Fiber Bragg grating sensors as a tool to evaluate the influence of filler on shrinkage of geopolymer matrices

Geopolymer matrices represent one of the main sustainable alternatives to ordinary Portland cement (OPC) and other clinker-based blended cements. Real scale applications are limited and a relevant amount of data is still needed to assess the early age and long-term behavior of these systems. Particularly, the early-age monitoring of geopolymers represent a key parameter for mix design optimization. Most of the available methods for the measurement of temperature evolution due to polycondensation kinetics and early age deformations are related to laboratory activities. The upscaling to in situ techniques represents a crucial step toward technological assessment. To this aim, authors propose to use Fiber Bragg Gratings (FBGs) embedded in the geopolymer matrices. Starting from a case study by authors related to the design of externally bonded fiber reinforced geopolymers for strengthening of existing structures, the matrix was optimized in terms of quartz filler content. The measurements carried out by means of FBG sensors allowed to reduce filler content respect to the abovementioned work. Particularly, quartz content can be reduced by 50%. The temperature associated to polycondensation was slightly below 65°C for the three studied systems, limiting the use of designed metakaolin geopolymer to non-massive structures, since thermal cracking could occur, unless further research will be able to assess the viability of retardants. The experimental results confirm that FBG represent an accurate method for simultaneous shrinkage and temperature measurements for geopolymers and the application in real scale structures for remote sensing could help to create database on inner temperatures and early age deformations.