PROPERTIES OF PORE STRUCTURE MEASURED BY STEP-BY-STEP MERCURY INTRUSION POROSIMETRY TEST

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1 Cementitious Composites, April 212, Amsterdam, The Netherlands PROPERTIES OF PORE STRUCTURE MEASURED BY STEP-BY-STEP MERCURY INTRUSION POROSIMETRY TEST Ryo YOSHIDA (1) and Toshiharu KISHI (2) (1) Depts. of Civil Engineering, Nagoya Institute of Technology, Japan (2) Institute of Industrial Science, University of Tokyo, Japan Abstract A unique Mercury intrusion porosimetry (MIP) method that can be used to determine volumes of continuative pore and discontinuative pore structure was proposed by authors. Using this method, a suitable maximum intrusion pressure can be selected. In this study, pore structure properties of hardened cement paste made under several different conditions, such as water to cement ratio, curing condition, age, carbonation, and binder type, were analyzed using the proposed method. It was found Blast furnace slag (BFS) paste cured in water, even 28-days-aged specimen, formed narrow threshold diameter whose size is less than 1 nm and stiff frame structure indestructible by the excessive high pressure intrusion. It was also found that total porosity of BFS paste is similar to that of Ordinary Portland cement (OPC) paste. However, BFS paste consisted of less continuative pores and more Ink-bottle pores than those of OPC paste. For OPC paste with water to cement ratio of.6, total porosity was decreased. Remarkable decrease of continuative pore with diameter of 2-3 nm, and increase of Inkbottle pore whose threshold diameter is less than 2 nm were observed as pore properties of carbonated OPC paste. 1. INTRODUCTION Hardened cementitious materials consist of various kinds of pores, like gel pores, capillary pores, air voids and so on, whose shape and size distribution is often called pore structure. However, ink-bottle relations are most characteristic pore structure of cementitious materials and govern drying shrinkage or ion absorption. Mercury intrusion porosimetry (MIP) test is one of the most widely used method to study the pore structure of cementitious materials. However, it is well known that the evaluation of pore structure by MIP is not totally accurate. The main reasons are the ink-bottle effect and the destruction of pores due to excessive high pressure intrusion. To solve these problems, a unique method that can be used to determine volumes of continuative pore and

2 Cementitious Composites, April 212, Amsterdam, The Netherlands discontinuative pore structure was proposed. Based on this method, a suitable maximum intrusion pressure can be selected. In this study, pore structure properties of hardened cement paste made under several different conditions, such as water to cement ratio, curing condition, age, carbonation, and binder type, were analyzed using this uniqe MIP method. 2. OUTLINE OF THE EXPERIMENT 2.1 Preparation of specimen Ordinary Portland cement (OPC) and Blast furnace slag (BFS) were used to make OPC and BFS cement pastes with water-to-binder (w/b) ratios of.45 and.6. BFS cement is a mixture of OPC (55%) and BFS (45%). These cement pastes were cast into a molds, the size of which were mm, and sealed with plastic bags, cured in water or air-exposed in a chamber for 14, 28 days or 22 months. Temperature and relative humidity in the chamber were maintained at 2 and 6%, respectively, and density of carbon dioxide was less than 1%. After curing, specimens were crushed into cubes of approximately 5 mm 3 by a chisel and a hammer. Finally, samples were soaked in acetone for 24 hours in order to stop cement hydration. After that, samples were dried in the D-dry machine for another 24 hours, before MIP tests. 2.2 Experimental program In this study, MIP measurements were conducted using AutoPore3 (Micromeritics Corporation). In measurement, equilibrium time was 1 seconds at each measuring points. Pore diameter was calculated with Washburn equation, θ = 13 and γ = 484 dyn/cm. Specimens were subjected to the intrusion steps of the unique MIP test. The intrusion steps shown in Table 1 were set based on the result of typical MIP measurement. 2.3 Determination of continuative pores and ink-bottle pores, Selection of suitable maximum intrusion pressure [1] A result of the unique MIP is shown in Fig.1. In the figure, dotted lines shows mercury intrusion process, and thinner lines show mercury extrusion process. Gaps caused by inkbottle effect between intrusion curves at.3 MPa can be seen in Fig.1. Table 1: Intrusion steps and pore diameter (S45: Seal-cured, w/c=.45) Intrusion steps Applied pressure [MPa] () Intrusion Steps Applied pressure [MPa] () 1 (32) 5(3) 6.3(4) 126(1) 2.3(4) 13(1) 7.3(4) 185(7) 3.3(4) 19(7) 8.3(4) 268(5) 4.3(4) 58(22) 9.3(4) 412(3) 5.3(4) 65(2) 1.3(4) 412(3)

3 Cementitious Composites, April 212, Amsterdam, The Netherlands.32 1 Extrusion S45 (Seal-cured, w/c=.45) Intrusion Trapped Hg in specimen (4).3MPa Envelope 5.4 Continuative capillary 3 2 pores Type Ink-bottle bet. Capillary pores & Air voids 1 S Envelope Type2 S Figure 1 : Whole process of MIP result measured by the original method[1] Figure 2 : Determination of continuative pores and inkbottle pores separately [1] Figure 3 : Selection of a suitable maximum intrusion pressure [1] Intrusion curves are rearranged into Fig.2 and Fig.3. In Fig.2, intrusion curve No.3-6 overlaps each other. This fit of intrusion curves made an envelope curve as shown by the thick line in Fig.2. The envelope shows similar mechanisms of mercury movement, indicating that mercury passes through the same series of continuative capillary pores from No.1-No.6, and the increase of the slope indicates that the mercury intrudes into the ink-bottle pores, like air voids or discontinuative capillary pores, and the increase of pore volume corresponds to the gap between intrusion curves shown in Fig.2. At high pressure levels, as shown in curves No.7-1, the threshold diameter became larger than the preceding one. Also, their slopes around 1 nm increased. This separation from the envelope indicates that mercury intrusion damage pores at excessive pressure. As can be seen in Fig.3, before intrusion step No.6, intrusion curves follow the envelope. During and after intrusion No.7, the separation was observed. Based on comparison of intrusion curves, it was supposed that intrusion step No.6 caused damage to the specimen. The pressure range at which damage of pores started was between maximum pressure of No.5 andno.6 ( MPa). Thus, by comparing the intrusion curves, a suitable maximum intrusion pressure can be selected. 3. DIFFERENCE OF PORE STRUCTURE BETWEEN OPC AND BFS CEMENT PASTE 3.1 Pore volume, threshold diameter, ink-bottle pore distribution Results of OPC and BFS paste cured in different conditions for 28 days are shown in Fig. 4. Upper graphs are results of OPC (O) and below graphs are those of BFS (B). And these graphs are lined in the follow order of curing condition: Left is water-cured (W), center is sealed-cured (S) and right is air-exposed (A) specimen. The differences of OPC and BFS pore structure were examined every curing condition. First, by comparing water cured specimens, it was found that threshold diameter of OW was about 3 nm, and BW formed narrow threshold diameter smaller than 1 nm. It can be said that the ink-bottle pore occupies most of tolal pore volume of BW. These ink-bottle pores are

4 Cementitious Composites, April 212, Amsterdam, The Netherlands OW45 OS45 OA ⅰ.1.1 ⅰ.1 ⅰ ⅲ ⅲ BW45 BS45 BA ⅰ.1.1 ⅰ ⅰ.1 ⅲ ⅲ Figure 4: Whole process of MIP result measured by the original method (Upper graphs are OPC paste, lower graphs are BFS paste with w/b =.45, cured in different conditions for 28 days. W : cured in water, S : sealed, A : Air-exposured).2.3 BW45 BS45 BA45 CH BS45-DG C-S-H BS45 15 mm Light gray (BS45-LG) Dark green (BS45-DG) BS45-LG 3 mm Figure 5: color of specimen (BFS paste with w/b =.45) θ [degree Cu-Kα] Figure 6: XRD chart of BS45-dark green and -light gray Cumlative pore volume [ml/ml] BS45-DG (Dark green) BS45 (Light gray) (BW45+BA45) Figure 7: Difference of BS45 between dark green part and light green part in MIP result

5 Cementitious Composites, April 212, Amsterdam, The Netherlands classified into three pore diameter range, (ⅰ) under 1 nm, ()1-1 nm and (ⅲ)over 1 nm. For BW, most of the ink-bottle pores are classified into (ⅰ) under 1 nm. Second, by comparing air-exposed specimens,oa and BA, it can be seen that their curves are almost same, and their threshold diameter is about 1 nm. Order of this threshold diameter is correspond to the distance between cement particles in cement paste. In addition, both specimens contain a large volume of pores lager than 1 nm. These threshold diameter and pore volume are the characteristics of air-exposed specimens. Third, by comparing sealed cured specimens, OS and BS, it was found that threshold diameters of both specimens were about 1 nm, and their curves have different inflection points. The inflection point of BS is about 1 nm, and that of OS is about 3 nm. The threshold diameter of sealed cured specimen similar to that of air-exposed specimen, and the inflection point of sealed cured specimen similar to the threshold diameter of water-cured specimen. The characteristics of water-cured and air-exposed specimens are reflected in those of sealed cured specimen. The characteristics of sealed cured specimen were examined by observation of the specimen s color as shown in Fig.5. For BFS cement paste, water cured specimen turned to dark green, air-exposed specimen did not change the color, light grey like OPC paste, and sealed cured specimen colors dark green and light gray. The difference of color is made by the state of ferrun or manganse oxidation which depends on existence of water in the specimen. The sealed cured specimen was broken into two specimens, one was dark green colored and the other was light gray colored specimen. Then, these specimens were examined by XRD and MIP testing. The XRD chart is shown in Fig.6. One can see that there is no difference between those colored specimens in XRD chart. The difference of color seems to depend on a small amount of ferrous oxide. The result of the unique MIP is shown in Fig.7. For dark green specimen, there was a small threshold diameter and large amount of ink-bottle pore which are the characteristics of pore structure for water cured specimen. For light gray specimen, there was a large threshold diameter and large amount of over-1 nm pores. This is the characteristics of pore structure for air-exposed specimen. These characteristics of water and air-exposed cured seems to be distributed in sealed cured cement paste. 3.2 Existence of BFS stiff frame structure indestructible by excessive high pressure intrusion Comparison of intrusion curves are shown in Fig. 8. Upper graphs are OPC and lower graphs are BFS. In these graphs, there are the envelope and separation of intrusion curves. As stated above, the envelope means that mercury intrudes into continuative pores without destruction of structure, and the separation means that mercury intrusion destructs pore structure when intrusion pressure is excessive. In this destruction, smaller pores un-intruded with mercury seems to be collapsed and lager pore structure intruded with mercury was enlarged. The maximum pressure of intrusion process which was caused the destruction is shown next tomark in graphs. For BW, there is little destruction, though it was pressured with 412 MPa. Comparing the maximum pressure between OPC to BFS on each curing condition, the maximum pressure of BFS is higher than that of OPC. Every intrusion curves branch out from the envelope. Before intrusion curves separate from the envelope, the volume of the branches in Fig.5 correspond to that of the ink-bottle

6 Cementitious Composites, April 212, Amsterdam, The Netherlands ⅰ ⅲ OW45 ⅰ ⅲ OS45 ⅰ ⅲ OA (185 MPa) separation 6(126 MPa) branch.1.1 7(185 MPa).1 envelope ⅰ ⅲ BW45 ⅰ ⅲ BS45 ⅰ ⅲ BA (412 MPa) (185 MPa).1 8(268 MPa) Figure 8: Comparison of intrusion curves for determination of continuative pore and ink-bottle pore and selection of suitable maximum intrusion pressure (Upper graphs are OPC paste, lower graphs are BFS paste with w/b =.45, cured in different condition for 28 days. W : cured in water, S : sealed, A : Air-exposured) pores in Fig.4. After the separation, the volume of the branches is larger than that of the inkbottle pores. It is indicated that specimen was compressed. However, the intrusion curves of BW don t separate from the envelope, and the branches correspond to the ink-bottle pores. It indicates that BW may have stiff frame structure indestructible by excessive high pressure intrusion. Considering the small threshold diameter of BW, productions of latent hydraulic property seems to precipitate on the surface of capillary pore wall, and make the frame structure thicker and stiffer. 4. PORE STRUCTURE OF OPC PASTE CARBONATED BY LONG TERM AIR- EXPOSURE 4.1 Pore volume, threshold diameter measured with typical MIP testing Air-exposed OPC pastes were examined with XRD. Small peak of calcite and large peak of portlandite were detected in 14- and 28-days-aged specimens. Large peak of calcite was detected and portlandite was not detected in 22-months-aged specimen. In this study, 14- and 28-days-aged specimens are regarded as non-carbonated and 22 months specimen is regarded as carbonated. Results of OPC paste with water to cement ratios of.6 with the unique MIP are shown in Fig.6. Thick lines show parts of the unique MIP which correspond to typical MIP test result.there is little difference between 14- and 28-day curves. The total pore volume and threshold diameter decrease from 28 days to 22 months. Particularly between 2 and 3 nm, pore volume decrease remarkably.

7 Cementitious Composites, April 212, Amsterdam, The Netherlands days.4 28 days.4 22 months ().6 (ⅰ) ().5 (ⅰ) Total pore Connective pore.5.2 Ink-bottle pore (ⅰ) : 2-3nm () : 1-2 nm.3 ().3 (ⅰ) Figure 9: Decrease of pore volume for carbonated OPC paste (w/c =.6, air-exposed curing) separation envelope branch 14 days days 22 months Figure 1: Comparison of intrusion curves for carbonated OPC paste (w/c =.6, airexposed curing) 4.2 Continuative pore, ink-bottle pore distribution measured with original MIP method Comparison of intrusion curves are shown in Fig.1. As stated above, the envelope (continuative pore), branches (ink-bottle pore) and separation (destruction) are obtained in this figure. The continuative pore curve in Fig.1 is shown in Fig.9 with a broken line. Comparing the curves of 28 days to 22 months in Fig.9, threshold diameter and volume of 2-3 nm pores decrease. Ink-bottle pore distribution also changed by carbonation. Comparing 22 months to 28 days, ink-bottle pore of 2-3 nm decrease, and ink-bottle pore of 1-2 nm increase. This decrease of ink-bottle pore is.2 ml/ml, and this increase of ink-bottle pore is also.2 ml/ml. The decrease of continuative pore is.7 ml/ml that is about 7% of the total pore volume decrease. These indicate that carbonation for OPC paste result in the decrease of 2-3 nm continuative pores and increse of under-2 nm ink-bottle pores. 4.3 Stiff frame structure formed by drying or carbonation The intrusion process that caused the destruction is shown in Fig.1. For 14 and 28 days case, No.7 intrusion process caused the destruction. For 22 months case, No.9 intrusion process caused the destruction. The maximum pressure of No.7 intrusion process is 185 MPa, and the maximum pressure of No.9 intrusion pressure is 268 MPa. Over these pressures, the intrusion curves separate from the envelope. This separation indicates the degree of the

8 Cementitious Composites, April 212, Amsterdam, The Netherlands destruction. There is large separation in the intrusion curves of 14 and 28 days, and there is small separation in the intrusion curve of 22 months. Air-exposed specimen cured for 22 months seems to make frame structure stiffer. When C-S-H gels get dried out, C-S-H gels condensed and polymerized[2], and the water dehydrated with drying or carbonation of C-S-H gels, makes un-hydrated cement re-hydrate. This polycondensation and re-hydration seem to make the frame structure stiffer. 5. SUMMARY In this study, pore structure of OPC and BFS paste were analyzed with the unique MIP method. BFS cement paste cured in water has smaller volume of continuative pore and smaller threshold diameter, compare to OPC paste. In addition BFS paste seems to have stiff frame structure indestructible by excessive high pressure intrusion. Decrease of threshold diameter and 2-3 nm continuative pore volume, and increase of under 2 nm ink-bottle pore volume were observed in carbonated OPC paste with w/c =.6. OPC paste with air-exposed for 22 months seems to forme stiff frame structure by C-S-H polycondensation and re-hydration. Properties of pore structures, for example distribution of ink-bottle pore and frame structure, can be analyzed by the unique MIP method. ACKNOWLEDGEMENTS We sincerely appreciate Professor Kiyoshi ASAGA for his valuable advice about MIP testing. We really thank Dr. Phan Huu Duy Quoc for valuables. REFERENCES [1] R.YOSHIDA and T.KISHI, ' Proposal of a new approach for determination of pore continuity and suitable intrusion pressure based on step-by-step mercury intrusion porosimetery test ', 1 st International conference on Microstructure Related Durability of Cementitious Composites, Vol.2 (28), [2] H.M.Jennings, A model for the microstructure of calcium silicate hydrate in cement paste, Cement and concrete research, Vol.3 (2)

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