Microstructural investigation of alkali silica and geopolymer gels

Abstract

For the past decades, alkali silica reaction (ASR) has been a major concern to concrete structures. Typified by the chemical interaction between reactive siliceous minerals from the aggregate and alkalis commonly provided by ordinary Portland cement (OPC), this deleterious process results in the formation of a hydrophilic amorphous gel known as alkali silica gel (A-S-G). Regarding its mitigation methods, it is well-known the beneficial effect of incorporating SCM over the concrete mixture. Moreover, there seems to be a tendency that alumina-rich SCM performs better when compared to its counterparts. Despite the acknowledged positive effect, there is no consensus on how aluminum mitigates ASR. Concurrently, a promising cementitious material generally referred to as geopolymer is receiving ever-growing interest due to its remarkable properties and low carbon footprint. It is characterized by the formation of an alkaline aluminosilicate gel (N-A-S-H), which is responsible for its binding properties. Due to its similarity both in terms of chemical composition and atomic structural ordering, a comparative study between A-S-G and N-AS- H is possible. Thus, the present study aims to investigate the aluminum incorporation into alkali silicate structures within the chemical composition in between that of A-S-G and NA- S-H phases. To accomplish this, hydrothermal synthesis method was employed, using solutions with appropriate quantities of Al(OH)3, Na2SiO3 and NaOH, mixed and left reacting at 60 °C for 30 days. Afterward, the synthetic gels were dried at 90 ºC for 15 days and ground in a ceramic ball mill. The resulting reaction products were characterized by means of XRD, FTIR, XRF, SEM/EDS and TGA/DSC. The XRD revealed the predominantly amorphous nature of the gels, yet a combination of increase in aluminum and alkali favored the formation of zeolite minerals. The TGA and FTIR analyses exposed the hydrophilic character of the gels and detected that the incorporation of aluminum reduced the weakly-bonded water absorption capacity, which is associated with the ASR expansion. Furthermore, the SEM/EDS spot analysis revealed the incorporation of aluminum in alkali silica gel with an Al/Si ratio of about 0,025 to 0,05. These findings indicate that aluminum species released from SCM can be incorporated into the non-calcium alkali silica gel chemical composition and, as a result, change its water absorption capacity.