Influence of carbonate on cement
Introduction
Scawtite (Ca7(Si6O18)(CO3).2H2O) is a calcium-silicate-carbonate-hydrate that forms
sporadically in calcium silicate or cement systems when exposed to carbonate and
temperature. Pure scawtite contains 5.80% CO2 (Luke et al.) or 5.28% CO2 (Kalousek
and Nelson). In other words, 1% CO2 corresponds to 19% scawtite and as such small
amounts of CO2, i.e. 1 to 2% will yield large amounts of scawtite. Publications that
mention the formation of scawtite or carbonate phases formed due to carbonation are
listed in the reference section. Table 1, gives a general synopsis of the testing parameters,
systems and stability temperatures at which scawtite was observed as well as CO 2
concentrations and performance characteristics.
Table 1. Conditions observed for scawtite formation and performance criteria if noted.
Ref
Harker
Kalousek
and Nelson
Cement
Temp
(°C)
CO2
(%)
CaO-SiO2-H2O-CO2
140 - 300
175 - 300
stoichiometric
0.34-0.37
1.32 – 1.74
350
0.9 – 2.0
Good performance
DATA PROVIDED
Trace – enhance
properties
Major
–
poor
performance
DATA PROVIDED
240
Class G
240
Trace
Portland Cement
or
Class J
215-315
300
CaO-MgO-SiO2-H2O
(Class J + chrysotile)
Eilers et al.
None determined
None determined
Class J
Class J
Roy et al.
Taylor
(Review)
Gallus et al.
Performance
Field of stability
Negligible
Scawtite formed
in equilibrium
with gyrolite or
truscottite
Increases with
CO2 content
Trace amounts
scawtite
Trace to major
amounts of a
carbonate phase
Presence
observed
Minor (10-30%)
C2S-S
Luke et al.
Scawtite
Formation
350
Up to 350
Minor
amounts
< 10%
(carbonate)
Trace - major
None determined
None determined
(Implication that
trace amounts of
scawtite gives
improved strength)
None determined
Compressive
strength low but
satisfactory and
decreased over
time. Permeability
satisfactory.
The data indicate that scawtite can form in the temperature range from 140-315°C at
concentrations from 0.9 to <10% CO2 from trace to major amounts and that it can to
some extent impact the compressive strength an permeability. The data is specific to the
various testing parameters and it is not clear as to where the limits are in terms of
carbonate addition, temperature and poor performance.
Synopsis of papers relating to the effect of carbonate on cement and scawtite
formation:
Harker investigated the parameters necessary for formation of scawtite using reagent
grade chemicals and found it to form at temperatures between 140 and 300ºC. The study
suggests that at lower temperatures calcite reacts with tobermorite to form scawtite and
with increasing temperature, the scawtite then reacts to give xonotlite and calcite. Eilers
et al. indicates that scawtite, can be formed at temperatures between 215 and 315ºC on
adding sodium carbonate, sodium bicarbonate or calcium carbonate at less than 10%
presumably by weight of cement to Portland cements containing 35% silica. Scawtite, in
trace to minor amounts, is reported to enhance the performance of cement whereas major
amounts are related to lower strength and increased permeability. The results furthermore
suggest that scawtite forms in the presence of xonotlite which conflicts somewhat with
Harker’s data and could relate to the effect of other components such as alumina or
alkalis in the cement, as indicated by Taylor and Roy. Luke et al. shows that the amount
of scawtite formed corresponds approximately to the amount of CO2 present and formed
at concentrations as low as 0.9 to 2.1% carbonate at 350ºC. Gallus et al. also noted the
formation of scawtite 240ºC in geothermal dry steam wells having 5000 ppm CO2 content
and was associated with gradual decrease in compressive strength over time although
over the 2 year study strengths were still apparently acceptable. It is also of note that in
this study, a Class J cement was used and Luke et al. reported that Class J cement tended
to contain small amounts of carbonate. Kalousek and Nelson, on a study of C2S-S
cements showed negligible amounts of scawtite when the CO2 content was 0.37% or less
whereas at 1.32 and 1.74% CO2, scawtite was observed over the 175-300°C temperature
range. At 175–200°C scawtite formed in equilibrium with gyrolite and at 250–300°C
scawtite was in equilibrium with truscottite. Scawtite apparently formed in total
replacement of xonotlite at these temperatures. Roy et al. observed scawtite in a CaOMgO-SiO2-H2O system at 300°C and 10,000 psi and noted that the material containing a
mixture of scawtite and xonotlite was stronger than the material containing pure
xonotlite.
References
Eilers L.H., Nelson, E.B. and Moran, L.K.: “High Temperature Cement Compositions
Pectolite, Scawtite, Truscottite or Xonotlite Which Do You Want?” paper SPE 9286
presented at the 55th Annual Fall Technical Conference and Exhibition, Dallas, Texas,
USA (September 21-24, 1980)
Gallus, J.P., Pyle, D.E. and Moran, L.K.: “Physical and Chemical Properties of Cement
Exposed to Geothermal Dry Steam,” paper SPE 7876 presented at the International
Symposium on Oilfield and Geothermal Chemistry, Houston, Texas, USA (January 2224, 1979).
Harker, R.I.: “Scawtite and Its Synthesis,” Mineral Mag., Vol. 34 (268) 1965, pp 232236.
Kalousek, G.L. and Nelson, E.B.: “Hydrothermal Reactions of Dicalcium Silicate and
Silica,” Cem. Concr. Res., Vol. 8, 1978, pp 283-290.
Luke, K., Taylor, H.F.W. and Kalousek, G.L.: “Some Factors Affecting Formation of
Truscottite and Xonotlite at 300 - 350ºC,” Cem. Concr. Res., Vol. 11 (2), 1981, pp197203.
Roy, D.M., White, E.I., Langton, C.A. and Gruzteck, M.W.: “Potential New High
Temperature Cements for Geothermal Wells,” paper SPE 7677 presented at the
International Symposium on Oilfield and Geothermal Chemistry, Houston, Texas, USA
(January 22-24, 1979).
Taylor, H.F.W. and Roy D.M.: “Structure and Composition of Hydrates,” 7th
International Congress on the Chemistry of Cement, Vol. 1, 1980, pII-2/1