1. Introduction
Autoclaved aerated blocks (AABs) have been used in load-bearing and non-load-bearing structures of buildings since the mid-1920s, because of its lower density, good fire resistance, excellent sound insulation and thermal insulation [
1], relative excellent impermeability [
2] and high resource utilization efficiency [
3,
4]. AABs are produced through the autoclaving of raw materials, which mainly include calcareous materials, siliceous materials, additives, expanding agents (normally aluminum powder), and water [
5]. Then, these autoclaved blocks would be cut to meet the suitable size for use. This would allow continuous capillary pores to be easily formed on the cut surface of AABs, while pores in the interior would be closed and discontinuous. This means that AABs can easily absorb water at the surface, but the degree of water absorption and relieving humidity is very slow [
6]. It was found that the single-sided continuous water absorption of AABs with a density of 500 kg/m
3 is 10% at 21 days. The equilibrium moisture content of AABs ranges within 3–4% at atmospheric environment with a relative humidity of 43%, the actual drying shrinkage value is 0.1–0.2 mm/m [
5]. However, the re-wetting of AABs by external moisture would increase the drying shrinkage. Hence, mortar for the AABs should be characterized by high water retention [
6,
7] for fresh mortar, and low water absorption capacity [
7] for hardened mortar.
Previous researchers have identified that cellulose and polymers (liquid resins, latexes, re-dispersible powders and water-soluble homopolymers or copolymers) can improve the water retention capacity of mortar [
6,
8]. Meanwhile, these also have a positive effect on the bond strength of the mortar [
9]. Compared to the reference sample, when the content of re-dispersible emulsion powder was 1.2%, the tensile bond strength increased by 18.9%, and when the cellulose content was 0.2%, the tensile bond strength increased by 85.3% [
10]. However, the high bonding strength of the plaster mortar was not necessary, because the AABs are lightweight, porous, and low-density materials. Without changing the surface characteristics of AABs, a thin layer of cement slurry coating was applied to the block surface. Ordinary low-strength mortar can produce a good bonding effect [
9,
11]. Hence, for the plastering mortar, cellulose and polymers play a more important role in preventing cracking, increasing toughness [
10,
12,
13,
14], and improving serviceability [
10]. Furthermore, the independent tiny spherical bubbles generated by the stirring of the mortar mixed with polymer and cellulose enhance the resistance of the mortar to damage caused by freeze-thaw cycles.
As presented in
Figure 1a, the typical insulation walls used in China consist of a load-bearing AAB wall (number 1), a plastering mortar layer (number 2), a thermal insulation layer (number 4), an anti-cracking protective layer (numbers 6, 7, and 8), and a finishing layer (number 9). Many facts [
15,
16,
17,
18] have proven that this compound structure is scientific. Because of the fact that the thermal conductivity of the masonry mortar used to bond AABs is much higher than that of AABs [
9], the existence of these masonry joints leads to the formation of “thermal bridges” [
19,
20], as presented in
Figure 1b. Therefore, it is necessary to reduce the heat loss caused by thermal bridges. The formation of the plaster layer of AABs from lightweight aggregates was considered to be the key to solving the “thermal bridges” effect [
21,
22]. Researchers have explored the potential of cork granules [
23], perlite microspheres [
24], expanded and vitrified small ball [
25], air entraining agents [
26,
27], expanded vermiculite [
28], phase change material [
29], and expanded polystyrene beads [
30]. Li et al. [
18] concluded that the heat transfer coefficient of plastering mortar is 0.48 W/(m·K), in which expanded perlite, vitrified microsphere, and 0.04% polypropylene fiber were added to prevent the shrinkage and cracking of the mortar.
The average density for commonly used AABs ranges within 400–600 kg/m
3. For the plastering of AAB walls, mortars made from lightweight aggregates are widely used at present, and the average density ranges within 1000–1400 kg/m
3. This brings a discrepancy in the deformation characteristics between the mortar and AAB, resulting in the debonding through internal stresses at the interface joint between the AAB wall and plastering mortar layer. In this view, the mortar for the plastering of AAB walls should be characterized by high toughness. Cyclic shear tests were employed by researchers to prove that plastering mortar with high toughness can improve the stability of the AAB wall structure [
31].
In this paper, in order to develop the plastering for AAB walls, the mortar was modified by compositing EVSB, fibers, ethylene-vinyl acetate (EVA), and hydroxypropyl methylcellulose (HPMC). Physical properties of the modified mortar, which included dry density, water absorption, water retention, compressive strength, and flexural strength were evaluated. The technical parameters and performance improvement mechanism were clarified in this study to provide a technical reference for the application field.
2. Experimental
2.1. Raw Materials
A compound cementitious system composed of P·II 52.5 type Portland Cement (Onada Cement Corp, Nanjing, China) and Class II fly ash (Ordos, China) was used in this experiment.
Table 1 shows the chemical composition of cement and fly ash.
Figure 2 presents the XRD pattern of fly ash. The physical properties and the particle size distribution of the expanded and vitrified small ball (EVSB) are listed in
Table 2 and
Table 3. Dry and clean quartz sand with a continuous particle size of 0.154–0.500 mm was used as aggregate for the mortar. Heavy calcium carbonate was used as the filling material, which has an average particle size of 0.0455 mm and a bulk density of 1080 kg/m
3. The appearance of hydroxypropyl methylcellulose (HPMC) was white powder with three viscosities of 50,000 mPa·s, 100,000 mPa·s, and 150,000 mPa·s. The re-dispersible emulsion powder appeared as a white solid powder was the ethylene-vinyl acetate copolymer (EVA VINNAPAS-5011L). The length of the polypropylene fiber used in this experiment was 3 mm and 6 mm, respectively.
2.2. Mix Proportion
The proportion of various materials in the mixture was calculated as the mass percentage. Cementitious materials, heavy calcium carbonate, and sand were mixed at a constant ratio (1.00:0.12:2.50). The additional levels of EVSB was 20%, 22%, 24%, 26%, and 28%, by mass of the binder, respectively. The dosage of the EVA was set as 1%, 2%, 3%, 4%, and 5% of the cementitious materials. The dosage of the HPMC with different viscosities was set as 0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% of the binder. The water/binder ratio (ratio of water to the total mass of fly ash and cement) was set as 0.68, 0.72, 0.76, 0.80, and 0.84. The cement and fly ash were weighed and mixed well (the mass ratio of cement and fly ash were 90:10, 80:20, 70:30, 60:40, 50:50). Then, the mixture was mixed with the heavy calcium carbonate, EVSB, EVA, and HPMC. Afterwards, water was mixed with the solid mixtures, and stirred at high speed for three minutes. Finally, the fibers were added into the above-mentioned mixture, and stirred for three minutes before molding.
2.3. Test and Characterization
The specimens with molds were placed in a standard curing box, with a temperature of 20 ± 2 °C and a relative humidity of more than 95% for 24 ± 1 h. Then, the specimens were released from the molds and were further cured until 7 d, 28 d, 90 d.
The water retention test was conducted in accordance to DIN 18555-7 [
32] using a filter-film allowing water to filter through, which is fixed on absorbent filter papers. The water retention rate was calculated based on the ratio of the mass of water absorbed by the filter papers. The consistence test for the workability evaluation was conducted in accordance to JGJ/T 70-2009 [
33], with the depth of a standard cone sinking into the mortar mixture during the specified time.
According to JGJ/T 70-2009 [
33], fresh mortar composites were cast into cubes molds, with a size of 70.7 mm × 70.7 mm × 70.7 mm, for the compressive strength test and freeze resistance test. After immersing in water for two days, specimens that were cured for 28 days underwent 25 freeze-thaw cycles. For each test, six specimens were tested and the average values were reported.
Based on GB/T 17671–1999 [
34], three specimens, with a size of 40 mm × 40 mm × 160 mm, were prepared for the flexural strength test and the average values were reported. Using a universal testing machine, the load–displacement curve was obtained from the test.
The thermal conductivity of the material was evaluated through the dry density of the mortar [
27]. The difference between the weight of the specimen under water-saturated and fully dried (dried at 105 °C to constant weight) conditions was used to calculate the water absorption of the hardened mortar. This is an intuitive approach to evaluate the porosity of mortars [
28]. According to Archimedes principle, the dry density of specimens obtained from the broken sample after the flexural strength test can be determined by weighing. These were calculated using the following equation:
where, A is the mass water absorption (%),
is the weight of the specimens under water-saturated condition (g),
is the weight of the specimens under fully dried condition (g),
is the drainage volume of the specimens under water-saturated condition (mm
3),
is the dry density of the specimen(kg/m
3).
An X-ray diffractometer (D max/RB Japan Rigaku Corporation) with a copper target (λ = 1.5418 Å, 40 kV, 30 mA) was used. The sample to be tested was dried to constant weight, and tested after grinding. The scanning diffraction 2θ angle was 5°~80°, and the scanning speed was 2°/min.
The mercury intrusion porosimetry was used to analyze the pore structure and pore size distribution of the test sample. The specimens were first broken into pieces, with a diameter of 2.5 to 5 mm. Then these were soaked in acetone solution, and finally taken out and dried before the test (Poremaster GT-60; Quanta chrome, Houston, TX, USA)