open porosity of cement pastes and their gas permeability

BULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES, Vol 64, No 4, 2016 DOI: 10.1515/bpasts-2016-0086 CIVIL ENGINEERING Open porosity of cement pastes and their gas permeability T TRACZ* Institute of Building Materials and Structures, Cracow University of Technology, 24 Warszawska St., 31-155 Kraków, Poland Abstract This paper presents the results of extensive research work on the open porosity and gas permeability of cement pastes Tests were conducted on cement pastes with different water/cement ratios and types of cement The three most popular cements in Poland from the CEM I, CEM II and CEM III groups were tested after the pastes had been cured for 90 days in laboratory conditions The scope of experiments included the assessment of open porosity determined using three different methods: comparing the bulk and specific densities, mercury intrusion porosimetry and saturating the material with water In addition, this article contains an analysis of the porosity characteristics based on the distributions produced by porosimetry examinations Gas permeability was determined using the modified RILEM-Cembureau laboratory method The results of the completed test allowed a quantitative determination to be made of the impact of water-cement ratio and type of cement used on open porosity assessed by various methods, and the influence of these parameters on the gas permeability of the paste The quantitative changes in the content of capillary pores and meso-pores in the cement pastes analysed are also presented Key words: cement paste, water-cement ratio, open porosity, helium porosity, MIP porosity, gas permeability Introduction Cement concrete is a non-uniform, composite material in which distributed grains of aggregate, most frequently stone, form inclusions, while the hardened cement paste is the matrix The characteristics of the hardened cement paste largely determine the characteristics of the concrete For this reason, the characteristics of the paste, and particularly of its open porosity, is of interest to many researchers [1‒7] Many methods exist for assessing this porosity, but the methods customarily used are subject to many limitations and thus provide incomplete information It is therefore justified to use several methods at the same time to assess the structure of open pores Information thus collected allows the results to be analysed in depth and better supported conclusions to be formulated As the effects of environmental substances, liquid or gaseous, usually have a negative effect on the in-service behaviour of the material, knowing its permeability allows its potential durability to be assessed Permeability, characterised by the coefficient defined below, is one of the measures of the accessibility of the porous structure of the material to external liquid and gaseous substances One of the phenomena occurring in cement materials is carbonation, generally caused by the penetration of CO2 into the material [8] As in reinforced concrete elements this phenomenon reduces or even eliminates the ability of the external reinforcement cover to protect steel bars, concrete should be sufficiently tight against CO2, in other words, of sufficiently low permeability for this gas So far, the most popular technique for measurement of cement material permeability has used water However, since the *e-mail: ttracz@pk.edu.pl new generation cement composites used in practice are particularly distinguished by their much reduced and qualitatively modified porosity, this measurement technique has become unsuitable Distinguishing the extent to which their internal structure is accessible to water has become very difficult or impossible because of their tight texture Measuring by methods based on gas flow provides a much more subtle ability to distinguish the extent to which the interior of the material is accessible Previously, such methods were mainly used for rock materials [9‒11] Determining the permeability of the material this way gives a more reliable representation of the extent of its accessibility to gaseous substances from the environment [12‒17] As the cement paste constitutes the most permeable component of concrete with stone aggregate, its permeability can provide information allowing the permeability of the entire composite to be predicted and designed The literature review carried out has shown that the permeability of cement paste determined using gas flows has not previously been of particular interest In addition, researchers did not examine the relationship between the paste porosity characteristic and permeability in much depth This article presents an attempt to quantitatively assess the impact of material factors on the characteristics of the open porosity and related permeability of pastes Permeability tests were carried out using an adapted RILEM-Cembureau method with a flow of gas (nitrogen) [18, 19] Experimental procedures 2.1 Material and specimen preparation The pastes analysed were produced using three types of cement, class 42.5 compliant with EN 197‒1 [20] Portland cement CEM I; Portland-fly ash cement CEM II/A-V and Slag cement CEM III/A The char- 775 Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM T Tracz acteristics of these cements are shown in Table Regardless of Every property of paste analysed in this article was evaluated the cement type used, the cement pastes also differed in their on a series of three specimens Irrespective of the type of cement used, the paste composiwater/cement ratio (0.3, 0.4, 0.5 and 0.6) Cement pastes were shown in prepared Table 1.inRegardless thethe cement type standard used, the[21] was the water-cement ratio This range of thewas water complianceofwith EN 196‒1 tions were the same (Table 2).wide The only variable the wacement pastes also differed in their water/cement ratio cementter-cement ratios represents most concrete compositions ratio This wide range of the water cement ratios (0.3, 0.4, 0.5 and 0.6) Cement pastes appliedrepresents in practice Table 1were prepared in most concrete compositions applied in practice compliance with the EN 196-1 standard [21] Chemical and physical characteristics of cements PARAMETERS Cement pastes characterized by water/cement ratio 0.3 and 0.4 were supplemented with superplasticizer in order Table CEM Composition of cement pastes to obtain similar fluidity of all suspensions This ensured III/A index of all3 samples Slight 42.5Ncomparable compaction w/c ratio Cement, kg/m Water, dm3/m3 sedimentation effect was observed only in the case of 0.30 water/cement 1605 ratio 0.6 As 483 it was cement pastes with mentioned above 0.40top and bottom 1383 all the cylindrical 554 30.0specimens were grinded so that to remove the layers 0.50 1217 608 6.2 which were not representative This procedure was 0.60 1083 650 1.7 especially important in the case of pastes with high water 50.3content CEM CEM Table I II/A–V Chemical and physical characteristics of cements 42.5 R 42.5 R PARAMETERS CEM CEM CEM Chemical Characteristics I II/A-V III/A (Oxide analysis, % m) 42.5 R 42.5 R 42.5N SiO2 23.2 Chemical Characteristics (Oxide analysis, % m) 18.6 SiO2 18.6 23.2 30.0 Al2O3 5.3 8.0 Al2O3 5.3 8.0 6.2 Fe2O3 2.9 3.3 Fe2O3 2.9 3.3 1.7 CaO 62.7 55.1 CaO 62.7 55.1 50.3 Cement pastes characterized by water/cement ratio 0.3 and MgO 1.50 1.59 4.98 MgO 1.50 1.59 4.98 0.4 were supplemented with superplasticizer in order to obtain SO3 3.22 2.96 2.41 SO3 3.22 2.96 2.41 Table Na2O 0.19 0.27 0.37 similar fluidity of all suspensions This ensured comparable Composition of cement pastes Na O 0.19 0.27 0.37 K2 O 0.96 1.12 0.70 compaction index of all samples Slight sedimentation effect 3 K 2O 0.96 1.12 0.70 eqNa2O 0.82 1.01 0.83 ratio onlyCement, Water, dmpastes /m3 with water/ was w/c observed in the kg/m case of cement Cl0.060 0.029 0.016 eqNa2O 0.82 1.01 0.83 1605mentioned above 483 top and bottom all cement0.30 ratio 0.6 As it was Physical Characteristics 0.40 1383 Cl– 0.060 0.029 0.016 the cylindrical specimens were grinded 554 so that to remove the Specific area 340 366 465 0.50 1217 608 Physical Characteristics (Blaine method), m²/kg layers which were not representative This procedure was espe0.60 1083 650 Setting time (minutes) Specific area 340 366 465 cially important in the case of pastes with high water content - start 176 221 (Blaine method), m²/kg 199 2.2 Methods - end 270 221 266 Setting time (minutes) 2.2 Methods Compressive strength, N/mm2 TheThe permeability of of pastes – start 199 176 221 2.2.1 Gas 2.2.1.permeability Gas permeability permeability pasteswas was de- after days 29.3 25.4 13.7 using the RILEM-Cembureau [18,19], – end 270 221 266 determined termined using the RILEM-Cembureau [18, 19], method method dedi- after 28 days 55.1 56.2 50.7 dedicated concretes,and and expressed the so-called Compressive strength, N/mm2 catedtoto concretes, expressed by thebyso-called permeability coefficient – after days 29.3 25.4 13.7permeability coefficient In order to eliminate microscopic defects from – after 28 days 55.1 56.2 50.7 The coefficient of permeability determinedusing The coefficient of permeability(k) (k)was was determined forming in the specimens because of shrinkage and using following equation: following equation: thermal effects, a decision was made to minimise the In order to eliminate microscopic defects from forming in $%& () volume of specimens Numerous trials showed that these the specimens because of shrinkage and thermal effects, a deci- (1) k = + ' + m$ (1) * & ,&' phenomena can be avoided in the case of cylindrical sion was made to minimise the volume of specimens Numerous samples 10 mm in diameter andphenomena about 60 mm in avoided height in It the case where: trials showed that these can be where: should beof emphasised that, 10 mm in the incase of material cylindrical samples diameter and aboutas60 mm in Q=V/t Q = V/t – the measured flow intensity /s], - the measured gas flowgas intensity [m3/s],[m uniform as cement paste, specimen dimensions ensure that height It should be emphasised that, in the case of material as Pa P – atmospheric pressure [1 bar = 10 Pa], a - atmospheric pressure [1 bar = 10 Pa], the requirements ofcement representativeness fulfilled These uniform as paste, specimenisdimensions ensure that the P - pressure P – pressure (absolute) [Pa] (absolute) [Pa] specimensrequirements were formed in rigid plasticistubes After of representativeness fulfilled Thesethe specimens A - cross-section A – cross-section area of the[m sample ], [m ], area of the sample tube waswere filled with paste, which was compacted by formed in rigid plastic tubes After the tube was filled with η – viscosity of the gas; η = 17,15 [Pa s] of the gas; h = 17,15 [Pa×s] shaking, both ofwas thecompacted tube wereby stopped to the tube η - viscosity paste,ends which shaking,with bothplugs ends of L – thickness of the sample [m] L thickness of the sample [m] prevent water from evaporating shouldwater be emphasised were stopped with plugs toItprevent from evaporating It should of be all emphasised thathigh the and fluidity of it allpossible pastes was high This is because the standard samples used to measure conthat the fluidity pastes was made is because the in standard samples used toinmeasure andthem madeprecisely it possible to compact them the precisely After 28 days, This crete permeability this method are 150 mm diameter The to compact After 28 days, specimens concrete permeability in this method are 150 mmmodified in the specimens the grinded ends wereand dry grinded measurements were taken using an appropriately were demoulded and were the demoulded ends wereanddry diameter The measurements were taken using an andproduce polishedatoheight produce 50 mm for the next device (Fig. 1) allowing measurements of small specimens polished to of a height 50 mm.of Then, forThen, the next device (seedifficulties Figure 1) allowing days,specimens the specimens were stored in in laboratory conditionsappropriately at 10 mm inmodified diameter The greatest were encountered 72 days,72 the were stored laboratory o o measurements of small specimens 10 mm in diameter a temperature of 20 C and relative humidity of 60±5%, after in adapting suitably tight chambers in which specimens of such conditions at a temperature of 20 C and relative humidity o The greatest difficulties were encountered in adapting which the samples were dried at a temperature of 40 C to cona small diameter are fixed These chambers must be absolutely of 60±5%, after which the samples were dried at a o tight chambers in whichwall specimens stantofweight at the relatively temperature C gas-tight where the chamber meets the of sidesuch wall aof the to constant weight low Drying at the of 40suitably temperature 40oC Drying made it possible to reduce othe impact of temperature on small the cylindrical specimen that These is tested.chambers When the cross-sectional diameter are fixed must be relatively low temperature of 40 C made it possible to change in characteristics of the pastes being analysed to absolutely the surface of the specimen and hence the amount of flowing gas, gas-tight where the chamber wall meets the reduce the impact of temperature on the change in greatest possible extent [3] All the tests presented were carried are so small, sealing thespecimen sample inthat the chamber isWhen of key imside wall of the cylindrical is tested characteristics of the pastes being analysed to the greatest out after 90 days of curing and drying in the above conditions portance for obtaining test results.and hence the the cross-sectional surface reliable of the specimen possible extent [3] All the tests presented were carried amount of flowing gas, are so small, sealing the sample in out after 90 days of curing and drying in the above the chamber is of key importance for obtaining reliable conditions Every property of paste analysed in this article 776 Bull Pol Ac.: Tech 64(4) 2016 test results was evaluated on a series of three specimens Unauthenticated in The test procedure was similar to that recommended Irrespective of the type of cement used, the paste Download Date | 2/9/17 4:00 AM [18,19] In essence, the test boils down to measuring the compositions were the same (Table 2) The only variable volume of gas (nitrogen) flowing through the specimen hin a specified time using calibrated tubes (burettes) of This method has been successfully implemented for erent volumes, equipped with a pump that allows an determinations of the density of many materials In the icator in the form of a soap bubble to be created in it tests, a chamber 19.1 mm in diameter was used, the uipping the device with a set of burettes with consolidation force amounted to 38 N and the conversion Open porosity of cement pastes and their gas permeability asurement volumes from to 100 ml ensured real factor was as recommended by the operating manual, i.e ge of measurement of the k coefficient for the 0.2907 cm3/mm [22] True sample densityvolume was determination determined Knowing using the a accurate helium mass of meability for paste specimens described in section 2.1 pycnometer In essence, this instrument measures theprocedure ging from 1x10-16 to 1x10-13 m2 Burettes were selected the sample, envelope density can by established The that single volume measurements of the flowing gas volume of the skeleton of the material tested with an in [22] in detail within a specified time using calibrated tubes (burettes) of was described This method has been successfully implemented for A measurement a for deuld take between 20 andwith 60 s.a The flow accuracy 0.0001 cm has This method differentapproximately volumes, equipped pump that allows an of determinations ofbeen thesuccessfully densitychamber of implemented manywith materials In the of the density many materials the tests, was used in theintests, with In the rated of 10a cm e was measured with an accuracy ±01 bubble s A diagram indicator in the form of aofsoap to be created in volume it terminations tests, chamber 19.1 of mm diameter was used, the a chamber 19.1 mm in diameter was used, the 40% consolidation cement paste specimens filling the chamber to about he deviceEquipping for permeability measurement and a detailed the device with a set of burettes with consolidation force amounted to 38 N and the conversion force amounted to 38 N that and the factor as recas recommended the as method wasconversion followed [23] was In manual, w of specimen fixing arevolumes shown infrom Figure measurement 1.to 100 ml ensured real factorinwas recommended by the operating i.e ommended by 3theQuantachrome operating manual, i.e 0.2907 cm3/mm [22] presented studies the Ultrapycnometer [22] range of measurement of the k coefficient for the 0.2907 cm /mm True density was determined using a helium pycnometer In 1200e2.1 was used True density was determined using a helium permeability for paste specimens described in section essence, this instrument measures the volume the skeleton Mercury pycnometer intrusion porosimetry (MIP) is ainstrument widelyofused In essence, this measures the ranging from 1x10-16 to 1x10-13 m2 Burettes were selected of assessing the materialthe tested with an accuracy of 0.0001 cm method microporosity characteristics of A mea3 tested with an so that single volume measurements of the flowing gas for volume of the skeleton of the material surement Regardless chamber withofa rated volume of 10 cm this was used in cement its 3many drawbacks, Aspecimens measurement with a would take between approximately 20 and 60 s The flowmaterials the accuracy tests, withof the0.0001 cementcm paste filling chamber the chamber method is considered to be of very valuable and to provide a was used in the tests, with rated volume 10 cm time was measured with an accuracy of ±01 s A diagram to about 40% as recommended in the method that was followed the lot of information about structure filling of the material beingto about 40% cement pastethespecimens the chamber of the device for permeability measurement and a detailed [23] In presented studies the Quantachrome Ultrapycnometer tested Because of the broad range of pore identification, as recommended in the method that was followed [23] In 1200e was used view of specimen fixing are shown in Figure this method presented is alsointrusion used to assess the Mercury porosimetry (MIP)microporosity is a widelyUltrapycnometer used method studies the Quantachrome structure of many other materials with a mineral skeleton, for 1200e assessing theused microporosity characteristics of cement matewas including rials advanced composites like High Regardless of its many drawbacks, this method consid-used Mercurycement intrusion porosimetry (MIP) is aiswidely Performance Concrete (HPC), Ultra High Performance Fig. 1 RILEM-Cembureau gas permeability measurement apparatus eredmethod to be very and to a lot of information about of forvaluable assessing theprovide microporosity characteristics Concrete the (UHPC) and Powder Concrete (RPC) and details of specimen fixing in a silicon tube chamber structure of Reactive the material being tested of the broad this cement materials Regardless of itsBecause many drawbacks, [24,25,26] range of pore is also used method is identification, considered tothis be method very valuable and to toassess provide a This the method is also successfully used to assess microporosity structure of many other materials with a min-being lot of information about the structure of the material porosity structure cement The test procedure was similar to that recommendedchanges in [18, in eralthe skeleton, including cement composites like High tested Because of advanced the of broad rangecomposites of pore identification, 19] In essence, the test boils down to measuring the volume Concrete (HPC), Ultracause High Performance Concrete subjected Performance to this the effect of factors which progressive method is also used to assess the microporosity of gas (nitrogen) flowing through the specimen and Reactive Powder Concrete (RPC) [24‒26] material (UHPC) destruction [27,14] Interesting relationship g RILEM-Cembureau gas permeability measurement apparatuswithin a specistructure of many other materials with a mineral skeleton, fiedoftime usingfixing calibrated tubestube (burettes) This method determined is also successfully used tointrusion assess changes in and details specimen in a silicon chamberof different volumes, between poreincluding structure by mercury advancedcement cement composites like High equipped with a pump that allows an indicator in the form of the porosity structure of composites subjected to the porosimetry and mechanical properties of cementitious Performance Concrete (HPC), Ultra Highdestruction Performance Open a soap porosity open pores by with bubbleThe to bepercentage created in it.ofEquipping the device a set effect of factors which cause progressive material composites was presented in [28] Concrete (UHPC) and Reactive Powder Concrete (RPC) buretteswas withdetermined measurement volumes to 100 ml ensured [27, 14] Interesting relationship between pore structure deume in theofpastes using threefrom methods: The principle of mercury intrusion porosimetry [24,25,26] real range of measurement of the k coefficient for the permeability termined by mercury intrusion and mechanical bulk density helium porosity (pH), comparing measurement isThis based on the fact,porosimetry that volume of to assess ‒16 method is composites also successfully used for paste specimens described in section 2.1 ranging from 1£10 properties of cementitious was presented [28] with true density; introduced mercury into material is directly related to incomposites changes in the porosity intrusion structureporosimetry of cement to 1£10‒13 m2 Burettes were selected so that single volume meaThe principle of mercury measureapplied pressure [1] Tests were ofconducted usingcause the progressive porosity determined on the basis of mercury subjected factors of which surements of the flowing gas would take between approximately ment is based to on the the effect fact, that volume introduced mercury Quantachrome Poremaster 60 mercury porosimeter with a relationship intrusion porosimetry measurements (pMIP); Interesting 20Fig and s The flow timegas was measured with an accuracy of intomaterial material is destruction directly related[27,14] to applied [1] Tests were 60 RILEM-Cembureau permeability measurement apparatus pressure Tests were pressure range from 0.1 to 400 N/mm porosity the basis of and details of ofon specimen fixing in amass silicon water tube chamber and §01 s.determined A diagram the device for permeability measurement conducted using the structure Quantachrome Poremaster mercury porobetween pore determined by60mercury intrusion using a Quantachrome Poremaster 60 N/  mercury ) are shown in Fig. 1 conductedsimeter saturation measurements (pWSfixing a detailed view of specimen with a pressure from 0.1 to 400 mmof 2Tests were porosimetry and range mechanical properties cementitious poroporosimeter with a pressure range fromin0.1 to 400 N/mm Open(pporosity The percentage of open pores by conducted Helium 2.2.2 porosity using was a Quantachrome Poremaster 60 mercury composites presented [28] H) was calculated using the of This range of pressure applied aided the identification volume in the pastes was determined using three methods: 2.2.2 Open porosity The percentage of open pores by volume simeterThe with a pressure 0.1 to 400intrusion N/ mm This range owing relationship: principlerange of from mercury porosimetry accessible toapplied mercury and of on diameters ranging in the- pastes was determined methods:bulk pores of pressure aided the identification of pores accessible to of comparing density helium porosity using (pH), three measurement is based the fact, that volume 23456 from nm to ca 0.25 mm This pressure is given by porosity (p comparing bulk density(2) with true den-3.75mercury and of diameters ranging from 3.75 nm to ca 0.25 mm −with 100 p/– =helium H),% vol true density; introduced mercury into material is directly related to the Washburn equation seenby[1] below: sity; 27849 Thisapplied pressure isas given the Washburn equation as seen below: pressure Tests were conducted using the porosity determined on the basis of mercury – porosity determined on the basis of mercury intrusion poere: ρbulk – bulk density [g/cm ], Quantachrome Poremaster 60 mercury porosimeter with a ,A B(DEF G) ); intrusion porosimetry measurements (p MIP dfrom = 0.1 to (3) (3) rosimetry measurements (pMIP); density (helium pycnometry) [g/cm ] ρtrue – true Tests were pressure range 400 N/mm I porosity determined on the basiswater of mass water – porosity determined on the basis of mass saturation conducted using a Quantachrome Poremaster 60 mercury saturation measurements (pWS) measurements (pWS ) – pore diameter [nm]; where: dwhere: – porosimeter pore ddiameter [nm]; The bulk density of porosity samples was determined by a using with a pressure range from 0.1 to 400 N/mm2 ) was calculated the g – surface Helium (p H calculated Helium porosity (pH) was using the following γ – tension surface tension of the mercury, 0.480 N/m; of the mercury, 0.480 N/m; wder pycnometry method, using the Micrometrics This ϕrange ofbetween pressure applied aided the identification following relationship:relationship: – angle the mercury and the pore wall, 130o; of f – pores angle between the to mercury and and the pore wall, oPyc 1360 This method is based in on displacement accessible mercury of diameters ranging – pressure [N/mm ] 130o;p3.75 23456 ory which an enables sample volume determination from nm to ca 0.25 mm This pressure is given by 100 % vol (2) (2) p/ = − 27849 sample, envelope ] p – the pressure [N/mm owing the accurate mass of the Washburn equation as seen below: It was assumed that the volume of pores accessible to water 3 described sity can by established Thedensity procedure was where: ρρbulk – bulk [g/cm ], (p ) can be treated as identical to bulk water saturation It WS where: – bulk density [g/cm ], bulk ,A B(DEF G) to It3 waswas assumed thatcalculated the volume of basis pores 22] in detail d =of accessible ρρtrue – true density (helium pycnometry) [g/cm ] therefore on the measured mass water (3) – true density (helium pycnometry) [g/cm ] I true ) can be treated as identical to bulk water water (pWS absorption (wa) and the bulk density of paste (ρbulk) using the The bulk density of samples was determined by a powder where:relationship d – pore diameter [nm]; The bulk density of samples was determined by a following pycnometry method, using the Micrometrics GeoPyc 1360 This g – surface tension of the mercury, 0.480 N/m; powder pycnometry method, using the Micrometrics method is based in on displacement theory which an enables pWSbetween  = wa £ρthe (4) bulkmercury f – angle and the pore wall, GeoPyc 1360 This method is based in on displacement o 130 ; theory which an enables sample volume determination p – pressure [N/mm2] Knowing the accurate mass of the sample, envelope Bull Pol Ac.: Tech 64(4) 2016 777 density can by established The procedure was3 described Unauthenticated It was assumed that the volume of pores accessible to in [22] in detail Download Date | 2/9/17 4:00 AM water (pWS) can be treated as identical to bulk water T Tracz Test results The results obtained for the paste properties tested are presented in Table These are averages from three measurements The test results obtained were characterized by high homogeneity, and the maximum noted variation coefficients did not exceed 5% The exception was the permeability test, where this factor was about twice as big, which is natural homogeneity of this feature Table Results of tests conducted Properties Water Coefficient Cement w/c True Helium MIP saturation of Bulk type ratio density density porosity porosity porosity permeability ρtrue pH pMIP pWS k ρbulk [g/cm3] [g/cm3] [% vol.] [% vol.] [% vol.] [10-16 m2] CEM I 42.5 R 0.30 0.40 0.50 0.60 1.744 1.628 1.495 1.398 2.308 2.250 2.165 2.116 24.4 27.6 30.9 33.9 17.3 20.5 23.5 26.5 31.4 36.5 41.4 44.5 2.73 3.95 10.50 24.70 CEM II/A-V 42.5 R 0.30 0.40 0.50 0.60 1.785 1.614 1.466 1.342 2.232 2.141 2.084 2.031 20.0 24.6 29.6 33.9 11.3 19.5 24.7 28.4 33.2 39.2 43.0 47.4 0.99 3.50 5.39 8.83 CEM III/A 42.5 N 0.30 0.40 0.50 0.60 1.727 1.586 1.391 1.346 2.214 2.098 2.071 2.017 22.0 27.4 32.8 33.4 13.3 19.5 26.4 30.4 30.0 35.5 39.2 45.0 1.70 2.79 3.57 6.63 Discussion 4.1 Open porosity, the w/c ratio and cement type The relationship between open porosity assessed by the three methods described above and w/c ratio is presented in Fig. 2 It is clearly visible that open porosity, regardless of the method by which it is determined, strongly depends on the water/cement ratio of the cement pastes tested As the w/c ratio increases, the open porosity of pastes obviously goes up A change of the w/c ratio from 0.3 to 0.6 was accompanied by as much as a two-fold increase of open porosity (e.g for a paste with CEM II, pMIP increased from 11.3 to 28.4% vol and for a paste of the CEM III cement, pMIP rose from 13.3 to 30.4 % vol.) It is also visible that this relationship is quasi-linear in nature The values of open porosity determined by the three methods are clearly different In every case, the highest values are achieved by porosity determined based on water saturation (pWS), and the lowest by that determined using mercury intrusion porosimetry (pMIP) A similar difference in the open porosity assessed by the three methods used was found by the authors of [5] On the one hand, such a high difference in open porosity assessed by mercury intrusion porosimetry and based on absolute water saturation can be explained by the fact that the cement paste contains pores smaller than 3.75 nm and greater than 0.25 mm, which cannot be identified by mercury intrusion porosimetry [2] On the other hand, water saturation porosity (pWS) did not 778 Fig. 2 The relationship between the helium porosity (pH), MIP porosity (pMIP), porosity determined based on water saturation (pWS) and the w/c ratio [29] reflect the real open porosity of the material because water, as a strongly polar liquid, is adsorbed by the cement gel as so-called interlayer water Its molecules slip in between the mineral layers of the C-S-H phase, thus increasing the distances between them and forming “additional porosity” This phenomenon is also the reason why cured cement materials swell when stored in water The results presented indicate that the type of cement has a significant impact on the open porosity of cement pastes analysed The relative difference in porosity was found to be caused by the type of cement used, regardless of the method by which this porosity was assessed, and was the higher the lower the Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM Open porosity of cement pastes and their gas permeability starts The critical diameter is the diameter of pores at which the distribution curve reaches the maximum, so this parameter demonstrates what diameters are the most frequent, and therefore dominant, in the structure of the material tested (see Fig. 4) In their publications, many researchers [1, 30, 31] have proven that a comparison of these two values is very useful when studying and analysing cement pastes with different w/c ratios Fig. 5 shows the dV/dlogD distribution curves for all pastes, grouped according to cement type Fig. 6 presents cumulative distributions Fig. 3 A summary table of the relationship between the helium porosity (pH), MIP porosity (pMIP), porosity determined based on water saturation (pWS) and the w/c ratio for all cements Fig. 4 Definition of critical and threshold pore radius [30] water/cement ratio of the paste tested In other words, along with decreasing water/cement ratio the greater impact of cement type on porosity was observed The greatest difference was observed in pastes with a w/c of 0.3 and porosity assessed by mercury intrusion porosimetry of 11.3 and 17.3% vol (pMIP = 6% vol.) for pastes with CEM II and CEM I, respectively In the remaining cases, the absolute difference between the porosity caused by the type of cement amounted to about 3.2% vol The results presented above, and their analysis, are based on assessing the total open porosity determined by various methods Mercury intrusion porosimetry was also used to determine pore distribution curves and two parameters characterising them: critical pore size, and threshold pore size The threshold pore size is defined as the diameter of pores at which significant filling of the system of open pores with mercury in the tested material Fig. 5 Differential pore size distribution of cement paste with different types of cement 779 Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM T Tracz capillary pores (d > 50 nm), we see it falling along with the w/c The intensity of occurrence of pores of this size also decreases Similar observations concerning both cement pastes and mortars were presented by the authors of the publications [1, 31‒33] Further analysis of the resultant distribution of the structure of pores identified by mercury intrusion porosimetry consisted in dividing the entire range of pores identified by the MIP into three classes [30, 34]: meso-pores (100 nm) The results of this analysis for the pastes made with the different cements tested are shown in Fig. 7 Fig. 6 Differential cumulative pore size distribution of cement paste at different type of cement The dV/dlogD distribution curves show a clear trend, particularly for cement pastes made with CEM II and CEM III cements where, as the w/c ratio decreases, the threshold pore diameter also falls within the range from 700 to 2000 nm Thus, as the w/c falls, not only does the total porosity decrease but also the dominant pores shift towards smaller diameters The research presented indicates that the dV/dlogD distribution may be bimodal in character If we compare the critical pore size within the range of 780 Fig. 7 Pore volume distribution of cement paste with pore classification Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM Open porosity of cement pastes and their gas permeability The analysis presented above confirms earlier observations that an increase of the w/c ratio causes a greater capillary porosity in all the cement pastes analysed For example, in pastes made with blast furnace cement at a w/c of 0.3, capillary pores accounted for 18%, while in a paste with the w/c of 0.6, this amount increased to 64% Obviously, as the number of capillary pores increases, the number of meso-pores, namely pores with a diameter below 50 nm, falls 4.2 Permeability, w/c ratio and open porosity Fig. 8 shows the relationship between the permeability coefficient of pastes made of different cements and the w/c ratio The type of cement can be said to have a significant impact on this relationship Just like the total open porosity and the share of capillary pores, permeability also increases along with the w/c [35] For pastes made with particular cements, this relationship can be described sufficiently accurately by the exponential regression equations shown in the figures (R2 > 0,9) Fig. 8 Coefficient of permeability vs w/c ratio [29] The greatest impact of the w/c ratio on permeability is visible in the case of pastes made with CEM I cement For pastes made with CEM II and CEM III, this impact is much weaker In addition, these pastes feature lower gas permeability The permeability coefficient of pastes made with CEM I and CEM II cements increases nine-fold as a result of the w/c rising from 0.30 to 0.60 In contrast, for pastes made with CEM III, the permeability increases only about four times Next, the dependencies of the permeability coefficient on the open porosity measured by helium pycnometry, mercury intrusion porosimetry and water saturation were analysed These relationships are illustrated by Figs 9‒11 and the regression equations presented there In all cases, a good correlation between permeability and open porosity determined by various methods can be found However, it should be noted that it is impossible to formulate a general dependency of cement paste permeability on its open porosity It turned out that pastes made with different types of cement had to be analysed separately Fig. 9 Coefficient of permeability of cement paste vs helium porosity [29] Fig. 10 Coefficient of permeability of cement paste vs MIP porosity [29] Fig. 11 Coefficient of permeability of cement paste vs water saturation porosity [29] 781 Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM T Tracz Diversification of analysed binders which consists in lack or presence of additives with pozzolanic or latent hydraulic properties brings about significant changes in the microstructure of hardened cement pastes Cement CEM II which contains high amount of reactive silica (fly ash) by pozzolanic reaction consume portlandite to produce additional amount of C-S-H phase In the case of pastes made with CEM III the C-S-H phase is one hand more compacted and less porous, but on the other hand due to age of tested specimens one can expect lower hydration degree of cement The test results obtained indicate that gas permeability of cement pastes is influenced not only by total open porosity but also on pores size distribution In other words, value of gas permeability dependance on both amount of open pores and pores diameter This could be clearly seen when cement pastes made with CEM I and CEM III, characterized by water/cement ratio 0.6 were compared Helium porosity is very similar and equals about 33%, but gas permeability is differentiated almost four times (see Table 3) Explanation to this phenomenon is volume fraction of capillary pores (> 50 nm), where in case of CEM I it equals 83% and CEM III it is 69%, related to total porosity It is difficult to clearly identify which of the presented methods for assessing the porosity is the most reliable and closest to the actual values These methods identifies different ranges of pore size Porosity measured by water saturation not reflect the actual values of open porosity of the material, which is associated with phenomenon described in Chapter 4.1 i.e with creation of “additional porosity” Helium porosity allows to measure widest range of pores in the material structure According to [23] very little atoms of helium can penetrate pores up to 0.25 nm in diameter Nevertheless, as one can see from presented analysis the dependency of the permeability on open porosity can be described by exponential regression equations (almost in all cases R2 > 0.9) Concluding remarks The research results presented and analyses completed support the following conclusions about the relationship between the open pore content and the composition of the paste and the impact of open porosity on permeability The value of the experimentally assessed content of open pores in cement paste clearly depends on the method used to measure it The highest values are produced when open porosity is treated as equivalent to the volumetric water saturation However, because water additionally slips in between the layers of the C-S-H phases and leads to the swelling of the material, the above value does not represent the real proportion of open pores to volume of the material, but a higher value The lowest values of open porosity are produced by mercury intrusion porosimetry The reason can be ascribed to the fact that this method does not identify pores that are smaller than 3.75 nm and greater than 0.25 mm However, this method does provide valuable information about pore size distribution An analysis of the results produced by it has shown that as the 782 w/c ratio increases, so does the share of capillary pores greater than 100 nm in diameter while the share of meso-pores, i.e ones smaller than 50 nm, decreases In addition, a general trend for the threshold pore diameter to decrease along with falling w/c ratio was observed A similar nature of changes applies to critical pore size and the intensity of occurrence of pores of this size, belonging to the range of capillary pores (> 50 nm) The dependency of the open pore content of cement pastes, determined by various methods, on the w/c ratio is quasi-linear The gas permeability of the paste depends on the type of cement, the w/c ratio and the associated open porosity (helium, MIP and water saturation porosity) The gas permeability of pastes can be estimated based on the value of the w/c ratio or their open porosity using the exponential regression functions presented above Presented above information about the open porosity of cement pastes made with the three most popular types of common cements can be useful for designing the composition of cement concretes which are required to offer the appropriate permeability References [1] R.A Cook and K.C Hover, “Mercury porosimetry of hardened cement pastes”, Cem Concr Res 29, 933–943 (1999) [2] S Diamond, “Mercury porosimetry An inappropriate method for the measurement of pore size distributions in cement-based materials”, Cem Concr Res 30, 1517–1525 (2000) [3] C Gallé, “Effect of drying on cement-based materials pore structure as identified by mercury intrusion porosimetry”, Cem Concr Res 31, 1467–1477 (2001) [4] G Hedenblad, “The use of mercury intrusion porosimetry or helium porosity to predict the moisture transport properties of hardened cement paste”, Adv Cem Based Mater 6, 123–129 (1997) [5] M Krus, K.K Hansen, and H.M Künzel, “Porosity and liquid absorption of cement paste”, Mater Struct 30, 394–398 (1997) [6] B.K Nyame and J.M Illston, “Capillary pore structure and permeability of hardened cement paste”, in 7th Int Congr Chem Cem Vol 3, p VI-181‒185, (1980) [7] D Winslow and D Liu, “The pore structure of paste in concrete”, Cem Concr Res 20, 227–235 (1990) [8] L Czarnecki and P Woyciechowski, “Modelling of concrete carbonation ; is it a process unlimited in time and restricted in space?”, Bull Pol Ac.: Tech 63, (2015) [9] C.L Zhang and T Rothfuchs, “Damage and sealing of clay rocks detected by measurements of gas permeability”, Phys Chem Earth 33, (2008) [10] J.C Stormont, “Conduct and interpretation of gas permeability measurements in rock salt”, Int J Rock Mech Min Sci Geomech Abstr 34, 648 (1997) [11] S Takeuchi, S Nakashima, and A Tomiya, “Permeability measurements of natural and experimental volcanic materials with a simple permeameter: Toward an understanding of magmatic degassing processes”, J Volcanol Geotherm Res 177, 329–339 (2008) [12] A.M Neville, “Properties of concrete”, Pearson Education Limited, London, (2011) [13] J Baron and J.P Ollivier, “Durablité des bétons”, Collect l’Association Tech l’Industrie Des Liants Hydraul Press I’ENPC (1992) Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM Open porosity of cement pastes and their gas permeability [14] W Kurdowski, “Cement and concrete chemistry”, Springer Netherlands (2014) [15] T Tracz and J Śliwiński, “Effect of cement paste content and w/c ratio on concrete water absorption”, Cem Lime Concr 3, 131‒137 (2012) [16] V Picandet, D Rangeard, A Perrot and T Lecompte, “Permeability measurement of fresh cement paste”, Cem Concr Res 41, 330–338 (2011) [17] A Pierre, A Perrot, V Picandet and Y Guevel, “Cellulose ethers and cement paste permeability”, Cem Concr Res 72, 117–127 (2015) [18] J.J Kollek, “The determination of the permeability of concrete to oxygen by the Cembureau method—a recommendation”, Mater Struct 22, 225–230 (1989) [19] RILEM Technical Recommendation, ”Permeability of concrete as a criterion of its durability”, Mater Struct 32, 174–179 (1999) [20] CEN, “EN 197‒1 Composition, specification and conformity criteria for common cements”, Brussels (2012) [21] CEN, “EN 196‒1, Methods of testing cement – Part 1: Determination of strength”, Brussels (2006) [22] Micrometrics Instrument Corporation, “GeoPyc 1360 operator’s manual”, (2001) [23] Quantachrome Instruments, “Ultrapycnometer 1000 operator’s manual”, (2007) [24] J Śliwiński and T Tracz, “Sorptivity of normal and high performance concrete”, Cem Wapno, Bet 1, 27‒33 (2007) [25] T Tracz and J Śliwiński, “Influence of type of cement on porosity and permeability of high performance concrete”, in Proc 7th CANMET/ACI Int Conf Durab Concr., Montreal, Canada (2006) [26] T Zdeb, “Ultra-high performance concrete – properties and technology”, Bull Pol Ac.: Tech 61, 183–193 (2013) [27] I Hager, “Behaviour of cement concrete at high temperature”, Bull Pol Ac.: Tech 61, 1–10 (2013) [28] X Chen, L Xu, and S Wu, “Influence of pore structure on mechanical behavior of concrete under high strain rates”, J Mater Civ Eng 28, 04015110 (2016) [29] T Tracz and J Śliwiński, “Influence of cement type and water-cement ratio on open porosity and gas permeability of cement pastesle”, in UKIERI Concr Congr Innov Concr Constr., Jalandhar, India, 461–470 (2013) [30] X Chen, S Wu and J Zhou, “Experimental study and analytical model for pore structure of hydrated cement paste”, Appl Clay Sci 101, 159–167 (2014) [31] H.N Atahan, O.N Oktar and M.A Tasdemir, “Effects of water-cement ratio and curing time on the critical pore width of hardened cement paste”, Constr Build Mater 23, 1196–1200 (2009) [32] X Chen, S Wu and J Zhou, “Experimental study and analytical model for pore structure of hydrated cement paste”, Appl Clay Sci 101, 159–167 (2014) [33] X Chen and S Wu, “Influence of water-to-cement ratio and curing period on pore structure of cement mortar”, Constr Build Mater 38, 804–812 (2013) [34] Q Zeng, K Li, T Fen-Chong and P Dangla, “Pore structure characterization of cement pastes blended with high-volume fly-ash”, Cem Concr Res 42, 194–204 (2012) [35] S Care and F Derkx, “Determination of relevant parameters influencing gas permeability of mortars”, Constr Build Mater 25, 1248–1256 (2011) 783 Bull Pol Ac.: Tech 64(4) 2016 Unauthenticated Download Date | 2/9/17 4:00 AM ... 4:00 AM Open porosity of cement pastes and their gas permeability [14] W Kurdowski, ? ?Cement and concrete chemistry”, Springer Netherlands (2014) [15] T Tracz and J Śliwiński, “Effect of cement. .. dependency of cement paste permeability on its open porosity It turned out that pastes made with different types of cement had to be analysed separately Fig. 9 Coefficient of permeability of cement. .. 38 N and the conversion Open porosity of cement pastes and their gas permeability asurement volumes from to 100 ml ensured real factor was as recommended by the operating manual, i.e ge of measurement