Civil Engineering Reference
In-Depth Information
An extensive study of the strength development in mortars contain-
ing Ca formate, Ca formate + Na formate and calcium chloride was carried
out by Geber.
[53]
2.7
Other Non-Chloride Accelerators
Many other non-chloride accelerators have been suggested which
are based on organic and inorganic compounds. Valenti and Sabateli
[56]
studied the effect of alkali carbonates, viz., Na
2
CO
3
, K
2
CO
3
, and
Li
2
CO
3
, on setting times and strengths of cements. At lower dosages,
sodium and potassium carbonates retarded the setting times of cement, but
at higher dosages (> 0.1%) they accelerated the setting. Lithium carbonate
acted as a setting accelerator at all concentrations studied. At concentra-
tions especially higher than 0.1%, the admixture generally increased the
compressive strength at 28 days. Addition of Na
2
CO
3
is reported to change
the intrinsic property of the hydrated cement. In the presence of Na
2
CO
3
,
smaller pores or radius 10-100 Å are decreased and pores of radius > 150
Å are increased slightly.
[57]
The reduction in the pores may be due to the
precipitation of CaCO
3
.
Calcium nitrate, calcium nitrite, and calcium thiosulfate are also
suggested as accelerators. Calcium nitrite accelerates the hydration of
cement as evident from the increased amounts of heat developed in its
presence. Calcium nitrite also increases the strength and accelerates the
setting times, but less effectively than Ca chloride. Table 4 compares the
setting times of concrete containing CaCl
2
and Ca(NO
2
)
2
.
[40][58]
Table 5
shows the strength development in concrete containing 2, 3, 4, and 5%
calcium nitrite.
[40][58]
Improvement is less significant at dosages greater
than 4%. Calcium thiosulfate increases the strength development of con-
crete at early ages. For example, at 1 day compared to a compressive
strength of 15.8 km/cm
2
for a reference mortar, that containing 1, 2, 3, 5, and
6.5% CaS
2
O
3
shows values of 17.1, 18.1, 18.8, 20.1, and 21.2 kg/cm
2
respectively.
[59]
Sodium thiosulfate also accelerates the setting time, but the
compressive strengths are slightly reduced with respect to the reference
concrete at dosages of 0.5 and 1.0%.
[60]
Depending on the dosage of NaNO
2
,
Rosskopf, et al., found that acceleration of setting varied between 0.5 and
2.3 hrs. At dosages of 0.1-1.0% NaNO
2
, the compressive strength of
concrete at 7 days was equal or slightly lower than that of the reference
concrete.
[60]
They also found that formaldehyde and paraformaldehyde in
small amounts (0.01-0.25%) reduced the setting time from 9.5 to 6.3 hours.
