Effects of Circulated Fluidized-Bed Fly Ash, Ground Granulated Blast-Furnace Slag and Coal Fly Ash on Properties of Mortars

Transcription

1 2017 2nd International Conference on New Energy and Renewable Resources (ICNERR 2017) ISBN: Effects of Circulated Fluidized-Bed Fly Ash, Ground Granulated Blast-Furnace Slag and Coal Fly Ash on Properties of Mortars Maochieh Chi, Jiang-Jhy Chang and Weichung Yeih ABSTRACT The amount of circulating fluidized bed combustion (CFBC) ash is increasing and the disposal cost of CFBC ash have also been increasing annually. Hence, developing a better utilization of CFBC fly ash with the by-products ground granulated blast furnace slag (GGBFS) and coal fly ash is an urgent and important task. In this study, seven mixes with different blended cements were prepared to cast mortar specimens. Flow test, setting times test, water absorption rate, compressive strength test and flexural strength test were performed to investigate the properties of mortars. Test results show that the addition of coal fly ash, CFBC fly ash, and GGBFS improves the workability.when the amount of cement replacement by CFBC fly ash, GGBFS, and coal fly ash was recommended less than 20% simultaneously, the compressive strength is higher than that of plain cement mortar. However, CFBC fly ash would results in a lower strength when adding up to 30%. Thus, the amount of CFBC fly ash replacement cement was recommended to be limited below 20% and the total amount of cement replacement was limited below 60%. INTRODUCTION The pollution from the electric power plant by coal combustion into the atmosphere has taken serious social attention due to the importance of air quality and environmental conservation. Recently, the amount of circulating fluidized bed combustion (CFBC) ash is increasing and the disposal cost of CFBC ash have also been increasing annually. Hence, developing a better utilization of CFBC fly ash with the by-products- ground granulated blast furnace slag (GGBFS) and coal fly ash is an Maochieh Chi, Department of Fire Science, WuFeng University, Taiwan Jiang-Jhy Chang, Weichung Yeih, Department of Harbor and River Engineering, National Taiwan Ocean University, Taiwan 90

2 urgent and important task. Circulating fluidized bed combustion (CFBC) is a very attractive environmental-friendly coal-combustion technology for coal burning and lower SO2 and NOx emissions[1, 2]. CFBC fly ash, coal fly ash and GGBFS are wastes from CFBC power plant, and have the high sulfur content. Thus, the utilization of these wastes in building materials would result in a great risk of destructive expansion. Particularly, when CFBC fly ash and GGBFS are used in cement based construction materials, the sulfur in CFBC fly ash and GGBFS would lead to great volume expansion[3-5]. However, several previous research have shown that CFBC fly ash, coal fly ash and GGBFS have the pozzolanic activity and self-cementitious properties and can be used in construction materials[5-7]. Though many attempts to use CFBC fly ash as cement or concrete admixture have been reported, the use of CFBC fly ash keeps limited because the CFBC fly ash cannot meet the requirements of the specification for pozzolans. Shen et al. [5] found that CFBC fly ash can be used as cement retarders and lead to lower strength. However, Chi and Huang pointed out that[7]cfbc fly ash has a positive effect on compressive strength, splitting tensile strength, and sulphate attack resistance of hardened roller compacted concrete. Chen et al. [8] reported that the finer CFBC fly ash is better to improve the compressive strength. Coal fly ash and GGBFS have the properties of self-cementitious and can set and hard with the addition of water. They are widely utilized in cement as the admixtures. These binders are characterized by their superior durability performance and low environmental impact [9-11]. Nevertheless, there are only a few investigations of the use of CFBC fly ash in cement as an admixture. In this study, CFBC fly ash, GGBFS and coal fly ash were used to replace part of Portland cement to investigate the properties of blended cement mortars. EXPERIMENTAL PROGRAM Materials Three kinds of materials, Type I ordinary Portland cement (OPC), circulated fluidized bed combustion (CFBC) fly ash and Class F coal-fired fly ash with the physical properties and chemical compositions of these materials listed in Table 1 were used in this study. CFBC fly ash and coal fly ash were obtained from the Mailiao Six Light Naphtha Cracker Plant, located in the Yunlin county of Taiwan. CFBC fly ash of gray-and-white powder passing No. 200 (75µm) accounts for about 86% of the particles. The specific gravity of CFBC fly ash is 2.76 and its blaine specific surface area is 3000 cm2/g. Coal fly ash and GGBFS were used for partial cement replacement by weight. Their specific gravity of coal fly ash and GGBFS are 2.39 and 2.88, respectively. The blaine specific surface area is 2370 cm2/g for coal fly ash, 6000 cm2/g for GGBFS. River sand with its fineness modulus, bulk density and absorption of 2.33, 2580 kg/m3 and 2.94%, respectively, was used as a fine aggregate. MIXES DESIGN AND SPECIMENS PREPARATION Mixing of Ordinary Portland cement (OPC) mortars and blended cement mortars with 536 kg of binder per cubic meter according to ASTM C192[12] was designed. 91

3 The liquid/binder ratio was kept at a constant of 0.5. Seven different mixtures were prepared with varying proportions of OPC, CFBC fly ash, coal fly ash and GGBFS to produce mortar specimens. The mix proportions are shown in Table 2. The specimens were cast and kept in steel molds for 24 hours, and then they were demolded and moved into a curing room at relative humidity of 80% RH and 25 until testing. The specimens were tested in triplicate sets until the time of testing. TABLE 1. PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF RAW MATERIALS (%WT). TABLE 2. Mix proportions of the blended cement mortar specimens. * Within mixture designation FxCyGz, x represents the level of replacement (in wt%) of coal fly ash, y represents the level of replacement (in wt%) of CFBC fly ash and z represents the level of replacement (in wt%) of GGBFS as cement. METHOD The flow test used to determine consistency of fresh mortars was conducted according to the ASTM C230-14[13]. This mortar sample is placed on a flow table and dropped 25 times within 15 seconds. As the mortar is dropped, it spreads out on the flow table. The initial and final diameters of the mortar sample are used to calculate flow. Flow is defined as the increase in diameter divided by the original diameter multiplied by 100.The test method determines the time of setting of hydraulic cement by means of the Vicat needle. The initial and final setting times were determined by a penetration resistance test according to ASTM C191-13[14].Water 92

4 absorption was made in accordance with ASTM C642[15].For the water absorption test, the cubes were first kept in an oven at 105±5 for 24 h and weighed (Wd). They were then immersed in water for 24 h and weighed again (Ws). Ws was taken as the saturated weight. It took up to 24 hours for the specimens. The water absorption (WA) was then calculated by the following formula[16, 17]: WA(%)=(Ws-Wd)/Wd 100%.The compressive strength tests of the specimens were conducted according to ASTM C109-11[18]. For each mixture, 50 mm cubes were prepared and three specimens of each mixture were tested at the ages of 7, 14, 28, and 91 days to determine the average compressive strength. RESULTS AND DISCUSSION Flow Workability of mortar is its ease of use measured by the flow of the mortar. Generally, laboratory mixed mortar should have a flow of approximately %. The flow of OPM and the blended cement mortar specimens is listed in Table 3. All blended cement mortars have the considerably higher flow ranged from 107 to 134.5% than OPM, with approximately 76% of flow. It indicates that increasing cement replacement with coal fly ash, CFBC fly ash, and GGBFS increases the flow of mortars, thus improves the workability of the mortars. TABLE 3. FLOW OF OPM AND THE BLENDED CEMENT MORTAR SPECIMENS (%). Mix no. Flow (%) OPM F1C1G F2C1G F2C2G F2C2G F1C3G F1C3G SETTING TIME Setting times of OPM and the blended cement mortar specimens are given in Table 4. It reveals that setting times were increased in blended cement mortar specimens compared with OPM except those of 30% cement replacement by CFBC fly ash. Increase in initial setting time was 21, 51, 58 and 97 minutes and final setting time was 30, 43, 68 and 124 minutes for F1C1G1, F2C1G2, F2C2G2 and F2C2G4 of blended cement mortar specimens compared with that of OPM specimen. Setting process was increased due increase in percentage of raw materials (coal fly ash, CFBC fly ash, and GGBFS). The higher setting time of specimen F2C2G4 is that the Blaine fineness of GGBFS is about two times high than the cement. 93

5 TABLE 4. INITIAL AND FINAL SETTING TIME OF THE BLENDED CEMENT MORTAR SPECIMENS. Mix no. Initial setting time ( hr : min ) final setting time ( hr : min ) OPM 3:19 4:45 F1C1G1 3:40 5:15 F2C1G2 4:10 5:28 F2C2G2 4:17 5:53 F2C2G4 4:56 6:49 F1C3G1 2:01 4:32 F1C3G2 1:58 4:28 WATER ABSORPTION RATE Table 5 lists the water absorption of OPM and the blended cement mortars. It can be seen that the water absorption rate of OPM is lower than that of the blended cement mortars. The water absorption rates of blended cement mortars varied from 9.24% to 12.72%, whereas the OPM mortars had the water absorption of 9.03%. Specimen F2C2G4 has the relatively lower water absorption rate than the others because GGBFS has a much finer particle size which can fill the pore and result in lower water absorption rate. TABLE 5. WATER ABSORPTION RATE OF OPM AND THE BLENDED CEMENT MORTAR SPECIMENS. Mix no. Water absorption rate (%) OPM 9.03 F1C1G F2C1G F2C2G F2C2G F1C3G F1C3G COMPRESSIVE STRENGTH The compressive strength development of OPM and mortars with coal fly ash, CFBC fly ash, and GGBFS at the ages of 7, 14, 28 and 91 curing days is shown in Figure 1. It can be seen that higher compressive strength was observed in specimen F1C1G1 than the others at all ages. At the age of 91 days, the compressive strengths of specimens F2C1G2 and F2C2G2 are higher than that of specimen OPM. In addition, when the specimens of cement replacement with CFBC fly ash are over 30%, their compressive strengths are much less than those of the others. It indicates that more cement replacement by CFBC fly ash in mixtures is unavailable to undergo pozzolanic reaction and hydration providing strength development. Thus, the amount of cement replacement by CFBC fly ash, GGBFS, and coal fly ash was recommended less than 20% simultaneously based on the present results. 94

6 Compressive strength (MPa) OPM F1C1G1 F2C1G2 F2C2G2 F2C2G4 F1C3G1 F1C3G Age (days) Figure 1. Effect of coal fly ash, CFBC fly ash and GGBFS replacement rate on compressive strength. CONCLUSIONS This study investigates the properties of mortars with CFBC fly ash, ground granulated blast furnace slag (GGBFS) and coal fly ash. On the basis of test results, the following conclusions should be drawn: (1) The addition of coal fly ash, CFBC fly ash, and GGBFS improves the workability. (2) The amount of cement replacement by CFBC fly ash, GGBFS, and coal fly ash was recommended less than 20% simultaneously based on the compressive strength results. In addition, CFBC fly ash would results in a lower strength when adding up to 30%. Thus, the amount of CFBC fly ash replacement cement was recommended to be limited below 20% and the total amount of cement replacement was limited below 60%. REFERENCES 1. Zhang, Z., Qian, J., You, C. and Hu, C., Use of circulating fluidized bed combustion fly ash and slag in autoclaved brick. Constr. Build. Mater. 35(2012) Chen, C.T., Nguyen, H.A., Chang, T.P., Yang, T.R. and Nguyen, T.D., Performance and microstructural examination on composition of hardened paste with no-cement SFC binder Constr. Build. Mater. 76(2015) Anthony, E.J. and Granatstein, D.L., Sulfation phenomena in fluidized bed combustion systems Prog. Energ. Combust. Sci. 27(2001) Anthony, E.J., Berry, E.E., Blondin, J., Bulewicz, E.M. and Burwell, S., Advanced ash management technologies for CFBC ash Waste. Manag. 23(2003) Shen, Y., Qian, J. and Zhang, Z., Investigations of anhydrite in CFBC fly ash as cement retarders Constr. Build. Mater. 40(2013) Song, Y., Qian, J. and Wang, Z., 'The self-cementing mechanism of CFBC coal ashes at early ages' J. Wuhan. Uni. Tech. - Mater. Sci. 24(3) (2008) Chi, M. and Huang, R., 'Effect of circulating fluidized bed combustion ash on the properties of roller compacted concrete' Cem. Concr. Compos. 45(2014)

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