Microstructural Assessment of the Effect of Sulfate Environments on the Mechanical Properties of Concrete

Chemical attacks through concrete pores destroy concrete structures and reduce their structural integrity in sulfate environments. The durability of concrete in harsh environments is a crucial issue worldwide [1–3]. Specifically, environments like coastalareas, saline-alkali lands and salt lakes contain many sulfate ions,which could penetrate into the concrete foundation facilities like pier, bridge and tunnel [4]. It is generally accepted that the ingression of sulfate ions in concrete causes serious deterioration, such as cracking, expansion and strength loss [5]. This phenomenon is mainly attributed to the formation of gypsum and ettringit [6,7]. Ordinary Portland Cement (OPC) concrete has long been used in construction of civil infrastructure and its deterioration over time due to sulphate attack has been widely observed and documented [1–4]. Investigations have revealed that the degradation of OPC concrete takes place due to reactions between cement hydration products and sulphate-bearing solutions. Degradation of concrete strength due to sulphate attack takes place when the calcium and hydroxide ions dissolve out of the matrix, causing an increase in porosity and permeability of the concrete surface [5].  Maintaining the durability of concrete structures against corrosion in acidic environments is an important challenge. Investigating concrete microstructure makes it possible to understand concrete porosity and structural composition in micro- and nano-scales, and makes concrete more concentrated, durable and strong. Accordingly, the present study is a microstructural analysis of the long- and short-term impacts of the conditions of different sulfate environments on concrete strength parameters.

 This study evaluated about 200 concrete samples. The samples were preserved for 3 months in simulated environments with 0.1%, 0.25%, 0.5%, 1%, 2.5%, 5% and 7.5% sulfuric acid concentrations. Compressive strength, weight percentage, ultrasonic wave, permeability and pH change tests were then performed after 1, 3, 7, 14, 28 and 90 days on all the samples in the preserving environment. Images from the scanning electronic microscope (SEM) test were used for microstructural analysis.

The results indicate that the strength of samples preserved in the sulfate environment compared to the control sample depended on the amount of sulfate. In the majority of sulfate attacks, the most vulnerable compounds to react with waterborne sulfate ions are calcium hydroxide (CH) and phases containing aluminium, such as AFm (e.g. monosulfate) and unreacted C3A. After 28 days, the compressive strength of samples preserved in the 5% concentration sulfuric acid sulfate environment was reduced by about 63% compared to control samples. This reduction in compressive strength is inversely related to the results from the ultrasonic test of samples preserved in the 5% percent concentration sulfate environment. The environment’s wave velocity increased by 27% after 90 days. Consequently, expansion and cracking result in severely compromised structural integrity of the attacked concrete. Cracking also leads to further propagation of the attack.  The increase in ultrasonic wave velocity of samples was accompanied by a loss of strength and mass due to destruction of concrete strength structures, including the C-S-H nanostructure, and formation of ettringite due to exposing samples to sulfate.