Sulfate Content
Understanding Sulfate Attack in Concrete, and cement sulfate content test methods
Introduction to Sulfate Attack
Concrete exposed to certain environments can suffer from sulfate attack. This phenomenon is common in areas in contact with seawater, industrial settings, sanitary sewers, and in regions with sulfate-rich soils and groundwater, notably in western parts of the United States, Canada's prairie provinces, and other global locations. Additionally, some aggregates containing gypsum can be a source of internal sulfate attack.
Mechanisms of Sulfate Attack
Sulfates, regardless of their origin, react with the hydration products of portland cement, primarily with the hydration product of tricalcium aluminate (C3A). During the hydration process, C3A forms monosulfate, and the sulfates present in the cement react with it to form ettringite. This reaction, which occurs post-concrete setting, is expansive and causes cracking in the hardened cement paste. However, not all ettringite formations are harmful; some may result from cycles of wetting and drying and are non-detrimental. The reaction eventually leads to the formation of gypsum, which also causes expansion and cracking.
Problems Arising from Sulfate Attack
Sulfate attack leads to various issues, including strength loss, expansion, surface spalling, and disintegration. The most aggressive form of sulfate is magnesium sulfate (MgSO4), which severely affects the concrete's calcium silicate hydrate gel, leading to softening and cohesion loss.
Variability in Sulfate Aggressiveness
Different sulfates exhibit varying levels of aggressiveness based on their associated cations. Calcium sulfate (CaSO4) is the least aggressive, sodium sulfate (Na2SO4) is moderately aggressive, and magnesium sulfate (MgSO4) is the most aggressive. These varying degrees of aggressiveness affect the concrete's stability and integrity in different ways.
Creating Sulfate-Resistant Concrete
To combat sulfate attack, it is essential to minimize vulnerable components in the concrete. This includes reducing C3A content and incorporating supplementary cementitious materials to react with calcium hydroxide, forming more robust calcium silicate hydrate. Type V cement, specifically designed for sulfate resistance, can be an effective solution, although its availability varies. The water-cementitious materials ratio is also crucial for reducing permeability and enhancing concrete performance against sulfate attack.
Evaluating Exposure Conditions
Assessing the severity of sulfate exposure in specific environments is critical. This evaluation, as defined by ACI 318, involves analyzing soil and water for sulfate content as per ASTM C1580. It's important to consider that some environments may present multiple exposure types, such as seawater containing both sulfates and chlorides.
Internal Sulfate Attack
Sulfates in aggregates, often in the form of gypsum, can lead to internal sulfate attack. This issue is prevalent in the Middle East and in aggregates made from recycled building materials. Removing the gypsum-contaminated fraction from the aggregate is sometimes the best approach to prevent long-term damage from internal sulfate attack.
Introduction to cement Sulfate content test methods
ASTM C1038/C1038M Standard Test Method for Expansion of Hydraulic Cement Mortar Bars Stored in Water
ASTM C563 Standard Guide for Approximation of Optimum SO3 in Hydraulic Cement
ASTM C1038/C1038M Standard Test Method for Expansion of Hydraulic Cement Mortar Bars Stored in Water
The ASTM C 1038 standard outlines a test method for determining the expansion of hydraulic cement mortar bars stored in water, primarily to evaluate the impact of sulfate content in the cement. This method involves creating and moulding specific-sized mortar bar specimens, storing them first in a moist environment and then in saturated lime water, and measuring their expansion at 24 hours and 14 days using a length comparator. The test is crucial in the construction industry for ensuring the quality and durability of cement, as excessive sulfate can lead to detrimental expansion. Precision and bias are noted, but no statement on bias is made due to the absence of suitable reference material, making this test essential for manufacturers to control sulfate levels and adhere to industry standards.
ASTM C265 Standard Test Method for Water-Extractable Sulfate in Hydrated Hydraulic Cement Mortar (Withdrawn 2017)
The ASTM C265 standard provides a method for measuring water-extractable sulfur trioxide (SO3) in hardened hydraulic cement mortar, essential for assessing unreacted sulfate levels and cement durability. This test involves preparing a specific mortar mixture, storing it in a controlled environment, and then analyzing the pulverized sample's filtrate for SO3 content using chemical analysis methods. The procedure is crucial for ensuring that hydraulic cements, especially those exceeding optimized SO3 limits in certain specifications, meet quality and durability standards. It also includes safety precautions due to the caustic nature of hydraulic cement mixtures. The test's precision and bias are documented, although no definitive statement on bias can be made due to the lack of reference samples.
ASTM C563 Standard Guide for Approximation of Optimum SO3 in Hydraulic Cement
The ASTM C563 test method provides a standardized procedure for determining the optimum sulfur trioxide (SO3) content in hydraulic cement to achieve maximum compressive strength. This method involves preparing mortar mixtures with varying amounts of calcium sulfate, molding them into cubes, and then testing their compressive strength at 24 hours, 3 days, and 7 days. The optimum SO3 content is calculated based on the strength differences between these mixtures. This procedure is crucial for cement manufacturers to ensure the production of high-quality cement that meets specific performance standards set by ASTM Specifications C150, C595, and C1157.