**3. Thermal activation method**

388 Heat Treatment – Conventional and Novel Applications

strength with age.

**Compressive strength (MPa)**

**OM (0%)**

**OSM/10 (10%)**

**OSM/20 (20%)**

**OSM/30 (30%)**

**2.3. Summary** 

MPa, which is 20% higher than that of OPC mortar. Furthermore, this will continue to gain

**3 days 7 days 28 days 56 days**

**Notes:** OSM/i= OPC-slag mortar for i% replacement with slag, OM= OPC mortar, SM= slag mortar.

**OSM/40 (40%)**

**OSM/45 (45%)**

level was taken as the optimum and used for the preparation of mortar samples.

In this section it was attempted to determine the best replacement level for slag. Different levels of replacement slag were used to make mixes, i.e. 10, 20, 30, 40, 50, 60, 70, 80, and 90%.

The use of different amounts of slag in OPC-slag mortars results in different compressive strengths. Before applying the proposed activation method it is required that several mortars are made with the use of different levels of slag to obtain indication of strengths. Based on this, it was decided to cast samples with 0, 10, 20, 30, 35, 40, 45, 50, 60, 70, 80, 90, and 100% replacement with slag to ascertain the optimum level. The specimens were prepared and hardened samples were tested at 3, 7, 28, and 56 days for compressive strength. Finally, it was revealed that the highest compressive strengths were obtained for samples having replacement level in the range of 40% and 50%. The results showed that there was some strength loss at 56 days compared to 28 days when 40% replacement level was used. In this case, the strength at 28 days was 64.9 MPa, which reduced to 57.9 MPa at 56 days giving a 10.8% loss in strength. In contrast no strength loss was observed at later ages when 50% replacement level was used. A comparison of the results obtained for both percentage levels of slag showed that 50% is the optimum. The strengths for both levels of replacement were very close but, the 50% replacement with slag did not show any subsequent loss in strength. Consequently, in the continuation of the research work this

**OSM/50 (50%)**

**OPC-slag mortar (slag content)**

**OSM/60 (60%)**

**OSM/70 (70%)**

**OSM/80 (80%)**

**OSM/90 (90%)**

**SM (100%)**

**Figure 1.** Variations of compressive strength for different slag contents (%)

**OSM/35 (35%)**

The objective of this research is to produce a data inventory of the early age mechanical properties, namely the compressive strength of mortars cured at different temperature, as well as the relationship between compressive strength with temperature and the relationship between the compressive strength of specimens cured in air and water at room temperature for 3 and 7 days, for 40% and 50% levels of cement replacement with slag. Thirty-seven mixes of OPC-slag mortars and two OPC mortars were prepared as control. For each mix, two factors are important for consideration. First, using a higher percentage of slag is desirable as it has some economic and environmental advantages and in addition, it helps to improve the durability of the mortars. Secondly, for early strength, it is clear that increasing the level of replacement slag causes early strength to be reduced, as the ggbfs has lower initial heat of hydration than that of OPC. In addition, for early strength the use of a low level of replacement slag is neither economic nor durable. Thus it is desirable to ascertain the optimum temperature and its duration that will give the highest early strength at 3 and 7 days. All the mix proportions made for water-binder and sand-binder ratios of 0.33 and 2.25, respectively, for 40% and 50% replacement level with slag.

## **3.1. Optimum temperature**

In this investigation the effects of different temperatures i.e. 50 ºC, 60 ºC, and 70 ºC were studied on the early strengths at 3 and 7 days of OPC-slag mortars by using 50% replacement with slag. The results are shown in Figure 2. It is clear that 60 ºC provided the most enhancements on early age strength therefore; it is selected as the optimum temperature.

The results obtained in the study for compressive strength based on duration of heat curing are given in Table 1. Based on this, it can be seen that the specimens have higher strengths at 3 and 7 days without use of heat curing and with use of heat curing for duration of 2 hours when they are cured in water compared to curing in air under room temperature.

This has been proven for both OPC-slag mortars with 50% OPC replacement with slag and OPC-slag mortars with 40% OPC replacement with slag. However, as soon as the duration of heat curing is increased to 4 hours and above, the aforesaid statement is reversed. The strength of specimens cured in the air under room temperature is improved compared to those cured in the water. It seems that this is due to the air temperature and high relative humidity of the room's air. As it will be seen in following study, it can be said that both combined effects of temperature and relative humidity are more efficient in strength improvement. Hence, it seems that probably the effect of temperature for duration of at least 4 hours beside the high relative humidity of room's air results in the higher strength for the specimens cured in the air under room temperature after heat curing. This fact is shown in Figure 3 (a) and (b).

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 391

**Figure 3.** Compressive strength versus duration of heat curing of OSMs/50 and OSMs/40 cured in water

(b) OPC-slag mortar with 40% OPC replacement with slag at 60 °C

**0 h 2 h 4 h 6 h 8 h 10 h 12 h 14 h 16 h 18 h 20 h 22 h 24 h 26 h**

**Duration of heating time- h**

(a) OPC-slag mortar with 50% OPC replacement with slag at 60 °C

**0 h 2 h 4 h 6 h 8 h 10 h 12 h 14 h 16 h 18 h 20 h 22 h 24 h 26 h**

**3 day-a c 3 day-w c 7 day-a c 7 day-w c**

**Duration of heat curing- h**

**3 day-a c 3 day-w c 7 day-a c 7 day-w c**

Based on the results given in Table 1, it can be seen that heat curing at 60 °C for 20 hours, the maximum early strength of 61.6 MPa is attributed to OPC-slag mortar with 50% replacement with slag. It can be seen that the increment percentage is by about 0.62% when compared to a heating time of 16 hours. This shows that if the duration of heat curing is

and air under room temperature.

**0**

**10 20**

**30 40**

**Comp. strength (MPa)**

**50 60**

**70**

**Comp. strength (MPa)**

**Figure 2.** The effects of different temperatures on early age strength of OSMs/50


**Table 1.** Compressive strength (f) versus duration of heat curing for OSMs/40 and OSMs/50 at 60 ºC

390 Heat Treatment – Conventional and Novel Applications

Duration (hours)

**0**

**10**

**20**

**30**

**Compressive strength (MPa)**

**40**

**50**

**60**

60 ºC

**Figure 2.** The effects of different temperatures on early age strength of OSMs/50

For OSMs/40 For OSMs/50 3 days 7 days 3 days 7 days ac wc ac wc ac wc ac wc

**50°C/9h-ac 50°C/9h-wc 60°C/6h-ac 60°C/6h-wc 60°C/9h-ac 60°C/9h-wc 70°C/9h-ac 70°C/9h-wc**

0 33.2 34.5 40.3 47.4 33.6 35.6 37.0 49.6 2 36.6 38.4 44.8 49.8 36.4 37.6 42.5 50.0 4 39.7 35.4 46.2 43.2 42.6 37.7 47.5 47.3 6 45.0 41.2 47.2 44.0 45.5 40.9 51.5 49.8 8 49.6 41.6 52.3 44.8 46.4 43.4 53.1 49.0 10 47.3 40.4 55.6 50.9 50.4 44.0 55.0 48.4 12 49.0 42.5 50.9 46.4 52.6 41.8 57.6 48.3 14 52.6 47.0 56.4 48.5 48.3 41.2 60.0 52.5 16 51.7 45.9 59.0 54.8 51.2 48.4 61.2 53.4 18 55.2 46.1 59.7 50.2 53.5 48.4 59.9 52.4 20 53.1 49.0 61.1 51.2 55.3 49.9 61.6 55.3 22 50.7 43.7 58.8 56.0 50.5 48.4 61.0 56.0 24 54.6 50.1 60.4 56.8 51.5 48.5 62.3 55.6 26 52.5 49.0 57.1 53.7 53.0 46.6 61.2 54.8 For optimum OSM/50 at six ages- air cured under room temperature f1= 15.5 f3= 55.1 f7= 61.4 f28= 71.2 f56= 69.6 f90= 73.6

**3 days 7 days**

**Age of curing (days)**

ac= air curing under room temperature; wc= water curing; all strengths are in MPa. **Table 1.** Compressive strength (f) versus duration of heat curing for OSMs/40 and OSMs/50 at

(a) OPC-slag mortar with 50% OPC replacement with slag at 60 °C

**Figure 3.** Compressive strength versus duration of heat curing of OSMs/50 and OSMs/40 cured in water and air under room temperature.

Based on the results given in Table 1, it can be seen that heat curing at 60 °C for 20 hours, the maximum early strength of 61.6 MPa is attributed to OPC-slag mortar with 50% replacement with slag. It can be seen that the increment percentage is by about 0.62% when compared to a heating time of 16 hours. This shows that if the duration of heat curing is 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 curing for the materials used in the study.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 393

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

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

f3 58.2 53.3 48.9 f7 59.9 59.9 54.5 f3/f7 0.97 0.89 0.90

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

Water 32 ºC

Water 25 -26 ºC

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

air curing under room temperature

no benefit to the engineering properties of concrete.

Curing regime

fi are strength in MPa.
