Page 417 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Jet Set-Based Cement Sysyems 393

The curing of Reg Set in air results in low strengths because the large
amounts of heat created in the reaction result in the drying out of the material,
which slows down the hydration reaction.
The compressive strengths for Reg Set cement during the first 24
hours are higher than those for ultra-rapid hardening portland cement. The
strength characteristics of the cements are similar beyond 24 hours.
Reg Set cement mortar with a citric acid retarder has greater
strength than Reg Set cement regulated with calcium sulfate hemihydrate.
The increased strength in Reg Set cement paste or mortar made with the
addition of citric acid is usually a result of an increased degree of hydration,
increased amount of ettringite, increased volume concentration of small
pores, and lower total porosity.
The strength development of a typical Reg Set concrete (w/c = 0.60,
water cured at 20°C) indicates that the strengths up to two years are superior
to those of portland cement concrete. [76] The strength at two years for
concrete with 20% Reg Set cement replaced with fly ash is slightly higher
than the strength of concrete containing no fly ash. Such fly ash concretes
are expected to exhibit low early strengths because of the lesser amounts of
the binding material.
Calcium lignosulfonate or calcium chloride can be used as alternatives
to calcium sulfate hemihydrate in order to control the setting time of Reg
Set cement. Calcium lignosulfonate (2.2 ml/kg) increases the compressive
strength of Reg Set cement paste by approximately 70% at 14 days. [77] Also,
a 1% addition of calcium chloride increases the compressive strength by
about 40% at 14 days. At ages greater than 14 days, free chloride may form
from the aluminate phases and accelerate silicate hydration. Both calcium
lignosulfonate and calcium chloride increase the time of setting from two
minutes up to 30 minutes depending on the dosage. The morphology may
also be affected because the N surface areas of Reg Set cement paste at w/
2
2
c = 0.60 are 12, 14, 20, and 21 m /g for 0, 1, 2, and 5% CaCl respectively.
2
Microhardness has been correlated with compressive strength for
several cementitious systems. [78] Figure 13 is a plot of microhardness
versus porosity for the following cement systems: hydrated Reg Set cement
paste with 0, 1, 2, and 5% CaCl and hydrated portland cement paste. [79] The
2
microhardness of Reg Set cement paste is significantly increased with the
addition of 1% CaCl , and increased further with additions of 2 and 5%
2
CaCl , for the porosity range studied. The increase in microhardness is
2
larger at higher porosities. The microhardness of hydrated portland cement
paste is significantly higher than that of Reg Set cement paste. The curve for
portland cement paste lies between the curves for Reg Set cement paste