- Email: [email protected]
Hydrated cement paste constituents observed with Atomic Force and Lateral Force Microscopy
Construction and Building Materials 25 (2011) 4299–4302
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Technical Note
Hydrated cement paste constituents observed with Atomic Force and Lateral Force Microscopy Alva Peled a,⇑, Jason Weiss b a b
Structural Engineering Department, Ben Gurion University of the Negev, Beer Sheva 84105, Israel School of Civil Engineering, Purdue University, West Lafayette, IN 47907, USA
a r t i c l e
i n f o
Article history: Received 30 December 2010 Received in revised form 20 April 2011 Accepted 21 April 2011 Available online 18 May 2011 Keywords: Atomic Force Microscope (AFM) Lateral Force Microscope (LFM) Cement Mortar Calcium silicate hydrates (CSH) Calcium hydroxide (CH)
a b s t r a c t Engineers have frequently used microscopy to better understand the structure of materials. Optical microscopy and scanning electron microscopy have been used for several decades to better understand the microstructure of cementitious composites. Very limited work has been performed to date in the cement area to study the structural characteristics of cementitious materials by Atomic Force Microscopy (AFM). This technical note describes observations from a series of images acquired using AFM as well as Lateral Force Microscopy (LFM). The objective of this work is to better understand how AFM and LFM techniques can be used as tools to better understand the nano and microstructure of cementitious materials. In this work the AFM and LFM techniques could distinguish between CSH particles and crystals of CH on the microscale. The LFM appears to be more sensitive to topographic changes and could more clearly distinguish between the different phases. Ó 2011 Elsevier Ltd. All rights reserved.
1. Background
2. Experimental
Atomic Force Microscopy (AFM) is a high resolution scanning probe microscope that provides a three-dimensional view of a sample surface. AFM works by moving a probe with a sharp tip at the end of cantilever across the surface of a specimen. As this tip is brought into contact with the surface the deflection of the tip can be measured and the topography of the surface is traced out [1]. Lateral Force Microscopy (LFM) is a derivative of AFM. In LFM the tip is maintained in contact with the sample surface and the ‘twisting of the cantilever’ is measured in addition to the vertical deflection which is more common from the AFM. The lateral deflection can be related to the friction of the surface. The LFM method is more sensitive to topographical variations than other methods. There is a growing interest to study the nano and microstructure characteristics of cementitious materials by the AFM techniques amongst the cement-based materials community [2– 5]. However the work up to date is still limited and even less work has been reported on the LFM. In this technical note both AFM and LFM investigations are introduced to provide detailed images and surface texture characteristics on the cement paste hydrated microstructure.
Mortar samples were prepared with a w/c (water to cement ratio) of 0.30% and 55% volume of natural river sand aggregate (details are provided in Table 1 and Ref. [6]). A mortar system was chosen in order to apply an equal level of polishing through the entire surface of the specimen using the stiff aggregate as the limit level. After mixing the mortar was placed in mold, vibrated and sealed. The sealed samples were maintained in an environmental chamber at 23 ± 1 °C until the sample reached an age of 28 d. At an age of 28 d the specimens were demolded, cut using a diamond tipped saw and placed in an oven at 50 ± 2 °C for 3 d to remove a large portion of the free water from the mortar. After drying, the samples were ground and polished with seven stages of increasing fineness, beginning with fine diamond wheel of 45 lm and ending with 250 nm diamond paste. The polishing process was based on the method developed by Diamond [7]. A low viscosity mineral oil was applied on the sample surface during polishing to avoid continues of hydration (if water was used) and as it could easily removed by acetone after the polishing process. The specimens were polished ‘‘as it is’’ (without epoxy impregnation) as the epoxy may penetrate into the cement paste pores, influencing the topographic characteristics measured by the AFM and even more by the LFM. Note that since the AFM and LFM techniques are sensitive to very slight differences in surface features, and the scanned region includes materials with different compositions the polishing method is critical. As all features in this work were
⇑ Corresponding author. E-mail address: [email protected] (A. Peled). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.04.066
4300
A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
Table 1 Mixture proportions (in SSD). Material
Proportions
Volume fraction of aggregates (%) w/c, by mass Cement (kg/m3) Water (kg/m3) Aggregate (kg/m3) SSD WRA (g/100 g cement)
55 0.3 727.0 218.1 1442.0 0.6
scanned in neighborhood region the polishing assumed to be similar and the differences between the scanned features can be related to differences in their surface structure. To expose uncarbonated area the polished samples were cut to slices approximately 5 mm tall as required for positioning in the AFM, using cooled low viscosity mineral oil to avoid moisture penetration and further hydration. The samples were then cleaned in an ultrasonic bath of acetone and stored in an oven at 50 ± 2 °C for several days. The atomic force microscope used in this study was a Digital Instruments CP-II AFM system (manufactured by Veeco) with a silicon tip and a cantilever arm having the following dimensions: T: 3.5–4.5 lm, L: 515–535 lm and W: 30–40 lm, with a resonance frequency of 19–24 kHz, and 0.9 N/m spring constant. The microscope was operated in contact mode to provide topographic maps of the cement paste surface with a set point of 180 nN and scan
speed of 0.5 Hz, using both AFM and LFM techniques. Three different scan ranges were used: 10 lm, 3 lm and 1 lm. During scanning the specimen was at room conditions (23 ± 2%, 45 ± 7% RH). The scanning electron microscope (SEM) used in this study was a R.J. Lee Personal scanning electron microscope. The SEM was used in backscattered (BS) mode with high vacuum condition to select the region to be scanned under the AFM. The goal was to select a typical zone of cement paste containing several features such as calcium hydroxide (CH), calcium silicate hydrates (CSH) and an un-hydrated cement particle. Energy dispersive X-ray (EDX) analysis was carried out to distinguish between the different features at the chosen region. The polished specimen was coated with thin layer of palladium in order to observe these elements under the SEM. After SEM observations and EDX analysis the coating was removed by acetone and the same specimen was immediately scanned under the AFM at the chosen region identify by the SEM and EDX.
3. Results and discussion SEM BS observations were conducted on the entire surface area of the polished specimen to search for a typical region of the cement paste to be scanned under the AFM and LFM. While numerous samples were examined, a typical region is presented in Fig. 1a. This area was chosen to be scanned under the AFM and LFM as it contains several distinguished elements within the cement paste
410 nm
0.5 V/Div
40 nm/Div
Fig. 1. SEM BS images: (a) entire observed area, (b) scanned area of different elements (presented by a point with a number); un-hydrated cement particle – Spectrum 1, CSH – Spectrum 2, and CH – Spectrum 3.
0
400
800 nm
230 nm
0
400
Scan range
Scanrange
(a)
(b)
800 nm
Fig. 2. Observation on the CSH area were Spectrum 2 in Fig. 1b was taken: (a) AFM, (b) LFM, exactly same area, 1 lm scan range.
4301
A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
Fig. 3. Observation on CSH, SEM image.
region: un-hydrated cement particles – the elements in white, CH phase located around one of the un-hydrated cement particle having light gray color, and CSH phase – darker gray in the figure. Higher magnification on this region using the SEM BS mode is presented in Fig. 1b, highlighting the different elements within the cement paste zone. Energy dispersive X-ray results on this region can clearly be identified using the element composition. The first point (labeled as 1 in Fig. 1b) indicates an un-hydrated cement particle age to the high content of calcium Ca = 53.3 and silica of Si = 12.5 where the oxygen is O = 34.2; The second point (labeled as 2 in Fig. 1b) indicates a CSH phase where silica and calcium are identified: Si = 11.5 and Ca = 48.3, with O = 40.2. The third point (labeled as 3 in Fig. 1b) identify calcium mixture due to the previous presence of calcium and oxygen are observed Ca = 47.3, O = 50.0 with hardly no silica (Si = 2.7). AFM and LFM techniques were used to provide topographic maps of this chosen region, focusing on the CSH and CH phases (shown as dots 2 and 3 in Fig. 1b). Fig. 2 shows the surface texture characteristics of the material located where Spectrum 2 was taken (in Fig. 1b), observed by AFM and LFM techniques. This area corresponds to CSH (as identified by EDX). The scale of color for the AFM images represents differences in surface heights, as each color deviation in Fig. 2a represents 40 nm, i.e., with a darker color corresponding to a lower surface and a lighter color corresponding to a higher surface. For LFM, different colors mean differences in voltage which represent
0.0
1.0
2.0 V/Div
4.0 V/Div
1 µm
2.0
Scan range
(a)
frictional resistance. For LFM the darker color corresponds to a lower voltage and smaller friction forces as lighter colors corresponds to a higher voltage and greater friction resistance. It is shown that the paste in this CSH zone is composed of clusters of grains with sizes in the range of about 200 nm in width and 400 nm in length. Such a structure is comparable with the description of Type III C–S–H phase as by Diamond [8]. The granular structure is observed in both AFM (Fig. 2a) and LFM (Fig. 2b) measurements; however these grains are much clearer with better separation when using the LFM technique as it is more sensitive to topographical variations. The amorphous arrangement of these CSH units is evident with the LFM image. This particulate nature of the CSH is also observed under the SEM at high magnification (60 K, Fig. 3). Note that similar samples not subjected to vacuum were also measured under the AFM and LFM, observing similar features as the CSH presented here. The LFM image captured where Spectrum 3 was taken (see Fig. 1b) represents the CH phase, as identified by the EDX, as shown in Fig. 4a, using a scan range of 3 lm. Hexagonal crystal morphology was observed at the range of 1 lm. These hexagonal units appear to be composed of smaller units. In general, the size of the CH units is about twice that of the CSH, 1 lm compared to about 400 nm, respectively. Higher magnification (lower scan range) on a specific unit (white dotted square in Fig. 4a) is presented in Fig. 4b. A relatively smooth and flat surface is observed for the CH as compared to the grainy structure of the CSH (Fig. 2b), both taken at the same scan range (1 lm). In Fig. 4b the CH unit is divided to two main smaller components lying on top of other units, suggesting a layered structure which is consistent with previous observations. When observing one of these components at smaller scan range (500 nm) at three-dimensional (3D) a difference in element height within the same unit is observed (Fig. 5), indicating either a thin coating around the crystal or an indented area at the top edge of the image. This type of information is unique to the LFM technique. The image presenting in Fig. 6 shows LFM topographic map of the square dots area in Fig. 1b. This region includes both zones that of CH and that of CSH. In this image these two phases of the cement paste were scanned together with exact same conditions. Fig. 6a was scanned at relatively large scan range of 10 lm and Fig. 6b was scanned at a smaller range of 3 lm. At the higher scan range (Fig. 6a) it highly noticeable that the CH region is flat and smooth and no grain structure is observed. On the other hand, the CSH area is relatively rough with surface with a grainy structure. This combined scan image clearly illustrates the difference in the microstructure of these two cement paste products. When comparing
3.0 µm
0
400
800 nm
Scan range
(b)
Fig. 4. Observation on the area were Spectrum 3 was taken (Fig. 1b), showing CH products: (a) 3 lm scan range, (b) 1 lm scan range, the doted square area in (a).
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A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
the two phases at lower scan range (higher magnification) the differences is still valid, rougher, bumpy and grainy surface of the CSH compared to smoother surface of the CH region, however at this scan range the surface of the CH also shows some particles structure but at much smaller range. This indicates that both regions, CSH and CH are assembled of particles but they are at different range, structure and size. It should be noted that the scanning was done at ambient air condition and therefore moisture is possible on the scan area as well as in between the tip and substrate which can influence the information collected at the scanned surface [1].
0.1 µm/Div
1.1 V/Div
4. Conclusions
0.1 µ m/Div Fig. 5. 3D LFM image at law scan range of 500 nm, the doted area in (Fig. 4b).
CH 4.0 V/Div
CSH
0.0
8.0 µ m
4.0
References
Scan range
(a)
CH
CSH
2.0 V/Div
0.0
1.0
2.0
In this paper Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) were used to examine the cement paste phase of a mortar. The AFM and LFM techniques can be used as a tool to identify different phases of a hydrated cement paste. LFM is more sensitive to topographic changes and could more clearly distinguish between the different scanned areas. AFM and LFM can provide advantages of high resolution digital three-dimensional morphological information that can be reached under room atmosphere where no vacuum is required, however the work is sensitive to polishing effects. In this work CSH particles and crystals of CH were observed. The measurements could distinguish between these two cement paste products. The CH region had a smoother surface as compared to the CSH as expected. At low scan range (below 3 lm) both CH and CSH regions exhibit particle nature, however the size of these particles is quite different, larger particles and more bumpy texture are observed for the CSH phase. The CH tends to have a more plate-like structure which is consistent with typical observations.
3.0 µm
Scan range
(b) Fig. 6. LFM images including both regions of CH and CSH products (the doted square area in Fig. 2) at two scan ranges: (a) 10 lm and (b) 3 lm.
[1] Butt H-J, Cappella B, Kappl M. Force measurements with the atomic force microscope: technique, interpretation and applications. Surf Sci Rep 2005;59:1–15. [2] Yang T, Keller B, Magyari E. AFM investigation of cement paste in humid air at different relative humidities. J Phys D: Appl Phys 2002;35(8):L25–8. [3] Yang T, Keller B, Magyari E. Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an atomic force microscope. J Mater Sci 2003;38:1909–16. [4] Papadakis VG, Pedersen EJ. An AFM–SEM investigation of the effect of silica fume and fly ash on cement paste microstructure. J Mater Sci 1999;34:683–90. [5] Bullard JW, Jennings HM, Livingston RA, Nonat A, Scherer GW, Schweitzer JS, et al. Mechanisms of cement hydration. Cem Concr Res, in press. [6] Peled A, Castro J, Weiss J. Atomic force microscope examinations of mortar made using water-filled lightweight aggregate. Trans Res Board 2009;2141: 92–101. [7] Diamond S. The microstructure of cement paste and concrete – a visual primer. Cem Concr Comp 2004;26:919–33. [8] Diamond S. Hydraulic cement pastes: their structure and properties. Proc conf held at Tapton Hall 2; 1976.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Technical Note
Hydrated cement paste constituents observed with Atomic Force and Lateral Force Microscopy Alva Peled a,⇑, Jason Weiss b a b
Structural Engineering Department, Ben Gurion University of the Negev, Beer Sheva 84105, Israel School of Civil Engineering, Purdue University, West Lafayette, IN 47907, USA
a r t i c l e
i n f o
Article history: Received 30 December 2010 Received in revised form 20 April 2011 Accepted 21 April 2011 Available online 18 May 2011 Keywords: Atomic Force Microscope (AFM) Lateral Force Microscope (LFM) Cement Mortar Calcium silicate hydrates (CSH) Calcium hydroxide (CH)
a b s t r a c t Engineers have frequently used microscopy to better understand the structure of materials. Optical microscopy and scanning electron microscopy have been used for several decades to better understand the microstructure of cementitious composites. Very limited work has been performed to date in the cement area to study the structural characteristics of cementitious materials by Atomic Force Microscopy (AFM). This technical note describes observations from a series of images acquired using AFM as well as Lateral Force Microscopy (LFM). The objective of this work is to better understand how AFM and LFM techniques can be used as tools to better understand the nano and microstructure of cementitious materials. In this work the AFM and LFM techniques could distinguish between CSH particles and crystals of CH on the microscale. The LFM appears to be more sensitive to topographic changes and could more clearly distinguish between the different phases. Ó 2011 Elsevier Ltd. All rights reserved.
1. Background
2. Experimental
Atomic Force Microscopy (AFM) is a high resolution scanning probe microscope that provides a three-dimensional view of a sample surface. AFM works by moving a probe with a sharp tip at the end of cantilever across the surface of a specimen. As this tip is brought into contact with the surface the deflection of the tip can be measured and the topography of the surface is traced out [1]. Lateral Force Microscopy (LFM) is a derivative of AFM. In LFM the tip is maintained in contact with the sample surface and the ‘twisting of the cantilever’ is measured in addition to the vertical deflection which is more common from the AFM. The lateral deflection can be related to the friction of the surface. The LFM method is more sensitive to topographical variations than other methods. There is a growing interest to study the nano and microstructure characteristics of cementitious materials by the AFM techniques amongst the cement-based materials community [2– 5]. However the work up to date is still limited and even less work has been reported on the LFM. In this technical note both AFM and LFM investigations are introduced to provide detailed images and surface texture characteristics on the cement paste hydrated microstructure.
Mortar samples were prepared with a w/c (water to cement ratio) of 0.30% and 55% volume of natural river sand aggregate (details are provided in Table 1 and Ref. [6]). A mortar system was chosen in order to apply an equal level of polishing through the entire surface of the specimen using the stiff aggregate as the limit level. After mixing the mortar was placed in mold, vibrated and sealed. The sealed samples were maintained in an environmental chamber at 23 ± 1 °C until the sample reached an age of 28 d. At an age of 28 d the specimens were demolded, cut using a diamond tipped saw and placed in an oven at 50 ± 2 °C for 3 d to remove a large portion of the free water from the mortar. After drying, the samples were ground and polished with seven stages of increasing fineness, beginning with fine diamond wheel of 45 lm and ending with 250 nm diamond paste. The polishing process was based on the method developed by Diamond [7]. A low viscosity mineral oil was applied on the sample surface during polishing to avoid continues of hydration (if water was used) and as it could easily removed by acetone after the polishing process. The specimens were polished ‘‘as it is’’ (without epoxy impregnation) as the epoxy may penetrate into the cement paste pores, influencing the topographic characteristics measured by the AFM and even more by the LFM. Note that since the AFM and LFM techniques are sensitive to very slight differences in surface features, and the scanned region includes materials with different compositions the polishing method is critical. As all features in this work were
⇑ Corresponding author. E-mail address: [email protected] (A. Peled). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.04.066
4300
A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
Table 1 Mixture proportions (in SSD). Material
Proportions
Volume fraction of aggregates (%) w/c, by mass Cement (kg/m3) Water (kg/m3) Aggregate (kg/m3) SSD WRA (g/100 g cement)
55 0.3 727.0 218.1 1442.0 0.6
scanned in neighborhood region the polishing assumed to be similar and the differences between the scanned features can be related to differences in their surface structure. To expose uncarbonated area the polished samples were cut to slices approximately 5 mm tall as required for positioning in the AFM, using cooled low viscosity mineral oil to avoid moisture penetration and further hydration. The samples were then cleaned in an ultrasonic bath of acetone and stored in an oven at 50 ± 2 °C for several days. The atomic force microscope used in this study was a Digital Instruments CP-II AFM system (manufactured by Veeco) with a silicon tip and a cantilever arm having the following dimensions: T: 3.5–4.5 lm, L: 515–535 lm and W: 30–40 lm, with a resonance frequency of 19–24 kHz, and 0.9 N/m spring constant. The microscope was operated in contact mode to provide topographic maps of the cement paste surface with a set point of 180 nN and scan
speed of 0.5 Hz, using both AFM and LFM techniques. Three different scan ranges were used: 10 lm, 3 lm and 1 lm. During scanning the specimen was at room conditions (23 ± 2%, 45 ± 7% RH). The scanning electron microscope (SEM) used in this study was a R.J. Lee Personal scanning electron microscope. The SEM was used in backscattered (BS) mode with high vacuum condition to select the region to be scanned under the AFM. The goal was to select a typical zone of cement paste containing several features such as calcium hydroxide (CH), calcium silicate hydrates (CSH) and an un-hydrated cement particle. Energy dispersive X-ray (EDX) analysis was carried out to distinguish between the different features at the chosen region. The polished specimen was coated with thin layer of palladium in order to observe these elements under the SEM. After SEM observations and EDX analysis the coating was removed by acetone and the same specimen was immediately scanned under the AFM at the chosen region identify by the SEM and EDX.
3. Results and discussion SEM BS observations were conducted on the entire surface area of the polished specimen to search for a typical region of the cement paste to be scanned under the AFM and LFM. While numerous samples were examined, a typical region is presented in Fig. 1a. This area was chosen to be scanned under the AFM and LFM as it contains several distinguished elements within the cement paste
410 nm
0.5 V/Div
40 nm/Div
Fig. 1. SEM BS images: (a) entire observed area, (b) scanned area of different elements (presented by a point with a number); un-hydrated cement particle – Spectrum 1, CSH – Spectrum 2, and CH – Spectrum 3.
0
400
800 nm
230 nm
0
400
Scan range
Scanrange
(a)
(b)
800 nm
Fig. 2. Observation on the CSH area were Spectrum 2 in Fig. 1b was taken: (a) AFM, (b) LFM, exactly same area, 1 lm scan range.
4301
A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
Fig. 3. Observation on CSH, SEM image.
region: un-hydrated cement particles – the elements in white, CH phase located around one of the un-hydrated cement particle having light gray color, and CSH phase – darker gray in the figure. Higher magnification on this region using the SEM BS mode is presented in Fig. 1b, highlighting the different elements within the cement paste zone. Energy dispersive X-ray results on this region can clearly be identified using the element composition. The first point (labeled as 1 in Fig. 1b) indicates an un-hydrated cement particle age to the high content of calcium Ca = 53.3 and silica of Si = 12.5 where the oxygen is O = 34.2; The second point (labeled as 2 in Fig. 1b) indicates a CSH phase where silica and calcium are identified: Si = 11.5 and Ca = 48.3, with O = 40.2. The third point (labeled as 3 in Fig. 1b) identify calcium mixture due to the previous presence of calcium and oxygen are observed Ca = 47.3, O = 50.0 with hardly no silica (Si = 2.7). AFM and LFM techniques were used to provide topographic maps of this chosen region, focusing on the CSH and CH phases (shown as dots 2 and 3 in Fig. 1b). Fig. 2 shows the surface texture characteristics of the material located where Spectrum 2 was taken (in Fig. 1b), observed by AFM and LFM techniques. This area corresponds to CSH (as identified by EDX). The scale of color for the AFM images represents differences in surface heights, as each color deviation in Fig. 2a represents 40 nm, i.e., with a darker color corresponding to a lower surface and a lighter color corresponding to a higher surface. For LFM, different colors mean differences in voltage which represent
0.0
1.0
2.0 V/Div
4.0 V/Div
1 µm
2.0
Scan range
(a)
frictional resistance. For LFM the darker color corresponds to a lower voltage and smaller friction forces as lighter colors corresponds to a higher voltage and greater friction resistance. It is shown that the paste in this CSH zone is composed of clusters of grains with sizes in the range of about 200 nm in width and 400 nm in length. Such a structure is comparable with the description of Type III C–S–H phase as by Diamond [8]. The granular structure is observed in both AFM (Fig. 2a) and LFM (Fig. 2b) measurements; however these grains are much clearer with better separation when using the LFM technique as it is more sensitive to topographical variations. The amorphous arrangement of these CSH units is evident with the LFM image. This particulate nature of the CSH is also observed under the SEM at high magnification (60 K, Fig. 3). Note that similar samples not subjected to vacuum were also measured under the AFM and LFM, observing similar features as the CSH presented here. The LFM image captured where Spectrum 3 was taken (see Fig. 1b) represents the CH phase, as identified by the EDX, as shown in Fig. 4a, using a scan range of 3 lm. Hexagonal crystal morphology was observed at the range of 1 lm. These hexagonal units appear to be composed of smaller units. In general, the size of the CH units is about twice that of the CSH, 1 lm compared to about 400 nm, respectively. Higher magnification (lower scan range) on a specific unit (white dotted square in Fig. 4a) is presented in Fig. 4b. A relatively smooth and flat surface is observed for the CH as compared to the grainy structure of the CSH (Fig. 2b), both taken at the same scan range (1 lm). In Fig. 4b the CH unit is divided to two main smaller components lying on top of other units, suggesting a layered structure which is consistent with previous observations. When observing one of these components at smaller scan range (500 nm) at three-dimensional (3D) a difference in element height within the same unit is observed (Fig. 5), indicating either a thin coating around the crystal or an indented area at the top edge of the image. This type of information is unique to the LFM technique. The image presenting in Fig. 6 shows LFM topographic map of the square dots area in Fig. 1b. This region includes both zones that of CH and that of CSH. In this image these two phases of the cement paste were scanned together with exact same conditions. Fig. 6a was scanned at relatively large scan range of 10 lm and Fig. 6b was scanned at a smaller range of 3 lm. At the higher scan range (Fig. 6a) it highly noticeable that the CH region is flat and smooth and no grain structure is observed. On the other hand, the CSH area is relatively rough with surface with a grainy structure. This combined scan image clearly illustrates the difference in the microstructure of these two cement paste products. When comparing
3.0 µm
0
400
800 nm
Scan range
(b)
Fig. 4. Observation on the area were Spectrum 3 was taken (Fig. 1b), showing CH products: (a) 3 lm scan range, (b) 1 lm scan range, the doted square area in (a).
4302
A. Peled, J. Weiss / Construction and Building Materials 25 (2011) 4299–4302
the two phases at lower scan range (higher magnification) the differences is still valid, rougher, bumpy and grainy surface of the CSH compared to smoother surface of the CH region, however at this scan range the surface of the CH also shows some particles structure but at much smaller range. This indicates that both regions, CSH and CH are assembled of particles but they are at different range, structure and size. It should be noted that the scanning was done at ambient air condition and therefore moisture is possible on the scan area as well as in between the tip and substrate which can influence the information collected at the scanned surface [1].
0.1 µm/Div
1.1 V/Div
4. Conclusions
0.1 µ m/Div Fig. 5. 3D LFM image at law scan range of 500 nm, the doted area in (Fig. 4b).
CH 4.0 V/Div
CSH
0.0
8.0 µ m
4.0
References
Scan range
(a)
CH
CSH
2.0 V/Div
0.0
1.0
2.0
In this paper Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) were used to examine the cement paste phase of a mortar. The AFM and LFM techniques can be used as a tool to identify different phases of a hydrated cement paste. LFM is more sensitive to topographic changes and could more clearly distinguish between the different scanned areas. AFM and LFM can provide advantages of high resolution digital three-dimensional morphological information that can be reached under room atmosphere where no vacuum is required, however the work is sensitive to polishing effects. In this work CSH particles and crystals of CH were observed. The measurements could distinguish between these two cement paste products. The CH region had a smoother surface as compared to the CSH as expected. At low scan range (below 3 lm) both CH and CSH regions exhibit particle nature, however the size of these particles is quite different, larger particles and more bumpy texture are observed for the CSH phase. The CH tends to have a more plate-like structure which is consistent with typical observations.
3.0 µm
Scan range
(b) Fig. 6. LFM images including both regions of CH and CSH products (the doted square area in Fig. 2) at two scan ranges: (a) 10 lm and (b) 3 lm.
[1] Butt H-J, Cappella B, Kappl M. Force measurements with the atomic force microscope: technique, interpretation and applications. Surf Sci Rep 2005;59:1–15. [2] Yang T, Keller B, Magyari E. AFM investigation of cement paste in humid air at different relative humidities. J Phys D: Appl Phys 2002;35(8):L25–8. [3] Yang T, Keller B, Magyari E. Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an atomic force microscope. J Mater Sci 2003;38:1909–16. [4] Papadakis VG, Pedersen EJ. An AFM–SEM investigation of the effect of silica fume and fly ash on cement paste microstructure. J Mater Sci 1999;34:683–90. [5] Bullard JW, Jennings HM, Livingston RA, Nonat A, Scherer GW, Schweitzer JS, et al. Mechanisms of cement hydration. Cem Concr Res, in press. [6] Peled A, Castro J, Weiss J. Atomic force microscope examinations of mortar made using water-filled lightweight aggregate. Trans Res Board 2009;2141: 92–101. [7] Diamond S. The microstructure of cement paste and concrete – a visual primer. Cem Concr Comp 2004;26:919–33. [8] Diamond S. Hydraulic cement pastes: their structure and properties. Proc conf held at Tapton Hall 2; 1976.