CAPITULO III ESTRUCTURA Y ELEMENTOS DEL MENSAJE PUBLICITARIO
3.8 Técnicas de comunicación visual
As shown in Figure 6A, bulk density gradually increases from 497 kg/m3 to 780 kg/m3 as the contents of Fe2O3 increases from 3% to 16%; the maximum particle density (1815.8kg/m3), the minimum water adsorption (8.4%), and the maximum porosity (58.6%) are obtained when the content of Fe2O3 is 10%. The liquid phases can be apparently increased by increasing the contents of Fe2O3 in the raw materials, which enhance the formation of FeO with higher viscosity and lower melting point (3Fe O + CO2 3 >820℃ 2Fe O + CO ;3 4 2
>820
3 4 2
Fe O + CO ℃ 3FeO + CO) [45]. But, it should be noted that a sinter with higher viscosity is not good for the release of gases and the formation of pores. 5%-8% is therefore selected as the optimal content range of Fe2O3 for production of ceramsite.
Ceramsite has higher bulk density and particle density as the CaO contents are in range of 2.75%-7%, indicating the presence of less intraparticle voids in ceramsite bodies, hence obtaining a lower water adsorption and porosity (as shown in Figure 6B). Higher water adsorption and porosity of ceramsite with lower density may affect the compressive strength and chemical stability of the lightweight ceramsite [12]. 2.75%-7% is selected as the optimal content range of CaO for production of ceramsite.
3 4 5 6 7 8 9 10 11 12 13 14 15 16
Bulk density Particle density Water adsorption Porosity
Bulk density Particle density Water adsorption Porosity
Figure 6. Effect of oxides contents on the physical characteristics of ceramsite (A-Fe2O3 and B-CaO).
(■ bulk density, □ particle density, ▲ water adsorption, × porosity).
The following tests (thermal analysis, XRD, morphological structures analyses, and compressive strength measurements) were conducted with ceramsite within each optimal contents range of the tested oxides (6%≤Fe2O3≤8% and 2.75%≤CaO≤7%).
3.2.2. Thermal properties (DT-TG) analyses
Two exothermic peaks of the mixtures with Fe2O3 contents of 5% are detected at 337.5
℃
and 473.2
℃
. Two exothermic peaks of the mixtures with Fe2O3 contents of 6% and one exothermic peak of the mixtures with Fe2O3 contents of 8% are detected at 336.7℃
, 462.5℃
,and 336.7
℃
, respectively. The intensity of exothermic peaks between 400-500℃
decreases as Fe2O3 contents increases and the peak disappears when Fe2O3 contents reach 8%. For each test, the detected exothermic peaks with significant weight loss below 500℃
are caused by the release of structural water and mixed gases (CO2, CO, SO2, etc.) (Figure 7A).Endothermic changes are observed and little substances are vaporized from the 3 mixtures above 900
℃
, which indicate the endothermic reaction is caused by the transformation of crystalline phases. Increase of Fe2O3 contents in the mixtures will profoundly influence the thermal properties and lead to more gas produced under the reaction of Fe2O3 at temperatures>820
℃
[45].0 200 400 600 800 1000
Figure 7. Effect of oxide contents on the thermal properties of ceramsite (A-Fe2O3 and B-CaO).
As shown in Figure 7B, it is interesting that there are two endothermic valleys at 280.6
℃
and 279
℃
for the mixtures with CaO contents of 5% and 7%, respectively. These endothermic valleys may partly attribute to the removal of water from hydrated products, which is likely to include most of calcium silicate hydrate (C–S–H) formed by the reaction of CaO, water, and silicate. From the results, it is clear that hydrated products formed at higher CaO contents are one of the first decomposed phases during the heating. There is little difference in DT-TG curves for the 3 mixtures above 300℃
. Weak weight loss for 3 mixtures are detected above 500℃
in TG analyses, which indicate that there is little volatilization of substances and the endothermic changes are caused by the oxidation of mostly inorganic substances and the transformation of crystalline phases [35].3.2.3. Crystalline phases (XRD) analyses
The main crystalline phases of the 3 ceramsite are similar to each other and Quartz (SiO2), kyanite (Al2SiO5), and Na-Ca feldspars [albite- Na(AlSi3O8), anorthite- Ca(Al2Si2O8)]
are the main crystalline phases of the 3 ceramsite (Figure 8). The formation of these
crystalline phases is thought to correspond to the endothermic changes above 900
℃
asdetected in the DT curves (Figure 7A). The enstatite [Mg2(Si2O6)], sillimanite [(Al1.98Fe0.2)SiO5], and ferrosilite magnesian [(Fe,Mg)SiO3] have a high probability to exist in the ceramsite with Fe2O3 contents of 8%, which can be attributed to the melting and diffusing of abundant Fe2O3 at 1000
℃
.Quartz, kyanite, and Na-Ca feldspars are the main crystalline phases of the ceramsite with CaO content of 2.75% (Figure 8). Na-Ca feldspars has already been formed in ceramsite with CaO content = 2.75%, as one of the minor crystalline phase, co-existing with quartz and kyanite. The most pronounced crystalline phases of the ceramsite with CaO contents of 5%
and 7% are Na-Ca feldspars, kyanite, and Quartz with a small amounts of enstatite [Mg2(Si2O6)] and sillimanite [(Al1.98Fe0.2)SiO5]. As the CaO contents increase from 2.75% to 7%, the intensity of XRD peaks of Na-Ca feldspars increases while the peaks of quartz and kyanite crystals tend to decrease and the amorphous phases increase in the ceramsite. As the total amount of quartz decreases, Na-Ca feldspars derived from kyanite dissolution either remain in glassy phase or are used to nucleate other crystals in ceramsite [35]. Better crystallization produces better strength and chemical durability [45], which implies that the compressive strength of ceramsite may decrease as CaO contents increase from 5% to 7%.
3.2.4. Morphological structures (SEM) analyses
It is observed that the ceramsite with Fe2O3 contents of 5% presents rough and densified surface and has some irregularly distributed approximately slit-shaped pores. It can be explained that the pores form as the residual glassy phase viscosity falls to a level when gas-forming inorganic decomposition reactions can produce the observed pores [44]. Ceramsite with Fe2O3 contents of 6% and 8% present rough surfaces with a few small pores. These are believed to be a result of the softening of the glassy phase present in the ceramsite, along with a simultaneous incomplete release of gas at 1000
℃
. Comparing the images in Figure 9 (A1-A3) reveals that the higher the Fe2O3 contents, the smaller and fewer the pores. The SEM observations for the 3 ceramsite are in general agreement with the trends shown by the physical data (Figure 6A). The above results indicate that ceramsite has better crystallization and melting of the bodies as 6%≤Fe2O3≤8%.As shown in Figure 9B1, ceramsite with CaO contents of 2.75% presents rough and densified surface and has a few irregularly distributed pores. Crystals with many-sided morphology and a few small pores are observed on the surface of ceramsite with CaO contents of 5%. For ceramsite with CaO contents of 2.75% and 5%, melting phenomena are observed on both of the crystalline surfaces. SEM image of ceramsite with CaO contents of 7% suggests that the majority of the coarse and fine minerals are located in close contact with the silicates matrix. The relative higher alkaline earth oxide content (CaO), present in the ceramsite will act as a fluxing agent during the sintering process [46, 47]. Besides better porosity, ceramsite surfaces are glossier when densification is more completed at lower CaO contents. This might partly explain the differences in compressive strength for the 3 samples because of ceramsite with denser surface generally have better strength. The differences in the microstructures (Figure 9B1, 9B2, and 9B3) are also suggested to be the reasons for the differences in their water absorption, because the size and quantity of pores are the chief determinants of water absorption.
S
Figure 8. Effect of oxide contents on the XRD patterns of ceramsite. Band labeling: A, albite-anorthite;
E, enstatite; F, ferrosilite magnesian [(Fe,Mg)SiO3]; K, kyanite; Q, quartz; S, sillimanite [(Al1.98Fe0.2)SiO5].
Figure 9. Effect of oxide contents on the morphological structures of ceramsite (A-Fe2O3 and B-CaO).
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Figure 10. Effect of oxides contents on the compressive strength of ceramsite (A-Fe2O3 and B-CaO).
3.2.5. Compressive strength analyses
The compressive strength of ceramsite with Fe2O3 contents of 5%, 6%, and 8% are 14.97MPa, 15.14MPa, and 15.67MPa, respectively (Figure 10A). Ceramsite with Fe2O3
contents of 5% has irregular-shaped elongated pores that decrease the compressive strength.
Because of its porous structure, which is caused the foaming reactions and the tiny pores produced during sintering, the resultant specimens have worse compressive strength property.
In contrast, ceramsite with a few pores formed at higher Fe2O3 contents lead to relatively higher compressive strength. The difference in crystalline phases may also explain the higher gain in compressive strength for ceramsite with 8% Fe2O3 as compared to ceramsite with 5%
and 6% Fe2O3. The Fe3+ may act as Al3+, replacing the Si4+ parent ions of silicates to be enclosed in the framework of silicates and lower the body strength.
It can be seen from Figure 10B that the compressive strength of ceramsite with CaO contents of 2.75%, 5%, and 7% are 15.13MPa, 14.26MPa, and 13.13MPa, respectively. Ca2+
may act as metal ion ionically bonded in the interstices of the silicate network to produce electrical neutrality that was broken due to the substitution of Si4+ by Al3+ [39]. Increasing the content of CaO usually makes the crystalline particles easier to form at a given temperature but increases its chemical reactivity in the silicate network. The compressive strength decreases as the content of CaO increase from 2.75% to 7%, which implies that excessive CaO exceeds the needed ions for producing electrical neutrality and lowers the body strength.
Another reason may be that the lower strength is thought to be due to the lower hardness of the resulting crystalline phases [45].
Figure 11. Effect of sintering temperature on the leaching of heavy metals.