Relationship between free chloride and total chloride contents in concrete

Linear relationships between free chloride and total chloride contents in concrete are proposed based on the results of several long-term exposure tests under marine environment for various cements, such as ordinary portland cement (OPC), high early strength portland cement (HES), moderate heat portland cement (MH), calcium aluminate cement (AL), slag cements of Types A (SCA) and B (SCB), and fly ash cement of Type B (FACB). A high chloride-binding ability is found for AL as compared to the other cements. Replacing the OPC with slag reduces the chloride-binding ability. The proposed linear relationships show reasonably good agreement with field data obtained from the wharf structures.     

Intrinsic differences in atomic ordering of calcium (alumino)silicate hydrates in conventional and alkali-activated cements

The atomic structures of calcium silicate hydrate (C–S–H) and calcium (–sodium) aluminosilicate hydrate (C–(N)–A–S–H) gels, and their presence in conventional and blended cement systems, have been the topic of significant debate over recent decades. Previous investigations have revealed that synthetic C–S–H gel is nanocrystalline and due to the chemical similarities between ordinary Portland cement (OPC)-based systems and low-CO₂ alkali-activated slags, researchers have inferred that the atomic ordering in alkali-activated slag is the same as in OPC–slag cements. Here, X-ray total scattering is used to determine the local bonding environment and nanostructure of C(–A)–S–H gels present in hydrated tricalcium silicate (C₃S), blended C₃S–slag and alkali-activated slag, revealing the large intrinsic differences in the extent of nanoscale ordering between C–S–H derived from C₃S and alkali-activated slag systems, which may have a significant influence on thermodynamic stability, and material properties at higher length scales, including long term durability of alkali-activated cements.     

Early activation and properties of slag cement

Early age activation of granulated blast-furnace slag (BFS) and blends of slag and portland cement (OPC) has been studied. Activators include sodium-hydroxide, sodium sulfate, alum (potassium aluminum sulfate), superplasticizer, and calcium aluminate cement. Heat of hydration, x-ray phase characterization, compressive strength, viscometry, pore size distribution and related characterization studies have been made. Activated BFS-cement mortars having equivalent strengths at 1 day to OPC have been prepared, which also have 28- and 90-day strengths exceeding those of the OPC. Mechanisms of activation are discussed.     

Role of alkaline nitrites in the corrosion performance of steel in composite cements

The influence of alkaline nitrites on the inhibition of corrosion of steel in binary and ternary cement environments was tested. pH measurements carried out for binary and ternary cement extracts showed that the alkalinity of the cement was not affected by making use of binary and ternary cements. Gravimetric measurements showed that the decrease in the corrosion rate of steel in different systems follows the order: Ternary > (OPC + PSC) > (OPC + PPC) > (PPC + PSC). Potential–time studies indicated that the ability to maintain the passivity of steel in different systems also follows the order as above. Potentiodynamic polarisation studies for steel in binary and ternary cement environments showed the favourable influence of the presence of higher amounts of chlorides. Nitrites of sodium, potassium and calcium act as anodic inhibitors and they compete with chloride ions for the ferrous ions at the steel to form a film of ferric oxide. An efficiency as high as 91% is obtained for the ternary system containing 1% chloride and 0.5% nitrite. The degree of surface coverage showed a maximum value for the ternary system (>0.9) even in the presence of a higher amount of chloride thereby indicating the better performance of the system.     

氯盐浸泡的水泥基材料的表面氯离子浓度

水泥基材料的表面氯离子浓度(cs)是Fick's第二定律寿命预测模型的1个重要参数,它通过将Fick's第二定律与氯离子浓度分布拟合而得到.当水泥基材料浸泡在一定浓度的氯盐溶液中时,水泥基材料的cs和浸泡溶液的氯离子浓度(c)之间的关系仍未得到很好的理解.一些发表的文献表明cs要高于c,Nagataki将其称为浓缩现象...     

Binding of chloride and alkalis in Portland cement systems

A thermodynamic model for describing the binding of chloride and alkalis in hydrated Portland cement pastes has been developed. The model is based on the phase rule, which for cement pastes in aggressive marine environment predicts multivariant conditions, even at constant temperature and pressure. The effect of the chloride and alkalis has been quantified by experiments on cement pastes prepared from white Portland cements containing 4% and 12% C₃A, and a grey Portland cement containing 7% C₃A. One weight percent calcite was added to all cements. The pastes prepared at w/s ratio of 0.70 were stored in solutions of different Cl (CaCl₂) and Na (NaOH) concentrations. When equilibrium was reached, the mineralogy of the pastes was investigated by EDS analysis on the SEM. A well-defined distribution of chloride was found between the pore solution, the C-S-H phase, and an AFm solid solution phase consisting of Friedel's salt and monocarbonate. Partition coefficients varied as a function of iron and alkali contents. The lower content of alkalis in WPC results in higher chloride contents in the C-S-H phase. High alkali contents result in higher chloride concentrations in the pore solution.     

Effect of temperature on pore solution composition in plain cements

Cement pastes with a water-cement ratio of 0.6 were prepared using three ordinary portland cements with C₃A contents of 2.43, 7.59 and 14%. Three levels of chlorides 0.3, 0.6 and 1.2% by weight of cement, derived from sodium chloride, were added through mix water. The pastes were allowed to cure in sealed containers at 20 and 70°C for 180 days and then subjected to pore solution extraction. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. Results show that increase in temperature from 20 to 70°C increased unbound chlorides and decreased hydroxyl ion concentration of pore solutions for all the three cements. The simultaneous increase in unbound chlorides and decrease in hydroxyl ion concentration drastically increased Cl⁻/OH⁻ ratio of the pore solution, thereby indicating an increase in corrosion risk. This adverse effect of increase in the Cl⁻/OH⁻ ratio of the pore solution with increase in temperature is higher in the high 14% C₃A cement than in the low C₃A cements, and is also higher for the low 0.3% chloride treatment level than the higher chloride inductions. Increase in temperature is also expected to cause an increase in ionic diffusion to steel embedded in concrete as well as in the rate of corrosion reaction. All these factors tend to increase corrosion risk of steel reinforcement in concrete with an increase in temperature.     

Corrosion of aluminium metal in OPC- and CAC-based cement matrices

Corrosion of aluminium metal in ordinary Portland cement (OPC) based pastes produces hydrogen gas and expansive reaction products causing problems for the encapsulation of aluminium containing nuclear wastes. Although corrosion of aluminium in cements has been long known, the extent of aluminium corrosion in the cement matrices and effects of such reaction on the cement phases are not well established. The present study investigates the corrosion reaction of aluminium in OPC, OPC-blast furnace slag (BFS) and calcium aluminate cement (CAC) based systems. The total amount of aluminium able to corrode in an OPC and 4:1 BFS:OPC system was determined, and the correlation between the amount of calcium hydroxide in the system and the reaction of aluminium obtained. It was also shown that a CAC-based system could offer a potential matrix to incorporate aluminium metal with a further reduction of pH by introduction of phosphate, producing a calcium phosphate cement.     

Tolerance limit of chloride for steel in blended cement mortar using the cyclic polarisation technique

The tolerance limit for chloride in ordinary Portland cement (OPC) and blended cements such as Portland pozzolana cement (PPC) and Portland slag cement (PSC) was assessed by cyclic polarisation. This study covers both cement extracts and mortar. The salient features of this investigation were: in extracts, the tolerance limit for chloride actually doubles for PSC when compared to PPC and OPC. The tolerance limit for chloride for various mortars follows the order: PSC > PPC > OPC. In OPC and PPC mortar, the repassivation potential (E _rep) shifted negatively with higher amounts of free chloride but in PSC mortar E _rep shifted positively (+590 mV) even in the presence of 5,000 ppm of free chloride. PSC takes longer time (50 days) to reach E _rep indicating perfect passivity maintained for the embedded steel.     

Development of rapid-set high-strength cement using statistical experimental design

Many applications like aircraft runway, busy roads, highway or motorways, water tank repair, etc. demand a cement that sets fast and gains the required strength in a few hours. Though there are few cements available to meet the requirements given above, most of them are very costly, like magnesium phosphate cement, jet cement, geopolymeric cement, etc. So, an attempt has been made to make cost-effective rapid-set high-strength cement having initial setting time of ∼15 min, final setting time of ∼30 min, 4 h cold compressive strength (CCS) of ∼12 MPa (minimum), 8 h CCS of ∼24 MPa and 1 day CCS of ∼40 MPa for the neat cement. The experiments were designed using orthogonal array technique in L₉ array with three factors, namely OPC/high-alumina cement/anhydrous calcium sulphate, fineness of the cement, and type of additives, at three levels each. The responses studied are initial setting time, final setting time, and CCS after 4, 8, and 24 h curing. The response data were analysed using analysis of variance (ANOVA) technique with a software package, ANOVA by Taguchi Method (ATM). In the case of setting time, fineness of the cement and OPC/HAC/anhydrous calcium sulphate ratio plays a significant role. Additive type and the OPC/HAC/anhydrous calcium sulphate are significant factors affecting the CCS at different ages. The confirmatory trial results clearly indicate that the setting time and CCS at different ages targeted were achieved using design of experiments.     

硫酸盐浸泡环境下CaO-Na2CO3激发矿渣砂浆性能退化及机理研究

以Na₂SO₄和MgSO₄溶液为侵蚀介质,研究了在浸泡环境下CaO-Na₂CO₃激发矿渣(CNS)砂浆和普通硅酸盐水泥(OPC)砂浆经硫酸盐侵蚀前后的抗折强度、抗压强度及不同深度处的SO^2-₄浓度,结合X射线衍射(XRD)、扫描电子显微镜(SEM)、压汞法(MIP)等测试方法分析了CNS砂浆和OPC砂浆的侵蚀产物及孔结构,对比讨论了Na₂SO₄和MgSO₄对CNS砂浆和OPC砂浆的侵蚀机理。结果表明:CNS砂浆的水化产物主要是低Ca/Si比的水化硅铝酸钙(C-A-S-H),不存在氢氧化钙,碳酸钙的填充作用使其孔结构优于OPC砂浆,并且在相同侵蚀环境下,CNS砂浆的抗硫酸盐侵蚀能力大于OPC砂浆;MgSO₄侵蚀环境下CNS砂浆的侵蚀产物主要是水镁石(腐蚀后期会带动试件表面的砂浆一起剥落)和无黏聚力的水化硅铝酸镁(M-A-S-H);与Na₂SO₄相比,MgSO₄对CNS砂浆的腐蚀性更强。     

Cement pastes alteration by liquid manure organic acids: chemical and mineralogical characterization

Liquid manure, stored in silos often made of concrete, contains volatile fatty acids (VFAs) that are chemically very aggressive for the cementitious matrix. Among common cements, blast-furnace slag cements are classically resistant to aggressive environments and particularly to acidic media. However, some standards impose the use of low C₃A content cements when constructing the liquid manure silos. Previous studies showed the poor performance of low-C₃A ordinary Portland cement (OPC). This article aims at clarifying this ambiguity by analyzing mechanisms of organic acid attack on cementitious materials and identifying the cement composition parameters influencing the durability of agricultural concrete. This study concentrated on three types of hardened cement pastes made with OPC, low-C₃A OPC and slag cement, which were immersed in a mixture of several organic acids simulating liquid manure. The chemical and mineralogical modifications were analyzed by electronic microprobe, XRD and BSE mode SEM observations. The attack by the organic acids on liquid manure may be compared with that of strong acids. The alteration translates into a lixiviation, and the organic acid anions have no specific effect since the calcium salts produced are soluble in water. The results show the better durability of slag cement paste and the necessity to limit the amount of CaO, to increase the amount of SiO₂ (i.e., reduction of the Ca/Si ratio of C-S-H is not sufficient) and to favor the presence of secondary elements in cement.     

Chloride binding into hydrated blended cements: The influence of limestone and alkalinity

Chloride binding into ten hydrated cement binders containing limestone and different pozzolanic materials was systematically investigated. The results showed that the amount of bound chloride in pozzolan-blended cements strongly depended on the content of aluminum in the binder. The presence of limestone in cement significantly decreased chloride binding capacity under the condition of common alkalinity (pH ≈ 13.5). The effect of limestone seemed to be related to decreased ability of chloride binding into carboaluminate phases that formed during the hydration of calcium carbonate-containing cement. Alkalinity of the synthetic pore solution generally influenced the course of binding with the effect being more pronounced at lower chloride concentrations. The presence of alkalis hindered the formation of chloroaluminate phases and decreased the chloride binding capacity. Lowering of the temperature resulted in a decreased amount of bound chloride. The reversibility of chloride binding was largely maintained under the condition of constant alkalinity.     

Study of chloride binding and diffusion in GGBS concrete

Ordinary Portland cement (OPC) and OPC/ground granulated blastfurnace slag (GGBS) 70%, with or without 5% sulfates, were widely investigated in respect to their pore structures, chloride diffusion coefficients, internal and external chloride-binding capabilities by expression method and leaching method and the microstructure analysis on Friedel's salt such as differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It can be concluded that GGBS can improve the pore structure of OPC and decrease the chloride diffusion coefficient greatly, and that sulfates do not do good for the pore structure and chloride diffusion for GGBS. GGBS increases the chloride-binding capability greatly without reference to the internal or external chloride and sulfates decrease the chloride-binding capability of GGBS greatly. It can be also found that the maximum intensity peak corresponding to Friedel's salt appears at about 8.0 Å, its endothermic effect appears at about 360 °C, its morphology is hexagonal slice whose size is about 2-3 μm; that the output of Friedel's salt formed by GGBS is much more than OPC; and that sulfates influence the output of Friedel's salt greatly. The corresponding mechanism was also analyzed.     

Thermodynamic modelling of the hydration of Portland cement

A thermodynamic model is developed and applied to calculate the composition of the pore solution and the hydrate assemblage during the hydration of an OPC. The calculated hydration rates of the individual clinker phases are used as time dependent input. The modelled data compare well with the measured composition of pore solutions gained from OPC as well as with TGA and semi-quantitative XRD data. The thermodynamic calculations indicate that in the presence of small amounts of calcite typically included in OPC cements, C-S-H, portlandite, ettringite and calcium monocarbonates are the main hydration products. The thermodynamic model presented in this paper helps to understand the interactions between the different components and the environment and to predict the influence of changes in cement composition on the hydrate assemblage.     

Porengrössenverteilung von Mörteln nach lagerung im wasser und in einer chloridlösung

For the purpose of the investigation of relations between the chloride diffusion into concrete and the pore structure of the hardened cement paste, the porosity of cement mortars were examined. The mortar specimens of four cements with a graded blast-furnace slag content (0 to 79 % by mass) prepared at w/c ratios 0,50 and 0,70 were stored in a concentrated NaCl solution and water respectively up to 2 years. The pore size distributions of the mortars were determined by means of mercury intrusion porosimetry and water absorption. The samples stored in water showed a relationship between the percentage of small (gel) as well as great (capillary) pores and the slag content of the cement and the w/c ratio of the mortar. After 6 and 12 months, the storing in the chloride solution effected an alteration of the pore structure reducing the total and gel pore volume and increasing the capillar pore volume. Later, the gel pore fraction was near to that of the specimens stored in water.     

The influence of different cements on chloride-induced corrosion of reinforcing steel

Corrosion of embedded steel in concrete may occur as a result of the depassivating effects of chloride ions. Two important parameters governing the risk of chloride-induced corrosion in cement matrices of varied compositions are believed to be: (i) the relative concentrations of chloride and hydroxyl ions in the pore electrolyte and (ii) the diffusivities of chloride ions. Measurements of these parameters for cement pastes of constant water/cement ratio and fixed total chloride content have been used to rank a series of Portland cements, slag blended cement and fly-ash blended cement in terms of their expected levels of corrosion protection. The validity of the predicted rank orders has been independently assessed by electrochemical monitoring of the corrosion rates of embedded steel electrodes by means of the method of linear polarisation.     

Effects of dosage and modulus of water glass on early hydration of alkali-slag cements

This paper examines the early hydration of alkali-slag cements activated with water glass with different n moduli and sodium metasilicate (Na₂SiO₃·5H₂O) in solution at 25 °C. The early hydration of alkali-activated blast furnace slag cements has been studied using isothermal conduction calorimetry. The cumulative heat of hydration increases by increasing the n modulus as well as the dosage of water glass, but is still lower than that of Portland cement. The compressive strength of normal-cured water glass slag cements is higher than Portland cement mortars. Drying shrinkage of alkali-slag cements is considerably higher than that of Portland cement. Consequently, industrial use of alkali-slag cement needs better understanding of the hardening mechanism and requires further research based on presented observations and results.     

Nonevaporable water from neat OPC and replacement materials in composite cements hydrated at different temperatures

Pastes of two neat OPC and three blended cements using GGBFS (60%), PFA (30%) and a volcanic ash (23%), were cured for up to 1 year at five temperatures. The degree of hydration of the OPCs was estimated by quantitative X-ray diffraction analysis and by measurements of nonevaporable water by thermogravimetry. A correlation between the results from these techniques is presented for the neat OPCSs. The correlation was used to estimate the contribution to the nonevaporable water from the cement replacement material fraction for the blended cements. According to the estimated data, the slag displayed a hydraulic nature retaining significant amounts of water in its hydrates, the slag nonevaporable water values as function of time varied with temperature and the patterns were similar to those of degree of hydration of the neat cement. The data estimated for the two pozzolanic materials indicated that their hydrates retained small amounts of water in spite of the CH consumption.     

Hydration and durability of sulphate-resisting and slag cement blends in Caron's Lake water

The problem of aggressive attack of sulphate and chloride ions has been of considerable scientific and technological interest because this attack is one of the factors responsible for damage to concrete. The corrosive action of chlorides is due to the formation of chloroaluminate hydrates, which causes softening of concrete. Sulphate ions can enter into chemical reactions with certain constituents of concrete, producing sulphoaluminate hydrates and gypsum, which cause the expansion of concrete. The aim of the present work is to study the hydration and the durability of mixed cement (sulphate-resisting and slag cement blends) pastes and mortars in Caron's Lake water. Different mixes of sulphate-resisting cement (SRC) with various proportions of slag cement were prepared and immersed in tap water for 3, 7, 28 and 90 days. The durability of the cement mortars was followed by curing the samples in tap water for 28 days (zero time) then immersed in Caron's Lake water for 1, 3, 6, 9 and 12 months. The hydration behavior was measured by the determination of the compressive strength, free lime, evaporable and nonevaporable water, total chloride and total sulphate contents at each curing time. The increase of substitution of SRC with blast-furnace slag cement (BFSC) up to 30% increases slightly the total pore volume. The free lime contents decrease sharply in the first months of immersion then slightly up to 1 year. The blended cement pastes made of SRC with BFSC up to 30 mass% have lower values of total chloride and total sulphate, while the mortars containing only SRC have lower values of compressive strength than those of all blended cement mortars at all curing ages of immersion under Caron's Lake water. Useful conclusions and recommendations concerning the use of 70 mass% of SRC with 30 mass% slag cement produces a highly durable mixed cement.