Nanoindentation Reveals How Acid and Alkali Quietly Weaken Granite at the Nanoscale

Granite has long been prized as one of the toughest building materials available to engineers. Its high hardness, abrasion resistance and apparent immunity to chemical attack have made it a mainstay of architectural flooring, underground tunnels and dam foundations. But a new study suggests that this reputation for indestructibility deserves a second look. Researchers from Shaanxi A&F Technology University and Xi’an University of Technology have shown, at the nanoscale, that ordinary hydrochemical environments—acidic, alkaline and saline solutions—can silently corrode the individual mineral grains inside granite, degrading the rock’s mechanical properties grain by grain. The work, published in Frontiers in Earth Science, offers one of the most detailed pictures yet of how water chemistry attacks rock at the scale where its strength actually lives.

The team, led by Man Lei and corresponding author Haibin Xue, took granite specimens from the Qinling Tunnel of the Hanjiang-to-Weihe River Valley Water Diversion Project in China, a massive engineering undertaking that brings water from the Yangtze basin to the Yellow River basin. All specimens came from the same rock mass, ensuring a consistent starting point. The researchers cut the granite into cuboids and then polished them on a metallographic polisher using abrasive papers ranging from 100 to 7000 mesh, with each grit applied for at least 10 minutes and the finest grit for no less than 20 minutes. The goal was to produce surfaces flat and smooth enough for nanoindentation testing, a technique that demands near-perfect specimen preparation. The polished samples were then ultrasonically cleaned in absolute ethanol and dried in an oven at 50 degrees Celsius for at least 24 hours to guarantee complete dryness before any chemical exposure began.

Once prepared, the granite specimens were immersed in 0.1 mol/L sodium chloride solutions adjusted to pH values of 3, 5, 7, 9 and 11, using hydrochloric acid and sodium hydroxide to set the acidic and alkaline conditions respectively. Distilled water at pH 7 served as a neutral control. The solutions were held in 500 mL wide-mouth bottles, with eight specimens laid flat on the bottom of each bottle in 250 mL of corrosive liquid. The immersion durations were 30, 60, 90 and 120 days, spanning four months of continuous chemical attack. The entire experiment ran at temperatures between 17 and 23 degrees Celsius, so thermal effects could be excluded from the analysis. After each soaking period, the corroded specimens were polished and dried again before nanoindentation tests were performed using an Agilent Nano Indenter G200 equipped with a Berkovich diamond tip.

Nanoindentation works by pressing a diamond tip into a material’s surface while continuously recording the applied load and the corresponding penetration depth. In the loading stage, the material undergoes both elastic and plastic deformation simultaneously. During unloading, the recovered elastic deformation provides the basis for calculating mechanical properties such as elastic modulus and hardness. The contact stiffness—the slope of the upper segment of the unloading curve at maximum penetration depth—is the key parameter that links the measured load-displacement data to the reduced modulus and, ultimately, the specimen’s true elastic modulus. The instrument used in this study had a load resolution of 50 nanonewtons and a displacement resolution of 0.01 nanometers, allowing extremely fine measurements of how each mineral grain responds to chemical corrosion. The researchers deployed grid indentation arrays with 100 indentation points per mineral per condition, spaced 100 micrometers apart, to generate statistically robust averages for quartz, feldspar and mica.

The results revealed that the pH of the solutions shifted noticeably during the early stages of corrosion as intensive water-rock reactions took place, before stabilizing in later stages. Neutral and alkaline solutions ended up weakly alkaline, attributed to the high alkali feldspar content of the granite generating hydroxide ions. Acidic solutions drifted toward near-neutral pH as hydrogen ions were consumed by reactions with the rock minerals. Precipitate morphologies also evolved over time: in the pH 3 solution, massive off-white floccules appeared within 10 days, peaked between 60 and 90 days, and declined thereafter, while the pH 11 solution produced abundant reddish-brown coarse particles as early as 10 days. Distilled water generated only slight turbidity and trace black precipitates over the full 120 days, confirming that pure water alone imposes minimal chemical damage on granite’s constituent minerals.

Scanning electron microscopy provided direct visual evidence of how each mineral grain corroded. After 120 days in the pH 3 solution, mica surfaces displayed abundant layered microcracks and uneven roughness, while feldspar formed step-like dissolution terraces along with fine cracks. In the pH 11 solution, quartz surfaces developed numerous irregular etch pits. These distinct corrosion signatures—microcracks, dissolution terraces and etch pits—correspond directly to the chemical compositions of the minerals and the concentrations of hydrogen and hydroxide ions in the surrounding solutions. Minerals with well-developed pore structures reacted faster with corrosive solutions, while those with poorly developed pores corroded at much lower rates, meaning that pore architecture dictated the kinetic rate of hydrochemical attack.

The nanoindentation data showed that elastic modulus and hardness degradation varied dramatically across the three minerals and across solution chemistries. In distilled water, quartz and feldspar suffered only minor modulus attenuation, but mica—due to its relatively high porosity—experienced severe degradation, with a rapid decrease during the first 60 days followed by a slower decline. In acidic solutions, quartz showed nearly linear modulus attenuation, while feldspar and mica degraded rapidly in the initial immersion stage before the rate declined. In alkaline solutions, all three minerals underwent rapid initial modulus loss that gradually slowed. The hardness results followed similar patterns: quartz suffered the most severe hardness loss under alkaline conditions, intensifying with rising alkalinity, whereas feldspar and mica underwent more pronounced hardness reduction in acidic solutions, worsening with increasing acidity.

To quantify the damage, the researchers adopted the Kachanov-Rabotnov damage variable framework and Lemaitre’s strain equivalence principle, defining scalar damage variables based on the ratio of pre- and post-corrosion elastic moduli and hardness values. The results were striking. After 120 days in the strongly acidic pH 3 solution, mica’s elastic modulus damage variable reached approximately 0.29, while its hardness damage variable climbed to 0.65—meaning mica’s hardness degraded more than twice as fast as its elastic modulus under acidic attack. For quartz, both damage variables increased with rising solution pH, confirming that hydroxide ions in alkaline solutions corrode quartz more severely. For feldspar, damage increased as pH decreased, indicating stronger corrosion by hydrogen ions. The elastic modulus damage variable exceeded the hardness damage variable for feldspar, suggesting its stiffness degrades faster than its hardness during immersion.

Using bivariate least-squares fitting, the team established damage evolution equations for elastic modulus and hardness as functions of both pH and soaking duration for each mineral, with coefficients of determination ranging from 0.979 to 0.997—exceptionally high fits that make the equations practical predictive tools. These equations allow engineers to estimate how much stiffness and hardness a given granite mineral will lose after a specified exposure time in a solution of known pH, without needing to run months of laboratory immersion tests. The researchers suggest the findings can provide a scientific reference for evaluating the mechanical degradation and durability of granite in acid rain and saline environments, conditions increasingly relevant as industrial development accelerates water pollution and yields hydrochemical environments with high concentrations of acids and alkalis.

The broader implications extend across geotechnical engineering, tunnel design and dam foundation safety. Long-term immersion in deep groundwater weakens the bonding force between mineral particles through water film lubrication, mineral dissolution and other effects, and prior studies have shown that clay hydration, pyrite oxidation and carbonate dissolution can further exacerbate internal structural degradation in other rock types. By isolating the micromechanical mechanism of granite’s degradation—reactions between solution ions and mineral grains that precipitate reaction products, raise porosity and generate cracks—the study reveals how macroscale weakening emerges from nanoscale chemistry. As underground infrastructure ages in chemically aggressive groundwater, understanding which minerals fail first, and under which pH conditions, could help engineers predict where and when granite structures will lose strength, before the damage becomes visible at the surface.

Subject of Research: Study on the micromechanical properties and damage model of granite under hydrochemical environments based on nanoindentation tests

Article Title: Study on the micromechanical properties and damage model of granite under hydrochemical environments based on nanoindentation tests

Article References: Lei, M., Xue, H., Dang, F.-N., & Ding, W. (2026). Study on the micromechanical properties and damage model of granite under hydrochemical environments based on nanoindentation tests. Frontiers in Earth Science, 14, Article 1874086. https://doi.org/10.3389/feart.2026.1874086

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1874086

Keywords: micromechanical, properties, damage, model, granite, under, hydrochemical, environments, based, nanoindentation, tests, scientific research