Trace elements leaching from cement mixtures containing fly ash. Nadya Teutsch and Olga Berlin Geological Survey of Israel

Transcription

1 Trace elements leaching from cement mixtures containing fly ash Nadya Teutsch and Olga Berlin Geological Survey of Israel WEACAU-III: Workshop on Environmental Aspects of Coal Ash Uses 12 December 2012

2 CLSM and Grout Fly ash can be utilized as a pozzolanic filler in two types of cement mixtures used in various sub-surface applications: 1. CLSM (Controlled Low Strength Material) is used to fill cavities (trenches, pits, etc.) as a replacement for soil, fine quarry products or natural sand fillings; similar to concrete composition but with different proportions; presence of coal ash (fine and globular particles) increases workability, improves flow properties and reduces water consumption. 2. Grout is designated to fill cracks in rocks and/or fill the gap between the casted concrete and the surrounding rock. The difference between CLSM and Grout is in the composition mixture, especially the cement content. Regarding coal fly ash - use in CLSM is less common, but the total amount consumed for substructure purposes is larger.

3 Research Goals In infrastructure applications, CLSM and Grout may come into contact with runoff and even groundwater. Therefore, the degree of contaminants leached from cement mixtures containing fly ash that are commonly used in Israel should be examined. Examination of these leachates and evaluation of the expected metal concentrations to be released will allow definition of criteria for fly ash usage in infrastructure applications.

4 Methodology The most contaminated (EN) coal ash was integrated into representative mixtures of CLSM and Grout used in Israel. Cubes of these mixtures were prepared and tested according to procedure EA NEN 7375:2004 the TANK TEST. Contribution of the ash was tested compared with mixtures without ash. Extraction process includes eight stages from six hours to 64 days and was performed on different curing times (7, 28 and 90 days) in duplicates. Trace elements were measured for each stage with ICPMS: Ag, As, B, Ba, Be, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, Th, U, V, Zn.

5 Cubes preparation Cubic blocks of 10 cm 3 of cement mixtures containing Columbian CerD coal fly ash ( ) & reference cubes were manufactured in the Standard Institute of Israel. The cubes were placed under controlled humidity (95 %) and temperature (22 C) conditions for curing periods of 7, 28 and 90 days. These periods were chosen for engineering reasons and are designed to follow concrete strengthening: 28 - typically, strengthening of the concrete is completed after 28 days if strength requirements at 28 cuing days are not satisfactory, the concrete must be tested after 90 days of curing. 7 - minimal curing period that represents immediate use of concrete before its strengthening. After 7 days cubes were transferred to regular room conditions.

6 Cubes composition CLSM - total of 12 cubes: duplicate for each curing periods (7, 28 and 90 days) 1 fly ash cement Sand 2 water total Admixture (20%) kg/m kg/m 3 Grout - total of 12 cubes: duplicate for each curing periods (7, 28 and 90 days) fly ash cement Sand 2 water Total Admixture (44%) An additional empty container with no cube served as procedural blank 2 fine sand 0-4 mm 3 air entrained agent Chemical Composition wt. % P2O5 SO3 SiO2 MnO Fe2O3 MgO Al2O3 CaO TiO2 LOI CLSM no ash < CLSM w/ash < Graut no ash < Graut w/ash

7 Procedure EA NEN 7375:2004 THE TANK TEST Following curing, the cubes were placed in a pre-cleaned (1M HNO 3 ) container on a plastic cylinder base (to ensure maximal water-surface contact). Required amount of water is 2-5 times the sample volume and at least 2 cm coverage on each side. Actual water amount was 3-4 L; no mixing of water. Extraction was conducted in 8 phases (6 hours to 64 days): 1. Ion-free water filling. 2. Emptying the water at the end of the period to another container and refill of the container. 3. Sampling for chemical analysis (0.45 μm filter). 4. Measuring the acidity (ph ± 0.05) and conductivity (± 1%) and emptying the water. Total time from casting to end of test days.

8 Conductivity (µs/cm) Conductivity of CLSM cubes no 7 ash 28 no 28 ash 90 no 90 ash Time (d)

9 Conductivity (µs/cm) Conductivity vs. ph - CLSM no 7 ash 28 no 28 ash 90 no 90 ash ph

10 Analysis of the eluates Ag, As, B, Ba, Be, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Th, U, V, Zn Concentrations of the elements were measured by ICPMS (µg/l). All blank test concentrations were under detection limits. For each extraction period measured and derived cumulative values were calculated. 1. determining the leaching per eluate fraction: A = 0.06 m 2 ; f = 10 6 ; V= volume of eluate 2. determining the measured cumulative leaching: 3. determining the derived cumulative leaching: Establishing whether material has been dissolved not dissolved. Leaching mechanism determination.

11 Reproducibility- Measured cumulative (6h-64d) CLSM cumulative (mg) B V Cr Co Ni Zn As Se Mo Cd Sb Ba #1 7 no #2 7 no #3 7 ash #4 7 ash #6 28 ash #7 28 ash #8 28 no #9 28 no #10 90 no #11 90 no #12 90 ash #13 90 ash Reproducibility B V Cr Co Ni Zn As Se Mo Cd Sb Ba 1/2 7 no /4 7 ash /7 28 ash /9 28 no /11 90 no /13 90 ash

12 Comparison between the 64 d cumulative release (mg/m 2 ) and the Dutch regulatory criteria* CLSM average values V Cr Ni Zn As Se Mo Cd Sb Ba ash ash - no ash ash ash - no ash ash ash - no ash no values: B, Co Graut average values V Cr Ni Zn As Se Mo Cd Sb Ba ash ash - no ash ash ash - no ash ash (#13) ash - no ash days curing - only up to 9 d stage no values: B *regulatory criteria for construction products as defined in the Dutch Soil Quality Decree (SQD, 2007)

13 leaching (mg/m2) leaching (mg/m2) leaching (mg/m 2 ) Graut 90d curing water replacement (up to 9d) Mo 4.0 Se Leaching time (days) Leaching time (days) Ba Leaching time (days)

14 Leaching mechanism determination of CLSM 10 7 days curing Se days curing days curing 1 Se Y derived Se Y measured

15 10 1 Leaching mechanism determination of Se - CLSM 7 days curing 7d CFa-b rc Sdrc Increment Increment Increment Increment Increment Increment Order Eluate fraction Increment a-b 1 Fractions 2 to 7 Increment 2-7 incl 2 Fractions 5 to 8 Increment 5-8 incl 3 Fractions 4 to 7 Increment 4-7 incl 4 Fractions 3 to 6 Increment 3-6 incl 5 Fractions 2 to 5 Increment 2-5 incl 6 Fractions 1 to 4 Increment 1-4 incl days curing 28 d CFa-b rc Sdrc Increment Increment Increment Increment Increment Increment CF > 1.5 Linear regression of log εn - log ti determine the slope rc & the sdrc days curing 1 Se Y derived Se Y measured d CFa-b rc Sdrc Increment Increment Increment Increment Increment Increment Depletion Surface wash-off Diffusion Dissolution Delayed diffusion or dissolution

16 Summary and Queries Trace elements leaching from cement mixtures of CLSM and Graut containing fly ash and reference materials were conducted to cubic blocks left for three different curing periods (90 d curing Graut has to be completed). Except for Selenium all cumulative values (mg/m 2 ) were below Dutch regulation criteria. Issues regarding the leaching process and establishing the leaching mechanism. Curing periods and conditions. Regulation criteria choice (Dutch). The nature of the derived cumulative leaching term Ԑ n *. Leaching mechanism determination according to the slope rc value.

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