# Surface Modification of Glass Textile for the Reinforcement of a Cement-Based Composite: A Review
## 4 Modification Techniques of Glass Fibers
### Inorganic Coatings  
When yarns are pre-impregnated with polymeric coatings, the penetration of the mortar within the filaments is prevented, so that the stress transfer between the filaments is no longer dependent on the ability of the mortar to penetrate into the yarn but rather from the bond at the interface between coating and matrix. In this case, the system is actually a glass-fiber polymer composite embedded in a cementitious matrix. Furthermore, since the coating prevents the penetration of the cementitious matrix into the yarn, the stress transfer between filaments strongly depends on the physical-mechanical propertiesof the coating itself. Moreover, some studies argue that the introduction of a polymeric layer between the reinforcement and the matrix can make the composite more sensitive to high temperatures and compromise its fire resistance [68,69]. Indeed, the reduction of the mechanical performance of FRCM composites exposed to temperature over 80 \({}^{\circ}\)C was reported for epoxy pre-impregnated carbon fabrics [68]. In order to overcome this issue, some studies propose the application of inorganic coatings instead of polymer based coatings [63,70,71,72,73,74,75,76,77,78]. [PERSON] at al. prepared a cement-in-polymer dispersion using a ultrafine grained cement with a particle size of <7 \(\upmu\)m and water-soluble polymers (poly vinyl alchol (PVA), poly(ethylene-co-vinyl acetate) (EVA), and poly(vinyl acetate) (PVAc)) [77,79]. They report that composites realized using the AR-Glass roving impregnated with the inorganic coating show maximum peak stress three times higher than the value obtained for the same sample reinforced with the untreated commercial fabric. The inorganic coating is also reported to change the pull-out behavior of the composite, which instead of experimenting with a sudden drop of the pull-out load after debonding displays a further increase. By conducing SEM-EDX analysis the authors could verify that the superior properties observed are due to the full penetration of the inorganic matrix into the yarn. Indeed, according to the author, the small amount of polymer contained in the applied coating (80% cement, 20% polymer) forms a thin film on the cement grains and the glass filament which dissolves when it comes in contact with the fresh cement-paste. In this way the non-hydrated cement contained in the inorganic coating can react with the Portland cement and improve the reinforcement embedding within the inorganic matrix. However, the high content of Portland cement contained in the coating provides a highly alkaline environment close to the glass fibers, which leads to a sever loss of the composite tensile strength over time. In another study the authors proposed the addition of active pozzolans in the coatings (metakaolin, and silicon dioxides), as a possible solution to decrease the pH in the vicinity of the glass filaments [80]. Although in general the increase of the pull-out load was lower compared to fibers coated with the cement/PVAc coating, the replacement of Portland cement with silica leads to a 1.6-fold increase in the residual tensile strength of the composite. [PERSON] et al. proposed a dry process for the pre-impregnation of glass yarn with inorganic powder, which makes use of a high-intensity alternating electrostatic field to facilitate the penetration of the powder into the yarn [70]. The process consists of passing the yarn covered with the powder between two electrodes where a strong alternative electrostatic field is created by applying high tension. When the yarn exits the dielectrics, the powder has homogeneously penetrated between the single filaments. Before casting the yarns are sprayed with water. The authors report an improved matrix penetration between the filaments which corresponds to superior tensile properties of the composite. Different studies propose the impregnation of the reinforcement with inorganic coating based on silica particles [63,72,75,76]. Silica coatings can be obtained by using silica particles of different dimensions.