磷石膏 - 高铝水泥双掺增强水泥固化有机质土强度室内试验研究

(1School of Transportation, Southeast University, Nanjing 210096, China)(2Sihong County Transportation Board, Sihong 223900, China)

Abstract：In order to improve the engineering properties of organic soil, a new stabilization agent is developed by the addition of phosphor gypsum and calcium aluminate cement. The artificial organic soil is applied in the study and a series of laboratory tests were carried out to explore new stabilization agents and determine the optimal dosage. Unconfined compressive strength (UCS) and the pH value of soil pore solution were measured. The influence of organic content, agent composition and curing time on the UCS of samples were also researched. The test results show that the UCS of stabilized organic soils by a new agent achieves approximately 800 and 1 200 kPa at 28 and 90 d curing time, respectively. The pH test results show that a high alkaline environment is a necessary and not a sufficient condition for high strength. The strength of stabilized soil is related to the hydration product of stabilization agent. The mechanism of strength formation was also explored by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) tests. A large amount of ettringite is produced to fill the large pores of organic soils, which contribute to the high UCS value of stabilized organic soils. The new agent can solidify the organic soil successfully as well as provide a new approach to treat the organic soil.

Keywords：organic soil; stabilization; strength; ettringite

Biography：Zhang Dingwen(1978—), male, doctor, professor, zhangdw@seu.edu.cn.

In China, 13 920 m2areas are covered with organic soil[1]. Soil stabilization is a widely used method for improving the geotechnical properties of soft soil by cementitious or pozzolanic binders. However, it is difficult to stabilize soil with a high organic matter content. This is mainly because organic matter has the tendency to hinder the hydration processes and related reactions are required for the development of strength. It is difficult to form calcium silicate hydrates (CSH) gel, which constitutes the strength of cement stabilized soil.

Organic soil is a type of heterogeneous soil formed by the putrefaction of organic matter such as plant remains, foliage, leaves, trunks, and so on[2]. Due to its low pH, big pores, high natural water content and compressibility of organic soils, it is difficult to treat effectively in the practical engineering. The organic substances in soil are divided into three main fractions: humin, humic acids and fulvic acids[3]. Humic acids and fulvic acids seriously delay the cement hydration process. Therefore, many researchers have proposed adding some additives to modify the properties of organic soil mixed with cement. For example, Tastan et al.[4]reported that fly ash has a positive effect on improving the UCS of cement solidified organic soils, and the enhancement degree mainly depends on the type of soil and fly ash. Hayashi et al.[5]indicated that a binder with high concentrations of sulfur trioxide and aluminum oxide has a positive effect on binding peat. Kolay et al.[6]established different correlations between the physical and geotechnical properties of peat and UCS of treated peat samples with different types of stabilizers including fly ash, quicklime and ordinary Portland cement. He also concluded that Portland cement may be the best choice compared with fly ash and quicklime.

Phosphor gypsum (PG) is an industrial by-product produced by phosphoric acid. About 5 t PG is generated along with production of 1 t phosphoric acid, and approximately 2×107t PG is generated every year in China. However, the reuse ratio of PG is less than 10%. Therefore, measures related to reusing PG should be adopted immediately.

This study investigates the effect of PG and calcium aluminate cement (CAC) additions on the UCS of cement stabilized organic soil based on a series of laboratory tests. Experimental studies were carried out on two artificial organic soils stabilized by several types of combined ordinary portland cement(OPC), PG and CAC. The influences of the element of stabilizing agent, organic matter content and curing time on UCS of samples were discussed. Mineral composition, pore characteristic and the microcosmic shape of stabilized soils were analyzed using XRD, MIP and SEM tests.

1 Materials and Methods

The tested soil is collected from the site of S121 highway in Sihong County, China. The physical parameters of the tested soil are shown in Tab.1.

In order to study the influence of organic matters, artificial organic soils were prepared. The fulvic acid was selected as the organic matter in this study. According to the ASTM D2487-11[7], an organic clay is a soil that is classified as a clay except that its liquid limit value after oven drying is less than 75% of its liquid limit value before oven drying. Therefore, the artificial mixed organic soil should take the liquid limit measurement compared with the inorganic soil. All the measurements of soil liquid limit used the Casagrande apparatus based on the standard of ASTM D4318-10e1[8]. The limit liquid of soil is shown in Tab.2. The inclusion of organic soil increases the liquid limit of soil. The liquid limit of artificial organic soil is greater than 25% of the soil without organic matter. Therefore, the two artificial soils are organic soils according to the ASTM D2487-11.

The selected binder is constituted by OPC, PG and CAC. The proportions of different binder types are listed in Tab.3. This proportion is on a dry soil weight basis. The chemical composition of each proportion is shown in Tab.4.

1.2 Sample preparation

Air-dried soil was passed through a 2 mm sieve and admixed with organic matter first. After the soil and organic matter were mixed homogeneously, OPC, PG and CAC powder were added proportionally into the organic soil. Then, certain water was added into the mixture. In order to stir the mixture homogeneously, the blender is stirred for 2 min with a velocity of (140±5) r/min and then stirred for 2 min at a velocity of (285±10) r/min. Then, the mixture was enclosed into a cylindrical plastic mold with a 5 cm inner diameter and 10 cm height hierarchically. The mold was vibrated until it was not bubbling up to the surface of the sample, which indicated that the sample was dense. All the specimens were cured in a standard curing room with a temperature of (20±2)℃ and relative humidity of 95%. The UCS of the sample and the pH of soil pore solution were measured after the sample was cured to 14, 28 and 90 d, respectively.

The details of samples can be seen in Tab.3. The organic contents used are 5% and 10%, and the target moisture content is 61.3% (i.e. the liquid limit of the soil with 10% organic content).

The UCS test was performed according to ASTM D2166/D2166M-16 with a strain rate of 1%/min[9]. The moisture content of the specimen was also measured after the UCS tests. All the specimens were dried in the 60 ℃ oven due to the presence of organic matter. After the UCS test, the pH of the soil pore solution was measured following the standard ASTM D4972[10]. Triplicate specimens were tested for UCS and pH, and the average values of three results are discussed in this study. The apparatus applied in the XRD analysis is Smartlab. The apparatus applied in the SEM analysis is HITACHI SU3500.

2 Results and Discussion

2.1 Physical properties of soils

The dry densities of stabilized soils with different organic matter are shown in Fig.1. The results indicate that adding PG and CAC into binders results in an increase of dry density of stabilized soils. In addition, the dry density of stabilized soils increases with the curing time, which indicates that a great amount of new compounds such as CSH and calcium aluminate hydrate(CAH) gels are formed as a result of hydration processes and continuous pozzolanic reactions. The CSH and CAH gels subsequently crystallize to bind the structure together.

OPC was first adopted only to solidify soil with a 5% organic content and the results are shown in Fig.2. Thesamples with a 10% organic matter have a negligible strength. The results indicate that the UCS of cement stabilized organic soils increases with the increase of cement content and curing time. However, the 90 d UCS of the stabilized sample is less than 300 kPa, which cannot meet the requirements of the design criteria. This mainly results from the existence of organic matters, which delay or inhibit the hydration of the cement. Axelsson et al.[11]pointed out that a low pH (i.e. less than 7) pore solution resulting from the existence of organic matters can retard strengthening reactions and decompose the products of cement hydration. Organic matters also form complexeswith alumino-silicates and with metal ions. Clare et al.[12]and Young[13]found that when cementitious binders, such as cement and lime, were mixed with soil, the organic matters in soil will absorb calcium ions. Following the hydration of a cementitious binder, calcium ions are more likely to be attached by the organic matters. Young[13]also reported that alumina ions can also form stable complexes with organic matter as well as calcium ions. Therefore, calcium and alumina ions are not sufficiently available to form CSH and CAH gel from hydrated reactions and pozzolanic reactions.

The UCS test results are shown in Fig.3. The UCS values of all samples increase with the curing time. The maximum value of 90 d UCS of the stabilized sample reaches 1 200 and 800 kPa for soil with 5% and 10% organic content, respectively. Using PG to replace part of the OPC leads to a remarkable increase of UCS values than using OPC alone for soils with organic content. For example, the 90 d UCS of S30-0-0, S25-5-0, and S20-10-0 are 270, 880 and 1 150 kPa, respectively. The UCS of organic soils stabilized by combining OPC and PG increases almost linearly in 90 d.

The test results also indicate that combining PG and CAC to replace part of the OPC leads to a significant increase of the 28 d UCS value than using OPC alone for soils with organic content. For example, after the addition of 2.5% CAC (S25-2.5-2.5), the 28 d UCS value is almost two times of those without CAC (S25-5-0). The organic soils stabilized by combining PG, CAC and OPC show an obvious UCS increase during the 28 d curing time, but the UCS increase rate slows down after the 28 d curing time. The 90 d UCS of the sample stabilized by combining PG, CAC and OPC is smaller than that stabilized by combining PG and OPC. The velocity of ettringite formation is determined by the concentration of Al3+source. Odler et al.[14]proposed that the high Al3+ion concentration accelerates the formation of ettringite and then forms the very early strength of cement stabilized soil.

The UCS of samples with 10% organic content is nearly two-thirds of UCS of samples with 5% organic content, which confirms that organic content has an enormous influence on the strength of solidified soils.

UCS of cement stabilized soil results from the CSH gel as the product of cement hydration. However, organic matters seriously delay or retard the process of cement hydration, so there is little CSH gel produced. The increase of UCS values of samples stabilized with combining OPC and PG is mainly due to the formation of the ettringite phases (Aft). The ettringite phases can be formed if there are enough S

ions in the solution of cementitious materials. The content of S

ions in OPC is very low, so the amount of the ettringite is negligible for samples stabilized using OPC alone. The additive of PG can compensate for the S

ions. Martínez-Ramírez et al.[15]demonstrated that there was a large amount of ettringite product if cement was mixed with gypsum. Ettringite fills the pores in soils effectively and the decrease of the porosity increases the UCS of the sample, especially for the organic soils known as their large porosity. So, the samples stabilized using OPC and PG (S25-5-0, S20-10-0) obtain much higher UCS than samples stabilized using OPC alone (S30-0-0).

After using half of the CAC to substitute PG, the samples reach a higher 28 d UCS values than samples without CAC. This can be attributed to the early hydration of CAC. CAC is mainly composed of tricalcium aluminate (C3A), which contributes to a rapid and strongly exothermic reaction with water and the high early strength of cement mortar. Young[13]investigated the effect of C3A on the initial setting process and early strength of cement mortar. So, the addition of CAC mitigates the delaying or retarding of the organic matter to a large extent, and increases the early strength of organic soils admixed with cement. C3A is the most reactive phase of Portland cement, and produces a rapid strength development due to the formation of metastable hexagonal hydrates. In the presence of PG, the main component calcium sulphate reacts with C3A, leading to the formation of ettringite. This increases the 28 d UCS of samples stabilized using OPC, PG and CAC (S25-2.5-2.5, S20-5-5).

2.3 pH of the soil pore solution

The pore solution pH measurement results of stabilized soils are shown in Fig.4. From Fig.4, it can be seen that the soil pore solution pH values are all below 11.5 and this high pH value fails to lead to the high UCS value. It is noted that the UCS reaches nearly 1 MPa with the pH value between 10.1 and 10.5. This result is inconsistent with previous findings.

Che et al.[16]reported that high strength of cement stabilized soils results from the high alkaline level of pH values, which should be more than 11.5. The pH value that can ensure that the CSH is stable is not less than 11.5. The different results can be attributed to the fact that different minerals contribute to the strength of soils using different agents. The OPC stabilized soil obtains high UCS value mainly by the CSH gel through the hydration of cement. However, for soils with some organic matter, the process of hydration of cement is delayed and it is difficult to form the CSH gel. With the addition of CAC and PG, the ettringite formation is accelerated, which contributes to the strength of stabilized soils with an organic content. Therefore, different pH values that make CSH and ettringite stable lead to the discrepancy. Babushka et al.[17]found that the pH value to ensure that the ettringite is stable is approximately 10.17. Tremblay et al.[18]indicated that, if the organic matters produce a pore solution pH lower than 9, the development of cementing products will be strongly affected and there is almost no strength gain. In this study, the pore solution pH value is more than 10, which ensures the formation of ettringite, and the samples exhibit a high UCS value.

2.4 Porosity of the solidified soils

As mentioned above, the large porosity of organic soil is effectively filled by the ettringite. The porosity of stabilized soils is shown in Fig.5. The porosity is calculated by

whereρis the density of stabilized soil;ωis the moisture content of stabilized soil;ρωis the density of purified water at 4 ℃;Gsis the composite specific gravity based on OPC, PG, CAC and fulvic acid mass percentages in the specimen.

It can be seen from Fig.5 that the porosity of soil with 5% organic matter is smaller than that of soil with 10% organic matter, which confirms that higher organic content results in a large porosity in soil. It is clear that the porosity is decreased by the addition of PG and CAC, and the results agree with the results of UCS. A lower porosity leads to a higher strength. For example, the porosity of soil S20-5-5 is decreased by approximately 4% compared with soil S30-0-0.

3 Microscopic Results

3.1 Mineral composition

Fig.6 shows the XRD results of stabilized soil after curing 14 d. The vertical axis of Fig.6 stands for the intensity of diffraction and the horizontal axis represents for the angle of diffraction (2θ). The diffraction angle has correspondence with the characteristic peakdvalue. Thus, the following equation is established,

wheren=0,±1,±2,…;λis the wave length of diffraction.

Huang et al.[19]pointed out that characteristic peakdvalues of ettringite are usually 5.61, 3.87 and 2.56, respectively. Moreover, thedvalue of 5.61 of ettringite is independent of the other mineral peak, so it can be distinguished clearly. The dominant mineral of sample S30-0-0 is quartz and ettringite is not found in all the characteristic peaks. However, the ettringite can be found in samples S20-10-0 and S20-5-5. With the addition of PG, the active Ca2+, Al3+and S

ions will form ettringite. Therefore, the ettringite is presented in sample S20-10-0, but the active Al3+source in OPC is finite and the amount of produced ettringite is limited, which can be seen in the latter SEM photographs.

3.2 Pore size distribution

In order to analyze the degree of filling, the MIP test is adopted to observe the microcosmic structure of pore. Fig.7 shows the effect of the addition of PG and CAC on the pore size distribution of specimens after 14 d. It can be seen from Fig.7(a) that the cumulative pore volume decreases with the addition of PG and CAC. The cumulative pore volumes of the samples S30-0-0, S20-10-0 and S20-5-5 are approximately 0.32, 0.30 and 0.24 mL/g, respectively. Due to the filling function of new mineral ettringite, the cumulative pore volumes of the sample decrease. A better packing and more close-grained soil structure is formed with the addition of PG and CAC. Fig.7(b) shows the differential pore volume distribution curves of the soils treated with different binders. The figure shows that the pore size mainly concentrates on 100 to 1 000 nm. After adding PG, the peak and width of the curve decrease compared with those of soil using OPC as a binder alone. With the addition of CAC, the peak and width of the curve decline further. It suggests that the pores of treated soils are filled effectively.

3.3 Scanning electron microscope analysis

Fig.8 exhibits the microstructures of treated soils after 14 d curing with 5 000 times magnification. The particles of stabilized organic soils using OPC alone are obviously separated with clear boundaries and a vast number of large pores are presented in the soil. There is almost no production of cement hydration and the soil and cement particles are wrapped by organic matters closely. Both the reaction of cement hydration and the pozzolanic reaction are retarded. Therefore, they exhibit a low UCS value. After PG is added into the binding system, a certain amount of ettringite is formed and large pores are filled. Therefore, the stabilized soil structure is becoming dense as shown in Fig.8(b), which suggests a high UCS value. And, the degree of density of the stabilized soil is enhanced further by adding CAC. A large amount of ettringite is produced in the reaction, as shown in Fig.8(c). The ettringite exhibits a short rod-like shape due to the short reaction time, and it is developed incompletely. CAC accelerates the formation of ettringite and enhances the early UCS of stabilized organic soils.

After the curing time reached 90 d, the effect of PG on the microstructures of treated soils can be seen clearly. From Fig.9, it is clear that the pores of soil sample S20-10-0 are bestrewed by a large amount of ettringite. Due to the long rod-like shape of ettringite mineral, it interweaves with soil particles to enhance the strength of thestabilized soil. The ettringite is developed completely due to long curing time (90 d). It can be seen that there is a large amount of ettringite and CSH interweaving with each other to exhibit a high UCS. This means that the delaying or retarding effect of organic matters becomes less obvious with the formation of CSH.

4 Conclusions

1) The UCS of stabilized organic soils improved with the addition of PG and CAC. The maximum value of 90 d UCS of stabilized sample reaches 1 200 and 800 kPa forsoil with 5% and 10% organic content, respectively.

2) A large amount of ettringite is produced to fill the large pores of organic soils, which contributes to the high UCS value of stabilized organic soils. CAC has the ability to enhance the early strength.

3) The pH value of soil pore solution is not the adequate parameter for predicting the strength of the stabilized organic soils. In order to ensure the stability of ettringite in soils, the pH value of soil pore solution should approximately range from 10.1 to 10.5.

4) XRD and SEM test results indicate that the main hydration product for stabilized organic soils with the addition of PG and CAC is ettringite, which contributes to the enhancement of UCS of stabilized organic soils. MIP test results show that the pores in organic soils are filled effectively by ettringite. Stabilized organic soils with the addition of PG and CAC have a lower porosity than that of soil stabilized using OPC alone.

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磷石膏-高铝水泥双掺增强水泥固化有机质土强度室内试验研究

(1东南大学交通学院, 南京 210096)(2泗洪县交通运输局, 泗洪 223900)

摘要：为了改善有机质土的工程特性,通过向水泥中外掺磷石膏与高铝水泥的方法，研发了一种新型的有机质土固化剂．通过室内试验研究了新型固化剂对人工配制有机质土强度的增强效果,并比选固化剂的最优配方．对不同配方固化剂固化有机质土开展了无侧向抗压强度试验和固化土孔隙液pH值测试．试验结果表明,外掺磷石膏与高铝水泥后固化有机质土的无侧限抗压强度可达800～1 200 kPa．pH值测试结果表明,高碱性环境是得到高强度的必要而非充分条件,其与构成强度的主要矿物密切相关．XRD,SEM和MIP试验结果证实新矿物钙矾石的生成与孔隙填充有利于提高固化有机质土体强度．该研究成果可为有机质软土的固化提供一种新的解决方法．

JournalofSoutheastUniversity(EnglishEdition) Vol．33，No．3，pp．309⁃315Sept．2017 ISSN1003—7985

Foundationitems：The National Natural Science Foundation of China (No. 51578148), the Project of China Communications Construction (No.2015-ZJKJ-26), the Fundamental Research Funds for the Central Universities, the Scientific Innovation Research of College Graduates in Jiangsu Province(No.SJLX15_0062).

Citation：Zhang Dingwen, Liu Ziming, Sun Xun, et al．Laboratory tests on enhancing strength of cement stabilized organic soil with addition of phosphor gypsum and calcium aluminate cement[J]．Journal of Southeast University (English Edition),2017,33(3):301-308．