Preparation and hardened properties of lightweight gypsum plaster based on pre-swelled bentonite

Construction and Building Materials 215 (2019) 360–370

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Preparation and hardened properties of lightweight gypsum plaster based on pre-swelled bentonite Jun Jiang a,b, Zhongyuan Lu a,⇑, Jun Li a, Yong Fan b, Yunhui Niu a a b

State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China

h i g h l i g h t s
Swelled bentonite promotes formation of micron-size pore in porous gypsum paste.
Formation of micron-size pore can improve thermal insulation performance of gypsum.
Swelled bentonite can enhance moisture absorption/desorption of gypsum paste.
Adding swelled bentonite into gypsum paste can generate slight humidity hysteresis.

a r t i c l e

i n f o

Article history: Received 13 December 2018 Received in revised form 1 April 2019 Accepted 23 April 2019 Available online 3 May 2019 Keywords: Lightweight gypsum plaster Pore structure Humidity control property Thermal conductivity Mechanical strength

a b s t r a c t Lightweight gypsum plasters (LGPs) were successfully prepared by adding pre-swelled bentonite to gypsum paste. As the content of bentonite slurry increased from 0% to 40%, the excessive water in the swelled bentonite could not be consumed completely, leading to an increase in the median size of pores in the LGP as well as increased porosity. The increasing bentonite content decreased the dry density, compressive strength, flexural strength and enhanced thermal insulation property, yielding LGPs with dry density, compressive strength, flexural strength and thermal conductivity of 784–1196 kg/m3, 3.9–11.7 MPa, 2.1– 3.9 MPa, and 0.116–0.143 W/m K, respectively. More importantly, the pores that formed in the montmorillonite layers of the bentonite enhanced the humidity-controlling ability of the LGPs. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Living comfort, which is related to indoor humidity and temperature, has been a high concern because suitable relative humidity (RH) and temperature for the indoor environment have a beneficial impact health and wellbeing [1–5]. Plenty of heating, cooling and humidity-controlling equipments are widely used to maintain comfortable living conditions. However, together with energy losses through exterior walls, the energy demand associated with equipments for controlling indoor environmental conditions results in tremendous energy consumption [5–7]. Many scientists and environmentalists have devoted considerable efforts to address this problem during the past decades [5,8–12], and using humiditycontrolling and thermal insulation materials is becoming popular due to their effectiveness in reducing energy consumption [13,14].

⇑ Corresponding author. E-mail addresses: [email protected] (J. Jiang), [email protected] (Z. Lu), [email protected] (J. Li). https://doi.org/10.1016/j.conbuildmat.2019.04.181 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Thermal insulation materials (expanded polystyrene, extruded polystyrene, porous cement-based material, aerogel, etc.) and humidity-controlling materials (diatomite, sepiolite, zeolite, polymer resin, etc.) have been developed to control temperature and humidity, respectively, inside buildings [15–20]. Gypsum plaster also has been extensively studied and is used for energy conservation in buildings, due to fire resistance, low cost, safety and aesthetic appearance, thermal insulation and humidity controlling abilities. However, the most important reason for the popularity of gypsum plasters is that calcium sulfate hemihydrates were commonly known as ‘‘green cements” due to their healthful contribution to environmental quality and pollution abatement by reducing CO2 emissions [21,22]. Unfortunately, conventional gypsum plasters are less thermally insulating and humidity controlling than other porous materials, and several studies have focused on improving the thermal insulation and humidity-controlling properties of gypsum plasters by adding modifiers, lightweight aggregates or air-voids [23–25]. Nevertheless, the high thermal conductivity and poor humidity control of lightweight gypsum plaster have stymied its development for indoor applications.

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

Increasing the number of small pores in a porous material can decrease the thermal conductivity of air as well as transmission of gaseous components [26]. This decrease results in improved thermal insulation properties. In addition, the increased and complex heat transfer path resulting from a higher proportion of small pores for any given porosity can also help reduce the thermal conductivity of porous materials [27]. Hardened gypsum plasters are composed of calcium sulfate dihydrate (CaSO42H2O), water, and micron-size voids, and the volume of voids can be increased by increasing the water content of fresh plaster. However, voids created in this way are coarse (relatively large) and always result in low strength and high thermal conductivity under the same porosity, due to short heat transfer path and relatively high thermal conductivity of air, additionally it also leads to the bleeding of fresh paste [26,28,29]. To achieve a higher proportion of small pores in plaster without sacrificing hardened properties, investigators have been examining other ways of introducing pores. Bentonite, composed mainly of montmorillonite, is widely used with inorganic binders together for meeting requirements of application [30–37], for example, bentonite is used as supplementary cementing material to address the shortage of raw materials and prepare green cement-based materials, bentonite/cement mortar is used for waterproofing, stabilization and solidification of heavy metals. However, bentonite with full water absorption may swell and result in structural damage of engineering construction, gypsum, lime, cement or their combinations are often used as stabilizers to hinder swelling. Conversely, utilizing this drawback, Jiang et al. [38] prepared nano-rich porous cement-based materials by adding pre-swelled bentonite for building thermal insulation, pore formation is mainly attributed to the fact that pre-saturated bentonite is composed mainly of several montmorillonite layers and a large amount of water [38,39]; water in pre-saturated bentonite cannot be completely consumed by the hydration reaction, finally forming tiny voids in matrix [38,39]. If pre-swelled bentonite is added to the gypsum paste, the capillary void volume of the plaster also may increase. Besides, bentonite can be used to prevent bleeding of plasters caused by excessive water because the montmorillonite layers have a strong attraction for free water [40]. Additionally, interlayer voids of bentonite may further increase the number of small pores [40]. Hence, pre-swelled bentonite may be used to increase the small pores content in plaster, resulting in porous gypsum that consists mainly of tiny pores. Compared to conventional gypsum plaster, the bentonite-amended plaster may show better thermal insulation properties due to decreased thermal conductivity of the air-filled pores and the longer heat transfer path they create. Moreover, the montmorillonite layers in bentonite-amended gypsum may also contribute to moisture absorption due to the numerous inter-layer spaces and presence of oxhydryls, leading to enhanced humidity-controlling ability. In this study, pre-saturated bentonite was added to gypsum with the aim of obtaining lightweight gypsum plasters (LGPs). The pore structure, thermal insulation, humidity-controlling and mechanical properties of LGPs were investigated. The findings of this study will provide a new method of preparing porous gypsum that has low thermal conductivity and enhanced humiditycontrolling properties.

by X-ray diffraction (Fig. 1). The particle size distribution of the hemihydrate plaster was obtained using a particle size analyzer and is shown in Fig. 2. Bentonite was purchased from Weifang Shengshi Co., Ltd. (Weifang, China) and was composed mainly of montmorillonite with quartz and illite, as shown in Fig. 1. The chemical compositions of bentonite and bassanite are shown in Table 1. A scanning electron micrograph (SEM) of bentonite, as well as the results obtained from energy-dispersive X-ray spectroscopy (EDX) of three randomly selected points on bentonite particle, are shown in Fig. 3. 2.2. Preparation The LGPs in this study were designed using a volumetric method. Table 2 describes the mix parameters of the LGPs. Fresh LGP slurry consisted of gypsum paste and bentonite slurry. Prior to the experiment, pre-saturated bentonite slurry (bentonite: water = 1:5, 1:10, 1:15, 1:20) was prepared by mixing tap water and bentonite for 1 h and then leaving the mixture undisturbed for 24 h to ensure complete hydration of the bentonite. Gypsum paste was prepared at 20 ± 2 °C in the laboratory. During this preparation, water was added to a vertical mixer followed by the addition of bassanite; the two components were mixed for 90 s until a lump-free homogeneous gypsum paste was obtained. For LGP slurries, different volume contents (0–0.8 m3) of presaturated bentonite (Table 2) were immediately added to the gypsum paste (0.2–1.0 m3) and mixed for 60 s until uniformly distributed. These pastes were subsequently placed in 40 mm  40 mm  160 mm and 300 mm  300 mm  30 mm molds, covered with plastic film, and cured at 20 ± 2 °C for 24 h in a 90% RH chamber. Afterward, the specimens were removed from the molds and cured at 20 ± 2 °C in the 90% RH chamber for 1 day, 7 days and 28 days. 2.3. Test methods The properties of the specimens were evaluated via compressive strength tests performed after 1 day, 7 days, and 28 days of curing, three samples (40 mm  40 mm  160 mm) of each composition were subjected to mechanical testing (TYE300, Wuxi Jianyi Instrument and Machinery Co., Ltd., Wuxi, China) at a loading rate of 2.4 kN/s and currency of ±1%, in accordance with Chinese standard GB/T 9776. Three 1-day-cured samples (300 mm  300 mm  30 mm) of each composition were used to

2. Material and methods 2.1. Materials Commercial bassanite (in accordance with the Chinese standard GB 9776-2008; Longyuan Gypsum Co., LTD, Jinmen, Hubei Province, China) was used as binder; the material was mainly composed of calcium sulfate hemihydrate (CaSO41/2H2O), as shown

361

Fig. 1. X-ray diffraction patterns of the raw materials.

362

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

To obtain the mineral phases, microstructures, pore parameters and N2 adsorption/desorption isotherms, the hydration of 1-daycured LGPs with 0–40% bentonite slurry and W/G of 0.6 was firstly stopped by immersion in anhydrous alcohol and then vacuum drying at 45 °C until the mass remained unchanged. Mineral phases of LGPs and the raw materials were characterized via X-ray diffraction (XRD) measurements performed with a PANalytical X’PertPRO diffractometer (copper target, step size 0.03°). The microstructures of LGPs (fracture surface) and bentonite particles were examined via SEM (MAIA3LMU, Tescan Brno, s.r.o, Brno, Czech Republic) in high vacuum mode under acceleration voltage of 8 kV and EDX (AMETEK, Inc., Berwyn, PA, USA) under acceleration voltage of 10 kV. Furthermore, pores in the samples were measured via mercury intrusion porosimetry (MIP; AutoPore IV 9500, Micromeritics Instruments Corp., Norcross, GA, USA). The applied intrusion pressure was increased from 0 to 30,000 psi. In general, pores are considered ideal cylindrical tubes with various diameters, and thus the intrusion pressure P can be related to the pore diameter d via the Laplace equation, which is given as follows: Fig. 2. Particle size distribution of the raw materials.

P¼ obtain thermal conductivity in accordance with Chinese standard GB/T 10294, using a heat flow conductometer (DRH-300, XiangtanXiangyi Instrument Co., Ltd., Xiangtan, China). Prior to the measurement, these slabs were polished, and dried or dried then put in 50 ± 5% RH chamber until the mass remained unchanged. Moisture adsorption/desorption performances were measured by the method ISO 12571. The main instrument used for the humidity control performance experiment was a temperature-controlled chest (TH-B, China) that provided a constant temperature environment. The humidity control experiments were conducted in the chamber, and the RH was controlled using a saturated salt solution with potassium sulfate (98% RH), sodium chloride (75% RH), magnesium nitrate (54% RH) and magnesium chloride (33% RH). The temperature was kept at 20 °C during the tests. Three 1-daycured samples of each composition were dried at 40 °C until the mass remained unchanged, then all samples were allowed to adsorb water vapor in different RHs until the mass change was less than 0.01%. After that, LGPs with fully absorbed water vapor in 98% RH chamber were gradually placed in different humidity levels in order from 98% RH to 33% RH until moisture desorption was complete. During these processes, an automatic recording balance was used to record the weight change of each sample, and the moisture content of samples was calculated using Eq. (1).

M ¼ ðmt  m0 Þ=m0  100%

ð1Þ

where M is the moisture content, m0 is the initial weight of the dried sample, and mt is the sample weight at time t. Raw materials (bentonite and bassanite) were dried at 45 °C until the mass remained unchanged for chemical composition, particle size distribution, mineral phase, microstructure and N2 adsorption/desorption measurement. The chemical compositions of these materials were then determined by means of X-ray fluorescence spectroscopy (XRF; Axios, PANalytical B.V., Almelo, Netherlands). Afterward, bentonite and bassanite particles were dispersed in anhydrous alcohol and their particle size distributions were determined under ultrasonic leaching using a Mastersizer 3000 (Malvern Instruments, Ltd., Malvern, England).

4ccosh d

ð2Þ

where c is the surface tension of mercury (0.48 N/m), and h is the contact angle of imperfect wetting between mercury and the pore surface (140°). The pore size distribution (PSD) is presented as a function of the intrusion volume V on the pore diameter d. The N2 adsorption/desorption isotherms of gypsum and LGP were measured via nitrogen adsorption/desorption (NAD; Autosorb-iQ, Quantachrome Instruments, Boynton Beach, FL, USA). 3. Results and discussion 3.1. Bulk density and physical appearance Bentonite slurry contains excessive water and when introduced into gypsum plasters, bleeding of fresh LGP may occur, resulting in significant volume variation and instability of the fresh LGP. Therefore, the actual volume will deviate from the design volume. Additionally, too much water in LGP may lead to severe volume change during drying, finally resulting in warping due to water migration and loss in hardened LGPs. Water, including mixing water in the gypsum and water in the bentonite slurry, plays the key role in bleeding and volume variation of fresh or hardened LGP. A suitable water content of the paste and ratio of bentonite to water are essential for overcoming these drawbacks. Therefore, bentoniteto-water ratio (B/W) of bentonite slurry and water-to-gypsum ratio (W/G) of gypsum paste should be identified for the preparation of LGPs, and the mix proportions in Table 2 are used to prepare LGPs for obtaining the suitable W/G and other parameters. Montmorillonite, the main component of bentonite, is a 2:1layer silicate comprised of silica and alumina sheets. The layer structure consists of two tetrahedral silica sheets with an octahedral alumina sheet sandwiched between the two silica sheets, and these sheets are strongly linked each other, while two close layers are bonded each other by van der Waals forces [41]. Owing to isomorphous substitution, the basal faces carry a permanent negative charge balanced by exchangeable cations (e.g., Na+, Li+, Ca2+) adsorbed at the layer surfaces. Furthermore, water molecules

Table 1 Chemical composition of raw materials. Compositions (%)

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2O

Na2O

Bassanite Bentonite

3.70 75.75

1.36 15.30

0.93 1.77

39.56 3.02

4.42 1.78

48.67 1.35

0.42 0.60

0.08 0.08

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

363

Fig. 3. Scanning electron micrograph and energy-dispersive X-ray spectroscopy analysis of bentonite particles.

Table 2 Mix parameters of lightweight gypsum plasters. Composition of per m3 Gypsum paste (m3) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

(Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum (Water-to-gypsum

Bentonite slurry (m3) ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio:

0, 0, 0, 0, 0, 0, 0, 0, 0,

0.2, 0.2, 0.2, 0.2, 0.2, 0.2, 0.2, 0.2, 0.2,

0.4, 0.4, 0.4, 0.4, 0.4, 0.4, 0.4, 0.4, 0.4,

0.6, 0.6, 0.6, 0.6, 0.6, 0.6, 0.6, 0.6, 0.6,

0.8) 0.8) 0.8) 0.8) 0.8) 0.8) 0.8) 0.8) 0.8)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

penetrate the region between two close layers and hydrate exchangeable cations, leading to adsorption of the water and swelling of the mineral layers [42]. In addition, water molecules can be limited by the surface of the montmorillonite [43]. Bentonite slurry with different B/W ratios was prepared based on the procedures described in Section 2.2 and then put into test tubes. Bentonite in water swelled and formed new montmorillonite layers or single layers when the B/W ranged from 1:5 to 1:20 [39]. However, due to the limited montmorillonite content in slurries, the ability for inhibiting water migration was limited. Fig. 4 illustrates the behavior of bentonite slurries with B/W of 1:5, 1:10, 1:15 and 1:20 after 24 h of standing. The volume of supernatant (which was mainly water) decreased significantly for B/W ranging from 1:20 to 1:5, and stratification was negligible at B/W less than 1:10. Supernatant may increase the amount of mix water in gypsum paste, resulting in weakened mechanical properties [28], thus, a B/W of 1:5 was used to prepare LGPs. All the fresh LGPs (mixture proportions are from Table 2) were firstly prepared to find proper W/G and content of bentonite slurry, as shown in Table 3, too low W/G ratios and high bentonite content could not be used to prepare LGPs due to difficulties with mixing and volume change. Volume change of samples can be mainly attributed to the fact that high content of bentonite slurry increases the water and decreases the content of calcium sulfate dihydrate in LGPs, which lead to overlapping difficulties of calcium sulfate dihydrate crystals, finally resulting in volume shrinkage for forming the skeleton during drying process. Fig. 5 shows the typical appearance of dried LGPs at 40 °C, which stems from samples in Table 3. For instance, the bentonite content ranged from 0% to 30% and 40% when W/G was 0.8 (W0.8B0, W0.8B10, W0.8B20 and W0.8B30) and 0.6 (W0.6B0, W0.6B10, W0.6B20, W0.6B30 and

Fig. 4. Bentonite slurries with different bentonite-to-water ratios.

W0.6B40), respectively, additionally, bentonite dosages from 20% to 50% at W/G 0.4 (W0.4B20, W0.4B30, W0.4B40 and W0.4B50) and 40% to 50% at W/G 0.2 (W0.2B40 and W0.2B50) were suitable for the preparation of LGPs, while only a bentonite proportion of 70% at W/G 0.0 (W0B70) could be used to fabricate LGP. The density of these samples is shown in Fig. 6. The density data indicated that the dry density of LGPs was variable at W/G of 0.8 (991 kg/m3 to 752 kg/m3), 0.4 (1218 kg/m3 to 845 kg/m3), 0.2 (1206 kg/m3 to 1075 kg/m3) and 0 (1308 kg/m3 to 1019 kg/m3). Besides, after a continuous decrease of dry density caused by increased bentonite slurry, dry density of LGPs increased sharply, due to the considerable volume change (Fig. 5) under a high content of bentonite slurry, such as LGP with bentonite slurry of 60% at W/G of 0.4. However, a W/G of 0.6 generated the largest density variation (from 1196 kg/m3 to 770 kg/m3), while the volume change occurred at bentonite slurry contents of 50%; therefore, the bentonite contents between 0% and 40% and W/G of 0.6 were suitable for preparing the LGPs, resulting in a dry density of 1196 to 784 kg/m3. Compared with other methods, such as, substitution of lightweight aggregates with low gravity for matrix and inducing

364

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

Table 3 Typical characteristics of lightweight gypsum plasters made from different water-to-gypsum ratios. W/G

0.8 0.6 0.4 0.2 0

Volume percentage of bentonite slurry per m3 (%) 0% p

10% p

20% p

30% p

40%

50%

60%

70%

80%

(W0.8B30) p

S (W0.8B40) p

(W0.6B20) p

(W0.6B30) p

(W0.6B40) p

S (W0.8B50) S (W0.6B50) p

(W0.4B20) N (W0.2B20) N (W0B20)

(W0.4B30) N (W0.2B30) N (W0B30)

(W0.4B40) p

(W0.4B50) p

(W0.2B40) N (W0B40)

(W0.2B50) N (W0B50)

S (W0.8B60) S (W0.6B60) S (W0.4B60) S (W0.2B60) N (W0B60)

S (W0.8B70) S (W0.6B70) S (W0.4B70) S (W0.2B70) p

S (W0.8B80) S (W0.6B80) S (W0.4B80) S (W0.2B80) S (W0B80)

(W0.8B0) p

W0.8B10) p

(W0.8B20) p

(W0.6B0) N (W0.4B0) N (W0.2B0) N (W0B0)

(W0.6B10) N (W0.4B10) N (W0.2B10) N (W0B10)

(W0B70)

p

Note: ‘‘N” represents difficulties to mold, ‘‘S” represents shrinkage, ‘‘ ” represents no significant drawback, content in bracket is mix ID of LGPs.

Fig. 5. Typical appearance of lightweight gypsum plasters.

3.2. Pore structure

Fig. 6. Dry density of lightweight gypsum plasters.

the formation of air voids in matrix, adding bentonite slurry into gypsum for LGP preparation can avoid the disadvantages caused by unstable air bubbles and achieve the preparation of LGPs with wide-range density [24,25].

Based on the results described in Section 3.1, a W/G of 0.6 and bentonite slurry content of 0–40% were used to prepare LGPs, as shown in Fig. 7. As the content of bentonite slurry increased from 0% to 40%, the LGP became more ‘‘porous” and the pore sizes within the plasters increased, despite there was no considerable change about solid phase. When bentonite slurry was 40%, the pore size increased significantly, and pore connectivity was enhanced. To analyze the air-void size quantitatively, the pore size distributions were measured by MIP. As shown in Fig. 8, the pore size increased as the LGP dry density decreased. Over the range of 0– 40% bentonite slurry, the dominant size in the pore size distribution curve increased from 1.7 lm to 3.3 lm. In addition, the cumulative pore volume increased as the dry density decreased, indicating that the porosity increased. The distributions corresponding to each LGP are quantified and compared by determining the volume of pores under different size ranges, as shown in Fig. 9, most pores were less than 10 lm in diameter. Total pore volume and pore volume in >10 lm, 100 nn-10 lm, 50 nm-100 nm increased from 0.44 mL/g to 0.88 mL/g (total volume), 0.01 mL/g to 0.03 mL/g, 0.43 mL/g to 0.84 mL/g and 0 mL/g to 0.01 mL/g, respectively, with the increasing of bentonite slurry. However, there are few pores below 50 nm. As shown in Fig. 10, the porosity (mercury-intruded pores) and median pore diameter increased

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

365

Fig. 7. Scanning electron micrographs of lightweight gypsum plasters.

Fig. 8. Pore size distribution of lightweight gypsum plasters. Fig. 10. Porosity and median pore size of lightweight gypsum plasters.

from 51.1% to 68.2% and 1.6 lm to 3.0 lm, respectively, at bentonite contents of 0% and 40%, respectively. In general, the pore sizes remained within the micrometer scale. Montmorillonite, the main mineral contained in bentonite, absorbed water and swelled, leading to the introduction of excessive water into the gypsum. However, water cannot be completely consumed by hydration of hemihydrate gypsum, due to W/G of 0.6

and water from bentonite slurry. Rather, some non-hydrated water occupies some capillary spaces, ultimately causing the formation of pores when the water is lost. In addition, with the increase of capillary space due to the introduction of excessive water into the plaster, the overlap of dihydrate gypsum becomes complex and loose, finally forming large capillary pores. Hence, the porosity and pore size increased as the content of pre-swelled bentonite in the LGPs increased. 3.3. Hardened properties

Fig. 9. Effect of bentonite slurry on pore volume of lightweight gypsum plasters.

3.3.1. Mechanical properties Fig. 11 shows the effect of bentonite slurry content on the compressive strength and flexural strength of LGPs. The compressive strength at 1 day, 7 days and 28 days decreased as the bentonite slurry content in the LGP increased from 0 to 40%. In fact, the compressive strength decreases were large: from 9.1 MPa to 2.5 MPa (1-day curing time), from 11.3 MPa to 3.8 MPa (7-day curing) and from 11.7 MPa, to 3.9 MPa (28-day curing). The flexural strength also decreased as the bentonite slurry content in the LGP increased from 0% to 40%, from 3.8 MPa to 1.3 MPa (1-day curing), from 4.6 MPa to 1.9 MPa (7-day curing) and from 4.9 MPa to, 2.1 MPa (28-days curing). Furthermore, both compressive strength and flexural strength increased as curing time increased to 28 days. For instance, the flexural strength of LGPs with 40%, 30%, 20%, 10% bentonite slurry contents increased from 1.3 MPa, 1.9 MPa, 2.5 MPa and 3.3 MPa, respectively, at 1-day curing time to

366

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

Fig. 11. Effect of bentonite slurry content on the mechanical properties of lightweight gypsum plasters.

2.1 MPa, 2.2 MPa, 2.6 MPa and 3.9 MPa at 28-days curing time, equating to increases of 61.5%, 15.7%, 4.0% and 18.2%, respectively. Likewise, the compressive strength of LGPs with 40%, 30%, 20%, 10% bentonite slurry contents increased from 2.5 MPa, 3.8 MPa, 5.5 MPa and 7.0 MPa, respectively, at 1-day curing time to 3.9 MPa, 5.1 MPa, 7.3 MPa and 8.5 MPa at 28-days curing time, equating to increases of 56.0%, 34.2%, 32.7% and 21.4%, respectively. Using data from Fig. 6, the relationships between dry density and compressive strength and flexural strength are shown in Figs. 12 and 13, respectively, indicating that both strengths increased significantly, for example, the compressive strength increased from 3.9 MPa to 11.7 MPa as dry density increased from 784 kg/m3 to 1196 kg/m3, and the flexural strength increased from 2.1 MPa to 4.9 MPa during the 28-day curing. Moreover, as shown in Figs. 12 and 13, because of tiny pore substitution of air-voids, LGPs also exhibited good mechanical performance, compared with other porous gypsum-based materials [44–47], and they all met the requirements of application used as gypsum plaster [48] Hence, the same samples were used in following study of thermal conductivity and humidity controlling. For any given curing time, the decreased strength of LGPs as the content of bentonite slurry increased can be mainly attributed to the increased porosity, additionally, as shown in Figs. 7 and 8, the pore size increased and the overlapping of calcium sulfate dihydrate became difficult with bentonite slurry increasing, finally aggravating the weakening of strength. However, the continuously increasing strength during long curing may be attributed to the completion of gypsum hydration [49]. 3.3.2. Thermal conductivity Fig. 14 indicates the effect of bentonite slurry content on the thermal conductivity of LGPs with W/G of 0.6. As the bentonite slurry content increased from 0% to 40%, the thermal conductivity of LGP decreased, despite only a slight change at 40% bentonite slurry. Specifically, thermal conductivity decreased 18.9% from 0.143 W/m K to 0.116 W/m K as the bentonite slurry content increased from 0 to 40%. Additionally, considering that the plasters were intended for indoor application, the dried LGPs were placed into a controlled-environment chamber at 50 ± 5% RH and a temperature of 20 ± 2 °C until their mass changed less than 0.1%, Fig. 14 shows that the thermal conductivity of these ‘‘indoor” specimens increased to varying degrees (11.5%, 15.2%, 17.0%, 7.9% and

Fig. 12. Effect of density on the compressive strength of lightweight gypsum plasters.

Fig. 13. Effect of density on the flexural strength of lightweight gypsum plasters.

Fig. 14. Effect of bentonite slurry on the thermal conductivity of lightweight gypsum plasters.

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

367

[25,45–47], this is mainly attributed to the tiny pores, capable of reducing the thermal conductivity of air [26] and yielding a longer heat-transfer path [29]. These two main effects result in the low thermal conductivity of LGPs with a given density.

Fig. 15. Effect of dry density on the thermal conductivity of lightweight gypsum plasters.

6.3% for bentonite slurry contents of 0%, 10%, 20%, 30% to 40%, respectively) compared with dried LGPs at the corresponding contents of bentonite slurry, however, they still present good thermal insulation performance and can meet the requirements of ISO 10456 [50]. Additionally, the thermal conductivity decreased from 0.169 W/m K to 0.123 W/m K, while it varied slightly for LGP with 40% of bentonite slurry. The effect of bentonite slurry on thermal conductivity of LGP is highly related to the pore structure and density of LGPs. Fig. 15 indicates that increased porosity (decreased density) decreased the thermal conductivity, from 0.143 W/m K to 0.116 W/m K and from 0.169 W/m K to 0.123 W/m K for dried LGPs and ‘‘indoor” LGPs, respectively. However, as shown in Fig. 7, high content of bentonite slurry (40%) resulted in enhanced pore connectivity of LGP, it may affect the continuous decrease of thermal conductivity, finally leading to a slight change of thermal conductivity [14]. Water plays an adverse effect on the thermal insulation property of LGP, because of its high thermal conductivity (0.6 W/m K); thus, the moisture absorption by plaster in a humid environment leads to the increased thermal conductivity compared with dried samples [51]. In general, compared with other porous gypsum-based materials, the LGPs maintained low thermal conductivity and exhibited good thermal insulation performance (Fig. 15)

3.3.3. Humidity controlling properties Fig. 16 shows that the humidity controlling ability of LGPs (samples with W/G of 0.6 and bentonite slurry of 10%, 20%, 30% and 40%) was better than that of hardened gypsum (sample of W/G 0.6 without bentonite slurry). Specifically, the absorption/ desorption of moisture increased at various humidities, when the bentonite content increased from 0% to 40%. At 98% RH, the absorption content was 8 mg/g (for bentonite slurry content of 0%) and increased by 296.3% to 31.7 mg/g (for bentonite slurry content of 40%). Under low humidity (54% RH), the increased content of bentonite slurry also contributed to a great increase in moisture absorption, from 2.4 mg/g (gypsum) to 12.2 mg/g (LGP with 40% bentonite slurry), specifically, compared with gypsum, moisture absorption of that LGP (40% bentonite slurry) increased by 408.3%, indicating potential in hindering the occurrence of high humidity in indoor space. In addition, the area of the hysteresis loops increased with the increased content of bentonite slurry,

Fig. 17. X-ray diffraction patterns of raw materials, gypsum and lightweight gypsum plaster.

Fig. 16. Moisture absorption/desorption of lightweight gypsum plasters.

368

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

Fig. 18. Energy-dispersive X-ray spectroscopy analysis of lightweight gypsum plaster.

Fig. 19. N2 adsorption/desorption isotherms of gypsum and lightweight gypsum plasters.

indicating that the humidity hysteresis was enhanced by bentonite, although the phenomenon was not significant overall. The humidity-controlling property of LGPs is highly related to the state of bentonite and the pore structure of LGPs. As shown in Fig. 17, the absence of a peak and the sharp ‘‘001” peaks corresponded to LGP and montmorillonite, respectively, indicating that montmorillonite in LGP existed as disordered single layers or an agglomeration of only a few layers, in contrast to its form in the bentonite itself [52,53]. A high-magnification SEM and EDX of the microstructure comprising the solid phase was used to confirm the results of XRD. EDX was firstly used to confirm the position of montmorillonite layers because montmorillonite typically contained Mg (Fig. 3), and then SEM was used to show the status of these layers in LGP, as shown in Fig. 18, pre-saturated bentonite consisted of layers that persisted on the walls of pores in LGP with 40% bentonite slurry, and the rearrangement of these layers into stacks as originally oriented in the bentonite was prevented in the dried LGP, this could be attributed to the layers segregation by dihydrate gypsum crystals. Under drying treatment, these layers adhered to and lay on the dihydrates (SEM in Fig. 18), such that more hydroxyl in montmorillonite layer surface was exposed to pores. Besides, the increase of bentonite slurry led to the increased porosity and formation of interconnected pore systems (Figs. 7 and 10), enhancing water vapor migration from outside to inner. These

factors enhanced moisture absorption and ultimately improved the humidity-controlling property of the LGPs. Moreover, the voids of internal layers in montmorillonite are micropores that are mainly responsible for contributing to the moisture absorption. Therefore, the humidity-controlling ability of LGPs was gradually enhanced as the content of bentonite slurry increased. As shown in Fig. 19, according to the classification system of the International Union of Pure and Applied Chemistry, the nitrogen adsorption–desorption isotherms of LGPs and gypsum are in accordance with typical Type II curves, indicating that LGPs and gypsum are mainly comprised of macropores, which also can be confirmed by MIP results (Fig. 8) [54]. However, LGPs also show more significant hysteresis loops compared with gypsum, indicating that LGPs consist of a few layer-like pores; these pores stem from the internal layer voids of montmorillonite and contribute to broadened hysteresis loops of moisture absorption/desorption [55], because hysteresis is associated with capillary condensation caused by micropores and mesopores inside samples. When the samples, which have completed the adsorption process, start to release water vapor to surrounding, the narrow pores (layer-like pores) blocked with condensed water lead to that the adsorbed water molecules can’t evaporate outward and then hysteresis occurs, thus it can be enhanced because of the increase of narrow pore when bentonite slurry increases in gypsum paste [55]. Due to the low proportions of bentonite used in LGPs in this study, the humidity hysteresis was not considerable, and the humidity-controlling ability of LGPs was finally enhanced. 4. Conclusions In this study, porous LGPs were successfully prepared by mixing pre-swelled bentonite slurry into gypsum paste. Owing to their mechanical, thermal-insulation and humidity controlling properties, LGPs obtained in this study have significant potential in the application of building energy conservation. (1) Suitable B/W of bentonite slurry and W/G of gypsum paste can be used to overcome volume variation and instability of fresh LGPs, finally resulting in successful preparation of LGPs with wide range of density. (2) Adding pre-swelled bentonite slurry into gypsum paste caused the increase of micron-size pore and size, due to presence of plenty of water in pre-swelled bentonite. Porosity of LGPs increased from 51.1% to 68.2% and median pore diameter increased from 1.6 lm to 3.0 lm as the content of bentonite increased from 0% to 40%.

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

(3) The decreasing of mechanical strength in LGPs didn’t occur during 28-day curing, indicating the possibility of good durability. Besides, the low thermal conductivity of LGPs in 50% RH environment indicated good thermal insulation performance. Specifically, compressive strength, flexural strength and dry density of LGPs were 3.9–11.7 MPa, 2.1– 3.9 MPa and 784–1196 kg/m3, respectively, which met the requirements of standard of gypsum plaster. Due to a high content of tiny pore, thermal conductivities were low, 0.123–0.169 W/m K in a 50% RH environment and 0.116– 0.143 W/m K in a dry environment, which could meet the requirements of energy conservation design. (4) The humidity-controlling ability of LGPs is enhanced by introducing bentonite slurry into gypsum paste and humidity hysteresis is not significant, the absorption of moisture increased by 296.3% in a 98% RH environment and by 408.3% in a 54% RH environment, respectively, when the bentonite content was 40%, showing the considerable potential in adjusting humidity of indoor environment. Declarations of interest None. Acknowledgement This work was supported by the National Key Research and Development Plan (2016YFC0701004), Science and Technology Projects of Sichuan Province (2018GZ0152,18YYJC0904); The first author would like to thank the Southwest University of Science and Technology for providing the scholarship for his research in UK. References [1] O.F. Osanyintola, C.J. Simonson, Moisture buffering capacity of hygroscopic building materials: experimental facilities and energy impact, Energy Build. 38 (2006) 1270–1282. [2] M. Woloszyn, T. Kalamees, M.O. Abadie, M. Steeman, A.S. Kalagasidis, The effect of combining a relative–humidity–sensitive ventilation system with the moisture–buffering capacity of materials on indoor climate and energy efficiency of buildings, Build. Environ. 44 (2009) 515–524. [3] Indoor Air Quality in Highly Energy Efficient Homes–a Review. Available online: https://www.thenbs.com/PublicationIndex/Documents/Details? Pub=NHBCFOUNDATION&DocId=294742 [4] J. Mlakar, J. Štrancar, Overheating in residential passive house: solution strategies revealed and confirmed through data analysis and simulations, Energy Build. 43 (2011) 1443–1451. [5] Z. Rao, S. Wang, Z. Zhang, Energy saving latent heat storage and environmental friendly humidity–controlled materials for indoor climate, Renew. Sust. Energy Rev. 16 (2012) 3136–3145. [6] X. Zhang, S.L. Wang, Energy consumption research and energy saving potential analysis for an office building Air Conditioning System. In: First international conference on building energy and environment, Dalian, China, 2008. [7] H. Yang, Z. Peng, Y. Zhou, F. Zhao, J. Zhang, X. Cao, Z. Hu, Preparation and performances of a novel intelligent humidity control composite material, Energy Build. 43 (2011) 386–392. [8] X. Huang, X. Chen, A. Li, D. Atinafu, H. Gao, W. Dong, G. Wang, Review shape– stabilized phase change materials based on porous supports for thermal energy storage application, Chem. Eng. J. 356 (2018) 641–661. [9] S. Naylora, M. Gillotta, T. Laub, A review of occupant–centric building control strategies to reduce building energy use, Renew. Sust. Energy Rev. 96 (2018) 1– 10. [10] G. Pérez, J. Coma, I. Martorell, L.F. Cabeza, Vertical greenery systems (vgs) for energy saving in buildings: a review, Renew. Sust. Energy Rev. 39 (2014) 139– 165. [11] J. Wu, Z. Zhang, Y. Zhang, D. Li, Preparation and characterization of ultra– lightweight foamed geopolymer (UFG) based on fly ash–metakaolin blends, Constr. Build. Mater. 168 (2018) 771–779. [12] Z. Huang, T. Zhang, Z. Wen, Proportioning and characterization of Portland cement–based ultra–lightweight foam concretes, Constr. Build. Mater. 79 (2015) 390–396. [13] Z. Hu, S. Zheng, M. Jia, X. Dong, Z. Sun, Preparation and characterization of novel diatomite/ground calcium carbonate composite humidity control material, Adv. Powder Technol. 28 (2017) 1372–1381.

369

[14] J. Jiang, Z.Y. Lu, Y.H. Niu, J. Li, Y.P. Zhang, Investigation of the properties of high–porosity cement foams based on ternary Portland cement–metakaolin– silica fume blends, Constr. Build. Mater. 107 (2016) 181–190. [15] W. She, Y. Chen, Y. Zhang, M.R. Jones, Characterization and simulation of microstructure and thermal properties of foamed concrete, Constr. Build. Mater. 47 (2013) 1278–1291. [16] T. Gao, T. Ihara, S. Grynning, B.P. Jelle, A.G. Lien, Perspective of aerogel glazings in energy efficient buildings, Build. Environ. 95 (2016) 405–413. [17] Z. Hu, S. Zheng, Y. Tan, M. Jia, Preparation and characterization of diatomite/ silica composite humidity control material by partial alkali dissolution, Mater. Lett. 196 (2017) 234–237. [18] J.C. Gonzlez, M. Molina-Sabio, F. Rodrlguez-Reinoso, Sepiolite–based adsorbents as humidity controller, Appl. Clay Sci. 20 (2001) 111–118. [19] A. Sagae, H. Wami, Y. Arai, H. Kasai, T. Sato, H. Matumoto, Study on a new humidity controlling material using zeolite for building, Stud. Surf. Sci. Catal. 83 (1994) 251–260. [20] L. Cao, H. Yang, Y. Zhou, F. Zhao, P. Xu, Q. Yao, N. Yu, Z. Hu, Z. Peng, A new process for preparation of porous polyacrylamide resins and their humidity control properties, Energy Build. 62 (2013) 590–596. [21] R.X. Magallanes-Rivera, C.A. Juarez-Alvarado, P. Valdez, J.M. Mendoza-Rangel, Modified gypsum compounds: an ecological–economical choice to improve traditional plasters, Constr. Build. Mater. 37 (2012) 591–596. [22] J.C. Rubio-Avalos, A. Manzano-Ramirez, J.G. Luna-Barcenas, J.F. Perez-Robles, E. M. Alonso-Guzman, M.E. Contreras-Garcia, J. Gonzalez-Hernandez, Flexural behaviour and microstructure analysis of a gypsum–SBR composite material, Mater. Lett. 59 (2005) 230–233. [23] C. Martias, Y. Joliff, B. Nait, A new composite based on gypsum matrix and mineral additives: hydration process of the matrix and thermal properties at room temperature, Thermochim. Acta 567 (2013) 15–26. [24] Y. Cheng, Q. Jiang, Y. Zhang, D. Li, Study of preparation and performance of gypsum–based light thermal insulation material, Mater. Rev. 4 (2014) 135– 147. [25] A. Bicer, K. Filiz, Thermal and mechanical properties of gypsum plaster mixed with expanded polystyrene and tragacanth, Therm. Sci. Eng. Prog. 1 (2017) 59– 65. [26] R. Baetens, High performance thermal insulation materials for buildings, in: F. Pacheco-Torgal, M.V. Diamanti, A. Nazari (Eds.), Nanotechnology in EcoEfficient Construction, Woodhead Publishing Limited, Philadelphia, 2013, pp. 188–206. [27] W. Cao, X. Cheng, L. Gong, Y. Li, R. Zhang, H. Zhang, Thermal conductivity of highly porous ceramic foams with different agar concentrations, Mater. Lett. 139 (2015) 66–69. [28] C. Zhu, J. Zhang, W. Yi, W. Cao, J. Peng, J. Liu, Research on degradation mechanisms of recycled building gypsum, Constr. Build. Mater. 173 (2018) 540–549. [29] Y. He, T. Xie, A review of heat transfer models of nanoporous silica aerogel insulation material, Sci. Bull. 60 (2015) 137–163. [30] S. Papatzani, E.G. Badogiannis, K. Paine, The pozzolanic properties of inorganic and organomodified nano–montmorillonite dispersions, Constr. Build. Mater. 167 (2018) 299–316. [31] S.A. Memon, R. Arsalan, S. Khan, T.Y. Lo, Utilization of Pakistani bentonite as partial replacement of cement in concrete, Constr. Build. Mater. 30 (2012) 237–242. [32] J. Mirza, M. Riaz, A. Naseer, F. Rehman, A.N. Khan, Q. Ali, Pakistani bentonite in mortars and concrete as low cost construction material, Appl. Clay Sci. 45 (2009) 220–226. [33] S.M. Opdyke, J.C. Evans, Slag–cement–bentonite slurry walls, J. Geotech. Geoenviron. 131 (2005) 673–681. [34] M. Katsioti, N. Katsiotis, G. Rouni, D. Bakirtzis, M. Loizidou, The effect of bentonite/cement mortar for the stabilization/solidification of sewage sludge containing heavy metals, Cem. Concr. Compos. 30 (2008) 1013–1019. [35] K. Takeshi, A. Ahmed, K. Ugai, Durability of soft clay soil stabilized with recycled Bassanite and furnace cement mixtures, Soils Found. 53 (2013) 155– 165. [36] I. Yilmaz, B. Civelekoglu, Gypsum: an additive for stabilization of swelling clay soils, Appl. Clay Sci. 44 (2009) 166–172. [37] N.K. Ameta, D.G. Purohit, A.S. Wayal, D. Sandeep, Economics of stabilizing bentonite soil with lime–gypsum, EJGE 12 (2007) 1–8. [38] J. Jiang, Z. Lu, J. Li, Y. Xie, K. Luo, Y. Niu, Preparation and properties of nanopore–rich lightweight cement paste based on swelled bentonite, Constr. Build. Mater. 199 (2019) 72–81. [39] M. Benna, N. Kbir-Ariguib, C. Clinard, F. Bergaya, Card–house microstructure of purified sodium montmorillonite gels evidenced by filtration properties at different pH, Prog. Colloid Polym. Sci. 117 (2001) 204–210. [40] O. Tan, A.S. Zaimoglu, S. Hinislioglu, S. Altun, Taguchi approach for optimization of the bleeding on cement–based grouts, Tunnelling. Underground Space Technol. 20 (2005) 167–173. [41] M. Segad, J. Bo, T. Akesson, B. Cabane, Ca/Na montmorillonite: structure, forces and swelling properties, Langmuir 26 (2010) 5782–5790. _ [42] G.E. Morris, M.S. Zbik, Smectite suspension structural behavior, Int. J. Miner. Process. 93 (2009) 20–25. [43] G.J. Ji, P.P. Zhang, G.L. Jiang, Processing and application of bentonite, 2nd.;., Chemical Industry Press, Beijing, China, 2013. [44] A. Colak, Density and strength characteristics of foamed gypsum, Cem. Concr. Compos. 22 (2000) 193–200.

370

J. Jiang et al. / Construction and Building Materials 215 (2019) 360–370

[45] M.S. Basßpınar, E. Kahraman, Modifications in the properties of gypsum construction element via addition of expanded macroporous silica granules, Constr. Build. Mater. 25 (2011) 3327–3333. [46] O. Gencel, J.J.D.C. Diaz, M. Sutcu, F. Koksal, F.A. Rabanal, G. Martinez-Barrera, W. Brostow, Properties of gypsum composites containing vermiculite and polypropylene fibers: Numerical and experimental results, Energy Build. 70 (2014) 135–144. [47] F. Hernández-Olivares, M.R. Bollati, M.D. Rio, B. Parga-Landa, Development of cork–gypsum composites for building applications, Constr. Build. Mater. 13 (1999) 179–186. [48] GB/T 28627–2012, Gypsum plaster, 2012 (in Chinese). [49] A.A. Khalil, A. Tawfik, A.A. Hegazy, M.F. El–Shahat, Effect of some waste additives on the physical and mechanical properties of gypsum plaster composites, Mater. Constr. 63 (2013) 529–537. [50] ISO 10456–2007, Building materials and products–Hygrothermal properties– Tabulated design values and procedures for determining declared and design thermal values, 2007.

[51] J. Liu, S. Xu, Q. Zeng, An investigation of thermal conductivity of cement–based composites with multi–scale micromechanical method, J. Build. Mater. 21 (2018) 293–298. [52] L.H. Fu, T.H. Cao, Z.W. Lei, H. Chen, Y.G. Shi, C. Xu, Superabsorbent nanocomposite based on methyl acrylic acid–modified bentonite and sodium polyacrylate: fabrication, structure and water uptake, Mater. Des. 94 (2016) 322–329. [53] G. Zhang, Y. Ke, J. He, M. Qin, H. Shen, S. Lu, J. Xu, Effects of organo–modified montmorillonite on the tribology performance of bismaleimide–based nanocomposites, Mater. Des. 86 (2015) 138–145. [54] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguezreinoso, J. Rouquerol, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (iupac technical report), Pure Appl. Chem. 38 (2016) 1051–1069. [55] L. Zhang, L. Yang, B.P. Jelle, Y. Wang, A. Gustavsen, Hygrothermal properties of compressed earthen bricks, Constr. Build. Mater. 162 (2018) 576–583.