THE EFFECT OF POLYPROPYLENE GLYCOLS ON THE PROPERTIES OF Fe-RICH ALKALI ACTIVATED MATERIALS

Inorganic polymers (IP) are emerging promising building materials due to their high performance and environmental advantages. Despite their technical advantages, such as rapid strength development and good fire and chemical attack resistance1,2,3, IP’s high susceptibility to shrinkage (2 – 4 times higher than the one of ordinary Portland cement3) often results in severe microcrack formation. The high content of unbounded water on alkali activation reaction products and the submicron porous structure of the consolidated polymers induces elevated capillary stresses which impose significant shrinkage during drying. To counteract such phenomena and increase the volumetric stability of inorganic polymers, shrinkage reducing agents have been employed in the formulations. Among them, organic compounds, such as polypropylene glycols (PPG), had been used to synthesize organic-inorganic hybrid polymers showing promising results in metakaolin and blast furnace slag-based systems4,5. PPG are non-ionic surfactants composed by hydrophilic heads and a hydrophobic chain and such amphiphilic character reduces the surface tension at the pore solution/air interface. Additionally, the introduction of PPGs is known to broaden the pore size distribution in IP structures, thus further contributing to the reduction of the internal capillary stresses. Yet, the effectiveness and effects of polypropylene glycols (PPG) on IP features are known to be widely dependent on the synthesis conditions used and their impact on Fe-rich systems remains largely unknown. Considering the expected growing worldwide production of Fe-rich residues and alkali activation as a possible large-scale valorization route, the present work explored the feasibility of using PPG to mitigate the shrinkage of Fe-rich IP. The effects of PPG molecular weight and dosage on the drying shrinkage, porosity, and mechanical properties of resulting IP are described herein. The obtained results showed that hybrid systems presented enhanced volumetric stability relative to pure inorganic matrices. IP volumetric stability is favored by the addition of organic polymers with shorter polymeric chains but mainly controlled by the organic phase content rather than its molar weight. Mercury intrusion porosimetry (MIP) data confirm that hybrid polymers have a broader pore size distribution than their inorganic counterparts, further complementing PPGs amphiphilic character. A maximum drying shrinkage reduction of 58% was achieved compared to pure inorganic samples. Nonetheless, the beneficial effect that PPG exerts on samples’ volumetric stability is accompanied by a consistent deleterious impact on strength development. Pure inorganic samples achieved flexural and compressive strength, after 28 days of curing at room conditions, of 7±1 and 100±12 MPa, respectively. Further, an abrupt decay on IP’s mechanical properties when PPG dosages higher than 3.0 wt% were introduced into the inorganic matrix was observed. A 5.0 wt% addition decreased the flexural and compressive strength, after 28 days of curing to a minimum of 2±0 and 52±2 MPa, respectively. Notwithstanding, the produced alkali activated materials presented reduced shrinkage and interesting mechanical features.