**4. Conclusions and recommendations**

## **4.1. Conclusions**

## *4.1.1. Introduction*

In this study 50% level of slag as the optimum was used as replacement for OPC. The thermal activation method (T) was used to activate ordinary Portland cement-slag mortars (OSMs).

## *4.1.2. Optimum replacement*

402 Heat Treatment – Conventional and Novel Applications

**Figure 7.** Thermal activation work phase

**4.1. Conclusions** 

*4.1.1. Introduction* 

**4. Conclusions and recommendations** 

In this study 50% level of slag as the optimum was used as replacement for OPC. The thermal activation method (T) was used to activate ordinary Portland cement-slag mortars (OSMs).

Determination of "optimum heat curing" based on the results obtained, i.e. at 60 °C for 20 hours

Determination of the compressive strengths of specimens at early ages, i.e. 1, 3, and 7 days

Making of OPC-slag mortars for 40% and 50% OPC replacement with slag

Heating of the specimens in water at 60 °C for durations of 2 to 26 hours

Curing of the specimens in water and air under room temperature for duration of 7 days

Recast optimum OSM/50 mix under "optimum heat curing" and determination of compressive strengths of the specimens up to 90 days

Determine the best relationship for strength development using optimum OSM/50 mix

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 loss phenomenon needs to be further investigated.

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

## *4.1.3. Thermal activation method*

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 advantages in arid regions for curing of concrete structures.

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 OPC mortar specimens cured in water, respectively.

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 specially in precast concrete industry.

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 mortars at 40% and 50% OPC replacement with slag are power equations. A relationship exists between the compressive strength of the specimens cured in air under room temperature and water for OPC-slag mortar at 40% and 50% OPC replacement with slag at 3 days, but not at 7 days. Generally, comparison of OPC mortars heated in water bath or oven showed that water bath heat treatment gave better results than those of oven heated. It was also observed that the specimens gave higher strengths in air under room temperature compared to water curing after heating in the water bath at 60 ºC for a duration of 20 hours. This is a significant finding with a major advantage in construction and is also economic since water bath heating can be practically implemented. However, oven heated pre-curing, results in higher strengths whenever the specimens are cured in water after heating.

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 405

SP (g)

Flow (mm)

Water (g)

No Mix name OPC

cured in air under room temperature, wc= water cured.

**Table 8.** Mix proportions of OPC-slag mortars for thermal activation method

(g)

Slag (g)

For OPC mortars, room temperature and water cured 1 OM- ac 1800 ----- 631.7 28 230 2 OM- wc 1800 ----- 631.7 30 230 For OPC-slag mortars for 40% replacement with slag , cured in water and air under room temperature 3 H0/0 720 480 421.11 28 225 4 H60/2 1440 960 842.22 82 230 5 H60/4,6 1440 960 842.22 90 230 6 H60/8,10 1440 960 842.22 79 230 7 H60/12,14 1440 960 842.22 79 230 8 H60/16 1440 960 842.22 82 230 9 H60/18,20 1440 960 842.22 73 230 10 H60/22,24,26 2160 1440 1263.33 70 220 For OPC-slag mortars for 50% replacement with slag, cured in water and air under room temperature 11 H0/0 600 600 421.11 35 230 12 H60/2 1200 1200 842.22 76 235 13 H60/4,6 1200 1200 842.22 91 225 14 H60/8,10 1200 1200 842.22 90 235 15 H60/12,14 1200 1200 842.22 73 235 16 H60/16 1200 1200 842.22 76 235 17 H60/18,20 1200 1200 842.22 62 225 18 H60/22,24,26 1800 1800 1263.33 60 220 OSMs/50 test for three sets of specimens in cured room temperature and in water 25 ºC and 32 ºC after heat treatment 60 ºC for duration 20 hours 19 H- 3 sets mix 900 900 631.7 35 225 OSMs/50 test for four sets of specimens cured in room temperature and in water after with and without use of heat treatment 60 ºC for duration 14 hours 20 H- 4 sets mix 1200 1200 842.22 50 225 For optimum OSM/50 at six ages, only room temperature cured 21 H60/20 900 900 631.7 43 230 H60/i,j,k means 60 ºC temperature with duration i, j, and k hours **Notes**: OSMs/50= OPC-slag mortars for 50% replacement with slag, OMs= OPC mortars, SP= super plasticizer, ac=
