of calcium-sulphoaluminate (CSA) type cements, Cem. Concr. Res. 84 (2016) 62– 75.