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Relationships between composition, structure and mechanical properties of very low porosity cementitious systems
CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 187-195, 1993. Printed in the USA. 0008-8846/93. $6.00+.00. 1993 Pergamon Press Ltd.
RELATIONSHIPS BETWEEN COMPOSITION, STRUCTURE AND MECHANICAL PROPERTIES OF VERY LOW POROSITY CEMENTITIOUS SYSTEMS Zhongzi Xu*, Mingshu Tang* and J.J. Beaudoin** * Deparunent of Silicate Engineering, Nanjing Institute of Chemical Technology, Nanjing, Jiangsu 210009, China ** Materials Laboratory, Institute for Research in Conslruction, National Research Council, Ottawa, Ore., Canada, K1A 0R6 (Communicated by M. Daimon) (Received Jan. 21, 1992)
ABSTRACT Relationships between composition, structure and mechanical properties of very low-porosity cementitious materials have been investigated. It has been found that the principle factor determining the development of su'ucture and strength is initial porosity which depends on the compacting pressure. The maximum degree of hydration was much higher than the limit described by Powers. A non-linear relationship between porosity and degree of hydration was found. The density of hydration products at an early age was found to be much lower that at later times. The hydration process appeared to proceed as ff pore space was present in the structure of cement paste. It was proposed that the presence of a large quantity of clinker in the mature structure may be a harmful factor on the stability and durability of very low-porosity cementitious materials, because the chemical energy stored in clinker would be released through hydration in confined space. The development of structure and properties has shown that porosity was reduced sharply and compressive strength increased considerably during the early stage of hydration. The relation between compressive strength and porosity can be described by Schiller's equation using parameters obtained from helium and mercury porosity. It was suggested that the helium porosity-compressive strength relation is suitable to describe the behavior of very low-porosity cementitious materials. INTRODUCTION Very low porosity high strength cement systems have been studied intensively in the last decade. Research results have shown that these new inorganic materials have many potential applications. It has been, however, difficult to apply them in practice. There are still concerns regarding stability, durability and large-scale fabrication techniques. In addition, relationships between composition, structure and properties have not been well established. It was felt that further examination of existing relationships for low-porosity cementifious materials would provide valuable insight into their behaviors[l]. 187
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Common characteristics of low-porosity cementitious materials such as compacted cement paste [2,3], MDF cement [4-6] and DSP materials [7], are dense structure and ultra high strength due to use of special fabrication procedures, very low water/cement ratio or organic admixtures. The initial water/cement ratios are much lower than the critical value 0.36 deduced from Powers' model of hardened Portland cement paste [8,9]. This is the minimum water/cement ratio at which cement can fully hydrate. A great quantity of unhydrated cement remains in mature pastes made with low water/cement ratio. This results in two problems: (a) a large quantity of chemical energy stored in clinker calcined at high temperature is wasted; (b) thermodynamic considerations indicate that chemically unstable substances present in the cement mineral phases may be unfavorable to the long-term stability and durability of the materials. For example, when hardened compacted cement pastes are used in the solidifying treatment of high radioactive wastes, there is no evidence to ensure that the materials can not deteriorate in geological time periods. Recent research on MDF cement-based materials has concentrated on the improvement of stability due to water absorption and swelling of water-soluble polymers. Large amounts of unhydrated cement in these materials may be harmful during later hydration periods due to increase of solid volume by 2.13 times. It appears that the long term success of many very low-porosity cementitious materials depend on reconsideration and modification of composition and structure. It is necessary, therefore, to increase understanding of the processes of formation and development of structure and relationships between properties and composition. The present paper will deal with factors affecting formation of structure, initial porosity, kinetics of hydration and their relation with mechanical strength. Powers theory of limiting water/cement ratio in the case of very lowporosity cement pastes will also be discussed. EXPERIMENTAL
Ordinary type 10 Portland cement was used. The Bogue calculation for the four major compounds gives 62%C3S, 14%C2S, 3%C3A and 16%C4AF. The density and fineness of cement were 3.19 g/cm3 and 3250 cm2/g respectively. Cylindrical cement paste specimens, with diameter 1 cm, were cast in a stainless steel mould. Compacting pressure of 300MPa was maintained for 3 minutes. The height of the specimen was 1.5 times the diameter. An optimal water/cement ratio of 0.09 was adopted; the mixing water exactly fdls the inter-particle spaces of cement. This experimental procedure is similar to Roy and Gouda's[2] without hot pressing. Specimens were hardened in laboratory environment for 24 hours after demoulding, then cured in water at 20°C. Compressive strength results were the average of 6 tested specimens. Pore structure was determined by means of mercury intrusion porosimetry and helium pycnometry methods. The porosimeter used was the Autoscan 60 made by Quanta Chrome Co. It detects pores in the range of 17xlO4 - 10 ~ radius. Specimens were pre-treated by drying at 105oC for 4 hours. The degree of hydration was determined by using a method based on Differential Scanning Calorimetry (DSC) as described in ref.[10]. RESULTS AND DISCUSSION
Modification of Powers' Theory for Low Porosity Pastes The initial structure of cement paste formed during early hydration is an important factor determining its properties. Dense structure promotes high values of mechanical properties and
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good durability. Ordinary Portland cement paste has a relatively high porosity and weak-bond structure due to relatively high water/cement ratio. This results in low strength and high permeability. Powers and his co-workers postulated a structural model of hardened cement paste describing relationships between the physical structure, degree of hydration and original water/cement ratio. Porosity, amount of hydrated products and unhydrated cement fractions can be calculated quantitatively. The initial structural parameter is water/cement ratio. It is not, however, a correctdescriptorof the initialstructureof very low-porosityPortland cement pastes, because the structurefor these system depends only on compacting pressure.Mixing water will be pressed out by high pressure if in excess, or enter from environment in the case of low pressure and absence of water. Therefore, the parameter expressing initial structure of very lowporosity cement pastes is initial porosity which depends on compacting pressure and cement particles size distribution. The original water/cement ratio can, therefore, be replaced for low-porosity paste system by initialporosityexpressed in volumetric terms. Consider the following relationship:
Wo/C P°= Wo/C + 0.32
(I)
where, Po - initial porosity; Wo/C - original water/cement ratio; 0.32 - specific volume of cement clinker. It is noted that Po in equation(I) is equal to the volume fraction of mixing water for normal water/cement ratio pastes. The optimum water/cement ratio should, however, be used to calculate Po for very low-porosity pastes. This implies that the mixing water can exactly fill the space among the compacted cement particles. The quantitative relations proposed by Powers for composition and structure of hardened cement pastes can, therefore, be modified for lowporosity pastes and written using equation (1) as: Fraction of gel Vs = 2.13 ( 1 - P o ) m
(2)
Fraction of unhydrated cement
Vm=(l-Po)(l-m)
(3)
P~=0.59( l - P o ) m
(4)
Porosity of gel
Porosity of capillary pores P~=Po- 1.13 ( 1 -Po) m
(5)
P=Po-0.54( 1 - Po)m
(6)
Total porosity
Maximum degree of hydration rnm~ -
po 1.13 (1-Po)
(7)
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where, m - degree of hydration. These equations provide a clearer physical description of the hydration process than the original [11]. The factor 2.13 represents the expansion of solid volume as clinker transforms into hydration products. The fraction of pores contained intrinsically in the gel is obtained using the factor 0.28x2.13=0.59. The increase in amount of solid volume from clinker to gel is obtained using the factor 0.54. The structural parameters of ordinary Portland cement paste are linearly related to initial porosity, and change with degree of hydration. The Powers' model proposes an intrinsic gel porosity of 28% and a limiting water/cement ratio for complete hydration below which the volume concentration of hydration products approach a constant value. This does not appear to be compatible with the case of very low-porosity cement paste,
Structure formation The initialporosity can be determined by porosimetry immediately afterdemoulding and drying the samples. Fig.1 shows the effectof compacting pressure on initialporosity.Porosity decreases with the compacting pressure up to 300MPa. In this region, apparent density of demoulded specimens increases greatly, which means the initialstructure becomes more dense with the pressure. It is unnecessary to continuously increase the pressure when particles pack together closely. An empirical equation can be obtained from Fig.1 by regression analysis as follows:
(s)
P o = A e x p (- B ae) +21
where, a e - compacting pressure in MPa; A = 29.1; B = 0.0054. The correlation coefficient for equation(8) is 0.991. The relationship between initial porosity and compacting pressure is important for choosing an applicable pressure in practice.
Relationships between Composition and Structure Degree of hydration of very low-porosity hardened cement pastes versus time is shown in Fig.2. According to equation(7), the maximum degree of hydration should be 0.33 for 26.9% initial 60
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Fig.2 Degree of hydration versus time for low-porosity cement pastes. porosity at 300MPa of pressure, The experimental results, however, show that degree of hydration of pastes cured in water for 90 days is more than 0.5. Considering the possibility that Ca(OH)2 may be slighdy carbonated, the actual value of degree of hydration is much higher than the theoretical limit in Powers' model. This would also call into question the validity of the 28% intrinsic porosity value for C-S-H gel in very low-porosity cement pastes. In fact, hydration is not stopped when capillary pores are consumed by the products. It is noted that the Feldman model for structure of cement paste recognizes the existence of capillary pores but not gel pores[12]. Equations (5) and (7) only remain relevant if the Feldman model is employed. Fig3 shows the relation between porosity and degree of hydration in very low-porosity Portland cement pastes; the dashed curve represents the variation of porosity with degree of hydration calculated from equations (5) and (6). It is clear that actual porosity is much lower than the theoretical value and has not a constant linear relation with degree of hydration. The different
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slopes occur before and after 7 days of hydration. Furthermore, if hydration of cement pastes follows equations(6) and (7), the final porosity and maximum degree of hydration should be 14% and 0.33 respectively for 26.9% initial porosity. In other words, hydration will be stopped when gel porosity equals 14% and no capillary porosity remains. There is no available space in the structure to accommodate the formation of products. Equation (5) underestimates porosity determined by helium displacement. This suggests that the Powers' expressions for capillary porosity is also inaccurate for low-porosity cement pastes. The present results have not only demonstrated that the theory is not correct in the case of very low-porosity cement pastes, but also it may possibly be fortuitous for the normal pastes at high water/cement ratio. A more detailed explanation is given by Xu and Tang in previous work[I,13]. It is noted that helium is capable of instantaneously penetrating space having least dimension of approximately 2.5 nm. Helium would, therefore, easily penetrate gel pore space if it is present. Further evidence of the inapplicability of Powers' theory for low-porosity pastes is given in Fig.4 and Table 1. It can be seen from Fig.4 that the development of porosity in cement paste does not follow Powers' equation; the actual values axe much lower than the theoretical especially in the early period of hydration. A possible reason can be seen in Table 1 which shows the peak positions of Ca(OH)2 . It is evident that Ca(OH)2 formed at early periods is amorphous or poorcrystalline, but becomes crystalline after 7 days. Therefore, the products of hydration vary gradually and a value of 28% intrinsic gel porosity may be incorrecL In addition, the method of determining porosity is very important in the case of very lowporosity cementitious materials. Mercury porosity is always much lower than helium porosity due to the presence of discrete or ink bottle pores inaccessible to mercury, "missing pores"[14]. 30
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COMPOSITION-STRUCTURE-PROPERTIES, LOW POROSITY
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193
Therefore, different empirical constants can be obtained from porosity - compressive strength
equations with helium and mercury porosity. This includes the intrinsic strength which is obtained by extrapolation from the empirical equations [15]. k is argued that helium porosity is a more correct structural parameter because some pores in very low-porosity cement pastes apparently cannot be penetrated by mercury Helium porosity is, therefore, used to describe the dependence of compressive strength of hardened cement paste.
Relationship between Porosity and Compressive Strength Mechanical properties of ccmendtious materials are usually controlled by pore structure, especially porosity. These relations have been described by several empirical equations, e.g. Schillefs. The general expression can be written as follows: P = A exp ( - B o¢ )
(9)
where, (~c" compressive strength; A, B - constant.
The development of compressive strength of very low-porosity Portland cement pastes is shown in Fig.5. Very high strength is achieved in the early period, and continuously increases with hydration. The strength at 1 day is 70% of that at 28 days. This result shows that a dense structure is formed in the early period, and spaces in the paste will be filled by further hydration at later periods. A more dense structure and high strength results. It was found, however, that the paste strength decreased to 161.SMPa at 6 months; helium porosity was measured to be 8.06%. The phenomenon was also observed with aluminate cement [1]. These results suggest that when the structure becomes very dense further hydration may be harmful to the structure; swelling or crystalizing pressures produced by products formation may occur. It is suggested that a large quantity of unhydrated cement may be an unfavorable factor on the stability and durability of very low-porosity cementitious materials. Fig.6 shows the relationship between compressive strength and porosity of very low-porosity cement pastes. Similar results described by Roy and Gouda [2] were obtained. The empirical equation obtained from regression analysis can be expressed as follows: 25O o._
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I
I
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Fig. 5 Development of compressive strength of very low-porosity cement pastes.
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250
OPC w/c=O.09
¢'1
.,-,'- 200 C r,n 0
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Fig.6 Relationship between compressive strength and porosity of very low-porosity cement pastes. P = 1149 exp ( -0.031 o c )
(10)
where, o c - compressive strength in MPa. The correlation coefficient of equation(10) is 0.991. It appears that Schiller's equation is applicable to the case of very low-porosity extending the applicability of traditional compressive strength - porosity equations. Porosity is still a principle factor determining mechanical behavior. CONCLUSIONS (1) The major parameter determining the formation and development of structure and properties of very low-porosity cementifious materials is initial porosity. It is dependent on compacting pressure, and not the initial water/cement ratio. The relation between initial porosity and compacting pressure can be expressed by an empirical equation as follows: Po = A exp ( -Ba e) + 12 (2) A clearer physical description of hydration process in low-porosity pastes is obtained using modified equations based on Powers' cement paste hydration theory. New evidence, however, confLrms that the intrinsic gel porosity concept may be invalid and that hydration is not stopped at a limiting value. It appears that Powers' model is not suitable for very low-porosity cement pastes, and may be fortuitous in the case of high water/cement rados. (3) Compressive strength of very low-porosity cement pastes increases rapidly up to 7 days of hydration. This is possibly related to the low initial porosity of the structure. The relationship between compressive strength and helium porosity can be expressed by Schiller's equation. Estimation of intrinsic strength by extrapolation from different porosity measurements should be considered carefully. (4) A large quantity of unhydrated cement in the mature structure of very low-porosity cementitious systems may be unfavorable for the stability and durability of these materials. It
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may be more important for the future of very low-porosity cementitious materials to develop a modified structure which is chemically stable. REFERENCES
[1] Xu Zhongzi, " Relationship between the composition, structure and mechanical properties of low-porosity cementitious materials and modification approach ", (Ph.D thesis), Nanjing Institute of Chemical Technology, July, 1988. [2] Roy, D.M., Gouda, G.R. and Brobowsky, A., Cem. Conor. Res., Vol.2, pp.349366(1972). [3] Gouda, G.R. and Roy, D.M., Cem. Concr. Res., Vol.5, pp.551-564(1975). [4] Birchall, J.D., Howard, A.J. and Kendall, K., Nature, VoL289, pp.388-389(1981). [5] Kendall,K, Howard, A.J. and Bin:hall, J.D., Phil. Trans. R. Soc., London, A310, pp.139-153(1983). [6] AlforcLN.McN., Cem. Contr. Res., Vol.11, pp.605-610(1981). [7] Hjorth, L., Phil. Trans. R. Sot:., London, A310, pp.167-173(1983). [8] Powers, T.C. and Brownyard, T.L., J. Amer. Ceram. Soc., Vol.43, No.2-6, pp.101669(1946-1947). [9] Powers, T.C., J. Amer. Ceram. Soc., Vol.41, No.l, pp.1-6(1958). [10] Ramachandran,V.S., Cem. Concr. Res., Vol.9, pp.677-684(1979) [ 11] Hansen, T.C., Materials and Structures, Vol. 19, No. 114, pp.423-436(1986). [12] Ramachandran, V.S., Feldman, R.F. and Beaudoin, J.J., " Concrete Science ", Heyden & Son Ltd., London, p.56(1981). [13] Xu 2~ongzi and Tang Mingshu, J. Chinese Silicate Soc., VoL19, No.2, pp.104111(1991). [14] Beaudoin, J.J. Cem. Concr. Res., Vol.9, pp.771-781(1979). [15] Xu Zhongzi and Tang Mingshu, J. Nanjing Institute of Chemical Technology, Vol. 12, No.4, pp.61-65(1990).
RELATIONSHIPS BETWEEN COMPOSITION, STRUCTURE AND MECHANICAL PROPERTIES OF VERY LOW POROSITY CEMENTITIOUS SYSTEMS Zhongzi Xu*, Mingshu Tang* and J.J. Beaudoin** * Deparunent of Silicate Engineering, Nanjing Institute of Chemical Technology, Nanjing, Jiangsu 210009, China ** Materials Laboratory, Institute for Research in Conslruction, National Research Council, Ottawa, Ore., Canada, K1A 0R6 (Communicated by M. Daimon) (Received Jan. 21, 1992)
ABSTRACT Relationships between composition, structure and mechanical properties of very low-porosity cementitious materials have been investigated. It has been found that the principle factor determining the development of su'ucture and strength is initial porosity which depends on the compacting pressure. The maximum degree of hydration was much higher than the limit described by Powers. A non-linear relationship between porosity and degree of hydration was found. The density of hydration products at an early age was found to be much lower that at later times. The hydration process appeared to proceed as ff pore space was present in the structure of cement paste. It was proposed that the presence of a large quantity of clinker in the mature structure may be a harmful factor on the stability and durability of very low-porosity cementitious materials, because the chemical energy stored in clinker would be released through hydration in confined space. The development of structure and properties has shown that porosity was reduced sharply and compressive strength increased considerably during the early stage of hydration. The relation between compressive strength and porosity can be described by Schiller's equation using parameters obtained from helium and mercury porosity. It was suggested that the helium porosity-compressive strength relation is suitable to describe the behavior of very low-porosity cementitious materials. INTRODUCTION Very low porosity high strength cement systems have been studied intensively in the last decade. Research results have shown that these new inorganic materials have many potential applications. It has been, however, difficult to apply them in practice. There are still concerns regarding stability, durability and large-scale fabrication techniques. In addition, relationships between composition, structure and properties have not been well established. It was felt that further examination of existing relationships for low-porosity cementifious materials would provide valuable insight into their behaviors[l]. 187
188
Z. Xu etal.
Vol.23,No. I
Common characteristics of low-porosity cementitious materials such as compacted cement paste [2,3], MDF cement [4-6] and DSP materials [7], are dense structure and ultra high strength due to use of special fabrication procedures, very low water/cement ratio or organic admixtures. The initial water/cement ratios are much lower than the critical value 0.36 deduced from Powers' model of hardened Portland cement paste [8,9]. This is the minimum water/cement ratio at which cement can fully hydrate. A great quantity of unhydrated cement remains in mature pastes made with low water/cement ratio. This results in two problems: (a) a large quantity of chemical energy stored in clinker calcined at high temperature is wasted; (b) thermodynamic considerations indicate that chemically unstable substances present in the cement mineral phases may be unfavorable to the long-term stability and durability of the materials. For example, when hardened compacted cement pastes are used in the solidifying treatment of high radioactive wastes, there is no evidence to ensure that the materials can not deteriorate in geological time periods. Recent research on MDF cement-based materials has concentrated on the improvement of stability due to water absorption and swelling of water-soluble polymers. Large amounts of unhydrated cement in these materials may be harmful during later hydration periods due to increase of solid volume by 2.13 times. It appears that the long term success of many very low-porosity cementitious materials depend on reconsideration and modification of composition and structure. It is necessary, therefore, to increase understanding of the processes of formation and development of structure and relationships between properties and composition. The present paper will deal with factors affecting formation of structure, initial porosity, kinetics of hydration and their relation with mechanical strength. Powers theory of limiting water/cement ratio in the case of very lowporosity cement pastes will also be discussed. EXPERIMENTAL
Ordinary type 10 Portland cement was used. The Bogue calculation for the four major compounds gives 62%C3S, 14%C2S, 3%C3A and 16%C4AF. The density and fineness of cement were 3.19 g/cm3 and 3250 cm2/g respectively. Cylindrical cement paste specimens, with diameter 1 cm, were cast in a stainless steel mould. Compacting pressure of 300MPa was maintained for 3 minutes. The height of the specimen was 1.5 times the diameter. An optimal water/cement ratio of 0.09 was adopted; the mixing water exactly fdls the inter-particle spaces of cement. This experimental procedure is similar to Roy and Gouda's[2] without hot pressing. Specimens were hardened in laboratory environment for 24 hours after demoulding, then cured in water at 20°C. Compressive strength results were the average of 6 tested specimens. Pore structure was determined by means of mercury intrusion porosimetry and helium pycnometry methods. The porosimeter used was the Autoscan 60 made by Quanta Chrome Co. It detects pores in the range of 17xlO4 - 10 ~ radius. Specimens were pre-treated by drying at 105oC for 4 hours. The degree of hydration was determined by using a method based on Differential Scanning Calorimetry (DSC) as described in ref.[10]. RESULTS AND DISCUSSION
Modification of Powers' Theory for Low Porosity Pastes The initial structure of cement paste formed during early hydration is an important factor determining its properties. Dense structure promotes high values of mechanical properties and
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good durability. Ordinary Portland cement paste has a relatively high porosity and weak-bond structure due to relatively high water/cement ratio. This results in low strength and high permeability. Powers and his co-workers postulated a structural model of hardened cement paste describing relationships between the physical structure, degree of hydration and original water/cement ratio. Porosity, amount of hydrated products and unhydrated cement fractions can be calculated quantitatively. The initial structural parameter is water/cement ratio. It is not, however, a correctdescriptorof the initialstructureof very low-porosityPortland cement pastes, because the structurefor these system depends only on compacting pressure.Mixing water will be pressed out by high pressure if in excess, or enter from environment in the case of low pressure and absence of water. Therefore, the parameter expressing initial structure of very lowporosity cement pastes is initial porosity which depends on compacting pressure and cement particles size distribution. The original water/cement ratio can, therefore, be replaced for low-porosity paste system by initialporosityexpressed in volumetric terms. Consider the following relationship:
Wo/C P°= Wo/C + 0.32
(I)
where, Po - initial porosity; Wo/C - original water/cement ratio; 0.32 - specific volume of cement clinker. It is noted that Po in equation(I) is equal to the volume fraction of mixing water for normal water/cement ratio pastes. The optimum water/cement ratio should, however, be used to calculate Po for very low-porosity pastes. This implies that the mixing water can exactly fill the space among the compacted cement particles. The quantitative relations proposed by Powers for composition and structure of hardened cement pastes can, therefore, be modified for lowporosity pastes and written using equation (1) as: Fraction of gel Vs = 2.13 ( 1 - P o ) m
(2)
Fraction of unhydrated cement
Vm=(l-Po)(l-m)
(3)
P~=0.59( l - P o ) m
(4)
Porosity of gel
Porosity of capillary pores P~=Po- 1.13 ( 1 -Po) m
(5)
P=Po-0.54( 1 - Po)m
(6)
Total porosity
Maximum degree of hydration rnm~ -
po 1.13 (1-Po)
(7)
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where, m - degree of hydration. These equations provide a clearer physical description of the hydration process than the original [11]. The factor 2.13 represents the expansion of solid volume as clinker transforms into hydration products. The fraction of pores contained intrinsically in the gel is obtained using the factor 0.28x2.13=0.59. The increase in amount of solid volume from clinker to gel is obtained using the factor 0.54. The structural parameters of ordinary Portland cement paste are linearly related to initial porosity, and change with degree of hydration. The Powers' model proposes an intrinsic gel porosity of 28% and a limiting water/cement ratio for complete hydration below which the volume concentration of hydration products approach a constant value. This does not appear to be compatible with the case of very low-porosity cement paste,
Structure formation The initialporosity can be determined by porosimetry immediately afterdemoulding and drying the samples. Fig.1 shows the effectof compacting pressure on initialporosity.Porosity decreases with the compacting pressure up to 300MPa. In this region, apparent density of demoulded specimens increases greatly, which means the initialstructure becomes more dense with the pressure. It is unnecessary to continuously increase the pressure when particles pack together closely. An empirical equation can be obtained from Fig.1 by regression analysis as follows:
(s)
P o = A e x p (- B ae) +21
where, a e - compacting pressure in MPa; A = 29.1; B = 0.0054. The correlation coefficient for equation(8) is 0.991. The relationship between initial porosity and compacting pressure is important for choosing an applicable pressure in practice.
Relationships between Composition and Structure Degree of hydration of very low-porosity hardened cement pastes versus time is shown in Fig.2. According to equation(7), the maximum degree of hydration should be 0.33 for 26.9% initial 60
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Compocting Pressure (MPG) Fig.l Relationship between initial porosity and compacting pressure for low porosity cement systems.
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1O0
(days)
Fig.2 Degree of hydration versus time for low-porosity cement pastes. porosity at 300MPa of pressure, The experimental results, however, show that degree of hydration of pastes cured in water for 90 days is more than 0.5. Considering the possibility that Ca(OH)2 may be slighdy carbonated, the actual value of degree of hydration is much higher than the theoretical limit in Powers' model. This would also call into question the validity of the 28% intrinsic porosity value for C-S-H gel in very low-porosity cement pastes. In fact, hydration is not stopped when capillary pores are consumed by the products. It is noted that the Feldman model for structure of cement paste recognizes the existence of capillary pores but not gel pores[12]. Equations (5) and (7) only remain relevant if the Feldman model is employed. Fig3 shows the relation between porosity and degree of hydration in very low-porosity Portland cement pastes; the dashed curve represents the variation of porosity with degree of hydration calculated from equations (5) and (6). It is clear that actual porosity is much lower than the theoretical value and has not a constant linear relation with degree of hydration. The different
[ ...~.... He 25' 20. 15
"~,.\
"~
~,
\
,
::
50
" - ~ = - 0
A.__ 0.2
0.4
0.6
Degree of hydration
Fig.3 Relation between porosity and degree of hydration of very low-porosity cement pastes
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slopes occur before and after 7 days of hydration. Furthermore, if hydration of cement pastes follows equations(6) and (7), the final porosity and maximum degree of hydration should be 14% and 0.33 respectively for 26.9% initial porosity. In other words, hydration will be stopped when gel porosity equals 14% and no capillary porosity remains. There is no available space in the structure to accommodate the formation of products. Equation (5) underestimates porosity determined by helium displacement. This suggests that the Powers' expressions for capillary porosity is also inaccurate for low-porosity cement pastes. The present results have not only demonstrated that the theory is not correct in the case of very low-porosity cement pastes, but also it may possibly be fortuitous for the normal pastes at high water/cement ratio. A more detailed explanation is given by Xu and Tang in previous work[I,13]. It is noted that helium is capable of instantaneously penetrating space having least dimension of approximately 2.5 nm. Helium would, therefore, easily penetrate gel pore space if it is present. Further evidence of the inapplicability of Powers' theory for low-porosity pastes is given in Fig.4 and Table 1. It can be seen from Fig.4 that the development of porosity in cement paste does not follow Powers' equation; the actual values axe much lower than the theoretical especially in the early period of hydration. A possible reason can be seen in Table 1 which shows the peak positions of Ca(OH)2 . It is evident that Ca(OH)2 formed at early periods is amorphous or poorcrystalline, but becomes crystalline after 7 days. Therefore, the products of hydration vary gradually and a value of 28% intrinsic gel porosity may be incorrecL In addition, the method of determining porosity is very important in the case of very lowporosity cementitious materials. Mercury porosity is always much lower than helium porosity due to the presence of discrete or ink bottle pores inaccessible to mercury, "missing pores"[14]. 30
A Hg -i-..o.-i--He 20
ffl
o a..
O~-O. 10
0
2
4
6
8
10
Hydration Time (days'/2) Fig. 4 Porosity development of very low-porosity cement pastes. Table 1. Ca(OH)2Peak positions in very low-porosity cement pastes* Hydration time (days) 1 3 7 14 21 28 Peak positions (°C)
443.8
*Pure Ca(OH) 2 crystal at 580°C
467.5
575.6
571.4
580.3
563.1
90 576.4
COMPOSITION-STRUCTURE-PROPERTIES, LOW POROSITY
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193
Therefore, different empirical constants can be obtained from porosity - compressive strength
equations with helium and mercury porosity. This includes the intrinsic strength which is obtained by extrapolation from the empirical equations [15]. k is argued that helium porosity is a more correct structural parameter because some pores in very low-porosity cement pastes apparently cannot be penetrated by mercury Helium porosity is, therefore, used to describe the dependence of compressive strength of hardened cement paste.
Relationship between Porosity and Compressive Strength Mechanical properties of ccmendtious materials are usually controlled by pore structure, especially porosity. These relations have been described by several empirical equations, e.g. Schillefs. The general expression can be written as follows: P = A exp ( - B o¢ )
(9)
where, (~c" compressive strength; A, B - constant.
The development of compressive strength of very low-porosity Portland cement pastes is shown in Fig.5. Very high strength is achieved in the early period, and continuously increases with hydration. The strength at 1 day is 70% of that at 28 days. This result shows that a dense structure is formed in the early period, and spaces in the paste will be filled by further hydration at later periods. A more dense structure and high strength results. It was found, however, that the paste strength decreased to 161.SMPa at 6 months; helium porosity was measured to be 8.06%. The phenomenon was also observed with aluminate cement [1]. These results suggest that when the structure becomes very dense further hydration may be harmful to the structure; swelling or crystalizing pressures produced by products formation may occur. It is suggested that a large quantity of unhydrated cement may be an unfavorable factor on the stability and durability of very low-porosity cementitious materials. Fig.6 shows the relationship between compressive strength and porosity of very low-porosity cement pastes. Similar results described by Roy and Gouda [2] were obtained. The empirical equation obtained from regression analysis can be expressed as follows: 25O o._
J
¢--
o~ 200
(D
"~ 03
0PC w/c=0.09
150
20'C
Q..
E o
100
i 0
I
20
I
I
40
I
I
60
Hydration Time
I
I
80
I
1O0
(days)
Fig. 5 Development of compressive strength of very low-porosity cement pastes.
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Z. Xu el al.
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250
OPC w/c=O.09
¢'1
.,-,'- 200 C r,n 0
.>-
150
r-~
E o
100
I
0
I
I
I
I
I
i
4 8 12 16 Porosity (% by He)
i
20
Fig.6 Relationship between compressive strength and porosity of very low-porosity cement pastes. P = 1149 exp ( -0.031 o c )
(10)
where, o c - compressive strength in MPa. The correlation coefficient of equation(10) is 0.991. It appears that Schiller's equation is applicable to the case of very low-porosity extending the applicability of traditional compressive strength - porosity equations. Porosity is still a principle factor determining mechanical behavior. CONCLUSIONS (1) The major parameter determining the formation and development of structure and properties of very low-porosity cementifious materials is initial porosity. It is dependent on compacting pressure, and not the initial water/cement ratio. The relation between initial porosity and compacting pressure can be expressed by an empirical equation as follows: Po = A exp ( -Ba e) + 12 (2) A clearer physical description of hydration process in low-porosity pastes is obtained using modified equations based on Powers' cement paste hydration theory. New evidence, however, confLrms that the intrinsic gel porosity concept may be invalid and that hydration is not stopped at a limiting value. It appears that Powers' model is not suitable for very low-porosity cement pastes, and may be fortuitous in the case of high water/cement rados. (3) Compressive strength of very low-porosity cement pastes increases rapidly up to 7 days of hydration. This is possibly related to the low initial porosity of the structure. The relationship between compressive strength and helium porosity can be expressed by Schiller's equation. Estimation of intrinsic strength by extrapolation from different porosity measurements should be considered carefully. (4) A large quantity of unhydrated cement in the mature structure of very low-porosity cementitious systems may be unfavorable for the stability and durability of these materials. It
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may be more important for the future of very low-porosity cementitious materials to develop a modified structure which is chemically stable. REFERENCES
[1] Xu Zhongzi, " Relationship between the composition, structure and mechanical properties of low-porosity cementitious materials and modification approach ", (Ph.D thesis), Nanjing Institute of Chemical Technology, July, 1988. [2] Roy, D.M., Gouda, G.R. and Brobowsky, A., Cem. Conor. Res., Vol.2, pp.349366(1972). [3] Gouda, G.R. and Roy, D.M., Cem. Concr. Res., Vol.5, pp.551-564(1975). [4] Birchall, J.D., Howard, A.J. and Kendall, K., Nature, VoL289, pp.388-389(1981). [5] Kendall,K, Howard, A.J. and Bin:hall, J.D., Phil. Trans. R. Soc., London, A310, pp.139-153(1983). [6] AlforcLN.McN., Cem. Contr. Res., Vol.11, pp.605-610(1981). [7] Hjorth, L., Phil. Trans. R. Sot:., London, A310, pp.167-173(1983). [8] Powers, T.C. and Brownyard, T.L., J. Amer. Ceram. Soc., Vol.43, No.2-6, pp.101669(1946-1947). [9] Powers, T.C., J. Amer. Ceram. Soc., Vol.41, No.l, pp.1-6(1958). [10] Ramachandran,V.S., Cem. Concr. Res., Vol.9, pp.677-684(1979) [ 11] Hansen, T.C., Materials and Structures, Vol. 19, No. 114, pp.423-436(1986). [12] Ramachandran, V.S., Feldman, R.F. and Beaudoin, J.J., " Concrete Science ", Heyden & Son Ltd., London, p.56(1981). [13] Xu 2~ongzi and Tang Mingshu, J. Chinese Silicate Soc., VoL19, No.2, pp.104111(1991). [14] Beaudoin, J.J. Cem. Concr. Res., Vol.9, pp.771-781(1979). [15] Xu Zhongzi and Tang Mingshu, J. Nanjing Institute of Chemical Technology, Vol. 12, No.4, pp.61-65(1990).