# Components of the Fracture Response of Alkali-Activated Slag Composites with Steel Microfibers
## 4 Conclusions  
Because of the lack of information concerning the fracture properties of composites with alkali-activated matrix which could be used as inputs for material models, the main aim of the present research was to assess the fracture parameters of alkali-activated slag composites with steel microfibers and quantify the contribution of the matrix of AAS composites to their fracture response. The following conclusions can be drawn based on the obtained experimental research results:  
* The compressive strength value increased by up to 40% in the case of composite with a steel microfiber content of more than 10%;
* The modulus of elasticity increased with the addition of steel microfibers by about 30-40%. The highest mean value was obtained for the composite with a 5% content of steel microfibers by weight. If a comparison is made between AAS matrix alone and the composite including steel microfibers, it can be seen that the values are almost the same.  
Generally, the addition of steel microfibers to AAS matrix should contribute to a lowering in the tendency to crack, and to an enhancement in the tensile properties of AAS based materials. The presented research results are in line with these suppositions:  
* The addition of steel microfibers to the AAS matrix caused the resistance to unstable crack propagation expressed here by the fracture toughness to gradually increase by up to 50% for the composite with a 20% content of steel microfibers;
* The steel microfiber-reinforced AAS composites proved to have a much better load carrying capacity after the maximum load is reached in comparison with the reference composite;
* The addition of steel microfibers to the AAS matrix caused the fracture energy value to gradually increase by up to 2.5 times the reference composite value for the composite with a 20% content of steel microfibers; the energy absorption mechanism is related to the de-bonding and pull-out of microfibers that bridge cracks.  
Based on the performed study, the addition of 10 to 15% by weight of microfibers to AAS composite is optimal from the point of view of the enhancement of the fracture properties of this composite. The obtained results can be used when designing alternative binders for Portland cement, as well as relevant input data for material models for the realistic numerical modeling of the response of structures or their parts which are made from this type of composite.  
Figure 10: Fracture toughness ratio (**a**) and load ratio (**b**) of AAS mortar with different amounts of steel microfibers.  
**Author Contributions:** Conceptualization, P.R.; methodology, H.S. and P.F.; software, P.F.; validation, P.R., P.F. and Z.K.; formal analysis, H.S. and P.F.; investigation, P.R., P.S. and H.S.; resources, P.R. and P.S.; data curation, H.S., P.F. and P.R.; writing--original draft preparation, H.S. and P.F.; writing--review and editing, P.R. and Z.K.; visualization, H.S.; supervision, P.R. and Z.K.; project administration, P.R.  
**Funding:** This researcher was funded by the Czech Science Foundation, project No. 16-00567S, and the Ministry of Education, Youth and Sports of the Czech Republic under \"National Sustainability Programme I\" (project No. LO1408 AdMa5 UP).  
**Conflicts of Interest:** The authors declare no conflict of interest.