Inorganic solidified foam (ISF) is a novel material for preventing coal fires. This paper presents the preparation process and working principle of main installations. Besides, aqueous foam with expansion ratio of 28 and 30 min drainage rate of 13% was prepared. Stability of foam fluid was studied in terms of stability coefficient, by varying water-slurry ratio, fly ash replacement ratio of cement, and aqueous foam volume alternatively. Light microscope was utilized to analyze the dynamic change of bubble wall of foam fluid and stability principle was proposed. In order to further enhance the stability of ISF, different dosage of calcium fluoroaluminate was added to ISF specimens whose stability coefficient was tested and change of hydration products was detected by scanning electron microscope (SEM). The outcomes indicated that calcium fluoroaluminate could enhance the stability coefficient of ISF and compact hydration products formed in cell wall of ISF; naturally, the stability principle of ISF was proved right. Based on above-mentioned experimental contents, ISF with stability coefficient of 95% and foam expansion ratio of 5 was prepared, which could sufficiently satisfy field process requirements on plugging air leakage and thermal insulation.
Coal fires are difficult, persistent, and costly problems worldwide in coal mining processes [
In this work, a novel material, ISF, with high closed porosity and uniform pore distribution was prepared via mixing aqueous foam and composite slurry consisting of fly ash, cement, and compound additives. In this process of preparation, the following two points are worthy of consideration. Firstly, stable aqueous foam is required for ISF to plug air leakage in mining applications. Furthermore, the stability of foam may be affected by foam generator parameters, surfactants, and their concentration [
In this paper, as a first step, we studied the preparation process of ISF and analyzed the working principles and the effects of key devices. As a next step, the aqueous foam with low drainage rate and high expansion ratio was prepared based on sodium dodecyl sulfate (SDS) solution and modified by the foam stabilizers such as cetrimonium bromide (CTAB), sodium chloride (NaCl), and lauryl alcohol (LA). Then the factors influencing the stability coefficient and foam expansion ratio of ISF were investigated. At last, through the observation on drainage of the bubble wall and the hydration products accelerated by calcium fluoroaluminate, the stabilization mechanism of foam fluid was proposed.
Constituent materials are listed below. Portland cement (PC) with the compressive strength of 64.5 MPa at 28 days, conforming to BSEN 197-1 type I cement [ Fly ash (FA) with a median particle size of 35 Calcium fluoroaluminate ( Redispersible polymer powder (PP): it is a kind of polymeric powder which can be easily reemulsified in water to reform liquid emulsion with essentially identical properties to the original emulsion. Water (W): its percentage was fixed in order to satisfy both the workability criterion and the controlled low strength materials (CLSM) recommendations for the insulation materials [ Lauryl sodium sulfate (SDS), Cetrimonium Bromide (CTAB), NaCl, and lauryl alcohol (LA). They were diluted with water in different ratios.
The basic preparation process can be divided into three parts including mixing the composite slurry, preparing aqueous foam, and mixing composite slurry, accelerator, and aqueous foam. We admixed cement, fly ash, and redispersible polymer powder together and got the blend of these three basic raw materials. Then, part of water was injected into the blend and composite slurry formed under the work of stirrer. The water-solid ratio was controlled slightly less than preset ratio. At the same time, the rest of the water was used to dilute the surfactant. Then, high pressure air was pumped into the foam generator and aqueous foam was produced. The next procedure was to mix composite slurry with aqueous foam in a self-made mixer, with some compound additives added. At last, foam fluid was produced and evolved into ISF at room temperature. The specific preparation procedures of ISF are schematically shown in Figure
The schematic of the preparation of ISF.
The main installations.
We chose the drainage rate to be 30 minutes since aqueous foam was produced, to reflect foam stability. After generation of aqueous foam, the initial foam mass,
The test instrument was one cylindrical gauge whose inner diameter was 100 mm and measuring range was 315 mm. The test procedure is as follows: (i) pour the fresh ISF into the test instrument and record the initial height,
Test procedure for foam expansion ratio of aqueous foam or foam fluid is as follows: (i) fill a container (volume and mass are known and designated by
The microstructure of aqueous foam and ISF fluid were observed by a Nomarski-type phase contrast interference microscope equipped with a digital camera, which can be used to take photomicrograph of the samples and the foams. A drop of sample was brought onto a microscope slide and the structure of bubble was observed. The bubble wall of ISF was investigated by scanning electron microscopy (SEM) (FEI QuantaTM 250 SEM system) with the size of test specimen being
The water and surfactant solution contact angle on the particles was measured using the gravimetric version of the Washburn method. The method is based on measuring the penetration rate of a wetting liquid into a packed bed of particles, which lead to the following equation [
In order to investigate the influencing mechanism of aqueous foam volume (FV), fly ash replacement for cement (FA), and water-solid ratio (W/S) on the stability of ISF, we conducted tests on different specimens. FV was controlled to vary from 2 V to 10 V with the increment being 2 V, FA changed as 10%, 20%
To develop fine, uniform, and stable inorganic solidified foam, the following two points deserve consideration. Firstly, the foam generator should be able to produce aqueous foam with uniform pore structure, high expansion ratio, and a certain stabilization time. Secondly, aqueous foam and composite slurry should contact thoroughly and then form stable foam fluid during the mixing process in the mixer. The schematic of key devices was shown in Figure
The schematic of foam generator and self-made mixer.
The main process of generating foams by the home-made foam generator is as follows: once foaming agent solution and high pressure air flow through the T-shape conduit of foam generator, the turbulent eddy is formed after mixing and enhanced by the porous medium which can be composed of multilayer meshes, powdered metal, or spherical glass particles, causing greater pressure drop due to their impediment. The more homogenous and denser aqueous foam is produced from down to up as the porosity of porous medium increases stepwise. The aqueous foam produced by mechanical agitation and home-made foam generator was as shown in Figure
The optical microscopic analysis diagram of aqueous foam.
Produced by mechanical agitation
Produced by the home-made foam generator
Mixer consists of chamber and hollow spiral pipe inside it. The high-speed composite slurry drives the impellers to rotate, and then foam slurry is stirred and delivered by hollow spiral pipe with helical blades. Vortex streets in this process can completely go into turbulence and cause vortex according to certain frequency. The loss of kinetic energy acts on the mixtures and a large number of foam fluids are formed. Aqueous foams pass into the mixer from the left body of hollow spiral pipe equipped with five aqueous foam outlets with an interval angle. Aqueous foams are added to slurry step by step, which reduce the broken rate of foam and increase foam slurry contact areas. This kind of mixing chamber can weaken the shock caused by larger flow of aqueous foam and is conducive for gas-liquid-solid to mix thoroughly.
From viewing of the technology process for preparing the ISF, the stability is mainly dependent on that of aqueous foam. Generally speaking, foam expansion ratio of aqueous foam should be more than 20. SDS is a widely used surfactant with strong foaming ability. Its change trends of 30 min drainage rate and foam expansion ratio with different SDS concentrations are depicted in Figure
Change trends of drainage rate and foam expansion ratio with different concentrations.
From Figure
In order to strengthen stability of aqueous foam, CTAB, NaCl, and LA were utilized as foam stabilizers. We studied modification effects on SDS aqueous foam under different concentrations of foam stabilizers ranging from 0.5% to 4.0%, whose concrete effects on foam expansion ratio and drainage rate are shown in Figure
Modifying effects of foam stabilizer on drainage rate and foam expansion ratio.
From Figure
Under the condition that the concentration of SDS is 2.5%, its foam expansion ratio decreases with the increasing concentration of CTAB. Because CTAB is a cationic surfactant while SDS is an anionic one, when these two surfactants are mixed, phase separation will occur due to intense electrostatic interaction and condensation of surfactant molecules [
The foam expansion ratio enlarges with the increase of NaCl concentration mainly because homo-ion could not only diminish the Critical Micelle Concentration (CMC) of the surfactant but also reduce surface tension of the solution and develop its foaming ability. The drainage rate decreases firstly and then ascends with the increase of NaCl concentration, the minimum of which is 26% at a concentration of 1.0%. The addition of NaCl to SDS solution enlarged its foaming ability to some degree and reduced its drainage rate, which could be explained that there is a threshold of added electrolyte on stratification phenomenon of foam film, above which the phenomenon is not observed [
The addition of LA could both prominently improve the foam expansion and greatly enhance the stability of aqueous foam. This is because the iceberg structure (a perfectly ordered structure formed by the LA molecules and water molecules) around the hydrocarbon chain in the alcohol makes it a spontaneous process for the alcohol to participate in the formation of micelle and thus bubble films are consolidated. The drainage rate of foam film will slow down with the rise of surfactant micelles in certain range of concentrations [
The prepared foam-forming solution containing SDS concentration of 2.5% and LA concentration of 2% possesses excellent foam expansion ratio with the value being 28 and the aqueous foam derived from the solution acquires the best stability with the value being 13%.
According to the results of 125 tests, it can be concluded that when FV is 8 V, FA is 30% and W/S is 0.4, and the ISF is in the best state with its foam expansion ratio and stability coefficient being 5 V and 90%, respectively. At the same time, some other test data was shown in Figure
The change curves of stability coefficient and foam expansion ratio with different factors. (a) Independent variable is FV and constants are 30 wt.% of FA and W/S of 0.4; (b) Independent variable is FA and constants are 8 V of FV and W/S of 0.4; (c) independent variable is W/S and constants are 8 V of FV and 30 wt.% of FA.
From Figure
To reduce cost, we use a small quantity of fly ash to replace cement. Figure
It is observed from Figure
The maximum stability coefficient is 90% based on the results of 125 groups of experiments. There are certain changes in its internal structure of foam fluid during the solidification from the fresh state. For a more in-depth study on the changes in the internal structure of the bubble, the fresh state of foam fluid (Figure
The fresh state of foam fluid.
Figure
According to Binks, the optimum contact angle for foam stabilization is about 90°, as at this value the energy to remove the particle from the interface has the highest value. Experimentally, the optimum contact angle interval, ensuring the highest foam stability, was found between 40 and 70° [
The bursting process of an unstable bubble.
From Figure
Based on the previous analysis, the apparent high stability against disproportionation is the most significant result, even considering the coagulated nature of the particles. Also, as with foam fluid, partial coagulation of particle networks on the surfaces of the bubbles is found to be advantageous for stability. It should be noted that the rate of drainage from the bubble wall is much faster than the rate of precipitation of the hydration products. So promoting the formation of hydration products is the correct way to delay and stop the burst of bubbles.
Accelerators influence the rate of cement hydration, leading to particles with a high degree of internetworking against disproportionation and to occurrence of greater retardation. So we conduct experiments on the concentration of calcium fluoroaluminate (
The foam stability coefficient versus concentration of accelerators.
In cement-based materials (e.g., ISF), the transformation process from a paste phase into a solid phase can be understood from the properties of their constituents. When
The SEM images of bubble wall.
In the SEM image obtained from the sample after solidification, the evolution of the primary cement hydration products is obvious. We can observe the formation of ettringite as rod-like crystals massively fill capillary pores. Surface products such as C–S–H gel can be observed as the major ISF microstructure component. CH as a pore product with a polycrystalline shape is another dominant cement hydration product. The SEM shows that the cement and fly ash particles are more connected and cement hydration products completely surround the particles.
This paper presents the manufacturing process of ISF which consists of mixing the composite slurry, preparing aqueous foam, and mixing them with accelerator. The foam generator can produce homogenous and dense aqueous foams due to the turbulent eddy which is formed and enhanced by the porous medium. A large number of foam fluids are formed by the self-made mixer in which turbulence and vortex were generated, and then aqueous foams were added stepwise to slurry. The aqueous foam with expansion ratio of 28 and 30 min drainage rate of 13% was obtained as a function of 2.5 wt.% SDS and 2 wt.% LA. The effects of FV, FA, and W/S on stability coefficient and foam expansion ratio were studied. And the results show that the optimum values of foam expansion ratio and stability coefficient were 5 V and 90%, respectively, by value of FV being 8 V, FA being 30%, and W/S being 0.4. The adsorption of CTAB and LA molecules on the surfaces of the particles changes their hydrophobicity with the contact angle from 166° to 78°. The mechanism concerning accelerating the hydration and reducing the drainage was proposed and verified based on the analysis of dynamic change of bubble wall. At last, ISF with stability coefficient of 95% and foaming expansion ratio of 5 was fabricated, which could sufficiently satisfy field process requirements of air sealing and thermal insulation.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (no. U1361213), the Fundamental Research Funds for the Central Universities (CUMT, 2014YC04), and the independent study projects of State Key Laboratory of Coal Resources and Mine Safety (SKLCRSM13X04).