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.     

Examination of mortar bars containing varying percentages of coarsely crystalline gypsum as aggregate

Contamination of aggregate sources by coarsely crystalline gypsum occurs frequently in the Middle East. Mortar bars were made which contained up to 5% gypsum (by weight of aggregate) in the form of aggregate pieces. The bars were made using three different cements of varying C₃A content and were stored at 20°C and 38°C. The results show that significant expansions do not occur within mortar bars if their total sulphate content lies below the present British Standard limit of 4% SO₃ by weight of cement. Sulphate Resisting Portland Cement (SRPC) can tolerate a higher level of contaminant gypsum than Ordinary Portland Cement (OPC). Temperature also effects the degree of expansion, especially in the case of high C₃A cement.     

Corrosion resistance in activated fly ash mortars

The question of whether reinforcing steel can be protected with activated fly ash cement as effectively as with Portland cement is explored in this study. Corrosion potential (E_corr) and polarisation resistance (R_p) values for steel electrodes embedded in Portland cement mortar and two fly ash mortars, respectively activated with NaOH and waterglass+NaOH solutions, are monitored. Chloride-free activated fly ash mortars are found to passivate steel reinforcement as speedily and effectively as Portland cement mortars, giving no cause to fear that corrosion may limit the durability of reinforced concrete structures built with these new types of activated fly ash cement. The polarisation curves and the response to short-term anodic current pulses (galvanostatic pulse technique) obtained further corroborate the full and stable passivation of the steel by the concrete manufactured with these binders.     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

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。     

Measuring the change in ultrasonic p-wave energy transmitted in fresh mortar with additives to monitor the setting

Research on ultrasonic methods to monitor the setting of concrete has mainly focussed on the wave velocity as a useful quantity. To investigate the application of also the wave energy as a parameter, an experimental program was set up to apply the ultrasonic wave transmission technique on several mortar samples containing air entrainer, blast-furnace slag or fly ash causing clearly different setting behaviour.The increase of the relative energy E/E_ref during setting is generally retarded if ordinary Portland cement is replaced by blast-furnace slag or fly ash. The mixtures with cement of a lower strength class or with large air content were difficult to test with the energy measurements since they were more sensitive to poor sensor contact due to shrinkage. For the other samples, the thresholds E/E_ref = 0.02 and 0.13 are proposed to easily determine respectively initial and final setting based on the ultrasonic energy measurements.     

Alternative blended cement with ceramic residues: Corrosion resistance investigation on reinforced mortar

Blended cements are largely used for concrete: they are usually considered cements with a low environmental impact, as they require less clinker than ordinary Portland cement (OPC). Different constituents can be used as supplementary clinker component usually leading to cement with high resistance to outdoor environment. Polishing residue (PR), coming from porcelain stoneware tiles production, can be successfully used as new constituent for blended cement, however its action for enhancing the durability of cement matrix must be assessed. With this purpose, electrochemical tests (half cell potential, impressed voltage and linear polarization techniques) have been carried out on steel reinforced mortar samples, prepared using a 25% PR based cement and 100% OPC as binder and exposed to a 3.5% NaCl solution. The corrosion resistance results and microstructure analysis highlight better durability performances for PR based cement than those exhibited by OPC, mainly for curing time > 28 days.     

Enhancement in early strengths of slag-cement mortars by adjusting basicity of the slag prepared from fly-ash of MSWI

The insufficient early strengths of cement mortars in which partial cement had been replaced by pulverized slag melted from municipal solid waste incinerator (MSWI) fly-ash were tackled in this study by adjusting the basicity of the slag through the addition of various amounts of CaCO₃ into MSWI fly-ash, melted into a ‘modified slag’, pulverized to partially replace cement. Increased basicity in the modified slag manifestly improves the early compressive strengths of cement mortar with 20% Portland cement replaced by the modified slag powder (20 wt.% CaCO₃ added). The 14-day and 28-day compressive strengths of the mortars evidently increased to nearly that of the reference specimen made of only Portland cement mortar. The 90-day compressive strength is even higher than that of the reference specimen. Porosity and Fourier transform infrared spectra (FTIR) analyses evidenced the improvement in early strengths by hydration while the enhancement in long-term strength by pozzolanic reaction in the CaCO₃ added slag-cement mortar.     

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.     

Hydration of calcium sulfoaluminate cement by a ZnCl2 solution: Investigation at early age

Hydration of calcium sulfoaluminate cement at early age was investigated as a function of the gypsum content of the binder, the thermal history of the material, and the ZnCl₂ concentration in the mixing solution. Early hydration was strongly accelerated by the presence of gypsum, but lower percentages of reaction were reached after 24 h. The slowing down effect induced by ZnCl₂, even at a concentration as high as 0.5 mol/L, was moderated compared to OPC but had a greater intensity in the absence of gypsum. Unlike what would have been expected for Portland cement, it was shown that the delay of a gypsum-free calcium sulfoaluminate cement resulted from the strong retardation caused by chloride anions, which was partly compensated by the accelerating effect of Zn²⁺ cations. The mineralogical observations revealed the precipitation of chloro–AFm phases such as Friedel's and Kuzel's salts, but no crystallized zinc-containing phases could be identified by XRD. The thermal history of the samples proved to be a key parameter. Applying a thermal cycle which reproduced the temperature rise and decrease occurring in a massive mortar block accelerated the rate of hydration and mainly modified the proportion of AFt versus AFm hydrates, especially when the binder had a gypsum content below 20%.     

Chloride resistance of high-performance concretes subjected to accelerated curing

The strengths and chloride penetration resistance of a series of high-performance concretes were measured after curing either at 23 °C or accelerated by heating to 65 °C. The results confirm that concretes containing silica fume (SF) or ternary blends of SF and ground granulated blast-furnace slag (GGBFS) exhibit improved chloride penetration resistance compared to those of plain Portland cement concretes. In addition, chloride penetration resistance of Portland cement concrete is adversely affected by accelerated curing. With the use of the ternary ordinary Portland cement (OPC)-SF-GGBFS binders, accelerated curing did not have detrimental effects on chloride penetration resistance and provided 18-h strengths in excess of 40 MPa.     

Employing of some hyperbranched polyesteramides as new polymeric admixtures for cement

Hyperbranched polyesteramides (HBPA₁ and HBPA₂) were synthesized by the bulk polycondensation of maleic anhydride (MA_n) as an A₂ monomer with both of diisopropanolamine (DIPA) and diethanolamine (DEA), respectively, as B′B₂ monomer. The prepared polymers were analyzed with IR, GPC, ¹H NMR, TGA, and DSC. The hyperbranched polyesteramides were applied as polymeric admixtures in two different types of cement, namely ordinary Portland cement (OPC) and Portland limestone cement (PLC). Several parameters were studied to evaluate the action of HBPA₁ and HBPA₂ polymers as cement admixtures. Adding HBPA₁ and HBPA₂ decreased the water of consistency and increased the compressive strength with no effect on the chemical composition of the cement phases. The combined water content and bulk density displayed the same trend as compressive strength. The IR spectra of the formed phases for the mixed cement pastes with HBPA₁ and HBPA₂ illustrated increased intensities of the absorption bands than those of the pristine cement pastes. The SEM photos showed that the incorporation of HBPA₁ and HBPA₂ in cement phases affected the morphology and microstructure of the formed hydrates. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A Review on Polymer/Cement Composite with Carbon Nanofiller and Inorganic Filler

Owing to non-toxicity and biocompatibility, water-soluble polymers (polyethylene glycol, polyvinyl alcohol, poly(N-isopropylacryl-amide), and polyvinylpyrrolidone) have different applications in civil engineering, paint industry, and environmental industries. Portland cement is made up of various kinds of components such as concrete, mortar, and stucco. The quality of cement can be enhanced by adding different fillers such as slag, silica fume, fly ash, and natural pozzolan. Carbon nanofillers (carbon nanotube and graphite) and inorganic fillers such as CaCO₃, Mg(OH)₂, talc, and mica are used in polymer/cement composites to enhance their properties. Polymer/cement composite with lightweight aggregates are best for several future composite applications.     

Autogenous healing of marine exposed concrete: Characterization and quantification through visual crack closure

Concrete can autogenously heal cracks potentially increasing construction durability. Studies quantifying this process are limited to fresh-water making the controlling parameters in sea-water unclear. Here we visually quantify the autogenous healing capacity of ordinary Portland cement (OPC) and blast-furnace slag (BFS) cement mortar specimens submerged in fresh- and sea-water. After 56 days, BFS cement specimens in sea-water healed 100% of cracks up to 104 μm, for OPC specimens it was 592 μm. In fresh-water, BFS cement specimens healed 100% of cracks up to 408 μm, while OPC specimens healed 100% of cracks up to 168 μm. Displaying greater healing efficiency OPC specimens in sea-water became weak, developing unacceptable losses in compressive strength. Differences in performance were attributed to the amount of calcium hydroxide in these mortars and specific ions present in sea-water. Visual crack-healing, therefore, should be assessed in conjunction with a material functional property.     

Corrosion initiation of reinforced concretes based on Portland or GGBS cements: Chloride contents and electrochemical characterizations versus time

The ingress of chlorides in cylindrical reinforced concretes based on ordinary Portland cement (OPC) or ground granulated blast furnace slag cement (GGBS) has been investigated together with the corrosion behaviour of the steel rebar. Chloride exposure was obtained by wetting and drying cycles during one to two years. The evolution of total and free chloride contents versus time of exposure shows that GGBS concretes induced a delay in chloride ingress. Corrosion initiation of steel was evaluated through nondestructive electrochemical measurements (half cell potential and linear polarization resistance) versus time of exposure. For GGBS concretes, corrosion assessment was not reliable based on the ASTM standard or RILEM recommendations. For OPC concretes, the transition from passive to active corrosion was studied considering a drop of potential or a corrosion current threshold value. Considering this latter, total and free chloride contents larger than 2.2% or 0.7% by weight of cement were estimated.     

Deicer-scaling resistance of phosphate cement-based binder for rapid repair of concrete

In this paper, a magnesium phosphate cement-based binder (MPB) was prepared by mixing MgO with mono-ammonium phosphate, borax and fly ash. The deicer-scaling resistance of MPB mortar and concrete and the bond strength loss between MPB paste and mortar with ordinary Portland cement (OPC) concrete were investigated. Experimental results show that MPB materials themselves have high deicer-frost resistance, which is not lower than that of OPC concrete with the air content of 4.5-6.5%. The bond strength loss between MPB materials and OPC concrete with the air-entraining agent is obviously lower than that between MPB and OPC concrete without the air-entraining agent, and the higher the air content in OPC concrete is, the smaller the loss is. Furthermore, the air-bubble parameters were analyzed, which indicate that MPB mortar and concrete can also obtain a reasonable air-bubble structure by chemical reactions.     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Chloride diffusion in partially saturated cementitious material

The paper proposes a combined application of composite theory and Powers' model for microstructural development for the estimation of the diffusion coefficient as a function of the moisture content of a defect-free cementitious material. Measurements of chloride diffusion in mortar samples (440 kg/m³ rapid-hardening Portland cement, w/c=0.5, maturity minimum 6 months) stored at 65% and 85% RH, as well as in vacuum-saturated mortar samples, illustrate the applicability of the method.     

Thermal analysis of borogypsum and its effects on the physical properties of Portland cement

Borogypsum, which consists mainly of gypsum crystals, B₂O₃ and some impurities, is formed during the production of boric acid from colemanite, which is an important borate ore. In this study, the effect of borogypsum and calcined borogypsum on the physical properties of ordinary Portland cement (OPC) has been investigated. The calcination temperature and transformations in the structures of borogypsum and natural gypsum were determined by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. Thermal experiments were carried out between ambient temperature and 500 °C in an air atmosphere at a heating rate of 10 °C min⁻¹. After calculation of enthalpy and determination of conversion temperatures, borogypsum (5% and 7%), hemihydrate borogypsum (5%) and natural gypsum (5%) were added separately to Portland cement clinker and cements were ground in the laboratory. The final products were tested for chemical analysis, compressive strength, setting time, Le Chatelier expansion and fineness properties according to the European Standard (EN 196). The results show that increasing the borogypsum level in Portland cement from 5% to 7% caused an increase in setting time and a decrease in soundness expansion and compressive strength. The cement prepared with borogypsum (5%) was found to have similar strength properties to those obtained with natural gypsum, whereas a mixture containing 5% of hemihydrate borogypsum was found to develop 25% higher compressive strength than the OPC control mixtures at 28 days. For this reason, utilization of calcined borogypsum in cement applications is expected to give better results than untreated borogypsum. It is concluded that hemihydrate borogypsum could be used as a retarder for Portland cement as an industrial side. This would play an important role in reducing environmental pollution.     

The influence of sulphates on chloride binding and pore solution chemistry

Ordinary Portland cement (OPC) and OPC/ground granulated blastfurnace slag (GGBS) 65% cements containing 2.0 to 9.0% sulphates derived from sodium sulphate and calcium sulphate were investigated in respect to their chloride binding properties and the concentrations of chloride and hydroxyl ions in the pore solutions. Chlorides derived from sodium and calcium chlorides were introduced at the time of mixing. The results indicate that calcium sulphate has a different effect on chloride binding and the pore solution chemistry than sodium sulphate. The slag cement has higher chloride binding capacities as a result of simple replacement for OPC, but at the same sulphate contents, the slag cement does not give the expected higher binding capacities, suggesting that the difference in sulphate content between the two cements may be the main reason for their different chloride binding behaviour.