Investigation of Alkali-Silica Reaction in Cement Concrete

Investigation of Alkali-Silica Reaction in Cement Concrete
1 Introduction
The alkali-silica reaction (ASR), often referred to as "concrete cancer," is one of the most significant internal deterioration mechanisms affecting the durability of cement concrete structures worldwide. It is a deleterious swelling reaction that occurs over time between the highly alkaline pore solution of cement paste and reactive amorphous silica present in certain aggregates, given sufficient moisture.
The reaction was first identified by Thomas E. Stanton in California during the 1930s, with his founding publication appearing in 1940. Since then, ASR has been recognized as a global problem affecting concrete structures ranging from dams and bridges to pavements and nuclear facilities. The reaction produces a hygroscopic alkali-silica gel that expands upon absorbing water, generating internal pressures that can exceed the tensile strength of concrete, leading to cracking, spalling, and potential structural failure.
The economic and safety implications of ASR are substantial. Damage caused by ASR can force the demolition of critical infrastructure, and the reaction is often observed in combination with other deterioration mechanisms such as corrosion and sulfate attack, accelerating overall degradation. Understanding the mechanisms, influencing factors, and mitigation strategies for ASR is therefore essential for designing durable concrete structures.
2 Mechanisms of Alkali-Silica Reaction
2.1 Fundamental Chemical Processes
The ASR is a multi-stage process involving several sequential reactions: (1) dissolution of metastable silica, (2) formation of nano-colloidal silica sol, (3) gelation of the sol, and (4) swelling of the gel. The overall reaction can be understood as a mineral acid-base reaction between alkali hydroxides (NaOH and KOH), calcium hydroxide (portlandite), and silicic acid.
The first step is the attack of siloxane (Si-O-Si) bonds in the aggregate by hydroxide ions. The alkali hydroxides dissociate in the pore solution, providing OH⁻ ions that break the siloxane bridges, depolymerizing the silica structure:
≡Si−O−Si≡+2OH−→2≡Si−O−+H2O≡Si−O−Si≡+2OH−→2≡Si−O−+H2O
The silicic acid formed is a weak acid and immediately reacts with additional hydroxyl ions to form alkali silicate (the ASR gel):
H4SiO4+2OH−→SiO32−+3H2OH4SiO4+2OH−→SiO32−+3H2O
A simplified representation of the complete reaction after exchange of alkali cations with calcium ions is:
Ca(OH)2+H4SiO4→Ca2++H2SiO42−+2H2O→CaH2SiO4Ca(OH)2+H4SiO4→Ca2++H2SiO42−+2H2O→CaH2SiO4
2.2 The Role of Calcium and Reaction Rims
Calcium plays a dual role in ASR. First, it participates in "alkali recycling"—exchanging with alkalis in the reaction products, which allows alkalis to return to the pore solution and continue attacking silica. Second, calcium influences the composition and properties of the ASR gel, determining whether it is expansive or non-expansive.
Research has demonstrated that the alkali silicate generated by ASR has no ability to generate expansive pressure unless the aggregate is tightly packed with a reaction rim. The reaction rim is slowly generated from the alkali silicate that covers the ASR-affected aggregate. Consumption of alkali hydroxide by the ASR induces dissolution of Ca²⁺ ions into the pore solution. The alkali silicate then reacts with Ca²⁺ ions to convert to an insoluble, tight, and rigid reaction rim. This rim allows penetration of alkaline solution but prevents leakage of viscous alkali silicate, so that the gel generated afterward accumulates within the aggregate to produce expansive pressure sufficient for cracking.
2.3 Gel Composition and Swelling
The ASR gel is a hydrated alkali silicate with the general formula Na₂SiO₃•nH₂O or K₂SiO₃•nH₂O. The gel is hygroscopic and swells by imbibing water from the surrounding pore solution. The swelling pressure developed during this process can reach up to 20 N/mm², far exceeding the tensile strength of normal concrete.
The composition of the gel is critical to its expansive behavior. Powers and Steinour proposed that reactions could be "safe" or "unsafe" depending on the relative amounts of calcium and alkalis in the reaction products. If a high calcium concentration is present in the pore water, a non-expansive gel containing a high amount of calcium is formed. However, if the calcium concentration is low, alkali-rich swelling gels are formed. This model, while often debated, remains a foundational concept in ASR research.

    Date: 2026-09-18 00:00:00.000000