# Effect of Incorporating Nanoporous Metal Phosphate Materials on the Compressive Strength of Portland Cement

Authors: Dawn M. Wellman; Kent E. Parker; Shas V. Mattigod; Glen E. Fryxell
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

Nanoporous metal phosphate (NP-MPO) materials are being developed for removal of contaminant oxyanions (As(OH)O32−, CrO42−, and TcO4−), and cations (mercury, cadmium, and lead) from water and waste streams.  Following sequestration, incorporation of metal laden NP-MPOs as a portion of cement formulation would provide an efficient and low-cost way to immobilize metal laden NP-MPOs in an easily handled waste form suitable for permanent disposal.  There are no known investigations regarding the incorporation of NP-MPOs in concrete and the effects imparted on the physical and mechanical properties of concrete.  Results of this investigation demonstrated that incorporating of NP-MPO materials requires additional water in the concrete formulation which decreases the compressive strength.  Thus, incorporation of NP-MPOs in concrete may not serve as an efficient means for long-term disposal.


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## Body

## 1. Introduction

Over the last decade, metal phosphate materials
have been studied extensively for their potential use in catalysis, ion exchange,
and phase separation [1, 2]. In contrast to silica-based materials,
functionality of metal phosphates is inherently present, and recent
advancements enabling control of the redox state of the metal center allow the reductive
capacity of the metal phosphate material to be controlled for redox sensitive
ion sequestration [3]. Nanoporous metal phosphate (NP-MPO) materials
are being developed for removal of contaminant oxyanions (As(OH)O32−, CrO42−, and TcO4−) [3] and cations (mercury, cadmium,
and lead), respectively, from water and various waste streams. Nanoporous materials offer an efficient way
of selectively sequestering many types of metals and radionuclides in a very small
volume [4–10]. The high surface area of a nanoporous
structure enhances mass transfer in sequestration applications and enables ions
to be highly concentrated in a very small volume of material. Following sequestration,
incorporation of
contaminant laden NP-MPOs as a portion of cement formulation would provide an
efficient, low-cost method to immobilize waste NP-MPOs in situ and would
contain it in an easily handled waste form suitable for permanent disposal.Concrete stabilization is a
process of mixing waste with a binder to reduce the contaminant leachability by
both physical and chemical means and to convert hazardous waste into
environmentally acceptable waste forms for disposal [11]. Cement is widely used for stabilization of
hazardous, radioactive, and mixed wastes [12] which serves to contain and
isolate the waste from the hydrologic environment and to act as an intrusion barrier. Any failure
of concrete stabilization may result in water intrusion and consequent
mobilization of contaminants from the waste packages by mass flow and/or
diffusion and may move into the surrounding subsurface environment.Portland cement is the most
commonly utilized cement because of its availability and low cost. Additional materials such as fly ash, natural
pozzolans, and blast furnace slag often supplement the cement matrix which
reduces cost and improves the performance of the concrete. Pozzolanic and mineral admixtures have
previously been studied with regards to the effects imparted on the physical
and mechanical properties of concrete [13, 14]. However, incorporating advanced materials
such as nanoporous metal phosphate materials in concrete and their effect on
the chemical, physical, and mechanical properties of concrete waste forms have not been
investigated.Compressive strength is
generally accepted as a principal measure of the quality of concrete, with
approximate relations between compressive strength and other mechanical
properties of concrete [15]. The
objective of this investigation was to evaluate mixture proportions for
incorporation of NP-MPOs in Portland cement and to quantify its effects on compressive strength of
the resulting concrete monolith.


## 2. Materials and Methods

The concrete composition used for this study was based on
the mixture composition specified for encasing low-level radioactive waste [15]. This
specification was used as the basis to
prepare a concrete for fabrication of test specimens. The specified composition includes
sulfate-resistant Portland Type I or Type II cement, a pozzolanic material
(Class F fly ash), fine and coarse aggregates, and steel fiber. Table 1 (column 3) presents the mixture composition specified
for encasing low-level radioactive waste [15]. Table 1 (column 4) presents the mass normalized composition
for encasing low-level radioactive waste; Table 1 (column 6) provides the mass normalized composition
for the laboratory concrete composition.Table 1
Material specifications and composition.
MaterialSpecificationsSpecified field compositionMass-normalized field specified compositionMaterial specifications used in revised laboratory mix comparisonMass-normalized laboratory compositionCementPortland Type I or Type I/II
sulfate-resistant cement381 kg/m30.27Portland  Type I and II0.27Fly AshClass F fly ash; nominal 15%
of cement by volume54 kg/m30.04Class F fly ash; nominal 20%
of cement by volume0.04Coarse aggregateno. 676 or equivalent (3/4”
nominal size)55% by volume0.04——Fine AggregateSand45% by volume0.51<2 mm sand0.51WaterNominal water:cement ratio:
0.4399 kg/m30.10Water-to-cement ratio: 0.50.13Steel FiberDeformed Type I, nominal length 2.5–3.8 cm
(1–1.5”)59 kg/m30.04Iron Particles, −200 mesh0.04Air Content—6.0±1.5%——6.0±1.5%Polyheed 997———Water-entraining agent0.007A laboratory-scale concrete mixture was prepared based on field-scale
specifications to prepare concrete for waste immobilization (Table1). Because of
the required small dimensions of laboratory test specimens, iron particles (−40
to +60 mesh) were used as the scaled-down steel fibers.  A particle size < 2 mm was used as the
aggregate in the laboratory composition.  Based on these modifications, a concrete mix
was prepared that consisted of Portland cement (Type I and II, ASTM C-150
compliant), Class F fly ash, scaled-down coarse aggregate, fine aggregate, iron
particles, and a water-entraining agent (Polyheed 997). The water-entraining
agent was included in the mix to facilitate the workability of the concrete.Concrete monoliths were prepared
with components added in the order: cement, fly ash, coarse aggregate, steel (if
applicable), NP-MPO (if applicable), Polyheed 997, MB AE 90, and water (Table1).  The concrete
was mixed in polypropylene containers for five minutes to ensure homogeneity of
the mixture. Polycarbonate bottles were used as the concrete molds for casting specimens. The molds were filled in the vertical
positions. After filling, the molds were
vibrated and lightly tapped on the laboratory bench until a significant
decrease in the release of air bubbles was observed. The forms were stored in a humidity chamber
for 5 days at which time the concrete monoliths were removed from the
molds. The monoliths were subsequently
stored in the humidity chamber for 23 days for a total of 28 days, as specified
in ASTM standard C109/C109M-02 [16].  Compression testing was conducted in
accordance with ASTM procedure C109/C109M-02 using a Carver Laboratory Press (Model M).


## 3. Results and Discussion

Table2 presents the mass normalized composition and physical
characteristics of the concrete specimens; Table 3 provides the calculated water/cement ratio and
calculated compressive strength of the concrete monoliths. The water to cement ratio is the mass-based
ratio of water to cement within the concrete composition. The compressive strength was calculated per ASTM procedure C109/C109M-02.
The experimentally measured force (lbs) necessary to produce structural
failure of the concrete specimens was used to calculate the respective force
per unit surface area (N/m2) (Table 3).Table 2
Composition and physical
characteristics of concrete specimens used in compression tests.
MaterialMass-normalized laboratory compositionMass-normalized composition: Core 1Mass-normalized composition: Core 2Mass-normalized composition: Core 3Mass-normalized composition: Core 4Mass Normalized Composition: Core 5Cement0.270.290.290.280.280.27Fly ash0.040.040.040.040.040.04Fine aggregate0.510.470.510.460.410.36Water0.100.140.140.170.190.22Iron particles0.040.04————NP-MPO———0.040.080.11Polyheed 9970.0070.0070.0070.0070.0070.007Air Content6.0±1.5%6.0±1.5%6.0±1.5%6.0±1.5%6.0±1.5%6.0±1.5%NP-MPO0.000.000.000.040.080.12Physical characteristicsLength, cm—4.174.354.414.624.62Diameter, cm—2.562.582.562.572.58Surface area, cm2—42.7144.2544.6046.1246.32Volume, cm3—20.5921.5721.7822.7022.86Density, g/cm3—2.212.082.062.021.99Table 3
Compressive strength of concrete specimens.
Core no.Calculated water/cement ratioForce (Newtons)Measured compressive strength (N/m2)(1)0.48204613.97×107(2)0.48111202.15×107(3)0.60124542.41×107(4)0.6875621.46×107(5)0.8144488.55×106The contribution of steel or iron particles on the durability of concrete has long been practiced.
The use of steel fibers increases the density and energy absorption capacity and
reduces cracking by bridging microcracks within the concrete matrix among
others [17–20]. Results
presented in this investigation indicate that incorporating iron particles into the
concrete matrix increases the bulk density by ∼6%. Substituting NP-MPOs into
the concrete matrix in place of iron particles decreases the bulk density of
the concrete monolith by ∼1.5% for every 4% increase in NP-MPO.Figure1 illustrates the effect of incorporating nanoporous
metal phosphate materials in Portland cement on the compressive strength. The results illustrate the inclusion of iron
particles within the concrete monolith which affords a composition with the greatest
compressive strength. Based on the water/cement ratio of the iron containing monolith, the calculated compressive
strength is 0.7 times greater than standard concrete [21]. Removing iron
particles from the concrete matrix decreased the compressive strength of the
test core ∼45%. Substituting 4% NP-MPOs
into the concrete matrix in place of the iron particles afforded a 40% decrease
in the compressive strength relative to the monolith containing iron. However, approximately a 10% increase in
compressive strength was measured for the concrete monolith containing 4%
NP-MPO relative to the concrete monolith without iron or NP-MPO. Further increasing the content of NP-MPOs in
the concrete matrix significantly decreased the compressive strength of concrete.Figure 1
Compressive strength of Portland cement with and without iron and/or nanoporous metal phosphate
materials as a function of water/cement ratio.NP-MPOs are synthesized via
surfactant-templated chemistry. In order
to remove the surfactant from the porous structure, NP-MPOs were calcined at∼500°C. As a result, the materials are extremely dry, absorb a significant
amount of water, and incorporation of them into the concrete matrix requires
additional water to be added to the composition (Table 2). Thus, by
increasing the proportion of NP-MPO incorporated into the concrete, the
water/cement ratio of the concrete increases, which decreases the compressive
strength of the concrete.


## 4. Conclusion

Incorporating metal laden
NP-MPOs as a portion of cement formulation has been proposed as an efficient,
low-cost, and low-temperature way to immobilize metal laden waste in situ and to contain it in an easily handled waste form
suitable for permanent disposal. This
investigation evaluated the effect of incorporating NP-MPOs in Portland cement
formulations on the compressive strength of the concrete waste form. The results demonstrate the exceptionally dry
nature of NP-MPO materials which
requires the addition of extra water to the overall concrete formulation. This, in turn, increases the water/cement
ratio and results in a decrease in the compressive strength of the concrete
waste form. Thus, the incorporation of
NP-MPOs in concrete may not serve as an efficient means for long-term
disposal. In order for concrete
stabilization to serve as a means for the long-term disposal of contaminant
laden NP-MPOs, further investigation would be necessary to determine the
optimal composition for incorporation.



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*Source: 238736-2008-03-16.xml*