International Concrete Abstracts Portal

Editors: Mohamed T. Bassuoni, R. Doug Hooton, and Thanos Drimalas

The papers presented in this volume were included in a three-part session sponsored by ACI Committee 201, Durability of Concrete, about sulfate attack on concrete at the ACI Convention in Philadelphia, PA, on October 23-24, 2016. In line with the practice and requirements of the American Concrete Institute, peer review, followed by appropriate response and revision by authors, has been used.

Deterioration of concrete due to sulfate attack is a complex process characterized by multiple damage manifestations including volumetric expansion, cracking, spalling, softening, and in some cases mushiness. Sulfate attack can generally be classified as internal or external to the cementitious matrix, and the underlying damage modes can be chemical or physical. The scope of papers involves a multitude of theoretical and experimental aspects of different forms of sulfate attack. Readers are urged to critically evaluate the work presented herein, in the light of the large body of knowledge and scientific literature on this durability topic.

Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-317

Cement production accounts for 5-7% of annual anthropogenic CO₂, making it one of the largest contributors to CO₂ emissions. The use of alternative cementitious materials and binders (ACMs) such as calcium sulfoaluminate cement (CSA), calcium aluminate cement (CAC), magnesium phosphate cement (MPC) and alkali-activated binders (AA) are one method of reducing the greenhouse gases associated with the production of concrete. These materials often can be produced at lower temperatures than ordinary portland cements (OPC) and have lower calcium contents, reducing the emissions associated with CO2 released from calcium carbonate during calcination. However, before alternative cements can become viable greener alternatives to ordinary portland cements their chemistry, microstructure, and resulting properties, with respect to durability must first be understood. Using standard mortar bar sulfate testing showed that CSA, AA, and blended CAC systems may be appropriate for use in high sulfate environments, while pure CAC and MPC systems should be avoided. However, additional characterization of the sulfate exposed samples, showing layers of altered microstructures in most samples, suggests that the mechanisms of expansion and degradation due to sulfate ingress, as they are understood for portland cement systems, may not be adequate for determining resistance to sulfate degradation for alternative binder systems. Therefore, additional study is warranted before recommendations on use of ACMs in sulfate-laden environments can be made.

Sulfate attack is one of the aggressive damage mechanisms that can jeopardize the durability of concrete structures. Several research studies have investigated the positive influence imparted by supplementary cementitious materials (SCMs) regarding the resistance of conventional concrete to sulfate exposure. However, the effects of SCMs on the sulfate resistance of two-stage concrete (TSC) has not been duly explored. In this paper, the durability of TSC mixtures incorporating different SCMs, including fly ash (FA), silica fume (SF) and metakaolin (MK), as partial replacement for ordinary portland cement (OPC) was investigated. Two different sodium sulfate exposure regimes were simulated: full immersion (conducive to chemical sulfate attack) and partial immersion combined with cyclic temperature and relative humidity (conducive to physical salt attack). Results show that TSC specimens incorporating FA achieved acceptable resistance to chemical sulfate attack, while incurring severe surface scaling under physical salt attack. Moreover, TSC specimens made with MK exhibited adequate resistance to both chemical and physical attacks. Surprisingly, TSC specimens incorporating SF deteriorated significantly due to abundant thaumasite formation. An attempt is made herein to delineate the mechanisms that result in deterioration.

Due to the increasing costs of maintaining deteriorating infrastructure, there has been an increased importance placed on the durability of new concrete structures. For marine structures and structures constructed in sulfate rich soils, sulfate attack can cause the structure to degrade over time. Historically, sulfate attack resistance has been evaluated using an expansion test method. However, in addition to expansion during sulfate attack, concrete can exhibit strength degradation without expansion. Resistance to sulfate attack was assessed using both expansion and strength degradation test methods for thirteen binder compositions. Results were compared to established criteria for expansion and proposed criteria for change in strength and were correlated to overall binder composition, considering the combination of three cement types and five supplementary cementitious materials (SCMs). Compressive strength degradation testing demonstrated that mix designs with a high initial CaO content, determined through oxide analysis of the cement and SCMs, performed well, presumably due to the formation of calcium hydroxide (CH) which served as a buffer to the decalcification of calcium-silicate-hydrate (C-S-H) in the formation of gypsum. However, high CaO contents led to poor performance on expansion testing due to the availability of large amounts of calcium hydroxide to react with sulfate ions to form expansive ettringite. Slag mix designs containing metakaolin performed well on both criteria.

External sulfate attack is often described by a diffusion-reaction mechanism which leads to the decomposition of hardened cement paste and cracking of concrete. In most studies, the linear expansion of mortar/concrete prisms is measured according to ASTM C1012. Even though this test can be used to determine the suitability of a mixture for specific sulfate exposure conditions, it does not provide insights on the actual degradation process. This paper presents a series of experiments performed to quantify the damage evolution on cement-based mortars with and without fly ash. Conventional expansion tests were conducted, followed by measuring the chemical and mechanical changes on the cross section of the specimens using EDS and microhardness techniques. The overall damage was further evaluated using a novel flexural fracture test on the specimens. It was observed that partial replacement of cement with class F fly ash reduced the level of mechanical damage in exposure to sulfate attack.