Effects of calcium ions on the alkali–silica reaction of seawater–sea sand concrete

This study investigated the composition of alkali–silica reaction (ASR) products formed in mortar and concrete that underwent accelerated ASR testing using two test methods: the accelerated mortar bar test (AMBT) and the simulated pore solution immersion test (SPSM). The composition of the ASR products formed in the accelerated tests was compared with those in a 25-year old bridge in New South Wales demolished due to ASR. Results showed that the ASR products inside an aggregate contained calcium (≈20%), silicon (≈60%), and alkalis (≈20%) regardless of the ASR test method used. The ASR products in the AMBT sample only contained sodium, whereas the ASR products in the SPSM test and the demolished bridge both contained significant amounts of sodium and potassium, which indicated that the type of alkali in the ASR product is largely affected by the dominant alkali in the pore solution. However, considering that the total alkali content (Na + K) in the ASR products was similar regardless of the ASR test method used, this suggests that the total alkali content has more influence on the rate of expansion than the type of alkali. The composition of the ASR products also notably varied depending on the location in the concrete. ASR products closer to the cement paste had a higher calcium and lower alkali content than those inside an aggregate, which suggests that the calcium as well as the alkali content of the ASR products plays a significant role in the degree of ASR expansion.

Influences of aggregate gradation on alkali-silica reaction of seawater and sea sand concrete

Nano-mechanical properties of alkali-silica reaction (ASR) products in concrete measured by nano-indentation

The combined effect of potassium, sodium and calcium on the formation of alkali-silica reaction products

In the last four years, a multidisciplinary study involving several research groups in Switzerland tackled a number of unsolved, fundamental issues about the alkali-silica reaction (ASR) in concrete. The covered topics include SiO2 dissolution, the characterization of various ASR products formed at different stages of the reaction in both concrete and synthesis, crack formation and propagation. The encompassed scale ranges from nanometers to meters. Apart from conventional techniques, novel methods for the field of ASR have been used, e.g. combination of scanning electron microscopy with dissolution experiments, combination of focused ion beam with transmission electron microscopy, several synchrotron-based methods, synthesis of ASR products for in-depth characterization, time-lapse X-ray micro-tomography combined with contrast-enhancing measures and numerical models of ASR damage based on realistic crack patterns. Key achievements and findings are the quantification of the effect of aluminum on dissolution of different silicates, the variance in morphology and composition of initial ASR products, the differences and similarities between amorphous ASR products and calcium-silicate-hydrate, the link between temperature and the structure of the crystalline ASR products, the behavior of the crystalline ASR products at varying relative humidity, ASR propagation in 4D and numerical modelling based on realistic crack patterns.

Stress-relaxation of crystalline alkali-silica reaction products: Characterization by micro- and nanoindentation and simplified modeling

The application of seawater and sea sand concrete (SWSSC) can reduce the construction period and cost of island infrastructure, but it may also bring the risk of alkali–silica reaction (ASR) owing to the presence of alkali ions in seawater and sea sand. To compare the characteristics of ASR between SWSSC and seawater and desalinated sea sand (DSS) concrete, the properties and the ASR products of mortar bars with different DSS content were studied. When the DSS proportion increased from 0% to 100%, the sodium (Na+), potassium (K+) and calcium (Ca2+) ion concentration of the specimens decreased by 22.6, 2.0 and 45.1 mg L−1, respectively, the pH decreased by 0.05 and the expansion of mortar bars was reduced by 0.16%. Desalination of sea sand could not eliminate the risk of ASR of SWSSC completely. The 14 days expansion of mortar bars with 100% DSS was 0.13%, and the precursors of ASR-P1 were observed by SEM. The experimental results of micriscopic tests all showed that with the increase of DSS proportion, the content of Na-shlykovite and ASR-P1 were reduced. A small amount of magnesium (Mg) in ASR products was detected. This study can provide a basis for the application of SWSSC in island infrastructure.

Impact of aggregate mineralogy and exposure solution on alkali-silica reaction product composition and structure within accelerated test conditions

Effect of calcium hydroxide on the alkali-silica reaction of alkali-activated slag mortars activated by sodium hydroxide

Amorphous and crystalline alkali silica reaction (ASR) products formed in aggregates of two different concrete mixtures exposed to the concrete prim test both at 38 °C and 60 °C have been analysed by scanning electron microscope with energy dispersive X-ray spectroscopy and by Raman microscopy. Additionally, amorphous ASR products were synthesized and analysed with Raman microscopy and 29Si nuclear magnetic resonance. Amorphous ASR products display a higher Na/K-ratio than crystalline ones. Both types of products display a structure dominated by Q3-sites (Si-tetrahedra with three bridging oxygen atoms typical for a layer structure) with a secondary amount of Q2-sites (Si-tetrahedra with two bridging oxygen atoms typical for a chain structure). Temperature in the CPT alters the structure of the crystalline ASR. While the Raman spectra of the product formed at 38 °C is identical to the one formed in concrete structures, the one of the 60 °C product corresponds to K-shlykovite.

An alkali-silica reaction (ASR) is a chemical process that leads to the formation of an expansive gel, potentially causing durability issues in concrete structures. This article investigates the properties and behaviour of ASR products in mortar with the addition of low-purity calcined clay as an additional material. This study includes an evaluation of the expansion and microstructural characteristics of the mortar, as well as an analysis of the formation and behaviour of ASR products with different contents of calcined clay. Expansion tests of the mortar beam specimens were conducted according to ASTM C1567, and a detailed microscopic analysis of the reaction products was performed. Additionally, their mechanical properties were determined using nanoindentation. This study reveals that with an increasing calcined clay content, the amount of the crystalline form of the ASR gel decreases, while the nanohardness increases. The Young's modulus of the amorphous ASR products ranged from 5 to 12 GPa, while the nanohardness ranged from 0.41 to 0.67 GPa. The obtained results contribute to a better understanding of how the incorporation of low-purity calcined clay influences the ASR in mortar, providing valuable insights into developing sustainable and durable building materials for the construction industry.

Evaluation of the biotransformation of alkali-silica reaction products by Alkalihalobacillus clausii and Bacillusthuringiensis

Formation of shlykovite and ASR-P1 in concrete under accelerated alkali-silica reaction at 60 and 80 °C

Atomic force microscopy characterisation of alkali-silica reaction products to reveal their nanostructure and formation mechanism

The use of Al-rich supplementary cementitious materials can effectively reduce the expansion of concrete caused by alkali-silica reaction (ASR). However, the role of Al in mitigating the formation and structure of ASR products is only poorly understood, since direct investigation of the reaction in the micro-scale veins of aggregates is difficult. Recent successful synthesis of ASR products provides a unique opportunity to directly assess the role of Al in ASR. In this study, the effect of Al on the formation and structure of ASR products synthesized at 40 and 80 °C is investigated. It is found that the presence of Al in concentration lower than 0.1 mM neither prevents ASR nor alters the structure of the crystalline ASR products formed at 40 °C. However, formation of the crystalline ASR product at 80 °C is lowered at higher Al content due to the formation of a zeolitic precursor.