**2.2. Replacement level**

This section reports on the testing of fourteen OPC-slag mortars (OSMs) and two control OPC mortars (OMs) and slag mortars (SMs). The main aim is to determine the level of cement replacement with slag to achieve higher early strength with reasonable flow. The variable is the level of ggbfs in the binder. Graded silica sand was used in all mixes. It was determined that the optimum level of replacement slag is within the range of 40% to 50% of OPC (Ahmed, Ohama, et al., 1999). The optimum level is defined as the replacement level of slag with the highest compressive strength, when used in the mortar while strength loss is the lowest.

## *2.2.1. Optimum replacement level*

It is intended to find the optimum cement at replacement level with slag that gives the highest early strength at 7 days and especially 3 days without the use of any activation method. From Figure 1 it is clear that whenever the level of replacement is more than 40% the early strength at 3 days will be reduced. It can also be seen that although for replacement levels 10%, 20%, and 30% the early strength at 3 days is approximately the same, but, generally with an increase in the slag level from 10% to 40% the early strength increases. However, the early strength at the 40% level is the highest, i.e. 42.4 MPa. Based on the results obtained it can be seen that by increasing the replacement level to more than 40%, the early strengths decreased significantly. This shows that the optimum level of replacement is 40% at 3 days. The same variations for 7-day strengths were observed albeit with slight changes. At 40% optimum level of replacement slag, the strength at 7 days is 55.8 MPa, which is 20% higher than that of OPC mortar. Furthermore, this will continue to gain strength with age.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 389

Finally, with comparison of the results obtained for compressive strengths the best level of replacement was determined at 50%. In determination of the best level two factors are considered i.e. early strength at 3 and 7 days and also strength loss that should be minimized.

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

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

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

when they are cured in water compared to curing in air under room temperature.

under room temperature after heat curing. This fact is shown in Figure 3 (a) and (b).

In following parts of the research the best level i.e. 50% was used as the optimum level.

0.33 and 2.25, respectively, for 40% and 50% replacement level with slag.

**3. Thermal activation method** 

**3.1. Optimum temperature** 

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

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

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

#### **2.3. Summary**

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%. Finally, with comparison of the results obtained for compressive strengths the best level of replacement was determined at 50%. In determination of the best level two factors are considered i.e. early strength at 3 and 7 days and also strength loss that should be minimized. In following parts of the research the best level i.e. 50% was used as the optimum level.
