Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 122

the sulfur and suppress the solid phase transition from the monoclinic form (S) to
the orthorhombic (S α ) form that occurs in elemental sulfur when cooled below 95°C.
Various modifiers have been investigated including dicyclopentadiene (DCPD) and
other organic oligomers, and silane coupling agents such as vinyltrimethoxysilane
(VTMS) and vinyltriethoxysilane (VTES). The USBM focused on the addition of
DCPD and got favorable results with mixtures of 95 wt% elemental sulfur, but found
that at reaction temperatures (120 to 140°C) the organic reagent was unstable and
depolymerized, resulting in a difficult-to-control exotherm and a viscous product.
This was alleviated by substituting an additive mixture of 2.5 wt% DCPD and 2.5
wt% of other higher molecular weight oligomers (trimer through pentamer), yielding
a stable, low-viscosity mixture that maintains the monoclinic crystalline form upon
cooling. This SPC formulation (also referred to as modified sulfur cement) was
patented and commercially licensed and is currently available from Martin Chemi-
cals (Odessa, TX) and GRC Chempruf Concrete (Clarksville, TN) for approximately
$0.12 per pound. 12
Work conducted at Pacific Northwest National Laboratory (PNNL) revealed an
effect of the thermal history and cooling rate on the crystalline structure, independent
of the chemical modifiers that are added. They claim that the role of the polymer
modification is to assist in the control of the microstructure on cooling, by facilitating
the formation of plate-like microcrystals found in beta sulfur rather than the larger
(millimeter- to centimeter-scale) crystals found in alpha sulfur. They found that very
slow cooling (< 1.5°C/min) resulted in formation of alpha sulfur regardless of the
polymer modification, whereas more rapid cooling (> 1.5°C/min) yielded the more
desirable beta form. The literature does not contain other references to the impacts
of cooling rate on the formation of stable sulfur. The Sulphur Cement Concrete
Design and Construction Manual indicates that SPC concrete forms attain 80% of
maximum strength within 1 day and most sulfur concrete data are reported following
a 1-day cooling time. 13
Concretes for construction applications formulated by the addition of aggregate
materials to SPC have been developed and extensively tested. The addition of high-
quality sand and quartz aggregate yields high-strength concretes that can be used in
a number of applications in place of conventional hydraulic cement concretes, e.g.,
road paving, concrete blocks, walls and floors, support columns, pipes, and sewer
systems. Compressive strengths of 48.2 to 68.9 MPa (7,000 to 10,000 psi) are typical
for sulfur polymer concretes and are attained within hours compared with weeks
for hydraulic portland cement concretes. Typical formulations and mechanical prop-
erties of SPC compared with portland cement concrete are summarized in Table
6.3.1. 14 Sulfur polymer concretes are extremely resistant to harsh chemical environ-
ments and can be used in applications where conventional concrete materials are
subject to degradation. For example, SPC is resistant to corrosive electrolytic solu-
tions, and mineral acids and salts that are found in many industrial applications (e.g.,
electroplating, metallurgical refining, and acid and battery production). In these
cases, precast or custom-poured SPC tanks, slabs, and foundations can provide
improved durability and performance. Figure 6.3.1 is a photograph of SPC and