Effect of Superabsorbent Polymers on Workability and Hydration Process in Fly Ash Cementitious Composites
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Karol S. Sikora 1 and Agnieszka J. Klemm 2
Abstract: The effects of three superabsorbent polymers (SAPs) on variations in the cement hydration process were studied by calorimetric measurements and the X-ray scanning technique. It was concluded that the influence of superabsorbent polymers on workability and autogenous shrinkage strongly depends on SAP water absorption/desorption kinetics. Gradual release of additional water stored by a SAP facilitated the formation of a denser and more homogenous cementitious matrix. The addition of fly ash to portland cement had a notable influence on the hydration process and microstructure development in mortars regardless of modifications by the SAP. Characteristic patterns attributed to pozzolanic reactions appeared to be related to the mortar densification measured by the X-ray absorption and quasi-adiabatic calorimetry techniques during the first 48 h. DOI: 10.1061/(ASCE)MT.1943-5533.0001122. © 2014 American Society of Civil Engineers. Author keywords: Internal curing; Superabsorbent polymers; Portland fly ash cement; Autogenous shrinkage; X-ray absorption.
Introduction Background The key issue in the technology of cementitious composites is the management of an optimal water supply. In order to ensure effective cement hydration, an adequate amount of water is required. Nevertheless, in terms of practical applications, an additional amount of free water is needed to enable correct mixing, placing, and compacting and to ensure time for transporting. The effects of additional water in a mix include increased porosity that may lead to increased shrinkage and creep and a reduction in strength and durability in severe environmental conditions. In turn, highperformance concretes are made with very low water-to-cement ratios (in a range of 0.21–0.33), which results in self-desiccation and, consequently, leads to autogenous shrinkage and finally to crack propagation, significantly decreasing mechanical properties and durability (Zhutovsky and Kovler 2012). Only a proper curing process of concrete is a practical solution to prevent these negative effects. There are two general philosophies of curing cementitious materials: external curing and internal curing. Various external curing techniques can be divided into two main groups: water curing including water ponding, water spraying, fog misting, and saturated coverings; and sealed curing including waterproof paper, plastic sheeting, and curing membranes (Kovler and Jensen 2005). Despite the diversity of external curing methods, this technique generally has many drawbacks and limits that result in serious problems. First, for the purpose of external curing, extra labor needs to be provided, increasing the costs of the final product. In addition, even 1 Postdoctoral Researcher, College of Engineering and Informatics, National Univ. of Ireland, Galway, University Rd., Galway, Ireland (corresponding author). E-mail:
[email protected] 2 Reader, School of Engineering and Built Environment, Glasgow Caledonian Univ., 70 Cowcaddens Rd., Glasgow G4 0BA, Scotland. E-mail:
[email protected] Note. This manuscript was submitted on August 28, 2013; approved on May 6, 2014; published online on August 7, 2014. Discussion period open until January 7, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561/04014170(13)/$25.00.
© ASCE
well-trained and well-equipped staff may not ensure a high quality of service and uniform distribution of water. A further limitation of the external curing is accessibility to the concrete surface. An extra water supply is more problematic in complex formworks. The final issue is the low permeability, especially in high-performance concretes, which reduces water movement from the surface inside the matrix, leading to uneven water distribution of composite subsequent zones. The unquestionable reason for external curing usage is that there is no need for any supplementary material to be added to the cementitious matrix, unlike in internal curing. So it has no implications on the price of a composite mix production. Nevertheless, due to the requirement of extra labor during the external curing process, the overall cost of the final product is likely to be higher than for an application of internal curing (Cusson et al. 2010). Therefore, the above issues suggest that the implementation of internal curing instead of external curing might be more beneficial. Internal curing (IC) is defined, according to the American Concrete Institute (ACI), as: “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self– desiccation” (ACI 2010). This concept has been researched over the last couple of decades, and many reports on internal curing are available (Weber and Reinhardt 1995; van Breugel and Vries 1998; Bentur et al. 1999; RILEM Report 41 2007; Bentz and Weiss 2011). In order to ensure sufficient water supply, various types of internal curing agents have been proposed. Apart from lightweight aggregates, they include water-saturated normal-weight aggregates (Hammer et al. 2004; Geiker et al. 2004), wooden derived products (Mohr et al. 2005), and recycled aggregates (Maruyama and Sato 2005). Jensen and Lura (2006) presented the detailed comparison of techniques and materials for internal water curing of concrete. One of the promising solutions is the modification of cementitious matrix by superabsorbent polymers (SAPs) (Jensen and Hansen 2001, 2002). Nevertheless, there is lack of awareness of the SAPs’ influence on properties of mixes with blended cements. Previously published studies were predominantly focused on the effect of SAP in composites based on portland cements without any main addition. Therefore, in order to contribute to the present knowledge, the main
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purpose of this research is to assess the effects of superabsorbent polymers on the rheology and hydration process of portland cement fly ash mortars.
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Superabsorbent Polymers Superabsorbent polymers are cross-linked hydrophilic networks with a high capacity for liquid uptake. The large number of water molecules is able to diffuse into void space inside a SAP threedimensional network with chemical cross-links. The process leads to the creation of polymer gel. In turn, the removal of water molecules results in SAP return to the initial state (Siriwatwechaklu et al. 2010). This ability of water storage and release inspired researchers to apply SAPs in cementitious matrices. The noticeable effect of SAP on workability and the hydration process is expected in mortars and concretes (Toledo Filho et al. 2012). Studies by Dudziak and Mechtcherine (2009), and Mönnig (2005) confirmed reduction in flow; however, SAPs might uptake water for a longer time, even after mixing finalization. The consequence of SAP application is the reduction of free water availability, which depends on water absorption rate and capacity. The swelling ratio and strength of the network are directly determined by the degree of cross-linking, the chemical structures of the monomers forming the SAP network, and external stimuli such as pH and ionic concentration in the surroundings (Siriwatwechaklu et al. 2010). Despite the fact that autogenous shrinkage has been extensively researched in recent years, only a few investigations on mortars are known (Mechtcherine and Dudziak 2012; Ribeiro et al. 2010; Schlitter et al. 2010; Klemm and Sikora 2012). The vast majority of studies have suggested that the addition of SAP to cementitious matrix may be an effective agent in mitigation of autogenous shrinkage (Bentz and Weiss 2011), which arises as a consequence of the hydration process (Davis 1940). The majority of published results indicated significant strain reductions in samples containing polymers, with greater magnitude for higher SAP content (Toledo Filho et al. 2012; Igarashi and Watanabe 2006; Dudziak and Mechtcherine 2008; Mechtcherine et al. 2008; Brüdern and Mechtcherine 2010; Esteves 2010; Ribeiro et al. 2010; Dudziak and Mechtcherine 2010; Igarashi et al. 2010; Schlitter et al. 2010; Lura et al. 2006). However, a small swelling has been reported when SAP addition exceeded 0.6% by weight of cement (Toledo Filho et al. 2012; Igarashi and Watanabe 2006; Lura et al. 2006). Although SAPs’ applications in concrete construction seem very promising (RILEM Technical Committee 225-SAP 2012), they might be ineffective in certain mixes when the consequences of supplementary materials can play a very important role (Klemm and Sikora 2012). Fly Ash Over the last couple of decades, fly ash has been commonly used as a supplementary material in blended cements. Due to its pozzolanic abilities, fly ash may positively influence the cementitious properties, despite having little cementitious value itself (ASTM 1995). Pozzolanic reaction of fly ash with calcium hydroxide (CH) usually starts between the third and the seventh day and leads to the formation of calcium silicate hydrate (CSH) gel and densification of microstructure. However, even after 91 days of hydration, part of fly ash remained unreacted (Malhotra and Mehta 2005; Mehta 1985). Mehta (1985) attributed the fly ash reactivity level to its particle size distribution. Reactivity was found to be directly proportional to the amount of particles below 10 μm, and inversely proportional to particles bigger than 45 μm. Furthermore, fly ash may noticeably alter the hydration process leading to lower heat of © ASCE
hydration (Feldman et al. 1990; Massazza 1993; Friasm et al. 2000). Li et al. (2004) have indicated that this phenomenon is due to fly ash inhibition abilities of calcium ions concentration incrementing in fresh paste, which consequently led also to the setting time extension. However, according to Bai and Wild (2002), the reduced portland cement content would not necessarily reduce the initial rate of heat evolution, since some pozzolans are known to accelerate cement hydration. Zhang (1995) and Termkhajornkit et al. (2005) have suggested that autogenous shrinkage increases with the progress of pozzolanic reactions. However, when the percentage of fly ash in a composite is high, the vast majority of its particles remain unreacted. It has given an indication that fly ash may reduce shrinkage and creep of high-volume fly ash concrete. Its unreacted particles, of higher modulus of elasticity than matrix, may act as microaggregates in cement pastes and could be considered as a composite material on a microscale. However, Berry et al. (1990) presented a contrary view, indicating shrinkage increase by fly ash addition. The recorded amount of ettringite in fly ash cement mixes was higher than in the mixes with CEM I in the first 5 h after mixing, leading to high water consumption. As a result, empty voids might appear and consequently the autogenous shrinkage becomes larger.
Experiment Materials and Mixes Portland–fly ash cement (containing minimum 30% of fly ash by weight) CEM II/B-V 32.5 R was used in this study (BS EN 197-1). Sizes of fly ash particles belonged predominantly to the range between 5 and 15 μm. Its chemical composition was conducted by X-ray fluorescence (XRF) analysis and the elements quoted as their oxides are presented in Table 1. The specific surface area identified by the Blaine method in accordance with BS EN 196-6:2010 was 3,350 cm2 =g. Fine sand with 99% of particles below 600 μm was used in the study. Three types of SAPs (later known as SAP A, SAP B, and SAP C) used in the study are cross-linked polymers. SAP A is a copolymer of acrylamide and acrylic acid, SAP B is a polymer based on acrylic acid, and SAP C is a modified polyacrylamide. All polymers have absorption capacity of 200–250 mL=g in demineralized water measured by the “tea bag” method. However, this technique is generally not appropriate for SAP absorbency assessment in cementitious environments (Esteves and Jensen 2012). Therefore, absorption characteristics were determined by defining slump for the mortar mix without SAP and calculating the additional amount of water added to the mortar mix with SAP, required Table 1. Chemical Composition of Cement by Oxide Weight Percentage (XRF) Oxide
Wt%
Na2 O MgO Al2 O3 SiO2 SO3 K2 O CaO TiO2 Cr2 O3 MnO Fe2 O3 ZnO
0 1.33 13.13 32.69 4.11 1.26 43.48 0.56 0.08 0.07 3.29 0.02
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Fig. 1. SEM of SAP: (a) dry; (b) wet
to maintain the defined slump. This is an assumption based on the idea that the swollen polymer gel does not affect the mortar rheology. Water absorption capacities (WACs), measured by the presented technique, differ significantly: approximately 10 g=g for SAP A, 5 g=g for SAP B, and 25–30 g=g for SAP C. All materials were prepared by grinding and screening to size of 63–125 μm with less than 10% of finer particles. Fig. 1 presents the scanning electron micrographs (SEMs) of used SAPs in dry and wet conditions (5 min after their contact with water). The bulk density of SAPs is between 500 and 700 g=L. For the purpose of this research, cement was mixed with fine sand at 1∶1 and 2∶1 sand/cement ratio (by weight). Throughout the investigation an even total water-to-cement ratio of 0.45 was maintained for all mixes: reference samples (R1 and R2) and samples with SAP additions (A1, B1, C1, A2, B2, and C2). Three types of SAPs were used in concentration of 0.25% by weight of cement content. The effective water-to-cement ratios for mixes containing SAPs were lower than total water-to-cement ratios due to water absorption by SAPs. Detailed information about mix composition is presented in Table 2. Experimental Techniques
mixes, the heat of hydration was determined by the quasi-adiabatic calorimeter (QAB) method [EN 196-9:2010 (EN 2010)]. The attempt was made to assess alterations of bulk densities of sealed samples undergoing the hydration process by the application of the X-ray absorption technique. The measurements in the X-ray absorption chamber commenced approximately 15 min after water addition, and the intensities I0 (incident X-ray intensity) and I (transmitted X-ray intensity) were measured. The natural logarithm of the detector count was used as a metric, which is inversely proportional to the density multiplied by the mass attenuation coefficient, assuming a constant incident X-ray intensity during the test. The increase in detector count represents density reduction or the attenuation coefficient. The sealed transparent containers (70× 70 × 70 mm) used for tests were located on a stand within the chamber, separated by a metal spacer, and a scanning sequence was performed. The scans across the samples were taken at approximately halfway up the container. The minimum achievable interval between the start of successive scans was 20–30 min. In addition, the established microstructural features of the mortars and SAPs were verified by examination under the SEM.
Results and Analysis
Autogenous shrinkage tests were performed according to ASTM C 1698-09 (ASTM 1999) using dilatometer with corrugated tubes (three samples of each mix) and results were recorded manually on a daily basis at early ages. For the purpose of this study, mixes (mortars and paste) were placed into the corrugated plastic tubes, having length of 420 5 mm and an outer diameter of 29 5 mm and stored in laboratory conditions. Measurements started at the time of final set obtained according to EN 196-3:2005+A1:2008 (EN 2008) (Table 3). Fresh mortar mixes were tested using the flow table in accordance with EN 1015–3:1999 (EN 1999b) to assess its consistency. Deformability was measured under gravity and stress applied by 15 jolts. Air content of fresh mortars was determined in accordance with EN 1015-7:1999 (EN 1999a) by the pressure method. The initial and final setting times of cement paste and mortar mixes with different sand-to-cement ratios were determined by the Vicat apparatus in accordance with EN 196-3:2005+A1:2008 (EN 2008). In order to monitor the hydration process of mortar
Physical Properties of Fresh Mixes The results confirmed that modification of mortar by SAP A and SAP C has significant effect on the workability. Flow values for different sand-to-cement ratios and both before and after jolts were reduced by SAP A and SAP C in comparison to the reference mortar. In turn, SAP B reduced those values only slightly. Table 3 shows the results of flow table, air contents, and setting times (initial and final) obtained for mortar mixes with different sand-tocement ratios. Initial and final setting times and differences between these values were relatively high, which likely resulted from the significant fly ash content in the mixes. Workability of fresh mixes is also strongly affected by air content. Higher air contents were recorded for mixes containing SAP A in comparison with the reference mortar. The SAP C also had increased air content that is more apparent with higher aggregate
Table 2. Cement Mortar Composition Mix code
R1
A1
B1
C1
R2
A2
B2
C2
Sand:cement ðWater=cementÞtot ðWater=cementÞeff SAP content (%)
1∶1 0.45 0.45 0
1∶1 0.45 0.425 0.25
1∶1 0.45 0.438 0.25
1∶1 0.45 0.375–0.388 0.25
2∶1 0.45 0.45 0
2∶1 0.45 0.425 0.25
2∶1 0.45 0.438 0.25
2∶1 0.45 0.375–0.388 0.25
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Table 3. Results of Physical Properties for Mortar Mixes Sand=cement ¼ 1∶1
Sand-to-cement ratio
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Mix code Flow before jolts (mm) Flow after 15 jolts (mm) Air content (%) Initial set (h:min) Final set (h:min)
R1 217.5 >250 0.45 5:37 13:38
A1 100 197.5 0.75 7:21 17:20
Sand=cement ¼ 2∶1
B1 192.5 >250 0.30 6:00 14:00
content. The SAP B had only very limited effect on air content. The air content values for SAP B were even slightly lower than for reference mixes with different sand-to-cement ratios. Standard consistence for CEM II used in the study was achieved for 0.30 water-to-cement ratio with initial setting time of 7 h 43 min and final setting time of 10 h 50 min. It was observed that additions
C1 103.8 202.5 0.50 5:50 13:47
R2 105 185 2.40 5:12 12:53
A2 100 112.5 3.55 6:45 16:57
B2 100 165 2.35 6:28 13:28
C2 100 113.5 3.20 5:49 13:20
of SAP A and SAP B prolonged both setting times in all mixes. In particular, the final setting time for cement paste with SAP A was 4 h 40 min longer than for the paste of standard consistence. The SAP C increased setting times little in comparison to the reference mixes. Heat of Hydration
(a)
(b)
Fig. 2. Temperature (°) versus time (h) mortar mixes with sand-tocement ratio (a) 1∶1; (b) 2∶1 first series in long term (first 7 days)
(a)
Measurements of heat of hydration were carried out for the first 7 days, and in order to confirm the patterns, the second series for each mix was recorded during the first 50 h after mix preparation. The curves presenting temperature changes as function of time for samples with sand-to-cement ratio (a) 1∶1 and (b) 2∶1 are shown in Fig. 2. During the first 15 h after preparation, the pattern of heat of hydration for the specimens A1 and B1 was almost the same as for the reference sample, starting with 20°C and reaching approximately 40°C. In the following hours the pattern changed. Higher temperatures were recorded for A1 and lower for B1 in comparison to the reference sample. The temperature generated by specimen C1 due to the heat of hydration was higher at the beginning by approximately 2°C in comparison with other samples. Later this difference increased, reaching a maximum of 10°C after 15 h. The peak intensity passed through a maximum at 20–26 h; first for Sample B1, and later, respectively, for R1 and C1, and for A1 after 26 h. Subsequently, the curve representing C moved closer to Curve A1 and after 30 h of hydration initiation, it followed the trend of Sample A1. Fig. 3 presents the temperatures of mortar mixes during the time of hydration progress, recorded for specimens with sandto-cement ratios (a) 1∶1 and (b) 2∶1. Temperatures for mortar mixes with sand-to-cement ratio 2∶1 after 20 h were reaching approximately 45°C in comparison to 55°C for Samples A2 and C2 with lower sand content. As anticipated, the effect of SAPs on the temperature changes of the mortars was rather insignificant when the sand-to-cement ratio was higher. The observed pattern of Sample A2 was the same as the reference sample. The values of temperatures generated by Samples
(b)
Fig. 3. Temperature (°) versus time (h) for mortar mixes with sand-to-cement ratio (a) 1∶1; (b) 2∶1 second series in short term (first 50 h) © ASCE
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C2 and B2 were slightly lower until 25 h and then Sample C2 followed a trend of R2. The B2 samples generated slightly higher temperature.
Fig. 5 presents the X-ray absorption results for mortar mixes with sand-to-cement ratio 2∶1 (a) for the initial 24 h and (b) for 6 weeks. The higher values have given an indication of less dense structure than for samples with 1∶1 sand-to-cement ratio, associated with a lower percentage of cement paste in the mixes. Moreover, the change of the densification trend was observed, similarly to samples with 1∶1 ratio, after about 15 h. The smallest differences were observed for the reference sample referring to highest density of the mix. An increase in densities was observed for samples modified by SAPs, although of much bigger magnitude and earlier for Sample A2. Results obtained after 6 weeks of curing for Samples A2 and B2 were comparable; however, Sample C2 showed less dense structure. This phenomenon was recorded at every point for the samples with sand-to-cement ratio 2∶1. It should be noted that the recorded values of detector count refer to the sealed system, comprising newly created hydration products, water absorbed by SAP, free water and pores, and some unreacted cement particles. However, soon after cement was mixed with water, the homogenous suspension of cement particles was created. This state occurred because hydration reactions proceeded and the system of separated phases was formed. Therefore, after the initial densification a steady decrease of detector count was observed. The process is very complex, particularly in the case
The profiles of measured attenuations of X-ray beam by samples with sand-to-cement ratio 1∶1 are shown in Fig. 4. Results represent recordings (a) from the initial 24 h for each sample approximated by polynomials of the fourth degree and (b) daily averages of recordings for each sample during 6 weeks. Due to the fact that samples were completely sealed during the measurements and there was no evaporation from their surfaces, desorption of SAPs was most probably delayed and not as intense as in normal curing conditions. It should be noted that by the time of first readings (approximately 15 min), a significant part of the mixing water could have been absorbed by the superabsorbent polymers. During the first 15 h after water addition, densification was observed. This trend changed in the following hours. In addition, the results for Sample A1 indicated lower density and for Sample B1 higher density than the reference sample. Mix containing SAP C followed a similar pattern to the reference sample during the first day of hydration. Between the second and the third weeks Sample C1 changed its trend and resembled more Sample A1. 8
R1
A1
B1
C1
Ln (Count)
7.8
7.6
7.4
7.2
7
0
3
6
9
12 Time [h]
(a) 8.2
R1
A1
B1
15
18
21
24
C1
8
Ln (Count)
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X-ray Absorption by Fresh Mortar
7.8
7.6
7.4
7.2 0
(b)
7
14
21 Time [days]
28
35
42
Fig. 4. X-ray absorption results for mortar mixes with sand-to-cement ratio 1∶1 for (a) initial 24 h; (b) during 6 weeks © ASCE
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8
R2
A2
B2
C2
7.6
7.4
7.2
7 0
3
6
9
12 Time [h]
(a) 8.2
R2
A2
B2
15
18
21
24
C2
8
Ln (Count)
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Ln (Count)
7.8
7.8
7.6
7.4
7.2 0
7
14
21 Time [days]
(b)
28
35
42
Fig. 5. X-ray absorption results for mortar mixes with sand-to-cement ratio 2∶1 for (a) initial 24 h; (b) during 6 weeks
of blended cements, and its understanding requires further thorough investigation. Autogenous Deformation The effect of SAP modification of mortar mixes with sand-tocement ratio 1∶1 was noticeable. In order to observe trends in results and provide clearness of graphs, the results were approximated by polynomials of the fourth degree. The values were the highest for the reference mix from the first days. However, the different SAPs did not affect the results to the same extent. After the first week, SAP A resulted in almost four times smaller shrinkage than obtained by the reference specimens. The SAP C gave three times reduction and SAP B 25% reduction after 7 days after specimen preparation. Between the first week and the third week autogenous shrinkage (AS) hardly changed. In the third week swelling of samples containing SAP A and SAP C started to cease. Sample B1 and the reference sample underwent continuous shrinkage with a small slope from the third week. The biggest shrinkage for Sample B1 was observed after 6 weeks and reached approximately 250 μm=m, in comparison to 280 μm=m for the reference sample after 5 weeks. The AS results are shown in Fig. 6. For specimens containing more sand (s=c ¼ 2∶1), relatively similar patterns of AS development were observed for all tested © ASCE
samples. During the first 5 days shrinkage of approximately 100 μm=m was recorded. From this point the effect of SAP A and SAP C was noticeable, as the shrinkage development stopped. In turn, Specimen B2 showed a very similar trend to the reference sample with slightly higher shrinkage for B2. Between the second and fourth weeks, a small swelling was recorded for A2 and C2 with no further linear changes observed later. In order to eliminate the effect of aggregates on shrinkage, the shrinkage measurements were carried out for cement paste samples [Fig. 6(c)]. The cement pastes were prepared according to the same procedure and materials, without fine sand addition, as the mortar mixes in this study. The pastes were labeled Rp for the reference paste and Ap, Bp, and Cp for the pastes containing relevant SAPs. In the first days some swelling was observed for Sample Ap. In contrast, relatively high shrinkage values were recorded for Sample Bp during the same time. Up to Day 4, shrinkage for Sample Cp had the same pattern as the reference. In the following days shrinkage for Cp was slightly higher than for Specimen Rp and ceased between the first and second weeks. Between the second and fourth weeks, Sample Cp was swelling to its initial value and no later changes were recorded. Between Weeks 3 and 6, shrinkage changed from 100 to 250 μm=m for the reference sample. Similar behavior to the reference sample was observed
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R1
A1
B1
C1
0 -50 -100 -150 -200 -250 -300 -350 -400
0
7
14
21 Time [days]
(a)
R2
A2
B2
28
35
42
28
35
42
28
35
42
C2
Autogenous shrinkage [µm/m]
50 0 -50 -100 -150 -200 -250 -300 -350 -400
0
7
14
21 Time [days]
(b)
Rp
Ap
Bp
Cp
50 Autogenous shrinkage [µm/m]
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Autogenous shrinkage [µm/m]
50
0 -50 -100 -150 -200 -250 -300 -350 -400
0
7
14
21 Time [days]
(c)
Fig. 6. Shrinkage results for mortar mixes with (a) sand-to-cement ratio 1∶1; (b) mortar mixes with sand-to-cement ratio 2∶1; (c) cement paste
for Sample Bp only with the higher difference by approximately 100 μm=m. After initial swelling a very low shrinkage was observed in specimens containing SAP A (not exceeding 50 μm=m). Analysis Superabsorbent polymer A (copolymer of acrylamide and acrylic acid) possesses a water absorption capacity of 10 g=g in cementitious composite, which is lower than SAP C (25–30 g=g) and higher than SAP B (5 g=g). The experimental results have indicated that SAP A absorbed water more quickly during the mixing process. It was confirmed by a significant flow reduction (approximately 30%) soon after mixing. However, it is likely that the majority of stored water was released in the first few hours. It was observed that additions of SAP A prolonged initial and final setting times in all mixes (by comparison with reference samples: 31 and 27% for samples with 1∶1 sand-to-cement ratio, 31 and 31% for © ASCE
samples with 2∶1 sand-to-cement ratio, and 4 and 35% for cement pastes). In turn, between the second and the fourth weeks some swelling was recorded in samples containing SAP A. This was the common feature for cement paste and both mortars with sand-to-cement ratios 1∶1 and 2∶1 suggesting that the rest of the water stored by SAP A was gradually released for at least 4 weeks ensuring hydration over that period of time. In order to investigate progress of the hydration process in the first hours, frequent measurements of the temperature due to the heat of hydration and X-ray absorption were carried out. The comparison of the recordings for mortars containing SAP A and the reference with 1∶1 and 2∶1 sand-to-cement ratios in the first 48 h is presented in Fig. 7. In general, the higher temperature was observed for denser mortars. However, the values for calorimetry measurements were delayed by approximately 5 h in relation to the densification. It was particularly visible after 15–20 h, when the peaks were observed for the X-ray absorption and heat of hydration results.
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cal A1
cal R1
cal A2
cal R2
xr A1
xr R1
xr A2
xr R2
60
6.8
7
50
7.2
7.4 30 7.6
Ln (Count)
Temperature [°C]
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40
20 7.8
10
8
0
8.2 0
6
12
18
24 Time [h]
30
36
42
48
Fig. 7. Comparison of temperature (cal) and X-ray absorption (xr) as a function of time for mortars containing SAP A and reference with 1∶1 and 2∶1 sand-to-cement ratio
Most probably the significant change in X-ray absorption pattern for all mixes after approximately 15 h from water addition and simultaneous temperature increase were attributed to ettringite formation in the early stage of the hydration process. Comprehensive SEM analysis for Specimen A revealed ettringite formations (shown in the Fig. 8) leading to a conclusion that the whole amount of ettringite was not converted into the calcium monosulfoaluminates as a consequence of the considerable calcium-to-tricalciumaluminate ratio in the cement used in the study. The intensive temperature increase observed from approximately the twentieth hour, might be associated with the specific cement composition, more precisely with pozzolanic reactions and the production of CSH. Moreover, large amounts of aluminates from fly ash could lead to reactions of CH with Al2 O3 resulting in creation of C4 AH13 . These observations were similar for all mixes, regardless of SAP addition, giving an indication of the predominant role of the cement type. It is also likely that SAP A released most of the stored water in the first hours and then further release was limited. However, SAP
A still could have retained some amounts of water and used it in the following days. Superabsorbent polymer of Type A proved to have a positive effect on AS behavior. The comparison of AS and the X-ray absorption recordings for samples with SAP A addition and the reference sample is presented in Fig. 9. Since the AS for SAP A diminished significantly during the first week and increased for the reference sample, it is likely that a majority of stored water was used during this period. It can therefore be assumed that SAP A supplied water to the cementitious matrix as soon as self-desiccation occurred. However, a small swelling up to the fifth week was observed indicating that water desorption by SAP A lasted at least up to this point. The AS results were consistent with the X-ray absorption measurements revealing similar tendencies. The AS development was measured in the corresponding periods of time to matrix densification determined by the X-ray absorption technique. The second superabsorbent polymer analyzed in this study (SAP B) is based on acrylic acid. Low water absorption capacity
Fig. 8. SEM micrographs showing ettringite of mortars containing SAP A for 1 week © ASCE
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as A1
as R1
as A2
as R2
xr A1
xr R1
xr A2
xr R2
-300
7.1 7.2
-200
7.4 Ln [Count]
Autogenous shrinkage [µm/m]
7.3
7.5 -150 7.6 -100
7.7 7.8
-50 7.9 0
8 0
7
14
21
28
35
42
Time [days]
Fig. 9. Comparison of autogenous shrinkage (AS) and X-ray absorption (xr) results for mortars containing SAP A and reference with 1∶1 and 2∶1 sand to cement ratio
of SAP B resulted in very limited effects on workability and the hydration process of mortars. The test results of flow show the reduction by approximately 10%. It might suggest that water absorption by SAP B proceeded relatively fast during mixing process (by comparison with 30% reduction for SAP A). Nevertheless, significant reduction in setting time (initial setting by 7% for 1∶1 and by 25% for 2∶1 sand-to-cement ratio, and by 17% for cement paste; final setting by 29% for 1∶1 and by 5% for 2∶1, and by 6% for
cal B1
cal R1
cal B2
cement paste) indicated that the total desorption occurred in the first few hours. The AS records confirmed this statement as SAP B was not able to provide reduction in AS protection over the period of 6 weeks. The relationship between the temperature due to the heat of hydration and X-ray absorption for mortars containing SAP B and the reference samples is shown in Fig. 10. Similar patterns in mixes with SAP A addition were observed. The peaks between fifteenth
cal R2
xr B1
xr R1
xr B2
xr R2
60
6.8
7
50
7.2
7.4 30 7.6
Ln (Count)
40 Temperature [°C]
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-250
20 7.8
10
8
0
8.2 0
6
12
18
24
30
36
42
48
Time [hours]
Fig. 10. Comparison of temperature (cal) and X-ray absorption (xr) as a function of time for mortars containing SAP B and reference with 1∶1 and 2∶1 sand to cement ratio © ASCE
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as B1
as R1
as B2
as R2
xr B1
xr R1
xr B2
xr R2
-300
7.1 7.2
-200
7.4 7.5
-150 7.6 -100
Ln [Count]
Autogenous shrinkage [µm/m]
7.3
7.7 7.8
-50 7.9 0
8 0
7
14
21
28
35
42
Time [days]
Fig. 11. Comparison of autogenous shrinkage (as) and X-ray absorption (xr) results for mortars containing SAP B and reference with 1∶1 and 2∶1 sand-to-cement ratio
and twentieth hours suggested that SAP B did not influence pozzolanic reactions of aluminates, and the cement constituents had a predominant role in the process. Superabsorbent polymer of Type B had a limited effect on microstructure development and autogenous shrinkage. The AS measurements for mixes containing SAP B revealed insignificant decrease for samples with 1∶1 sand-to-cement ratio and negligible increase for samples with 2∶1 ratio. In turn, the increase of up to 20% by comparison with the reference samples was observed for cement paste indicating a possible negative effect on shrinkage behavior. It could cal C1
cal R1
cal C2
possibly be attributed to a little disturbance in water distribution (uneven distribution) and/or permanent retention of some water molecules inside the polymer network. Fig. 11 shows the comparison of AS and the X-ray absorption results for mortars containing SAP B and the reference sample with 1∶1 and 2∶1 sand-to-cement ratios. Its lower absorption/desorption abilities could not reassure a continuous supply of water; hence, the performance of SAP B mortars was comparable to the reference samples not containing polymers. The third superabsorbent polymer analyzed here was based on modified polyacrylamide. The SAP C had the highest water
cal R2
xr C1
xr R1
xr C2
xr R2
60
6.8
7
50
7.2
7.4 30 7.6
Ln (Count)
40 Temperature [°C]
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-250
20 7.8
10
8
0
8.2 0
6
12
18
24 Time [hours]
30
36
42
48
Fig. 12. Comparison of temperature (cal) and X-ray absorption (xr) as a function of time for mortars containing SAP C and reference with 1∶1 and 2∶1 sand-to-cement ratio © ASCE
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as C1
as R1
as C2
as R2
xr C1
xr R1
xr C2
xr R2 7.2
-300
7.3 7.4 -200
7.5 7.6
-150
7.7 -100
7.8
Ln [Count]
Autogenous shrinkage [µm/m]
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-250
7.9
-50
8 0 8.1 50
8.2 0
7
14
21
28
35
42
Time [days]
Fig. 13. Comparison of autogenous shrinkage (as) and X-ray absorption (xr) results for mortars containing SAP C and reference with 1∶1 and 2∶1 sand-to-cement ratios
absorption capacity in mortars, but the results of flow and setting time have given an indication that the absorption rate differs from both SAP A and SAP B. Despite over double water capacity than SAP A, SAP C reduced the flow to lower extend (approximately 8%). Moreover, SAP C had slightly increased setting times in comparison to the reference mixes. However, it increased setting time less than both SAP A and SAP B. This may have suggested that SAP C released less water in the first hours. Swelling observed during the second and fourth weeks for samples containing SAP C pointed out a full usage of water reservoirs. Water may have been gradually released during this period promoting hydration process. The observations of mortar mixes by calorimetry and X-ray absorption measurements suggested that SAP C did not significantly affect hydration during the first 48 h. Fig. 12 presents comparison of the temperature due to the heat of hydration and the X-ray absorption results for mortars containing SAP C and the reference sample with 1∶1 and 2∶1 sand-to-cement ratios. For Sample C, like other samples (R, A, and B), the peaks of the curves attributed to a possible ettringite formation and aluminates with portlandite were visible between the fiftieth and twentieth hours. This feature might be associated with the significant role of cement composition. Superabsorbent polymer of Type C proved to have a positive effect on AS behavior, showing similarities to the effect given by SAP A. The comparison of AS and the X-ray absorption results for mortars containing SAP C and reference sample with 1∶1 and 2∶1 sand-to-cement ratios is shown in Fig. 13. Unlike SAP A, SAP C performed more intensively, indicating an increased water release after the second week and beyond the fifth week. This phenomenon might be attributed to delayed water desorption by SAP C. The above analyses of the effect of SAP on rheology and the hydration process confirmed that the type of superabsorbent polymer and consequently its WAC and water absorption/desorption kinetics were the most influential features. The SAPs with the high WAC, of at least 10 g of water per 1 g of polymer in a pore solution (SAP A and SAP C), proved to be efficient in diminishing AS. These polymers were able to retain some part of the absorbed water and released it gradually in the following weeks. Despite the similar effect on AS, it seems that the absorption/desorption behavior of © ASCE
SAP A and SAP C was much different. The significant reduction of workability by SAP A was an indication of a very quick rate of water absorption. Nonetheless, noticeable limitation in AS during the first week suggested reduction of self-desiccation by a substantial water supply from collapsing SAP A. In turn, the insignificant effect of SAP C on workability and AS cease, delayed by approximately a week by comparison to SAP A, were recorded. These observations suggested the delayed water absorption/desperation kinetics in the studied pore solutions. The rather insignificant effect of SAP B was attributed to its low water absorbtion capacity (5 g=g). In addition, most probably SAP B released all or a vast majority of the stored water during the first hours after mix preparation. Although the influences of superabsorbent polymers on workability and AS were clearly visible, the hydration process observation by the X-ray absorption and QAB calorimetry revealed the insignificant effects during the first 48 h. Pronounced similarities of the results for samples modified by SAPs and the reference samples suggest that cement type had the predominant role in the hydration process.
Conclusions Based on the investigations presented in this study, the following conclusions can be formulated: • It was confirmed that the effects of superabsorbent polymers on rheology, autogenous shrinkage, and microstructure strongly depend on SAP type and water absorption/desorption kinetics. • In general, SAPs with high water absorption capacity (minimum 10 g of water per 1 g of polymer in pore solution) proved to be efficient in diminishing autogenous shrinkage (SAPs A and C). These polymers are able to retain part of the absorbed water and release it gradually during a period of 4 weeks. Superabsorbent polymers with low water absorption capacity (5 g of water per 1 g of the polymer) have a limited effect on microstructure alteration and hence autogenous shrinkage (SAP B). The vast majority of the absorbed water is released during the first hours
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after mix preparation. Nonuniform distribution of water and/or permanent retention of some water molecules inside the polymer network may lead, in some cases, to a small increase in autogenous shrinkage. • Total water absorption capacity is not a sole criterion for selection of appropriate SAP in cementitious system. The kinetics of the process and its compatibility with the hydration process are of paramount importance. Despite higher full WAC for SAP C (25–30 g of water per 1 g of polymer in pore solution) the process of water absorption/desorption is delayed in the studied cementitious mixes, resulting in smaller flow reduction than for SAP A (10 g of water per 1 g of the polymer). This may bring significant benefits in practical applications in the construction industry by not affecting workability and in consequence placing and compacting during the first hours after preparation. Composites containing such SAPs could also benefit from reduced shrinkage and increased compressive strength. • Fly ash addition to cement has a notable influence on hydration process and microstructure development in cementitious composites regardless of modifications by SAP. Characteristic patterns attributed to pozzolanic reaction manifested themselves in pronounced similarities of the mortar densification measured by the X-ray absorption and calorimetry during the first 48 h. In principle, the higher temperature is observed for denser mortars. The intensive temperature increase observed at approximately the twentieth hour and soon after peak appearance, might be associated with the specific cement composition. Moreover, the higher amounts of aggregates in the matrix do not influence the density of the CSH gel in the interfacial transition zone in composites with fly ash cements. Since sizes of fly ash particles are mostly below 15 μm, they act as effective fillers for voids in the vicinity of aggregate and play a dominant role in ensuring the packing of cement particles (“wall effect”). • Despite the complexity of interactions between various types of SAPs and supplementary cementitious materials, superabsorbent polymers may prove to be very effective in internal curing. Careful selection of the SAP with desorption characteristics compatible with the hydration process is a critical issue. Diversity in types of commercially available cements leads to differences in kinetics of the hydration process and hence different water requirements. Since water absorption capacity and the rates of both water uptake and release depend on the SAP type, the adjustment to certain cements may be very complex.
Acknowledgments Provision of superabsorbent polymers by the BASF Construction Chemicals GmbH for the purpose of this study is gratefully acknowledged.
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