Preparation of Novel Core-shell Non-sintered Lightweight Aggregate and Its Application in Wallboard for Better Properties

PANG Chaoming, ZHANG Chunpeng, MENG Xinxin,2, PAN Jinlong

(1. School of Material Science and Engineering, Southeast University; Jiangsu Key Laboratory of Construction Materials, Nanjing 211189,China; 2. Commercial Aircraft Corporation, Shanghai Aircraft Manufacturing Co., Ltd., Shanghai 200436, China; 3. School of Civil Engineering, Southeast University; Key Laboratory of Concrete and Prestressed Concrete Structure of the Ministry of Education, Nanjing 211189,China)

Abstract: Due to the relatively high density of conventional non-sintered lightweight aggregate(NLA),a low-density core-shell NLA(CNLA) was developed. Moreover, two types of porous lightweight aggregate concrete (PLAC) for wallboard were designed, using both foam and lightweight aggregates. The effects of LA on lightweight concrete workability, compressive strength, dry shrinkage, and thermal conductivity were studied and compared. The bulk density of CNLA can be lowered to 500 kg/m3, and its cylinder crushing strength is 1.6 MPa. PLACs also have compressive strengths ranging from 7.8 to 11.8 MPa, as well as thermal conductivity coefficients ranging from 0.193 to 0.219 W/(m·K-1). The CNLA bonds better to the paste matrix at the interface transition zone, and CNLA concrete has a superior pore structure than SLA concrete, resulting in a 20%improvement in fluidity, a 10% increase in strength, a 6% reduction in heat conductivity, and an 11% decrease in drying shrinkage.

Key words: core-shell non-sintered lightweight aggregate(CNLA); porous lightweight aggregate concrete(PLAC); low density; thermal insulation; drying shrinkage; pore structure

1 Introduction

Thermally insulating lightweight materials have become more prevalent in modern construction and architecture. Lightweight aggregate concrete (LAC) is a type of concrete that comprises lightweight aggregate(LA) and has a density of less than 1 900 kg/m3, which is much less than the density of standard concrete[1].Additionally, LAC has lower thermal conductivity than conventional concrete due to its higher porosity[2]. Recent research has focused on decreasing the density of LAC while increasing its thermal insulation properties.Techniques include increasing the LA content[3]and using ultralight porous LA[4]. Ke[5]and Gao[6]found that a significant impediment to casting concrete with ultralight LA was that the aggregate floated. If the aggregate density is too high or too low, the aggregate will sink or float. To prevent aggregate from sinking or floating, the apparent density of LA should match the cementitious matrix’s dry density.

Foam concrete is another type of lightweight concrete in which an appropriate foaming agent is used to create foam in the cement paste[7,8]. Foam concrete has low apparent density and low thermal conductivity;however, it suffers from low strength, high dry shrinkage, low crack resistance and is easily absorbed by water[9,10]. Additionally, foam concrete’s thermal insulation would degrade significantly as a result of water absorption.

Porous lightweight aggregate concrete (PLAC)can be prepared by using LA and introducing large numbers of micropores into the concrete. It shrinks less and cracks less than foam concrete, and it is less dense and more insulating than LA concrete. PLAC has a reasonable apparent density, a high strength, good thermal insulation[11,12,13,14]and excellent sound absorption[13,15].SLA or NLA, crushed waste clay bricks[15], or light aerated concrete waste[14]can all be used as PLAC aggregates. Micropores can be introduced by using a foaming agent[12,14,15]or created by an air-entraining agent[13].

Commercial LAs made primarily of clay are sintered at approximately 1 200 ℃, however sintering requires a significant amount of energy. NLA production,which is produced primarily from industrial wastes and cured at room temperature or less than 100 ℃, decreases clay consumption, saves energy, and reduces emissions. Norliaet al[16]used ordinary Portland cement and a foaming agent to prepare a lightweight aggregate with a loose bulk density of 813 kg/m³ and compressive strength of 7.83 MPa. Penget al[17]created NLA from dredging sediment with a bulk density of 850 kg/m³ and compressive strength of 5 MPa. They added a pore-forming agent to the NLA[16]to decrease density and improve sound absorption, but the bulk density remained at approximately 850 kg/m3. Although various measures have been taken to reduce the density of NLA, the bulk density of NLA is generally greater than 800 kg/m³. Frankovicet alproduced LAC using 100%NLA and found that it had a dry density of 1 490 kg/m3and thermal conductivity of 0.73 W/m/K, respectively,which are 35% and 46% less than those of conventional concrete[18]. Due to NLA’s increased density and thermal conductivity when compared to SLA, its use in lightweight concrete is limited, and as a result, there is less research interest in NLA. To make NLA economically viable, its bulk density must be reduced and its thermal insulation characteristics improved.

To further reduce the density and improve the thermal insulation of NLA, a core-shell non-sintered lightweight aggregate (CNLA) with relatively low bulk density ranging from 500 to750 kg/m3has recently been developed[19]. Our exhaustive analysis of the literature revealed only a single study of low-density CNLA by Feras[20]. Feras produced CNLA with bulk densities ranging from 510 to 650 kg/m3and crushing strengths ranging from 1.0 to 2.5 MPa. The CNLA was composed of cores of expanded perlite particles surrounded by an outer shell of perlite powders mixed with fly ash and cement[21]. When the surface layer was recoated with cement and silica fume, the compressive strength of the CNLA with a bulk density of 608 kg/m3increased to 3.55 MPa[22]. Also, Shang developed a new CNLA with a wet density of 1 340 kg/m3and crushing strength in a cylinder of 2.46 MPa, which is formed of an inorganic fly ash-cement shell and an organic sodium alginate core[23].

Most studies on NLA have focused on the reuse of industrial waste, neglecting the improvement of its own properties and the enhancement of LAC. In this study, a novel low density CNLA was prepared and studied, which has spherical expanded polystyrene(EPS) as the core, surrounded by an outer shell layer consisting of cementitious material as the shell layer.EPS can significantly lower the density and water absorption of the LA while simultaneously increasing its thermal insulation properties. Cementitious materials can reinforce LA and increase its compatibility with concrete. Furthermore, two sets of samples of PLAC were prepared using either core-shell CNLA or SLA.The two sets of samples were analyzed and compared in terms of their characteristics and properties. This work aims to develop a method for producing low density, very cheap NLA and to demonstrate its application in PLAC.

2 Experimental

2.1 Raw Materials

In this investigation, Portland cement P.II 42.5R was used for NLA and PLAC. The cement, produced by Jiangnan Xiaoyetian Cement Co., Ltd, has a specific surface area of 362 m2/kg. Additionally, class 1 fly ash(FA) with a 95% water requirement and 1.5% loss on ignition was used for NLA and PLAC. FA was produced by Nanjing China Resources Thermal Powder Co.,Ltd.The chemical composition of the cement and fly ash was determined by X-ray fluorescence, as shown in Table 1.

To promote the activity of fly ash, Lime with a 60% effective CaO content was used. Lime was produced by Guangde United Calcium Industry Co.,Ltd.

A commercial vegetable protein foaming agent was used to create micropores for PLAC which was produced by Changzhou Merit Building Materials Co.Ltd. The foaming agent was dissolved in water(1:30), and the foam was produced at high pressure;foam properties are shown in Table 2.

Table 1 Cement composition determined by X-ray fluorescence/wt%

Table 2 Foam properties

The SLA with particle size 5-10 mm was used,and it had a water absorption of 12.5%, an apparent density of 967 kg/m³, a bulk density of 498 kg/m³, and a cylinder compression strength of 1.5 MPa.

2.2 Specimen preparation and mixture proportion

CNLA was made using a core-shell structure that included spherical EPS particles as the core and a cementitious material surrounded by peripheral as the shell. The EPS surface was sprayed with 15% water-glass solution for interfacial treatment because of the surface hydrophobicity of EPS. The cementitious material consists of cement, fly ash and lime. After carefully mixing all of the powder in specified proportions, the well-mixed powder and processed EPS were combined in a pelletizer at a tilt angle of 45°. The pelletizer machine was turned on and rotated at a speed of 25-30 r/min while 15% to 25% of water was sprayed gently over the EPS ball, which grew gradually enveloped in the powder. When the powder wrapping thickness was up to 1.0-1.5 mm, the sample was removed and then placed in a standard curing room with a temperature of (20±2) ℃ and relative humidity of 95% for more than 28 days, yielding the product as shown in Fig.1.

PLAC was prepared using prefabricated foam as follows. The lightweight aggregate was presoaked for 1 h (SLA) or 15 min (CNLA); the aggregates absorb water at different rates. Cement, water (reduced by the quantity of water used for foaming), water reducer, and presoaked LA were steadily mixed for 2 min. The prefabricated foam was added to the mixer, and the fresh PLAC was mixed for 2 min. During this stage, the difference between the actual wet apparent density and the theoretical wet apparent density was controlled to be ＜50 kg/m3. The fresh PLAC was put into the molds.The specimens were demolded after curing for 24 h in the atmosphere and placed in a standard curing room with a temperature of (20±2) ℃ and relative humidity of 95% for specified ages, depending on the subsequent testing.

Fig.1 Core-shell non-sintered LA: (a) pelletizer machine; (b) diagram of core-shell; (c) photo of EPS core and fly ash-cement shell; (d) photo of core-shell non-sintered LA.

2.3 Test methods

The cylinder crushing strength was used to assess the load bearing capacity of LA according to Chinese Standard GB/T 17431.1-2010. The sample was loaded into a steel cylinder with an inner diameter of 115 mm and a height of 145 mm, vibrated densely and scraped flat, then pressed with an indenter with an outer diameter of 113 mm at a speed of 300-500 N/s. The load is recorded when the height of the sample is reduced by 20 mm, and the load is divided by the compressive area of 10 000 mm2to obtain the cylinder compressive strength. Take the average value of the three measurements.

A conventional concrete slump test using a truncated cone with a size of 100 mm at the upper bottom,200 mm at the lower bottom, and 300 mm height, was used to characterize workability; the slump cone was only once filled up without tamping because of high fluidity of concrete mixtures.

For each group of PLAC, three cubes with sides of 100 mm were used to assess compressive strength.According to Chinese Standard GB/T 50081-2019, a loading force of 3-5 kN/s was employed.

Drying shrinkage of PLAC was measured according to Chinese Standard GB/T 50082-2009 with prisms of size 100 mm×100 mm×400 mm. After curing for 3 d in a standard curing room at a temperature of (20±2) ℃and relative humidity of 95%, the specimens were moved into a dry curing room at a temperature of (20±2)℃ and relative humidity of 65±5%.

The thermal conductivity of PLAC was measured using two specimen prisms of 300 mm × 300 mm × 30 mm, based on steady-state heat transfer method according to Chinese National Standard GB/T 10294-2008.The heated side was set to 35 ℃, while the chilly side was set to 15 ℃. Before heating, the equilibrium time was set to 2 hours.

An Autopore IV 9500 automated mercury intrusion porosimeter (manufactured by American Micromeritics Instrument Co.,Ltd) was used to determine the pore structure of the PLAC matrix. The measurement size ranged from 3.6 nm to 360 nm, with a 20-second balance time.

Using X-ray computed tomography (X-CT) developed by German YXLON Co.,Ltd., the large pore structure of a specimen prism 50 mm×50× mm50 mm was estimated. The X-ray voltage and current were 80 kV and 0.20 mA, respectively, with a detector resolution of 62 m/pixel and a resolution of 1 024×1 024 pixels.

3 Results and discussion

3.1 Properties of CNLA

CNLA is an excellent lightweight aggregate with qualities that allow you to modify the core size and adjust shell performance to match design specifications.The cylinder crushing strength of CNLA with a diameter of 4-6 mm as the core and a cementitious material with a thickness of around 3.0 mm as the shell was in the range of 2.5-7.0 MPa when the density was in the range 580-720 kg/m3[24]. The properties of CNLA,at the same density or strength, greatly exceed those of the CNLA produced by Feras, who used expanded perlite particles as the inner core[22]. The shell thickness of CNLA was controlled to roughly 1.0-1.5 mm in this experiment, with a fixed cement dosage of 40%, fly ash dosage of 54%, and lime dosage of 6% (approximately 10% of the total content of fly ash and lime), and its performance was compared to that of SLA, as shown in Table 3.

The thickness of the CNLA shell layer has a significant impact on its performance. When CNLA’s shell thickness is around 3.0 mm, its bulk density with a particle size of 4.75-9.5 mm is 651 kg/m3and its cylinder crushing strength is 6.12 MPa[25]. However, if the shell thickness is furthermore decreased to 1.0-1.5 mm, the loose bulk density of CNLA is reduced to approximately 500 kg/m³ while the cylinder crushing strength is dramatically reduced to 1.6 MPa.

Naturally, the bulk density of CNLA can be lowered further by increasing the inner core diameter of the EPS spheres. However, the sidewall concrete is typically just 20-30 mm thick. It is necessary to add steel wire mesh in the middle, with the light aggregate particle size less than 10 or 15 mm. As a result, reducing the thickness of the shell layer to around 1.5 mm should be a preferable method for lowering the density of CNLA.

3.2 Mixture proportion and workability of PLAC

PLAC was designed to be used as thermal insulation wallboard, so it had to have a low density, good thermal insulation, and sufficient strength. Low-cost manufacturing and the use of easily available materials were critical. Additionally, each version of PLAC was separated into two moderate dry density grades of 1 000 and 1 200 kg/m3, which were suitable for wallboard use and had desired compressive strengths of 7.5 and 10 MPa, respectively. Adjust the foam contentfrom 35% to 30% of PLAC to change the density level from 1 000 to 1 200. To reduce PLAC shrinkage and production costs, a 20% volume ratio of natural sand with a density of 2 600 kg/m3and a fineness modulus of 2.3 was employed as the fine aggregate. With a fixed 20% FA mass content in the total binder, the mixing proportions of PLAC were calculated to obtain these values (Table 4). In mixtures with the same density, the volume contents of LA were the same, but the mass contents of LA were slightly different due to the difference in densities. PLAC fluidity was ensured using a polycarboxylic acid water reducer.

Table 3 Properties of LA

Table 4 Mix proportions of concrete samples

PLAC slump and slump flow diameter are shown in Table 5.

Table 5 Slump and slump flow diameter of PLAC

The slump tests showed that SLA is unstable and tends to float upward. Thus the slump and slump flow diameter performance of PLAC with SLA cannot be improved by an additional water reducer. Fig.2 shows that neither segregation nor delamination occurred in PLAC with CNLA. Table 5 shows that both the slump and slump flow diameters of PLAC with CNLA were greater than those of PLAC with SLA for density levels 1 000 or 1 200. There are two reasons for this: CNLA is closer to spherical, making it easier to push fresh concrete. Furthermore, even though both LAs were presoaked and excess moisture was removed before use, the SLA became saturated more slowly and continued to absorb water during mixing, reducing the water content of the PLAC matrix, whereas the CNLA is heavily capillary and absorbs water quickly, usually within 10 minutes. As a result, PLAC with SLA had a lower slump and slump flow than that of PLAC with CNLA. Additionally, when concrete containing SLA is subjected to high pumping pressure, a portion of the water enters the interior of the light aggregate, resulting in a poor flow of concrete and making pumping construction difficult[25]. However, due to the rapid absorption of water, the concrete with CNLA will not face the same difficulty.

3.3 Compressive strength

The samples were dried in a 105 ℃ oven after seven days of curing. The density change as a function of drying time is depicted in Fig.2.

After 24 hours, the density of both types of PLAC had decreased significantly. From 24 to 72 hours, the density of PLAC with SLA did not change much,whereas the density of PLAC with CNLA fell progressively. This means that the water release rate from concrete with CNLA is slower than concrete with SLA,i e, the water release rate from CNLA in concrete is also slower than SLA. This is because the outer shell of CNLA is thicker and more numerous than the outer shell of LA, and the pores are predominantly microporous.

Fig.2 Density of PLAC with CNLA and SLA over 5 days

Fig.3 Compressive strength of PLACs

Fig.3 depicts the compressive strength change during PLAC curing for several aggregates and two density levels.

The data for each aggregate-density category are quite similar in Fig.3. Compressive strength grows fast for both types of PLAC from 3 to 7 days, at a rate of 52% for PLAC with SLA and 42% for PLAC with CNLA. Between 14 and 28 days, the compressive strength of both types of PLAC grows gradually: the rate of growth is 15.6% for PLAC with CNLA and only 3% for PLAC with SLA. Internal curing of lightweight aggregates is a well-documented phenomenon[26,27].The released water aid in the continued internal curing of the concrete, resulting in a rise in the compressive strength of PLAC with CNLA from 14 to 28 days, exceeding the compressive strength of PLAC with SLA.PLAC with CNLA performs slightly better compressive strength at each density level.

3.4 Drying shrinkage

Drying shrinkage is a significant issue in the manufacturing of lightweight concrete. Three data points were collected from each sample group, and because the data were little variance, the average of the three data points was calculated. Dry shrinkage during dry curing is illustrated in Fig.4 for both types of PLAC at 1,3, 7, 14, 21, and 28 days.

Fig.4 Drying shrinkage of PLAC using CNLA or SLA

Fig.4 shows the difference in drying shrinkage for two different types of PLAC at two different densities.PLAC has a low drying shrinkage, ranging between 0.07 and 0.12 percent at 28 days. Roslan discovered that the drying shrinkage of foamed concrete with densities of 600, 1 000, and 1 400 kg/m3is typically between 0.1% and 0.35% of the total volume of the hardened matrix[28]. Drying shrinkage of foamed lightweight structural concrete with a density of 1 900 kg/m3ranges from 0.12% to 0.15% at 28 d when glycol is used to reduce shrinkage[29]. These data demonstrate that PLAC outperforms foamed concrete or foamed lightweight structural concrete in terms of drying shrinkage.

Because the 1 000 density level concrete contains more foam, the drying shrinkage of the 1 200 density level LAC is significantly smaller than that of the 1 000 density level concrete. As a result, the low-density PLAC contains larger interior pores, resulting in an increased water absorption loss. The drying shrinkage of PLAC with CNLA is smaller than that of PLAC with SLA at each density level. Most researchers agree that using presoaked lightweight aggregate as an internal curing agent reduces cementitious composites’ drying shrinkage[30,31]and that increasing the LA content promotes internal curing[32]. Water released from LWA increases hydration and modifies the pore structure[33].CNLA releases more water over a longer period than SLA does due to its greater water absorption and slower moisture release. Additionally, CNLA is also a cement-based material that is compatible with the matrix,implying that it may link more strongly to the matrix and hence resist drying shrinkage.

3.5 Thermal conductivity

Table 6 shows the thermal conductivity of the two types of PLAC.

Table 6 Thermal conductivity of PLAC with CNLA or SLA

All PLAC exhibit good thermal insulation, with thermal conductivity values of less than 0.22 W/(m·K)from Table 6. Tajra reported thermal conductivity values of 0.300-0.317 W/(m·K) for LAC blended at a similar density but with higher strength; it was also more expensive[22]. Jones determined the thermal conductivity of foam concrete with dry densities of 1 000-1 200 kg/m3to be 0.23-0.42 W/(m·K)[34]. Ganesan produced foamed concrete samples with densities in the range of 700 to 1 400 kg/m3, using additives, and thermal conductivity values in the range 0.24-0.74 W/(m·K), with thermal conductivity corresponding to density[35]. In general, PLAC produced by mixing LA and foam into the concrete mix had high thermal insulation.The thermal conductivity of PLAC with a density of 1 200 kg/m3was greater than that of PLAC with a density of 1 000 kg/m3because there were fewer pores.The cement paste was thus more compact in the denser PLAC, and the heat was transferred quickly. The thermal conductivity of PLAC with CNLA was less for PLAC with SLA because the EPS foam balls used in CNLA acted as thermal insulation, resulting in lower thermal conductivity for the composite PLAC with CNLA.

3.6 Pore structure

Fig.5 depicts the cumulative porosity and the relationship between pore size and the log differential of the PLAC samples with CNLA and SLA. The pore size distribution and pore diameters of the samples are shown in Table 7.DAverageandDMedianare average pore diameter and median pore diameter, andDProbableis the most probable pore diameter. Fig.6 shows the internal microstructure of the samples obtained by X-CT.

Table 7 shows that the porosity of PLAC with density level 1 200 was much less than that of PLAC with density level 1 000 and that the average pore diameter and the most probable pore size diameter were also less. The table also shows that porosity was very different between the two types of PLAC. The most probable pore size of PLAC with SLA was 77.1 nm and of PLAC with CNLA was 50.4 nm. The average pore size of PLAC with CNLA was also less than that of PLAC with sintered LA.

Table 7 Porosity of PLAC determined by mercury intrusion

Fig.5 Cumulative porosity of samples: (a)pore size; (b) pore size distribution

Fig.6 Two-dimensional X-CT slices of concrete specimens

Pores less than 50 nm diameter had little effect,but pores greater than 50 nm diameter had more damaging effects. Table 7 shows that about 50% of the pores in PLAC with CNLA and about 10% of the pores in PLAC with SLA fell into the nondamaging category. The two-dimensional slices in Fig.7 show that the aggregates were evenly dispersed. These results are consistent with the test results from mercury intrusion porosimetry. CNLA has a better performs in absorbing and releasing water, and CNLA slowly releases the absorbed water as an internal curing agent and provides water for further cementitious hydration. The release of water makes the aggregate structure more compact and reduces the porosity of the concrete.

Micropores in the PLAC were mainly distributed in LA. The test results show that there were more micropores in the PLAC with CNLA and that their size distribution range was less than that in the PLAC with SLA thus the micropores were more homogeneous. Because the CNLA core was EPS, the porosity of PLAC with CNLA included only data from the matrix and the aggregate shells of CNLA. When the porosity of PLAC with SLA was compared with that of PLAC with CNLA in the X-CT images, the former was 24.0%-25.4% greater.

Fig.6 shows the SEM pictures. The interface transition zone link between CNLA and the paste matrix was denser than that between SLA and the matrix,as shown in the photos. This is because the shells of CNLA are also cement paste and therefore similar in substance to the matrix. Feras attributed the increased density of the interface transition zone to internal curing caused by absorbed water stored within the aggregate[31]. The quantity of water stored in CNLA was much greater than that stored in SLA, which resulted in PLAC with CNLA having better pore structure, better mechanical properties, and lower shrinkage than PLAC with SLA.

Fig.7 SEM images of PLAC with CNLA and SLA

4 Conclusions

a) CNLA when EPS spheres are used as core is an excellent lightweight aggregate. When the thickness of the shell layer is kept to around 1.5 mm, the loose bulk density can be as low as 500 kg/m³.

b) A low-cost PLAC with good performance was created by combining readily available ingredients such as cement, fly ash, a small amount of natural sand, and lightweight aggregate. It had a dry density of 1 000 to 1 200 kg/m3, a compressive strength of 7.8-11.8 MPa,shrinkage of 0.07%-0.12%, and thermal conductivity coefficients of 0.193-0.219 W/(m·K-1).

c) PLAC with CNLA performs better in wallboard than PLAC with SLA due to its low density, high thermal insulation, and enhanced strength. Slump flow was approximately 20% more in the CNLA concrete than in the SLA concrete, 28 d compressive strength was approximately 10% stronger, drying shrinkage was approximately 10% less, and thermal conductivity was approximately 10% less. CNLA has the potential to be used successfully in the production of lightweight concrete at a low cost.

d) When compared to the PLAC with SLA, the CNLA shells bonded strongly to the paste matrix at the transition zone interface. PLAC with CNLA had fewer pores and a better pore structure than that of PLAC with SLA, according to mercury intrusion porosimetry and X-CT.