This paper discusses the feasibility of preparing soil stabilizer which is circulating fluidized bed combustion ash-based, supplemented with carbide slag and desulfurization gypsum, composed entirely of complete industrial wastes. The results show that CFBC ash has better pozzolanic activity than fly ash. When stabilizer total content is 10% and the ratio of CFBC ash : carbide slag : desulfurization gypsum is 7.2 : 1.8 : 1, compressive strength of stabilized soil can reach the maximum of 2.12 MPa at the age of 28 d of curing. Stabilizer can meet the strength requirements of cement-soil mixing pile composite foundation and cement-soil mixing pile waterproof curtain.
With the increasing amount of coal mined and gangue heaped, countries encourage utilizing the combustion of gangue and low calorific value coal to generate power and ban permanent gangue heap. Circulating fluidized bed combustion (CFBC) is widely used due to the ability of utilizing low calorific value coal, fully combust, and high desulfurization efficiency [
Taking advantage of the hydration characteristic of industrial wastes to produce soil stabilizer is one of the most important ways to recycle industrial wastes. Research shows that stabilizer can be prepared with entirely industrial wastes (like gangue, CS, and phosphor gypsum); the 28 d compressive strength of stabilized soil can reach 2 MPa and can be 3 times higher compared to that stabilized by cement in the same content [
Research shows that the structure of clay is formed by soil particle groups integrated with clay particles and holes inside [
Soil sample: the soil sample (ST) was taken from Nanhai Park in Taiyuan City, Shanxi Province. The physical properties of the soil sample are presented in Table
Physical mechanical properties of tested soil.
Soil sample | Physical properties | |||||
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ST | 20.40 | 2.61 | 0.66 | 0.89 | 0.259 | 0.167 |
Notes:
CFBC fly ash was retrieved from Shanxi thermal power plant. The 80
XRD patterns of CFB ash.
CS was retrieved from Tianjin acetylene plant, the desulfurization gypsum was retrieved from Guizhou Hongfu Industrial Development General Co., Ltd. Chemical compositions of raw materials are shown in Table
Chemical compositions of raw materials.
Compositions/% | SiO2 | Al2O3 | CaO | SO3 | Fe2O3 | Others |
---|---|---|---|---|---|---|
CFBC ash | 50.11 | 28.26 | 8.85 | 3.99 | 4.72 | 4.07 |
PFA | 48.18 | 37.88 | 3.42 | 1.52 | 4.22 | 4.78 |
CS | 2.84 | 2.16 | 68.99 | 0.76 | 0.15 | 25.10 |
FGDG | 3.61 | 0.13 | 31.30 | 42.70 | 0.02 | 22.24 |
Notes: self-made admixture LLY is a kind of inorganic liquid. The LLY content is 0.5% of stabilizer by weight.
Weigh stabilizer and water according to the mix proportions, then put them into the agitator kettle, and mix them by electrical mixing machine for 60 seconds. Weight soil according to the proportion, put it into the agitator kettle, mix it at low speed for 30 seconds, and then mix it at high speed for 60 seconds. Use scraper to scrape the stabilized soil pasted on wanes and wall into the agitator kettle, continuing to mix for 60 seconds. Compact the soil-admixture samples into steel molds with 50 mm × 50 mm × 50 mm in three layers; then, molds were vibrated for 60 s on jolting table (ZT-1 × 1). Demold after compaction for 24 hours, then move the samples into standard curing chamber with curing temperature 20°C ± 2 and curing humidity 95%.
The compressive strength tests of the samples were conducted according to Chinese standard test methods of soils for highway engineering (JTJ051-93). Three specimens of each mixture were tested to investigate the average compressive strength.
Table
Stabilizer components and stabilized soil strength.
Mixture | CFBC ash |
CS |
PFA |
FGDG |
Compressive strength (MPa) |
---|---|---|---|---|---|
1-1 | 5 | 5 | 0.69 | ||
1-2 | 6 | 4 | 0.75 | ||
1-3 | 7 | 3 | 0.87 | ||
1-4 | 8 | 2 | 1.04 | ||
1-5 | 9 | 1 | 0.66 | ||
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2-1 | 2 | 8 | 0.22 | ||
2-2 | 1 | 9 | 0.21 | ||
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3-1 | 7.2 | 1.8 | 1 | 2.12 | |
3-2 | 6.4 | 1.6 | 2 | 1.70 | |
3-3 | 5.6 | 1.4 | 3 | 1.45 | |
3-4 | 4.8 | 1.2 | 4 | 1.03 |
As shown in Table
Most of PFA particles are dense and smooth. By contrast, CFBC ash is mainly composed by coarse and irregular particles, which are more favorable to perform the pozzolanic activity [
As shown in Table
CFBC ash is various in chemical compositions; hence, lots of experiments would be done to research the optimum proportion between CFBC ash and CS. To reduce the workload, in this paper, put forward an equation to estimate the optimum proportion between CFBC ash and CS.
Assume that the main hydration product CSH is formed with CaO/SiO2 molar ratio at 1.2. The reaction is shown as follows:
Active Al2O3 and Fe2O3 react with CaO to form C3(A,F)H6 which reacts with CaSO4·2H2O to form ettringite (AFt). The proportion is calculated by the following equation: Unit mass SiO2 hydration reaction needs unit mass: Unit mass Al2O3 hydration reaction needs unit mass: Unit mass Fe2O3 hydration reaction needs unit mass: Unit mass Al2O3 combines maximum mass: Unit mass Fe2O3 combines maximum mass:
Then unit mass CFBC ash hydration reaction needs unit mass CaO and CaSO4·2H2O can be calculated by
According to the proportion of all kinds of oxides, the theoretical optimum proportion of CFBC ash and CS can be calculated by (
Based on the optimum proportion of CFBC ash : CS = 8 : 2, discuss the optimum proportion of FGDG in soil stabilizer. The effect of FGDG on stabilized soil compressive strength can be revealed by changing the proportion of FGDG in stabilizer. As is shown in Table
Stabilized soil is usually formed by uniformly mixing loose and porous soil particle groups. In order to obtain the highest strength of stabilized soil, stabilizer should not only bind the loose soil particles, but also squeeze and fill the pores [
As is shown in Table
CFBC ash hydration requires high water demand and low strength at early age; in particular, the free lime and anhydrite contained in CFBC ash can produce volume expansion during hydration, which severely limits the resource utilization of CFBC ash in concrete. However, all these characters will not be disadvantages if the CFBC ash is used in preparing soil stabilizer. The usage of soil stabilizer is mixing dry stabilizer powder or stabilizer mortar with soil. For mixing dry stabilizer powder with soil, high water requirement is beneficial to construction and increases strength. For mixing stabilizer mortar with soil, increment on water requirement, which will not bring significant effect on the compressive strength of stabilized soil, is quite little compared to soil’s water content, according to the national technology standard, the standard compressive strength of stabilized soil should be taken at 90 d; therefore, the low compressive strength of CFBC ash at early age is not a disadvantage. The volume expansion during hydration can just fill the pores in stabilized soil, which is beneficial to increase the compressive strength.
According to the preliminary test results obtained from this experiment and the analysis of the characteristics of CFB above, it is completely feasible to utilize CFBC ash to make the stabilizer. Stabilizer with better technical performance is expected to be made, which is CFBC ash-based and composed of complete industrial wastes, with further optimization.
In this paper, stabilizers which are prepared by mixing CS with CFBC ash and PFA, respectively, are used to stabilize soil. The compressive strength of CFBC ash stabilizing soil is 3–5 times that of PFA stabilizing soil in the strength test at 28 d, which shows that CFBC ash has better pozzolanic activity than fly ash. There is an optimum proportion of the ratio between CFBC ash and CS and the FGDG content. The compressive strength of stabilized soil peaks at 2. 12 MPa at the age of 28 d, when total mixing proportion of stabilizer was 10% and CFBC ash : CS : FGDG = 7. 2 : 1. 8 : 1. Using stabilizer which is CFBC ash-based, supplemented with CS and FGDG, composed of complete industrial wastes, can meet the engineering strength requirements.
The authors declare that there is no conflict of interests regarding the publication of this paper.