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ASCEDevelopmentofUltralightweightCementCompositeswithLowDensityandHigh-SpecificStrengthUsingHollowGlassMicrospheres

Ultra lightweight Cement Composites

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ASCEDevelopmentofUltralightweightCementCompositeswithLowDensityandHigh-SpecificStrengthUsingHollowGlassMicrospheres

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  1. See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/352031662 Development of Ultralightweight Cement Composites with Low Density and High-Specific Strength Using Hollow Glass Microspheres Article  in  Journal of Materials in Civil Engineering · June 2021 DOI: 10.1061/(ASCE)MT.1943-5533.0003739 CITATIONS 5 READS 199 5 authors, including: Shukai Cheng Wuhan Institute of Technology 43PUBLICATIONS   1,204CITATIONS    SEE PROFILE All content following this page was uploaded by Shukai Cheng on 14 December 2021. The user has requested enhancement of the downloaded file.

  2. Development of Ultralightweight Cement Composites with Low Density and High-Specific Strength Using Hollow Glass Microspheres Feixiang Chen1; Shukai Cheng, Ph.D., M.ASCE2; Guozhi Zhang, Ph.D.3; Shanglei Chen4; and Ronghui Yang5 Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. Abstract: This paper investigated hollow glass microspheres (HGMs) on the density, mechanical properties, and durability of ultralight- weight cement composites (ULCCs). The influences of HGMs on the microstructure developments of ULCCs were also studied. The results indicated that the ULCCs had a 1-day density ranging from 778 to 948 kg=m3and 28 days compressive strength ranging from 22.9 to 33.1 MPa. The density of ULCCs was decreased with the increasing amount of HGMs, while the mechanical properties were reduced accordingly. The newly developed ULCCs with an HGM-to-binder volume ratio of 5∶1 and water-to-binder of 0.58 has a 1-day density of 778 kg=m3and 28 days compressive strength of 22.9 MPa, whose specific strength was 0.029 MPa=Kg=m3. Furthermore, the obtained ULCCs showed a good resistance to chloride ion penetration. The microstructure demonstrated that the pozzolanic reaction of HGMs resulted in denser hydration products, which exhibited beneficial impacts on the durability of ULCCs. DOI: 10.1061/(ASCE)MT.1943- 5533.0003739. © 2021 American Society of Civil Engineers. Author keywords: Ultralightweight cement-based composites (ULCCs); Hollow glass microspheres (HGMs); Density; Compressive strength; Chloride ion penetration; Microstructure. Introduction 2019), or foam slag (Chung et al. 2019). The main benefit of using LAC is to reduce self-weight of structural elements, allowing smaller sizes and less reinforcing materials (Blanco et al. 2000). Thus, the requirements for offshore foundations and transportation cost can be significantly decreased. Ultralightweight cement composites (ULCCs), as a novel light- weight cement-based materials composite, have been developed with a unit weight (less than 1,500 kg=m3) of at least 40% less than that of natural aggregate concrete (NAC) and a compressive strength up to about 65 MPa (Wu et al. 2015). Due to their high specific strengths, ULCCs have attracted widespread research interests in offshore and marine applications (Dalai et al. 2014). Besides, ultralightweight is also an important characteristic of ULCCs, which can also be applied to those offshore and marine structures. To achievelow density, fly ash cenospheres (FACs) were used as a filler material in the ULCC mix (Huang et al. 2018). The FACs are lightweight, inert, and hollow microspheres, which are acquired as residue from ash power plants (Kwan and Chen 2013). Blanco et al. (2000) developed the concrete containing FACs with a density of 1,510 kg=m3and 28 days compressive strength of 33.03 MPa. Wu et al. (2015) prepared the ULCCs incor- porating FACs and the properties of ULCCs were compared to the cement paste and concrete with the same water to binder ratio. The results showed that the mechanical properties of ULCC were de- creased with the reduction of density, while the compressive strength and flexural tensile strength of ULCCs was similar to that of control group samples. In addition, ULCCs contained FACs ex- hibited better load cycles and crack pattern characteristics than that of LAC with similar strength (Sohel et al. 2018). Furthermore, Hanif et al. (2016) investigated the effect of FACs dosage and aero- gel on the mechanical and thermal properties of ULCCs. It was found that combined FACs with aerogel could lead to strong mechanical performance of ULCCs as well as thermal properties. Some studies reported that the incorporation of supplemental ce- mentitious materials such as fly ash (Patel et al. 2019), silica fume, The construction for islands and reefs infrastructures is a necessary prerequisite for carrying out scientific research, resource prospec- tion, navigation, and other activities in the open sea of China. However, the most essential material used for those infrastructures is cement-based materials, especially lightweight cement-based materials (Thomas and Bremner 2012) that can be widely used in the construction of large floating structures such as artificial floating islands and offshore floating platforms (Neramitkornburi et al. 2015). Lightweight aggregate concrete (LAC) is a repre- sentative lightweight cement-based material with strength higher than 17 MPa and density less than 1,920 kg=m3(ACI 2003), which is produced using lightweight aggregate such as expanded shale (Kramar and Bindiganavile 2013), expanded clay (Ahmad et al. 1Engineer, Master, CCCC Second Harbour Engineering Co. Ltd., Jinyinghu Rd. 11, Dongxihu District, Wuhan City 430040, China. Email: 834178732@qq.com 2Lecturer, School of Civil Engineering and Architecture, Wuhan Institute of Technology, Hongshan District, Wuhan City 430074, China (corresponding author). Email: chengsk@whut.edu.cn 3Professor, Senior Engineer, CCCC Second Harbour Engineering Co. Ltd., Jinyinghu Rd. 11, Dongxihu District, Wuhan City 430040, China. Email: 452762256@qq.com 4Engineer, Master, CCCC Second Harbour Engineering Co. Ltd., Jinyinghu Rd. 11, Dongxihu District, Wuhan City 430040, China. Email: 957844217@qq.com 5Engineer, Master, CCCC Second Harbour Engineering Co. Ltd., Jinyinghu Rd. 11, Dongxihu District, Wuhan City 430040, China. Email: 542471817@qq.com Note. This manuscript was submitted on July 14, 2020; approved on November 9, 2020; published online on March 29, 2021. Discussion period open until August 29, 2021; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE 04021124-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  3. Table 1. Chemical compositions and physical properties of OPC, SF, and FAC and nanosilica (Du 2019) had better mechanical strength and filling properties in the ULCCs containing FACs. Apart from FACs, hollow glass microspheres (HGMs) are com- monly used as a kind of filler due to their low density, low thermal conductivity, high strength, and excellent chemical stability (Ranjbar and Kuenzel et al. 2017). The particle size of HGMs is smaller and the density is lower compared with FACs. Besides, HGMs are environmentally friendly due to their higher proportion of recycled glass and less energy requirements (Dalai et al. 2014). Recently, different types of HGMs were prepared with different lightweight cementitious materials (Al-Gemeel et al. 2018). Zhuge et al. (2014) found that the addition of 10 % by volume HGMs had a slight influence on the compressive strength of lightweight engineered cementitious composites (LW-ECCs), while the density of LW-ECCs reduced by 6%. Aslani and Wang (2019) prepared LW-ECCs with three different types of HGMs, whose research showed that the density and compressive strength of the LW-ECC mixture containing HGMs were decreased by 10% and 3%, respec- tively. Wang et al. (2020) developed ultralightweight engineered cementitious composites (ULW-ECCs) by using HGMs, whose density is less than 1,400 kg=m3, compressive strength is higher than 30 MPa, and flexural strength reaches 14.4 MPa, respectively. However, most investigations focused on development of the density and mechanical properties of ULCCs by using different FACs/HGMs and their applications in the cement-based materials. Few studies have developed the ULCC-incorporated HGMs with density less than 1,000 kg=m3and higher specific strengths. More- over, the influence of HGMs on the durability and microstructure of ULCCs is lacking. In this study, the HGMs are used to prepare the ULCCs and then evaluate the feasibility of using HGMs in designed ULCCs to satisfy requirements for offshore floating platforms, whose den- sity is lower than 1,000 kg=m3and compressive strength is higher than 20 MPa, respectively. Experiments on the properties such as density, mechanical properties, water absorption, and chloride ion penetration of the designed ULCCs are conducted. The effects of HGM-to-binder ratio and water-to-binder ratio on the physical and mechanical properties of ULCCs are investigated. Moreover, the failure modes and microstructure of ULCCs are discussed, respectively. Chemical composition (% by weight) OPC SF FAC HGM Al2O3 SiO2 CaO MgO Fe2O3 Na2O K2O SO3 L.O.I Physical properties Specific gravity (kg=m3) Specific surface area (m2=kg) Water demand ratio (%) 5.69 21.27 60.15 2.41 3.16 0.14 0.69 3.66 3.95 0.25 94.65 0.36 0.47 0.15 0.13 0.84 0.69 2.29 35.65 58.72 0.22 0.85 2.86 0.49 0.66 0.11 0.44 0.42 83.00 9.04 0.26 0.03 6.96 0.01 0.15 1.22 Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. 3,210 380 100 2,300 21,800 115 580 457 94 260 — 102 Note: L.O.I = loss of ignition. Fig. 1. Particle size distributions of OPC, SF, and FAC. Table 2. Physical properties of HGM Unit weight (g=cm3) True density (g=cm3) Crush Moisture content (%) Experiment Investigations D90 (μm) 87 compressive (MPa) 0.26 0.38 30 0.40 Materials In this study, the ULCCs are prepared to gain a compressive strength higher than 20 MPa and a low volume-weight (less than 1,000 kg=m3) by adopting cementitious materials and HGMs. Ordinary Portland cement (OPC, type I 42.5), loose silica fume (SF), and FACs were used as the cementitious materials. The chemical compositions and physical properties of the raw materials are presented in Table 1 with Zetium (PANalytical B.V., Almelo, Netherlands) X-ray fluorescence (XRF) and the Blaine method, respectively. Particle size distribution of the cementitious materials is presented in Fig. 1 by using Malvern Mastersizer 2000 laser diffraction (Malvern, UK). HGMs were used as aggregate in this study, whose true density and bulk density were 380 and 260 kg=m3, respectively. The physical properties of HGMs are listed in Table 2. The photograph and morphology characteristics of HGMs are shown in Figs. 2(a–c). The HGMs present a spherical and smooth textured particle [Fig. 2(b)]. These HGM particles are well rounded and hollow from the inside. It can be seen from Fig. 2(c) that the HGM particle is easy to crush and has a shell thickness of several microns. Fig. 3 shows the mineral composition of HGMs by using X-ray diffraction (XRD) and amorphous silica mineral such as cristobalite has been identified in the XRD pattern. A polycarboxylate-based superplasticizer (SP) compound with de- foaming agents was used for all mixtures, having a water-reducing rate of 35%. Hydroxymethyl cellulose and organosilicon defoamer are used as thickening agents and defoaming agents, whose amounts account for 0.05% of the total binder. Mix Design In the ULCC mixtures, the HGM particles are dispersed into the cementitious material system as aggregate. Thus, the volume ratio of HGMs to cementitious material determines the bulk density of ULCCs. The workability and mechanical properties of ULCCs will be decreased as well as bulk density when the volume ratios are increased. Lower volume ratios will increase the workability and © ASCE 04021124-2 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  4. Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. (a) Fig. 3. XRD pattern of HGMs. the water to binder ratios were 0.35, 0.43, 0.58, 0.65, and 0.78, respectively. For the ULCCs sample preparation, the OPC, SF, and FACs were first dry mixed in a 30-L mixer for about 5 min before water and SP were added. Then, the HGMs were incorporated and mixed for another 5 min until a homogeneous mixture was obtained with a suitable workability. According to BS EN 1015-3 (BSI 1999), the ULCCs mixtures were measured and the flow value ranged from 70 to 90 mm. The samples were cast into a different mold without vibration. Then, they were covered with plastic films, and de- molded within 48 h. Afterward, the samples were cured in a stan- dard room with temperature of 25°C±2°C and relative humidity of 98%±2%. (b) Experimental Program Density Test The density of hardened specimens for ULCC mixtures with the size of 100 × 100 × 100 mm after being demolded for 1 day were measured according to BS EN 12390-7 (EN 2009b). Mechanical Properties Test According to BS EN12390-3 (EN 2009a), the compressive strength measurements of ULCC mixtures were conducted. Each group with three samples (size of 100 × 100 × 100 mm) was tested at 7 and 28 days. Flexural performance of UCLL-3 at 28 days was determined using 100 × 100 × 400 mm prisms with third-point loading. In addition, elastic modulus of UCLL-3 samples (cylinder Φ100 × 200 mm) at 28 days was tested. (c) Fig. 2. (a) Photograph of HGMs; (b) micrograph of HGMs; and (c) crush HGMs. Water Absorption Test Three specimens of the designed ULCC mixtures (100 × 100 × 100 mm cubes) were carried out for capillary water absorption ac- cording to ASTM C642-97 (ASTM 2007) by measuring increase in mass of the specimens as a function of time with one surface ex- posed to water. The mass increase of all samples at different ages was calculated using Eq. (1) Δm ¼wn− w0 w0 mechanical properties of ULCCs, butthe bulkdensity also gain. On the other hand, the water consumption and admixture types also affect the properties of ULCCs. Thus, depending on the mixture proportion, the water to binder ratio, the utilization of supplement cementitious materials, and chemical admixtures, fivedifferent pro- portions of ULCCs mixture are listed in Table 3. The HGM-to- binder volume ratio of ULCC mixtures ranged from 3 to 7 and × 100% ð1Þ © ASCE 04021124-3 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  5. Table 3. Mix proportion of ULCCs Binder (kg=m3) Specimen number HGM-to-binder volume ratio 3∶1 4∶1 5∶1 6∶1 7∶1 HGM (kg=m3) Water (kg=m3) SP OPC FAC SF (% by weight) ULCC-1 ULCC-2 ULCC-3 ULCC-4 ULCC-5 710 533 426 355 305 189 142 114 95 81 48 35 28 23 20 380 380 380 380 380 331 305 329 308 316 3 3 3 3 3 Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. where w0is the mass of oven-dry sample in air (g) and wnis mass of surface-dry sample in air after immersion (g). it exhibits the lowest density when the HGM-to-binder volume ra- tio is 7∶1. This is attributed to the fact that when the HGMs are used as aggregate in ULCCs, they decrease the volume of cement paste. This decrease is primarily due to the high air content associated with hollow properties of HGMs. In addition, the spherical shapes of HGMs are difficulty packed completely, and therefore, many air voids may remain in the ULCCs and result in the reduction of the density (Hanif et al. 2017b). Moreover, it is observed that the ULCCs show a significant density reduction rate when the HGM- to-binder volume ratio rises from 3∶1 to 4∶1. However, the density of ULCCs shows slight reduction, which ranges from 730 to 778 kg=m3when the replacement level of the HGM ratio is in- creased from 5∶1 to 7∶1. This is probably ascribed to the addition of HGMs gradually approaching the optimized packing of the indi- vidual granular ingredients in the cementitious system based on the most closely packed principle. On the other hand, as presented in Table 3, when the HGM-to-binder ratio is 3∶1, the binder’s weight is 947 kg=m3, but it decreases to 710 kg=m3at a 4∶1 volume ratio. The decrease from 947 to 710 kg=m3is a big change. However, the binder addition only changed from 710 to 568, 473, and 406 kg=m3at the value of 5∶1, 6∶1, and 7∶1, respectively. The big change of binder content directly influenced the density of the hard- ened sample. The compressive strengths of the ULCC specimens at 7 and 28 days are shown in Fig. 6. It is observed that the compressive strengths of ULCCs gradually decrease with the increasing HGM content. The incorporation of HGMs shows negative effects on the compressive strength of ULCCs because the compressive strength of samples decreases with the addition of HGMs. Besides, in order to maintain good workability, the water-to-binder ratio of ULCCs increases with the increasing of HGM content, whereas the com- pressive strength of samples reduces accordingly. For instance, the Rapid Chloride Migration Test The chloride ion penetration of ULCC mixtures at 28 days was evaluated by calculating the chloride migration coefficient accord- ing to NT Build 492 (EN 1999). For each ULCC mixture, three samples (size of Φ100 × 50 mm) were measured and calculated for the chloride ion diffusion coefficient based on the chloride ion penetration depths. Microstructure The crushed ULCC samples were performed for morphology char- acteristics. The microstructure changes on the interfacial transition zone (ITZ) of ULCCs were observed and analyzed by JSM-IT300 (JEOL, Tokyo) scanning electronic microscopy (SEM). Results and Discussion Density and Compressive Strength of ULCCs Fig. 4 shows the floating test of the designed ULCCs. It is clear that the density of ULCCs is lower than pure water (1,000 kg=m3). The density of each designed ULCCs has significantly lower density ranging from 730 to 948 kg=m3as shown in Fig. 5. It is clear that the HGM aggregates have obvious impacts on the density of the designed ULCCs. For all ULCCs samples, the density is decreased with the HGM volumes increasing. Thus, the ULCCs with the HGM-to-binder volume ratio of 3∶1 have the highest density, while Fig. 4. Floating test of ULCCs. Fig. 5. Dry density of ULCCs. © ASCE 04021124-4 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  6. lower than 20 MPa when the HGM-to-binder volume ratio increase by 6∶1 and 7∶1. This is probably attributed to that the low density HGMs were easy to crush, which cannot enhance the compressive strength of ULCCs (Wang et al. 2020). The HGMs are easily dam- aged and thus result in negative impacts on the composite strength of ULCCs. In addition, as a result of a high degree of sphericity of HGMs with a smooth surface, low surface energy, and stable chemical properties, this makes it difficult to form a strong bond energy in contact to the interface of the cement matrix. Therefore, the incorporation of HGMS will decrease the binding force be- tween the hydration products of cement, resulting in a reduction of the mechanical properties of ULCCs. The density of the designed ULCCs plotted against the com- pressive strengths are presented in Fig. 7. According to Fig. 7, there is a linear correlation between the compressive strength and density of ULCCs, with the coefficient of determination ðR2Þ ¼ 0.94887. However, it should be noted that when the designed density is too high or too low, the required water to binder ratios may be beyond the range of 0.35–0.78. On other hand, it can be seen that the density of the ULCCs samples is less than 1,000 kg=m3and compressive strengths are higher than 20 MPa, which exhibits a higher strength-to-weight ratio in the range from 0.029 to 0.035 MPa=kg=m3when the HGM-to-binder-volume ratio are 3∶1, 4∶1, and 5∶1, respectively. The reason is attributed to the pres- ence of amorphous silica in HGMs, which can possess some degree of pozzolanic activity and contribute to enhancing a higher specific strength (Hanif et al. 2017a). Among them, the ULCC-3 has the lowest dry density of 778 kg=m3and 28 days compressive strength of 22.9 MPa, whose specific strength is 0.029 MPa=Kg=m3. In summary, the density and compressive strength of the designed ULCC-3 can meet the requirement of offshore engineering constructions. The density and mechanical properties of the designed ULCC-3 samples compared to various researchers are summarized in Table 4. It is clear that the designed ULCC-3 samples have more than 30% reduction of density than the previous studies. However, the compressive strength and flexural strength of the designed ULCC-3 samples are similar to the results of various strengths of ULCCs as reported in literature. Moreover, it can be seen that the designed ULCC samples show desirably low density with adequately higher strength thanthat of conventional LAC and NAC. The results indicate that HGMs can be efficiently utilized as light- weightaggregateincementitioussystems.Thegreatreductioninthe density with excellent mechanical characteristics signifies broad applicabilityofULCCs.Furthermore,asucceededmethodtodesign the mix proportions of ULCCs can be used to effectively determine the mixture proportions of ULCCs in the terms of the designed unit weight, compressive strength, and workability (Wang et al. 2014). Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. Fig. 6. Compressive strength of ULCCs. Fig. 7. Relationship between compressive strength and density of ULCCs. maximum compressive strengths at 7 and 28 days are 20.3 and 33.1 MPa,respectively, which is observed from ULCC-1. The good strength properties of ULCCs may be ascribed to that the HGMs have hard and stiff shells, which can control void sizes in the cement matrix; meanwhile, the cement matrix can provide “three- dimensional” confinement to the HGMs (Krakowiak et al. 2020). However, the samples exhibited a compressive strength of 28 days Table 4. Comparison of physical properties and mechanical properties of ULCC-3 and some studies as reported in the literature 28-day compressive strength (MPa) 28-day flexural strength (MPa) Modulus of static elasticity (GPa) Water/binder ratio Density (kg=m3) Mix types ULCC-3 Blanco et al. (2000) Wang et al. (2018) Du Hongjian (2019) Sengul et al. (2011) Topçu and Uygunoglu (2007) Losiewicz et al. (1997) LAC NAC 0.58 0.30 778 22.9 5.0–33.0 8.7–37.2 21.5–28.4 0.1–28.8 6.0 0.55–2.88 63 58 3.1 2.3 — 1,090–1,510 1,259–1,610 1,075–1,240 354–1,833 900 760–867 1,870 2,380 2.1–5.1 6.1–15.4 — — — — 6.0 6.2 0.40–0.57 0.24–0.32 0.55 0.2 1.2 0.38 0.38 7.1–25.0 — — — — 20.1 35.5 © ASCE 04021124-5 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  7. Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. Fig. 9. Chloride diffusion coefficient of ULCCs. Fig. 8. Water absorption of ULCCs. Water Absorption The water absorption of the ULCC sample measured as a function of up to 48 days is presented in Fig. 8. It can be seen that the water absorption of ULCCs in the first 3 days is fast and a clear distinc- tion is observed among all ULCC mixtures. Then, the increased rate decreases gradually, and the samples continue to absorb water within 3–48 days. Obviously, the water absorption of ULCCs in- creases with the HGM-to-binder volume ratio increasing. The sorp- tivity of concrete can absorb and transmit water by capillarity, which is affected by its composition and capillary pore structures (Spiesz et al. 2013). These results show that the increased HGM content in the matrix would increase the sorptivity of ULCCs. This is mainly due to the weak binding forces between HGMs and the surrounding cement matrix, which makewater easy to diffuse in the pores, while the water will cause swelling in the cement matrix, thus resulting in worse binding forces and further accelerating water transport in the pores. As a result, the water absorption will accelerate in the interfacial transition zone (ITZ) between HGMs and the surrounding cement matrix. (a) Chloride Ion Penetration Fig. 9 presents the chloride diffusion coefficient of ultrahigh per- formance concrete mixtures at 28 days by using a rapid chloride migration test. The chloride diffusion coefficients for ULCC-1, ULCC-2, ULCC-3, ULCC-4, and ULCC-5 are 1.8 × 10−12, 2.2 × 10−12, 2.6 × 10−12, 5.2 × 10−12, and 5.6 × 10−12m2=s, re- spectively. It is observed that the chloride diffusion coefficient of ULCCs increases when the HGM content is increasing. The chloride permeability of ULCC-1, ULCC-2, and ULCC-3 is mod- erate based on the evaluation index of chloride permeability, while the chloride permeability of ULCC-4 and ULCC-5 is ranked high. These results also indicate that the increasing amount of HGMs decreases the resistance to chloride permeability of ULCCs. How- ever, it can be seen that ULCCs containing HGMs show a lower resistance to chloride ion penetration and have a similar chloride ion penetration as NAC. These results are inconsistent with the pre- vious study, which showed that the addition of HGMs did not in- duce the increase of chloride permeability in high strength LAC (Rheinheimer et al. 2017). This is probably due to internal closed-pores in HGM aggregate, which will not facilitate the chlo- ride transportation and hence the chloride permeability of ULCCs (b) Fig. 10. Typical failure mode of ULCC specimens under uniaxial com- pression: (a) ULCC-3; and (b) ULCC-5. © ASCE 04021124-6 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  8. Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. (a) (b) (c) (d) (e) Fig. 11. SEM image of ULCCs: (a) ULCC-1; (b) ULCC-2; (c) ULCC-3; (d) ULCC-4; and (e) ULCC-5. © ASCE 04021124-7 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2021, 33(6): 04021124

  9. is limited. In addition, the HGM particles cannot provide a channel for chloride ions due to their dense and impermeable surfaces. The microstructure of HGMs is even, although porous, and the pores are not interconnected, thus reducing the total permeability. Therefore, the incorporation of appropriate content of HGMs in the cement matrix will not significant increase the chloride permeabil- ity of ULCCs. 2. The water absorption of ULCCs is increased with the increasing amount of HGMs. This is mainly due to theweak binding forces between HGMs and the cement matrix, which makes it easier for water diffusion in the pores. However, the ULCCs exhibit excellent resistance to chloride ion penetration. This is probably due to the low open porosity of HGMs and dense ITZ between HGMs and the surrounding cement matrix. 3. SEM analysis indicates that the ULCC-incorporated HGMs achieve a higher mechanical strength at lower density levels. The ULCCs shows a crumbling structure with obvious air voids and cracks. Moreover, the HGMs also affect the interfacial bonding between HGMs and the surrounding cement matrix due to their spherical structure and smooth surface. 4. This study showed that HGMs may be considered as lightweight performance filler for offshore floating platforms constructionin open islands. However, the low-strength HGMs are easier to crush under compressive load, which makes them unlikely to enhance the compressive strength of cement-based materials. Besides, the damaged HGMs can generate voids and affect the composite porosity, thus leading to low composite strength. However, they could provide better durability of ULCCs, which could be applied to enhance the performance of structures and reduce construction cost. Failure Modes Downloaded from ascelibrary.org by WUHAN UNIVERSITY OF TECHNOLOGY on 10/14/21. Copyright ASCE. For personal use only; all rights reserved. The failure modes of ULCC samples under uniaxial compression tests are presented in Figs. 10(a and b). The ULCC specimens show the initiation of multiple cracks as shown in Fig. 10(a), which are formed at the surface of samples and then developed parallel to the direction of the applied load. Finally, the failure appearance of sam- ples is found with the loading cycles increased [Fig. 10(b)]. These phenomena can be attributed to the smooth surface and sphere structure of HGMs, which will decrease the bonding strength be- tween HGMs and the surrounding cement matrix (Sohel et al. 2018). As a result, a high ductility and multiple cracks behavior of ULCC specimens is founded. Nevertheless, it is also observed that there was no spalling failure and brittle behavior in the failure mode for ULCC samples. In addition, it can be seen from the frac- ture surface of ULCC samples that no large pores are present in the cement matrix and the textures of the cement matrix are homo- geneous, which exhibit that the samples show a relatively dense structure. Data Availability Statement All data, models, and code generated or used during the study appear in the published paper. Microstructure Figs. 11(a–e) show the SEM image for all ULCC mixtures at 28 days. It is observed that there are more pores and cracks in the cement matrix with the increasing amount of HGMs. The hy- dration products of the ULCC-1 sample are more homogeneous and denser in Fig. 10(a), compared to that of other ULCC mixtures. In comparison, the ITZ between the HGMs and cement matrix for ULCC-4 and ULCC-5 are relatively weak and more crushed HGMs are observed at higher magnification. The crushed HGMs have adverse influence on the composite strength of ULCCs and will also lead to high porosity of ULCC samples as shown in Figs. 11(d and e). When larger amounts of HGMs are introduced into the ULCC mixture, the surrounding cement matrix presents a loose and uncompacted structure, in which large cracks can be observed for the ULCC-5 sample. These results indicate that the changes in microstructure of ULCCs are consistent with strength development and resistance to chloride ion penetration. Acknowledgments This research was financially supported by National Key Research & Development Program of China (No. 2016YFB0707003). References ACI (American Concrete Institute). 2003. Guide for structural lightweight- aggregate concrete. ACI 213. Farmington Hills, MI: ACI. Ahmad, M. R., B. Chen, and S. Farasat Ali Shah. 2019. “Investigate the influence of expanded clay aggregate and silica fume on the properties of lightweight concrete.” Constr. Build. 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