Water-resistance properties of high-belite sulphoaluminate cement-based ultra-light foamed concrete treated with different water repellents

Construction and Building Materials 228 (2019) 116798

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Water-resistance properties of high-belite sulphoaluminate cement-based ultra-light foamed concrete treated with different water repellents Chao Liu a, Jianlin Luo a,b,⇑, Qiuyi Li c,b, Song Gao a,b, Zuquan Jin a,b, Shaochun Li a,b, Peng Zhang a,b, Shuaichao Chen a a

School of Civil Engineering, Qingdao University of Technology, Qingdao 266033, PR China Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao University of Technology, Qingdao 266033, PR China c School of Architecture Engineering, Qingdao Agricultural University, Qingdao 266109, PR China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


We compared the waterproof effect of

four types of PWR and two types of LWR on ULFC.
The PWR of CS with 4 wt% dosage reduced the WV and Rf of ULFC most effectively.
The 72 h WV of ULFC using soaked method with HSO and KH550 was reduced to 4.4 wt%.

a r t i c l e

i n f o

Article history: Received 20 June 2019 Received in revised form 3 August 2019 Accepted 24 August 2019

Keywords: Ultra-light foam concrete Water repellents

a b s t r a c t The high water absorption (Wv) of FC, especially ultra-light FC (ULFC), tends to increase the structure burden, reduce the strength, reduce the thermal and sound insulation performance, and make the frost resistance deterioration, which limits its wide development in prefabrication insulation panel industry. In order to effectively reduce the WV at varied water soaking time of ULFC, four types of powdery water repellents (PWR) including calcium stearate (CS), zinc stearate (ZS), polysiloxane (PS) and redispersible latex powder (RDL) were doped into high-belite sulphoaluminate cement-based ULFC with dosages varying from 0.5 wt% to 4.0 wt% to prepare water-resistant ULFC with the dry density (qd) from 270 kg/m3 to 300 kg/m3. Two types of liquid water repellents (LWR) of methyl polysiloxane resin (MPR) and hydrogenated silicone oil (HSO) were used to treat the pristine ULFC by the soaking or surface coating

Abbreviations: FC, Foamed concrete; ULFC, Ultra-light foamed concrete; HBSC, High belite sulphoaluminate cement; FA, Fly ash; NSP, Naphthalene-based superplasticizer; PWR, Powdery water repellents; LWR, liquid water repellents; CS, Calcium stearate; ZS, Zinc stearate; PS, Polysiloxane; RDL, Re-dispersible latex powder; MPR, Methyl polysiloxane resin; HSO, Hydrogenated silicone oil; KH550, c-aminopropyl triethoxy silane coupling agent; qd, Dry density; fcu, Compressive strength; ft, Flexural strength; kc, Thermal conductivity; WV, Volume water absorption rate; Rf, Strength loss coefficient; RS, Solidfied rate; h, Contact angle; wFA, FA replacement for HBSC; W/B, Water-binder ratio; wNSP, NSP fraction of binder; wFOAM, Foam fraction of binder; SEM, Scanning electronic microscopy; XRF, X-ray fluorescence spectrometer; XRD, X-ray powder diffraction. ⇑ Corresponding author at: P.O. 221 Fushun Rd. 11, Qingdao 266033, PR China. E-mail address: [email protected] (J. Luo). https://doi.org/10.1016/j.conbuildmat.2019.116798 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

2 Water resistance Volume water absorption Pore structure Water-proof mechanism

C. Liu et al. / Construction and Building Materials 228 (2019) 116798

method to explore the water-resistant effect of sole LWR or the combination of LWR and CS on the ULFC. A silane coupling agent (KH550) was further used to promote the LWR to solidify the formation of waterresistant film. The connected pores of ULFC doped with CS, ZS, PS and RDL are effectively reduced; and the pore walls of ULFC doped with CS, ZS and PS become more compact than the control ones. CS, ZS and PS dosage all can reduce the WV of ULFC, and the ULFC doped with 4 wt% CS has the lowest WV of 23.6 wt%, and the strength loss coefficient after saturation is close to 0. The water-resistant effect of LWR is superior to that of PWR, the soaking method is better than surface coating method in reducing WV, and the ULFC treated with HSO and KH550 has the lowest 72 h WV of 4.4 wt% owing to its capillary clogging mechanism, and the corresponding contact angle can reach 125.87°. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, the application of foam concrete (FC) become more and more common in our life. Because FC can provide thermal insulation, reduce noise, absorb and disperse seismic loads, it is mainly used in retaining wall, foundation backfill, pipeline backfill, environmental cover, garbage cover and foundation treatment in engineering, besides building energy saving [1–6]. However, it also has many shortcomings, such as low strength, high dry shrinkage, high water absorption, poor durability, etc. [4]. The so-called FC is usually made by the physical method of making the foaming agent solution into a foam, and then adding the foam to the slurry made of binder, admixture, and water, mixing and stirring uniformly and pouring molding, and curing outside under natural conditions, standard maintenance room or a steam curing box [7–10]. Many scholars have studied the influences of the types of foaming agents, foam dosages, foaming schemes, chemical reactions of binder and foam-stabilizing methods on the properties of the foam and the finally prepared FC. Falliano et al and Liu et al compared the properties of foam and FC prepared by protein foaming agent and synthetic foaming agent, and found that compared with synthetic foaming agents, the produced foam of protein foaming agents was less stable, but the prepared FC had higher compressive strength (fcu), because during the initial mixing, the unstable foam did not increase too much air volume simultaneously, making the binder system more fluidity [11,12]. Panesar found that protein foaming agents were more likely to produce smaller isolated spherical bubbles, but that the conductivity of FC produced by synthetic foaming agents was more stable to the change in air content [13]. Tian et al. found that the bulk density and compressive strength of the FC showed a linear relationship with the foam dosage [14]. Hajimohammadi et al used H2O2 as chemical foaming agent to prepare one-part mix geopolymers, they combined the stable foaming process with the rapid geopolymer reaction to form evenly distributed small pores by controlling the mixing design of geopolymers, and found that the strength of foamed geopolymer was determined by the inherent strength of the geopolymer skeleton and pore size and distribution [15]. Hajimohammadi et al further explored the effect of the alkali reaction on characteristics of the foam and alkali activated slag FC, and found that the water content of binder was the key factor affecting the foam stability and distribution, and the reaction rate of binder and the size of gel particles determined the pore size and distribution uniformity of FC [16]. Hajimohammadi et al. also used Xanthan gum as a foam stabilizer to aggregate the liquid film to improve the foam stability, enhanced the pore size distribution of FC and improved the mechanical properties of the cellular structure [17]. A lot of researches on FC have been launched to improve its pore structure and physical properties by partially replacing cement with types of mineral admixtures or doping additional components into FC. Kearsley and Wainwright replaced cement

with high content of fly ash (FA) to improve the late strength of FC [18]. Jiang et al. found that adding a suitable amount of superplasticizer into FC could reduce the viscosity of the slurry, resulting in the decrease of the number of broken bubbles and the reduction of the big pores, and an excessive amount of superplasticizer would increase the pore size [19]. Batool et al used FA, silica fume (SF), and metakaolin to partially replace cement to explore the thermal conductivity (kc) of FC, and found that SF had the best effect in reducing the kc of FC [20]. Khan et al added polypropylene fiber into FC to increase the flexural strength (ft) and tensile strength [21]. Falliano et al placed bi-directional grids of glass fibers close to the bottom external face of the FC beams and further added short polymer fibers into the binder to explore the ft of FC beams with different densities, and found that the bi-directional grids of glass fibers could increase the ft of FC beams up to 1700% and the doped polymer fibers could only increase the ft of FC beams up to 31% [22]. Hulimka et al also placed two types of bi-directional composite reinforcing fiber mesh in tensile zone of FC, and found the carbon grids were better than basalt grids in improving the ft of FC [23]. Prabha et al. and Luo et al. studied the effect of carbon nanotubes (CNTs) in FC on its properties, and found that CNTs could enhance the stability of FC slurry and the strength of cured FC, improve the pore structure, and reduce the kc [24–26]. Some scholars also explored the relationship between the physical properties and pore structure, or the relationship between the curing conditions and the mechanical properties of FC. Sang et al found that the dry density (qd) and kc decreased with the increase of porosity, the fcu and volume water absorption rate (WV) increased with the increase of porosity [27]. Falliano et al found that the extrudable FC with smaller pore size had lower kc than the aerated autoclaved concrete or classical FC at the same density grade [28]. Falliano et al also found that the mass water absorption rate decreased with the increase of the qd of FC, but the WV increased with the increase of the qd of FC [29]. Kearsley and Wainwright found that the water vapor permeability of FC increased with the increase of porosity and the FA dosage of FC [30]. Kozlowski et al explored the fracture and mechanical properties of FC varying qd and found that the higher the qd was, the higher were the fracture energy and maximal tensile stress [31]. Falliano et al compared the mechanical properties of lightweight FC under different curing conditions and found that the ft and fracture energy of lightweight FC cured in air were significantly higher than that cured in water, but the fcu increased less [32]. As known, ordinary concrete will be damaged by chloride ion erosion, freeze–thaw and carbonization if it is kept in an erosive environment for a long time, which will lead to the decrease of durability [33–35]. And the main culprit in reducing the durability of concrete is water. Cao et al explored the influence of silane impregnation on the water absorption and freeze–thaw resistance of concrete with different strength grades, and found that the lower the initial water content of concrete was, the better was

C. Liu et al. / Construction and Building Materials 228 (2019) 116798

the freeze–thaw resistance performance [36]. Ma et al found that the water absorption of concrete treated with waterproof treatment was lower than that of concrete without waterproof treatment, and the content and speed of chloride ions entering the concrete were also reduced [37]. Yin et al found that the content of chloride ions and sulfate ions in concrete of different depths under water head pressure increased significantly compared with the concrete under free immersion, which greatly reduced the durability of the concrete [38]. These studies have shown that the water itself entering concrete would reduce the durability of concrete, or water as a transmission medium would promote harmful chloride or sulfate ions into the concrete inside, which will also seriously affect the durability of concrete. In terms of the FC, especially ULFC, as a material with high porosity, is sensitive to the environmental humidity and water intrusion, which severely limit the wide application of FC product [39–41]. Sun et al found that the fcus of FCs with a qd of 600 kg/m3 were reduced by 15%– 30% after 40 freeze–thaw cycles after immersion in water [42]. Nambiar and Ramamurthy found that the dry shrinkage of FC was related to the moisture content of FC, the loss of moisture can contribute to the dry shrinkage of FC [43]. Chen et al found that the total weight of FC with the qd of 550 kg/m3 would increase by about two-fold after water absorption, which significantly reduced load bearing capacity [44]. Del Coz Diaz et al found that the thermal insulation performance of FC decreased with the increase of moisture content of FC or decreased with the increase of relative humidity of the environment [45]. Therefore, the water-resistant treatment of FC before application is essential for its long-term behavior. There are four main types of water repellents commonly used in construction industry: fatty acid metal salts, paraffin latex, silicones and redispersible polymers [46–48]. Commonly used water-resistant treatment technology mainly includes doping method, surface coating method, and soaking method. As known, the moisture of concrete includes capillary absorption of liquid water and water vapor absorption in high humidity. Although the FC products in this paper will be used as the inner insulation panels, when they are in a humidity environment for a long time or undergo the rainy season, a small amount of moisture will inevitably penetrate into the inside, causing capillary water absorption phenomenon of the FC panel. [49]. The dry FC is in an unsaturated state, and in the absence of external pressure, the water enters the interior of the FC mainly through capillary attraction [50]. Surface tension exists at each interface of the solid–liquid interface, and water diffuses along the surface of the capillary to form a meniscus, and the meniscus bulges toward the wet side. The wetting of a material is one of the main characteristics of its water repellent properties, and the wetting can be studied by the sessile drop contact angle (h) measurement [51]. The h is the angle between the solid surface and the liquid–gas interface, with the limiting condition that 0°
3

[52,53]. The capillary force diagram of a single cylindrical capillary hole when it absorbs water and when it is drowning and contact angle diagram of a single circular solution on a solid surface is shown in Fig. 1 [39,40,52,54]. A lot of researches on the water-resistant treatment of building materials have been implemented. Zhu et al found that recycled aggregate concrete impregnated with silicone on the surface had better water resistance than untreated natural aggregate concrete [55]. Flores-Vivian et al prepared a water-resistant emulsion, and enriched the emulsion with polymethyl-hydrogen siloxane oil hydrophobic agent as well as metakaolin (MK) or SF, and finally applied it into cement mortar bricks, and they found that the corresponding h of cement mortar bricks containing PVA fiber treated by MK-based emulsion was up to 156° [56]. Al-Kheetan et al used three types of water repellents of fluoropolymer, silicate resin and sodium acetate crystallizing material to treat concrete pavement and found that the sodium acetate had the best water-resistance [57]. Wong et al studied the effect of waste paper sludge ash (PSA) on the water-resistant properties of concrete and found that the sample surface coated with PSA showed excellent water repelling and self-cleaning characteristics [58]. Izaguirre et al added two types of powdery water repellent (PWR), sodium oleate, and calcium stearate, into aerial lime-based mortars respectively, and found that both of them could help plug larger pores and filled smaller pores to a certain extent to achieve the water-resistant effect [59]. Tkach et al used a composite modified with water repellents to reduce the water absorption and capillary penetration of concrete by 3–3.5 times [60]. Tittarelli and Moriconi found that water-resistant treatment of the concrete surface could effectively improve the corrosion resistance of galvanized reinforcing steel inside [61]. Medeiros and Helene found that the silane/siloxanebased water repellent was effective only when the concrete was in an unsaturated state, while in a saturated state and water under pressure did not work well [39]. Liu and Hansen treated the surface of the concrete with silane and found that silane built a waterresistant barrier against capillary attraction by increasing the h [40]. Lanzon and Garcia-Ruiz added six water repellents into the mortar and evaluated its capillary water absorption and found that the mortar with sodium oleate showed the best impermeability [62]. So many researches on water repellents, but most of them were applied onto ordinary concrete and mortars, and the researches of water-resistant treatment on FC were not enough. Most people paid more attentions to the lightweight and excellent thermal insulation properties of FC. Only a few scholars have studied the water absorption of FC and waterproofed it. Ma and Chen compared the water-resistant properties of three types of PWR by doping them into Portland cement-based FC with density class of 550 kg/m3 and fitted the strength loss curve of FC after water absorption [44]. Nambiar and Ramamurthy studied the sorption characteristics of FC and concluded that the sorption depends on the filler type, density, and pore structure and also on permeation mechanisms [63]. Kearsley and Wainwright explored the porosity

Fig. 1. The capillary force diagram of a single cylindrical capillary hole [22,23].

4

C. Liu et al. / Construction and Building Materials 228 (2019) 116798

and permeability of FC and found the water vapor permeability of FC increased with increasing porosity and ash content [30]. Therefore, it is far from enough to study the water absorption of FC and water-resistant treatment, especially for ULFC who has ultra-low density, high porosity, and high water absorption. The binder of ULFC is high-belite sulphoaluminate cement (HBSC) and FA. Indeed, HBSC has the advantages of high early strength, fast growth of late strength, fast condensation speed, low dry shrinkage and good impervious property [64]. By using HBSC in FC, it is beneficial to promote the setting time of the FC to prevent film collapse, improve the early strength of FC, and prevent the dry crack of FC before demolding. Appropriate FA dosage can improve the flowability of fresh FC slurry and reduce the density of FC. Moreover, since the thermal conductivity (kc) of FA is lower than that of cement, the incorporation of FA in cement can effectively reduce the kc of FC [20]. Meanwhile, HBSC can be sintered with various industrial wastes, which effectively realizes the reuse of industrial wastes, reduces the cost of HBSC production, and conforms to the sustainable concept [65,66]. In the present study, the water-resistant influences of four types of PWR doping in HBSC-based ULFC and the influences of two types of liquid water repellents (LWR) coating on or soaking in the ULFC blocks were comprehensively studied, and the influences on the physical performances of the ULFC are further explored. These consequences will provide an effective basis for the final preparation of water-resistant ULFC that meets the requirements of prefabricated thermal insulation panels.

detail listed in Table 1. A type of plant protein foaming agent, branded with Leran (obtained from Leran Trading Co., Ltd., Shijiazhuang, China) was used to produce foam and its corresponding physicochemical properties are listed in Table 2. The naphthalene-based superplasticizer (NSP, bought from Shanghai Chenqi chemical company, Shanghai, China) was used to improve the flowability of fresh ULFC slurry. Four types of PWR, calcium stearate (CS, obtained from Dinghai Plastic Chemical Co., Ltd., Dongguan, China), zinc stearate (ZS, obtained from Dinghai Plastic Chemical Co., Ltd., Dongguan, China), polysiloxane (PS, obtained from Zhengzhou Chengxin Chemical Co., Ltd. Zhengzhou, China) and re-dispersible latex powder (RDL, obtained from Mingyu Chemical Co., Ltd., Zhengzhou, China) were directly doped with the fresh ULFC slurry. Two kinds of LWR, methyl polysiloxane resin (MPR, obtained from Hubei Xinsihai Chemical Co., Ltd., Xiangyang, China) and hydrogenated silicone oil (HSO, obtained from Hubei Xinsihai Chemical Co., Ltd., Xiangyang, China) were used for surface coating on or soaking in ULFC blocks. And a gammaaminopropyl triethoxy silane coupling agent (KH550, obtained from Hubei Xinsihai Chemical Co., Ltd., Xiangyang, China) was added into two MPR and HSO solutions at a 5 wt% fraction to explore the effect of KH550 on solidified rate and water-resistant effect of MPR and HSO to ULFC. The water was tap water. The physicochemical performances of water repellents are shown in Table 3.

2. Experimental

In order to focus on exploring the water-resistant effect of PWR on ULFC, the FA replacement for HBSC (wFA), the water-binder ratio (W/B), NSP fraction of binder (wNSP) and foam fraction of binder (wFOAM) were fixed at 15 wt%, 0.5, 0.6 wt% and 15.5 wt%, respectively. Four types of PWR of CS, ZS, PS, and RDL were doped in different fractions to prepare water-resistant ULFC, and the mix proportion was shown in Table 4. In this study, the foaming method for foaming and the preparation process for preparing ULFC was physical foaming method, prefoamed mixing method, respectively. The HBSC, FA, NSP and the PWR were weighed proportionally and then added into a vertical

2.2. Mix proportion and preparation of ULFC

2.1. Raw materials The HBSC (acquired from Polar Bear Building Material Co., Ltd, Tangshan, China) sintered with solid industry wastes and Grade-I FA (acquired from Shandong Huadian power plant, Qingdao, China) were used as the binder and their chemical compositions characterized by an X-ray powder diffraction (XRD, D8 Advance type, Bruker Inc., Karlsruhe, Germany) and an X-ray fluorescence spectrometer (XRF, XRF-1800 type, Shimadzu Corp., Kyoto, Japan),

Table 1 Chemical compositions of HBSC and grade-I FA. Ingredient

CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

TiO2

Loss

HBSC Ingredient FA

51.54 SiO2 60.98

13.80 Fe203 6.70

15.34 Al2O3 24.47

1.52 CaO 4.90

2.08 MgO 0.68

14.21 f-CaO 0.58

0.71 SO3 0.52

0.38 Loss 1.86

Table 2 The physicochemical performances of foaming agent used in this study. Brand

pH

Appearance

Decomposition temperature (°C)

Density

Foam multiple

1 h settling distance (mm)

1 h bleeding volume (mL)

Leran

8.2

Light yellow liquid

100

1.107

24

11

96

Table 3 The physicochemical performances of water repellents. Abbr. Name

Appearance

Density (g/cm3)

Average particle size (lm)

pH

Viscosity (25 °C, mm2/s)

Refractive index (25 °C)

CS ZS PS RDL MPR HSO KH550

White powder White powder White powder White powder Colorless liquid Colorless liquid Colorless liquid

1.08 1.09 0.35 0.50 1.10 0.98 0.95

75 75 75 80 – – –

7–9 7–8 10–12 5–8 6–7 6–8 7–10

– – – – >1000 5–300 –

– – – – 1.38–1.42 1.39–1.42 1.42–1.43

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C. Liu et al. / Construction and Building Materials 228 (2019) 116798 Table 4 Mix proportion of ULFC doping different PWRs. Series

Group No.

wFA (wt%)

W/B

wNSP (wt%)

wFOAM (wt%)

wCS/wZS/wPS/wRDL (wt%)

Control CS/FC

Con CS1 CS2 CS3 CS4 CS5 CS6 ZS1 ZS2 ZS3 ZS4 ZS5 ZS6 PS1 PS2 PS3 PS4 PS5 PS6 RDL1 RDL2 RDL3 RDL4 RDL5 RDL6

15 15

0.5 0.5

0.6 0.6

15.5 15.5

15

0.5

0.6

15.5

15

0.5

0.6

15.5

15

0.5

0.6

15.5

– 0.5 1.0 1.5 2.0 3.0 4.0 0.5 1.0 1.5 2.0 3.0 4.0 0.5 1.0 1.5 2.0 3.0 4.0 0.5 1.0 1.5 2.0 3.0 4.0

ZS/FC

PS/FC

RDL/FC

rotating mixer special for FC (STSJ-15L type, Zhejiang Geotechnical Instrument Manufacturing Co. Ltd., Shaoxing, China) for dry mixing at speed of 60 rpm/min for 1.5 min until the color of the whole dry binder system is uniform. After dry mixing, the weighed water was slowly added into the binder to further mix at speed of 60 rpm/min for 2 min until there was no agglomeration in the slurry. Simultaneously, the foam was produced by a foam generator (BL168-8 type, Hefei Baile Energy Equipment Company, Hefei, China) and added into the slurry to further mix at 60 rpm/min speed for another 2 min until the white foam was not visible in the mixed slurry, and both the top and bottom of the slurry appeared to be the same color and look smooth. Noting that, while foaming, the dilution ratio of foaming agent and valve angles of foaming machine suction was constantly set at 1:30 and 60°, respectively, and the corresponding pressure of the air compressor during foaming process was fixed at 0.5 MPa, thus, the resulting foam density almost remained at 187 ± 3.0 kg/m3. The fresh ULFC slurry was poured into pre-brushed oil cubic steel molds with the size of 100  100  100 mm3 and special prism molds for thermal conductivity testing with the size of 300 mm  300 mm  35 mm. After 12 h incubation under ambient environment, the ULFC specimens were removed from the molds, and placed in a standard curing room with temperature around 22 °C and relative humidity greater than 90 R.H. till 7 d. It’s noting, the ULFC specimens were prepared in accordance with ‘‘Foamed concrete” JG/T 266–2011 standard (Beijing, China) [67]. 2.3. Water-resistance process of ULFC Firstly, a ULFC specimen with the size of 100  100  100 mm3 in group Con or group CS6 was cut into 8 blocks with the same size of 45  45  45 mm3 by a hacksaw, respectively, and treated with MPR and HSO after the surfaces of the test blocks were scraped smooth with a blade. Then, we compared the water-resistant effect of the MPR and HSO by coating on the surface of the ULFC blocks and soaking in the ULFC blocks. It’s noting, the the blocks before treated with MPR and HSO were the derived from the specimens for fcu and kc in group Con or group CS6, the qds was accordingly consistent with each other. When using surface coating method,

the LWR were brushed with a dosage of 200 mL/m2 on the six surfaces of ULFC blocks in each group [57]. Whereas when using the soaking method, the ULFC blocks should be completely immersed in LWR for 1 h. 3. Characterizations 3.1. Dry density The cured ULFC specimens were put into a vacuum drier (BG2140 type, Shanghai Boyi Equipment Factory, Shanghai, China) and dried to a constant weight at 55 °C. And the ULFC blocks treated with MPR and HSO were also cured in the vacuum drier to constant weight at 55 °C. Three cube specimens of each group were weighed and their average was recorded as the final mass. The qd of ULFC could be obtained after dividing its apparent volume of the specimen. 3.2. Compressive strength The fcus of the dried cube ULFC specimens were tested by an automatic cement bending and compression testing machine (DYE-300S type, Dejiayi Test Instrument Co., Ltd., Wuxi, China). Due to the low expected strength value of the ULFC specimens in this test, if the loading speed was too high, the test blocks would be crushed instantly, resulting in inaccurate value. Therefore, the loading speed was set at 0.1 kN/s. The loads at break of three test specimens were recorded in each group. The fcu of ULFC at 7 d could be calculated by Eq. (1),

fcu ¼

F0 S

ð1Þ

where, fcu was the compressive strength of ULFC, F0 was the average load of three test specimens for each group, S was the contact area between the test machine and specimen. 3.3. Thermal conductivity The dried ULFC specimen with the size of 300 mm  300 mm  35 mm was placed between the two plates

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of a flat thermal conductivity detector (DR-3030 type, intelligent thermal conductivity meter, Beijing Hangjian Huaye Technology Development Co., Ltd., Beijing, China) and clamped, and the size parameters and qd of the specimen was pre-set in accordance with the criteria ‘‘Thermal insulation-determination of steady-state thermal resistance and related properties-guarded hot plate apparatus” (GB/T 10294-2008, China) [68]. The average of 3 specimens in each group was taken as final kc. The hot plate temperature, cold plate temperature, recording time and power estimating time was set at 35 °C, 15 °C, 120 min and 30 min, respectively. The kc could be calculated by Eq. (2),

kc ¼

ud S  DT

ð2Þ

where, kc was the thermal conductivity (W/mk), u was heat flow rate (J/s), d was average thickness of specimen (m), S was average area of specimen (m2), and DT was temperature difference between the hot and cold plates (°C). 3.4. The solidified rate and volume water absorption rate The solidified rate (RS) of ULFC blocks after treated with LWR could be calculated by Eq. (3),

RS ¼

M2  M1 M1

ð3Þ

where, RS was the solidified rate of ULFC blocks treated with LWR, M1 was the pristine mass of ULFC blocks, M2 was the mass of ULFC blocks treated with LWR after solidified. The volume water absorption rate (WV) of ULFC specimen was tested in accordance with JG/T 266-2011 standard (Beijing, China) requirements [67]. Firstly, the dried ULFC specimen was placed in a plastic container higher than it and top pressed with a heavy brick to prevent the ULFC specimen from floating after adding water. Secondly, the water was added to the plastic container at 1/3 height of the specimen to soak for 24 h, and during this period, the 1 h, 4 h, 8 h, 12 h and 24 h WVs of the ULFC specimen were tested. Thirdly, the water was added till 2/3 height of the specimen to soak for another 24 h, and during this period, the 36 h and 48 h WVs of the ULFC specimen were tested. Finally, the water was added beyond 30 mm of the specimen to soak for another 24 h, and during this period, the 60 h and 72 h WVs of the ULFC specimen

were tested. After that, the ULFC specimen was removed from the water and dried the surface with a cloth before weighing the mass. The testing process for WV was shown in Fig. 2, and the WV could be calculated by Eq. (4),

WV ¼

MN  M0 V ql

ð4Þ

where, WV was the volume water absorption rate of the specimen after soaking in water, MN was the total mass of the specimen after soaking in water for N h, M0 was the absolute dry mass of the specimen before soaking, V was the volume of the specimen, ql was the density of the soaked liquid, here ql was 1000 kg/m3 of water. 3.5. SEM and XRD analysis In order to further verify the effect of water repellents on the performance of ULFC, scanning electronic microscopy (SEM, S3500N type, Hitachi, Tokyo, Japan) was employed to characterize the microstructure of ULFC, and X-ray powder diffraction (XRD, D8 Advance type, Bruker Inc., Karlsruhe, Germany) was employed to characterize the crystal patterns of main hydration products of ULFC. 3.6. Strength loss coefficient After testing the WV for 3 d soaking, the soaked specimens were immediately tested for its fcu, and the strength loss coefficient (Rf) could be obtained by Eq. (5),

Rf ¼

f0  f3 f0

ð5Þ

where, Rf was the strength loss coefficient of the ULFC before and after water absorption, f0 was the fcu of dried ULFC before soaking, f3 was the fcu of ULFC soaked for 3 d. 3.7. Wetting contact angle (h) A video optical contact angle tester (LS150 type, Shanghai Suolun Information Scientific Co. Ltd., Shanghai, China) was employed to measure the hs between the red ink drops and the surfaces of ULFC specimens.

Fig. 2. Flow chart of water absorption test for ULFC.

C. Liu et al. / Construction and Building Materials 228 (2019) 116798

7

4. Results and discussions 4.1. Effect of four types of PWR on qd, fcu and kc The test results of different fractions of CS, ZS, PS and RDL on the

qd, fcu and kc of ULFC are shown in Table 5. There is no regular change in the qd of ULFC in all groups, which means that the four types of PWR have no obvious effect on the qd of ULFC, as shown in Table 5. The qds of ULFC vary from 270 kg/m3 to 300 kg/m3, which all meet the qd range of ULFC according to JG/T 266–2011 standard [67]. But Table 5 demonstrates there are slight differences in the qd of the ULFC doped with different types of PWR. On the whole, the qds of ULFC doped with ZS, PS, and RDL are higher than that without PWR or doped with CS, which may because ZS, PS and RDL have poor compatibility with foam, and they tend to disperse on the surface of slurry to prevent a few foams from integrating into the slurry or make it burst, resulting in a higher density of fresh ULFC slurry. As shown in Fig. 3 and Table 5, with the increase of the dosage of four types of PWR, the fcu of ULFC firstly increases and then decreases. Among them, the ULFC doped with CS reached the maximum fcu of 0.50 MPa when the dosage is 2.0 wt%, while the ULFC doped with ZS, PS, and RDL reached the maximum fcu of 0.54 MPa, 0.52 MPa and 0.49 MPa respectively when the dosage is 1.5 wt%. There are two main reasons for this phenomenon: ① The diffraction peak of ettringite that providing mainly strength of ULFC doped with PWR is a little higher than the ULFC in group Con. [59,60], as shown in Fig. 4; ② The PWR in the fresh ULFC slurry affects or participates in hydration reaction, which alters the pore structure and pore wall morphology of the ULFC [69], as shown in Fig. 5. It can be roughly observed from the peak height of XRD images in Fig. 4 that the content of ettringite in group CS4, ZS3, PS3, and RDL3 is relatively higher than that in group Con, and the ettringite is the main hydration product of HBSC-based ULFC providing early strength [44]. The corresponding pore structure further zoomed at 40 times magnification and the pore wall morphology further zoomed at 1000 times magnification can be revealed in Fig. 5. The ULFC of group Con has large difference in pore diameter,

Fig. 3. The effect of PWR with varied dosage on fcu of ULFC.

Fig. 4. XRD patterns of ULFC in group Con, CS4, ZS3, PS3 and RDL3.

Table 5 The qd, fcu and kc results of ULFC doped with varied PWRs. Series

Group No.

qd (kg/m )

fcu (MPa)

kc (W/mk)

Control CS/FC

Con CS1 CS2 CS3 CS4 CS5 CS6 ZS1 ZS2 ZS3 ZS4 ZS5 ZS6 PS1 PS2 PS3 PS4 PS5 PS6 RDL1 RDL2 RDL3 RDL4 RDL5 RDL6

280.9 ± 3.1 273.9 ± 2.4 274.1 ± 2.2 276.3 ± 2.6 285.9 ± 2.9 286.8 ± 3.5 282.9 ± 3.4 279.9 ± 1.9 276.3 ± 2.6 298.3 ± 3.9 283.1 ± 2.5 299.1 ± 2.6 295.0 ± 2.8 280.3 ± 3.0 287.1 ± 3.1 296.7 ± 3.5 295.3 ± 2.8 293.6 ± 2.6 286.7 ± 3.2 270.5 ± 1.9 282.9 ± 2.8 296.6 ± 2.8 289.1 ± 2.5 278.6 ± 3.6 298.7 ± 2.4

0.42 ± 0.007 0.46 ± 0.017 0.45 ± 0.010 0.48 ± 0.010 0.50 ± 0.017 0.45 ± 0.007 0.46 ± 0.010 0.45 ± 0.007 0.46 ± 0.003 0.54 ± 0.020 0.46 ± 0.010 0.48 ± 0.007 0.43 ± 0.003 0.43 ± 0.010 0.42 ± 0.003 0.52 ± 0.020 0.48 ± 0.010 0.49 ± 0.017 0.46 ± 0.010 0.47 ± 0.007 0.46 ± 0.017 0.49 ± 0.010 0.41 ± 0.003 0.38 ± 0.003 0.39 ± 0.010

0.0739 ± 0.0002 0.0772 ± 0.0008 0.0760 ± 0.0007 0.0749 ± 0.0009 0.0764 ± 0.0005 0.0730 ± 0.0003 0.0744 ± 0.0008 0.0751 ± 0.0002 0.0745 ± 0.0007 0.0833 ± 0.0011 0.0769 ± 0.0004 0.0813 ± 0.0008 0.0766 ± 0.0006 0.0740 ± 0.0004 0.0748 ± 0.0004 0.0745 ± 0.0003 0.0755 ± 0.0009 0.0764 ± 0.0009 0.0747 ± 0.0001 0.0686 ± 0.0002 0.0778 ± 0.0005 0.0786 ± 0.0008 0.0762 ± 0.0006 0.0702 ± 0.0003 0.0788 ± 0.0005

ZS/FC

PS/FC

RDL/FC

3

irregular shape and more connected pores, and the pore wall structure is too loose. The pore diameters of the group CS4 cover relatively smaller range, and the connected pores turn fewer, and the pore walls are closely distributed with rod-like and plate-like hydration products with a thin layer of gel covering them. According to the XRD patterns in Fig. 4, the rod-like hydration products and gels may be ettringite and calcium aluminum silicate. The pore diameter of the group ZS3 is several times larger than others. Due to the hygroscopic property of ZS, after mixing with foam, it will absorb the water between foam liquid bridges and the water in foam liquid films, and accelerate the drainage speed of foam and reduce the liquid film thickness, causing two or several bubbles to merge together. Thus, the average diameter of the bubble in slurry increases, and the average pore diameter of ULFC also increases [70]. The pore wall is thicker, the hydration products on the pore wall are bonded together by thick layer of gels, which may be the combination of calcium silicate and calcium aluminum silicate according to XRD analysis in Fig. 4. And such pore morphologies of ULFC specimens doped with ZS render most of fcus be higher than those of other groups. The pore size of group PS3 is basically the same as that of group Con, but there is a large amount of rod-shaped ettringite on the pore wall, which results in the relatively higher fcu. The pores in group RDL3 are relatively closed, and there are many hydration products being similar to tree

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roots and hemp ropes tightly entangled on the wall of the pore, which guarantees the high fcu. However, when the fcu reaches the peak, it decreases with the increase of the dosage of four types of PWR. Because when the dosage of PWR is too high, they will gather together to adhere the surface of the binder to form a water-resistant barrier between water and the binder, which weakens the cement hydration [44]. According to the XRD analysis of groups Con, CS6, ZS6, PS6, and RDL6 in Fig. 6, excessive dosage of PWR mainly hinders the formation of ettringite. Thus, the fcu of ULFC will be accordingly reduced by excessive dosage of PWR. It can be seen from Fig. 7, the kc of ULFC does not change regularly with the type and dosage of PWR. That is to say, the addition of PWR would not regularly affect the kc of ULFC when other conditions are constant. The kc of the ULFC doped with CS and PS, and the ULFC without PWR almost do not fluctuate remarkably along with the dosage of PWR. Because the qd of ULFC doped with CS in Series CS/FC differs little, and so does Series ZS/FC. However, the kc of ULFC doped with ZS and RDL fluctuates greatly. This may be resulted from the uncertain change of number of connected pores in the prepared ULFC with different dosages of ZS and RDL [27]. As shown in Fig. 7, when the RDL dosage is 0.5 wt% and 3 wt%, the qd of obtained

ULFC is 270.5 kg/m3 and 278.6 kg/m3, respectively, which are the lowest qd in Series RDL/FC. And the final measured kc is 0.0686 W/mk and 0.0702 W/mk, respectively, which are also the lowest kc in Series RDL/FC. On the contrary, when the ULFC doped with RDL has the highest qd, the corresponding kc is also the largest one. This rule is also consistent with the ULFC doped with ZS in Series ZS/FC as shown in Table 5 and Fig. 7. As far as we know, the kc is not directly related to the qd, but the qd will affect the heat flow rate (u) during the test, and the u has a direct functional relationship with the kc, which can well explain the phenomenon that the kc changes with the qd. And it can be seen from Fig. 7 that the kc of ULFC doped with ZS is generally higher than that of Con group or ULFC doped with other types of PWR. This is because the pore wall between the pores of the ULFC doped with ZS is much thicker than that of other series, which leads to the increase of pore wall proportion and the decrease of porosity [19], as demonstrated in Fig. 5. 4.2. Effect of four types of PWR on WV and Rf As shown in Table 6 and Fig. 8, with the increase of soaking time, the WV of all groups of ULFC firstly increases and then tends

Fig. 5. The pore structures and morphologies of ULFC in group Con, CS4, ZS3, PS3 and RDL3.

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9

Fig. 5 (continued)

Fig. 6. XRD patterns of ULFC in group Con, CS6, ZS6, PS3 and RDL6.

to be stable. With the addition of CS, ZS and PS, the WV of ULFC is significantly lower than that of the group Con, and the higher is the dosage, the lower the WV is. However, the WV of ULFC doped with RDL is not always lower than that of the group Con, and some WVs are higher than that of the group Con. This phenomenon can be explained by SEM microstructure of ULFC shown in Fig. 5. In the group Con, the ULFC has many connected pores and its pore wall structure is loose, many capillary pores with large pore size are exposed on the surface, and the water will not only enter into the ULFC through many connected pores, but also rapidly diffuse to all corners of the ULFC through large pore capillary pores by capillary force. It can be seen from the black curve in Fig. 8 that the capillary water absorption power is the most intense within 1 h, and the 1 h WV can reach 70 wt% of the final WV. The connected pores of ULFC doped with CS, ZS and PS reduce, and the surfaces

Fig. 7. The effect of PWR with varied dosage on kc of ULFC.

of the pores become denser, the capillaries with large pores are effectively blocked, leaving only small pore capillaries exposed on the surface [50,59], which effectively reduces the power of capillary water absorption and water absorption speed within 1 h soaking. Although the connected pores of ULFC doped with RDL reduce, there are still many voids on the pore walls, which is conducive to a large amount of water penetrating into the interior of ULFC. The height of the water rising along the capillary in ULFC of group Con, CS6, ZS6, PS6 and RDL6 soaked in water at 1/3 height of the specimens for 1 h can be seen in Fig. 9. The rise height of water in ULFC doped with CS and ZS is the lowest one, while that doped with PS is slightly higher. However, there is almost no difference in the height of water between the ULFC doped with RDL and the ULFC of group Con, and after 1 h soaking, the water has nearly risen along the capillary to the top. The 1 h WV shown in

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Table 6 The WVs of ULFC doping PWR at various soaked time and the Rf after water absorption. Series

Group No.

Control CS/FC

Con CS1 CS2 CS3 CS4 CS5 CS6 ZS1 ZS2 ZS3 ZS4 ZS5 ZS6 PS1 PS2 PS3 PS4 PS5 PS6 RDL1 RDL2 RDL3 RDL4 RDL5 RDL6

ZS/FC

PS/FC

RDL/FC

WV (wt%)

Rf

1h

4h

8h

12 h

24 h

36 h

48 h

60 h

72 h

34.0 18.3 13.8 8.9 8.9 8.1 2.0 13.2 11.6 9.5 10.2 7.4 4.7 22.1 20.9 21.9 18.9 14.8 13.7 33.9 34.8 31.8 30.4 34.2 35.8

36.8 19.8 15.6 11.1 11.0 10.9 7.3 16.3 13.7 12.7 12.6 10.5 8.2 25.3 24.9 24.1 20.5 16.3 14.9 36.5 39.3 34.9 33.3 36.8 40.5

37.0 20.5 16.1 12.0 11.7 11.8 8.1 16.4 14.2 12.9 13.0 11.4 8.9 27.6 26.2 25.4 22.9 17.2 16.5 38.2 40.5 36.9 35.3 37.6 41.0

38.5 20.7 17.2 12.9 12.5 12.8 8.8 16.7 14.6 13.1 14.6 12.2 9.1 28.1 27.0 26.9 24.2 18.3 16.8 39.6 40.6 38.2 36.0 38.0 41.5

38.7 21.0 18.3 13.2 12.7 13.1 9.5 16.8 14.9 13.6 14.6 13.3 9.6 28.5 29.1 27.9 24.3 19.4 18.1 40.3 40.7 38.2 37.2 38.7 41.7

47.0 27.9 21.5 17.9 16.4 14.4 11.9 17.9 17.5 17.5 18.0 17.0 15.7 30.8 31.4 29.0 27.0 21.0 18.3 44.4 46.4 42.1 43.4 46.6 48.3

47.1 27.9 23.4 18.3 17.9 15.3 12.6 19.7 17.6 18.2 18.3 18.1 15.7 31.7 32.2 29.0 27.2 21.6 19.3 47.1 46.5 48.6 44.8 48.3 51.2

48.0 30.0 26.9 25.9 26.2 26.7 22.8 26.6 27.4 25.6 25.5 28.5 25.9 37.4 34.0 32.2 30.9 29.8 27.6 50.9 47.5 50.6 46.5 47.3 52.3

48.2 30.1 27.5 27.6 26.2 27.3 23.6 26.9 27.7 25.7 26.6 28.5 26.0 37.8 34.1 32.5 32.0 30.9 27.9 52.0 49.9 52.6 47.7 50.7 53.6

0.20 0.22 0.17 0.32 0.21 0.05 0.02 0.19 0.70 0.58 0.56 0.51 0.65 0.23 0.12 0.30 0.72 0.67 0.65 0.60 0.65 0.76 0.62 0.67 0.64

Fig. 8. The Wv curves of ULFC doped with or without PWR at varied soaking time.

Table 6 and Fig. 8 is basically consistent with the rising state of water in Fig. 9, which intuitively shows the influence of different PWR on the WV of ULFC. The gap between each line in Fig. 8 a) is greater than that in Fig. 8 b), c) and d), which indicates that the WV of ULFC is the most sensitive to the change of CS dosage. In

other words, CS dosage is the most beneficial to the waterresistant effect of ULFC. The ULFC doped with CS has the lowest WV with 1 h WV and 72 h WV being 2.0 wt% and 23.6 wt%, respectively, when the dosage is 4 wt%. It can be seen from Fig. 8, the WVs of ULFCs doped with

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Fig. 9. The water rising height of ULFC in group Con, CS6, ZS6, PS6 and RDL6 after 1 h water soaking.

PWR show two plateaus from horizontal to upward at 24 h and 48 h watering, but the ULFCs in group Con almost reach the state of saturated water absorption only at 36 h. Because the ULFC without PWR has strong capillary force and can absorb a large amount of water in a short time [44], it is close to saturation after soaking in the water at 1/3 height of the specimen for 24 h, and the water can pass through any part of the ULFC to achieve full saturation by the capillary when adding water to 2/3 height of the specimen. Thus, it will not absorb much water when the specimen is completely immersed in water. Nevertheless, the PWR dosage can weaken the capillary force by changing the pore structure or condensing into a waterproof layer on the pore surface [52], making it difficult for ULFC to absorb water to reach saturation state before it is completely immersed in water.

In order to explore the influence of water on the strength loss of ULFC in different group, the fcu is tested immediately after completion of the WV test, and the results could be seen from Table 6 and Fig. 10. The Rf of ULFC doped with CS shows an opposite trend to that doped with ZS, PS and RDL. Although the WV of ULFC doped with ZS and PS decreases with the increase of the dosage, but it is bad for retaining the strength after water absorption. Although the RDL dosage can improve the fcu of ULFC in dry state within a certain dosage range, it is not conducive to waterproof and will increase Rf after water absorption. On the contrary, CS dosage can not only improve the fcu of ULFC in a certain dosage range but also reduce the WV of ULFC as the increase of its dosage, in particularly, the 3 d WV of ULFC doped with CS at dosage of 4 wt% can be dropped to 23.6 wt%. More importantly, with the increase of CS dosage, the Rf of ULFC decreases gradually after water absorption, and the Rf is close to 0 when the CS dosage is 4 wt%. Thus, compared with the other three types of PWR, CS is more suitable for the production of water-resistant ULFC, and the CS dosage of 4 wt% could be selected to effectively reduce the WV in the case of meeting the strength requirements. 4.3. Effect of LWR on WV of ULFC

Fig. 10. Strength loss of ULFC doped with PWR after saturated water absorption.

In order to further reduce the WV of ULFC and explore the difference in the water-resistant effect of LWR on ULFC doped with or without CS, the ULFC specimens of group Con and group CS6 are selected for next experiment. Both the two types of LWR of MPR and HSO are used to explore their water-resistant effect on ULFC by means of surface coating and soaking method under the condition of adding or not adding KH550. The test results of RS and WV are shown in Table 7, and the WVs of ULFC treated with MPR, HSO along with varied soaking time are shown in Fig. 11, Fig. 12, respectively. The hs and the state of water drops on the surfaces of ULFC blocks are accordingly shown in Table 8 and Fig. 13, respectively.

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Table 7 The RS and WV of ULFC treated with LWR or/and KH550 at varied soaking time. GroupNo.

RS(wt%)

ConSM ConCM CSSM CSCM ConSMK ConCMK CSSMK CSCMK ConSH ConCH CSSH CSCH ConSHK ConCHK CSSHK CSCHK

22.6 7.6 18.7 8.8 19.8 2.7 14.1 1.7 27.0 5.8 23.8 3.6 28.5 8.2 30.1 8.4

WV (wt%) 1h

4h

8h

12 h

24 h

36 h

48 h

60 h

72 h

3.7 2.6 2.6 2.3 2.2 2.2 3.8 1.4 3.5 24.4 2.0 23.6 1.0 1.3 0.8 0.9

4.2 3.1 3.3 3.1 2.7 2.7 3.9 2.0 4.1 26.2 2.5 24.4 1.0 2.0 1.2 1.4

4.7 3.9 3.3 4.5 3.0 2.8 3.9 2.5 4.4 26.2 2.5 25.4 1.1 2.2 1.2 1.7

7.2 4.0 3.8 5.4 3.0 4.1 4.7 3.2 4.8 27.6 4.2 25.8 1.4 2.3 1.4 1.7

8.0 4.3 4.6 6.0 3.2 4.1 4.7 3.5 4.9 28.3 4.7 25.9 1.7 2.4 1.6 1.8

9.1 5.8 10.1 8.8 4.8 6.4 6.2 19.3 7.1 29.0 6.6 27.4 3.1 4.2 2.5 3.4

12.6 6.8 10.7 10.6 5.8 6.3 6.8 22.9 8.0 29.4 7.4 27.4 3.5 4.9 2.8 4.2

13.7 10.0 14.2 13.7 7.0 22.0 9.1 26.3 12.2 31.8 8.3 31.0 4.2 5.3 3.6 5.9

17.8 13.3 15.4 16.2 7.7 23.6 10.8 27.4 13.4 32.3 8.8 31.5 4.9 5.8 4.4 7.3

Noting that, ConSM—Soaking group Con with MPR; CSCH—Coating group CS6 with HSO; CSSMK—Soaking group CS6 combined with MPR and KH550, the other groups are named by analogy.

Table 8 The hs between the water drops and the surfaces of varied group ULFC blocks.

Fig. 11. Effect of MPR treatment on WV of different ULFC block at varied soaking time.

Fig. 12. Effect of HSO treatment on WV of different ULFC block at varied soaking time.

Group No.

h (°)

Con ConCM/ConCMK ConSM/ConSMK ConCH/ConCHK ConSH/ConSHK CS6 CSCM/CSCMK CSSM/CSSMK CSCH/CSCHK CSSH/CSSHK

2.35 109.35/96.34 110.44/120.10 90.51/119.28 109.73/125.22 65.41 109.55/99.11 112.05/117.39 90.88/120.02 116.22/125.87

As shown in Table 7 and Fig. 11, the WVs of ULFC after MPR treatment are less than 5 wt% in 1 h soaking, and the WVs increase slowly in 24 h soaking. The same trend can also be found in the WVs of ULFC after HSO treatment (Fig. 12), and the WVs of 6 groups of ULFC blocks are around 5 wt% in 24 h soaking, which indicates that the ULFCs after either MPR or HSO treatment both have a good water-resistance. Indeed, capillary suction is an unsaturated transport process by means of capillary force, which is a function of surface tension of the wetting liquid and its h with a pore of a radius [39,40]. MPR and HSO are two types of silicone LWR, they can form a water-resistant lining on the surface of ULFC to achieve an waterresistant effect similar to the lotus effect [71], which increases the h between the surface of ULFC and the water above 90° by reducing the attraction between them, causing the pressure difference reversed and requiring additional force to allow water to penetrate the pores [39]. The h between the water and the surface of ULFC treated with MPR or HSO can reach more than 90°, as shown in Table 8, which indicates MPR and HSO can effectively build the water-resistant barrier by increasing h between the substrate and water. It’s noting, the h of group Con and CS6 is 2.35° and 65.41°, respectively, which is far less than 90°, the ULFC without MPR or HSO treatment has a great molecular attraction between the water and its interface by capillary rise and a concave meniscus, the water will spontaneously enter the inside ULFC. In order to better observe the surface water accumulation effect of MPR and HSO, 3 drops of red ink are dropwise added to the sur-

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Fig. 13. Water droplet state on the surfaces of the control and ULFC blocks after MPR or HSO treatment.

face of group Con, ConSM, ConSH, CS6, CSSM and CSSH ULFC block, respectively. As revealed in Fig. 13, the red ink on the surface of group Con is immediately diffused into the interior of ULFC, and the red ink on the surface of group CS6 begins to disperse, whereas the red ink drops on the ULFC with MPR and HSO treatment are all gathered into a spherical water ball, and the h between the water ball and the interface is greater than 90°. After surface treated by MPR and HSO and cured, some connecting pores of the ULFC are blocked and the surface of the pore wall become smooth as if a membrane attached to the surface, as verified by the corresponding pore structures and wall morphologies (Fig. 14). As presented in Figs. 11 and 12, the WV of different group ULFC begins to show a large difference after 24 h soaking owing to water soak height further increasing. Indeed, the water-resistant effect (WV) of LWR on ULFC by means of surface coating (Fig. 11) can be effectively distinguished, the WV curves representing groups CSCMK and ConCMK rise sharply after 24 h and 48 h soaking, respectively, while the other two WV curves just rise steadily. This indicates that the addition of KH550 in MPR renders the RS reduction, and a large amount of MPR volatilization and a relatively thin water-resistant film formation, resulting in rapidly decreasing of the surface tension when soaked in water [61,72]. Water eventually breaks through the defense line and enters the ULFC in large quantities in a short time. And the hs of group ConCMK and CSCMK are 96.34° and 99.11°, which are about 13° and 10° lower than that of group ConCM and CSCM, respectively, which can well explain the decrease of surface tension. The WV of group CSCMK is above that of group ConCMK and shows a fast-rising trend earlier, so do those of groups CSCM and dzCM. Actually, the group CSCM ULFC is doped with a large amount of CS, the pore structure is relatively closed owing to CS is not involved in hydration, and just attached to the surface of the ULFC, to eventually form a small range of water-resistant layer, which will reduce the depth of penetration of MPR in the ULFC and reduce the thickness of the waterresistant layer [62]. Overall, the WV of ULFC soaked in MPR is lower

than that coated with MPR on the surface of ULFC, and the WV of ULFC soaked in MPR with KH550 is lower than that without KH550. As known, after 1 h immersion of ULFC in MPR, the MPR may enter almost all parts of ULFC, even if a part of MPR will volatilize after curing, which determines the water-resistant effect is not the penetration depth of MPR, or the amount of formed water-resistant film, but the density of the formed film. The denser is the water-resistant film, the greater the surface tension is. And the hs of group ConSMK and CSSMK are 120.10° and 117.39°, which are about 10° and 5° higher than that of group ConCM and CSCMK, respectively, as verified in Table 8 and Fig. 13. As shown in Fig. 12, the ULFC treated with HSO has a low WV, and the WVs all rise slowly except groups ConCH and CSCH. Since it is difficult for HSO to solidify without KH550, and the surface coating method makes its penetration depth be smaller, the WV of ULFC in group ConCH and CSCH is higher, the addition of KH550 can increase the RS of HSO, and reduce the WV owing to the water-resistant film formation and the surface tension increase. As shown in Table 8, the hs of group ConCHK and CSCHK are about 28° and 30° higher than that of group ConCH and CSCH, respectively, which indicates the surface tension of the formed water-resistant film under the synergy effect of HSO and KH550 is larger, resulting in lower WVs of ConCHK and CSCHK than group ConCH and CSCH. It’s worth to pointing out, the WV of ULFC in the other 6 groups is generally lower than that of the ULFC treated with MPR, as shown in Figs. 11 and 12. As known, the water-resistant film formed by HSO after solidified is particularly smooth and can cover most of the capillary pores [62,63], as verified in Fig. 14 (ConSH and CSSH), it is naturally difficult for HSO to evaporate during solidifying, but the RS accordingly increases. The product formed after HSO solidified takes up a larger volume in ULFC, which reduces the space that water can occupy, and the smooth water-resistant layer can effectively prevent liquid water and most of the water in the gaseous state from entering the ULFC to greatly reduce the WV, as schematically depicted in Fig. 15 a). However, MPR can pen-

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Fig. 14. The pore structures and wall morphologies of the control and ULFC blocks after MPR or HSO treatment.

etrate into the substrate without clogging the capillary channels, the active groups in MPR interact with each other to form a dendritic, chain or network molecules, forming a network waterresistant siloxane film, as shown in Fig. 14 (ConSM and CSSM). Although it can effectively prevent the most liquid water from penetrating, the gaseous water can enter and exit freely, as schematically revealed in Fig. 15 b). Therefore, MPR is easy to volatilize during the solidifying process and reduce the RS, and

the product formed after MPR solidification accounts for a small volume in ULFC. Therefore, the water-resistant effect of HSO is better than that of MPR, and the WV of ULFC treated by the soaking method is lower than the surface coating method. In all groups of ULFC treated with LWR, the ULFC without any PWR but treated with HSO by coating method in group ConCH presents the worst waterproof effect with the 72 h WV being up to 32.3 wt%, the ULFC doped

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Fig. 15. Water-resistant mechanism of ULFC treated with a) HSO; b) MPR.

with 4 wt% CS and treated with HSO by soaking method simultaneously presents the best waterproof effect with the 72 h WV being only 4.4 wt%. Considering that extremely high RS will greatly increase the qd of ULFC, which may affect the other physical properties (kc, etc) of ULFC, the group ConCHK is chosen as suitable one for preparing ULFC, whose RS and WV are only 8.2 wt% and 5.8 wt% respectively. 5. Conclusions and prospects (1) Compared with other three types of PWR, the qds of ULFC doped with CS are the lowest. With the increase of the dosage of four types of PWR, the fcu of ULFC firstly increases and then decreases. Among them, the fcu of ULFC doped with ZS is the greatest, and up to 0.54 MPa. The kc of ULFC is insensitive to CS and PS dosage, positive to ZS dosage, while negative to RDL dosage, and low to 0.0686 W/mk. (2) When the dosage of CS, ZS, PS and RDL is 2.0 wt%, 1.5 wt%, 1.5 wt%, and 1.5 wt% respectively, the amount of ettringite in the hydration products of ULFC shows some increase, whereas when the dosage of four types of PWR increases to 4 wt%, the amount of ettringite decreases, which well reflects the change trend of the fcu. The pore wall structures of ULFC doped with CS, ZS and PS are accordingly more compact than those of ULFC doped with RDL or without PWR. (3) The WV of ULFC after doping PWR sharply increases within 1 h water soaking. CS, ZS and PS dosage can significantly reduce the WV of ULFC by reducing the connected pores or partially gathering on the surface of ULFC, and the higher is the dosage of CS, ZS and PS, the lower the WV is. Among them, CS dosage has the best water-resistant effect, and the WV of ULFC can be reduced to 23.6 wt% with 4 wt% dosage. With the increase of ZS, PS and RDL dosage, the Rf of ULFC increases, but CS dosage effect is on the contrary, the corresponding Rf is close to 0 with 4 wt% dosage. (4) The WV of ULFC treated with MPR and HSO can be reduced to less than 5 wt% within 1 h water soaking, and the h between water droplets and the surface is greater than 90°. The WV of the ULFC treated with soaking method is lower than that with the surface coating method, but soaking method will render the RS increase. The CS dosage is bad for the infiltration of MPR on the surface of ULFC, which weaken the waterresistant effect of MPR. (5) KH550 incorporation can reduce the RS of ULFC with MPR but increase the RS with HSO. The combination of KH550 and MPR is easy to volatilize, resulting in the reduced water-resistant effect of ULFC by surface coating method.

The water-resistant effect of HSO on ULFC by soaking method is better than that of MPR, and the WV of ULFC treated by HSO and KH550 can be reduced to 4.4 wt% after 72 h soaking, and the corresponding h can reach 125.87°. It can be concluded from §4.2 that the ULFC doped with 1.5 wt% PS has both high fcu and low kc, and the kc is less affected by the dosage of PS. It can be concluded from §4.3 that the ULFC specimens treated with HSO and KH550 by surface coating method has both low WV and low RS. Thus, ULFC doped with 1.5 wt% PS and treated with HSO and KH550 by surface coating method simultaneously can achieve favorable waterproof performance on the basis of ensuring high fcu and low kc. ULFC itself has low fcu and high WV, and this scheme can achieve good results. However high-density grade FC has higher fcu and lower WV than ULFC. If this scheme is extended to high-density grade FC, it will have a better effect. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was partially supported by grants from the Natural Science Foundation of Shandong Province (No. ZR2017ZC0737 and ZR2018MEE043), National Natural Science Foundation of China (No. 51878364, 51878366, and 51578297), Key Technology Research and Development Program of Shandong Province (Public welfare type) (No. 2019GSF110008), the National ‘‘111” project, and first-class discipline project funded by Education Department of Shandong Province. References [1] B. Yuan, C. Straub, S. Segers, Q.L. Yu, H.J.H. Brouwers, Sodium carbonate activated slag as cement replacement in autoclaved aerated concrete, Ceram. Int. 43 (2017) 6039–6047. [2] M.R. Jones, A. McCarthy, Preliminary views on the potential of foamed concrete as a structural material, Mag. Concr. Res. 57 (2005) 21–31. [3] B. Chen, W. Zhen, Experimental research on properties of high-strength foamed concrete, J. Mater. Civ. Eng. 24 (2011) 113–118. [4] A.H. Azimi, Experimental investigations on the physical and rheological characteristics of sand-foam mixtures, J. Non-Newtonian Fluid Mech. 221 (2015) 28–39. [5] M. Frenzel, M. Curbach, Shear strength of concrete interfaces with infralightweight and foam concrete, Struct. Concr. 19 (2018) 269–283.

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