Blast Furnace Slag (BFS) is a latently hydraulic material used as additive or substitute for Portland cement. The market share for BFS cement in the Netherlands is currently more than 60%. Previous research proved the high resistance of slag-rich concrete in aggressive environments such as exposure to acid, chloride ingress or sulphate attack, but also the poor resistance against carbonation. The carbonation of a Blast Furnace Slag (BFS) cement paste involves the conversion of calcium silicate hydrate gel (also known as C-S-H) to porous silicate hydrates. As a result, it can be expected that the strength of the matrix decreases. Consequently, the carbonated BFS concrete is vulnerable to scaling under the combined load of freezing–thawing and de-icing salt. The disintegration of the concrete surface results in a higher vulnerability to other chemical attacks decreasing its durability. Sodium monofluorophosphate (Na-MFP) gained attention as surface treatment as a corrosion inhibitor. Recent studies revealed the recovering effect of Na-MFP on the microstructure of carbonated BFSC pastes with respect to their frost-salt scaling durability. Recently published, a study by Kempl and Çopuroğlu (2016) indicated that the impregnation of hardened carbonated cement pastes with aqueous Na-MFP resulted in a recovery of the initial pH of 98.85% and 79.81%, of an untreated cement paste for CEM I and CEM III/B, respectively. Their results brought insights on the reaction of Na-MFP solution with the cement matrix carbonated or non-carbonated. The goal of this thesis was to investigate the influence of applying sodium monofluorophosphate (Na-MFP) on the microstructural and micromechanical properties upon a surface treatment for carbonated BFS cement with different slag concentrations. The main objective of this study is split into two parts: 1) to investigate the penetration of 25% Na-MFP solution as surface treatment into the CEM III /B cement mortar or cement paste under different curing conditions and carbonation periods, and 2) to characterize the influence of the solution on the micromechanics and microstructure of the cement paste samples. The cement types used in this study were: 1. Ordinary Portland Cement (CEM I 42,5 N) which serves as a reference 2. Blast Furnace Slag Cement with a slag content of 67% (CEM III/B 42,5 N HSR LH) 3. Ternary blended cement (SP Mix) with a slag content of 55%, 30% CEM I 52,5 R and 15% fly ash. For the first part of the study polarized light microscopy (PLM), Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX) and stereomicroscope were used for analysing the penetration of the Na-MFP solution into the cement matrix from the surface. For the second part, analysing the effect of the Na-MFP solution on the microstructure of the cement paste the following tests was applied: Nano-indentation (NI), Mercury Intrusion Porosimetry (MIP), and Thermogravimetric Analysis/Differential Scanning Calorimetry (TGA/DSC). The penetration results indicate that applying the Na-MFP solution once on the surface after the CEM III /B mortar sample (W/C=0.45) was carbonated was more effective than before carbonation. The penetration depth increased at increasing carbonation period. The curing methods, wet or sealed, do not show a significant influence on the penetration depth. The penetration depth for mortar samples was higher compared to the paste sample with the same water-cement ratio (W/C), cement type and carbonation period. Applying the Na-MFP solution on the surface of ternary blended cement paste, as used in the research (SP Mix), resulted in a higher resistance to carbonation compared to slag cement paste with the same W/C and carbonation period. The results of the microstructure analysis showed that the Na-MFP solution improved the strength (the elastic modulus and the hardness), permeability (decrease in the porosity) and provided high resistance against carbonation. The study showed that applying the Na-MFP solution before carbonation increased the porosity for the CEM III/B and SP Mix cement paste especially, with W/C =0.6. The recover capillary porosity due to carbonation was 38% and 26% for CEM III/B and SP MIX cement paste, respectively. In conclusion, the microstructural properties of the treated cement matrix were affected by the water - cement ratio in conjunction with the condition of the cement paste, whether the sample was carbonated or non-carbonated.