# A Low-Autogenous-Shrinkage Alkali-Activated Slag and Fly Ash Concrete
## 3 Results and Discussion
### Strength of Concrete  
The strength development of AASFICMK concrete is shown in Figure 6 with the plain AASF concrete for comparison. It can be seen that with the incorporation of SAPs and MK, the compressive and splitting tensile strength of AASFICMK concrete was generally lower than that of AASF concrete. The reduced strength was contributed by both SAPs and MK. To provide internal curing to the concrete, extra liquid was added during mixing to be absorbed by the SAPs (see Table 4). The SAPs after absorption would act as liquid reservoirs during reaction and also as defects due to the large voids left when the liquid was released. The increased porosity of the concrete led to reduced strength [11]. Besides, the incorporation of MK was found to hinder the reaction rate in the acceleration period and could therefore reduce the strength in the very early age [17], although its impact on the 28-day strength was minor. When SAPs and MK were added together, their reducing effects were combined. Nonetheless, the 1-day compressive strength of AASFICMK concrete reached 2.1 MPa, which enabled a successful demolding at that age. The 28-day compressive strength of AASFICMK concrete reached 51 MPa, which was already sufficient for most structural uses as specified, for example, in the standard ACI 318 [33].  
Besides strength values, the splitting tensile strength-to-compressive strength (f\({}_{\text{f}}\)/f\({}_{\text{c}}\)) ratio is also an important parameter that allows for the estimation of f\({}_{\text{t}}\) by knowing f\({}_{\text{c}}\) or vice versa [34]. The ratio also provides insight into the stress type (compression or tension) to which the concrete is more prone. The f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratio of AASFICMK concrete is compared with that of AASF concrete in Figure 7. On the first day, the f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratio of AASFICMK concrete was lower than that of AASF concrete which was probably because that the bonding between the aggregate and the paste in AASFICMK was still weak due to the retarding effect of MK and SAPs on the early-age reaction rates of AASF [10,11]. After the first day, however, the f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratios of AASFICMK concrete were always higher than those of AASF concrete. The higher f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratio of AASFICMK indicates that the incorporation of MK and SAPs could improve the tensile resistance of AASF concrete.  
According to [31,35], a low f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratio is related to the development of microcracking in the concrete, for example in the paste surrounding aggregates, which harms the tensile strength more than the compressive strength of concrete. As shown in Figures 3 and 4, the incorporation of SAPs and MK reduced the autogenous shrinkage and the cracking potential of AASF paste. Therefore, the development of microcracking in AASFICMK concrete was supposed to be less severe than in AASF concrete. This may be the main reason why AASFICMK concrete showed a higher f\({}_{\text{f}}\)/f\({}_{\text{c}}\) ratio than the  
Figure 5: Slump (**a**) and flowability (**b**) of AASFICMK concrete.  
plain AASF concrete. Whether the combination of SAPs and MK can reduce the autogenous shrinkage and the potential for cracking of concrete at the macro level will be verified in the next sections.  
Figure 6: Compressive (**a**) and splitting strength (**b**) of AASFICMK concrete in comparison with AASF concrete. For splitting strength, the error bar is shown in the diagram, but it is too small to be clearly distinguished from the marker.  
Figure 7: Splitting tensile strength-to-compressive strength (f\({}_{\text{t}}\)/f\({}_{\text{c}}\)) ratios of AASFICMK concrete, in comparison with AASF concrete.