**3.3. Ettringite formation**

In this study four extra sets of specimens were tested for SEM images and EDX analyses of H- 4 sets mix given in Table 5 after curing for 7 days. SEM images and EDX analyses were tested on four specimens of the four sets. Each specimen was analyzed to determine if ettringite was present. If ettringite was detected its morphology was noted. It is impossible to quantify ettringite because of the scaling factor and depth of uncertainties of the electron microscope. However, it is significant to note that very small samples measuring approximately 5 mm in diameter yielded large quantities of ettringite. Ettringite was identified visually and from the EDX analysis. The morphologies of the observed ettringite are summarized in Table 5. Samples typically produced ettringite with similar morphologies including lamellar and needles. Ettringite was found in cavities and in the cement matrix.


**Table 5.** Summary of scanning electron microscopy morphology for H- 4 sets mix after 7 days curing

The mechanism of DEF expansion is a highly debated issue. Ettringite Crystal Growth Theory and Uniform Paste Expansion Theory are the two predominant theories. (Shoaib, Balaha, et al., 2000) and (Wang, Pu, et al., 1995) suggested the Ettringite Crystal Growth Theory, which attributes the expansion to pressure exerted by the growing ettringite crystals in the micro cracks between the cement paste and the aggregate.

(Wang, Scrivener, et al., 1994) proposed the Uniform Paste Expansion Theory, which suggests that the concrete expands and then the ettringite forms in the newly created gaps. (Yang, R., Lawrence, et al., 1996) found no evidence to support the Uniform Paste Theory concluding that the ettringite present in the mortar produced the expansion. (Lewis, M. C. & Scrivener, 1996) suggests that both mechanisms are possible and depending on the environmental condition one may be more prevalent. Although other mechanisms have been suggested, expansion most probably results from crystal growth pressure. There are differences of opinion as to whether expansion in mortars or concretes is driven by growth of ettringite crystals at aggregate interfaces or by processes occurring in the paste. If the later expands, gaps will be formed around aggregate particles (Wang, Scrivener, et al., 1994), and ettringite or other phases may recrystallize in them, simultaneously or subsequently. Based on the paste expansion theory the widths of peripheral cracks are proportional to aggregate size; cracks at the interfaces are initially empty. Assuming that expansion occurs through 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 Figure 5.

396 Heat Treatment – Conventional and Novel Applications

**3.3. Ettringite formation** 

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

In this study four extra sets of specimens were tested for SEM images and EDX analyses of H- 4 sets mix given in Table 5 after curing for 7 days. SEM images and EDX analyses were tested on four specimens of the four sets. Each specimen was analyzed to determine if ettringite was present. If ettringite was detected its morphology was noted. It is impossible to quantify ettringite because of the scaling factor and depth of uncertainties of the electron microscope. However, it is significant to note that very small samples measuring approximately 5 mm in diameter yielded large quantities of ettringite. Ettringite was identified visually and from the EDX analysis. The morphologies of the observed ettringite are summarized in Table 5. Samples typically produced ettringite with similar morphologies including lamellar and needles. Ettringite was found in cavities and in the cement matrix.

Set No. Curing regime Ettringite formation Set 1 With heat curing, air cured Thick and long needles Set 2 With heat curing, water cured Needles with lamellar Set 3 Without heat curing, air cured Needles with lamellar Set 4 Without heat curing, water cured Needles with dense lamellar Heat treatment was done after demoulding at 60 °C in duration of 20 hours. **Table 5.** Summary of scanning electron microscopy morphology for H- 4 sets mix after 7 days curing

The mechanism of DEF expansion is a highly debated issue. Ettringite Crystal Growth Theory and Uniform Paste Expansion Theory are the two predominant theories. (Shoaib, Balaha, et al., 2000) and (Wang, Pu, et al., 1995) suggested the Ettringite Crystal Growth Theory, which attributes the expansion to pressure exerted by the growing ettringite crystals

(Wang, Scrivener, et al., 1994) proposed the Uniform Paste Expansion Theory, which suggests that the concrete expands and then the ettringite forms in the newly created gaps. (Yang, R., Lawrence, et al., 1996) found no evidence to support the Uniform Paste Theory concluding that the ettringite present in the mortar produced the expansion. (Lewis, M. C. & Scrivener, 1996) suggests that both mechanisms are possible and depending on the environmental condition one may be more prevalent. Although other mechanisms have been suggested, expansion most probably results from crystal growth pressure. There are differences of opinion as to whether expansion in mortars or concretes is driven by growth of ettringite crystals at aggregate interfaces or by processes occurring in the paste. If the later expands, gaps will be formed around aggregate particles (Wang, Scrivener, et al., 1994), and ettringite or other phases may recrystallize in them, simultaneously or subsequently. Based on the paste expansion theory the widths of peripheral cracks are proportional to aggregate size; cracks at the interfaces are initially empty. Assuming that expansion occurs through

in the micro cracks between the cement paste and the aggregate.

strength of water cured specimens is different for the durations of 3 and 3 to 7 days.

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, Soutsos, et al., 2006). It can grow up to 100μ*m* long for high w/c ratios. The particle-size of 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 with the start of expansion (Barnett, Soutsos, et al., 2006).

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 sets mixes are given in Table 6.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 399

Water cured (25 ºC - 26 ºC )

Room temp. Cured (32 ºC)

Type of curing regime

Figure 5, (d) Figure 5, (c) Figure 5, (b) Figure 5, (a)

Room temp. Cured (32 ºC)

f7 53.7 38.7 57.6 64.0

f28 61.6 44.5 63.2 64.2

f7/f28 0.87 0.87 0.91 1.00

f28/f7 1.15 1.15 1.10 1.00

Based on the given data in Table 6 it is evident that the highest strength is obtained from air curing at room temperature after heat curing at 60 ºС for 14 hours. Generally speaking, the

The strength of specimens cured in air at room temperature after subjected to heat curing is the highest. This is followed by the specimens cured in water after heat curing and specimens cured under the same conditions but without heat curing. The lowest strength was obtained for the specimens cured in air at room temperature without heat curing. It was also observed that the thickest and the longest ettringite crystals were formed in the

Similar to the strength at 7 days, it is seen that among the four curing regimes the strength of specimens at 28 days is the highest for those cured at room temperature after heat curing at 60 °C for 14 hours. From Table 6 it can be seen that the strength growth (i) at 28 to 7 days in different curing regimes is as follows: for without heat and curing in air at room temperature i= 1.15 and for curing in water i= 1.15. Use of heat curing at 60 ºС for 14 hours

From the results observed it is clear that in the cases without use of heat curing, the strength growth of 28 to 7 days is noticeable for both curing at room temperature and in water. The relative strength growth is on average about 1.15. This shows that there is a continuous hydration process progression for duration of 7 to 28 days. It seems that the latent potential of the specimens is gradually released, whilst whenever the specimens are heated, the whole latent potential is suddenly released during the initial days (in duration of the first days) due to the temperature effect. In the case with heat treatment, it is seen that the strength gain is completely different for the specimens cured at room temperature and in water. It is

followed by curing in air at room temperature i= 1.00 and for curing in water; i= 1.10.

Without use of heat curing Heat curing at 60 ºC for 14 hours

Curing & Strength (MPa)

SEM images

Water cured (25 ºC - 26 ºC )

**Table 6.** Compressive strength (f) at 7 and 28 days for H- 4 sets mix

specimens with the highest strength as shown in Figure 5.

results obtained can be discussed according to the different curing regimes.

**Figure 5.** SEM images and EDX analyses for H-4 set mix specimens


**Table 6.** Compressive strength (f) at 7 and 28 days for H- 4 sets mix

398 Heat Treatment – Conventional and Novel Applications

**Figure 5.** SEM images and EDX analyses for H-4 set mix specimens

(a) Set No. 1 (b) Set No. 2

(c) Set No. 3 (d) Set No. 4

Based on the given data in Table 6 it is evident that the highest strength is obtained from air curing at room temperature after heat curing at 60 ºС for 14 hours. Generally speaking, the results obtained can be discussed according to the different curing regimes.

The strength of specimens cured in air at room temperature after subjected to heat curing is the highest. This is followed by the specimens cured in water after heat curing and specimens cured under the same conditions but without heat curing. The lowest strength was obtained for the specimens cured in air at room temperature without heat curing. It was also observed that the thickest and the longest ettringite crystals were formed in the specimens with the highest strength as shown in Figure 5.

Similar to the strength at 7 days, it is seen that among the four curing regimes the strength of specimens at 28 days is the highest for those cured at room temperature after heat curing at 60 °C for 14 hours. From Table 6 it can be seen that the strength growth (i) at 28 to 7 days in different curing regimes is as follows: for without heat and curing in air at room temperature i= 1.15 and for curing in water i= 1.15. Use of heat curing at 60 ºС for 14 hours followed by curing in air at room temperature i= 1.00 and for curing in water; i= 1.10.

From the results observed it is clear that in the cases without use of heat curing, the strength growth of 28 to 7 days is noticeable for both curing at room temperature and in water. The relative strength growth is on average about 1.15. This shows that there is a continuous hydration process progression for duration of 7 to 28 days. It seems that the latent potential of the specimens is gradually released, whilst whenever the specimens are heated, the whole latent potential is suddenly released during the initial days (in duration of the first days) due to the temperature effect. In the case with heat treatment, it is seen that the strength gain is completely different for the specimens cured at room temperature and in water. It is observed that there is no significant strength growth at 28 days compared to 7 days. In fact, it can be said that when the specimens are cured at room temperature after heat curing, the highest strengths are achieved during the first 7 days. This is a major advantage to the precast concrete industry. However, when the specimens are cured in water after heat curing more time is needed to achieve maximum strength. This shows that curing in water is not the best way to cure the specimens after heat treatment from the standpoint of strength gain at early ages. Hence, the comparison of curing regimes at room temperature and in water shows that the best curing regime after heat curing is air curing under room temperature, especially for the precast concrete industry. It is also seen that in the case without heat curing, on average, the strength at 7 days is 87% of the strength at 28 days for both curing at room temperature and in water, and with heat curing this ratio is increased on average to 95%. This means that heat curing improves the strength at 7 days by an average of about 9%.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 401

**f OM-wc = 6.1673ln(t) + 35.141 R² = 0.9738**

**f T-OSM50-ac = 5.2039ln(t) + 50.664 R² = 0.9311**

the relationships can approximately be forecasted the behaviour of the phenomenon at the later ages. In this research based on the results obtained for the OPC-slag mortars with 50% OPC replacement with slag under thermal activation method have been determined to

Comparison of all the relationships shows that the most appropriate form of equation to describe the variations of strength versus age of curing is a logarithmic function in the form of *f= a\*Ln (t) + b;* where *R2* is the coefficient of determination, *a* and *b* are constants for a specified mortar, f is compressive strength in MPa and t is the age of specimens in

**Figure 6.** Strength development curve fitting for the optimum OPC-slag mortar activated using thermal

0 10 20 30 40 50 60 70 80 90

**fOM-ac fOM-wc fOSM50-wc fT-OSM50-ac**

**Age (t)- days**

For different duration several temperatures such as 50 °C, 60 °C, and 70 °C were used. Based on the results obtained it can be deduced that heat curing at 60 °C for 20 hours is the optimum. It should be noted that 40% and 50% levels of replacement of OPC with slag were used and the results were compared. In addition to temperature, the effects of relative humidity were also studied. Finally, the results of compressive strength at 3 and 7 days versus heat curing were determined for 40% and 50% levels of replacement. It was recognized that the formation of long and thick crystals of ettringite was the main reason for significant strength improvement at early ages when the specimens were heated and then cured in air under room temperature. The flowchart of thermal activation method is shown

forecast the variations of compressive strengths versus age of curing.

The best fitted curves strength developments are shown in Figure 6.

**f OM-ac = 4.3065ln(t) + 36.771 R² = 0.6617**

**fOSM50-wc = 10.911ln(t) + 23.731 R² = 0.9472**

days.

activation method and OPC mortars

**3.6. Summary** 

**Compressive strength (f)-**

 **MPa**

in Figure 7.

## **3.4. Strength loss**

The compressive strength loss of mortars and concretes containing supplementary cementitious materials is quite common. In the process of this research, strength loss has been observed several times for some of the mortars prepared. This is because for a variety of reasons; some of the observed reasons in the study are as follows:

Several researchers reported that a high temperature improves strength at early ages (ACI, 2001; Al-Gahtani, 2010; Shariq, Prasad, et al., 2010). At later ages, the important number of formed hydrates had no time to arrange suitably and this caused a loss of ultimate strength. This behaviour has been called the crossover effect (Powers, T.C., 1947). For OPC it appears that the ultimate strength decreases nearly linearly, with curing temperature (Ramezanianpour & Malhotra, 1995).

Generally, the reason for the loss of strength can be due to internal or external factors. The internal factors are those linked to the chemical composition of the reacted products. The most efficient external factors are due to the variability of specimens and testing procedures. Another factor having high importance is the effect of temperature. The initial curing temperature has a significant effect and can reduce or increase strength at later ages. It seems that the main reason for strength loss at later ages is due to lack of inside water in specimens to complete the hydration and pozzolanic process progression. Usually for the duration of 1 to 28 days, the inside water due to the mixing water is available and adequate for the hydration process; however, beyond 28 days it is reduced and then insufficient for the process of hydration and pozzolanic reaction to progress; hence, strength loss occurs.

## **3.5. Strength development**

Customarily, whenever it is wanted to understand the behaviour of a phenomenon, it is accepted to model its behaviour by the use of a diagram or mathematical relationship. Using the relationships can approximately be forecasted the behaviour of the phenomenon at the later ages. In this research based on the results obtained for the OPC-slag mortars with 50% OPC replacement with slag under thermal activation method have been determined to forecast the variations of compressive strengths versus age of curing.

Comparison of all the relationships shows that the most appropriate form of equation to describe the variations of strength versus age of curing is a logarithmic function in the form of *f= a\*Ln (t) + b;* where *R2* is the coefficient of determination, *a* and *b* are constants for a specified mortar, f is compressive strength in MPa and t is the age of specimens in days.

The best fitted curves strength developments are shown in Figure 6.

**Figure 6.** Strength development curve fitting for the optimum OPC-slag mortar activated using thermal activation method and OPC mortars

#### **3.6. Summary**

400 Heat Treatment – Conventional and Novel Applications

average of about 9%.

**3.4. Strength loss** 

(Ramezanianpour & Malhotra, 1995).

**3.5. Strength development** 

observed that there is no significant strength growth at 28 days compared to 7 days. In fact, it can be said that when the specimens are cured at room temperature after heat curing, the highest strengths are achieved during the first 7 days. This is a major advantage to the precast concrete industry. However, when the specimens are cured in water after heat curing more time is needed to achieve maximum strength. This shows that curing in water is not the best way to cure the specimens after heat treatment from the standpoint of strength gain at early ages. Hence, the comparison of curing regimes at room temperature and in water shows that the best curing regime after heat curing is air curing under room temperature, especially for the precast concrete industry. It is also seen that in the case without heat curing, on average, the strength at 7 days is 87% of the strength at 28 days for both curing at room temperature and in water, and with heat curing this ratio is increased on average to 95%. This means that heat curing improves the strength at 7 days by an

The compressive strength loss of mortars and concretes containing supplementary cementitious materials is quite common. In the process of this research, strength loss has been observed several times for some of the mortars prepared. This is because for a variety

Several researchers reported that a high temperature improves strength at early ages (ACI, 2001; Al-Gahtani, 2010; Shariq, Prasad, et al., 2010). At later ages, the important number of formed hydrates had no time to arrange suitably and this caused a loss of ultimate strength. This behaviour has been called the crossover effect (Powers, T.C., 1947). For OPC it appears that the ultimate strength decreases nearly linearly, with curing temperature

Generally, the reason for the loss of strength can be due to internal or external factors. The internal factors are those linked to the chemical composition of the reacted products. The most efficient external factors are due to the variability of specimens and testing procedures. Another factor having high importance is the effect of temperature. The initial curing temperature has a significant effect and can reduce or increase strength at later ages. It seems that the main reason for strength loss at later ages is due to lack of inside water in specimens to complete the hydration and pozzolanic process progression. Usually for the duration of 1 to 28 days, the inside water due to the mixing water is available and adequate for the hydration process; however, beyond 28 days it is reduced and then insufficient for the process of hydration and pozzolanic reaction to progress; hence, strength loss occurs.

Customarily, whenever it is wanted to understand the behaviour of a phenomenon, it is accepted to model its behaviour by the use of a diagram or mathematical relationship. Using

of reasons; some of the observed reasons in the study are as follows:

For different duration several temperatures such as 50 °C, 60 °C, and 70 °C were used. Based on the results obtained it can be deduced that heat curing at 60 °C for 20 hours is the optimum. It should be noted that 40% and 50% levels of replacement of OPC with slag were used and the results were compared. In addition to temperature, the effects of relative humidity were also studied. Finally, the results of compressive strength at 3 and 7 days versus heat curing were determined for 40% and 50% levels of replacement. It was recognized that the formation of long and thick crystals of ettringite was the main reason for significant strength improvement at early ages when the specimens were heated and then cured in air under room temperature. The flowchart of thermal activation method is shown in Figure 7.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 403

For the slag used in this investigation, the optimum cement replacement level from viewpoint of high early strength was proven within the range of 40% - 50%. By using the optimum level of replacement slag, i.e. 50%, noticeable strength levels of OPC-slag mortars are achievable without the use of any activation method. In contrary to OPC-slag mortar for 40% replacement with slag, the other mortars made with different levels of replacement slag have shown higher 56-day strength compared to strength at 28-day, but the OPC-slag mortar for 40% OPC replacement with slag gives less 56-day strength compared to strength at 28-day by about 10.8%; namely compressive strength loss about 10.8%. Thus this strength

The highest growth of 56-day strength compared to 28-day are attributed to slag mortars and OPC-slag mortars for 10% replacement with slag (OSM/10) as 31.5% and 21.7%, respectively. This shows that the strength of mortar including the highest level of replacement slag will be improved at later ages more than others. However, it is well known that the ultimate strength of slag mortars is not significant compared to the strength of the others at the same ages. Therefore,

It has been shown that the strengths of specimens cured in water at 3 and 7 days for OPCslag mortar with 40% and 50% OPC replacement of slag, without and with use of heating for duration of 2 hours, are more than those cured in air under room temperature. However, as soon as the heating duration is increased to 4 hours and more, this effect is reversed. This is a new finding with a major advantage in precast concrete industry and also has many

Based on the experimental results obtained in the study, it can be concluded that there is an optimum temperature for each specific material to obtain high early strength. It was determined that 60 °C is the optimum. Heating duration is also very important for obtaining high early strength. For the slag used in the study, duration of 20 hours is optimum. Usually, as heating time increases towards the optimum, the compressive strength will be increased.

The maximum strengths obtained at 3 and 7 days for OPC-slag mortar with 50% OPC replacement of slag cured in air under room temperature are 55.3 and 61.6 MPa, respectively. It can be seen that these are 21.8% and 20.0% more than those of OPC mortar specimens cured in air under room temperature, and 26.1% and 29.0% more than those of

It was proven that whenever the mortar is heated larger than the optimum duration, it could be seen that this will not lead to an increase in early strength. According to the results of the study and other researches, it can be deduced that the thermal activation is one of the most efficient and applicable techniques for activation of OPC-slag mortars. This is well known

The results obtained show that the best relationship of compressive strengths versus heating duration of the specimens cured in air under room temperature and water for OPC-slag

it can be deduced that the slag mortars are the best only from viewpoint of durability.

*4.1.2. Optimum replacement* 

*4.1.3. Thermal activation method* 

loss phenomenon needs to be further investigated.

advantages in arid regions for curing of concrete structures.

OPC mortar specimens cured in water, respectively.

specially in precast concrete industry.

**Figure 7.** Thermal activation work phase
