Investigation of Alkali-Silica Reactions in Different Cement Systems by Thermodynamic Modelling

Alkali silica reaction (ASR) is a crucial concrete durability problem that causes swelling and degradation in the process of service. It involves the chemical reaction between the reactive silica content of aggregates used in concrete, and alkali sources in the pore solution or from the aggregate itself. This problem, also named as “concrete cancer”, has been reported for more than 80 years with respect to its mechanism and mitigation methods. However, due to the variable material compositions and different testing conditions and duration time, controversy between opinions is always presented in the literature. Therefore, the aim of this study is to use thermodynamic modelling instead of experimental methods to investigate ASR, eliminating the effects of variabilities mentioned above.
Thermodynamic parameters of shlykovite-type ASR products, which have similar microstructure and chemistry to field-observed ASR products, were firstly refined and built up in this study. New pH values of two shlykovite products gained from GEM-Selektor v.3 software were implemented and used in the calculation of solubility products. Other thermodynamic parameters were also introduced through a correlation based on formula unit volumes. Binary phase diagrams were used to show the relationship between oxide concentrations and phases formed. Without alkali ions, only C-S-H and amorphous silica were found. Increasing the concentration of alkalis leads to the formation of shlykovite-type products. However, there is a maximum concentration of alkalis that enables the formation of ASR products due to the suppression of calcium solubility.
The well-agreed method to mitigate ASR is the incorporation of supplementary cementitious materials (SCMs). The study here systematically selected four types of SCMs (ground granulated blast furnace slag (GGBFS), silica fume (SF), metakaolin (MK) and fly ash (FA)) and explained each role in mitigating ASR via thermodynamic modelling. A newly-designed two-step modelling method was used to separate the process of hydration and ASR, to establish the relationship among amount of portlandite, alkali contents in the pore solution, and the volume of ASR products formed for the first time.
The simulation results indicated that the consumption of Ca(OH)2 and the reduction of alkali content in the pore solution decrease the volume of K- and Na-shlykovite products formed. The former aspect is responsible for the hydration via latent hydraulic or pozzolanic reactions, while the latter is attributed to a dilution effect and formation of more of the alkali-bearing end-member of the C-S-H solid solution. However, Al-rich SCMs (MK and FA) have weaker ASR-resisting performance based on the modelling outcomes, which contrasts with experiments. This does not mean that this modelling fails to work in these materials. Rather, this indirectly supports the idea that the role of Al is to slow the dissolution of reactive silica from aggregates, rather than forming Al-bearing zeolites.
Lastly, by changing the types, dosages, and modulus of activators (NaOH, Na2SO4, Na2CO3, Na2SiO3), the thesis investigates ASR in alkali-activated slag cements. Compared with Portland cement, there is no K-shlykovite and much less Na-shlykovite predicted to form in the simulations of alkali activated cements. Na-shlykovite was formed only with 0.5-2% and 8% equivalent Na2O added as NaOH, and only formed at the lowest dosage simulated (2%) for the rest of the activators. For waterglass activation, the volume of ASR products increases when increasing the waterglass modulus. Combining the results of solution chemistry and phase assemblages, the higher concentration of Ca in the pore solution after hydration is the main reason for forming Na-shlykovite. Even though there is a much higher concentration of alkali in the pore solution of alkali-activated cement, less portlandite was found and this avoids triggering the process of ASR.
The results shown in this study prove that it is plausible to observe and analyse the formation of ASR products and reaction process by using thermodynamic modelling. The improved thermodynamic database for shlykovite-type products at ambient temperature fills in gaps because of the lack of the understanding about ASR products, and improves the accuracy of ASR simulation. The thesis provides new thermodynamic insights into the mechanism by which SCMs enhance concrete durability against ASR. Raw materials with unknown compositions may also have a new and time-saving way to predict expansion, instead of depending on slow laboratory testing methods. This advantage is also reflected in different cement systems.