Reaction of Calcium Hydroxide with Allophane-Kaolinite Clay Minerals

Clay Science, Vol. 3, Nos. 1-2, pp. 11-36, 1967.

REACTION OF CALCIUM HYDROXIDE WITH ALLOPHANE-KAOLINITE CLAY MINERALS

Tomoko KOBASHI

The Railway Technical Research Institute

1. Introduction

Volcanic ash soils are distributed widely in our country. These soils are unique in their mechanical properties, and their moisture contents are very high, frequently causing troubles during and after construction. It is generally accepted that such properties are mainly ascribed to the nature of clay minerals contained in these soils. Although various attempts have been made to chemically stabilize volcanic ash soils, effective methods are very few. The lime-treatment is expected to stabilize soils by such mechanisms as changing soil surface properties so as to decrease cohesion, increasing binding forces by the pozzolanic reaction and consuming excess water as combined water. The lime-treatment is suitable for the use in the field, and is used widely. But little is known of the nature of reaction or of reaction products, because reaction mechanism and reaction rate vary with the kind of soil. Basic studies have been made of the reaction of allophane with calcium hydroxide in solution by ARIIZUMI (1961), with the result revealing that the reaction product is hydrated gehlenite which was recognized in metakaolin-calcium hydroxide solution by STRATLING (1940). The object of this study is to investigate the reaction between allophane-kaolinite group and calcium hydroxide with water content similar to that of volcanic ash soils in natural conditions, the relation between the consumption of calcium hydroxide and the formation of reaction products, and the effects on the soil mechanical properties.

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2. Clay samples

Clay samples used in this study, their abbreviated names and their sources are as follows :

Halloysite group API Clay Mineral Standard Halloysite (H)-Arrowhead Mine, Spruce Pine, North Caroline, USA. API Clay Mineral Standard Halloysite No. 13 (H13)-Dragon Iron Mine, Eureka, Utah, USA. API Clay Mineral Standard Halloysite No. 12 (H12)-North Gurdi- ner Mine, Bedford (Huron), Indiana, USA. Chosen Kaolin (Hc)-Keisho Nanda, Korea.

Kaolinite group API Clay Mineral Standard Kaolinite No. 5 (K5)—Carmar Pit, Bath, South Carolina, USA. API Clay Mineral Standard Kaolinite No. 9 (K9)—Canada del Camino, Mesa Alta, New Mexico, USA. API Clay Mineral Standard Kaolinite No. 7 (K7)—Dixie Rubber Pit, Bath, South Carolina, USA. Gairome Clay (Kg)-Gifu, Japan.

Allophane group Kanuma Soil (Aka)-Shimotsukeosawa, Tochigi, Japan. Kokubunji Soil (Ako)-Kokubun ji, Tokyo, Japan. Morioka Soil No. 1 (Ami)-Shiriuchi, Iwate, Japan. Morioka Soil No. 5 (Am5)-Shiriuchi, Iwate, Japan.

The clay particles less than 5 microns in size were extracted by siphoning from soil suspension after proper sedimentation.

2.1. X-ray diffraction analysis 2.1.1. Experimental condition The X-ray diffraction patterns were obtained with a Rigakudenki diffraction unit, under the following measuring conditions : (12) 13

Target : Cu, Filter : Ni, Voltage : 30 kV, Current : 15 mA,

Scale factor : 8, Time const.: 2, Multiplier : 1, Scanning

speed : 2•‹/min., Divergence slit : 1•‹, Scattering slit : 1•‹ , Receiving slit : 0.3 mm.

2.1.2. Results

Fig. 1 shows the results of X-ray diffraction analysis of the clay samples.

(1) Halloysite group (Fig. 1-1)

He and H12 are more hydrated than H and H13, as indicated by the strong 10A reflexion. Besides clay minerals, H12and He contains gibbsite and quartz, respectively.

- Halloysite group —

Fig. 1-1. X-ray diffraction patterns of clay samples.

(2) Kaolinite group (Fig. 1-2) The samples range from well-crystallized to disordered kaolinite

(13) 14 in the following order ; K5, K9, K7 and Kg, which are examined by reflections that start at 4.4A and range over large angles. ,In K5, which is highest in crystalinity, clear separation of 4.48A, 4.37A and

4.19A is noticed, whereas in disordered samples the separation becomes indistinct. The half maximum breadths of K5, K9, K7 and Kg are 0.37•‹,

0.40•‹, 0.45•‹ and 0.80•‹, respectively.

Fig. 1-2. Kaolinite group.

(3) Allophane group (Fig. 1-3) Aka shows typical allophane curves, having no peak except a diffuse one at 3.25A. Ako is similar to Aka but has quartz peaks. In -the case of A im, hydrated halloysite develops to some degree and (14) 15

small quantities of vermiculite and chlorite are contained. In Am5, hydrated halloysite develops progressively.

Fig. 1-3. Allophane group.

2.2. DTA

2.2.1. Experimental condition

The apparatus used was a Shimazu DT-2A type unit. The heat-

ing rate was 10•Ž/min. and the quantity of sample packed in a sample

holder was 0.5 g in every case.

2.2.2. Result

(1) Halloysite group

Fig. 2 shows endothermic peaks at 130-150•Ž due to the loss of

absorbed water, an endothermic peak at about 600•Ž due to the loss

of hydroxyl group, and an exothermic peak at about 990•Ž attribut-

able to the formation of a new phase. It is recognized that the

endothermic peaks at 130-150•Ž are larger in hydrated samples .

(2) Kaolinite group

Fig. 2 shows the result similar to the halloysite group, but the second endothermic peak and the exothermic peak are more sharp

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Fig. 2. DTA of clay samples.

(16) 17 and the first endothermic peak is smaller, although this peak becomes larger with lowering of crystalinity.

(3) Allophane group

Aka shows a typical allophane curve. Amt has a week endothermic

peak at 535•Ž, showing the presence of hydrated halloysite. In Am5 this peak is larger than in Arm.

2.3. Infrared absorption analysis

2.3.1. Experimental condition

The apparatus used was a Perkin-Elmer 21 type unit and the

experimental conditions were as follows:

Prism: NaCl, Phase: KCl disk, Resolution: 927 Response: 1-1, Gain: 6.5, Speed: 2 min/300 cm-1 Suppression: 0

2.3.2. Result Fig. 3 shows the results of the hydroxyl group. (1) Halloysite group (Fig. 3-1) Fig. 3-1 shows absorptions at about 3700 cm-1 and 3600 cm-1 due to hydroxyl groups in the clay mineral lattice, and an absorption at about 3450 cm-1 due to absorbed water, but this is inclined to increase with progress of hydration. (2) Kaolinite group (Fig. 3-1) The absorption in the range of 3700 cm-1 to 3600 cm-1 is found at about 3640 cm-1 in well-ordered kaolinite, that is, it is recognized clearly in K,, and in K9 it occurs as a shoulder in this range but it is not present in Kg. (3) Allophane group (Fig. 3-2) Ako shows a typical curve of allophane, having the very broad absorption in the range of hydroxyl groups. Am1 has a shoulder at about 3650 cm-1; in Am5 this absorption becomes clear, showing the presence of hydrated halloysite.

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Fig. 3-1. Infrared absorption spectra of clay samples.

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Fig. 3-2.

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3. Reaction products

3.1. Experimental method

(1) Mixing The samples were treated with 5 to 40% calcium hydroxide and.

85 to 200% water by weight of oven-dried clay sample.

At first, air-dried clays and calcium hydroxide were blended to-

gether in dry state, then distilled water was added to the dry mix and mixed well until a homogeneous mixture was obtained. The

mixed samples were sealed in containers to prevent the moisture

from evaporating, and were cured at 30•Ž. After curing, the samples

were dried rapidly in a vacuum desiccator, then were ground in an

agate mortar. Powdered samples were studied by means of X-ray

diffraction analysis, differential thermal analysis and infrared absorp-

tion analysis. The measuring conditions of these analyses were similar

to those for clay samples.

(2) Measurement of reflection intensity

(a) Preparation of samples

Care was taken so as to prepare the samples under as much

identical conditions as possible. The vacuum-dried samples after

curing were crushed in an agate mortar to such an extent that they

became very smooth to the "feel". Then the powder samples were

placed in a cavity of glass sample holder, tamped gently with an edge

of spatula, and the surplus powder was sliced off with a blade, then

the sample surface was pressed gently but firmly with a flat surface

of glass.

(b) Correction to instrument stability

Making allowance for reflection intensity changes with time which.

were caused by the instrument stability of X-ray tube, G. M. counter,

etc., because the measurement extended over the long periods, the

reflection intensity of sample was corrected by the intensity change

of a silicon external standard which was measured at each measuring

time.

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3.2. Result 3.2.1. Identification of reaction product (1) X-ray diffraction analysis The main reaction product identified in all systems treated with calcium hydroxide of less than 40% was hydrated gehlenite, except the cases of Aka and Ako. Fig. 4 shows the diffraction pattern of H treated with 20% of calcium hydroxide and 115% of water and cured for 2 months (H-20%Ca(OH)2-115%H2O-2 Month). The hydrated gehlenite lines were detected clearly and the calcium hydroxide lines (for example, 4.93A) disappeared. Hydrated gehlenite (C2ASHn) was recognized by STRATLING5) as the reaction product of metakaolin and calcium hydroxide solution, and was described in detail by ARIIZUMI1)-4) as the reaction product of allophane and calcium hydroxide solution. Table 1 shows the diffraction lines and identification of other reaction products, although identification is usually difficult because all of the lines are weak.

Table 1.

The 3.05A line is possibly correlated with calcium carbonate because carbonation occurred more or less during the sample preparation, although precautions were taken. Any line of these products showed no such clear change against calcium hydroxide content or curing period as in the case of hydrated gehlenite.

(2) DTA Fig. 5 shows the result of Hc-20%Ca(OH)2-115%H2O-1 Month. The strong endothermic peak detected at 210•Ž is correlated with hydrated gehlenite, according to the result of high temperature X-ray diffraction analysis of the same sample which showed that the hydrated gehlenite lines disappeared after heating at this temperature, probably

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Fig. 5. DTA of Hc treated with 20% of calcium hydroxide and 115% of water and cured for 1 month. indicating dehydration.

In addition, weak endothermic peaks were detected at 270•Ž, 315•Ž,

400•Ž as shown in Fig. 5. Fig. 7 shows endothermic peaks of various clays between 100 and 400•Ž against various curing periods. The endothermic peak at 270•Ž was detected in many systems and showed

an inclination that the intensity increased with curing period. GOTO

and others (1962) recognized the 270•Ž endothermic peak, together with

the 215•Ž endothermic peak, of hydrated gehlenite in the experiment

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of granulated slag-calcium hydroxide solution, and stated that this peak was due to the decomposition of C4AH13 and C3AH8-12, which was assured by the presence of 8.3A and 7.8A lines in X-ray diffrac-

tion analysis. Although 8.2A and 7.6A lines was also recognized in

this experiment as stated above, their correlation with the 270•Ž

endothermic peak were not clear because these lines were very weak and overlapped with the tail of 7A line of halloysite and disordered

kaolinite.

The endothermic peaks that appeared at temperatures higher than

270•Ž are considered to indicate dehydration, but these peaks are very

weak and their change in each system is not clear, so that the iden-

tification is difficult, similar to the case of the X-ray diffraction

analysis.

The endothermic peak at 310•Ž of H12 is due to dehydration of

gibbsite.

In Fig. 5, the endothermic peak at about 580•Ž due to the ex-

pulsion of water from the clay mineral lattice decreased with rising temperature, showing partial decomposition of the clay mineral lat-

tice by the heat treatment.

With respect to the exothermic peak at about 990•Ž, the intensity

decreased and the peak became broad. But in the case of a clay

mineral of which the exothermic peak was weak, like Ami, the inten-

sity became strong in constrast, showing the appearance of gehlenite

at about this temperature, which was supported by the result of high

temperature X-ray diffraction analysis of the same sample . Consequ- ently it is considered that hydrated gehlenite becomes X-ray amor-

phous by the dehydration corresponding to the endothermic peak at

210•Ž but changes to gehlenite when heated at about this temperature .

(3) Infrared absorption analysis

Except a small and diffuse absorption at 1450-1500 cm-1, new ab-

sorptions were not detected. The absorption at 1500 cm-1 was recog-

nized also in the reaction of allophane-calcium hydroxide solution by

ARIIZUMI.

(24) 25 3.2.2. Change of calcium hydroxide and hydrated gehlenite with curing period Since it was recognized that the main reaction product was hydrated gehlenite, the changes of calcium hydroxide and hydrated gehlenite with curing period were investigated by the X-ray diffraction analysis and DTA. (1) X-ray diffraction analysis (a) Halloysite and kaolinite groups Fig. 6 shows the intensity change of the 4.93A line of calcium hydroxide and 12.6A line of hydrated gehlenite with curing period. The intensity of the 4.93A line was compared with that of powder mixture without addition of water. The intensity of the 12.6A line was relative so that its absolute value is meaningless. Fig. 6-1 shows the results of halloysite group treated with 20% calcium hydroxide and 115% water. The intensity of the calcium hydroxide line decreased rapidly, and the line disappeared completely after 7-14 days. Formation of the hydrated gehlenite was similarly rapid. In hydrated samples, such as He and H12, hydrated gehlenite was detected after one day and became constant after 7 days for H12 and 14 days for Hc. In the case of less hydrated samples, hydrated gehlenite was detected after 3 days for H and 5 days for H13, and became constant after 14 days for H and 30 days for H13. In the case of kaolinite group as shown in Fig. 6-2, the decrease of calcium hydroxide and the formation of hydrated gehlenite were slow. Especially in well-ordered samples this inclination was remark- able. In K5, calcium hydroxide did not completely disappear and hydrated gehlenite was not detected even after 6 months. However, in disordered samples the reaction became rapid. In K9, calcium hydroxide disappeared completely and hydrated gehlenite was detected after 6 months. In K7, hydrated gehlenite was detected after 30 days. In Kg, calcium hydroxide disappeared after 2 months and hydrated gehlenite was detected after 14 days and became constant after 2 months. Fig. 6-3 shows the result of similar examination when the mixing

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―Halloysitegroup―

Fig. 6-1. The intensity change of the calcium hydroxide line (4.93A) and the hydrated gehlenite line (12.6A) with curing period. ratio of calcium hydroxide was changed. Similar to the case of 20% calcium hydroxide treatment, the intensity change of calcium hydroxide was inclined to be rapid at the start and then to become comparatively slow and linear. Hydrated gehlenite was inclined to be detected after a certain quantity of calcium hydroxide was consumed, as revealed later, and then it increased with decrease of calcium (26) 27

•\ Kaolinite group•\—

Fig. 6-2. hydroxide. The curing period expended until hydrated gehlenite was first detected was inclined to become longer with the increase of the calcium hydroxide content. For example, in Kg this period was 7, 14, 30 days for 10, 20, 40% of calcium hydroxide, respectively. In addition, the formation rate of hydrated gehlenite was slow in the sample treated with 40% calcium hydroxide. As stated above, hydrated gehlenite was detected after a certain

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Fig. 6-3. quantity of calcium hydroxide was consumed, and this quantity per 1 g of clay was 0.08, 0.1-0.14 and 0.15-0.16 for 10, 20 and 40% of calcium hydroxide, respectively. The hydrated gehlenite intensities attaining to constant value are

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0.2 and 0.3-0.4 for 10% and 20% of calcium hydroxide contents, respectively. For 40% of calcium hydroxide, the intensity did not attain to constant value, but in HL it attained to 0.5 after 2 months. Although the accuracy of the intensity measurement could not be very high since the intensity was measured by the method mentioned in 3-1 and calcium hydroxide was consumed more or less in reactions different from the hydrated gehlenite formation, the hydrated gehlenite intensity per 10% of calcium hydroxide is considered to be about 0.2. (b) Allophane group For allophane group treated with less than 40% calcium hydroxide, in contrast with halloysite or kaolinite group, calcium hydroxide reacted with clay very raidly and after 30 minutes the calcium hydroxide lines already disappeared, but hydrated gehlenite was not detected even after 6 months. In Anil of allophane hydrated halloysite mixture group, which was treated with 20% of calcium hydroxide, the calcium hydroxide decreased rapidly, similar to the cases of Aka and Ako, but hydrated gehlenite was detected after 3 hours and became constant after 5 hours. In Am5 in which hydrated halloysite develops more progressively than in Ami, the reaction became slow, that is, hydrated gehlenite was detected after one day and became constant after 7 days. The intensities attaining to constant value per 10% of calcium hydroxide were 0.05 for Aml, and 0.1 for Am5, which were smaller than for halloysite and kaolinite groups. (2) DTA (a) Halloysite and kaolinite groups Fig. 7 shows the change in the 200-400°C temperature range with curing period. As mentioned in 3.2.1., the endothermic peaks at 210°C and 270°C show the dehydration of hydrated gehlenite and C4AH12 or C3AH8_12, respectively. The endothermic peaks at temperature higher than these peaks seem to show the dehydration of hydrated reaction products but cannot be identified clearly. (29) 30

Koolinite group Holloysite group

Temperature (° C)

Fig. 7. DTA curves between 100-400•‹C.

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It was recognized that the endothermic peak at 210°C increased with curing period. Comparing this change with that of the hydrated, gehlenite peak in X-ray diffraction analysis, hydrated gehlenite was detected earlier in DTA than in X-ray diffraction analysis. In Kg, hydrated gehlenite was not detected before 14 days in X-ray diffraction analysis, whereas in DTA it was detected after one day and became distinct after 3 days. Also in K5, hydrated gehlenite was not detected before 6 months in the X-ray diffraction analysis, though in DTA it was detected after one month. (b) Allophane group Fig. 7 shows the result of Ako-20 Ca(OH)2-115%H2O-2 Month. The endothermic peak at 210°C was not detected, and higher temperature peaks were not clear. The result of Aka was similar to that Ako. Am' showed endothermic peaks at 205°C and 280°C and also weak endothermic peaks at higher temperatures.

4. Unconfined compressive strength of clay treated with calcium hydroxide

4.1. Experimental method The clay samples examined in this test were HL, Kg and Ako of less than 5 microns. The mixing ratio of calcium hydroxide was 20% and that of water was 85, 115 or 145%. The samples, mixed by a method similar to that stated in 3.1.1., were put into a container which was separated into six chambers of which inner size is 3.7 cm x 2.0 cm x 2.0 cm, and then were compacted ten times with a tamper when water content was low, or were poured in the container when water content was high. Then they were sealed with a polyethylene film and were cured at 30°C. After curing, excess samples were sliced off which a blade. The testing apparatus was a Maruto model No. S 38 B unit of oil pressure type. The compressing speed was about 1% per minute.

4.2. Results Fig. 8 show the change of compressive strength with the curing

(31) 32 period. For Hc, the compressive strength measured immediately after mixing was zero as the water content is high in comparison with the optimum moisture content of the sample, but the strength attained to 10 kg per cm2 after 7 days and increased with curing time. However, the rate of increase slowed after 14 days.

Fig. 8. Unconfined compressive strength of . clays treated with 20% of calcium hydroxide.

For Kg, the strength immediately after mixing was zero in all cases of 85, 115 and 145% of water contents. The strength of the samples treated with 85 and 115% of water contents became measurable after 7 days and that of the sample treated with 145% of water content was measurable after 14 days, and thenceforth the strength increased almost linearly. On the other hand, the strength of Ako scarcely increased.

5. Discussion

5.1. Formation of hydrated gehlenite As a result of the experiments on clay minerals of less than 5 microns, it was recognized that in the case of allophane, calcium

(32) 33 hydroxide reacted very rapidly when the content of calcium hydroxide was less than 40°0, but the reaction products were not detected. On the other hand, in the case of kaolinite and halloysite, calcium hydroxide reacted slower than in the case of allophane, and hydrated gehlenite was detected as the main reaction product. In addition, the change of calcium hydroxide was inclined to be rapid at the start and then to become comparatively slower almost linearly. Hydrated gehlenite was detected after a certain quantity of calcium hydroxide was consumed, and it became constant after the calcium hydroxide lines had disappeared completely. STRATLING stated that in the reaction of metakaoline and calcium hydroxide solution, the calcium hydroxide was adsorbed by metakaoline in the first place, then was diffused into the metakaoline lattice, and finally it was coordinated so as to form the crystalline product. This mechanism was recognized in the reaction of allophane-calcium hydroxide solution by ARIIZUMI. It seems that the experimental result can be explained by assuming that the surface area of clay mineral related to the adsorption is an important factor in addition to the mechanism stated above. Of course, the surface area related to the adsorption is not the total one but one which has a charac- teristic nature. According to the assumption, when the quantity of calcium hydroxide is smaller than the quantity which is required for a certain surface area of each clay mineral, the concentration of calcium hydroxide adsorbed is poor per unit area and so hydrated gehlenite cannot be formed. Since allophane has a very large area, hydrated gehlenite cannot be formed when the content of calcium hydroxide is less than 40%. On the other hand, in the case of halloysite and kaolinite, hydrated gehlenite can be formed even when calcium hydroxide is less than 10%, because the area is small. This assumption can be supported by the experiment which detected hydrated gehlenite in Ako of less than 420 microns even when the content of calcium hydroxide was 20%. (The experiments about the reaction rate, etc., have not yet been made.) Moreover, the

(33) 34 unconfined compressive strength, which is related to the formation of hydrated gehlenite as will be discussed later, develops in Ake,of less than 420 microns in contrast with the sample of less than 5 microns. For the sample of less than 420 microns, it can be assumed that the surface area related to the reaction per 1 g of clay became smaller, so that hydrated gehlenite was formed and then the strength develops even when the content of calcium hydroxide was 20%. In Kg and treated with 5% of calcium hydroxide, hydrated gehlenite was detected in the former but not in the latter. This suggests that the content of calcium hydroxide was deficient in H. which has a larger surface area related to adsorption than Kg. Consequently, it is probable that in the formation of hydrated gehlenite the surface area of clay mineral is a more important factor than the weight. The writer intends to proceed with the experiments on the surface area and the nature, of each clay mineral hereafter. The curing periods when hydrated gehlenite was detected were longer in X-ray diffraction analysis than in DTA. In the case of Kg treated with 20% of calcium hydroxide, the diffraction peaks were detected after 14 days, but the endothermic peaks at 210°C had already been detected in DTA after one day. In the case of Ks treated with 20% of calcium hydroxide, the diffraction peak was not detected even after 6 months, but the endothermic peak at 210°C was detected after one month. As the endothermic peak at 210°C point to the expulsion of crystal water, it is considered that an intermediate reaction product of hydrated gehlenite was formed, which was X-ray amorphous but had a closely allied bond to the crystallized form. In the case of allophane under this experimental condition, even the intermediate reaction product could not be formed because the 210°C peak was absent.

5.2. Relation between the formation of hydrated gehlenite and the unconfined compressive strength In the case of He and Kg where hydrated gehlenite was formed the strength developed, whereas in the case of Ako without formation

(34) 35 of hydrated gehlenite the strength scarcely developed. Moreover, the strength developed more rapidly in He where hydrated gehlenite increased rapidly, than in Kg, so that it is clear that the development of the strength is correlative with the formation of hydrated gehlenite. The strength change of Kg was almost linear, but the gradient of changed after 14 days. The intensity change of hydrated .gehlenite in X-ray diffraction analysis for treated with the same condition (Fig. 6-3) shows constant value after 14 days, so that it is considered that the hydrated gehlenite formation has a correlation with the unconfined compressive strength. Although the unconfined compressive strength of other clay samples was not measured, the degree of reaction was investigated by measuring the penetration of a needle per unit time by the use of the cement setting testing apparatus. As a result, it was recognized that the degree of penetration decreased with the increase of hydrated gehlenite. It seems that the intermediate reaction product of hydrated gehlenite contributes to the development of strength, because Kg treated with 20% of calcium hydroxide and 115% of water showed that the strength developed between 1 and 14 days, and in this period X-ray diffraction diagrams showed no peak but the endothermic at 210°C was present in DTA. Also in the case of K5, no diffraction peak was ,detected after 6 months but the endothermic peak at 210°C was detected after one month, and the degree of penetration decreased in this period. In the case of Ami of allophane-hydrated halloysite mixture group, the decrease of calcium hydroxide and the increase of hydrated .gehlenite were both rapid and the degree of penetration decreased rapidly. It seems that the reaction was rapid because of the very low crystallinity of hydrated halloysite contained in Anil. As the constant values of hydrated gehlenite peaks in X-ray analysis were smaller than those of halloysite or kaolinite, it seems that a part of calcium hydroxide was absorbed by allophane in such a concentration as not to enable formation of hydrated gehlenite. In comparison with

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Ami, Am5 reacted slowly and the constant value of intensity of hydrated ' gehlenite peak was large, thus indicating the increasing in crystallinity and content of halloysite.

6. Concluding remarks

The main reaction product of allophane-kaolinite treated with calcium hydroxide is hydrated gehlenite, and its formation determines the effect of stabilization. Moreover, it is suggested that the surface area is the important factor, on which the writer intends to make further experiments. The writer is indebted to Dr. A. ARIIZUMI for his valuable sug- gestion to initiate this study.

References

1) ARIIZUMI, A. (1961). Dobokukeukyujo Hokoku, 110, 97. 2) ARIIZUMI, A. (1961). ibid., 115, 112. 3) ARIIZUMI, A. (1961). ibid., 111, 95. 4) ARIIZUMI, A. (1961). ibid., 111, 105. 5) STRATLING, W. (1940) . Zement, 29, 475. 6) GoTO, K., HANADA, M. and MIYAIRI, H. (1962). Semento Gijitsu Nenpo, 16, 162 (1962).

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