- Email: [email protected]
Investigation of residual internal stresses of the I and II modes in cement-hardening structures
Colloids and Surfaces A: Physicochemical and Engineering Aspects 160 (1999) 163 – 170 www.elsevier.nl/locate/colsurfa
Investigation of residual internal stresses of the I and II modes in cement-hardening structures L.M. Rybakova a, E.A. Amelina b,*, L.I. Kuksenova a, E.D. Shchukin b a
Blagonra6o6 Institute of Mechanical Engineering, Russian Academy of Sciences, Moscow, Russia b Colloid Chemistry, Moscow State Uni6ersity, 119899 Moscow, Russia
Abstract Since the discovery of X-ray diffraction techniques, this has enabled the detection of internal microstresses (of mode II) caused by capillary forces during the removal of pore-confined water for the first time. The effect of polyethylene glycol and polypropylene glycol on residual internal stresses was studied quantitatively. The results were discussed within the basic concepts of physicochemical mechanics. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cement; Residual internal stresses; X-ray diffraction; Organic additives
1. Introduction Residual stresses developing both in the process of the formation of a disperse structure and at the stage of the operation of a material play a significant role among the factors that in many respects determine the operational characteristics of cement materials. These stresses can also arise in the finished material under the effect of external conditions (in the course of its operation). The internal stresses arising during the formation of the structure are primarily microstresses (stresses of the II mode): they are localised and
* Corresponding author. Tel.: +7-95-939-5386; fax: + 795-135-8916. E-mail address: [email protected] (E.D. Shchukin)
balanced in microvolumes of the structure. Their presence, if their level is sufficiently high, may produce a dramatic effect during the subsequent operation of the material. As a rule, in the course of manufacture (and operation) of the cement-based materials, macrostresses (stresses of the I mode) also arise when different microscopic parts of the material are in different stressed states. These stresses exist primarily due to the effect of capillary forces. Shrinkage and cracking are mainly associated with such stresses. When efficient regulation methods for cement material properties are developed, including those using organic additives it is important to estimate the level of residual mode I and II stresses at different stages of formation and use of cement stone.
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 3 5 6 - 8
164
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Earlier, Shchukin et al. proposed an effective X-ray diffraction method for the determination of the residual internal stresses of the II mode [1,2]. Unlike the conventional method applied in the physics of metals, the new method is based on comparison of the physical width of lines on the X-ray patterns of the stressed structure with those of a powder resulting from the ‘soft’ disintegration of the structure, in which only the contacts between particles are broken (while the dispersity of the material remains invariable). As a result of disintegration such as this, the stresses localised in the contact zone are relaxed, causing narrowing of the lines on the X-ray pattern. This method was applied to measure and analyse the microstresses in the hardening structures of monomineral binders, such as hemihydrate gypsum, and magnesium and calcium oxides [3,4]. The crystals of hydrates of these binders give fairly distinct and identifiable lines on the X-ray patterns. It has been shown that the residual internal microstresses can reach the level of the strength of structures, and their magnitude substantially depends on the conditions of the hardening process (the values of supersaturation, dispersity of the starting binder, water to solid ratio, the presence of a filler, etc.). In this work, the X-ray diffraction method was applied to measure the residual internal stresses of the I and II mode in cement-hardening structures for the first time. In distinction from magnesium oxide and gypsum, cement is a much more complex product because of its multicomponent nature, high dispersity (and a low degree of crystallinity) of hydration products, and diversity of hydration forms. The structure of cement is so complicated that it often seems impossible to decipher even the X-ray patterns of cement without stresses hydrated in a large quantity of water under the conditions of continuous mixing because of insufficient resolution of the diffraction pattern recorded on the conventional apparatus [5,6]. Therefore, the procedure had to be substantially improved, including the elaboration of the sample preparation technique with allowance for the specific features of the object and adjustment of the corresponding apparatus.
2. Methods and materials An X-ray chamber operating according to the Debye–Scharrer principle was designed and developed for the X-ray diffraction analysis of cement samples. Its specific feature consists in the system of collimating devices (the size of the slits amounts to approximately 10 mm), arranged at strictly fixed distances on the path of an incident beam of X-rays, and in their specific design, as well as in the adjustment conditions. In general, these features allow recording of a high resolution diffraction pattern [7]. The X-ray patterns thus obtained register different structural elements of cement with a complex crystallochemical structure in the form of multiple distinct interference lines. These lines seem to be separated even when they are recorded within a very narrow range of diffraction angles. Physical broadening b(hkl) of Xray lines and their shift Du serve as the experimental basis for the determination of residual internal stresses sII and sI, respectively. An important issue is the choice of the line to be analysed. Due to complexity and instability of the cement crystalline structure in the process of hardening CaCO3 line which is always, from the beginning, available in cement due to its carbonisation in air, has been chosen for observation and analysis. CaCO3 is not subject to chemical changes in hardening and it is retained even in completely hydrated cement. Being included in the structure of calcite stone without direct participation in the process of its formation, calcite plays the role of a natural ‘filler’, absorbing the stresses that are developed in such a structure. The circumstance that the line changes its width and position in hardening, allows us to consider calcite a natural indicator of stresses appearing in hardening cement. For an independent estimation of internal stresses, an additional indicator of stresses was introduced into the composition of the samples, namely BaSO4 microcrystals (with the size 0.1 mm) with an easily determinable size of the regions of coherent scattering. The standards for comparison were cement powders hydrated in a large quantity of water under the conditions of continuous mixing.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Residual internal microstresses (mode II stresses) were determined from broadening (b) CaCO3 and BaSO4 lines. Using measurements of b values microdeformations Dd/d were defined and stresses were estimated as sII :E (Dd/d); E is Young’s elasticity modulus of crystal indicators. The error of b value (and consequently Dd/d) obtained by special methods was not \10%. Residual macrostresses were defined from the shift of the line (alteration of the diffraction angle Du) in the ‘stressed’ samples in comparison with the standard ones i.e. without stresses; stresses were estimated as sII : − (E/m)ctgu°Du; u° is the diffraction angle of fully hydrated cement powder, m is the Poisson coefficient; when Du B 0 the stresses are tensile, sI \0; when Du \0 the stresses are compressible, sI B0. The distances between the intensity peaks on chosen X-ray pattern phase lines with clear resolution have been measured with the help of a comparator 20 times for every line. The errors were: 9 3 ×10 − 4 and 6× 10 − 4 rad for CaCO3 and BaSO4, respectively. Table 1 The values of physical broadening (b) and microdeformations Dd/d of BaSO4 crystals in the samples of cement (w/s= 0.3) with different duration of hardening (H =100%) Hardening time (days)
b×103 (rad)
(Dd/d)×103
sII (Mpa)
7 28
1.45 0.78
0.78 0.45
45 26
Table 2 The shift of X-ray lines (Du) of CaCO3 and BaSO4 in cement samples under the influence of capillary forces in the process of drying Hardening time (days)
7 28 120
Cement; w/s= 0.35, drying at H = 20%
Shift of CaCO3 lines (Du×103 (rad))
Shift of BaSO4 lines (Du×103 (rad))
2.8 1.0 0.25
8.7 3.5 1.4
165
Samples were moulded from the mixture of portland cement with water in the shape of a prism (10× 6×4) mm3 and were subject to hardening in the atmosphere of saturated water vapour (H=100%). Organic additives with practically equal molecular mass (: 400) but with different degree of hydrophilicity — hydrophilic polyethylene glycol (PEG) and more ‘hydrophilic’ polypropylene glycol (PPG) — have been used. Portland cement and organic additives were the courtesy of Grace company. Macrostresses (mode I stresses) were generated by drying moistened samples (at H= 20% and in the atmospheric air). To maintain constant humidity in the process of exposing to X-rays the samples were preliminarily quickly covered with the film of NC-222 varnish. Similar tests have been conducted with the samples of monomineral binder — gypsum semihydrate.
3. Results and discussion X-ray investigations showed that the values of b and Du are very sensitive to changing the conditions in which the samples were prepared: duration of hardening, the ratio of water to a solid phase (w/s), availability of additives, etc. As follows from Table 1, for the samples produced from 100% cement b and Dd/d values (and sII accordingly) decrease in hardening. On the contrary, for the samples produced from the mixture of cement and sand with the ratio 1:3 with w/s = 0.45 the level of stresses sII after 28 days of hardening is higher than after 7 days. It can be seen (Table 2) that drying of the samples and the resultant appearance of capillary forces cause a noticeable shift of the diffraction lines of BaSO4 and CaCO3 which is characteristic of the level of residual microstresses (mode I stresses). The observed decrease in stresses with the increasing of the sample hardening time may be associated both with a decrease in the total amount of the pore-confined water in the course of hydration and with possible relaxation of stresses in the surface layer due to its microcracking.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
166
Table 3 The shift of X-ray lines in the samples of hardened (at H= 100%) gypsum Sample
Shift of gypsum lines (Du×103 (rad))
Gypsum, air drying0.35 Gypsum+BaSO4, 1.0 drying at H = 20%
than the strength of this sample measured during three-point bending tests (: 10 MPa). Calculated macrostress sI values were also too high. Hereby values of stresses depend on what indicator has been chosen to conduct the estimate. There can be several reasons for too high values of sI and sII. One of the reasons is that the modulus of minerals BaSO4 and CaCO3 used for calculations can be substantially different from the modulus of those small crystals that were used as stress indicators in cement. The issue of a too high level of defined stresses is an open one and requires special investigation. Nevertheless, the proposed method enables one to estimate the influence of the conditions under which cement materials were produced and used on the value of residual stresses of mode I and II, which largely define the quality of a material. It enables one to describe the efficiency of chemical modificators (organic additive effect) and to reveal the mechanism of their influence. The character and the degree of altering mode I and II stresses due to additives do not depend on the indicator whose line broadening and shift were used to estimate their values but are defined only by the type and concentration of the additive. So, to characterise the efficiency of the additive effect on the level of residual stresses quantitatively s/s°, i.e. stress values in the samples with additives (s) to the stresses in the control samples without additives (s°) ratios have been used. For control samples this ratio is equal to one (Table 4). The experiments have shown that both additives decrease microstresses; the higher the concentration of additives, the more substantial is the decrease. Hereby PPG additives diminish stresses
Shift of BaSO4 lines (Du×103 (rad)) 1 3.1
A similar method was also applied to detect the residual macrostresses of the I mode in the samples of hardened (at H = 100%) gypsum. The error of Du determination was 9 2 ×10 − 4 rad. It is evident from Table 3 that the shift and accordingly the level of residual macrostresses mode I depend on the degree of dehydration of the samples. To estimate accurately the values of sI and sII, one should know the exact value of the modulus of elasticity (E) of just that component whose X-ray lines (width and shift) were analysed. An approximate estimation of sII for the samples of gypsum (E= 103 MPa) yields the value sII = 1.9 MPa. Taking into consideration that the strength (at the three-point bending test) of the corresponding samples amounts to approximately 3.0 MPa, the obtained estimates of sII seem to be quite reasonable (the values of the microstresses approach the level of the macroscopic strength of the corresponding samples). In case of cement such estimates result, to our mind, in higher values. When standard values of mineral BaSO4 elasticity modulus are used (E = 5.8 ×104 MPa [8]) sII is equal to 26 MPa, that is considerably higher
Table 4 The effect of additives on mode II stresses (microstresses) in the hardening structure of cement and gypsum Cement (w/s=0.35)
Gypsum (w/s= 0.45)
Concentration of additive (wt%)
Without additives
1% PPG
3% PPG
3% PEG
3% PPG
3% PPG
3% PEG
Sample age (days) sII/s °IIa
1
7 0.88
7 0.59
7 0.84
120 0.52
1 0.19
1 0.58
a
sII, microstresses in the sample with additive; s °II, microstresses in the sample without additive.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Fig. 1. The effect of (a) PEG and (b) PPG additives on the average strength of cement specimens in (curves 1 and 2) compression and (curves 3 and 4) three-point bending tests. Curves 1 and 3 correspond to additive-free specimens. Concentrations of PEG and PPG are 5 and 3%, respectively.
in a higher degree than PEG. The strength data for cement specimens containing 1% of the cement mass have shown that the additives of this concentration do not affect the strength of the specimen after various hardening time. When the concentration of additives is 23%, the strength of the cement specimens decreases at initial stages and it actually does not change at later stages of hardening (Fig. 1).
167
Thus, the final effect of the additives on the strength of hardening structures can be reduced to two main interrelated factors: the formation of bridging contacts and internal stresses [2]. At the initial stages of hardening, the stabilising effect of the additives (impeded bridging) is predominant, thus reducing the strength. At further stages of structural development, reduction in internal stresses has a significant part, providing conditions for the increase in strength. The balance of these factors at low (B 1%) PPG and PEG contents provides equal strength of specimens with or without additives. Reduced strength at high additive contents (ca. 3–5%) seems to be associated with the predominance of the first factor (stabilisation of nuclei and impediment of their bridging). Residual mode I stresses (macrostresses) arising due to capillary forces were defined using the samples that were hardening at 100% humidity and before placing them into the X-ray camera they were exposed to an atmosphere of low humidity: in air (for several minutes) and above saturated solution of potassium acetate (i.e. at 20% humidity) for 24 h. During short exposure to air only a surface layer is dried; during 24 h exposure to the dry atmosphere (H=20%) drying takes place in deeper layers and the most part of moisture is extracted from the pores. Table 5 shows the results for the samples processed by surface dehydration. It can be seen that PPG additives substantially increase tensile stresses in the surface layer, in the presence of PEG the stresses also grow but not as much. After more complete dehydration the picture somewhat changes. The data for the samples from which the main quantity of moisture from the pores has evaporated are presented in Table 6. In this case PPG additives are also seen to increase tensile stresses. In the samples with PEG additives stresses change the sign: tensile stresses become compression ones. These data are compared with the results of additive effect on alteration of hardening structure strength during their dehydration, i.e. under the conditions when mode 1 stresses are developed. In Table 7 the ratios of dehydrated sample strength (Pd) to the strength of samples with
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
168
moisture (Pm) reflecting the degree of strength increase (or decrease) when moisture is eliminated from the pores are presented. In the presence of 3% PPG additives the strength of dehydrated samples is lower than that of the samples with moisture. It is especially noted for the samples that were hardening for 7 days. In the presence of PEG it does not happen i.e. the strength increases after drying. The picture described can be related to the fact that substantial increase in tensile stresses in the presence of PPG results in the partial destruction of the structure facilitated by residual microstresses (mode II stresses) available in it.
We can assume that under the influence of macrostresses partial relaxation of internal microstresses accompanied by the formation of microcracks in the most stressed parts of the structure takes place being the reason for decrease (or for the lack of increase) of sample strength during their dehydration. Table 8 presents ratios of microstresses in dry and hydrated samples that characterise the degree of mode II stress alteration under the influence of macrostresses. It can be seen that after dehydration microstresses are decreased in all samples but in the presence of 3% PPG this decrease is more considerable than for the samples without additives and with the addition of PEG.
Table 5 The effect of additives on the macrostresses s *I that appear in cement and gypsum samples during their short-term surface dehydration Binding material
Cement (w/s=0.35)
Additive
–
1% PPG
3% PPG
3% PEG
–
3% PPG
–
3% PPG
3% PEG
Sample age (days) a *° s */s I I
7 1.0
7 6.0
7 7.0
7 2.8
120 1.0
120 10
1 1.0
1 4.0
1 2.0
a
Gypsum (w/s= 0.45)
The s *° values correspond to samples without additives. I
Table 6 The influence of additives on mode I stresses (macrostresses) (sI) that appear in cement and gypsum samples after their deep dehydration in a dry atmosphere (H = 20%) Binding material
Cement (w/s=0.35)
Gypsum (w/s= 0.45)
Additive
0%
1% PPG
3% PPG
3% PEG
3% PPG
3% PPG
3% PEG
Sample (days) sI/s °I
1
7 1.7
7 1.8
7 −0.6
120 6.8
1 1.7
1 1.3
Table 7 The effect of additives on the change in the strength (Pd/Ph) of cement samples as a result drying Additive concentration (%)
PPG Sample age (days)
0 1 3 5
PEG
2
7
28
7
1.4 1.0 0.9 –
1.0 1.1 0.5 –
1.0 1.0 0.8 –
1.3 1.1 – 1.2
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
169
Table 8 The effect of additives on the change in internal mode II microstresses during the development of mode I macrostresses upon dehydration of cement samples Additive
–
1% PPG
3% PPG
3% PEG
–
3% PPG
Sample age (days) s dII/s hIIa
7 0.58
7 0.46
7 0.20
7 0.40
120 0.86
120 0.52
a
s dII/s hII, microstresses (mode II) of dry and humid samples, respectively.
Various effects of additives can be due to different hydrophilicity of these substances i.e. their ability to retain water. It is confirmed by the results of dehydration rates of hydrated samples and water vapour sorption rates of dehydrated sample investigation and also by the results of water mobility in cement study by the method of nuclear magnetic resonance relaxation. As a result in the samples with PPG in the process of pore humidity elimination high humidity gradients appear which results in the development of higher macrostresses. Moisture retaining capacity of PEG provides more uniform dehydration. It is natural that to explain different influence of these additives on macrostresses it is possible to be confined not only to the reasons presented. Additives can substantially influence other parameters which can define development of capillary forces. For example, they influence dispersivity of hydrated particles and deformability of material and what is more important, geometry of pores.
more complete disclosure of the chemical modifier impact mechanism and for preliminary estimate of efficiency and expediency of their purposeful use. 2. PPG and PEG significantly reduce microstresses in cement-hardening structures. This reduction is more pronounced with more hardening time and a higher additive concentration, and PPG is much more effective than PEG. 3. Macrostresses caused by capillary forces during the removal of pore-confined water were studied and different influence of PPG and PEG on the nature and level of mode I macrostresses was shown.
Acknowledgements The authors are grateful to Professor Max J. Setzer for his interest in the present work, discussion and valuable remarks which will be taken into consideration in further investigations.
References 4. Conclusions 1. The developed technique is very sensitive and can be extremely helpful to provide the estimate of various physico-chemical impacts, conditions of preparation and use of materials on residual internal stresses. Its further development and improvement to provide use together with other tests (mechanical and deformational) opens up new prospects for
[1] E.D. Shchukin, S.I. Kontorovich, J.G. Malikova, L.M. Rybakova, M. Rovinsky, P.A. Rehbinder, Dokl. AN SSSR 173 (1967) 139 (in Russian). [2] E.D. Shchukin, E.A. Amelina, S.I. Kontorovich, in: J. Skalny (Ed.), Materials Science of Concrete, vol. III, American Ceramic Society, Westerville, 1992, p. 1. [3] J.G. Malikova, S.I. Kontorovich, L.M. Rybakova, E.D. Shchukin, Cristallografiya 13 (1968) 642 (in Russian). [4] S.I. Kontorovich, J.G. Malikova, E.D. Shchokin, Colloid J. 32 (1970) 224 (in Russian). [5] E. Gartner, L. Myers, J. Am. Ceram. Soc. 76 (1993) 1521.
170
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
[4] S.I. Kontorovich, J.G. Malikova, E.D. Shchokin, Colloid J. 32 (1970) 224 (in Russian). [5] E. Gartner, L. Myers, J. Am. Ceram. Soc. 76 (1993) 1521. [6] L.S. Zevin, L.M. Kheiker, X-ray Methods for Testing of Building Materials, Stroyizdat, Moscow, 1965 (in Russian).
.
[7] L.M. Rybakova, L.I. Kuksenova, Structure and Wear Resistance of Metal, Mashinostroenie, Moscow, 1982 (in Russian). [8] F. Birch, G.E. Scharer, H.P. Spicer, in: A.P. Vinogradov (Ed)., Handbook of Physical Constants, Inostranaya Literatura, Moscow, 1949 (in Russian).
Investigation of residual internal stresses of the I and II modes in cement-hardening structures L.M. Rybakova a, E.A. Amelina b,*, L.I. Kuksenova a, E.D. Shchukin b a
Blagonra6o6 Institute of Mechanical Engineering, Russian Academy of Sciences, Moscow, Russia b Colloid Chemistry, Moscow State Uni6ersity, 119899 Moscow, Russia
Abstract Since the discovery of X-ray diffraction techniques, this has enabled the detection of internal microstresses (of mode II) caused by capillary forces during the removal of pore-confined water for the first time. The effect of polyethylene glycol and polypropylene glycol on residual internal stresses was studied quantitatively. The results were discussed within the basic concepts of physicochemical mechanics. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cement; Residual internal stresses; X-ray diffraction; Organic additives
1. Introduction Residual stresses developing both in the process of the formation of a disperse structure and at the stage of the operation of a material play a significant role among the factors that in many respects determine the operational characteristics of cement materials. These stresses can also arise in the finished material under the effect of external conditions (in the course of its operation). The internal stresses arising during the formation of the structure are primarily microstresses (stresses of the II mode): they are localised and
* Corresponding author. Tel.: +7-95-939-5386; fax: + 795-135-8916. E-mail address: [email protected] (E.D. Shchukin)
balanced in microvolumes of the structure. Their presence, if their level is sufficiently high, may produce a dramatic effect during the subsequent operation of the material. As a rule, in the course of manufacture (and operation) of the cement-based materials, macrostresses (stresses of the I mode) also arise when different microscopic parts of the material are in different stressed states. These stresses exist primarily due to the effect of capillary forces. Shrinkage and cracking are mainly associated with such stresses. When efficient regulation methods for cement material properties are developed, including those using organic additives it is important to estimate the level of residual mode I and II stresses at different stages of formation and use of cement stone.
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 3 5 6 - 8
164
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Earlier, Shchukin et al. proposed an effective X-ray diffraction method for the determination of the residual internal stresses of the II mode [1,2]. Unlike the conventional method applied in the physics of metals, the new method is based on comparison of the physical width of lines on the X-ray patterns of the stressed structure with those of a powder resulting from the ‘soft’ disintegration of the structure, in which only the contacts between particles are broken (while the dispersity of the material remains invariable). As a result of disintegration such as this, the stresses localised in the contact zone are relaxed, causing narrowing of the lines on the X-ray pattern. This method was applied to measure and analyse the microstresses in the hardening structures of monomineral binders, such as hemihydrate gypsum, and magnesium and calcium oxides [3,4]. The crystals of hydrates of these binders give fairly distinct and identifiable lines on the X-ray patterns. It has been shown that the residual internal microstresses can reach the level of the strength of structures, and their magnitude substantially depends on the conditions of the hardening process (the values of supersaturation, dispersity of the starting binder, water to solid ratio, the presence of a filler, etc.). In this work, the X-ray diffraction method was applied to measure the residual internal stresses of the I and II mode in cement-hardening structures for the first time. In distinction from magnesium oxide and gypsum, cement is a much more complex product because of its multicomponent nature, high dispersity (and a low degree of crystallinity) of hydration products, and diversity of hydration forms. The structure of cement is so complicated that it often seems impossible to decipher even the X-ray patterns of cement without stresses hydrated in a large quantity of water under the conditions of continuous mixing because of insufficient resolution of the diffraction pattern recorded on the conventional apparatus [5,6]. Therefore, the procedure had to be substantially improved, including the elaboration of the sample preparation technique with allowance for the specific features of the object and adjustment of the corresponding apparatus.
2. Methods and materials An X-ray chamber operating according to the Debye–Scharrer principle was designed and developed for the X-ray diffraction analysis of cement samples. Its specific feature consists in the system of collimating devices (the size of the slits amounts to approximately 10 mm), arranged at strictly fixed distances on the path of an incident beam of X-rays, and in their specific design, as well as in the adjustment conditions. In general, these features allow recording of a high resolution diffraction pattern [7]. The X-ray patterns thus obtained register different structural elements of cement with a complex crystallochemical structure in the form of multiple distinct interference lines. These lines seem to be separated even when they are recorded within a very narrow range of diffraction angles. Physical broadening b(hkl) of Xray lines and their shift Du serve as the experimental basis for the determination of residual internal stresses sII and sI, respectively. An important issue is the choice of the line to be analysed. Due to complexity and instability of the cement crystalline structure in the process of hardening CaCO3 line which is always, from the beginning, available in cement due to its carbonisation in air, has been chosen for observation and analysis. CaCO3 is not subject to chemical changes in hardening and it is retained even in completely hydrated cement. Being included in the structure of calcite stone without direct participation in the process of its formation, calcite plays the role of a natural ‘filler’, absorbing the stresses that are developed in such a structure. The circumstance that the line changes its width and position in hardening, allows us to consider calcite a natural indicator of stresses appearing in hardening cement. For an independent estimation of internal stresses, an additional indicator of stresses was introduced into the composition of the samples, namely BaSO4 microcrystals (with the size 0.1 mm) with an easily determinable size of the regions of coherent scattering. The standards for comparison were cement powders hydrated in a large quantity of water under the conditions of continuous mixing.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Residual internal microstresses (mode II stresses) were determined from broadening (b) CaCO3 and BaSO4 lines. Using measurements of b values microdeformations Dd/d were defined and stresses were estimated as sII :E (Dd/d); E is Young’s elasticity modulus of crystal indicators. The error of b value (and consequently Dd/d) obtained by special methods was not \10%. Residual macrostresses were defined from the shift of the line (alteration of the diffraction angle Du) in the ‘stressed’ samples in comparison with the standard ones i.e. without stresses; stresses were estimated as sII : − (E/m)ctgu°Du; u° is the diffraction angle of fully hydrated cement powder, m is the Poisson coefficient; when Du B 0 the stresses are tensile, sI \0; when Du \0 the stresses are compressible, sI B0. The distances between the intensity peaks on chosen X-ray pattern phase lines with clear resolution have been measured with the help of a comparator 20 times for every line. The errors were: 9 3 ×10 − 4 and 6× 10 − 4 rad for CaCO3 and BaSO4, respectively. Table 1 The values of physical broadening (b) and microdeformations Dd/d of BaSO4 crystals in the samples of cement (w/s= 0.3) with different duration of hardening (H =100%) Hardening time (days)
b×103 (rad)
(Dd/d)×103
sII (Mpa)
7 28
1.45 0.78
0.78 0.45
45 26
Table 2 The shift of X-ray lines (Du) of CaCO3 and BaSO4 in cement samples under the influence of capillary forces in the process of drying Hardening time (days)
7 28 120
Cement; w/s= 0.35, drying at H = 20%
Shift of CaCO3 lines (Du×103 (rad))
Shift of BaSO4 lines (Du×103 (rad))
2.8 1.0 0.25
8.7 3.5 1.4
165
Samples were moulded from the mixture of portland cement with water in the shape of a prism (10× 6×4) mm3 and were subject to hardening in the atmosphere of saturated water vapour (H=100%). Organic additives with practically equal molecular mass (: 400) but with different degree of hydrophilicity — hydrophilic polyethylene glycol (PEG) and more ‘hydrophilic’ polypropylene glycol (PPG) — have been used. Portland cement and organic additives were the courtesy of Grace company. Macrostresses (mode I stresses) were generated by drying moistened samples (at H= 20% and in the atmospheric air). To maintain constant humidity in the process of exposing to X-rays the samples were preliminarily quickly covered with the film of NC-222 varnish. Similar tests have been conducted with the samples of monomineral binder — gypsum semihydrate.
3. Results and discussion X-ray investigations showed that the values of b and Du are very sensitive to changing the conditions in which the samples were prepared: duration of hardening, the ratio of water to a solid phase (w/s), availability of additives, etc. As follows from Table 1, for the samples produced from 100% cement b and Dd/d values (and sII accordingly) decrease in hardening. On the contrary, for the samples produced from the mixture of cement and sand with the ratio 1:3 with w/s = 0.45 the level of stresses sII after 28 days of hardening is higher than after 7 days. It can be seen (Table 2) that drying of the samples and the resultant appearance of capillary forces cause a noticeable shift of the diffraction lines of BaSO4 and CaCO3 which is characteristic of the level of residual microstresses (mode I stresses). The observed decrease in stresses with the increasing of the sample hardening time may be associated both with a decrease in the total amount of the pore-confined water in the course of hydration and with possible relaxation of stresses in the surface layer due to its microcracking.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
166
Table 3 The shift of X-ray lines in the samples of hardened (at H= 100%) gypsum Sample
Shift of gypsum lines (Du×103 (rad))
Gypsum, air drying0.35 Gypsum+BaSO4, 1.0 drying at H = 20%
than the strength of this sample measured during three-point bending tests (: 10 MPa). Calculated macrostress sI values were also too high. Hereby values of stresses depend on what indicator has been chosen to conduct the estimate. There can be several reasons for too high values of sI and sII. One of the reasons is that the modulus of minerals BaSO4 and CaCO3 used for calculations can be substantially different from the modulus of those small crystals that were used as stress indicators in cement. The issue of a too high level of defined stresses is an open one and requires special investigation. Nevertheless, the proposed method enables one to estimate the influence of the conditions under which cement materials were produced and used on the value of residual stresses of mode I and II, which largely define the quality of a material. It enables one to describe the efficiency of chemical modificators (organic additive effect) and to reveal the mechanism of their influence. The character and the degree of altering mode I and II stresses due to additives do not depend on the indicator whose line broadening and shift were used to estimate their values but are defined only by the type and concentration of the additive. So, to characterise the efficiency of the additive effect on the level of residual stresses quantitatively s/s°, i.e. stress values in the samples with additives (s) to the stresses in the control samples without additives (s°) ratios have been used. For control samples this ratio is equal to one (Table 4). The experiments have shown that both additives decrease microstresses; the higher the concentration of additives, the more substantial is the decrease. Hereby PPG additives diminish stresses
Shift of BaSO4 lines (Du×103 (rad)) 1 3.1
A similar method was also applied to detect the residual macrostresses of the I mode in the samples of hardened (at H = 100%) gypsum. The error of Du determination was 9 2 ×10 − 4 rad. It is evident from Table 3 that the shift and accordingly the level of residual macrostresses mode I depend on the degree of dehydration of the samples. To estimate accurately the values of sI and sII, one should know the exact value of the modulus of elasticity (E) of just that component whose X-ray lines (width and shift) were analysed. An approximate estimation of sII for the samples of gypsum (E= 103 MPa) yields the value sII = 1.9 MPa. Taking into consideration that the strength (at the three-point bending test) of the corresponding samples amounts to approximately 3.0 MPa, the obtained estimates of sII seem to be quite reasonable (the values of the microstresses approach the level of the macroscopic strength of the corresponding samples). In case of cement such estimates result, to our mind, in higher values. When standard values of mineral BaSO4 elasticity modulus are used (E = 5.8 ×104 MPa [8]) sII is equal to 26 MPa, that is considerably higher
Table 4 The effect of additives on mode II stresses (microstresses) in the hardening structure of cement and gypsum Cement (w/s=0.35)
Gypsum (w/s= 0.45)
Concentration of additive (wt%)
Without additives
1% PPG
3% PPG
3% PEG
3% PPG
3% PPG
3% PEG
Sample age (days) sII/s °IIa
1
7 0.88
7 0.59
7 0.84
120 0.52
1 0.19
1 0.58
a
sII, microstresses in the sample with additive; s °II, microstresses in the sample without additive.
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
Fig. 1. The effect of (a) PEG and (b) PPG additives on the average strength of cement specimens in (curves 1 and 2) compression and (curves 3 and 4) three-point bending tests. Curves 1 and 3 correspond to additive-free specimens. Concentrations of PEG and PPG are 5 and 3%, respectively.
in a higher degree than PEG. The strength data for cement specimens containing 1% of the cement mass have shown that the additives of this concentration do not affect the strength of the specimen after various hardening time. When the concentration of additives is 23%, the strength of the cement specimens decreases at initial stages and it actually does not change at later stages of hardening (Fig. 1).
167
Thus, the final effect of the additives on the strength of hardening structures can be reduced to two main interrelated factors: the formation of bridging contacts and internal stresses [2]. At the initial stages of hardening, the stabilising effect of the additives (impeded bridging) is predominant, thus reducing the strength. At further stages of structural development, reduction in internal stresses has a significant part, providing conditions for the increase in strength. The balance of these factors at low (B 1%) PPG and PEG contents provides equal strength of specimens with or without additives. Reduced strength at high additive contents (ca. 3–5%) seems to be associated with the predominance of the first factor (stabilisation of nuclei and impediment of their bridging). Residual mode I stresses (macrostresses) arising due to capillary forces were defined using the samples that were hardening at 100% humidity and before placing them into the X-ray camera they were exposed to an atmosphere of low humidity: in air (for several minutes) and above saturated solution of potassium acetate (i.e. at 20% humidity) for 24 h. During short exposure to air only a surface layer is dried; during 24 h exposure to the dry atmosphere (H=20%) drying takes place in deeper layers and the most part of moisture is extracted from the pores. Table 5 shows the results for the samples processed by surface dehydration. It can be seen that PPG additives substantially increase tensile stresses in the surface layer, in the presence of PEG the stresses also grow but not as much. After more complete dehydration the picture somewhat changes. The data for the samples from which the main quantity of moisture from the pores has evaporated are presented in Table 6. In this case PPG additives are also seen to increase tensile stresses. In the samples with PEG additives stresses change the sign: tensile stresses become compression ones. These data are compared with the results of additive effect on alteration of hardening structure strength during their dehydration, i.e. under the conditions when mode 1 stresses are developed. In Table 7 the ratios of dehydrated sample strength (Pd) to the strength of samples with
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
168
moisture (Pm) reflecting the degree of strength increase (or decrease) when moisture is eliminated from the pores are presented. In the presence of 3% PPG additives the strength of dehydrated samples is lower than that of the samples with moisture. It is especially noted for the samples that were hardening for 7 days. In the presence of PEG it does not happen i.e. the strength increases after drying. The picture described can be related to the fact that substantial increase in tensile stresses in the presence of PPG results in the partial destruction of the structure facilitated by residual microstresses (mode II stresses) available in it.
We can assume that under the influence of macrostresses partial relaxation of internal microstresses accompanied by the formation of microcracks in the most stressed parts of the structure takes place being the reason for decrease (or for the lack of increase) of sample strength during their dehydration. Table 8 presents ratios of microstresses in dry and hydrated samples that characterise the degree of mode II stress alteration under the influence of macrostresses. It can be seen that after dehydration microstresses are decreased in all samples but in the presence of 3% PPG this decrease is more considerable than for the samples without additives and with the addition of PEG.
Table 5 The effect of additives on the macrostresses s *I that appear in cement and gypsum samples during their short-term surface dehydration Binding material
Cement (w/s=0.35)
Additive
–
1% PPG
3% PPG
3% PEG
–
3% PPG
–
3% PPG
3% PEG
Sample age (days) a *° s */s I I
7 1.0
7 6.0
7 7.0
7 2.8
120 1.0
120 10
1 1.0
1 4.0
1 2.0
a
Gypsum (w/s= 0.45)
The s *° values correspond to samples without additives. I
Table 6 The influence of additives on mode I stresses (macrostresses) (sI) that appear in cement and gypsum samples after their deep dehydration in a dry atmosphere (H = 20%) Binding material
Cement (w/s=0.35)
Gypsum (w/s= 0.45)
Additive
0%
1% PPG
3% PPG
3% PEG
3% PPG
3% PPG
3% PEG
Sample (days) sI/s °I
1
7 1.7
7 1.8
7 −0.6
120 6.8
1 1.7
1 1.3
Table 7 The effect of additives on the change in the strength (Pd/Ph) of cement samples as a result drying Additive concentration (%)
PPG Sample age (days)
0 1 3 5
PEG
2
7
28
7
1.4 1.0 0.9 –
1.0 1.1 0.5 –
1.0 1.0 0.8 –
1.3 1.1 – 1.2
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
169
Table 8 The effect of additives on the change in internal mode II microstresses during the development of mode I macrostresses upon dehydration of cement samples Additive
–
1% PPG
3% PPG
3% PEG
–
3% PPG
Sample age (days) s dII/s hIIa
7 0.58
7 0.46
7 0.20
7 0.40
120 0.86
120 0.52
a
s dII/s hII, microstresses (mode II) of dry and humid samples, respectively.
Various effects of additives can be due to different hydrophilicity of these substances i.e. their ability to retain water. It is confirmed by the results of dehydration rates of hydrated samples and water vapour sorption rates of dehydrated sample investigation and also by the results of water mobility in cement study by the method of nuclear magnetic resonance relaxation. As a result in the samples with PPG in the process of pore humidity elimination high humidity gradients appear which results in the development of higher macrostresses. Moisture retaining capacity of PEG provides more uniform dehydration. It is natural that to explain different influence of these additives on macrostresses it is possible to be confined not only to the reasons presented. Additives can substantially influence other parameters which can define development of capillary forces. For example, they influence dispersivity of hydrated particles and deformability of material and what is more important, geometry of pores.
more complete disclosure of the chemical modifier impact mechanism and for preliminary estimate of efficiency and expediency of their purposeful use. 2. PPG and PEG significantly reduce microstresses in cement-hardening structures. This reduction is more pronounced with more hardening time and a higher additive concentration, and PPG is much more effective than PEG. 3. Macrostresses caused by capillary forces during the removal of pore-confined water were studied and different influence of PPG and PEG on the nature and level of mode I macrostresses was shown.
Acknowledgements The authors are grateful to Professor Max J. Setzer for his interest in the present work, discussion and valuable remarks which will be taken into consideration in further investigations.
References 4. Conclusions 1. The developed technique is very sensitive and can be extremely helpful to provide the estimate of various physico-chemical impacts, conditions of preparation and use of materials on residual internal stresses. Its further development and improvement to provide use together with other tests (mechanical and deformational) opens up new prospects for
[1] E.D. Shchukin, S.I. Kontorovich, J.G. Malikova, L.M. Rybakova, M. Rovinsky, P.A. Rehbinder, Dokl. AN SSSR 173 (1967) 139 (in Russian). [2] E.D. Shchukin, E.A. Amelina, S.I. Kontorovich, in: J. Skalny (Ed.), Materials Science of Concrete, vol. III, American Ceramic Society, Westerville, 1992, p. 1. [3] J.G. Malikova, S.I. Kontorovich, L.M. Rybakova, E.D. Shchukin, Cristallografiya 13 (1968) 642 (in Russian). [4] S.I. Kontorovich, J.G. Malikova, E.D. Shchokin, Colloid J. 32 (1970) 224 (in Russian). [5] E. Gartner, L. Myers, J. Am. Ceram. Soc. 76 (1993) 1521.
170
L.M. Rybako6a et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 160 (1999) 163–170
[4] S.I. Kontorovich, J.G. Malikova, E.D. Shchokin, Colloid J. 32 (1970) 224 (in Russian). [5] E. Gartner, L. Myers, J. Am. Ceram. Soc. 76 (1993) 1521. [6] L.S. Zevin, L.M. Kheiker, X-ray Methods for Testing of Building Materials, Stroyizdat, Moscow, 1965 (in Russian).
.
[7] L.M. Rybakova, L.I. Kuksenova, Structure and Wear Resistance of Metal, Mashinostroenie, Moscow, 1982 (in Russian). [8] F. Birch, G.E. Scharer, H.P. Spicer, in: A.P. Vinogradov (Ed)., Handbook of Physical Constants, Inostranaya Literatura, Moscow, 1949 (in Russian).