Compressive Strength of Binary and Ternary Blended Cement Mortars

TIChE International Conference 2011 November 10 – 11, 2011 at Hatyai, Songkhla THAILAND Paper Code: Paper Code

Compressive Strength of Binary and Ternary Blended Cement Mortars Containing Fly Ash and Silica Fume Under Autoclaved Curing W. Wongkeo1, P. Thongsanitgarn1, A. Chaipanich1* 1

Advanced Cement-Based Materials Research Unit, Department of Physics and materials, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand *e-mail: [email protected]

Abstract – Cement industry is a one of the major sources of environmental pollution therefore the reduction of cement demand should be improved. Fly ash and silica fume is a by-product of industries and it should be reused to reduce the waste pollution. Thus, this study investigated the use of fly ash and silica fume as a cement replacement in binary and ternary blended cements on compressive strength and physical properties of mortar. Autoclaved curing at 130 C and 20 psi of pressure for 9 h was used in this study. The results show that the compressive strength of binary blended cement mortar with FA tends to decrease with increased FA replacement and shows compressive strength lower than PC control. However, compressive strength of binary blended cement mortar with SF was improved and shows compressive strength higher than that of PC control. The compressive strength of ternary blended cement mortar was higher than binary blended cement at the same level replacement and it increases with increased SF replacement. Moreover, ternary blended cement mortar containing 10%SF by weight contribute in giving compressive strength higher than PC control. The incorporation of FA with SF can enhance workability of blended cement mortar containing only SF replacement. Keyword: Fly ash, Silica fume, Blended cement, Autoclaved, Compressive strength.

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TIChE International Conference 2011 November 10 – 11, 2011 at Hatyai, Songkhla THAILAND

1.

Introduction

Cement and concrete has been high demand both in building and infrastructure development. Cement industry is a one of the major sources of environmental pollution. Main pollutions of cement productions include; cement dust, air pollution, water pollution, solid waste pollution noise pollution, ground vibration and resources depletion due to raw material extraction. Moreover, CO2 emission from burning process of cement industry is the main cause of global warming. Therefore, the reduction of cement demand must be discussed to improve environmental pollution from cement industry. Nowadays, blended cement with pozzolanic materials such as fly ash (FA), granulated blast furnace slag (GGBS), silica fume (SF) and other natural pozzolan materials is widely used in cement and concrete construction by replacing part of cement especially, FA and SF is by-product from industry. FA is a by-product from the combustion of coal fired thermoelectric power plants. FA consists of silicon dioxide (SiO2), aluminium oxide (Al2O3) and iron oxide (Fe2O3). However, FA normally also contains high proportions of heavy metals and other trace elements such as Br, Cr, Cd, Co, Cu, Fe, Mn, and Pb which is the environmental toxins [1]. SF is a by-product of ferrosilicon alloy and silicon metal industry. The chemical composition of SF consists of amorphous SiO2 more than 80% which it is high pozzolanic reaction In recent years, binary blended cement at high volume fly ash (HVFA) has been widely studied. However, the strengths of HVFA concrete are lower than that of plain cement (Portland cement concrete) at early age [2-6]. Previous studies, the incorporation of FA and SF in ternary blended cement have been found to offer enhanced properties of blended cement concrete than blended cement with only FA. Thomas et al. [7] studied the effect of ternary blends containing FA and SF on the strength and durability of concrete. It is concluded that the ternary blends cement with FA and SF offer significant advantages over binary blends cement and even greater enhancements over plain portland cement. Gesoglu and Ozbay [8] investigated the effects of using FA, GGBFS and SF as a supplementary cementitious materials in binary, ternary, and quaternary blends on the fresh and hardened properties of self-compacting concretes (SCCs). It is reported that the ternary blended cement with FA and SF show higher compressive strength than binary blended cement with FA at the same level replacement. Also reported by Guneyisi et al. [9] who studied the strength and drying shrinkage of self-compacting concretes incorporating multi-system blended cement. The parts of experimental designs show that the compressive strength of ternary blended cement (PC+FA+SF system) was higher than that of binary blended cement (PC+FA system) at the same

replacement level. Nochaiya et al. [10] studied the utilization of fly ash with silica fume and properties of Portland cement–fly ash–silica fume concrete. It is also reported that the utilization of silica fume with fly ash was found to increase the compressive strength of concrete at early ages. Autoclaved is curing method at high temperature and high pressure steam. The curing temperature of this curing exceeds 100 C. The range of curing temperature normally used in autoclaving is 160 to 210 C at steam pressure of 6 to 20 atm [11]. The chemistry of hydration changes under these conditions have substantially different properties from products cured below 100 C. In the absent of reactive silica (PC only) the α-C2SH phase is formed whereas tobermorite (C5S6H5) phase is formed on continued heating in the presence of reactive silica [12]. The change of hydration product has lead to strength gain and properties development in the short time of curing. Thus, in this study investigated the used of FA and SF as a cement replacement in binary and ternary blended cement. Autoclaved curing at 130 C and 20 psi of pressure for 9 h was and compressive strength and physical properties of mortar were also tested.

2.

Research methodology

Ordinary Portland cement type 1 (OPC) Fly ash (FA) obtained from Mae Moh power plant in Lampang, Thailand and undensified silica fume (SF) grade 920-U produced by Elkem were used this study. The chemical compositions PC, FA and SF powder are given in Table 1. River sand with specific gravity of 2.65 was used in the fine aggregate of mortar. Table 1. Chemical composition of materials used in this research. Chemical composition (%) SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O P2O5 TiO2 SO3 LOI

OPC 20.64 4.85 63.62 3.17 1.14 0.51 0.81 0.32 0.21 2.75 2.08

Materials FA 34.67 18.01 21.52 14.24 2.47 0.96 2.74 0.21 0.38 4.64 0.06

SF 93.55 0.56 1.13 0.17 0.75 0.14 1.05 0.53 0.002 1.01 1.06

In this study, binary and ternary blended cement mortar was used FA and SF as a Portland cement replacement. FA was used to replace part of cement at 50%, 60% and 70% by weight while SF was used to replace part of cement at 5% and 10% by weight, respectively. The fine aggregate to binder ratio of 2.5 and water to binder ratio of 0.5 were used. The mix proportions and mix design are summarized in Table 2. The mortar mixes were mixed and cast into 50x50x50 Paper Code-2


mm moulds and compacted using vibrating table. The mortar specimens were stored in moulds for 24 h. After demoulded, these specimens were exposed to autoclaved at 130 C and under pressure 20 psi for 9 h. The compressive strength was tested after end of cooling periods of autoclaved circle. The apparent density, volume of permeable pore space (voids) and water absorption were also measured. Table 2. Mix proportion of blended cement mortar. Mixes

OPC 562 281 225 169 534 506 281 281 225 225 169 169

OPC 50FA 60FA 70FA 5SF 10SF 45FA5SF 40FA10SF 55FA5SF 50FA10SF 65FA5SF 60FA10SF

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FA 281 337 393 253 225 309 281 365 337

Materials (kg/m3) SF Aggregate 1405 1298 1276 1254 28 1395 56 1385 28 1298 56 1299 28 1277 56 1277 28 1255 56 1256

Water 281 281 281 281 281 281 281 281 281 281 281 281

Results and discussion

Workability property of binary and ternary blended cement mortar was evaluated through the measurement of flow table. Flow table of binary blended cement mortar with SF decreased with increasing SF replacement especially at 10%SF by weight (Table 3). However, flow table of ternary blended cement mortar with FA and SF replacement was increased, compared to binary blended cement mortar with only SF replacement. Moreover, flow table of ternary blended cement mortar increased with increasing FA replacement. Thus, the workability of blended cement mortar with only SF replacement can be improved by incorporating FA replacement. The apparent density, volume of permeable pore space (voids) and water absorption of binary and ternary blended cement mortar under autoclaved curing for 9 h are shown in Table 4. It was observed that the apparent density of binary blended cement mortar with FA tend to increase with increasing FA replacement. In addition, the apparent density of all binary and ternary blended cement mortar was higher than PC control. In high temperature and pressure curing, the α-dicalcium silicate hydrate (α-C2SH) is formed in PC control mixture while tobermorite is formed in blended cement. Tobermorite phase has a larger volume of structure than α-C2SH phase which cause a decrease in porosity and increase in the density of blended cement mortar [11]. Thus, the present of tobermorite lead to increased apparent density of binary and ternary blended cement mortar. Voids and water absorption of binary blended cement mortar with FA replacement was higher than PC

control while voids and water absorption of binary blended cement mortar with FA replacement was lower than PC control. In ternary blended cement mortar, voids and water absorption increased with increasing FA replacement. However, voids and water absorption of ternary blended cement mortar tend to decreased with increasing FA replacement. This is due to the filler effect by hydration products and SF particle. Furthermore, voids and water absorption of all ternary blended cement mortar was higher than PC control. This may causes the distribution and agglomerated of unreacted FA and SF. Table 3. Flow table and Ca/Si ratio of blended cement mortar. Mixes OPC 50FA 60FA 70FA 5SF 10SF 45FA5SF 40FA10SF 55FA5SF 50FA10SF 65FA5SF 60FA10SF

Flow table (mm) 105.75 -* -* -* 86.75 67.75 123.5 79.25 -* 86.25 -* 104.75

Ca/Si ratio 3.08 1.54 1.32 1.12 2.83 2.60 1.36 1.21 1.17 1.04 0.99 0.88

*cannot measured due to sample was extended over the dimension of flow table pan Table 4. Apparent density, voids and absorption of blended cement mortar.

Mixes OPC 50FA 60FA 70FA 5SF 10SF 45FA5SF 40FA10SF 55FA5SF 50FA10SF 65FA5SF 60FA10SF

apparent density (g/cm3) 2.47 2.56 2.57 2.59 2.51 2.52 2.53 2.54 2.55 2.56 2.56 2.55

Voids (%) 21.65 24.70 24.07 25.13 21.81 20.64 23.52 23.25 24.94 24.59 26.19 25.37

Water absorption (%) 11.18 12.82 12.32 12.96 11.13 10.34 12.17 11.93 13.05 12.72 13.88 13.35

Fig. 1 shows the compressive strength of binary blended cement mortar under autoclaved curing for 9 h. It was found that compressive strength of binary blended cement mortar with FA tends to decrease with increased FA replacement. In addition, binary blended cement mortar with FA shows compressive strength lower than PC control (OPC mix). This is due to the dilution effect and low pozzolanic reaction of FA. However, compressive strength of binary blended cement with SF increase with increased SF replacement and show compressive strength higher than that of PC control (OPC mix). This is due to higher pozzolanic reaction of SF and tobermorite formation. The structure Paper Code-3


of C-S-H phases was changed under high temperature and pressure of autoclaved curing. The α-dicalcium silicate hydrate (α-C2SH) is formed instead of an amorphous calcium silicate hydrate (C-S-H) in PC control mixture while tobermorite (C5S6H5) is formed in blended cement. Tobermorite phase has a larger volume of structure than α-C2SH phase which cause a decrease in porosity and increase in the compressive strength. The tobermorite formation in high temperature curing was controlled by Ca/Si ratio. Meller et al. [13] shows phase diagram of relationship between temperature and CaO/SiO2 mole ratio. It is reported that tobermorite phase is formed at the CaO/SiO2 ratio between about 0.5-1.5 and at temperature above 110 C. However, it was observed that 50FA mix show compressive strength similar to 60FA mix. It can be found that CaO/SiO2 ratio decreased with increased FA replacement and lead to tobermorite formation. However, Ca/Si ratio of 70FA mix was 1.12 which suitable to tobermorite formation but the compressive strength can be improved similar to 50FA and 60FA mix. This is due to the low pozzolanic reaction and the tobermorite formation cannot give compressive strength equal to the dilution effect.

Fig. 2. Compressive strength of ternary blended cement mortar under autoclaved curing for 9 h.

In addition, it was founded that the 45FA5SF and 55FA5SF mixes contribute compressive strength equal to PC control while the 65FA5SF mixes shows compressive strength lower than PC control due to the higher dilution effect. Moreover, ternary blended cement mortar containing 10%SF by weight (40FA10SF, 50FA10SF and 60FA10SF mixes) give compressive strength higher than PC control.

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Fig. 1. Compressive strength of binary blended cement mortar under autoclaved curing for 9 h.

Fig. 2 shows the compressive strength of ternary blended cement mortar under autoclaved curing for 9 h. When compared to binary blended cement, it was found that compressive strength of ternary blended cement mortar increase with increased SF replacement at the same level replacement. This was due to the higher pozzolanic reaction of SF than FA and tobermorite phase was formed. It was observed that ternary blended cement mortar had a CaO/SiO2 ratio between 0.88-1.36 which suitable to tobermorite formation and thereby increased the compressive strength of ternary blended cement mortar with increasing amount of SF replacement.

Conclusions

In this study focused on the compressive strength of binary and ternary blended cement mortar using FA and SF as a cement replacement under autoclaved curing for 9 h. It can be concluded the workability of blended cement mortar with only SF replacement can be improved by incorporating FA replacement. The compressive strength of binary blended cement mortar with FA at 50%, 60% and 70% by weight was lower than PC control and tend to decreased with increased FA replacement. While the compressive strength of binary blended cement mortar with SF at 5% and 10% by weight was higher than PC control increased with increased SF replacement. For ternary blended cement, the compressive strength of ternary blended cement mortar with SF at 5% and 10% by weight was higher than binary blended cement mortar with FA at the same level replacement. In addition, the 45FA5SF and 55FA5SF mixes contribute compressive strength equal to PC control while the 65FA5SF mixes shows compressive strength lower than PC control. Moreover, ternary blended cement mortar containing 10%SF by weight contribute compressive strength higher than PC control.

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Acknowledgement and References

I would like to thank the Office of the Higher Education Commission, Thailand for supporting by Paper Code-4


grant fund under the program Strategic Scholarships for Frontier Research Network for the Ph.D. Program Thai Doctoral degree for this research. The Graduate School, Chiang Mai University is also acknowledged. We also wish to thank the National Research University Project under Thailand's Office of Higher Education Commission for financial support. [1]

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