(c) w/s = 0.60

1
A new cement system for nuclear waste
immobilisation calcium sulfoaluminate cement
Q. Zhou and N B Milestone Immobilisation Science
Laboratory, Department of Engineering Materials,
University of Sheffield, UK M. Hayes Nuclear
Science and Technology Services, UK

• Figure 2 compares the two systems at 1, 3, and 7
  days. The Rockfast
• system showed much less corrosion of Al metal.
  The low pH value of
• the system about 10.5 probably plays an important
  role but the amount
• of free water available is also important. The
  reasons for colour change
• of both systems are not clear but could be due to
  oxidation and/or
• carbonation after being broken. Corrosion
  products of BFS/OPC system
• include bayerite AH3, stratlingite C2ASH8,
  gehlenite C2AS and
• monosulfateC3A.CS.12H2O2. The amount of corrosion
  products in the
• Rockfast system has too small to carry out XRD
  analysis, but it will be
• investigated further.
• 4. Conclusions
• Ettringite is the main hydration product of CSA
  cement. By adjusting the amount of sulfate, w/s
  ratio, the mixing properties can be controlled.
  Depending on the amount of sulfate added,
  different reactions occur as follows
• If there is sufficient CS, reaction (1) and
  (2) occurs,
• C4A3S 2CS 38H C6AS3H32 2AH3
  (1)
• 3CA 3CS 38H C6AS3H32 2AH3
  (2)
• If there is insufficient CS, a third reaction
  (3) occurs
• C4A3S 18H C4ASH12 2AH3
  (3)
• A previous study indicated that if excess lime
  is present, a fourth reaction (4) can occur
  giving a higher yield of ettringite, which
  normally causes expansion

• 1. Introduction
• Composite cement systems of OPC with BFS or PFA
  are widely used in current operations of nuclear
  waste immobilisation. However, these systems
  have several disadvantages, notably their high pH
  cause the corrosion of reactive metals leading to
  generation of hydrogen gas and possible
  expansion. There are several other cementing
  systems which have different chemistries that may
  provide some advantages for particular wastes.
• Calcium Sulfoaluminate (CSA) cement is one such
  new cement. It has been extensively developed and
  used as a construction cement in China for about
  30 years1. The main properties include
• Reduced energy consumption
• Low CO2 emission during manufacture
• Low alkalinity pH 10 11
• High early strength development
• Shrinkage compensation for OPC
• Very good durability under normal service
  conditions excellent
• durability for marine construction
• 2. Materials
• A commercially available CSA cement, Rockfast
  450, was provided by Lafarge Cement UK. The
  specific surface area is 450m2/kg with specific
  gravity 2.7g/cm3. The chemical and compound
  composition of clinker is shown in Table 1. It is
  a C3S and C2S free product.
• Anhydrite was supplied by Chance Hunt Ltd. UK
  with CaSO4 content gt93.6(by weight)
• Table 1 Chemical and compound composition of
  Rockfast clinker ( by weight)

(c) w/s 0.60 The fluidity
increased with decreasing amounts of anhydrite
and increased with w/s ratio. The temperatures
increased with the mixing time. The more rapidly
the fluidity dropped, the quicker the temperature
increased. pH values of all mixes over the
different time were between 10.5 and 11. Bleeding
were low, in the range of 0.03 to 0.17 ml/100 ml
grout. X-ray diffraction analysis XRD was used
to identify the phases after hydration. The main
hydration product is ettringite, its formation
increased with the hydration time, while
anhydrite decreased. Gehlenite present in the
clinker is inert during the hydration up to 28
days. C4A3S was identified when a high ratio of
Rockfast was used, R/A 80/20, otherwise it
tended to all react. Gibbsite tends to
crystallise when hydration occurs at 60oC.
Monosulfate forms when insufficient sulfate is
present. Al corrosion trials The corrosion
behaviour of BFS/OPC and CSA/anhydrite cements
was compared by running parallel experiments of
two systems embedded with Al rods (99.5 in
purity).
Chemical composition Chemical composition Compound composition Compound composition
SiO2 3.6 C4A3S 57
Al2O3 47.4 CA 17
Fe2O3 1.4 C12A7 1
CaO 38.0 CS 0.5
MgO 0.3 C2AS 16
SO3 7.5 C4AF 4
TiO2 2.2 CT 4
K2O 0.16 CaO 0.3
LOI 0.3 C3S 0
Free lime 0.3 C2S 0
3. Results and Discussion Mixing
properties According to theoretical calculations,
to complete the hydration of Rockfast, 31.2Wt.
anhydrite is needed at w/s ratio of 0.67.
Rockfast was mixed with various ratios of
anhydrite 65/35, 70/30,75/25 and 80/20. The w/s
ratios were 0.75, 0.67 and0.60. The fluidity of
mixings, measured by a Colflow apparatus, is
shown in Figure 1

Figure 2. Visual observation of Al corrosion in
two systems BFS/OPC90/10, w/s0.33, R/A75/25,
w/s0.67.