Thermal and Waterproof Properties of Foamed Concrete with Nano SiO 2 Aerogel and Organosilicon Waterproofing Agent

This paper aimed to improve the thermal and waterproof properties of foamed concrete through the synergic work of nano SiO 2 aerogel powder (NSAP) and organosilicon waterproofing agent (OWA). Hence, several series of foamed concrete with a 0–3.0% of OWA and with 1–4% of NSAP addition were developed. The results show that OWA can decrease the dry bulk density, compressive strength, water absorption rate, and thermal conductivity. With the addition of NSAP, the water absorption rate reached a minimum value of 9.2% at 3.0% NSAP, while the thermal conductivity continued decreasing. Through the microscopic pore size distribution, XRD, and SEM, it was found that the addition of NSAP had no effect on the type of hydration product, but it can increase the average pore size and the amount of macropore and weaken the interfacial adhesion between hydration products. Comparing the heat transmission model, the presence of NSAP increased the heat transmission pathway and resistance, resulting in lower thermal conductivity.


Introduction
Foamed concrete is a lightweight porous material prepared by introducing premanufactured aqueous foam into the cement or mortar paste [1], with light weight, good thermal insulation and sound insulation performance, re resistance, low cost, and recycling waste [2,3]. However, the foamed concretes generally show a poor thermal conductivity, low mechanical strength, and high water absorption rate, which greatly restricts its application in construction industries; thus, a low-density foamed concrete with excellent properties is di cult to prepare [4,5]. As foamed concrete or other porous materials are easily damaged by water hydrolysis, it can be blended with nanoaerogel or nanomaterials to solve the problem [6][7][8].
Kistler discovered aerogels 80 years ago [9] by complex synthesis in supercritical drying conditions. Depending on the silica source and the preparation process, aerogels have signi cant physical, thermal, optical, and acoustic properties [10]. To overcome the problems of foamed concrete, some experiments [11,12] were attempted to enhance the thermal transfer and moisture resistances of foamed concrete by merging with aerogel technologies. e thermal conductivity of foam concrete with hydrophobic aerogel is 0.08 W/m·K approximately, which corresponds to be 30-50% for conventional foam concrete [13]. And the waterproof capacity of foam concrete with hydrophobic aerogel was signi cantly improved, resulting in exhibiting 75% lower water absorption at an age of 24 hours when compared with the conventional foam concrete [13].
Nano SiO 2 aerogel is a porous and solid material with the lowest thermal conductivity and density. It is widely used in heat insulation, aerospace, environmental protection, new energy, and other elds [14]. In recent years, it has been found that adding nano SiO 2 aerogel powder (NSAP) into the foamed concrete can e ectively reduce the bulk density and thermal conductivity of foamed concrete [15]. e aerogel foamed concrete (AFC) presented a lower thermal conductivity and bulk density than normal foamed concrete. However, the waterproo ng properties of AFC have not been significantly improved with the addition of NSAP, which can damage its thermal insulation performance when the AFC was contacted with water or in a humid environment. Moreover, when the AFC was used in cold or severe cold areas, the strength was hugely reduced due to the alternate freezing and thawing cycles [16], resulting in the damage of the foamed concrete structure.
From the literature, it was concluded that the addition of supplementary cementitious materials and fibers could significantly improve the mechanical properties. e waterproofing agents can improve the water resistance, and the NSAP can reduce its bulk density. However, the addition of NSAP fillers simultaneously with waterproofing agents in foamed concrete to reduce their thermal conductivity and improve their water resistance has not been engaging in the literature, which restricts its application in the construction industry. erefore, more attention should be paid to determine the synergic work between NSAP and waterproof materials on the thermal insulation performance and water resistance of foamed concrete. In this study, the influence of NSAP and organosilicon waterproofing agents on the thermal and waterproofing properties of AFC was studied.

Materials.
e ordinary Portland cement (PC) used in this study was P·O 42.5R conforming to the Chinese Standard for ordinary Portland cement. e fly ash (FA) used in this study was grade Ӏ FA, which conforms to the Chinese Standard for fly ash used for cement and concrete. e physical and chemical properties of PC and FA are given in Table 1. e density of PC and FA is 3080 kg/m 3 and 2340 kg/ m 3 , respectively. NSAP is white and translucent produced by Suzhou Rexiang Nano Technology Co., Ltd., with an average particle size of 15 um, a thermal conductivity of 0.015 W/ (m·K) (25°C), and a density of 0.08 g/cm 3 . Foaming agent is HTQ-1 compound foaming agent produced by Henan Huatai New Material Technology Co., Ltd. e accelerator is a selfmade composite accelerator, and hydroxypropylmethylcellulose (HPMC) is from Shandong Gomez Chemical Co., Ltd. e water reducing agent is polycarboxylic acid superplasticizer (SP) with a water reducing rate of 35% and a solid content of 50%. Organosilicon waterproofing agent (OWA) was from Wacker Chemical Co., Ltd.

Preparation of Specimens.
First, the specimens with OWA at 0%, 0.1%, 0.5%, 1.0%, and 3.0% of the scale specimens were studied. en, the specimens with OWA at 0.5% while NSAP ranging from 1.0% to 4.0% were also studied. e wet density of all AFC pastes was controlled from 280 kg/m3 to 300 kg/m 3 . e mix designs of AFC are shown in Table 2. In order to ensure good performance of aerogel foam concrete, 10wt% fly ash is added in the mixture.

Mixing Procedure.
e mixing procedures of AFC were as follows. Firstly, the PC, FA, HPMC, accelerator, OWA, and NSAP, according to the mix proportions in Table 2, were uniformly dry-mixed for 2-3 min. en, the tap water and SP were added to the premixed powders and mixed for 3 min. Finally, the preformed foam was added to the paste and mixed for 3 min. Additionally, the following specimens were cast to determine the mechanical properties: the cubic specimens 100 × 100 × 100 mm 3 for testing the compressive strength and the dry bulk density. In addition, prismatic specimen 300 × 300 × 30 mm 3 was used to test the thermal conductivity. All the specimens were cast into molds and covered with cling film to prevent water evaporation. After 72 h, the specimens were demolded and kept in a standard curing room (20°C, 100% relative humidity) for 28 days. Before tests, each specimen was dried in an oven at 60°C to a constant weight and cooled to room temperature.

Test Procedures.
e 28 days compressive strength of the AFC was determined by a sensitive multifunctional testing system machine with a 200 KN capacity. e bulk density of the AFC was calculated by dividing the dry mass of the AFC to its volume (100 × 100 × 100 mm 3 ) at the age of 28 days. e water absorption rate was calculated based on the difference in the mass before and after immersion in water for 24 h. e thermal conductivity (k) of AFC was measured in accordance with ASTM C518 Standard Test Method for Steady-State ermal Transmission Properties by means of the Heat Flow Meter Apparatus on HFM 436 by NETZSCH. e pore structure of the AFC was measured by the specimen of 100 mm × 100 mm × 100 mm cut in half, and the section was the surface to be measured. DJCK-2 crack width meter manufactured by Jingmao Instrument was used to photograph the structure of the cut face. e size of the section was 11 × 9 mm, and the magnification was 60 times. Finally, Namo Measurer software was used for statistical analysis of pore structure parameters. e micromorphology of AFC was observed by scanning electron microscope (SEM). Due to the poor conductivity of the sample, the sample was sprayed with gold before testing. e phase composition of AFC was analyzed by Powder X-ray Diffraction analyzer (XRD, Shimadzu DX-6100).

Influence of OWA Content on Fundamental Properties.
e fundamental properties of the AFC with different OWA contents are presented in Figure 1. It was observed that with the increasing dosage of OWA from 0.1% to 3.0%, the dry bulk density and compressive strength were significantly decreased. is is because the hydrophobic cement particles adsorb on the gas-liquid interface of the bubble, which could prevent the bubble from further growing and merging [17] and delay the hydration reaction, resulting in reduced compressive strength of specimens [18,19]. Figure 2(a), with the increase in OWA content from 0.1% to 3%, the water absorption rate of AFC was significantly decreased from 16.9% to 9.2%. As shown in Figure 2(b), the thermal conductivity of AFC was obviously reduced from 0.104 W/(m·k) to 0.073 W/ (m·k). It was seen that with the content of OWA more than 0.5%, the decline tendency of water absorption rate and thermal conductivity became much less obvious. e results indicated that the optimum content of OWA is 0.5% for improving the waterproofing properties of AFC. According to the result of Hai-li [20], the aerogel is effective in enhancing the moisture resistance of foam concrete and forming hydrophobicity on the interfacial substrate of air pores.

Influence of OWA Content on Water Absorption Rate and ermal Conductivity. As shown in
When OWA content was kept as 0.5%, the bulk density, compressive strength, water absorption rate, and thermal conductivity of the AFC were 300 kg/m 3 , 0.6 MPa, 12%, and 0.08 W/(m·k), respectively. e OWA content was kept constant as 0.5% in the following mix proportion. Figure 3 presents the influence of NSAP content on water absorption rate and thermal insulation of AFC. It can be seen from Figure 3(a) that with the addition of NSAP from 0% to 4.0%, the water absorption rate first slightly decreased and then significantly increased, reaching a minimum value of 9.2% at 3.0% NSAP. is is because with the increase in the content of small particle NSAP, the consistency and viscosity of foamed concrete slurry increase, resulting in a decrease in the number of connected pores. On the other hand, the microaggregate effect of NSAP reduces the number of capillary pores in the cement paste of foamed concrete [21,22]. Once when the content of NSAP exceeded 3.0%, the reduction of C-S-H amount in hydration products led to an increase in a number of connected pores. As shown in Figure 3(b), with the increasing content of NSAP from 0% to 4.0%, the thermal conductivity was decreased from 0.074 W/(m·K) to 0.055 W/ (m·K). e reasons for the decrease in thermal conductivity are as follows. Firstly, the NSAP with high void fraction decreased the heat transmission route in AFC. Secondly, the thermal conductivity of NSAP is 0.019 W/(m·K), which is lower than the thermal conductivity of air (0.024 W/(m·K)) [23]. irdly, concrete materials will become rougher, more loose, and  Advances in Materials Science and Engineering porous when NSAP particles are used [24]. us, the thermal conductivity was decreased.

Pore Structure Analysis.
e microstructure photographs and pore size distribution of AFC-2 and AFC-7 cross section are shown in Figure 4. It was seen that the AFC-2 sample has a small pore size and thick pore wall, while the AFC-7 has a larger average pore size and more macropore amount. It indicated that the addition of NSAP could significantly increase the average pore size and macropore amount of specimens due to the poor combination of NSAP and cement pastes. e results also can explain why the thermal conductivity was decreased [25].

XRD.
e X-ray diffraction analysis ( Figure 5) proved that the CH (PDF 84-1269) and ettringite (PDF 72-1907) are the main crystalline compounds in all the specimens with or without NSAP. It was also seen that at the peak of 29 degree, C-S-H existed in both AFC-2 and AFC-7.
ese results indicated that the main hydration products of AFC are CH, C-S-H, and ettringite. Moreover, the addition of NSAP almost had no effect on the hydration products of foamed concrete.

SEM.
e SEM images and EDS results of the specimens are shown in Figure 6. As shown in Figures 6(a) and 6(b), the hydration products of AFC-2 and AFC-7 were CH, C-S-H, and ettringite, which further proved the results of XRD. It is seen from Figure 6(b) that there are a large number of NSAP on the surface of AFC hydration product. e EDS results ( Figure 7 and the detection position is marked with the red box in Figure 6) of AFC-2 andAFC-7 samples also proved that the particles covering the surface of the hydration product were NSAP. By comparing the SEM images of AFC-2 and AFC-7, it was obvious that the presence of NSAP on the hydration products can weaken the interfacial adhesion between hydration products [26].

Heat Transmission Model.
e heat transmission model of foamed concrete with or without NSAP is presented in Figure 8. It was observed that due to the presence of NSAP, the heat transmission pathway was significantly increased, resulting in more time to complete the heat transfer process [27,28]. According to equation (1) [29], under the same total heat, material thickness, temperature difference, and heat transmission area, the longer the heat transmission time, the lower the thermal conductivity. Furthermore, as reported in the literature, the thermal conductivity of gas is much lower than that of solid [28]. When the heat was transferred to the surface of the NSAP, the resistance transmitted through the pores of NSAP increased sharply owing to the NSAP being filled with medium gas. erefore, the AFC has a lower thermal conductivity compared with ordinary foamed concrete.
where λ is the thermal conductivity, Q is the total heat, a is the material thickness, A is the heat transmission area, T 2 − T 1 is the temperature difference, and t is the heat transmission time.

Conclusion
(1) As OWA content increased, the dry bulk density, compressive strength, water absorption rate, and thermal conductivity of the foamed concrete were decreased. e foam concrete with 3.0% OWA has the best waterproof and thermal insulation performance. (2) As NSAP content increased, the water absorption rate firstly decreased and then increased while the thermal conductivity was decreased. e AFC with 3.0% NASP not only has low thermal conductivity but also has the lowest water absorption. (3) e hydration product of AFC was CH, C-S-H, and ettringite. Moreover, the addition of NSAP almost had no effect on the type of hydration product. However, the average pore size and the amount of macropore were significantly increased, and the interfacial adhesion between hydration products was weakened. (4) In comparison to the heat transmission model, the presence of NSAP could significantly increase the heat transmission pathway and resistance, resulting in the reduction of thermal conductivity.
Data Availability e data are generated from experiments and can be available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.