1、1Experimental Study on Sandstone Freezing-thawing Damage Properties under Condition of Water ChemistrAbstract. For the purpose of researching the freezing-thawing cycle pHysical mechanics properties of sandstone in various chemical solutions, taking the red sandstones from Shandong as the example, f
2、reezing-thawing cycles experiments under the condition of H2SO4 solution (pH1.5), NaOH solution(pH12.5), NaCl solution (pH7, mass fraction is 4%) and water were conducted. The nuclear magnetic resonance (NMR) technique was used to test the porosity of rock samples after freezing-thawing cycles. Braz
3、ilian splitting test was also conducted to test the samples with different times of freezing-thawing cycles and soaking solutions. Results show that the quality change of samples in various solutions is different. The mass of sample in water increased, however, the mass change of the sample in other
4、 three solutions showed a firstly increasing and then decreasing tendency. The porosity distribution in rock changed obviously after different times freezing-thawing cycles. After 30 times freezing-thawing cycles, the porosity in H2SO4 solution, NaOH solution, NaCl solution 2and water has increased
5、by 151.1%, 85.443%, 39.388%, and 17.976% respectively. With the increase of freezing-thawing cycles times, tensile strength of the rock reduced, but the damage properties were different in various solutions. The research can provide some mechanical parameters basis to physical mechanics properties o
6、f sandstones. Keywords: Water Chemistry; Rock Mechanics; Freezing and Thawing Cycles; Porosity; Brazilian Splitting Test 1 Introduction Research shows that that the pore water in rocks is in the state of freezing and thawing if temperature alternates positively and negatively, and thus the physical
7、and mechanical properties of rocks will be affected under the effect of freezing and thawing. In addition, the occurrence of freezing-thawing damages to rocks is closely linked with the water chemistry environment. Therefore, the studies on the physical and mechanical properties of rocks under the c
8、ondition of water chemical environment and temperature effect are of great importance to know well the deformation and strength properties of rocks. Many foreign and domestic scholars have carried out many 3studies on the freezing-thawing problems of rocks. In the mechanical properties of rocks unde
9、r the effect of water chemistry, many foreign and domestic scholars also have conducted studies. Ding Wuxiu and Feng Xiating 6 tested the rocks corrosive in different chemical solutions, and the results showed that the tie between mineral grains was disturbed and granules were corroded under the eff
10、ect of chemical solutions, so that the strength of rock was significantly reduced and the structure of rock was damaged. Shang Liansheng 7, 8 made a three-point bending test on the fracture mechanics indexes KIC and of two granites and two sandstones with different chemical properties under the acti
11、on of an aqueous solution. Sondergeld 9 studied the changes of Berea sandstones relevant physical parameters after freezing-thawing cycles under the condition of saturated salt water. Zhang Jizhou 10 made a circulated freezing-thawing test on three rocks in two water chemistry environments (saturate
12、d distilled water, and soaked and corroded with nitric acid solution of 1%), finding the freezing-thawing damage to rocks was more much more severe under the acid condition than that under the pure water chemistry condition. Karfakis 11 studied the effect of chemical solutions on rock fracture tough
13、ness 4property. Feucht 12 studied the laws of different solutions such as NaCl, CaCl2, and H2SO4 on the crack surface friction factor and strength of prefabricated crack sandstone. By choosing the red sandstone in Heze of Shandong, the freezing-thawing test under different water chemistry condition
14、was studied, the porosity was measured using the rapid, lossless nuclear magnetic resonance (NMR) technology 13-15, and also the physical and mechanical properties of rocks after freezing-thawing were studied using Brazilian splitting test. 2 The freezing-thawing cyclic test 2.1 Test rock sample and
15、 test instrument Red sandstone was chosen in Heze of Shandong, which was hazel and presented with a granular detrital structure. According to the Water Resources and Hydropower Engineering Rock Testing Regulations (SL 264-2001) 16, the sandstone was made into standard cylinder specimen with a diamet
16、er of (501) mm and a height of (301) mm. There were 36 samples in total, which were numbered with C1C9, D1D9, E1E9, and F1F9 (i.e. acid, alkali, salt, and water environment groups), as shown in table 1. No.1No.3, No.4No.6, and No.7No.9 were corresponding to 10, 20, and 30 freezing-thawing cycles. In
17、 the freezing-thawing cycling test, freezing-thawing 5testing machine TDS-300 was applied, and its working principle was freezing in air and thawing in water so that the cycle time and freezing-thawing temperature were automatically implemented in the freezing-thawing process; nuclear magnetic reson
18、ance imaging analysis system AniMR-150 manufactured in Shanghai new McLaren electronic technology co., LTD was applied in the nuclear magnetic resonance experiment, and its specification was as follows: main magnetic field (0.30.05T), RF pulse frequency (230MHz), magnet temperature control (2535), m
19、agnet evenness (35.0ppm), and RF power (300W). Microcomputer control electro-hydraulic servo universal testing machine SHT4206 manufactured in Shanghai SANS was used in Brazilian splitting test, and the electrical system of it possessed three control modes (force control, deformation control, and di
20、splacement control); the universal testing machine was with a double-space lower oil cylinder structure and the maximum test force of 2000 KN. 2.2 Experimental method (1) Freezing-thawing cycling test Rock sample groups C, D, E, and F were immersed respectively into solutions H2SO4 (pH1.5), NaOH (pH
21、12.5), NaCl (pH7, and mass fraction=4%), and water, and then taken 6out after they were saturated for 48 hour. Then, the moisture on the surfaces of the samples was dried and weighted for obtaining the initial quality of each sample. Then, the rock samples were immersed into their corresponding liqu
22、id, and then the rock samples and liquid were placed in freezing-thawing testing machine for a freezing-thawing cycling test; the parameters of the freezing-thawing cycling test were as follows: freezing temperature (-20), thawing temperature (20 ), and freezing and thawing processes kept for 4h (i.
23、e. a freezing-thawing cycle of 8h). Samples 13 and samples 46 were taken out respectively after 10 times and 20 times of freezing-thawing to implement Brazilian split test, while samples 79 were taken out and weighted after each freezing-thawing cycle, and then applied in nuclear magnetic resonance
24、test. (2) Nuclear magnetic resonance test In the nuclear magnetic resonance test, the rock samples weighted after freezing and thawing cycles were wrapped using preservative film, and then put in the carrying bed of nuclear magnetic resonance (NMR) imaging analyzer AniMR-150 so as to implement the p
25、orosity nuclear magnetic resonance (NMR) measurement. (3) Brazilian split test 7Samples were saturated in a corresponding solution, and then taken out after 48h and dried in the same environment, and then applied in Brazilian split test. 3 Analysis on the experimental result 3.1 Quality variation Th
26、e quality variation of the samples after freezing-thawing cycles was shown in table 1. Under the effect of different water chemical environment, the effect of freezing-thawing cycles on the red sandstone damage degradation was different. The quality of the sample was increased after 10 times of free
27、zing-thawing, but the increasing quality amount of the sample in different water chemistry solution was different. The increasing quality amount of the sample in solution NaCl was the largest and the average increasing amount was 0.92 g, and the average increasing amount percentage was 0.642%, follo
28、wed by the quality increase of the sample in NaOH; the quality increases of the samples in H2SO4 and water were 0.23g and 0.29g, respectively. After 20 times of freezing-thawing cycle, the quality of the samples in water continued to increase, with an average increase of 0.06g; the quality of the sa
29、mples in H2SO4, NaOH, and NaCl tended to decline (except E8), among which the quality of sample D7 was reduced the most greatly, up 8to 4.77g. After 30 times of freezing-thawing cycle, the quality of the samples in water still increased; the quality of the samples in other three solutions were reduc
30、ed, and the quality of sample D7 was reduced the most greatly, up to 16.07g. Through the test, it was found that there were free particles and turbid substances in solutions NaOH and NaCl when samples were saturated in different water chemical solutions before freezing-thawing cycle test was impleme
31、nted. From the comparison on the quality variation of red sandstone samples in four chemical conditions (see figure 3), it was seen that the quality loss of the red sandstone immersed in NaOH, NaC, and H2SO4 after the freezing-thawing cycles was greater than that of the red sandstone samples in wate
32、r. In figure 4, the contrastive photos of sample D7 before and after freezing and thawing were shown, finding the samples surface corrosion degree was serious after freezing and thawing and a large number of particles fall off. From figure 3, it was seen that the quality of each sample would increas
33、e after 10 times of freezing-thawing cycle, suggesting the frost heaving and thawing effect of ice made the micro-pore in rock constantly increased after each freezing under the action of freezing and thawing cycle, and also new micro-pore produced in the samples, 9so that moisture migrated toward t
34、he inside of rock. After freezing and thawing cycles, the quality variation and the appearance of the samples in the testing process were observed, and then the freezing-thawing damage degradation modes of red sandstone in NaOH, NaCl, and H2SO4: particles-pore damage mode and its damage degradation
35、process was the existence of rocks micro-pore the emergence and desquamation of the free particles on the surface the softening of the surface the emergence of new micro-pore the constant expansion of micro-pore further softening: one-step softening and loosing a large number of particles fall off m
36、oisture migrated to the inside pore extension and connection constantly deepened freezing-thawing damage. 3.2 Porosity change In table 2, the mean value of the nuclear magnetic resonance porosities after samples underwent different times of cycles in different solutions was shown. Therefore, it was
37、seen that the porosity of each sample in different solution increased along with the increase of freezing-thawing cycles. Along with the progress of freezing-thawing cycle, rock porosity increased, suggesting the porosity change of rock was greatly affected by the freezing-thawing cycles; along with
38、 the 10increasing number of freezing-thawing cycles, the production and expansion of new pores in rock were accelerated, and also the porosity change was obvious. The different rock porosity changes in four water chemistry solutions showed that the damage from freezing-thawing test to the samples in
39、 different solutions was different: the damage was the largest to the samples in NaOH and NaCl, followed by that in H2SO4; the degree of damage in water was minimal. 3.3 Rock stress characteristics Based on Brazilian split test, the tensile strength of red sandstone samples in different solution and
40、 after different times of freezing-thawing cycles was obtained, as shown in table 3. From table 3, it was seen that the tensile strength of red sandstone was reduced significantly along with the increase of the times of freezing-thawing cycle, suggesting the tensile strength of red sandstone was greatly affected by freezing-thawing damage. The tensile strengths of red sandstone in different times of cycle and different solutions were fit using polynomials, so that the relationship curve and fitting relation between tensile strength and freezing-thawing cycles in different
