**3.2. Temperature and humidity effects**

From the H- 3 sets mix as shown in Table 2 the effects of ambient temperatures and relative humidity were considered on the strength improvement of the specimens. The specimens were first made and demoulded 24 hours after casting, and then heated at 60 °C for 20 hours and finally, cured in three curing regimes, i.e. at room temperature, in water at 25 ºС - 26 ºС, and in water at 32 ºС. After 3 and 7 days the strength of specimens was determined. The results are given in Table 2.

Based on the given data in Table 2, it is clear that for curing regimes with different temperature and the same relative humidity, higher strengths are attributed to higher temperature regimes. For curing regimes with different relative humidity and the same temperature, higher strengths are attributed to lower relative humidity regimes. From the comparison of the three curing regimes it is seen that the strengths of specimens cured in water at 32 ºС are less than those cured at room temperature. This shows that the increase of early age strength is a function of both temperature and relative humidity effects. In fact, it can be deduced that neither relative humidity nor temperature has the highest strength improvement alone, but rather a combination of both effects are effective on strength development. From the results it is clear that the trend of strength development at 7 days is similar to that at 3 days. It is seen that the strength of specimens at 7 days cured at room temperature and in water at 32 ºС are the same, which shows that the effect of relative humidity over duration of 3 to 7 days is more than that of temperature. It is seen that the highest strengths at 3 and 7 days are attributed to curing at room temperature.

The percentages of strength growth (i) for duration of 3 to 7 days can be obtained as follows: For air cured specimens under room temperature i= 0.97, for water at 25 – 26 ºС i= 0.90, and for water cured at 32 ºС i= 0.89; where i= ratio f3/f7. These results show that whenever the specimens are cured at room temperature, about 97% of the strength is achieved at 3 days. This is a major advantage in the precast concrete industry when the specimens are cured at room temperature. For the specimens cured at room temperature, the maximum relative humidity attainable was 85%, while for the specimens cured in water it was 100%.

Based on the results obtained it is evident that the effect of temperature on the strength improvement at 3 and 7 days is higher than that of relative humidity. This is because the strengths of the specimens cured at room temperature and in water at 32 ºС are higher than those in water at 25 ºС at 3 and 7 days by about 14% and 10%, respectively.

It can be seen that with the use of heat curing at 60 ºС for 20 hours the strength at 3 days is, on average, about 97% of the strength at 7 days for specimens cured at room temperature, while, on average, this ratio is about 90% for curing in water at 25 ºС and 32 ºС. This shows that curing at room temperature after heat curing improves the early strength at 3 days extensively, which is very cost effective and applicable in the precast concrete industry. This result also shows that the heat treatment is a useful and efficient method for the activation of ordinary Portland cement-slag mortars and concretes which requires only slat duration and without the use of water to cure the specimens. An elevated curing temperature accelerates the chemical reaction of hydration and increases the early age strength. However, during the initial period of hydration an open and unfilled pore structure of cement paste forms which has a negative effect on the properties of hardened concrete, especially at later ages (Fu, Y., 1996; Neville, A.M. , 2008). Hardened mortars and concretes can reach their maximum strength within several hours through elevated temperature curing. However, the ultimate strength of hardened mortars and concretes has been shown to decrease with curing temperature (Carino, 1984). It was found that by increasing the curing temperature from 20 ºC to 60 ºC and the duration of heat curing to 48 hours causes a continuous increase in compressive strength (Brooks & Al-kaisi, 1990). Studies by (Hanson, 1963; Pfeifer & Marusin, 1991; Shi, 1996) have shown that there is a threshold maximum heat curing temperature value in the range of 60 ºC to 70 ºC, beyond which heat treatment is of little or no benefit to the engineering properties of concrete.

392 Heat Treatment – Conventional and Novel Applications

curing for the materials used in the study.

**3.2. Temperature and humidity effects** 

results are given in Table 2.

increased over 20 hours, the increase at 7 days strength is not appreciable. Hence, it can be deduced from Figures 3 and 4 that heat curing at 60 °C for 20 hours is the optimum heat

From the H- 3 sets mix as shown in Table 2 the effects of ambient temperatures and relative humidity were considered on the strength improvement of the specimens. The specimens were first made and demoulded 24 hours after casting, and then heated at 60 °C for 20 hours and finally, cured in three curing regimes, i.e. at room temperature, in water at 25 ºС - 26 ºС, and in water at 32 ºС. After 3 and 7 days the strength of specimens was determined. The

Based on the given data in Table 2, it is clear that for curing regimes with different temperature and the same relative humidity, higher strengths are attributed to higher temperature regimes. For curing regimes with different relative humidity and the same temperature, higher strengths are attributed to lower relative humidity regimes. From the comparison of the three curing regimes it is seen that the strengths of specimens cured in water at 32 ºС are less than those cured at room temperature. This shows that the increase of early age strength is a function of both temperature and relative humidity effects. In fact, it can be deduced that neither relative humidity nor temperature has the highest strength improvement alone, but rather a combination of both effects are effective on strength development. From the results it is clear that the trend of strength development at 7 days is similar to that at 3 days. It is seen that the strength of specimens at 7 days cured at room temperature and in water at 32 ºС are the same, which shows that the effect of relative humidity over duration of 3 to 7 days is more than that of temperature. It is seen that the

highest strengths at 3 and 7 days are attributed to curing at room temperature.

humidity attainable was 85%, while for the specimens cured in water it was 100%.

those in water at 25 ºС at 3 and 7 days by about 14% and 10%, respectively.

The percentages of strength growth (i) for duration of 3 to 7 days can be obtained as follows: For air cured specimens under room temperature i= 0.97, for water at 25 – 26 ºС i= 0.90, and for water cured at 32 ºС i= 0.89; where i= ratio f3/f7. These results show that whenever the specimens are cured at room temperature, about 97% of the strength is achieved at 3 days. This is a major advantage in the precast concrete industry when the specimens are cured at room temperature. For the specimens cured at room temperature, the maximum relative

Based on the results obtained it is evident that the effect of temperature on the strength improvement at 3 and 7 days is higher than that of relative humidity. This is because the strengths of the specimens cured at room temperature and in water at 32 ºС are higher than

It can be seen that with the use of heat curing at 60 ºС for 20 hours the strength at 3 days is, on average, about 97% of the strength at 7 days for specimens cured at room temperature, while, on average, this ratio is about 90% for curing in water at 25 ºС and 32 ºС. This shows that curing at room temperature after heat curing improves the early strength at 3 days Based on the given data in Table 1 it can be seen that the highest strengths at 3 and 7 days of OPC-slag mortars for 40% replacement with slag and OPC-slag mortars for 50% replacement with slag is attributed to the specimens cured in air under room temperature as:


**Table 2.** Compressive strengths (f) at 3 and 7 days for three curing regimes of H- 3 sets mix

OPC-slag mortars for 40% OPC replacement with slag: f3= 55.2 at 18 hours and f7= 61.1 MPa at 20 hours; OPC-slag mortars for 50% replacement with slag: f3= 55.3 and f7= 61.6 MPa, the both for 20 hours. The 3 and 7 days strengths of OPC mortars' specimens cured at room temperature and in water are f3= 45.4, and f7= 51.4 MPa, and f3= 43.8, and f7= 47.8 MPa, respectively. It is noted that the maximum 3 and 7 days strengths of OPC-slag mortars for 40% replacement with slag and OPC-slag mortars for 50% replacement with slag specimens are 21.7% and 19.0% which are 21.8% and 20.0% more than those of OPC mortars' specimens cured at room temperature at the same age, respectively. It is seen that there is strength loss at 56 days compared to 28 days by about 2.2%. This has been previously reported by other researchers (Kosmatka, Panarese, et al., 1991). The main objective of elevated temperature curing is to achieve early strength development. However, it is generally acknowledged that there is also strength loss as a result of heat curing (Bougara, Lynsdale, et al., 2009). Another mix proportion of OPC-slag mortars for 50% replacement with slag was made by using the optimum heat curing at 60 ºC for 20 hours and specimens were tested at ages of 1, 3, 7, 28, 56, and 90 days. To determine the trend of strength development for the mentioned mortar cured at room temperature, the regression technique was used. The equations obtained for this mortar and also OPC mortar cured in water are as below:

$$f\_{"\text{-}w=5.2128"}\text{ Lu (t)} + 50.644 \text{ with } R^{\text{\textquotedblleft}} \text{ 0.9305} \tag{1}$$

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 395

It is also seen that the best fit curve at 3 and 7 days strengths are power equations. According to the results obtained in the study, it can be said that thermal activation is one of

No Age (days) Power regression relationship Curing 1 3 f OSM/50 = 1.3991\* x 0.9147 ; R2= 0.8857 air 2 7 f OSM/50 = 0.4548\* x 1.2047; R2= 0.9334 air 3 3 f OSM/50 = 2.098\* x 0.8064; R2= 0.7349 water 4 7 f OSM/50 = 5.9897\* x 0.5511; R2= 0.6897 water

**Notes:** OM= OPC mortar, T= OPC-slag mortar for 50% OPC replacement with slag made by using the optimum heat curing, f= Compressive strength of the specimens in MPa, ac and wc denote air and water curing under room

**0 7 14 21 28 35 42 49 56 63 70 77 84 91 Age (t)- days**

**f T-ac = 5.2039\*Ln(t) + 50.664; R<sup>2</sup>**

**f OSM/50-wc = 10.911\*Ln(t) + 23.731; R<sup>2</sup>**

**f OSM/50-wc f T-ac f OM-wc**

**f OM-wc = 6.1673\*Ln(t) + 35.141; R<sup>2</sup>**

 **= 0.9311**

 **= 0.9472**

 **= 0.9738**

**Figure 4.** Strength development for OPC mortar and OPC-slag mortar made by using the optimum heat

Based on the results presented in Table 4, it can be seen that there is an acceptable power relationship at 3 and 7 days strengths between OPC-slag mortars for 50% OPC replacement with slag and OPC-slag mortars for 40% OPC replacement with slag for the specimens cured

The given relationships in Table 4 were determined by using the regression technique. Based on the relationships it is seen that the coefficient of determination R2 of regression for relationships between the strengths at 3 and 7 days of water cured OPC-slag mortars for 50% OPC replacement with slag and OPC-slag mortars for 40% OPC replacement with slag is small. This shows that there is no acceptable relationship between the strengths of water cured specimens. However, there is a proper relationship for those cured at room temperature.

x= compressive strength of OSM/40 in MPa, R2= coefficient of determination. **Table 4.** Relationships between compressive strengths (f) of OSMs/50 and OSMs/40

the best techniques for the activation of OPC-slag mortars.

temperature, respectively.

**Compressive strengt (f)- MPa**

at room temperature.

curing

For OPC-slag mortar cured in air under room temperature, made using optimum heat curing at 60 ºC for 20 hours, and

$$f\_{\text{M\\_uv}=6.1673''\text{ Ln (t)}+35.141\text{ with R\text{\text\textquotedblleft}R\text{\textquotedblright}}}\text{ }}\text{ }\tag{2}$$

For OPC mortar cured in water; where f is compressive strength in MPa, ac and wc denote air and water curing under room temperature, respectively and t is the age of the specimen in days. The best fit curves are shown in Figure 4.


**Table 3.** Relationship between compressive strength (f) versus heat duration

The relationships between compressive strength and duration of heat curing at room temperature and in water for OPC-slag mortars at 40% OPC replacement with slag and OPC-slag mortars at 50% OPC replacement with slag is shown in Table 3. It can be seen that the best equations are binomial and attributed to the specimens cured at room temperature. It is also seen that the best fit curve at 3 and 7 days strengths are power equations. According to the results obtained in the study, it can be said that thermal activation is one of the best techniques for the activation of OPC-slag mortars.


**Table 4.** Relationships between compressive strengths (f) of OSMs/50 and OSMs/40

394 Heat Treatment – Conventional and Novel Applications

curing at 60 ºC for 20 hours, and

in days. The best fit curves are shown in Figure 4.

Binomial relationships

below:

N o Age (d)

development for the mentioned mortar cured at room temperature, the regression technique was used. The equations obtained for this mortar and also OPC mortar cured in water are as

 *fT- ac= 5.2128\* Ln (t) + 50.644 with R2= 0.9305* (1)

For OPC-slag mortar cured in air under room temperature, made using optimum heat

 *fOM- wc= 6.1673\* Ln (t) + 35.141 with R2= 0.9738* (2)

For OPC mortar cured in water; where f is compressive strength in MPa, ac and wc denote air and water curing under room temperature, respectively and t is the age of the specimen

For OPC-slag mortars for 40% replacement with slag, i.e. OSMs/40

For OPC-slag mortars for 50% replacement with slag, i.e. OSMs/50

X= heat duration in hours, f is compressive strength in MPa, R2 is coefficient of determination, CR= curing regime, d= days

The relationships between compressive strength and duration of heat curing at room temperature and in water for OPC-slag mortars at 40% OPC replacement with slag and OPC-slag mortars at 50% OPC replacement with slag is shown in Table 3. It can be seen that the best equations are binomial and attributed to the specimens cured at room temperature.

1 3 f= -0.0455X2+ 1.865X+ 32.921; R2= 0.9261 f= 0.6825 X + 38.651

2 3 f=-0.0131X2+ 0.8806 X + 34.825; R2= 0.8502 f= 0.5391 X + 36.191

3 7 f= -0.0347 X2 1.5959 X + 40.621; R2= 0.9107 f= 0.6927 X + 44.234

4 7 f= 0.0163 X2- 0.0222 X + 46.327; R2= 0.6305 f= 0.4011 X + 44.634

5 3 f= -0.0487 X2+ 1.9196 X + 34.298; R2= 0.9271 f= 0.6526 X + 39.366

6 3 f= -0.0184 X2+ 0.9857 X + 35.234; R2= 0.8492 f= 0.5066 X + 37.154

7 7 f= -0.0479 X2+0.1079 X + 38.56; R2= 0.9598 f= 0.8628 X + 43.54

8 7 f= 0.0115 X2+ 0.0108 X + 48.789; R2= 0.7742 f= 0.3089 X + 47.597

**Table 3.** Relationship between compressive strength (f) versus heat duration

Linear relationships

R2= 0.7545

R2= 0.8252

R2= 0.8068

R2= 0.5815

R2= 0.7212

R2= 0.7954

R2= 0.8291

R2= 0.7232

CR

air

wate r

air

wate r

air

wate r

air

wate r

**Notes:** OM= OPC mortar, T= OPC-slag mortar for 50% OPC replacement with slag made by using the optimum heat curing, f= Compressive strength of the specimens in MPa, ac and wc denote air and water curing under room temperature, respectively.

**Figure 4.** Strength development for OPC mortar and OPC-slag mortar made by using the optimum heat curing

Based on the results presented in Table 4, it can be seen that there is an acceptable power relationship at 3 and 7 days strengths between OPC-slag mortars for 50% OPC replacement with slag and OPC-slag mortars for 40% OPC replacement with slag for the specimens cured at room temperature.

The given relationships in Table 4 were determined by using the regression technique. Based on the relationships it is seen that the coefficient of determination R2 of regression for relationships between the strengths at 3 and 7 days of water cured OPC-slag mortars for 50% OPC replacement with slag and OPC-slag mortars for 40% OPC replacement with slag is small. This shows that there is no acceptable relationship between the strengths of water cured specimens. However, there is a proper relationship for those cured at room temperature.

This appears to be due to the behaviour of specimens cured in water, which are still not clear and that is specific for the duration of 7 days. This means that the effect of water on the strength of water cured specimens is different for the durations of 3 and 3 to 7 days.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 397

*m* long for high w/c ratios. The particle-size of

crystal growth pressure, significant growth pressures could not be obtained in relatively large cracks and pastes expand, albeit slowly. Three factors influencing expansion will be considered namely chemistry, paste microstructure, and mortar or concrete microstructure. Proportionality between crack width and aggregate size, which can only be explained by paste expansion, was first reported by (Johansen, Thaulow, et al., 1993). For the H- 4 sets mix 24 hours after casting, the first two sets of the specimens were demoulded and without heat curing but were cured in water and air under room temperature for 7 days, respectively. Another two sets were heated at 60 ºC temperature for 14 hours and then cured the same as the former sets. Ettringite was observed in all of the sets of specimens i.e. with and without heat curing, but with different amounts of crystal size. The results are shown in

The morphology and crystal size of ettringite varies under the different curing conditions the specimens were subjected to. Most of the SEM observation shows that ettringite is normally a slender, needle-like crystal with a prismatic hexagonal cross-section. Its size depends on w/c ratio, that is, the effective space that ettringite is able to occupy (Barnett,

μ

Al-bearing agents is also a main factor affecting the size of ettringite. Large particles of Albearing agents form a large amount of small ettringite crystal and the period of expansion can last longer. Small particles will produce large size crystals quickly at any early stage because of the large surface area and fast reaction rate. The ettringite crystals will also be smaller in size in the presence of calcium hydroxide (CH) (Barnett, Soutsos, et al., 2006). The form of ettringite is relevant to studies of the mechanism of expansion. (Lerch, Ashton, et al., 1929) reported that synthetic ettringite consists of long slender needles that often form sphere-like. (Mehta, P.K., 1976) also reported the presence of spheroid ettringite. (Ogawa & Roy, 1982) found that during the hydration, the ettringite formed as very small irregular particles around Al-bearing particles in the early stage, and then changes to long needle-like crystals arranged radially around the Al-bearing particles. This formation was associated

Comparison between Figure 5 (a), (b), (c) and (d) show that for each test and in all curing regimes ettringite crystals were detected. For the four sets of tests, the ettringite crystals formed under room temperature curing, i.e. Figure 5 (a), and were more and bigger than those under water curing. Probably, this is the reason for the higher strength improvement of the specimens cured in air under room temperature compared to curing in water. It can be observed that the thickest and the longest ettringite crystals are attributed to the specimens that were cured in the heating process at 60 ºC for 14 hours after casting, and then cured at room temperature for 7 days as shown in Figure 5 part (a). The strength obtained at 7 days for the specimens was about 64 MPa. With the comparison of SEM images and the strengths obtained from the four sets of specimens, it can be deduced that heat curing at 60 ºC for 14 hours increases the rate of ettringite formation and thus the early strength. Whenever there is a greater quantity of ettringite formed it contributes to the higher strength. The details obtained for the compressive strengths at 7 and 28 days, of H- 4

Figure 5.

Soutsos, et al., 2006). It can grow up to 100

with the start of expansion (Barnett, Soutsos, et al., 2006).

sets mixes are given in Table 6.
