1、Atomic force microscopy （AFM） was used to observe particle shape and surface texture details of normal portland cement and supplementary cementing materials （silica fume,low-calcium fly ash,and high-calcium fly ash）.The latter maerials mixed with cement were examined after prolonged hydration.Signif
2、icant innovative information on particle shape and hydrated paste microstucture was obtained.Conventional microscopy techniques,such as scanning electron microscopy（SEM）,cannot provide such detailed images and surface texture characteristics of the fine materials （especially silica fume ）and of the
3、product microstructure.AFM showed ,for the first time ,that silica fume particles are primarily composed of two complimentary parts（hemispheres or semicylinders）.Nano-size particles were found in all materials.A relatively smooth product surface was observed in the hydrated cement paste.The hydrated
4、 surface of the addition-cement pastes presented small spheroid bulges,giving an additional roughness as was measured by AFM.A sufficient correlation of this microscopical quantitative information with macroscopical engineering and durability properties of cement products is also presented.1.Introdu
5、ctionFor technical and economic reasons,new materials with pozzolanic and cementitious properties have been mixed with cement during the last years.Among these materials are industrial by-products such as fly ash from coal-burning electric power plants,slags from metallurgical furnaces,silica fume （
6、or microsilica） from electric arc furnaces producing silicon and ferrosilicon alloys,and some naturally active materials such as volcanic tuffs.Their activity is mainly due to te reaction of their active constituents with Ca（OH）2 produced from cement hydration（pozzolanic activity）and the formation o
7、f hydrated products with binding properties.The exact stucture and chemical formula of these products are still unknown.The detailed knowledge of cement paste microstructure is of great importance for the understanding and predictionof cement applicationsperformance.Visual examination,optical micros
8、copy,and scanning electron microscopy（SEM） have been extensively used in microstucture research of hardened cement paste and concrete,providing additional understanding of macroscopical properties.Electron microprobe analysis studies of hardened cement pastes have contributed to the compositional ch
9、aracterization of hydration products and spatial information.Transmission energy microscopy（TEM） enables the identification and analysis of features on a significantly submicrometre scale.However,these techniques most commonly investiate specimens under high vacuum,and thus alteration of damage on m
10、icrostructure morpology may occur.Furthermore,some of these materials,such as silica fume,with a grain size distribution in the 0.02 to 0.3um range,are too small to be observed in detail by SEM.Techniques with stronger magnification capabilities would be very useful in these cases,as well in the bet
11、ter conception of the pure cement paste microstructure. By atomic force microscopy （AFM）,a sharp tip is scanning over a sample surface and three dimensional images haveing resolution at nanometer level are obtained at atmospheric conditions （room temperature,humidity,and ambient pressure）.AFM has be
12、en used to produce atomic resolution images of both conductors and nonconductors.In the present decade,AFM has also been used in the study of cement surface microstucture,especially because surfaces can be imaged under aqueous solutions at normal conditions.AFM has been applied to investigate the su
13、rface of alite alone and of alite covered with an organic admixture and has shown that the surface roughness of the alite decreased markedly after reaction with the organic admixture.AFM was also used to investigate the early period of portland cement hydration ,and a membrane/osmosis model was prop
14、osed.A combination of nuclear magnetic resonance（NMR）and AFM showed that the hydration rate is highly correlated with the roughness of the gel surface.For fly ash particles,AFM showed two types of spheres ,dark,large ones （approximately 100um）with numerous craters on their surface and clear,small on
15、es （approximately 10 um）with smooth surface. In the present work ,AFM was used to observe particle shape and surface texture details of cement silica fume,fly ashes ,and hydrated mixtures.Distinct micrographs of 1um1um were taken ,providing information on the particle shape of the additions and micr
16、ostructure of the hydrated mixtures.The same materials were examined under SEM to obtain a general overview,and the comparative use of these two methods（SEM-AFM） is discussed.Finally,the correlation between the microstructure and the macroscopical engineering and durability properties of cement prod
17、ucts is discussed.2.Experimental procedure2.1.Materials and sample preparationThree typical cementitious and/or pozzolanic additions were examined;a silica fume ,a low-calcium fly ash,and a high-calcium fly ash.The silica fume （SF）originated from Norway（Elkem Materials A/S,Kristiansand ）and is a typ
18、ical highly pozzolanic material.The low-Ca fly ash （FL） was produced is Denmark （distributed by Danaske I/S,Aalborg）and is categorized as normal pozzolanic material.The high Ca fly ash （FH）was produced in Greece（Public Power Corporation,Ptolemais）and is a cementitious mineral admixture.Thus,the choi
19、ce of these materials covers almost all the range of cementitious-pozzolanic by-products used in concrete.SF and FL were used as they were delivered from the producers,whereas FH was pulverized prior to use,to meet the FL mean particle size.A normal portland cement （350m2/g Blaines fineness） was use
20、d.The main physical properties and chemical analyses of the materials ,determined by X-ray sedimentation technique,showed that the mean particle diameter of fly ashes was 13um,similar to the cement particles.SF particles,as reported in the literature,are about 100 times smaller in size（0.1um average
21、 diameter）.Four paste specimens were cast in small plastic containers.The pastes were made of normal portland cement（control）,cement plus 10% silica fume,cement plus 20% low-calcium fly ash ,and cement plus 20% high-calcium fly ash.A water-to-cement ratio（W/C）of 0.5 was retained for all pastes.First
22、,the cement amount （10g）was added and then the corresponding amount of the additive.These materials were mixed by hand for two min;then the corresponding amount（5g） of the water was added and the fresh paste was further mixed for 2 min.One day after the casting,1ml of water was added to all specimen
23、s.The containers were hermetically sealed and placed at 20 constant temperature.Microscopy analyses were performed after six months.2.2.Atomic force microscopyMaterial particles and paste samples were examined by atomic force microscope（RasterscopeTM 4000,Danish Micro Engieering A/S）running in nonco
24、ntact mode （0.1nN force）.The particles in the cement ,silica fume,and fly ashes were dispersed by ultrasonic treatment;the silica fume in distilled water and the other components in acetone in order to avoid hydartion.The specimens were prepared by leaving one drop of the suspension to dry at room c
25、onditions on a block of highly oriented pyrolytic graphite（HOPG）.The samples from te pastes were removed by hammer stroke from the paste specimens,oven-dried at 105 for 24h.Small pieces of material were then glude onto AFM sample holders and slightly polished in dry condition.All specimens were exam
26、ined at room conditions （20,1atm and 40-60% relative humidity）.2.3.Scanning electron microscopy All of the above particles and pastes were also examined by means of scanning electron microscope（LEO 435 VP）.Surface micrographs of 8,000 magnification size were obtained and can be used as a general ove
27、rview of the materials.3.Results and discussion3.1.Cement ,silica fume ,and fly ash particlesTypical SEM micrographs in Figs 2-5.As observed by SEM（Fig.1a）,cement particles have and irregular polygonal shape.Particle sizes range from 15 to less than 0.5um.From AFM （Fig.2）,some particles of size 1 to
28、 0.5um n diameter are rounded,with a globular surface,whereas others are polygonal.However ,some irregular particles of size less than 0.1um in diameter were observed（Fig.2,right）. Silica fume particles are too small in size to be imaged in detail by SEM（Fig.1b）.From AFM（Fig.3）,the silica fume inves
29、tigated has particle diameters of about 0.1um,in agreement with previous reported dimensions.Two particle shapes are present,one spheroid and one cylindrical.It is very characteristic of the material that all particles are composed of two complimentary parts（hemispheres or semi-cylinders）.This featu
30、re can be explained from the production of silica fume,where the reduction of quartz to silicon at about 2000 produces a gaseous SiO,which is transported to lower temperatrues,where it is oxidized and condensed. This particular shape can help in SF identification in cement paste during the hydration
31、 process.In general,fly ashes consist of glassy spheres of various sizes.Due to the lower proportion of surface deposits consisting of alkali sulphate crystals,FL tends to show a cleaner appearance in SEM （Fig.1c）.For the FH,many of the particles are plerospheres containing numerous smaller particle
32、s,but after grinding,smaller size particles are produced having an irregular shape like the cement particles （Fig.1d）.Using AFM technique,it is observed （Fig.4）that the FL consists primarily of large spheroid particles （approximately 3um）with a smooth surface,like found previously,and of smaller（in one dimension）.These nano-size particles may be correlated with the early pozzolanic activity of the fly ashes. 3.2.Paste microstructure SEM micrographs for all mature pastes are summarized
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