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Concrete structure is prone to corrosion and weathering when built near marine environment. The greater damage on the concrete can be observed when it involves wet-dry action such as tidal waves combine with the existence of aggressive ions such as sulphate and chloride in seawater. The objective of this study is to review on the mechanism of sulphate, chloride attack toward concrete, parameters that influence the wet-dry action and identify the overview of research trends. The mechanism of sulphate and chloride attack during wet-dry action had reciprocal inhibiting effect on concrete and the penetration level for each ion also vary. The physical and mechanical damage of concrete exposed to wet-dry action also influence by wet-dry ratio, number of cycles and temperature during drying process. The main compound detected during exposure period are Friedel salt, ettringite, mirabilite and thenardite which can cause concrete delamination and spalling. [ABSTRACT FROM AUTHOR]

By modeling the areas of elastic and microplastic deformations of high-strength self-compacting concrete was established of increased crack resistance by the criterion of crack opening due to restrained shrinkage of concrete with the addition of metakaolin. This is explained to the reaction of metakaolin and CaSO4‧2H2O of cement with the formation of additional ettringite and low-calcium hydrosilicates with a decrease of shrinkage deformations and fragility of fracture. The positive effect on the shrinkage of concrete of replacing part of the cement by mineral additives is also explained by the reduction of autogenous shrinkage of concrete due to decreasing cement content, and reduction of drying shrinkage of concrete due to adsorption binding of capillary moisture by fillers. The introduction of mineral additives to replace of cement leads to a decrease of fragility of high-strength self-compacting concretes (increasing of area microplastic deformations) due to increased content of fine crystalline calcium hydrosilicates and gel-like neoformation. [ABSTRACT FROM AUTHOR]

The results of the study of the effect of metakaolin on the stability of ettringite in cement systems with high, R2O content are presented. According to the results of DTA and X-ray diffraction analysis, it was found that the increased content of R2O promotes the decomposition of ettringite with the formation of monosulfate, even under normal temperature conditions. The introduction of metakaolin, which contains active aluminosilicates that bind Na+/K+ ions from the pore solution, shifts the balance toward the formation of primary ettringite and reduces the content of monosulfate, which prevent further crystallization of delayed ettringite. [ABSTRACT FROM AUTHOR]

Using physicochemical research methods, has been established that the ability of the developed composites to retain a significant amount of chemically bound water at a temperature of about 300°C is explained by the synthesis of highly basic calcium hydrosulfoferrites and mixed calcium hydrosulfoaluminoferrites based on mechanically activated 3-valent iron oxide. Such new formations are able to hold chemically bound water at higher temperatures than their counterpart, ettringite. On model systems, using physicochemical methods of analysis (X-ray phase, chemical and differential thermal), it was found that mechanical activation (grinding and temperature exposure) in the presence of atmospheric oxygen accelerates the process of spontaneous transition of the ferrous form of iron (which is contained in metal corrosion products and in rocks) into the ferric oxide by about 1000 times. The obtained oxide of 3-valent iron is characterized by an amorphous structure, which accelerates the formation of highly basic iron-containing compounds by three times, compared to their synthesis based on iron oxide, which is in a stable state. It is shown that the optimal parameters of mechanical thermal activation are achieved at a fineness of grinding 3000...3500 cm2/g and a treatment temperature of 200...300°C in atmospheric oxygen. [ABSTRACT FROM AUTHOR]

Portland cement hydration in normal conditions is a complex problem. A detailed description of cement hydration processes includes the clinker dissolution, diffusion process, surface phenomena of new phase nuclei, quantification of solid phases, growth in hydration products, porous structure, and moisture, depending on curing time and water/cement ratio. Thermodynamic simulation of Portland cement hydration based on the Lothenbach model, substantially assists in solving these problems. Gibbs energy minimization thermodynamic software is used to evaluate in equilibrium conditions the quantitative content of both initial clinker minerals and hydration products (hydrated cement, portlandite) for a long time. This paper presents code GEMS simulation results of the phase content during Portland cement hydration, in terms of the Lothenbach model, in the absence of such mineral additives as calcite, lime, anhydride, gypsum, hemihydrate, potassium sulphate. The water/cement ratio is proposed to be 0.3, 0.4, and 0.5. It is found that the final products mostly contain calcium silicate hydrate gel of a complex composition, portlandite, nonsoluble clinker minerals, hydrogarnet, monosulfate, and moisture distributed in the pore space; ettringite being absent. It is shown that the pore space grows with increasing water content. And even after 1250-day curing, the contribution of nonsoluble clinker is 20–25 g among 150 g of hydrated Portland cement. [ABSTRACT FROM AUTHOR]

This study demonstrates how sulphate attack on exposed marine concrete structures causes ettringite formation in the water environment. Creating ettringite is linked to concrete's ability to repel and expand. The early formation of ettringite, which happens immediately, is the process that controls stiffening without damaging expansion and is linked to the management of time in the Portland cement paste environment (within hours). Though it does not show any reaction in a marine environment, this mechanism will cause a harmful expansion later after several seasons of exposure (DEF). Delayed ettringite formation (DEF) formed at late stages due to cracking and spalling in forms. For both external and internal attacks on sulphate-containing cells, DEF can be tested. Magnesium sulphate (MgSO4) must first be converted before being used another way (ettringite) to make hydrated calcium aluminate sulphate. DEF may occur at marine concrete structures, but it is not linked to sulphate attack but instead to seawater chloride intrusion, which causes corrosion of the concrete structures. As a result, the reaction of DEF is linked to sulphate and another mechanism that can contribute to concrete cracking and spalling. [ABSTRACT FROM AUTHOR]

In this study, three industrial by-products (ultrafine fly ash, ground granulated blast-furnace slag (ggbs) and circulating fluidized bed co-fired fly ash) were used to produce ternary cementless composites without using alkali activators. The finenesses of ultrafine fly ash, ggbs and co-fired fly ash were 33,800, 5830 and 5130 cm2/g, respectively. The composite material was developed by mixing supplementary cementing materials of different particle sizes and exploiting the high-alkaline properties of the co-fired fly ash to develop a substantial hardening property like cement. The test specimens were made in the form of pastes and the water-to-cementitious-material ratio for the test was fixed at 0.55. The test results show that the flowability of the six different mixtures could be up to 120% and the setting time could be controlled within 24 h. At 60% of the ggbs proportion, the setting time could be held for 8 h. The compressive strength of each proportion reached 7 MPa at 7 days and 14 MPa at 28 days. The water-cured specimens exhibited better strength behavior than the air-cured specimens. Scanning electron microscopy found the main components of strength growth of the specimens to be hydrated reactants of C-A-S-H or ettringite. The results of the XRF analysis show that the specimens responded to higher compressive strengths as the Ca/Si and Ca/Al ratios increased. [ABSTRACT FROM AUTHOR]

The effective application of the evils in the construction, including road, an important area of research is to identify the characteristics of physical and chemical processes and their hydration mechanism composed gold-cement binders. By chemical, mineralogical, derivatographic, spectroscopic and X-ray analyzes established physico-chemical properties of fly ash of thermal power hydroremoval Rudnenskiy AO SSGPO (Kazakhstan). It was determined that the hydraulic ash is acidic, with a high content of SiO2 and Al2O3 (𝚺85.8%), has a second dispersion class, with a true and bulk density of 2.95 g / cm³ and 795 kg / m³, respectively, and is itself inactive It is represented by quartz (23%), mullite (5%), vitreous matter (66%) and coal (6%). As the second component, Portland cement of the grade ПЦ500Д0Н is used, which plays a dominant role in the composition of the ash-cement stone, since it provides lime (during hydration) to activate the vitreous matter of the ash. During cement hydration mainly the following new phases arise: lime - is formed in the form of basal spherulitic plates, uniaxial, negative with light refraction indices: no = 1.577 and nс = 1.544; thin needles of ettringite with no = 1.464 and ns = 1.460; granular mass of calcium hydrosilicates; individual prismatic calcite crystals with ng = 1,684 and np = 1,531, formed as a result of lime carbonization. Moreover, ettringite crystals (1.1-1.2 μm in size) and a gel-like mass of submicrocrystalline structure related to calcium hydrosilicates of the CSH and CSH types (I and II), as a rule, are located around the grains of the initial cement, and lime crystals (size 15 -20 microns) - apart. The mechanism of hydration of hardened ash-cement binders is shown, from which it follows, firstly, that among the sets of aluminosilicate phases arising from cement hydration, only lime activates the ash, and secondly, that from the phase composition of acidic ash consisting of quartz, mullite, vitreous matter and coal, only the vitreous substance under normal conditions interacts with lime - a product of cement hydration. The function of acid ash in hardening under normal conditions, ash-cement binders is as follows: particles of quartz (partly mullite) become the active center of crystallization of hydrated phases; the microsphere interacts with high speed with lime - the product of cement hydration, as a result of which aluminosilicate hydrate phases form in the system; following the microsphere with lime (possibly simultaneously), the vitreous phase of the ash interacts with the formation of the same aforementioned aluminosilicate phase. [ABSTRACT FROM AUTHOR]

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Dry Building Mortars (DBM) are new in terms of building technology, and they sometimes substitute different kinds of concrete and mortar mixes. DBM have been successfully used in construction and their performance has been as efficient as the one of commercial mortar mixes, i.e. they boost labour efficiency, and bring down material consumption. They can be also kept in stock for a long time, and shipped with no compromise in quality. Today DBM are based on cementing components. And some famous mix mortars are based on gypsum, polymer, and some other types of cementing components which contain fine-dispersed additives of ground slag, fly ash, and raw sludge. These mix mortars are applied as smoothing, aligning, waterproof, and safety types of coverage. This article studies the possibility of involving slag cementing components in manufacturing DBM. Due to their high waterproof resistance in salt-affected water, these sorts of mixes may be used as protective coatings and plasters for concrete frames in corrosive medium. Slag Cementing Components (SCC) are hydraulic cementing agents, which harden both in water and in open air. They are produced by mixing electric-furnace slag or furnace clinkers with some solutions of alkaline metals (to trigger alkaline reaction), or by mixing together all these agents. [ABSTRACT FROM AUTHOR]

Sulphate precipitation from mine water as ettringite, Ca6Al2(SO4)3(OH)12·26H2O, is generally indicated to be implemented as a second treatment step, following the heavy metal precipitation or as a third step, after gypsum seeded crystallization. The whole process may be seen as complicated by the operators, which always prefer simpler flow sheets, with fewer steps and with lower investment cost, but this treatment process sequence is justified by sludge management depending on type and by the economic reasons related to the aluminium required input. However, for some mine water sources, a single step precipitation process can be considered, specifically for sources with sulphate concentration below 2000 mg/L, out of which those containing more than 50 mg/L of aluminium, are of special interest. Single step metal and sulphate ions precipitation can have the benefit of using the aluminium already present in the mine water for the ettringite formation. This was experimentally tested for a Romanian mine water source having typically 350 to 500 mg/L iron, 80 to 100 mg/L aluminium and sulphate below 2000 mg/L. Results for continuous flow reaction system and batch laboratory tests were similar, leading to the conclusion that aluminium from the mine water is responsible for a relevant proportion of the precipitated sulphate. Of certain interest is the settling behaviour of the composite sludge, consisting of metal hydroxy compounds and ettringite. [ABSTRACT FROM AUTHOR]

Sulphate is the specific anion present in the acid rock drainage (ARD). Although sulphate is considered benign or with low toxicity, some countries have set limits for the wastewater discharged to natural receivers, often 500 mg/L or 600 mg/L. By conventional treatment of the ARD with calcium hydroxide, some gypsum can precipitate, depending on the ARD initial sulphate content, but the conformity for sulphate will not be achieved, because the concentration correlated to gypsum solubility exceeds the limit value by roughly three times. An experimental study has been carried out to assess sulphate precipitation via ettringite process from acid rock drainage originating from closed non-ferrous mine site in the North Romania mining area. Tests for sulphate precipitation, using monocalcium aluminate cement as source of reactive aluminium for the ettringite formation, showed important differences regarding sulphate precipitation yield between batch and continuous flow reaction systems, due to reaction system itself. For continuous stirred tank reactor system (CSTR) a total space-time of 2.6 h is required when operating with cement dose equivalent to 265 mg Al for 1 g sulphate in order to safe meet the sulphate discharge limit of 600 mg/L for mine water with initial c(SO42-) = 2650 mg/L and at a temperature of 15 °C. [ABSTRACT FROM AUTHOR]

The use of the Nonlinear Impact Resonance Acoustic Spectroscopy (NIRAS) method to monitor the evolution of damage due to delayed ettringite formation (DEF) is examined. In practice, the temperature of concrete during casting of precast concrete members or massive concrete structures may reach higher than 70°C which can provide suitable conditions for damage to occur due to DEF, particularly in concrete which is subsequently exposed to wet environments. While expansion - often in excess of 1% - is characteristic of DEF, the evolution of damage begins with microcracking. Unfortunately, there is no standard to test the susceptibility of materials or material combinations to DEF. On the other hand, NIRAS shows great sensitivity to the detection of microcracks and has been successfully applied to concrete to detect thermal and alkali silica reaction in concrete. In this preliminary research, the NIRAS method is used to discriminate among mortar samples which are relatively undamaged and those in the early stages of DEF. The results show that NIRAS could be a reliable and robust method in the detection of microcracks due to DEF. [ABSTRACT FROM AUTHOR]

In the present work, we study the transverse relaxation time evolution of water molecules confined inside cement paste during the hydration process. The cement paste under study was manufactured with different water-to-cement ratios and using two types of cement: gray cement (CEM I 52.5 R) having a high content of magnetic impurities and white cement (CEM I 52.5 N) with lower amount of magnetic impurities. The two cement types were chosen in order to better distinguish the surface contribution to the relaxation process. On this basis a relationship between porosity evolution, ettringite formation and the transverse relaxation time evolution was established. It was also observed that the increase in the water-to-cement ratio better reveals the ettringite formation. [ABSTRACT FROM AUTHOR]

Circulating fluidized bed combustion (CFBC) fly ash/slag is a waste from CFBC power plant and has such a high sulfur content, which calculated to SO3 is usually higher than 5% by weight, that there is a high risk of volume instability when made into building materials curing at room temperature. Use of CFBC fly ash/slag in production of load-bearing brick by pressing and autoclaving process was investigated in this study. The optimum water requirement, pressing pressure, precured duration, autoclaving pressure and duration were determined and long-term volume stability was evaluated by measuring variation of the length of specimen in dry-shrinkage and wet-expansion. The hydration products of the adobe and autoclaved brick were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscope (SEM). It is shown that CFBC fly ash and slag could account for 97% of the total mass of the brick and the brick under optimum test condition, whose compressive strength is up to 14.3 MPa, had good long-term volume stability. There was few dihydrate gypsum and ettringite, which could result in volume expansion, but more C-S-H gel in the autoclaved brick to avoid ruinous voluminal expansion. It is suggested that CFBC fly ash and slag are promising used to produce high-quality bricks by autoclaving. [ABSTRACT FROM AUTHOR]

Tailings produced by mine operations are disposed of in large impoundments or mixed with binders and used as backfill in underground openings. They may include a high percentage of sulphide minerals that could oxidize and produce sulphate-rich acidic waters. Since the binders are a combination of cement and pozzolans, they are vulnerable to sulphate attack, a process well studied in the concrete industry, and in which ettringite is produced and strength is compromised. Current tests for sulphate attack are limited to measuring expansion that results from ettringite formation. This paper presents a method of assessing sulphate attack by dissolving the ettringite formed in a mixture of ethylene glycol and methanol, and calculating it from the aluminum concentration in the solution. Results indicate that the method is able to quantify the mineral, and is well suited in assessing the durability of different binders against sulphate attack. [ABSTRACT FROM AUTHOR]

A calcium sulphate rich industrial by-product (locally known as 'Galligu') was treated with a Stabilisation Solidification (S/S) process in order to increase the bearing capacity and examine the potential for expansion due to ettringite formation and hence increase the utility of the material and prevent it being land filled. The waste material is a by-product from the historic alkali production industry in the North West of England (UK) where it poses a significant environmental risk and obstacle for the redevelopment of brownfield sites. The treatment process (S/S) involves the mixing of hydraulic binders with the waste product. The binders used in this study include ordinary Portland cement (OPC), lime and ground granulated blast furnace slag (GGBS). Expansion potential was monitored in a purpose built rig, capable of maintaining the samples at a constant temperature and high humidity. Results for the different binders were compared with samples that have known expansive tendencies. Ettringite formation was followed using x-ray-diffraction. Strength was measured through unconfined compression tests. [ABSTRACT FROM AUTHOR]