1—StrengthCompressive and flexural

1—Strength
Compressive and flexural strength gain characteristics of
concrete containing slag cement can vary over a wide range.
Compared with portland cement concrete, the use of Grade 120
slag cements typically results in reduced strength at early
ages (one to three days) and increased strength at later ages
(seven days and beyond) (Hogan and Meusel 1981). Use of
Grade 100 results in lower strengths at early ages (1 to 21 days)
but equal or greater strength at later ages. Grade 80 typically
gives reduced strength at early ages, although, by the 28th
day, the strength may be equivalent to or slightly higher than
a 100% portland cement mixture.
The extent to which slag cement affects strength depends
on the slag activity index of the particular slag cement and
the fraction in which it is used in the mixture. Figure 5.1 indicates that the mortar strength development of 50% blends
depends upon the grade of slag cement as defined in ASTM
C 989. Consistent and stable long-term strength gain beyond
20 years has been documented for concrete made with portland
blast-furnace slag cement (Type IS) while exposed to moist
or air curing (Wood 1992).
Other factors that can affect the performance of slag
cement in concrete are w/cm, physical and chemical characteristics of the portland cement, and curing conditions. As
seen in Fig. 5.2, the percentage of strength gain, relative to
portland-cement concrete, with Grade 120 slag cement is
greater in mixtures with a high w/cm than in mixtures with a
low w/cm (Fulton 1974; Meusel and Rose 1983). Malhotra
(1980) also noted the same trend.
The temperature at which concrete is cured will have a
great effect on strength, particularly at early ages. Concrete
containing slag cement responds well to elevated temperature curing conditions (Roy and Idorn 1982). In fact, strength
exceeding that of portland-cement concrete at 1 day and
beyond has been reported for accelerated curing conditions
(Hogan and Meusel 1981; Fulton 1974; Lea 1971).
Conversely, lower early-age strength is expected for
concrete containing slag cement when cured at normal or
low temperatures.
The proportion of the slag cement used affects the strength
and rate of strength gain as noted in Fig. 5.3. When highly
active slag cements have been used, the greatest 28-day
strengths are found with blends as high as 65% slag cement
(Fulton 1974; Hogan and Meusel 1981; Meusel and Rose
1983). Where early-age strengths are concerned, the rate of
strength gain is generally inversely proportional to the fraction
of slag cement used in the blend. Figure 5.4 compares compressive strength development of various blends of slag cement and
portland cement with a portland-cement mixture only