Recent developments of cement chemistry in China

104

CERAMICS

INTERNATIONAL.

“01 8. n. 3. 1982

Recent Developments of Cement Chemistry in China Invited Review Paper XUE JUNGAN Cement

Research

Institute, Research Institute of Building People’s Republic of China

In recent years, much work has been done by the cement chemists in China centering on energy saving in cement and concrete making and improving the properties of both.

1 - THE CHEMISTRY

OF CEMENT

CLINKER

In cement manufacture the energy consumption in clinker burning makes up 80% of the total consumption. For the purpose of saving energy, attention has been given to producing CS and C2.S under lower temperature and utilizing inferior raw materials and fuel. The Section of Physical Chemistry of the Cement Research Institute (CRI) under the Research Institute of Building Materials (RIBM)’ by using fluorspar-gypsum composite mineralizer has achieved in reducing the burning temperature of Portland cement by 150-2OO’C. The 28-day compressive strength of the clinker attains 60 N/mm*. The output of kiln is raised by 16% and the coal consumption reduced by 13%. Liu Baoyuan ef aL2 studied the clinker formation with a high temperature X-ray diffractometer. They observed the formation of two intermediate phases, 3C2S3CaS04CaF2 and 2C2S.CaS04, which appeared at 900°C and 1050°C, respectively. At 12OO“C the two phases disappeared and CS was formed. Hence, the temperature of formation of alite dropped by 150-200°C. Ren Xiangtai et aL3 studied the influence of the amount of gypsum and fluorspar addition on the clinkering temperature when they were used as a composite mineralizer. In their view appropriate amount of addition and due proportion of the two mineralizers are preferable in order to acquire a favorable rate of formation of C$S in a full-size kiln. Wang Tiandi4 used coal cinder and coal gangue as well as fluorspar and gypsum composite mineralizer to reduce the clinkering temperature. Owing to the low clinkering temperature, C$S and &A& replaced C2S and &A. The cement thus made has high early strength. Wang Chenghua et a/.’ made high early strength cement under low temperature in a similar way using fly ash as raw material. They found that the high early strength component varied with the CaF2/S03 ratio. With the ratio greater than 0.39 the main component was ClrA7CaF2, while with the ratio smaller than 0.39 it was C4A.S. CaC12 has been used as a mineralizer to reduce the temperature of formation of C3S in the Tianjin Research Station of Building Materials and the section of Physical Chemistry of CRI of RIBM. Su Muzhen’ reported the clinkering temperature of the raw meal with CaCl2 might be as low as 1150-12OOOC. She found that the eutectic mix formed at 625-635OC enhanced the decomposition of CaC03. Besides, the intermediate phase C2S*CaC12 formed below 950°C melted incongruently at about 1OOO’C and the liquid phase formed helped in accelerating the formation of ZZ phase, whose XRD pattern was similar to that of r-alite Calr(Si,Al)~OIxCI as reforted by Noudelman etd7’. Yang Nanru er al. have obtained p-&S by heating to 950°C the calcium silicate hydrates synthesized hydrothermally under 100°C. This p-&S has better hydraulicity than the normal p-C2S, and its early and long-term strengths are much higher than the ones synthesized under high

Materials

temperature. On the basis of the above findings Min Panrong and Yang Nanru et a/.9 have made a low temperature synthesized fly ash cement. The clinkering temperature of the cement containing 70% fly ash is 700-850°C. The initial set is lo-30 min and the 1,3 and 28-day compressive strengths are 1o-20, 15-25 and 35-45 N/mm’, respectively. The cement has been used on a trial basis in making jet concrete in mines. In the field of the chemistry of special cement clinker, studies have been made in recent years on rapid hardening cement and expansive self-stressing cement based on &A$ and CIIA,CaF2. Deng Junan et al.10have studied the 1influence of A/S, lime saturation value,_A/S and temperature on the course of formation of &AS, p-C2S, C2AS and 2CS.CaSO4 in the system CaO-A1203-Si02-SO,. They have given the technical conditions for producing clinkers with &AS and p-C2S as their main constituents, and on the basis of the clinker a series of new-type cements have been made possessing the characteristics of high early strength, low expansion, expansion and self-stressing. The 12-hr, 1 and 3-day compressive strengths of the sulfoaluminate rapidhardening cement are 30, 40 and 50 N/mm*, respectively, and the self-stressing value of the sulfoaluminate selfstressing cement concrete is 6.0 N/mm’. The Research Group of quick setting and rapid hardening sand-cast cement of CRD, RIBM” made clinker similar to Japan’s superrapid hardening cement clinker, its composition being C3S 50.5-53.7010, C1,A7CaF2 21.2-26.00/o, CzS 6.2-10.60/o and C4AF 4.6-9.6010. Small amounts of C4A3S are present in the clinker. The Research Group has also made quick setting and rapid hardening cement clinkerI with CL1A7CaF2 (72.4%) and PCS (17.6%). At present the cement made from CILA7CaF2 and CS or C2S is used in rush repairs where 6-hr concrete strength is needed and also as moulding sand where one-hour strength is needed. The study on the matching of portland cement clinker minerals with &A$ and C11A,CaF2 has attracted the attention of many cement workers in China. The purpose of their study in general is to produce cement with high early strength and low energy consumption as well as to utilize low grade raw material and fuel.

2 - THE CHEMISTRY

OF CEMENT

HYDRATION

In the field of the chemistry of cement hydration some theoretical research is under way in recent years; besides, studies have been made in connection with the exploitation and application of special cements and concretes. Chu Ji’an13 analysed the movement of ion groups in the hydration of CS, C2S and CaS04. He established the equation for the instantaneous ion concentration field of cementitious materials. In the same time he observed the electric load distribution as a function of time in the induced period. He offered an explanation for the division of cementitious materials into hydraulic materials such as calcium silicates and the materials that harden in air such as gypsum. Huang Chengyi et al.“,” studied the influence of pore

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105

IN CHINA

size distribution of cement paste on its strength and permeability with BET adsorption and mercury porosimetry. He explained in terms of pore structure the strength degression of hardened high alumina cement paste at ~ooo-1200°C, the strength increase of polymer impregnated concrete and the improvement of permeability resistance of expansive cement concrete. Zhao Yuping et al. I6 studied the effect of MF water reducing agent (with sodium polymethyl-sulfonate as its main constituent) on the hydration of portland cement. They showed that the chief function of MF water reducing agent is to achieve a better dispersion for cement particles and hence the mixing water may be reduced. The structure of hardened cement paste is, at the same time, improved in favor of higher paste strength and better resistance to frost, permeability and corrosion. Gu Dezhen et a/.” and Chen Manqing et al. ‘* arrived at the same conclusion with sodium sulfonated naphthalene formaldehyde condensate as water * reducing agent. Zhang Feipeng et al.” determined the solubility product of C3A,3CaS04,31Hz0, C3ACaC12.10H20, CsACaC03.11Hz0 and CjA.CaS04.12Hz0 to be 1.1 x 10m4’, 1 .o x 1o-3o, 1.4x lo-” and 1.7x 10m2*, respectively, by which they clarified their views on the formation and stability of the double salts in cement paste. Lou Zonghan et a/.20 studied the formation of ettringite in slag cement and supersulfated cement and suggested the most favorable condition for the formation of ettringite in slag cement. The Research Institute of Water Conservancy and Electric Power of Changjiang, Zhejiang University and CRD of RIBM developed low expansive slag cement, its linear expansion being 0.2%, and 7-day heat of hydration 40 kcallkg. The cement is suitable for huge concrete projects. Self-stressing cement concrete pipeline in water, oil and gas transportation has the merits of saving energy and corrosion resistance as compared with cast iron and steel pipeline. A systematic study has been made in CRI of RIBM on the correlation of the hydration products of aluminate selfstressing cement2’, sulfoaluminate self-stressing cement22 and silicate self-stressina cementZ3 with exoansion. strenath and self-stressing value_Chang Yiyuz4 by selective solutkn method and Liu Baovuan et al.” bv XRD determined remnant gypsum content’in aluminate aid silicate self-stressing cement paste, thus bringing about the possibiliry of studying the influence of ettringite on the properties of self-stressing cement quantitatively. On the basis of a study of the influence of morphology, amount and compatible phases of ettringite (both formed in the hydration of individual phases and in self-stressing cement pastes) on strength, expansion and self-stressing value, Xue Jungan, Chen Wenhao, Tong Xueli et ai.26-27 expounded the characteristics of the expansion of ettringite formed under unsaturated CaO solution. Under such a condition, they stressed, the ettringite expanded and gel phase was formed concomitantly, thus resulting in a cement paste with fine and close texture, high strength and self-stressing value. At present, the self-stressing value of the concrete made from high-stressing value aluminate cement has attained 8 N/mm’. It is being used on a trial basis in a power station with waterhead 200 m. In order to save energy and obtain inexpensive expansive cement for making shrinkage compensating concrete, Wu Zhongwei and Wang Yansen” suggested to make expansive cement from uncalcined alunite instead of from high alumina cement or sulfoaluminate clinker. Wu Zhongwei29 discussed the deformations of expansive concrete in the process of expansion and contraction as well as the relation between the degree of reinforcement of the concrete and its shrinkage compensating effect and proposed a new idea for the effect. Zhang Hanwen et aL30 studied the factors that influenced the strength degression of high alumina cement. They hold that the transformation of CAHlo into C3AHa bringing about a higher porosity in the hardened paste is the chief one. They Put forward an experimental formula to correlate the lowest

value of concrete strength with cement composition, water cement ratio and ambient condition. Cheng Wenhao, Tong Xueli et al.” compared the hydration of rapid hardening cements made from CllA7.CaF2 and C4A3S. They consider that the ettringite formed at an early stage imparts the rapid hardening properties to the cements. Besides, the gel phase associated with the ettringite also plays an important role. The cement hydration products of low density and deep oil-well cement obtained hydrothermally under 150-200% have been studied by the Oil-Well Cement Research Group of CRI of RIBM”. The workers of the group attribute the low strength of pure oil-well cement to the formation of high C/S ratio calcium silicate hydrates C2SH(A) and C2SH(C). With the addition of an appropriate amount of silica sand, the hydration products turn into low C/S ratio calcium silicate hydrates CSH(B), which causes a rise in the strength of hardened cement paste. Zuo Wanxin” reported on the rapid set and rapid hardening aluminate cement. The 1 and 3-hr strengths of the concrete made from the cement were 20 and 30 N/mm’, respectively. Glass fibre reinforced cement composite, as a material being questioned for many years owing to the low durability of glasses in cement paste, has many good properties. After a hiatus of some ten years, with the advent of light building materials, the composite once again arouses the interest of many science workers33. Apart from the normal way of increasing the alkali resistance of the glass fibre and coating it34, Xue Jungan ‘7 pointed out that in the phase diagram of the system_ C-A-CS-H, the invariant point of &A.SCS.H32, AH3 and CSH2 has the lowest CaO concentration. Thus, the chemical attack on glass fibres by the cement having the above three compounds rn?z be reduced appreciably. Lu studied the low-pH-value ceBao-San, Xu Wenxia et al. ment. It has been manufactured in batches and used on a trial basis in some projects. Chang Peixing and Xu Wenxia et al.” studied the mutual reaction of the cement and glass fibres. They have shown that the attack on glass fibres by the above cement is only one twentieth of that by Portland cement. In the composite that combines the cement with alkaliresistant glass fibres, the durability of the fibres in the cement is unparalleled. Wang Youyun3’ reviewed the study and application of polymer concrete in China. He pointed out that in ancient China there were three kinds of polymer concrete, namely, lime mixed with glutinous rice paste, lime mixed with blood matter and oil and grease impregnated clay brick. Lately, Han Yulan et a/.39 have made polymer impregnated concrete whose compressive strength is 120-l 80 N/mm’, and bending strength 20-30 N/mm’. It is extraordinarily superior in chemical resistance and hence it may be used in projects where high strength and resistance to chemical attack are required. The mechanism of the strength increase of polymer impregnated concrete using methyl methacrylate was studied in CRI of RIBM4’. No chemical reaction was found between polymer and cement paste. Thus, the strength increase is considered to be due mainly to physical action. Tang Mingshu4’ compared the alkali-silica reactions in Portland cement, high alumina cement and supersulphated cement. He clarified his view on the mechanism of inhibition of alkali-silica reaction by blended materials in the light of Ca(OH)2. He suggested the use of cement without Ca(OH)2 or with only low Ca(OH)2 content to prevent alkali-silica reaction initiated by alkalies from other sources.

3-THE UTILIZATION AND RESEARCH WASTE AND NATURAL REACTIVE

OF INDUSTRIAL MATERIALS

In recent years, remarkable progress has been made in the application of industrial wastes and natural materials with potential reactivity.

XUE

106

JUNGAN

_

The composition,

both chemical

and mineralogical,

of fly

ash from 36 power stations in China, has been determined by Shanghai Research Institute of Construction and Shenxi Research Institute of Construction42r”3. A study has been made on the content of glass, magnetic glasses, mullite, quartz and carbon in different varieties of fly ash as well as the factors that affect the strength of hydrothermally synthesized products. They have thus suggested a formula that relates the strength of product with the content of < 48 urn particles, glass, alkali, SO3 and magnetic glass beads. Liu Huakun et a1.44studied the phases of high lime fly ash. They are glasses of B-C& %A:, C2F, CaS04 and free-lime. With the addition of appropriate amounts of gypsum and sodium chloride, ettringite and calcium silicate hydrates are formed and thus the paste hardens. Sheng Danshen et a/.45discussed the influence of fly ash on the strength of cement concrete in terms of morphology effect, the effect of reactivity and micro-aggregate effect. They suggested a formula to predict the strength of concrete with fly ash addition. The key to the utilization of steel-slag in cement lies in tackling the problem of inconstancy of volume. Zhang Peixing et a/.46 studied the mineral phases of steel-slags of openhearth furnace, converter and electric furnace. Tang Mingshu et aL4’ studied the effect of the state of crystallization of MgO in the converter, openhearth furnace and electric furnace slag on the soundness of autoclaved steel-slag cement. They consider that RO phase is more stable than periclasse and is not liable to cause unsoundness. Lo Shousun” and Ye Gongxin49 on the other hand consider that when MgO enters into solid solution with RO phase, the soundness of cement is decided by the ratio of MgOlFeO + MnO. With the ratio smaller than 1, the soundness of cement is not affected, while with the ratio greater than 1, the cement is unsound. Wang Yuji et aL5’ analysed the top-blown converter slag, its main phases being C& C2S, C2F, RO phase and free-CaO. For the steel-slags with C3S content > 40% and free-lime < 3010,steel-slag cement can be made by adding an appropriate amount of gypsum. In China, granulated blast furnace slag is usually added to the steel-slag cement to guard against unsoundness. Yang Yongqi and Li Rei etaL5’ proposed to produce steel slag zeolite cement in which zeolite is used in place of granulated slag. Sun Shushan et aL5’ suggested Ca0/Si02+ PzOs as a criterion to assess the reactivity of steel slag and classified them into groups with low, medium and high reactivity, their CaO/Si02+ P205 ratios being < 1.8, 1.8-2.5 and > 2.5, respectively. An excellent review on structure and characterisation of pozzolanas and of fly ashes was presented by R. Sersale to the 7th Congress on the Chemistry of Cement53. He considers that with more labile crystalline structure of the zeolitic type, due to the more open porous structure, the attack by the lime saturated solution, starting on the external surface where the links are less intense, would penetrate more easily inside. And in his view the lime combination after short attack times cannot be totally ascribed to the calcium-alkali exchange and the zeolite minerals are subjected to a hydrolysis process which brings silicate and aluminate ions into the solution, and they react with Ca2’ ions, forming the very low solubility product phases-calcium silicates and aluminates. Guo Jingxiong et aP4 studied the mechanism of reaction between zeolite and cement paste. They consider the reaction of zeolite in cement paste to be that of acid and alkali reaction rather than exchange of ions, and the process of reaction to be the initial detachment of alumina from the framework of zeolite in saturated lime solution followed by liberation of active silica from the remains of zeolite, The active silicate then reacts with Ca(OH)2 to form hydrated calcium silicate. The Zeolite Research Group of CRI of RIBM” evaluated six natural zeolites in China. The cements with zeolite were found to have almost identical 28-day strength with that of slag cement, but their 3-day strength is relatively lower. Great water requirement is considered to be the cause of low strength. Zeolite is effective in improving the

property of shaft kiln cement with high freelime content in so far as soundness is concerned. Wu Sumei56 utilized natural alunite as grinding aid in clinker grinding. The formation of ettringite with the addition of alunite has enabled more CaO to be taken up in the hydration. As a result, the danger of unsoundness due to high freelime content of shaft kilns may be reduced. Besides, the early strength of the cement is improved to some extent. Xu Wenxia et a!.57 added into natural anhydrite 1530% of clinker with C4A3S and p-C2S as its main mineral compositions. With the clinker as activator ettringite and other hydration products are formed, which imparts hydraulicity to the anhydrite. Wu Zhaozheng er al.” studied the mineralogical constituents of kiln dust. They are calcium carbonate, dolomite, quartz, feldspar, mica, glass beads and carbon; not fully reacted raw mix, such as f-CaO, p-C25 C2F, C4AF, mullite, and dehydrated clay; and the CaS04, Na2S04 and K2S04 formed as a result of the reaction between flue gas and kiln feed. They arrived at the following conclusions: (1) the reactivity of kiln dust matches that of fly ash and granulated slag; (2) the free-lime and the sulfate of potassium and sodium in the kiln dust are not detrimental to the properties of cement; (3) with the addition of 5% kiln dust into normal Portland cement and 8% into slag cement, the properties of the cements remain unchanged. To produce autoclaved silicate products with industrial wastes is also an approach to saving energy. He Erzhang59 studied the hydration products of the mixture of fly ash, selfigniting coal gangue, fluidized furnace slag and coal cinder with lime and gypsum reacted hydrothermally under 100 to 175%. The correlation of the hydration products with the variety of industrial waste, autoclaving condition and mix proportion were discussed. In the field of research in cement chemistry, priority has been given to the practical aspects. We are still weak so far as fundamental research and microstructural studies are concerned. Provided that the research in these aspects is further enhanced, more achievements are to be expected.

ACKNOWLEDGMENT The author expresses his gratitude to Mr. Sun Guokuang for the translation of this paper from Chinese into English.

REFERENCES 1. The Section of Physical Chemistry of CRI of RIBM, Reducing the Temperature of Formation of Clinker Using Fluorspar-Gypsum Composite Mineralizer, September, (1979, Unpub.) 2. LIU BAOYUAN and LI XIUYING, Abstract, Symposium of the Annual Meeting of the Cement Committee of the Chinese Silicate Society (ASAMCC), 139 (1980, Distributed only in China) 3. REN iIANGTAI and ZHOU SHIFEN, ASAMCC, 134 (1980) 4. WANG TIANDI. ASAMCC. 28 /1980) 5. WANG CHENGHUA, SU ERdA anb WU RUIJIN, ASAMCC, 30 (1980) 6. SU MUZHEN, ASAMCC, 143 (1980) 7. NOUDELMAN, M. BIKBAOU, A SVENTSITSKI and V. ILLUKHINE. 7th International Conaress on the Chemisttv of Cement (7th iCCC), Vol. Ill, V-169 (1580, Paris) 8. YANG NANRU and ZHONG BAIXI, ASAMCC, 130 (1980) 9. MIN PANRONG, YANG NANRU and ZHONG BAIXI, ASAMCC, 28 (1980) 10. DENG JUNAN, GE WENMIN, SU MUZHEN and LIXIUYING, 7th ICCC, Vol. IV, 381 (1980, Paris) 11. Research Group of Quick Setting and Rapid Hardening SandCast Cement, CRI of RIBM, Journal of the Chinese Silicate Societv (JCSS17.131.223 (1979) 12. CHEN-\iVENHbd,‘l%NG ‘XUEil and XU JIZHI, Journal of the Research Institute of Buildina Materials IJRIBM)...-.[31. 42 (1980), Distributed only in China 13. ZHU JIAN, ASAMCC, 152 (1980) LI DEHOU and GUO SHUYUN, The 14. HUANG CHENGYU,

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Microscopical Test and Their Applications, 11 (1979) HUANG CHENGYI, ZHANG QIYUAN and YAN JIMIN, JCSS, 7 (1979) 333 ZHAO YUPING, LIU MEIYING and CHEN MIN, JCSS, 8 (1980) 3. GU DEZHEN, XIONG DAYU and LU ZHANG, ASAMCC, 161 (1980). CHEN MANQING, SHEN JIANMEN and PENG XIAN, AsAMCC, 14(1980). ZHANG FEIPENG, ZHOU ZHIFA and LOU ZONGHAN, 7th ICCC, Vol. II, II-88 (1980, Paris). LOU ZONGHAN, XU XIANYU AND HAN REN, 7th ICCC, Vol. II, II-82 (1980. Paris). CRI of RIBM, JCSS, 8 (1978) 15. CRI of RIBM. JCSS, 8 (1978) 123 The section of Physical Chemistry of CRI of RIBM, JCSS, 8141, 242 (1978). ZHANG YIYU, the Microscopical Tests and Their Applications, 109 (1979). LIU BAOYUAN. LU BUOYAN and WU CHANGFA. The Microscopical Tests and Their Applications, 1 (1979). XUE JUNGAN, CHEN WENHAO, TONG XUELI, ZHAO YUPING and XU JIZHI, JCSS, 7 (1979) 58. XUE JUNGAN, CHEN WENHAO, TONG XUELI, ZHAO YUPING and XU JIZHI, 7th ICCC, Vol. Ill, V-33 (1980, Paris). WU ZHONGWEI and WANG YANSHENG, 7th ICCC, Vol. Ill, V-27 (1980, Paris). WU ZHONGWEI, The Shrinkage Compensating Concrete, (1980). ZHANG HANWEN, CHEN JINGCHUAN, BAI REIFEN and YOU LAILU, JCSS, 8 (1980) 259. The Oil-Well Cement Research group of CRI of RIBM, JCSS, 7 (1979) 168. ZUO WANXIN, The Achievements on Cement Research for the Last Thirthv Years. 1. CRD of RIBM. (1979. Distributed onlv in . China) ’ SHEN RONGXI, JCSS, 8 (1978) 199. The Fiber Reinforced Concrete Research Group of Shandong Cement Product Institute, JCSS, 7 (1979) 178. XUE JUNGAN, JCSS, 8 (1978) 317. LU BAOSHAN, XU WENXIA, ZHANG PEIXIN and XUE JUNGAN, An Investigation on Low-pH-value Cement for Glass Fiber Reinforced Concrete, JCSS, 9 (1981) 309.

PEIXIN, xu WENXIA, LU BAOSHAN and XUE 37. ZHANG JUNGAN, A Study on the Reaction Between Low-pH-value Cement and Glass Fiber, (to be pub.). 38. WANG YOUYUN, Third International Congress on Polymers in Concrete, Vol. I, 46 (1981, Japan). 39. HAN YULAN, XU YAPING and BAI REIFEN, Metallurgical Construction, [3], 53 (1980, Distributed only in China). 40. CRI of RIBM, JCSS, 8, [1,2], 61 (1978). 41. TANG MINGSHU and HAN SUFEN, 7th ICCC, Vol. II-94 (1980, Paris). 42. Shanghai Research Institute of Construction and Shenxi Research Institute of Construction, ASAMCC, 149 (1980). 43. Shanghai research Institute of Construction and Shenxi Research Institute of Construction, ASAMCC, 41 (19809). 44. LIU HUAKUN, LU ZHONGYA and LIN SHENGJIE, 7th ICCC, Vol. Ill, IV-7 (1980, Paris). 45. SHEN DANSHEN AND ZHANG YINJI, JCSS, 9 (1981) 57. 46. ZHANG PEIXIN, ZHOU SHIFEN and SU MUZHEN, JRIBM, 121, 47 (1979). 47. TANG MINGSHU, YUAN MEIQI, HAN SUFEN and SHEN XING, JCSS, 7 1979) 33. 48. LUO SH 6 USUN. 7th ICCC. Vol. II, Ill-25 (1980. Peric’ 49. YE GONGXIN, ASAMCC, 155 (1980). ’ -“““ 50. Wang YUJI AND YE GONGXI, 7th ICCC, Vol. II, Ill-19 (1980, Paris). 51 YANG YONGQI, LI REI, CHANG SHIFENG, SHU MINDI and GOU JINSHONG, ASAMCC, 9 (1980). 52. SUN SHUSHAN, WAN YUJI, CHEN YOUMIN. LI SHUANGQUAN, WEI KUISHI and WANG ZHIJUN, ASAMCC, 11 (1980). 53. R. SERSALE, 7th ICCC, Vol. I, IV-I (1980, Paris). 54. GUO JINGXIONG and LIANG CHUNLIN, JCSS, 8 (1980) 244. 55. The Zeolite Research Group of CRD of RIBm, ASAMCC, 42 (1980). 56. WU SUMEI, ASAMCC, 17 (1980). 57. XU WENXIA and Lu BAOSHAN, JRIBM, [l], 34 (1979). 58. WU ZHAOZHENG, ASAMCC, 33 (1980). 59. HE ERZHANG, JCSS, 7 (1979) 201.

Received November 2, 1981; Final text received February 2, 1982.