Study on water absorptivity and frost resistance performance of self-ignition coal gangue autoclaved aerated concrete

Self-ignition coal gangue (SCG) used as one of precursors to fabricate aerated autoclaved concrete (AAC). Aiming at studying water absorptivity and frost resistance performance of self-ignition coal gangue aerated autoclaved concrete (SCGAAC), three-period water absorbing tests and freezing-thawing tests were carried out and the corresponding results were recorded and analyzed. In order to modify the water absorptivity of SCGAAC, foam stabilizer was applied to adjust pore structure while calcium stearate was expected to change hydrophilic feature of CG. It was demonstrated that the compressive strength of SCGAAC containing foam stabilizer or calcium stearate declined at different levels, although the porosity became lower slightly. For water absorptivity, foam stabilizer failed to decrease the water content at any period and even increased water absorbing rates. Calcium stearate controlled water absorbing rate successfully but the ultimate water content hardly reduced. All of the SCGAAC samples exhibited intact appearance after 50 freezing-thawing cycles and showed excellent frost resistance performance. Three models were proposed to predict water absorptivity and frost resistance performance of SCGAAC and the corresponding prediction results matched test resulted well.


Introduction
In recent years, autoclaved aerated concrete (AAC) has been commonly used as construction material by virtue of its functional performance [1][2][3][4][5][6], in particular, thermal insulation and energy efficiency [7][8][9]. AAC comprised of cement, fly ash, lime and finegrained aggregates has been produced in Asia, Europe and Americas since it once turned into creation in Sweden in 1930s [10]. However, its excellent thermal and sound insulation properties are shadowed when considering of high water absorption. It was reported that the mass loss and compressive strength loss of water-saturated AAC reached 1.5% and 16.6% after suffering from 50 freeze-thaw cycles [11]. Frost resistance of AAC deteriorated with the interior moisture increasing and it was easy to generate frostheaving cracks and denudation [12]. Meanwhile, Miloš Jerman [13] also observed that a thermal conductivity coefficient of AAC in capillary water saturation became as much as six times of that in a dry state.
As a porous product, pore structure has a strong effect on the moisture distribution of AAC [14,15]. In the past few decades, some principles related to water transportation in porous AAC system were presented [13][14][15][16][17][18]. For example, it was found that water vapor diffusion and capillary suction dominated in dry conditions and high humidity circumstances, respectively. Water absorbing and transmitting caused by capillarity in porous materials like AAC were defined as a sorptivity process which was governed by the unsaturated flow theory [8]. The overall capillary absorption was well described as two processes: a capillary absorption in aerated pores achieved gravitational equilibrium rapidly; and a slow capillary absorption into the matrix pores [19].
With the intention of consuming self-ignition coal gangue (SCG) in construction materials, AAC using self-ignition coal gangue (SCGAAC) as one of precursors was fabricated and studied [20]. Mechanical performance and failure mechanism of the SCGAAC significantly depended on its micro structure as well as formation of reaction products, especially on the generation of tobermorite. In order to further elaborate the properties of SCGAAC with high humidity, water absorption and frost resistance were studied. Specifically, the compressive strength and mass change of SCGAAC samples subjected to freeze-thaw cycles were tested and the corresponding prediction models related to porosity were proposed.

Raw materials
The precursors of SCGAAC comprised of SCG, ordinary Portland cement (PC), lime (L) and gypsum (G). The chemical composition of the raw materials was listed in Table 1.
In this study, the SCG was resourced from Heilongjiang, China. The as-received SCG aggregates with a particle size of 10-30 mm were milled to form SCG powder which sieved through an 80 μm sieve. Naphthalene-based superplasticizer (FDN) was selected to adjust the slurry expansion (SE) of SCGAAC pastes. Aluminum powder (AP) was used as a foaming agent in AAC system.

Chemical agents and mix ratios
Two chemical admixtures added into the mixes when fabricating SCGAAC samples. A foam stabilizer (F) was used to reduce bubble cracking during the stirring process and optimize pore structure of hardened AAC. The other chemical agent, calcium stearate (C), was regarded as a waterproof agent to decrease water absorption. The two chemical agents were expected to reduce water absorption in different ways. Aiming at improve the pore structure of AAC, the foam stabilizer mainly stopped bubbles merging and reduced connected pores. In addition, calcium stearate achieved a hydrophobic AAC matrix that was instrumental in absorbing little water. The dosage of the chemical agents and the mix ratios of SCGAAC were presented in Table 2.

Specimens
The mix procedures of SCGAAC was applied as the method mentioned in a preceding study [20]. Slurry expansions (SE) was a measurement index reflecting workability of fresh AAC pastes and it was found that the SE had an influence on the density and pore structure of hardened AAC [20]. In this study, all of the fresh pastes were controlled the SE at 250, 290 and 320 (±5) mm respectively through adjusting the dosage of FDN. The SCGAAC samples were moulded in 100 mm 3 cube metal moulds. The fabrication process and the curing protocol referred to [20].

Test methods
Super depth of field microscope (SDFM) was applied to characterize the pore structure of SCGAAC. In order to accurately identify the pores and matrix, the samples for the pore characterization were pretreated through ink painting and titanium dioxide powder filling. Fig. 1 showed the contrast of the original sample and pretreated samples. The The measuring process of water content and saturated water absorptivity conforms to GB/T 11969-2008 [21]. The water absorptivity WR can be calculated as Eq. (1): where Mg is the mass of sample after water absorption and M0 is the mass of sample in absolute dry status.
Frost resistance performance of SCGACC was tested according to GB/T 11969-2008 [21]. One freezing and thawing cycle contained two stages: the water-soaked samples froze in air at -20±2 ℃ for 4 h; and then the temperature rose to 20±2 ℃ and the samples immerged in water for 4 h to thaw. The compressive strength and mass change were recorded after the SCGAAC samples were subjected to 50 freezing and thawing cycles.

Pore structure of SCGAAC
According to the captured pictures of SCGAAC slices, three main pore parameters were computed by the OLYMPUS Stream software connected with the SDFM, including porosity, pore-size distribution and average length-width ratio (ALwR) of the pores. The results related to the pores of the SCGAAC samples were shown in Table 3. It was observed that porosity increased with the sample SE becoming larger. For the samples without any chemical agents, the porosity rose by approximate 16% when the SE increased from 250 mm to 320 mm. The f-1 sample showed the lowest pore content (45.93%) and the highest density (750 kg/m 3 ). The porosity increased obviously as the SE was enlarged, especially the percentage of the pores within 0.5-2 mm.
The chemical agents improved the pore structure of SCGAAC at different levels through lowering the porosity and mesopores. The foam stabilizer was a type of surfactant that reduced the surface tension of the slurry and forming stable bubbles. Much more bubbles formed and existed easily in the thick slurry with 250 mm SE when the F agent was used. As the slurry becoming thinner (320 mm SE), a part of bubbles inclined to crack in low viscosity pastes and the F agent intensified this tendency. Hence, the F agent adjusted the bubble status and content in the slurry with different SE and the thick slurry witnessed an obvious effectiveness. The agent C also showed an effect on adjusting pore structure of the SCGAAC samples. For all of the samples with different SE, the samples containing the C agent had a lower porosity than those containing the F agent. It was found that the small and large pores increased while the mesopores reduced.

Mechanical properties of SCGAAC
Compressive strength was tested to assess the mechanical properties of the SCGAAC.
As exhibited in Fig. 3, the chemical admixtures showed an influence on the compressive strength. The foam stabilizer almost brought no harm in compressive strength of the SCGAAC while calcium stearate obviously decreased the compressive strength. This was because that the density of the SCGAAC containing calcium stearate was lower than that of the others. The corresponding density results were listed in Table 2. The foam stabilizer kept small bubbles suspending in the slurry rather than rising up and cracking through adjusting the surface tension of fresh slurry. The bubbles evenly distributed in the matrix to form relatively dense system, which resulted in the high compressive strength.  Table 3, had the lowest density. It was reasonable to infer that calcium stearate had a negative influence on the matrix of SCGAAC.

Water absorptivity of SCGAAC
Water-absorbing rates of the SCGAAC samples were shown in Fig. 4. The water absorption curves for the f-1, f-2 and f-3 presented the identical trends. During the three water uptake periods, the samples with the 290mm SE obtained the highest water content.
The water content of these three samples after 72-hour absorbing were at a similar level, ranging from 47.3% to 52.3%. Although the foam stabilizer enhanced the compressive strength, the water absorptivity failed to be controlled by using the foam stabilizer. As shown in Fig. 4, f-4, f-5 and f-6 absorbed more volume of water in every period compared to the counterparts without any admixture. For the f-6 sample, the water content after 72 h was slightly lower than 60%. Two potential reasons explain this results.
The first one is the formation of interconnecting capillary pores because of using the foam stabilizer, and the other one is the matrix becoming more hydrophilic. Compared to the foam stabilizer, calcium stearate effectively controlled the water absorption of the SCGAAC within 48 h. Although the 72-hour water contents were near the others, the water-uptake of the SCGAAC comprising calcium stearate was obviously slow during the first 24 h. Calcium stearate was a strong waterproof agent for the SCGAAC, which can be seen according to the slopes of water absorption curves in Fig. 4.
For the samples f-1 to f-6, the water absorption rates were variable over the three periods, where the initial water uptake slopes were steep and the rates gradually tended to be flat at last. When it came to the samples with calcium stearate, the trend of water absorption was different. The samples kept absorbing water with an invariable rate over the whole duration. The lowest water absorption of the SCGAAC came from the f-8 sample with 37.9% of the water content after 72 h.
The different trend of water absorption was mainly from hydrophobicity caused by calcium stearate. It was difficult for water to infiltrate into the CG matrix when the water was initially on contact with the matrix. It took time to permeate into the matrix during the immerging period. For the samples modified by calcium stearate, moisture transport along the pore path because of capillarity was a minor factor for water absorbing process.
Although the driving force for water uptake was capillarity, it was water-holding capacity that decided how much moisture can be transported up. The SCGAAC samples without calcium stearate absorbed water quickly when water was on contact with the matrix and the water content for the local matrix under the water line nearly reached saturation over the first period. The rate of moisture transport was slower than that of water uptake, therefore, the curves for f-1 to f-6 showed decreased slopes. For the f-7 to f-9 samples, water absorbing rates kept a constant throughout, which demonstrated that the water content for the matrix was unsaturated and it reached a dynamic equilibrium between the moisture transport and water uptake. Therefore, calcium stearate was an effective waterproof agent for SCGAAC and it was efficient to decrease water absorbing rate and reduce water content at early time.  temporarily slowed water absorbing of SCGAAC, the following moisture transport through pore channels was hindered notably.

Frost resistance of SCGAAC
In order to further examine the difference caused by freezing-thawing cycles, the samples experienced 50 freezing-thawing cycles dried at 105 ℃ to reach a constant mass.
The absolutely dried mass were recorded and the compressive strength were tested, and the corresponding comparisons were shown in Fig. 6. It can be seen, in Fig. 6 (a), the absolutely dried mass change Δmd/m was a positive value for the each sample, which meant growth of the SCGAAC matrix happened. This weight increment likely attributed to cement hydration during the freezing-thawing period. The autoclave curing protocol for the SCGAAC was heating at 196 ℃ (1.1MPa) in water vapor for 8 h, during which most precursors would react and generate reaction products. However, CG inclined to hold and consume water as much as possible when the precursors were mixed with water.
It took hours for cement particles to hydrate and the free water would be insufficient for complete hydration taking place at that time. The subsequent freezing-thawing tests appropriately supplied water for the unhydrated cement. This was the most likely reason for the absolutely dried weight increment of the SCGAAC.
The compressive strength of the SCGAAC was also different with the counterparts experienced freezing-thawing cycles. As shown in Fig. 6

Predictive models
On the basis of the test results, the water absorptivity of the SCGAAC could be related to the matrix as well as pore parameters. Herein, the fine pore (＜0.5mm) fraction and the ALwR were applied to fit a predictive model of water absorptivity of SCGAAC. The proposed model was shown as Eq. (2): where Ws is the 72-hour water absorptivity of the SCGAAC; P0.5 is the percentage of fine pores (＜0.5mm); and R1 is a parameter related to effectiveness of chemical agents, equal

Conclusions
Focusing on the water absorptivity and frost resistance performance of SCGAAC, 9 SCGAAC samples with different SE and chemical agents were tested, including porosity, water content and rate, and mass change experienced freezing-thawing cycles etc.
According to the tests and corresponding analysis, the main conclusions were drawn as follow: 3) SCGAAC had high water absorptivity due to the porous matrix and high water uptake capacity of CG. Foam stabilizer slightly increased the water absorption and water absorbing rates at any period, especially the rate of first 24-hour period. The water absorbing rate became flat gradually with time. The water absorbing rate became obviously slow because of the water proof property of calcium stearate and it showed a constant rate throughout the whole duration. 5) The models proposed for predicting water absorptivity, mass change and strength change matched well to the corresponding test results. These models would reasonably predict water absorbing ability and frost resistance performance through some basic physic parameters.