Causes and mechanisms of decorative stone diseases

Shanghai Jiani Material Technology Co., Ltd. is a professional company specializing in the production of stone tiles.

English Translation

Stone defects stem from diverse causes, including inherent properties of the stone itself, improper handling during processing and transportation, flawed construction procedures, and wear and tear arising from daily service. Close observation reveals that defects occurring at each stage present distinct characteristics.

1. Defects arising during stone processing and transportation are mainly caused by faulty processing equipment and inadequate protective measures in transit.

Sandblasting is a common stone finishing technique. Under compressed air pressure, iron grit is blasted onto stone surfaces to create a rough textured finish. Tiny shattered iron particles tend to remain embedded on the treated surface. Exposed to rainwater afterwards, the stone will gradually develop yellow discoloration and rust stains. We encountered this exact issue during the defect remediation project for honeycomb composite stone cladding on the new main building of Beijing Normal University.

The rooftop of the building features a steel-framed translucent ceiling. Subject to strict load-bearing restrictions, over 2,000 square meters of aluminum honeycomb composite stone panels were adopted. The stone veneer of these panels is merely 3 to 5 millimeters thick, making flame finishing infeasible, so sandblasting became the only viable surface treatment method.

Rust stains emerged widely on the stone immediately after delivery to the construction site. The manufacturer applied special stone rust remover and eliminated the yellow rust marks. We recommended implementing stone protection treatment after rust removal, yet the suggestion was not adopted. During the period from scaffold dismantling to final project acceptance, extensive rust discoloration reappeared on the stone.

The stone is installed on the cornice more than 30 meters above the ground, with a translucent roof overhead and no accessible footholds. Rust removal and protective treatment became extremely difficult and hazardous work, pushing up construction costs and prolonging the project schedule.

2. Stone defects are closely associated with construction techniques and stone varieties. Fundamentally, water as well as salts and alkalis contained in cement serve as the primary culprits. Cement acts as the major cementitious material for wet-laid stonework. It contains calcium ions and sodium ions, which react with water during mixing to form calcium hydroxide and sodium hydroxide, alongside abundant inherent salts. These soluble substances permeate into stone together with moisture. Wet-installed stone surfaces tend to darken for an extended period, and improper routine maintenance will eventually lead to water spot formation.

The formation mechanism of water stains is illustrated by the following comparative experiment. Six granite slabs were divided into two groups of three specimens each. The first group was soaked in purified water for 24 hours, then sealed with plastic tape on the back and side surfaces. The second group was immersed for the same duration in an aqueous solution mixed with 10% saline-alkali substances and a small amount of cement, followed by identical sealing treatment.

All specimens were baked under high-temperature lamps. Water marks faded on all samples three days later. After one week, specimens soaked in purified water regained their original dry color, while noticeable water stains persisted on those treated with saline-alkali mixed solution. Full color recovery took two weeks for the mixed solution group. Repeated trials showed that the drying cycle grew progressively longer for stone exposed to saline-alkali liquid. In contrast, once the sealing tape was removed, internal moisture evaporated rapidly and water stains vanished within several days.

The experimental results explain the formation process of water spots. Salts, alkalis and incompletely cured components in cement infiltrate stone with water during wet construction, spreading through natural fissures toward the surface. Moisture evaporates into the air, leaving calcium hydroxide, sodium hydroxide, calcium carbonate and sodium carbonate trapped inside the stone pores. Accumulated saline-alkali deposits gradually block pores, hindering internal moisture evaporation. Coupled with improper daily maintenance, water stains consequently form. Removing backside attachments facilitates outward moisture evaporation and eliminates surface stains.

Moreover, alkaline and saline substances from cement trigger oxidation of iron elements inside stone, generating ferric oxide and resulting in yellow discoloration.

3. Stone damage frequently occurs during long-term service. Dry-hung stone curtain walls are widely applied on exterior walls of modern high-rise buildings. This installation method effectively avoids common drawbacks of wet laying such as alkali efflorescence, rust stains and white bloom. Consequently, many construction parties mistakenly believe stone protection treatment is unnecessary.

Relevant data reflects potential risks. Research on the stone lions at Lugou Bridge in Beijing indicates that surface weathering over the past 50 years has matched the deterioration level naturally accumulated over 700 to 800 years. Some stone carvings have weathered severely, losing distinct facial and bodily contours.

According to the 2002 atmospheric environment report released by Beijing Municipal Environmental Protection Bureau, there were 162 days with air quality below Grade II that year, among which 95% of pollution was attributed to sulfur dioxide and suspended particulate matter.

Severe pollution from industrial sulfur dioxide and airborne dust poses long-term threats to dry-hung stone facades. These pollutants accelerate surface weathering, altering stone color and texture, which deserves adequate attention.

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