# Formulation of Alkali-Activated Slag Binder Destined for Use in Developing Countries
## 3 Results  
### Mechanical Strength  
Figures 4 and 5 represent the flexural (Rf) and compressive (Rc) strengths of all the mortars studied for 7, 28 and 90 days curing, respectively. After analyzing the flexural strength results separately for the three studied mixtures, namely NaOH (SH series), Na\({}_{2}\)CO\({}_{3}\) (SC series), and NaOH and Na\({}_{2}\)CO\({}_{3}\) mixtures (SHC series), it highlights the following results: (a) for NaOH, an increase in Rf took place between 7 and 28 curing days. For the 90 days curing time, Rf decreased with increased activator molarity; (b) for Na\({}_{2}\)CO\({}_{3}\) as an activator after 7 curing days, all the mortars had similar flexural strength values, except for the mortar with the lower quantity activator. After 28 curing days, all the values were similar, and strength slightly decreased in all the mortars after 90 curing days except for the mortar SC3; (c) when the NaOH and Na\({}_{2}\)CO\({}_{3}\) mixture was used as the activator, the mortar with the lesser Na\({}_{2}\)CO\({}_{3}\) quantity obtained the lower strength value at 7 curing days, but no significant difference among the three types of mortars was observed after 28 and 90 curing days. The flexural strengths after 28 curing days of the mortars were higher than after 90 curing days. The decreased flexural strength from 28 days to 90 days was attributed to the shrinkage cracks, in line with what [PERSON] et al. [24] suggested. Many of the mortars with the three different activators reached a flexural strength between 8 and 10 MPa. When the three activator mixtures types were compared to one another, it concluded that the mortars containing Na\({}_{2}\)CO\({}_{3}\) had higher flexural strength than those activated with only NaOH. Figure 5 represents the Rc of all the studied mortars. The obtained results showed that Na\({}_{2}\)CO\({}_{3}\) was the activator that yielded the highest Rc values. After studying the results for each tested activator, we highlight the following observations:  
1. When NaOH is used, for the studied molarities range, there was no global correlation between the molarity of the NaOH solution and the Rc for the three studied curing days. An increase in Rc was observed with curing time. For any mortar, the compressive strength at 90 days exceeded 30 MPa. [PERSON] et al. [25] reported for NaOH that variation in the quantity of NaOH within 3-11% of the range (per slag weight) had very little effect on strength. In the present study, the NaOH percentages varied between 4.8% (SH3) and 8% (SH5). For the wider variations in the NaOH concentration, other authors obtained significant differences in compressive strength. For example, [PERSON] et al. [26] made mortars with BFS activated with NaOH and KOH: for the NaOH-activated mortars, those activated with 4 M of NaOH (6.4% per slag weight) at  
Figure 5: Compressive strength (Rc) of mortars for 3, 28 and 90 curing days. 65 \({}^{\circ}\)C for 7 days obtained a compressive strength of 17.47 MPa compared to 31.11 MPa obtained by the mortar activated with 12 M of NaOH (19.2% per slag weight). 2. The Na\({}_{2}\)CO\({}_{3}\)-activated mortars displayed much better mechanical performance than the corresponding NaOH-activated ones. The Rc values at 90 curing days fell within the 40-55 MPa range; that is, about twice that for NaOH. An increase in Rc was observed when increasing the quantity of Na\({}_{2}\)CO\({}_{3}\) as the activator (SC 2.5, SC 3 and SC 3.5). These results are consistent with other authors, such as [PERSON] et al. [27], who obtained the best compressive strengths for the mortars with the biggest Na\({}_{2}\)CO\({}_{3}\) quantity (analyzed range between 3 and 5% for Na\({}_{2}\)O wt %). It is remarkable that for 7 curing days, the Rc values were higher than those for the NaOH-activated mortars after 90 days. This means that the activation rate of slag by Na\({}_{2}\)CO\({}_{3}\) was high, and good early age mechanical performance was achieved.