جستجوی مقالات مرتبط با کلیدواژه "اترینگایت" در نشریات گروه "عمران"

One of the common ways to increase the life of concrete is the use of pozzolans, some pozzolans that have higher reactivity than ordinary cement lead to greater resistance and less permeability of concrete by reducing concrete pores. Structures made with ordinary Portland concrete generally do not have proper performance in harsh environmental conditions and destructive aggressive factors. Sulfate attack is the process of concrete destruction due to the expansion caused by sulfate reactions inside the concrete, and in the long term, it causes a decrease in cohesion, occurrence of cracks and collapse in the concrete structure, one of the effects of which is the reduction of concrete strength. The addition of metakaolin reduces the porosity in concrete, as a result, concretes containing metakaolin have lower permeability compared to normal concrete. In this research, calcined clay was used as pozzolan, first the soil was heated to a temperature of 700 degrees Celsius for 3 hours so that the calcination process takes place, then calcined clay along with a percentage of limestone powder and microsilica was replaced by cement. The purpose of this combination for cement materials is to achieve a mixture design that the concretes made by it have better resistance than normal concretes against aggressive and corrosive environmental conditions. In this study, ettringite crystals were formed on the samples processed in sodium sulfate solution, which were less in the pozzolanic samples than in the control sample. At early ages, the capillary absorption coefficient for samples containing calcined clay is higher than the control samples, but this difference is greatly reduced as the samples age and the pozzolanic reactions become more complete. The electrical resistance values of the samples also increased with the passage of time and the increase in pozzolan replacement percentage. Also, in all the experiments, the addition of microsilica fills the empty spaces in the concrete, because the microsilica particles, being smaller than the pozzolan particles of the clay used and the faster reactivity of microsilica produces more silicate gel in a shorter period of time than clay pozzolan and makes the concrete denser. In this research, 10 mixed designs were used in 2 ratios of water to cement materials: 0.35 and 0.4. In each proportion of clay in percentages of 10 and 20%, limestone powder in percentages of 30 and 20%, respectively, and microsilica along with the combination of soil and lime in 7% by weight as powder materials were replaced by cement. In order to check the properties of the prepared soil, XRD test was performed to ensure the amorphousness of the soil particles and the presence of pozzolanic compounds on it. Also, to check and analyze the durability of concrete, sulfate resistance, capillary absorption and electrical resistance tests were used on 10 cm cube samples at the age of 28 and 90 days. In this research, designs containing 20% calcined clay, 20% lime, and 7% microsilica have the best performance in pozzolanic designs and are introduced as optimal pozzolanic designs in the conducted experiments.

In this study, the kinetics role of gypsum is evaluated in limestone calcined clay cement (LC3) during exposure to a 30 g/l [[EQUATION]] solution. Previous studies considered the chemical aspects of cement, and seldom of them take into account the kinetics aspects. In this study, geochemical code PHREEQC along with kinetics and rate keyword data block was used to evaluate the chemical and kinetics aspects of cementitious material, simultaneously. The simulation results are compared with experimental and modeling results to the confirmation of present model accuracy. Kinetics diagrams for samples with the various cement replacement percentages and ratio of calcined clay to cement replacement percentage (CC/CC+L) are evaluated to get more information about the kinetics path of gypsum production during sulfate attacks. The results show that three levels can be considered as the kinetics change of gypsum. The first stage of gypsum production gets longer by the increment of CC/CC+L ratio. It means that gypsum is produced with delay for the mixture with a higher CC/CC+L ratio. The availability of more aluminum ions due to the presence of more calcined clay leads to the continuation of ettringite production instead of gypsum formation. In LC3 cement the maximum amount of produced gypsum is reduced as well as the rate of gypsum production. The stress due to the gypsum formation is applied to the cement matrix with the delay because of the prolongation of the first stage.

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.

Today, one of the most important indices in the design, construction, and operation of concrete structures is paying particular attention to sustainable development and environmental issues so that the useful life of structures is increased in order for economic savings and environmental protection. Accordingly, researchers have proposed permanent concrete durability idea along with strength concrete and improved the strength and durability of concrete and also reduced cement consumption by replacing natural additives. In this regard, to assess the durability of pozzolan-containing concrete against sulfate attacks, in this paper, a variety of additives including zeolite, fly ash, metakaolin, as well as micro and nano silica was used as an alternative for a part of cement. In order to investigate the effect of different types of sulfates, concrete was placed within four saturation environments containing iron, magnesium, sodium, and calcium sulfates and the control saturation sample was made in limewater to compare all the designs. The samples were evaluated using compressive strength test and ultrasonic test at the ages of 7, 14, 28, 60, 180, and 360 days. The results showed the concrete containing micro and nano silica had maximum strength against sulfate attacks and the devastating effects of magnesium, sodium, and calcium sulfates and low effect of iron sulfate were respectively demonstrated. Investigating the velocity of ultrasonic waves also showed that wave velocity increased by increasing the age from 180 to 360 days and increasing the formation of Ettringite, a destructive substance, in concrete. The highest effect of increased velocity was observed by increasing Ettringite in the calcium sulfate samples.