EFFETS DE L’INCORPORATION D’ADJUVANTS MINÉRAUX SUR LES PROPRIÉTÉS DE CIMENTS GÉOPOLYMÈRES A BASE DE SCORIES VOLCANIQUES

Volcanic ashes are raw materials from geological deposits with a range of chemical compositions. When combined with suitable alkali activators they can be converted to geopolymers cement at ambient temperature. In this work we have investigated the possibility of use bauxite and oyster shells as mineral admixture in volcanic ashes, to enhance the properties of geopolymers synthesized. Different methods of analyses such as Fourier Transform Infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and Scanning Electron Microscopy (SEM) were used to assess the variation of setting time, linear shrinkage and 28 days compressive strength of geopolymers paste. The bauxite and the oyster shells were characterized using inductively coupled plasma (ICP-AES), thermal analyses (DSC/ATG), FTIR and X-ray diffractometry. The results of these analyses has showed that bauxite and oyster shells are respectively source of Al2O3 and of CaO, and can compensate the deficiencies of these oxides in volcanic ashes. Adding mineral admixture dissolve slowly in high alkaline medium. Addition of about 20% of bauxite or 10% of oyster shells is seen to decrease the setting time respectively from 415 to 275 min and 195 min. Linear shrinkage decrease with percentage of bauxite or of oyster shells added. Efflorescence is reduced by adding 10% of bauxite. 28 days compressive strength of geopolymeric materials increase respectively for 4.77 and 7.52% for 10% of bauxite or 20% of oyster shells added. More than these percentage additive has a deleterious effect on compressive strength due to crystalized mineralogical phases of the admixture.

Elaboration et caractérisation de ciments géopolymères à base de scories volcaniques

Two types of volcanic ashes referenced as ZD (volcanic ashes from Djoungo) and ZG (volcanic ashes from Galim) were collected from two Cameroonian localities. They were characterized (chemical and mineralogical compositions, amorphous phase content, particle size distribution and specific surface area) and used as raw materials for the synthesis of geopolymer cements at ambient temperature of laboratory (24 ± 3 °C). The synthesized products were characterized by determining their setting time, linear shrinkage and compressive strength, X-ray Diffraction, Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy. The mineralogical composition, the amorphous phase content, the particle size distribution, the specific surface area of the volcanic ashes as well as the mass ratio of the alkaline solution (sodium silicate / sodium hydroxide) were the main parameters which influenced the synthesis of geopolymers with interesting characteristics at ambient temperature (24 ± 3 °C). The volcanic ashes (ZD) whose mineralogical composition contained anhydrite, low amorphous phase content and low specific surface area led to long setting times. Moreover, its products swelled and presented cracks due to the formation of ettringite and these resulted in low compressive strengths (7 to 19 MPa). The volcanic ashes (ZG) containing high amounts of amorphous phase and high specific surface area led geopolymers with setting times between 490 and 180 minutes and compressive strength between 7 and 50 MPa at ambient temperature of laboratory. The properties of geopolymers were improved when elaborated with a mixture of volcanic ashes and metakaolin (ZD–MK and ZG–MK). For geopolymers obtained from ZD–MK, the setting time was between 500 and 160 minutes while it was between 220 and 125 minutes for geopolymers obtained from ZG–MK. The compressive strength greatly increased between 23 and 68 MPa and 39 and 64 MPa for geopolymers obtained from ZG –MK and ZD–MK respectively. A comparative study of the properties of mixtures of metakaolin–alumina and volcanic ash–alumina based geopolymers shows that metakaolin is a good source of Al2O3 and SiO2 and which highly reactive with alkaline solution and produces geopolymers with better characteristics compared to volcanic ash based–geopolymer. The properties of volcanic ash–based geopolymer were also improved when amorphous alumina was incorporated into the volcanic ash. This additive is used to compensate the deficiencies in Al2O3 content in the volcanic ash. Compare to when volcanic ash is used alone 20 to 40 % incorporation of this additive corresponded to increases of the compressive strength by 18.1 % for metakaolin-alumina based-geopolymers and by 32.4 % for volcanic ash-based geopolymers.

DEVELOPMENT OF POROUS CONCRETE GEOPOLYMER USING HYDROGEN PEROXIDE AS BLOWING AGENT.

Metakaolin and volcanic ashes respectively called MK and ZG were used as aluminosilicate raw materials for the synthesis of porous geopolymers. The hydrogen peroxide was used as a blowing agent. The geopolymer prepared were characterized by Spectroscopy Fourier Transform Infrared (SFTI), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and the determination of some physical properties such as bulk density, total porosity, volume shrinkage, compressive strength and thermal conductivity was also carried out. The presence of pores in the geopolymer obtained is function of the percentage of hydrogen peroxide added, causing the concomitantly decrease in compressive strength, apparent density and thermal conductivity. The use of hydrogen peroxide as a blowing agent gave some thermal insulation properties to these geopolymers which could be close to the properties of some commercial insulation materials.

Characterization of a Coal Fly Ash-Cement Slurry by the Absolute Foam Index

This dissertation established a standard foam index: the absolute foam index test. This test characterized a wide range of coal fly ash by the absolute volume of air-entraining admixture (AEA) necessary to produce a 15-second metastable foam in a coal fly ash-cement slurry in a specified time. The absolute foam index test was used to characterize fly ash samples having loss on ignition (LOI) values that ranged from 0.17 to 23.3 %wt. The absolute foam index characterized the fly ash samples by absolute volume of AEA, defined as the amount of undiluted AEA solution added to obtain a 15-minute endpoint signified by 15-second metastable foam. Results were compared from several foam index test time trials that used different initial test concentrations to reach termination at selected times. Based on the coefficient of variation (CV), a 15-minute endpoint, with limits of 12 to 18 minutes was chosen. Various initial test concentrations were used to accomplish consistent contact times and concentration gradients for the 15-minute test endpoint for the fly ash samples. A set of four standard concentrations for the absolute foam index test were defined by regression analyses and a procedure simplifying the test process. The set of standard concentrations for the absolute foam index test was determined by analyzing experimental results of 80 tests on coal fly ashes with loss on ignition (LOI) values ranging from 0.39 to 23.3 wt.%. A regression analysis informed selection of four concentrations (2, 6, 10, and 15 vol.% AEA) that are expected to accommodate fly ashes with 0.39 to 23.3 wt.% LOI, depending on the AEA type. Higher concentrations should be used for high-LOI fly ash when necessary. A procedure developed using these standard concentrations is expected to require only 1-3 trials to meet specified endpoint criteria for most fly ashes. The AEA solution concentration that achieved the metastable foam in the foam index test was compared to the AEA equilibrium concentration obtained from the direct adsorption isotherm test with the same fly ash. The results showed that the AEA concentration that satisfied the absolute foam index test was much less than the equilibrium concentration. This indicated that the absolute foam index test was not at or near equilibrium. Rather, it was a dynamic test where the time of the test played an important role in the results. Even though the absolute foam index was not an equilibrium condition, a correlation was made between the absolute foam index and adsorption isotherms. Equilibrium isotherm equations obtained from direct isotherm tests were used to calculate the equilibrium concentrations and capacities of fly ash from 0.17 to 10.5% LOI. The results showed that the calculated fly ash capacity was much less than capacities obtained from isotherm tests that were conducted with higher initial concentrations. This indicated that the absolute foam index was not equilibrium. Rather, the test is dynamic where the time of the test played an important role in the results. Even though the absolute foam index was not an equilibrium condition, a correlation was made between the absolute foam index and adsorption isotherms for fly ash of 0.17 to 10.5% LOI. Several batches of mortars were mixed for the same fly ash type increasing only the AEA concentration (dosage) in each subsequent batch. Mortar air test results for each batch showed for each increase in AEA concentration, air contents increased until a point where the next increase in AEA concentration resulted in no increase in air content. This was maximum air content that could be achieved by the particular mortar system; the system reached its air capacity at the saturation limit. This concentration of AEA was compared to the critical micelle concentration (CMC) for the AEA and the absolute foam index.

Current Regulating and Supporting Services: Nutrient Cycles

The study forest regulates nutrient cycles as a supporting ecosystem service mainly via retention in the biosphere and the soil organic layer. How tight the nutrient cycles are depends on environmental conditions. In this chapter, we focus on the roles of (1) deposition from the atmosphere, (2) soil moisture regime, and (3) conversion to pasture in the nutrient cycle. Between 1998 and 2010, there were a seasonal deposition of salpetric acid, an episodic deposition of Ca and Mg from Sahara dusts, and a continuous increase in reactive N inputs related to Amazonian forest fires, the El Niño Southern Oscillation cycle, and the economic development, respectively. Simultaneously, soils became increasingly drier enhancing nutrient release by mineralization. An increasing number of rain storms could considerably increase the export of N and base metals (K, Ca, Mg) via fast surface-near lateral transport in soil. Land-use change from forest to pasture introduces alkaline ashes and grass-derived organic matter. The resulting increases in soil pH and nutrient and substrate supply increase nutrient cycling rates because of enhanced microbial activity.

Landscape Evolution of the Dry Valleys, Transantarctic Mountains: Tectonic Implications

There are different views about the amount and timing of surface uplift in the Transantarctic Mountains and the geophysical mechanisms involved. Our new interpretation of the landscape evolution and tectonic history of the Dry Valleys area of the Transantarctic Mountains is based on geomorphic mapping of an area of 10,000 km(2). The landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution. Following a lowering of base level about 55 m.y. ago, there was a phase of rapid denudation associated with planation and escarpment retreat, probably under semiarid conditions. Eventually, downcutting by rivers, aided in places by glaciers, graded valleys to near present sea level. The main valleys were flooded by the sea in the Miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast. There has been remarkably little landform change under the stable, cold, polar conditions of the last 15 m.y. It is difficult to explain the Sirius Group deposits, which occur at high elevations in the area, if they are Pliocene in age. Overall, denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland, which is in good agreement with the results of existing apatite fission track analyses. It is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy. Most crustal uplift (2-4 km) is inferred to have occurred in the early Cenozoic with 400 m of subsidence in the Miocene followed by 300 m of uplift in the Pliocene.

(Table 1) Stratigraphic position, abundance, shape, color, and refractive indices of glass shards of selected samples from DSDP Leg 60 Holes vitric muds

Precisely determined refractive indices of glass shards from 32 ash-rich, volcaniclastic sediments, mostly turbidites interbedded with nonvolcanic sediments in the Mariana Trough, range from 1.480 to 1.585 (corresponding to SiO2 ca. 75 to 49%), with most in the range 1.500 to 1.540 (SiO2 ca. 70-62%) and a second, smaller mode between ca. 1.560 and 1.585 (57 to 49% SiO2). Shards are almost exclusively colorless from 1.480 to ca. 1.530, light brown with minor colorless and green tones between 1.530 and 1.560, and dominantly brown at higher refractive indices. Tubular pumice shards are more common at higher silica percentages and non- to poorly-vesicular cuniform shards at low SiO2 values, but there is no clear correlation between shape and composition of shards. About half of the samples have bimodal shard populations with silica differences ranging up to 20 percent; unimodal layers have a range of up to about 7 percent SiO2. Of 21 samples in which one type of shard dominates, seven have the main mode in the rhyolitic composition (>69% SiO2), eight in the intermediate range (56 to 69% SiO2), and five in mafic composition (SiO2 <53%). These unusually abundant mafic shards occur mainly in site survey piston cores, SP-IA and 4E, and in Holes 454, 456, 458, and 459B. These are the sites closest to the present arc. Hole 453, containing by far the most vitric tuff turbidites, shows a gradual increase in silica content of ash layers upward to the hole from Cores 36 to 19 (about 4.6 to 3.0 Ma). A drastic decrease in ash-rich beds in the younger (Pleistocene) part of this hole was noted by the shipboard party (see site chapter, Site 453) and was interpreted by them as indicating increasing distance from the arc volcanoes as the trough opened. The increase in silica in ashes from the early to the late Pliocene at Site 453 could be interpreted in the same way and might indicate that the trough started to open in early Pliocene time.

(Table 1) Geochemistry of ash layers at DSDP Leg 57 Holes

The study of the main characteristics of ash layers in Leg 57 cores shows that they are suitable for an analysis of the effect on eruptive activity of their distribution. We found (1) sediment recovery good and ash layers numerous; (2) sedimentary environment generally free from terrigenous clastic material; (3) reworking limited; (4) volcanic glass very acidic, ranging from rhyolitic to rhyodacitic composition; and (5) alteration and diagenesis negligible above the lower Miocene. The curves of explosive volcanic activity in Holes 438, 439, and 440 display two stages of high activity: an early one around 16 m.y. and a late one starting 5 m.y. B.P., both stages being separated by an upper Miocene quiescence. Detail in these results is limited by the chemical composition of the glass and accounts only for trends in explosive acid volcanism. Nevertheless, results are roughly in agreement with other data from the Northwest Pacific, although some discrepancies in the correlation of intensity of the episodes occur. The data from Leg 57 support the hypothesis of synchronous pulses in explosive volcanism.

Chemistry of particles from the Toba ash layer

Two cores from the southern South China Sea contain discrete ash layers that mainly consist of rhyolithic glass shards. On the basis of the SPECMAP time scale, the ash layers were dated to ca. 74 ka, the age of the youngest Toba eruption in northern Sumatra. This link is supported by the chemical composition of the glass, which is distinct from volcanic glass supplied from the Philippines and the northern South China Sea, but is almost identical with the chemistry of the Toba ash. The youngest Toba ash layers in the South China Sea expand the previously known ash-fall zone over more than 1800 km to the east. The dispersal of ashes from Sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that (1) the Toba eruption probably happened during the Southeast Asian summer monsoon season, and (2) the volume of erupted magma was larger than previously interpreted.