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MATERIALS AND INTERFACES Influence of Quartz Particle Size on Hydrothermal Solidification of Blast Furnace Slag Zhenzi Jing,*,† Emile Hideki Ishida,† Fangming Jin,† Toshiyuki Hashida,‡ and Nakamichi Yamasaki† Graduate School of EnVironmental Studies and Fracture and Reliability Research Institute, Tohoku UniVersity, Aoba 6-6-20, Aoba-ku Sendai 980-8579, Japan
Solidification of blast furnace water-cooled slag (BFWS) has been carried out using a hydrothermal processing method, in which the BFWS could be solidified in an autoclave under saturated steam pressure (1.56 MPa) at 473 K for 12 h by the addition of quartz. Experimental results showed that the strength development of the solidified specimen was significantly affected by the curing time and quartz particle size. With increasing curing time, the amount of tobermorite increased, and the formed tobermorite, in turn, enhanced the tensile strength. The particle size of the quartz added exerted a significant influence on the strength development, and the optimal quartz content for the strength development appeared to depend on the quartz particle size. The results also suggested utilization of finer quartz had the advantage of reducing the addition of quartz. Introduction Blast furnace slag (BFS), an industrial solid waste or byproduct generated in the process of iron ore reduction in blast furnaces, has been successfully utilized for the manufacture of portland slag cement, which is widely used to produce construction materials. BFS, of higher lime content, is well-known to have a hydraulic activity when mixed with water. BFS, therefore, may also have potential as a useful resource for producing construction materials directly, like portland slag cement. The direct use of BFS for the manufacture of construction materials, undoubtedly, will offer both energy savings and cost reductions. In our previous work, BFS was solidified hydrothermally by tobermorite formation, and the addition of quartz was favorable for the tobermorite formation. The solidified body in our study was shown to possess a reasonable strength (tensile strength of 6-7 MPa), and therefore the hydrothermal solidification technology may have such a potential for producing construction materials directly from BFS. The hydrothermal solidification technique is considered to have the capability of reducing energy consumption due to the lower hydrothermal autoclaving temperature range typically (423-475 K). Ishida1 described how the energy required for the hydrothermal solidification (at 423 K) of earth ceramics (i.e., ceramics is made from earth by hydrothermal solidification) is only about 1/6 that of energy needed for fired ceramic tiles. Some work has been reported on the influence of quartz addition or quartz particle size on the mechanical properties during the hydrothermal process. For autoclaved aerated concrete (AAC) products, the crystallinity of the tobermorite increased with increasing quartz particle size, and the degree of reaction also depended on the quartz particle size.2 For autoclaved cement-ground quartz mixtures, different quartz particle sizes added led to different strength development.3 A * To whom correspondence should be addressed. Tel.:/Fax: +8122-795-4398. E-mail:
[email protected]. † Graduate School of Environmental Studies. ‡ Fracture and Reliability Research Institute.
few percent addition of silica fume greatly changed the pattern of the strength development and the microstructure for OPC (ordinary portland cement)-slag cement.4 As mentioned above, the strength development clearly depended not only on the amount of quartz but also on the quartz particle size. However, the influence of quartz particle size on the strength development for hydrothermal solidification of BFS has not been reported extensively in the literature. The objective of the present work is to investigate the strength development for the hydrothermal solidification of BFS with the addition of quartz during the autoclave process. In particular, the influence of the quartz particle size on the strength development is examined in order to find out the appropriate processing conditions for the hydrothermal solidification of BFS. Experimental Section BFS can be divided into air-cooled and water-cooled blast furnace slag according to the cooling method. Blast furnace water-cooled slag (BFWS) comprises nearly 80% of the volume of total blast furnace slag in Japan, which is widely used for the manufacture of portland slag cement. The BFWS used throughout in this study was supplied from Sumitomo Metals Ltd., Japan. The BFWS was ground with a ball mill to obtain a BET specific surface area of 370 m2/kg. Quartz from northeastern Japan was used as an additive for the hydrothermal solidification of BFWS. The quartz was ground in the ball mill for different times, i.e., 20, 40, and 60 h, and then classified into three particle size ranges, i.e., coarse, medium, and fine according to their milling time of 20, 40, and 60 h, respectively. The particle size distributions determined by laser diffraction technology (X100, Microtrac) and their main parameters of the ground quartz and BFWS powders are shown in Figure 1 and Table 2, respectively. Chemical compositions of BFWS and quartz determined by X-ray fluorescence (XRF; RIX3100, Rigaku) are shown in Table 1. BFWS powder mixed with ground quartz was used as starting material. The starting material (20 g) was first mixed manually in a mortar with 5 mass %
10.1021/ie060461e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
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Figure 2. Hydrothermal apparatus used for curing compacted specimens. Table 1. Compositions of BFWS and Quartz Used (mass %)
SiO2 CaO Al2O3 MgO SO3 Mn2O3 TiO2 Fe2O3 ignition loss
BFWS
Quartz
32.6 44.7 15.8 3.5 1.2 0.3 1.1
89.7 0.9 7.0
0.3 0.6 1.5
0.8
Table 2. Main Parameters of the Ground Quartz and BFWS Powders Used
coarse quartz medium quartz fine quartz BFWS b
D(10) (µm)a
median size D(50) (µm)b
D(90) (µm)b
5.3 4.2 3.6 9.7
36.6 29.8 23.5 46.3
69.9 55.4 47.7 115.5
a D(10) means 10% of the powder particles are smaller than this value. D(90) means 90% of the powder particles are smaller than this value.
stainless steel hydrothermal apparatus used for curing the demolded specimens is shown in Figure 2, in which the spring is used to press the Teflon lid tightly. After autoclaving, all the solidified specimens were dried at 343 K before testing. The solidified disk-shaped specimens (30 mm diameter × 20 mm height) were used to measure the tensile strength by employing the Brazilian testing method.5 The Brazilian tests were conducted in an Instron universal testing machine (M1185) at a crosshead speed of 0.2 mm/min. The measured maximum load, Pmax, was substituted into the following equation to calculate the tensile strength, σt:
σt ) 2Pmax/πdt
Figure 1. Particle size distributions of quartz and BFWS powders. (1) Coarse quartz, (2) medium quartz, (3) fine quartz, and (4) BFWS.
distilled water (1 mL), and then the mixture was compacted into a disk-shaped mold (30 mm diameter × 120 mm height) by compaction pressure of 30 MPa. The demolded specimens were subsequently autoclaved under saturated steam pressure (1.56 MPa) at 473 K, up to 12 h. The Teflon (PTFE) lined
where d ()30 mm) is the specimen diameter and t ()20 mm) is the specimen thickness. Three specimens were tested for each hydrothermal processing condition, and the experimental results presented in this study are the averaged data. After the Brazilian testing, the crushed specimens were investigated for phase analysis by X-ray diffraction (XRD, MiniFlex, Rigaku), for microstructure by a scanning electron microscope (SEM S-4100, Hitachi), and for pore diameter distribution by a mercury intrusion method (Poremaster 33P, Quantachrome).
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Figure 3. Effect of curing time on strength development of solidified specimens with addition of 20 mass % coarse quartz. Figure 5. Pore size distributions of solidified specimens synthesized with addition of 20 mass % coarse quartz for different curing times.
Figure 4. XRD patterns of solidified specimens synthesized with addition of 20 mass % coarse quartz for different curing times.
Results and Discussion Our previous research showed that BFWS could be solidified hydrothermally with formation of tobermorite and the addition of quartz was favorable to the tobermorite formation. Tobermorite is a calcium silicate hydrate mineral of the ideal composition of Ca5(OH)2Si6O16‚4H2O. The dissolved silica reacts with calcium to form tobermorite during the hydrothermal process. The formed fibrous tobermorites were shown to bond slag particles together and fill in the spaces between particles tightly, thus enhancing the strength of the solidified body. However, the effect of quartz particle size on the strength development is still unknown. The strength development for BFWS solidified specimen with 20 mass % quartz content (coarse) at 473 K during the autoclave process was investigated first. As shown in Figure 3, the tensile strength increases with increasing curing time; the increase is initially slow during the first 3 h and then becomes rapid for the longer curing times. The process was also investigated by XRD analysis (Figure 4). Few new crystals form for the curing time of 1-3 h compared with that without curing (0 h), reflecting that little reaction occurred in such a short curing time. Above 6 h, however, a trace of the phase corresponding to 1.1 nm tobermorite becomes distinct, and the amount of 1.1 nm tobermorite and the crystallinity increase with increasing curing time. These results suggest that the strength development depends on the formation of calcium silicate hydrate (CSH), i.e., 1.1 nm tobermorite. A detailed investigation of the microstructure change with increasing curing time was conducted by measuring the pore diameter distributions in the specimens (Figure 5). Before
Figure 6. SEM photograph of solidified specimen with addition of 20 mass % quartz (coarse).
hydrothermal treatment (curing time of 0 h), the pore diameter distribution gives the highest frequency peak at 7 µm. The pore dimension may correspond to the space between BFWS particles in the as-compacted specimen. The peak remains almost the same with increasing curing time up to 3 h, and tends to shift toward a finer diameter at the curing time of 6 h. In addition, for the micropore diameter distribution (0.1 µm) shifts more toward a finer diameter and a new peak of micropores (
