Effect of shrinkage reducing admixture on early

Cement and Concrete Composites 92 (2018) 82–91

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Effect of shrinkage reducing admixture on early expansion and strength evolution of calcium sulfoaluminate blended cement

T

Iman Mehdipour, Kamal H. Khayat∗ Center for Infrastructure Engineering Studies, Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Calcium sulfoaluminate Expansion Hydration Relative humidity Shrinkage reducing admixture Strength evolution

This research examines the influence of presence of shrinkage reducing admixture (SRA) on expansion, hydration, and strength evolution of calcium sulfoaluminate-based expansive agent (CSA-based EX) blended with ordinary portland cement (OPC) in the absence of initial moist curing. The performance of inclusion of SRA in OPC-CSA system and without moist curing was compared to the moist-cured OPC-CSA system made without any SRA. The investigation uses the measurements of cement hydration from isothermal calorimetry, autogenous and drying volume changes, internal relative humidity, X-ray diffraction (XRD), and thermogravimetry, as well as compressive strength to characterize the expansion and hydration of OPC-CSA mortar systems. The results indicate that under unsealed conditions, the expansion and strength evolution of OPC-CSA system is significantly sensitive to the presence of moist curing. Mortar containing 15% CSA-based EX resulted in no expansion and had comparable magnitude of total deformation to that of the plain OPC system when no moist curing was adopted. This is linked to the: (i) reduced ettringite formation; and (ii) decreased stiffness which translates to the lower resistance to deformation. Analysis of calorimetry, XRD pattern, and thermogravimetry indicates the synergistic effect of using SRA in OPC-CSA system, which contributes towards higher early-age expansion and increased later-age strength compared to the similar system made without any SRA. In the absence of moist curing, the OPC-CSA system containing SRA exhibited 20% higher non-evaporable water content than OPC-CSA system alone, confirming the effect of SRA addition on hydration reaction progress of OPC-CSA system. The outcomes of this research provide insights into formulation of CSA-based suspensions with the aim of reduced shrinkage, when insufficient initial moist curing is applied.

1. Introduction Self-desiccation and external drying in cement-based materials result in the development of shrinkage-induced tensile stresses that can increase the risk of early-age cracking. To overcome such problems, the expansive agent (EX) [1–4] which compensates shrinkage through the expansion-induced stress, and/or shrinkage reducing admixture (SRA) [5–8] which decreases the surface tension of pore solution can be incorporated. Among the various EXs, the use of calcium sulfoaluminate (CSA) cements are receiving increasing attention given their shrinkage compensation [4,9–14] and environmentally friendly features associated with manufacturing process. The environmentally friendly feature is on account of a lower embodied energy and CO2 emissions in the production of CSA cements compared to the ordinary portland cement (OPC) [15–19]. The main phase of CSA cement is ye'elimite (C4 A3 Sˆ ), which reacts with calcium sulfate to form ettringite and aluminium hydroxide [20–22]. The expansion mechanism in CSA cement is

∗

Corresponding author. E-mail address: [email protected] (K.H. Khayat).

https://doi.org/10.1016/j.cemconcomp.2018.06.002 Received 5 December 2017; Received in revised form 31 May 2018; Accepted 4 June 2018 Available online 06 June 2018 0958-9465/ © 2018 Elsevier Ltd. All rights reserved.

strongly related to both the supersaturation level with respect to ettringite and the distribution and confinement of ettringite crystals [9,10]. The induced-expansive stresses due to the formation of relatively high ettringite nucleation rate can be explained by both the “swelling” and “crystal growth pressure” theories [4,9,10,23–25]. According to the swelling theory, the colloidal ettringite particles can promote expansion due to the adsorption of polar molecules of water, resulting in inter-particle repulsion and swelling. Based on the crystal growth theory, the crystallization pressure induced by ettringite crystals can cause expansion in matrix. A number of parameters can influence the early-age expansion induced by CSA cements, such as the degree of hydration of ye'elimite [26], sulfate content [4,20–22], presence of lime [27,28], particle size distribution [4,29], pore structure [24,30], curing condition [25], and curing temperature [26]. The effectiveness of incorporating CSA cements to fully form ettringite crystals and compensate shrinkage of cement-based materials is significantly affected by the availability of


I. Mehdipour, K.H. Khayat

nominal maximum size of 5 mm, density of 2500 kg m−3, and water absorption of 0.6% was employed. The grading of sand was in compliance with ASTM C33 [46]. A polycarboxylate-ether (PCE) based high-range water reducer with a solid content of 23% and a density of 1050 kg m−3 was used. The dosage of PCE dispersant was adjusted to reach a given equivalent fluidity in all mortars. All experiments were carried out on concrete equivalent mortars (subsequently referred to as mortars) formulated from environmentallyfriendly high-performance concrete with a relatively low binder volume fraction of 10% [47]. The concrete equivalent mortar is proportioned using the same concrete composition, except that the coarse aggregate fraction is replaced by an additional quantity of sand to secure equivalent total surface area of solid particles to the concrete [47,48]. All mortars were prepared with a fixed w/b of 0.40 and volumetric sand-to-binder ratio of 2.6. As presented in Table 2, four different mortars were formulated including: (i) the plain OPC mortar; (ii) the OPC mortar with replacement of 5% concentration of mixing water with SRA (subsequently designated as OPC-SRA); (iii) mortar with 15% replacement of OPC with CSA-based EX (subsequently designated as OPC-CSA); and (iv) mortar in which 15% of OPC and 5% concentration of mixing water were replaced by CSA-based EX and SRA, respectively (subsequently designated as OPC-CSA-SRA). In order to evaluate the effect of moist curing on expansion and strength evolution of CSA-based EX, mortar mixtures were subjected to two different curing conditions including: (i) 6 days of moist curing before exposure to air drying at 23 ± 1 °C and 50% ± 3% RH; and (ii) continuous air drying at 23 ± 1 °C and 50% ± 3% RH (i.e., no moist curing). The SRA and EX replacements were considered as parts of the mixing water and binder content (by mass replacement), respectively. The mixing sequence consisted of homogenizing sand for 60 s, before introducing half of the mixing water. The OPC and CSA-based EX (if used) were then added and mixed for 30 s followed by the PCE dispersant diluted in the remaining water. After 30 s of mixing, the SRA (if used) was incorporated, and the mortar was mixed for 3 min at a constant shear rate of 140 revolution-per-min (rpm), followed by a 30 s mixing at a high shear rate of 285 rpm. The mortar was remained at rest for 2 min for fluidity adjustment before remixing for another 3 min at shear rate of 140 rpm, as outlined in ASTM C305 [46].

water that is provided through the mix design water-to-binder ratio (w/ b), external moist curing, and/or internal curing provided by watersaturated porous materials. The w/b required for complete hydration of CSA cement to form ettringite is higher than that of OPC system [31,32]. For instance, Cheung and Leung [33] investigated the effect of CSA-based EX on the drying shrinkage of high-strength fiber-reinforced cementitious composites made with various w/b contents of 0.19, 0.3, and 0.4. The authors concluded that the incorporation of CSA is more effective at compensating shrinkage for mixtures prepared with higher w/b. Collepardi et al. [34] found that the reaction of CSA-based EX with water needs higher moist curing of 5–7 days compared to 1–2 days for calcium oxide-based EX to complete its hydration and develop its expansion potential. The scanning electron microscopy (SEM) observations conducted by Yan et al. [35] showed that the colloidal ettringite crystals can significantly swell when they adsorb more water under moist curing condition. Limited studies have demonstrated the effect of using SRA in OPCCSA systems on further improvement in shrinkage mitigation [36]. The reduced surface tension of pore fluid and lower rate of water evaporation due to the SRA addition result in a lower capillary stress development as well as a higher internal relative humidity in matrix compared to the similar system made without SRA [36–41]. For example, Meddah et al. [42,43] reported that the combination of CSAbased EX and SRA is highly efficient shrinkage mitigating approach to eliminate shrinkage of high performance concrete. In addition to decreased surface tension and water evaporation features, SRA addition also alters the chemistry of the pore fluid which can induce early-age expansion shortly after setting in these systems [7,44,45]. This is attributed to the increase in portlandite supersaturation level of pore solution, which induces crystallization pressure, thus contributing towards an expansion in matrix [45]. Based on the review presented herein, in spite of extensive research carried out on hydration and expansion characteristics of CSA cements, limited information still exists on the synergy between SRA and CSAbased EX, especially when insufficient moist curing is adopted. Consequently, this study elucidates the influence of the presence of SRA on early-age expansion and strength evolution of OPC-CSA mortar systems in the absence of initial moist curing. As such, a set of experimental observations including the hydration kinetics of cement using isothermal conduction calorimetry, X-ray diffraction (XRD), thermogravimetric analysis, scanning electron microscopy (SEM), autogenous and drying deformations, internal relative humidity, as well as compressive strength were employed with the aim of better understanding of volume changes occurring in OPC-CSA systems.

2.2. Experimental procedures Mini-slump cone test: The mini-slump cone test was used to evaluate the flow spread of mortars [49,50] and adjust the PCE dispersant dosage to achieve a target fluidity of 220 ± 20 mm. A truncated mini-cone with a top diameter of 70 mm, a bottom diameter of 100 mm, and a height of 60 mm was used. The mini-slump flow results for the OPC, OPC-CSA, and OPC-CSA-SRA systems were 235, 210, and 220 mm, respectively. The PCE dispersant demand of the investigated mortars to achieve a target fluidity (i.e., 220 ± 20 mm) is presented in Table 2. Viscosity measurement: The coaxial cylindrical rheometer was used to evaluate the effect of SRA addition on viscosity of stilled water solution. The test procedure consists of a step-wise (10 steps of 5 s each) decrease of the rotational velocity from 18 rpm to 1.5 rpm. The measured viscosity of 0.00089 Pa s for the distilled water at a temperature of 23 ± 0.5 °C was used as a standard reference for determining the change in viscosity due to the SRA addition. Hydration kinetics: The rate of heat evolution and extent of heat release occurring during hydration of the mortars was determined using isothermal conduction calorimetry (Calmetrix I-CAL 8000) at a constant temperature of 20 ± 0.1 °C for 72 h. The thermal power and measured energy were used to examine the coupled effect of CSA-based EX and SRA on hydration kinetics of cement. Time-dependent volume changes: Autogenous deformation (i.e., internal drying) of the mortar systems was determined in accordance with ASTM C1698 [46]. For each mixture, two corrugated samples were

2. Experimental approach 2.1. Materials and mixture proportions A Type I/II OPC conforming to ASTM C150 [46] and a commercial CSA-based EX (manufactured by CTS company) were used. The oxide compositions of the OPC and EX is presented in Table 1. A commercially available SRA composed of propylene glycol ethers was incorporated in this study. Continuously graded natural sand with a Table 1 Oxide composition of OPC and CSA-based EX (wt.%). Oxide

OPC

CSA-based EX

SiO2 Al2O3 Fe2O3 CaO MgO SO3 CaCO3

19.32 4.38 3.15 62.53 2.63 3.31 3.21

8.36 7.60 1.30 51.10 0.11 28.23 –

83



Table 2 Mixture proportions and fresh characteristics of the evaluated mortars. Mixture ID

SRA (wt.% by water)

CSA-based EX (wt.% by binder)

OPC

0

0

OPC-SRA OPC-CSA

5 0

0 15

OPC-CSA-SRA

5

15

Curing condition

PCE dispersant demand (wt.% by binder)

(i) continuous air drying (ii) 6-d MC + air drying continuous air drying (i) continuous air drying (ii) 6-d MC + air drying continuous air drying

Setting time (hours) Initial set time

Final set time

0.45

6.85

8.95

0.41 0.75

8.33 5.33

11.05 7.12

0.72

6.85

8.82

Notes: SRA: shrinkage reducing admixture; EX: expansive agent; MC: moist curing; Air drying: maintained at 23 ± 1 °C and 50% ± 3% RH. Codification: OPC-CSA-SRA = 85% OPC + 15% CSA-based EX + 5% SRA.

Simultaneous instrument (Model 429) was used to identify and measure various phases present in OPC-CSA systems subjected to various moist curing conditions. The mass loss (TG) and differential mass loss (DTG) patterns at 28 days were used to quantify the phases present in the systems. The powdered samples were extracted from prismatic paste specimens (i.e., in the absence of aggregate particles) measuring 25 × 25 × 285 mm3. Specimens were crushed, immersed in isopropanol for seven days to arrest hydration, and subsequently dried for one day using vacuum drying. Samples were then ground and passed through a 0.075 mm sieve. For TGA, samples were heated from 20 to 1000 °C in a nitrogen gas flow (to prevent carbonation) at a heating rate of 10 °C.min−1. The non-evaporable water content of the investigated systems was determined as the difference between the mass measurements at 950 °C and 105 °C, and corrected for the loss on ignition of the cement powder (based on its mass fraction in the paste) and the calcium carbonate content (650–800 °C). X-ray diffraction (XRD) analysis was also conducted on powdered samples (similarly extracted to TGA) at 28 days using a Philips X'pert diffractometer in a θ–θ configuration using CuKa (k = 1.54 Å) radiation. Samples were scanned between 5° and 90° (2θ) in continuous mode with an integrated step scan of 0.025° (2θ) using a PiXcel detector with a time per step of 150 s. Scanning electron microscopy: Samples measuring 15 × 15 mm were used for scanning electron microscopy (SEM) examinations using a Hetachi S4700-SEM. After 28 days of hydration, the slice cuts were extracted from the mortar samples and immersed them in isopropyl alcohol to suppress further hydration along with drying at 60 °C in a vacuum oven until reaching a constant mass. The samples were ground to achieve a relatively smooth surface, then mounted in epoxy resin and subjected to further grinding and polishing to secure smooth surface followed by coating with carbon. Compressive strength: The 7-, 28-, and 91-day compressive strengths of mortars were evaluated using 50-mm cube specimens, as outlined in ASTM C109 [46]. Specimens were cast in one lift without applying any mechanical consolidation given the self-consolidating ability of mortar mixtures [53,54]. After demolding at 24 h, specimens were subjected to saturated lime water at 21 ± 2 °C for different durations of 0 and 6 days. Following the moist curing period, all

prepared using polyethylene tubes. The initial reading (time zero) for the autogenous shrinkage corresponded to the time of final set determined in compliance with ASTM C403 [46]. Total deformation (combination of both internal and external drying) was evaluated in the prismatic specimens measuring 25 × 25 × 285 mm3 in accordance with ASTM C157 [46] using a digital type extensometer. After demolding at 24 h, the initial length of specimens was recorded. In order to evaluate the effect of moist curing on total deformation of OPC-CSA systems, the specimens were subjected to different moist curing durations of 0 and 6 days before exposure to air drying at 23 ± 1 °C and 50% ± 3% RH until 150 days, when all length change measurements reached the steady-state condition. It is important to note that the term “total deformation” used in this study refers to as the deformations resulting from both the autogenous drying and external drying. The uncertainty values for autogenous and drying deformation measurements were determined to be 4% and 3%, respectively. Internal relative humidity: The variations in internal relative humidity of the investigated systems were monitored from the final setting time to the age of 10 days using cast-in sensors under sealed condition. This system is based on measurements taken at localized holes inside a cement-based material specimen [51]. In this study, this system was integrated with inserted hollow PVC tubes prior to casting to measure the RH of fresh sample under sealed conditions. Multiple small holes were provided on the humidity sensor which allow the moisture flow transmission and avoid moisture condensation. The sequence of measurement procedure is depicted in Fig. 1.The measurement procedure involves casting mortar into prism measuring 75 × 75 × 285 mm3, then inserting two hollow PVC tubes with diameters of 20 mm and embedded depth of 40 mm into the fresh mixture. Following the final set, cast-in sensors are placed at the bottom of PVC tubes in contact with the mortar surface. This creates an internal macropore with the diameter/length of the PVC tube, intended to be in hygrothermal equilibrium [52]. To secure sealing condition, the specimens were wrapped using two layers of self-adhesive aluminum sheet to prevent any moisture loss due to the external drying, and subsequently stored them in temperature and humidity controlled room at 23 ± 1 °C and 50% ± 3% RH up to 10 days. Thermogravimetric and X-ray diffraction analyses: A Netzsch

Fig. 1. Schematic illustration test setup for internal relative humidity measurement in mortars. 84



followed by a plateau in shrinkage after 8 days. However, the OPC-CSA system exhibited sharp expansion with a peak expansion of 210 μm/m at 3 days, after which this system underwent a monotonic shrinkage to reach 45 μm/m shrinkage. Compared to the pure OPC mortar, the OPCCSA system had 85% lower extent of autogenous shrinkage at 50 days. It is also noted that the OPC-CSA system reached the steady-state shrinkage after 40 days compared to 8 days for the plain OPC mixture. This implies that the incorporation of CSA-based EX not only reduces the ultimate magnitude of autogenous shrinkage, but also extends the time corresponding to steady-state shrinkage (i.e., reduced shrinkage rate). For instance the rate of shrinkage for the OPC-CSA-SRA was approximately 10 μԐ/day compared to 47 μԐ/day for the plain OPC mixture. The synergistic effect of using CSA-based EX with SRA resulted in a larger extent of expansion, followed by a higher net expansion at 50 days than that of the corresponding mixture without any SRA. While both the OPC-CSA and OPC-CSA-SRA systems experienced expansion shortly after setting, the OPC-CSA-SRA system exhibited slower rate (corresponding to time from final set to peak expansion) and higher magnitude of expansion as well as longer duration of expansion compared to the OPC-CSA mixture alone. For instance, the OPC-CSA-SRA mortar had 67% longer time required to achieve peak expansion, 55% higher peak expansion, and had longer expansion duration before achieving steady-state condition than that of the OPC-CSA system. Further, the OPC-CSA system exhibited a peak expansion of 210 μm/m at 3 days, after which this system underwent a monotonic shrinkage. However, the inclusion of SRA in OPC-CSA system resulted in higher magnitude of peak expansion of 325 μm/m at 5 days. The rate of expansion (corresponding to time from final set to peak expansion) for the OPC-CSA-SRA was quantified to be 65.0 μԐ/day compared to 70.0 μԐ/ day for the corresponding without any SRA. The slower rate of expansion, that was determined for the time duration between the time of final set to expansion peak, for OPC-CSA-SRA system can be due to the delay in hydration kinetics of OPC-CSA system, which can slow down the dissolution rate of sulfate [45,60]. This is based on the results of pore solution composition reported by Sant et al. [45] who noted that the plain OPC paste exhibited initially a higher SO42− concentration compared to similar mixture containing SRA. The enhanced expansion and reduced autogenous shrinkage associated with using SRA in OPCCSA system are attributed to: (i) lower surface tension of pore fluid, which reduces the capillary stresses due to the self-desiccation [37–41]; (ii) higher degree of saturation in paste matrix for a longer duration,; and (iii) higher supersaturation level of pore solution with respect to

specimens were exposed to air drying at 23 ± 1 °C and 50% ± 3% RH until testing age. The results of compressive strength represent the average of three replicate specimens. The uncertainty was determined to be on the order of 2% based on measurements of three replicate samples. 3. Results and discussion 3.1. Setting behavior of OPC-CSA systems The results of initial and final setting times of the evaluated systems are provided in Table 2. The setting time behavior of mixtures can be divided into two categories: (i) mortar made with CSA-based EX resulted in shorter setting times compared to the plain OPC mortar; and (ii) mortar incorporating SRA exhibited longer set times than the corresponding mixture without any SRA. For instance, the OPC-CSA system had 23% and 21% shorter initial and final setting times, respectively, compared to the pure OPC mortar. This occurred as a result of the hydration reaction of ye'elimite, which initiates rapidly to form ettringite which consumes a substantial amount of water [55]. Upon faster hydration and formation of ettringite, the solid-to-solid connectivity evolves and formation of solid network (i.e., microstructural formation) increases, thus leading to shorter initial and final set times. On the other hand, OPC-CSA mortar containing SRA exhibited 30% and 23% longer initial and final setting times, respectively, compared to the similar mixture without any SRA. For the SRA-containing system, the addition of SRA reduces the polarity of mixing water. This results in lowering the affinity of alkalis (e.g., K2SO4 and Na2SO4) to dissolve and ionize in the mixing water. Consequently, the resulting pore solution contains a smaller concentration of alkali ions compared to the pore fluid of a plain cement paste. The reduced alkali concentration of the pore solution in mixture containing SRA leads to an increase in calcium ion concentration, thus expecting a delay in cement hydration [56,57]. This is supported by the results of early-age hydration kinetics (see Fig. 4). The retardation in the mixture containing SRA may also be partly related to delayed dissolution of C3A, due to the lower alkalicontent in the pore solution [58,59]. 3.2. Autogenous and total deformation of OPC-CSA systems Fig. 2(a) shows the autogenous deformation profiles from the time of final set as a function of specimen age for the investigated mortars. The OPC system experienced continuous and rapid shrinkage that was

Fig. 2. Profiles of the: (a) autogenous deformation from the time of final set as a function of specimen age (positive and negative values represent expansion and shrinkage, respectively.) and (b) internal relative humidity of the investigated mortar systems. The specimens were under sealed condition and maintained at 23 ± 1 °C and 50% ± 3% RH. 85



from the time of demolding (i.e., at 24 h) for different curing conditions. The efficacy of CSA-based EX to generate its expansion potential and compensate total deformation (i.e., combination of internal and external drying) is strongly sensitive to the existence of moist curing (i.e., external water supply). In the absence of moist curing, the OPCCSA system experienced no expansion and had comparable shrinkage trend to that of the plain OPC mortar. However, when 6 days of moist curing was adopted, the OPC-CSA exhibited a peak expansion of 570 μm/m at 6 days, after which it was followed by monotonic shrinkage to reach −260 μm/m at 150 days. Such shrinkage value was 75% lower than that observed for similar system when no moist curing was applied. In the presence of moist curing, even the plain OPC mixture shows a slight expansion of 70 μm/m at 6 days (see Fig. 3(a)), after which the system shrinks rapidly and consistently upon exposure to air drying. Although, the plain OPC mixture subjected to 6 days of moist curing underwent an expansion until the end of moist curing, this mixture exhibited solely 10% lower shrinkage at steady-state condition compared to the similar mixture without exposure to moist curing. This distinction in performance of OPC-CSA with respect to the presence of moist curing is attributable to relatively high water consumption of CSA cement to complete its hydration and form ettringite. As mentioned earlier, the expansive stresses induced by ettringite results from the simultaneous action of crystallization pressure due to the increased ettringite supersaturation level and swelling pressure caused

portlandite and ettringite that can induce crystallization pressure, which could lead to an expansion in matrix [7,44,45]. Fig. 2(b) shows the effect of SRA and CSA-based EX on internal relative humidity profiles for the investigated mortars under sealed condition. As expected, the internal relative humidity of the evaluated systems decreased with the hydration progress; however, the rate and extent of reduction were observed to vary with the presence of CSA and SRA compared to the pure OPC system. This reduction in the internal relative humidity under sealed condition is driven by water consumption during hydration. The OPC-CSA system underwent the fastest reduction and the lowest extent of internal relative humidity at 10 days. The OPC-CSA system containing SRA exhibited 6% higher relative humidity at 10 days than that of the similar mixture devoid of any SRA. Similar results have been observed with previous research studies for the OPC pastes containing SRA [37–39,61,62]. This is attributed to the delay in cement hydration with SRA addition, which in turn maintains a higher degree of saturation for a longer period of time. As the reduction in internal relative humidity is delayed in SRA system, the expansion potential occurring in the OPC-CSA system due to the formation of ettringite is enhanced, which can increase the expansion potential. In addition, in such system, a lower extent of produced expansion is compensated by simultaneous shrinkage induced by self-desiccation in mixture incorporating SRA. Fig. 3(a) shows total deformation profiles of the evaluated systems

Fig. 3. (a) Total deformation profiles from the time of demolding (i.e., 24 h) of the investigated mortars as a function of drying time under different curing conditions. Positive and negative values represent expansion and shrinkage, respectively. (b) The profiles of mass loss as a function of drying time for the investigated mortar systems. 86



measurement of a concentration of 5% SRA (by mass) in distilled water solution exhibited a viscosity of 0.0092 Pa s compared to 0.00089 Pa s for the distilled water. The enhanced viscosity can result in a reduction in transport properties (based on the Stokes Einstein relation and Fick's second law), thus lowering fluid flux and maintaining higher degree of saturation in matrix [37,66,67].

by water adsorption of ettringite crystals [10,23–25]. Therefore, the water supplied by moist curing is crucial to promote hydration and generate expansion potential in CSA-based EX system. In addition to the chemical effects, the higher shrinkage experienced in OPC-CSA system with no moist curing can also be attributed to the lower stiffness of this system due to the incomplete hydration, which translates to lower resistance to deformation as compared to the corresponding mixture subjected to moist curing. In the absence of moist curing, the addition of SRA in OPC-CSA system was effective at mitigating total shrinkage deformation compared to the OPC-CSA system alone. For example, the OPC-CSA system containing SRA resulted in a reduction in total deformation of 50% at 150 days than that observed for the similar mixture without any SRA. A closer look at the total deformation in Fig. 3(a) reveals that the OPCCSA system containing SRA exhibited an expansion of 84 μm/m at 2 days even in the absence of moist curing. In addition to lower surface tension of pore fluid, the addition of SRA can increase the supersaturation level of pore fluid with respect to ettringite and portlandite crystals, which could lead to a larger expansion potential in OPC-CSA system [7,45]. Fig. 3(b) depicts the mass loss of the investigated systems when exposed to drying condition (i.e., 23 ± 1 °C and 50% ± 3% RH). It can be noticed that the presence of SRA in the OPC-CSA system increased the rate of water evaporation at early-age compared to the similar system without any SRA. However, this trend was not carried over to the later-age; the OPC-CSA-SRA system exhibited lower rate of water evaporation compared to the OPC-CSA system alone. It should be noted that the changes in moisture gradient (i.e., water removal) of specimen can be related to the surface tension and viscosity of pore fluid. With decreasing surface tension of pore fluid, more water is expected to remove from specimen exposed to drying condition. Therefore, in the context of surface tension, more water should be lost from specimen made with SRA. The initial mass loss rates for the OPC-CSA, OPC-SRA, and OPC-CSA-SRA were quantified to be 1.24, 1.56, and 1.64%/day. However, based on the mass loss results, the OPC-CSA-SRA system exhibited lower mass loss compared to the similar system without any SRA at later-age. The distinction in initial and secondary trends for system containing SRA can be attributed to: (i) hydration evolution at later age (as supported by isothermal calorimetry data observed in Fig. 4) and (ii) increase in viscosity of pore solution and formation of viscous layer (i.e., concentrated solution) at the surface of specimen containing SRA which can reduce long term water transport [39]. This is in agreement with previous research studies that the inclusion of SRA can reduce transport properties [63–65]. The viscosity

3.3. Hydration characteristics of OPC-CSA systems Fig. 4 shows the heat flow profiles normalized per gram of cement for the investigated mortar systems under isothermal condition without the presence of moist curing. Compared to the pure OPC mixture, the incorporation of CSA in OPC-CSA system resulted in faster hydration of cement as indicated by leftward shift of heat evolution curve and larger extent of heat flow at the main hydration peak. This observation is consistent with setting time (Table 2), which confirms that the use of CSA-based EX accelerates the cement hydration. Expectedly, the addition of SRA in OPC-CSA system retarded the cement hydration. This was marked by reduced slope of acceleration regime and main heat evolution peak. However, this delay gradually decreased and was not carried over to later stages, so that OPC-CSA-SRA system had slightly higher heat release after 72 h than that of the corresponding mixture without any SRA. The OPC-CSA-SRA mixture exhibited 6% larger extent of heat release after 72 h compared to the OPC-CSA system alone, as shown in Fig. 4(b). The enhanced cumulative heat evolution at later ages for OPC-CSA system containing SRA can be related to progress of hydration of ye'elimite in OPC-CSA blend. This hypothesis is corroborated with XRD pattern data (see Fig. 5), which shows that in the absence of moist curing, the OPC-CSA system containing SRA exhibited larger intensity of ettringite peak (and lower peak intensity for unreacted ye'elimite) compared to the OPC-CSA system alone. The intensity of ettringite peak at 28 days for moist cured OPC-CSA system significantly differs from that observed for the similar mixture when no moist curing was adopted (see Fig. 5). In the absence of moist curing, no significant intensity peak associated with ettringite was identified for the OPC-CSA system. This distinction in XRD pattern confirms the superior expansion behavior induced by ettringite formation for the moist cured OPC-CSA system compared to the corresponding mixture that had no moist curing. Furthermore, in the absence of moist curing, the OPC-CSA sample exhibited the highest unreacted (i.e., unhydrated) ye'elimite peak. However, this peak was not detected for the similar sample when moist curing was adopted. Based on the quantitative XRD results at 28 days, the amount of ettringite (mass %) was determined to be 1.2%, 14.2%, 3.8% and 10.8% for the plain OPC

Fig. 4. Hydration kinetics of the OPC and OPC-CSA systems up to 72 h: (a) heat flow rate and (b) cumulative heat release. 87



peak in the range of 20–200 °C compared to the plain OPC system, potentially suggesting higher extent of ettringite formation in the presence of CSA-based EX. In the absence of moist curing, the OPC-CSA system containing SRA exhibited larger peak in the range of 20–200 °C compared to the corresponding mixture devoid of any SRA. These observations are consistent with our XRD pattern data shown in Fig. 5. The non-evaporable water content at 28 days for the evaluated systems under different curing conditions is depicted in Fig. 6(b). The nonevaporable water content (i.e., chemically bound water) is generally considered to be one of the key indicators of hydration reaction progress in cement [68,69]. The highest non-evaporable water content was observed for OPC-CSA system subjected to 6 days of moist curing, whereas in the absence of moist curing, this system exhibited the lowest non-evaporable water content. In the absence of moist curing, the OPCCSA system incorporating SRA exhibited 20% higher non-evaporable water content compared to the similar system devoid of any SRA, confirming the effect of SRA addition on hydration progress of OPCCSA system. Fig. 7 shows SEM observations for the OPC-CSA system with 6 days of moist curing (6-day MC) and OPC-CSA-SRA without any moist curing after 28 days of hydration. SEM images confirmed the presence of prismatic ettringite needles for both mortar systems, although quantity of ettringite crystals appears to be different. The SEM-EDS spectrum for image 1a revealed that Ca, S, Al, and O are the main elements, confirming that the hydration products are mainly composed of ettringite and aluminum hydroxide intermixed with C-S-H/Ca(OH)2. In the presence of moist curing, the OPC-CSA system indicated larger quantities of ettringite intermixed with C-S-H gel compared to the OPC-CSA-SRA mixture without any moist curing. Furthermore, ettringite formation in the moist cured OPC-CSA system appears to be more colloidal (cluster), which is expected to have a significant influence on the expansion characteristics of CSA-based EX systems [12,70].

Fig. 5. Representative XRD patterns at 28 days for the OPC and OPC-CSA paste systems under different curing conditions. Here, E: ettringite, CH: portlandite, Hc: hemicarboaluminate, and Y: ye'elimite. The HC peak observed in XRD pattern is on account of the presence of limestone (CaCO3) in the OPC, albeit in minor quantity (i.e., 3.21 mass%).

(6-d MC), OPC-CSA (6-d MC), OPC-CSA (air drying), and OPC-CSA-SRA (air drying). As expected, the largest ettringite amount was obtained for the OPC-CSA blend when 6 days of moist curing was applied. In the absence of moist curing, the OPC-CSA system indicated 74% lower ettringite amount compared to the corresponding system with moist curing of 6 days, thus substantial expansion was evident for the moistcured OPC-CSA system. It is also important to point out that in the presence of moist curing, the OPC-CSA system had smaller portlandite peak compared to that of the plain OPC mixture. The residual CH contents at 28 days for the plain OPC and OPC-CSA mixtures were quantified to be 15% and 11%, respectively (derived from TGA traces shown in Fig. 6). To further elaborate on the effect of moist curing and SRA addition on hydration of OPC-CSA blends, the TGA profiles of mortars after 28 days of hydration under different curing conditions is shown in Fig. 6(a). The evaluated systems exhibited distinguishable peaks at 130 °C and 450 °C, corresponding to decomposition of C-S-H/ettringite and dehydroxylization of CH, respectively. From Fig. 6(a), it can be noticed that moist-cured OPC-CSA system exhibited the greatest peak in the range of 20–200 °C compared to the similar mixture without any moist curing. This is attributed to the larger extent of C-S-H/ettringite formation when moist curing is adopted. In the case of 6 days of moist curing, mortar containing 15% CSA-based EX was shown to have larger

3.4. Compressive strength evolution of OPC-CSA systems Fig. 8 shows the evolution of compressive strength as a function of time for the plain OPC and OPC-CSA systems under different curing conditions. As Expected, mortars subjected to 6 days of moist curing developed higher strength as well as faster rate of strength gain compared to the corresponding mixtures that had no moist curing. The enhanced hydration and improved solid-to-solid connectivity due to the moist curing are responsible for this effect. The effect of moist curing on strength gain is more evident in the OPC-CSA systems. In the presence

Fig. 6. (a) Representative DTG traces at 28 days for the OPC and OPC-CSA systems at different curing conditions and (b) non-evaporable water content at 28 days determined from DTG traces. 88



Fig. 7. SEM images coupled with EDS pattern of the OPC-CSA systems at 28 days under unsealed condition in the: (a) presence of 6 days of moist curing and (b) presence of 5% SRA and without moist curing. All SEM images were acquired in the secondary electron mode, while operating at an acceleration voltage of 15 kV. The magnification of each image is 20,000×.

example, the OPC-CSA-SRA mixture exhibited 6% lower 7-day strength compared to the OPC-CSA mixture without any SRA; however, the OPCCSA-SRA system had 20% and 27% higher strength at 28 and 91 days, respectively. This drop in 7-day strength in the presence of SRA can be attributed to the slower dissolution of alkalis in pore fluid, and hence slower rate of strength development compared to the similar mixture without any SRA [7,38,40]. Higher strengths observed at 28 and 91 days in OPC-CSA system containing SRA can be due to the: (i) larger amount of formation of space-filling ettringite in this system (see Fig. 5); and (ii) enhanced hydration kinetics, as evidenced by TGA data (Fig. 6(b)) that shows larger amount of non-evaporable water in OPCCSA-SRA system as compared to the similar system devoid of any SRA. 4. Summary and conclusions A comprehensive investigation was carried out to elucidate the effect of using SRA on expansion and hydration characteristics, as well as strength evolution of CSA blended cements when no initial moist curing is applied. The performance of inclusion of SRA in OPC-CSA system and without moist curing was compared to the moist-cured OPC-CSA system made without any SRA. The results indicate that the expansion stresses induced by CSA under unsealed condition is strongly influenced by the presence of moist curing. The OPC-CSA system resulted in no expansion and had comparable trend and magnitude of total deformation to that of the plain OPC system when no moist curing was adopted. This suppression in performance is linked to the lack of water available to complete hydration and form ettringite in OPC-CSA system. This was supported by weaker intensity of ettrinigite peak derived from both DTG curve and XRD patterns. In addition to shrinkage, the mechanical properties (i.e., compressive strength) of OPC-CSA system were found to be significantly sensitive to the presence of moist curing. The moist cured OPC-CSA system exhibited 22% and 52% higher 7- and 91-day compressive strengths, respectively, than that of the similar mixture that had no moist curing. Under both sealed and unsealed conditions, addition of SRA in OPCCSA system resulted in a larger magnitude of expansion, followed by lower rate and magnitude of shrinkage than that of the similar mixture without any SRA. The synergistic effect of using SRA and CSA-based EX was confirmed by higher intensity of ettringite peak derived from XRD pattern as well as larger peak corresponding to C-S-H/ettringite determined from TGA profiles compared to the corresponding mixture made without any SRA. In the absence of moist curing, the SRA addition resulted in an increased strength in OPC-CSA system at later ages compared to the similar mixture without any SRA. Although the use of SRA in OPC-CSA system suppressed the heat flow peak value and slope of acceleration regime determined from isothermal calorimetry, this

Fig. 8. Compressive strength evolution for the OPC and OPC-CSA systems under different curing conditions.

of moist curing, the rate of strength gain for the OPC-CSA system was 1.04 MPa/day compared to 0.80 MPa/day for the plain OPC system. This is attributed to the higher amount of water required to complete hydration in OPC-CSA systems compared to the plain OPC. While water-to-cement ratio of 0.40 is theoretically sufficient to secure complete hydration of pure cement, the complete reaction of ye'elimite, gypsum and water to form ettringite and aluminium hydroxide requires a higher water-to-binder ratio of 0.55 (and 0.78 for pure ye'elimite reacting with 2 mol of anhydrite 0.78) for CSA clinker blended with sufficient gypsum [71–73]. In the present study the water-to-binder ratio of 0.4 is insufficient for complete hydration, and hence the adoption of externally supplied water (i.e., moist curing) has dominant role in progressing hydration kinetics CSA system. For example, the moist cured OPC-CSA system exhibited 22% and 52% higher 7- and 91day compressive strengths, respectively, than that of the similar mixture without any moist curing. However, for the plain OPC system, the extents of strength reduction due to absence of moist curing were 20% and 32% at 7 and 91 days. The lack of moist curing on strength reduction for the OPC-CSA systems further magnifies at later ages. It is important to note that in the presence of moist curing, the OPC-CSA system containing 15% CSA-based EX exhibited equivalent strength at both 7 and 91 days to that of the plain OPC mortar. The addition of SRA in OPC-CSA mixture resulted in a lower strength at 7 days compared to the OPC-CSA system alone, while this strength reduction at 7 days was not carried over to the later ages. For 89



delay gradually decreased and was not carried over to the later ages. The OPC-CSA-SRA system had 6% higher heat release after 72 h than corresponding mixture without any SRA. It is, therefore, concluded that the synergy between SRA and CSA-based EX can promote the expansion and strength characteristics of CSA-based systems, when no moist curing is adopted.

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