ESTIMATION OF SLAG CONTENT AND WATER-TO-CEMENTITIOUS MATERIAL RATIO OF HARDENED CONCRETE

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1 ESTIMATION OF SLAG CONTENT AND WATER-TO-CEMENTITIOUS MATERIAL RATIO OF HARDENED CONCRETE K. Sisomphon*, National University of Singapore, Singapore M. H. Zhang, National University of Singapore, Singapore S. Q. Zhang, W.R. Grace Pte Ltd, Singapore F. Qi, National University of Singapore, Singapore H. K. Kor, National University of Singapore, Singapore 31st Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 06, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 31 st Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 06, Singapore ESTIMATION OF SLAG CONTENT AND WATER-TO-CEMENTITIOUS MATERIAL RATIO OF HARDENED CONCRETE K. Sisomphon*, National University of Singapore, Singapore M. H. Zhang, National University of Singapore, Singapore S. Q. Zhang, W.R. Grace Pte Ltd, Singapore F. Qi, National University of Singapore, Singapore H. K. Kor, National University of Singapore, Singapore Abstract This paper presents preliminary results of a research on the determination of slag content using chemical analysis method and w/cm of hardened concrete using petrographic method. The determination of the slag content was based on cement pastes with w/cm of 0.40 and different slag contents, and the determination of w/cm was based on concrete with a fixed slag content and different w/cm ratios. Results obtained indicate that the methods have potential for estimation of blast-furnace slag content and w/cm ratio of hardened concrete. Keywords: ground granulated blast-furnace slag, chemical composition, petrography, porosity, selective dissolution, w/cm 1. Introduction Ground granulated blast-furnace slag (referred to as slag from this point on) has been used increasingly in construction for improving the long-term durability of concrete. However, the use of the slag to replace cement may result in lower strength at early ages depending on percentage and quality of the slag used in concrete. If the batching of the slag relative to cement is not accurate, longterm behaviors of concrete may be affected as well, e.g. resistance of concrete to de-icing salt scaling. In situations where there are disputes, determining the quality of concrete, water-tocementitious ratio (w/cm), and quantity of the slag used in already hardened concrete is necessary. Analytical methods were proposed by various researchers for determining mix proportion of hardened concrete. In work of Hooton and Rogers [1], the X-ray diffraction technique of ignited mixture to determine the slag content in hardened concrete was studied. The method involves ignition of the mortar fraction of concrete at 950 o C to 1050 o C to devitrify unreacted slag. Thereafter, the resulting crystalline melilite component is compared to that in an ignited sample of blast-furnace slag from the same source. This method is applicable to slag with very fine particle size distribution. More importantly, this technique can be applied only to slag which converts mainly to melilite after the high temperature devitrification. Hooton and Rogers [1] also proposed a method to determine slag content by optical microscopy on thin sections made from the hardened concrete. This method involves the preparation of thin sections of the concrete and determination of the content of slag particles by point counting.

3 Grantham [2] examined results of quantitative X-ray fluorescence (XRF) analysis of concrete, and applied a mathematical approach to obtain the content of the individual components. The method has been successfully employed by a number of UK laboratories and provides another possible way of resolving the composition of concrete in which it is suspected that mis-batching may have occurred. However, this method can only be used in situations where the chemical composition of original ingredient materials of concrete is known. Petrographic methods have been used to determine w/c of hardened concrete for many years [3-8]. In this method, thin sections with fluorescent dye were prepared from concrete to be examined. Intensity of green tone of the concrete thin sections under optical microscope is compared with that of standard thin sections with known w/c ratios. From the comparison, the w/c of the concrete to be examined may be estimated. Most of the works published so far are concentrated on Portland cement concrete. This paper presents preliminary results of a research on the determination of slag content using chemical analysis method and water-to-cementitious material ratio (w/cm) of hardened concrete incorporating slag using petrographic method. For the preliminary study, the determination of the slag content was based on cement pastes with w/cm of 0.40 and different slag contents, and the determination of w/cm was based on concrete with a fixed slag content and different w/cm ratios. 2. Experimental Investigation 2.1 Materials, mix proportions, and curing of cement pastes and concretes For the determination of slag content in cement pastes, a normal Portland cement (NPC1) and slag were used. For the determination of w/cm of concrete, another Portland cement (NPC2) and a Portland blast-furnace slag cement (PBFSC-B) were used. According to manufacturer, the slag content in the PBFSC-B was 65% by mass. Chemical compositions of these materials are shown in Table 1. Table 1 - Chemical composition of cements and slag used Oxide Content (% by mass) NPC1 Slag NPC2 PBFSC-B CaO SiO Al 2O Fe 2O Na 2O K 2O MgO SO For the determination of slag content, four cement paste mixtures were prepared with a controlled w/cm ratio of 0.40 and slag contents of 0, 30, 50 and 70% by mass of total cementitious materials. The pastes were cured in a sealed condition in the first 24 hrs followed by curing in a fog room with a relative humidity of 100% and temperature of 30 o C until 28 and 91 days. At these ages, the pastes were broken into small pieces, and dried in a vacuum oven at 40 o C until constant weight was reached. The samples were then ground into fine powders passing through 75 m sieve. The powder samples were kept in glass bottles and stored in a desiccator for analyses. For the determination of w/cm ratio, ten concrete mixtures with w/cm ratios of 0.30 to 0.70 with an increment of 0.10 were prepared. Five of the concretes were made from Portland cement NPC2 and another five were made from the blast-furnace slag cement. The mix proportions of the concretes are summarized in Table 2. After 24 hours curing in moulds (100x100x100-mm) covered with plastic sheet at ambient temperature (about o C), the specimens were demoulded and cured in water until 28 and 56 days. For preparation of thin sections of concrete, small samples (60x40xmm) were cut from the cubes and dried in a vacuum oven at 40 o C for about 15 hrs and at 50 o C for another 3 hrs followed by drying in a regular oven at 105 o C for about 3-4 hrs. Thin sections of the concrete were prepared and analyzed in accordance with Nordtest Method NT BUILD 361, 1991 [9].

4 Table 2 - Mix proportion of concrete Materials (kg/m 3 ) Mixture Superplasticizer NPC2 PBFSC-B Water Sand Gravel (ml/100kg cement) NPC NPC NPC NPC NPC PBFSC PBFSC PBFSC PBFSC PBFSC Determination of slag content in hardened cement paste The slag content was determined based on chemical analysis. The total cementitious materials in cement paste mixtures consisted of two components, NPC and slag. The relationship among the oxide content in NPC, slag, and mixed cementitious materials can be written as follows. (1 R) R Eq. (1) CM NPC slag where R ω CM ω NPC ω slag % of slag by mass of total cementitious materials [% by mass] oxide content in cementitious materials (cement and slag) [% by mass] oxide content in cement [% by mass] oxide content in slag [% by mass] Calcium oxide, SiO 2, Al 2 O 3 and Fe 2 O 3 are the major components in cementing materials. Among those, CaO is easily dissolved in most chemical treatments, whereas Fe 2 O 3 has relatively low content. Hence, the ratio of silicon dioxide-to-aluminum oxide content SiO 2 was chosen for calculation and Al2O3 determination of slag content according to the relationship in Equation (2). Si SiNPC 1R Sislag R Al Al 1R Al R CM NPC slag Eq. (2) where R % of slag by mass of total cementitious materials [% by mass] mass] Si Al Si NPC Al NPC Si slag Al slag CM SiO 2 to Al 2 O 3 content in the cementitious materials [% by mass / % by SiO 2 content in NPC [% by mass] Al 2 O 3 content in NPC [% by mass] SiO 2 content in slag [% by mass] Al 2 O 3 content in slag [% by mass]

5 To calculate the slag content R, Equation (2) can be rearranged as demonstrated in the equation shown below. Si SiNPC AlNPC Al CM R Si Si Si Al Al NPC slag slag NPC Al CM Eq. (3) If the raw materials are available or if the composition of the cement and slag used in the concrete are known, the slag content R can be calculated from Equation (3) after determining the composition of the cement paste in concrete which has a strong relationship to that of the original cementitious materials. In this study, X-ray fluorescence (XRF) technique was used to determine the chemical composition of the cement, slag, and the dried samples of the cement pastes. A Thermo Electron ARL 9800 XRF spectroscopy system was used for the chemical analyses. If the original cement used for the concrete is not available and the composition of the cement is unknown, estimation may be made since the cement is an industrial product and has to meet standards. If the original slag used for the concrete is not available and the composition of the slag is unknown, selective dissolution techniques on cement paste samples can be applied to obtain slag samples. The principle of the methods is to dissolve a cement paste sample in specific solvents which attack unhydrated cement components and hydration products except for unreacted slag which is retained as. A selective dissolution process originally proposed by Luke and Glasser [10] was used in the current study to obtain slag sample from the hardened cement pastes. Details of the process can be found in Reference [10]. The process was originally proposed to determine the reaction degree of the slag. Among the major oxides, only CaO is significantly dissolved by the dissolution. If it is assumed that the content of SiO 2 and Al 2 O 3 of the slag is similar to that of the original slag, the content of SiO 2 and Al 2 O 3 of the original slag may be estimated and used for the calculation of slag content in cement pastes. 2.3 Determination of w/cm of hardened concrete Fluorescence microscopy was used to determine the w/cm of concrete [3-8]. The principle of the method is that the green tone (GT) intensity of a thin section of the concrete under fluorescence microscope is related to capillary porosity of the concrete. The higher capillary porosity, the higher GT intensity. The capillary porosity is further related to the w/cm of the concrete. If standard thin sections of concrete cured for 28 days with known w/cm ratios, slag quality, and slag content are available, the w/cm of a concrete can be estimated by comparing the GT intensity of the thin section of the concrete being investigated to that of the standard thin sections of the concrete with similar slag quality and content and similar curing. In this study, thin sections of the concretes with w/cm of 0.30 to 0.70 were prepared. The GT intensity of the thin sections was determined at a magnification of 100 times by using a standard polarizing microscope LEICA DMLP with 100-watt tungsten-halogen light source and a blue excitation filter and a yellow barrier filter. Both the polarizer and analyzer were removed from the light path and a condenser was adjusted to give Köhler illumination. The microscope examination of the thin sections was carried out in a semi-dark room. A JVC 3-CCD color video camera was used to collect images, and an image analysis software was used to determine the GT intensity. The green tone value was determined at 10 different randomly selected paste areas through the thin section, and the mean GT was then calculated. Details can be found in Reference [3]. From the above information, relationships between GT intensity and w/cm can be established. The relationships of the control Portland cement concrete and concrete made from the blast-furnace slag cement were compared.

6 3. Results and Discussion 3.1 Estimation of slag content in cement pastes The chemical composition of the cement pastes determined from XRF is presented in Tables 3. The chemical composition of the pastes can also be calculated from that of the raw materials (Table 1) by Equation (1), and the results are given in Table 4. Table 3 - Chemical compositions of the hardened cement pastes determined by XRF technique (a) Cement paste samples cured for 28 days (b) Cement paste samples cured for 91 days Oxide Content (% by mass) NPC 30%Slag 50%Slag 70%Slag Oxide Content (% by mass) NPC 30%Slag 50%Slag 70%Slag CaO CaO SiO SiO Al 2O Al 2O Fe 2O Fe 2O Na 2O Na 2O K 2O K 2O MgO MgO SO SO Table 4 - Chemical compositions of the cement pastes calculated from the raw materials Oxide Content (% by mass) NPC 30%Sla g 50%Sla g 70%Sla g CaO SiO Al 2O Fe 2O Na 2O K 2O MgO SO The content of SiO 2 and Al 2 O 3 calculated is plotted against that determined from hardened paste for both 28 and 91 days samples (Fig. 1). The SiO 2 and Al 2 O 3 calculated and determined by XRF seem to have good relationships. Linear regression lines were drawn for the SiO 2 and the Al 2 O 3 and the gradients represent the correlation factors. Empirical relationships between the oxide contents calculated and determined for the hydrated cement pastes can be established and shown in Equation (4), where Si cp and Al cp are SiO 2 and Al 2 O 3 contents in the hardened cement paste, respectively. Oxide contents calculated from binders (%wt) Line of equality Al 2O 3 y = x SiO 2 y = x Oxide contents measured from pastes (%wt) 28-day 91-day Fig.1 - Correlation of the oxide contents in cement pastes

7 calculated from raw materials and determined by XRF method Si Al CM 0.94Si 1.1Al cp cp Eq. (4) By substituting Equation (4) into Equation (3), the slag content R can be determined as shown in Equation (5). 0.94Si cp SiNPC Al NPC 1.1Al cp R 0.94Si cp Si Si Al Al 1.1Al cp NPC slag slag NPC Eq. (5) Estimation of slag content R when cement composition is known If the cement used for the concrete is available or the chemical composition of the Portland cement used is known, there are two possible scenarios for determining the slag content depending on whether the chemical composition of the slag is known or unknown. In cases that the composition of both the cement and slag is known, the slag content R can be directly estimated by Equation (5). However, if the composition of slag is unknown, the selective dissolution process is required to determine the composition of the slag. Table 5 presents the composition of slag samples obtained using the selective dissolution process in comparison with that of the original unreacted slag. The results indicate that SiO 2 and Al 2 O 3 contents in the s are similar to those in the original slag for most mixes except for the sample 30Slag- 91days. Nevertheless, the SiO 2 /Al 2 O 3 ratio of the s for all the mixes was similar to that of the original slag and the difference was not more than 13% (Fig. 2). Table 5 - Chemical composition of slag samples after selective dissolution Oxide Content (% by mass) Original slag 30% 28d, 50% 28d, 70% 28d, 30% 91d, 50% 91d, 70% 91d, CaO SiO Al 2O Fe 2O Na 2O K 2O SiO2/Al2O days 91 days Pure Slag 30Slag 50Slag 70Slag 30Slag 50Slag 70Slag MgO SO Fig.2 - SiO 2 /Al 2 O 3 ratios of the s and original unreacted slag The estimated slag content is plotted against the actual content when the composition of the slag is known or unknown (Fig. 3). The method yields reasonably good estimation of the slag content with an absolute error of estimation within 5% by mass of cementitious materials regardless whether the composition of slag is known or unknown.

8 Estimated %by mass of cementitious materials % -2.5% -5% Actual %by mass of cementitious materials 28-day 91-day Estimated %by mass of cementitious materials % -2.5% -5% Actual %by mass of cementitious materials (a) Known slag (b) Unknown slag Fig. 3 - Verification of estimation when the composition of the cement is known Estimation of slag content R when cement composition is unknown In practice the cement used for the concrete to be investigated may not be available after the construction is completed and the composition of the cement is unknown. In such cases the chemical composition of the cement needs to be estimated. Because Portland cements are industrial products, and their composition has to meet requirements of standards. The chemical composition of the cement can thus be roughly estimated. Information of the chemical composition of normal Portland cements used in Structural and Concrete Laboratory at NUS over the past five years were collected, and their SiO 2 and Al 2 O 3 contents are shown in Fig. 4. From the available data, six-point boundaries of SiO 2 -Al 2 O 3 contents were drawn. 28-day 91-day SiO2 (% by mass) A D 22 B E 21 C F Al 2O 3 (% by mass) 5-year data Average Boundary NPC1 Fig. 4 - Chemical composition of normal Portland cements used in Structural and Concrete Laboratory at NUS over the past 5 years Similar to the cases where the chemical composition of cement is known, there are two possible scenarios for determining the slag content depending on whether the composition of slag is known or unknown. Figure 5 presents the estimated slag contents vs. the actual ones. To study the sensitivity of estimated cement composition in order to estimate the slag content R, calculation was performed by substituting the oxide content of cement with the 6-point boundary values, and the results are also shown in Fig. 5. As expected, the estimation error for the slag content was higher than the cases where the cement composition is known. The absolute error of estimation is within 10% by mass of cementitious materials in most cases except for some values calculated from the boundaries A, E and F. Normally, relationships among oxides in Portland cement must be controlled by several parameters called cement modulus to achieve a complete clinkering process in manufacturing. Index of activity,

9 SiO 2 /Al 2 O 3, is one of the moduli which has to be controlled whereby the value would fall under a specified range. From Fig. 4, if considering the boundary in terms SiO 2 /Al 2 O 3 as presented by the slope of dash lines from origin, the boundaries of the Portland cement from 5-year collected data would be at Boundaries C, D and E, whereas one point has been considered as outlier shown within a circle. Excluding this outlier, the error of estimation will be lower than 10% by mass of cementitious materials for all cases % +5% -5% % +5% -5% Estimated % by mass of cementitious materials % Boundary A Boundary B Boundary C Boundary D Boundary E Boundary F Estimated % by mass of cementitious materials % Boundary A Boundary B Boundary C Boundary D Boundary E Boundary F Actual % by mass of cementitious materials Actual % by mass of cementitious materials (a) Known slag (b) Unknown slag Fig.5 - Verification of estimation when cement composition is unknown 3.2 Estimation of water-to-cementitious materials ratio (w/cm) of concrete Figure 6 shows the relationship between the green tone (GT) intensity and w/cm. It is clear that the GT intensity has good correlation with the w/cm of the concrete. However, the GT intensity of the concrete made with the slag cement is lower compared with that of the control Portland cement concrete of equivalent w/cm, indicating that the slag concretes are denser than the control concretes. It is also noted that the GT intensity of the concrete cured for 56 days was generally lower than that cured for 28 days. The longer curing promotes further hydration which decreases porosity and thus the intensity of the fluorescent light passing through a concrete thin section. Regression analyses of the data for the control Portland cement concrete and concrete made with the slag cement cured for 28 days provide two relations between the w/cm and GT intensity shown in Equations (6) and (7) with correlation coefficient of and , respectively. w/ cm GT (for the control Portland cement concrete) Eq. (6) w/ cm GT (for the concrete with the slag cement) Eq. (7) y = x R 2 = w/cm ratio PBFC NPC 28d NPC 56d NPC 28d PBFC 56d PBFC 0.30 y = x R 2 = Green tone value Fig. 6 - Correlation between GT and w/cm ratio (the regression lines are based on 28-day data)

10 Assume that the source and quality of the slag used in concrete in a local area is reasonably consistent. If that is the case, the w/cm of the concrete with slag may be estimated if the standard thin sections of concrete cured for 28 days with known slag contents and w/cm ratios are available as long as the curing of the concrete to be examined is similar to that of the standards. The w/cm of the concrete may be estimated by comparing the GT intensity of the concrete to be investigated with that of the standards if the slag content in the concrete is known. Table 6 presents the GT value of the concrete experimentally determined, the estimated w/cm calculated based on Equations 6 and 7, and the error of estimation. The absolute error of estimation for the w/cm of the concrete cured for 56 days was > 0.10 for a number of cases, particularly for the concrete with the slag cement. This is probably due the fact that longer moisture curing results in greater hydration and pozzolanic reaction for the concretes which reduces capillary porosity, particularly for the concrete with the slag cement. Reduced capillary porosity would decrease the GT value of thin section of the concrete. This results in underestimation of the w/cm of the concrete. Therefore, in order to estimate the w/cm of the concrete with reasonable accuracy, the curing of the concrete to be investigated, particularly those with slag, should be similar to that of the standards. Age Actual w/cm 28 days 0.30 Mixture Green tone value Table 6 - Error of estimation of w/cm Estimated Error Mixture w/cm Green tone value Estimated w/cm NPC PBFC days Error Summary and Conclusions For the preliminary study, the determination of the slag content was based on cement pastes with w/cm of 0.40 and different slag contents using a chemical analysis method, and the determination of w/cm was based on concrete with a fixed slag content and different w/cm ratios using a petrographic method. Based on the results, the following conclusions may be drawn: 1. The slag content in hardened concrete may be estimated by the chemical analysis method. However, the accuracy of the estimation depends on whether the chemical composition of the cement and slag used in the concrete to be investigated is known. In cases that the composition of the cement is known, the absolute error of estimation of the slag content is within 5% by mass of cementitious materials regardless whether the composition of slag is known or unknown. In cases that the composition of the slag is unknown, a selective dissolution process may be used to estimate the ratio of SiO 2 /Al 2 O 3. In cases where the composition of the cement is unknown, the absolute error of estimation of the slag content is within 10% by mass of cementitious materials for most of the cases. It is recommended, therefore, to keep chemical composition of the cement used in record for various construction projects. 2. For both concretes made with the slag cement and Portland cement, there are good correlations between the green tone (GT) intensity and w/cm. However, the GT intensity of the concrete made with the slag cement is lower compared with that of the control Portland cement concrete of equivalent w/cm. The GT intensity of the concrete cured for 56 days was generally lower than that cured for 28 days. Assume that the source and quality of the slag used in concrete in a local area is reasonably consistent, the w/cm of the concrete with slag may be estimated with reasonable accuracy if the standard thin sections of concrete cured for 28 days with known slag contents and w/cm ratios are available as long as the curing of the concrete to be investigated is similar to that of the standards. Further research is needed to determine the slag content in concrete samples. The challenge is to separate the fine aggregate particles from the cement pastes.

11 References [1] Hooton, R. D. and Rogers, C. A., 1995, Determination of Slag and Fly Ash Content in Hardened Concrete, Cement, Concrete and Aggregates, CCAGDP, Vol.17, No.1, pp [2] Grantham, M. G., 1995, Determination of Slag and Pulverized Fuel Ash Hardened Concrete The Method of Last Resort Revisited, Cement, Concrete, and Aggregate, CCAGDP, Vol.17, No.1, pp [3] Zhang, S. Q. and Zhang, M. H., 05, Application of petrography for determining the quality of concrete cured in tropical environment, Cement and Concrete Research, Vol.35, Issue 7, pp [4] St John Donald, A., Poole Alan, W. and Sims, I., Concrete petrography, A Handbook of Investigation Techniques, John Wiley & Sons Inc., New York, 00. [5] Liu, J. J. and Khan, M. S., Comparison of Known and Determined Water-Cement Ratios Using Petrography, Water-Cement Ratio and Other Durability Parameters Techniques for Determination, ACI SP-191-2, pp , 00. [6] Mayfield, B., The Quantitative Evaluation of the Water/cement Ratio Using Fluorescence Microscopy. Magazine of Concrete Research, no. 150, pp , [7] Jakobsen, U.H., Understanding the Fetures Observed in Concrete Using Various Fluorescence Impregnation Techniques. Proc. th ICMA, Mexico, pp , [8] Jakobsen, U. H., Laugesen, P. and Thaulow, N., Determination of Water-Cement Ratio in Hardened Concrete by Optical Fluorescence Microscopy, Water-Cement Ratio and Other Durability Parameters Techniques for Determination, ACI International SP-191-3, 00. [9] Nordtest Method, Concrete, Hardened: Water-Cement Ratio, NT Build 361, 1991 [10] Luke, K. and Glasser, F. P., 1987, Selective Dissolution of Hydrated Blast Furnace Slag Cements, Cement and Concrete Research, Vol.17, pp [11] British Standards BS 1881 Part 124, 1988: Testing Concrete, pp