Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Performance of lightweight aggregate concrete containing slag after 25 years in a harsh marine environment

This paper reports the results of a study on concretes containing lightweight aggregate (LWA) retrieved from the tidal zone of a marine exposure site. In terms of chloride resistance, the LWA concrete performed equivalently to similar concretes of the same age produced with normal density aggregate that were retrieved from the same site 2 years earlier. The partial replacement of Portland cement with slag led to substantial reductions in chloride penetration and the chloride diffusion coefficient. However, at w/cm ≥ 0.50, the incorporation of slag resulted in increased surface deterioration (scaling) attributed to freezing and thawing. Concrete with LWA, silica fume and w/cm = 0.33, showed better-than-expected performance with regard to resistance to chloride-ion penetration and it is speculated that this may be partly attributed to “internal curing” provided by the LWA which reduces the impact of self desiccation. Further studies are needed to confirm this phenomenon.     

Steam-cured concrete incorporating mineral admixtures

This paper explores the potential benefits of steam-cured concrete, particularly on mixes incorporating mineral admixtures. Twenty mixes with various combinations of Portland cement, fly ash (FA), slag and silica fume (SF) were investigated. For each mix, specimens were either standard-cured in a water bath of 27 °C or steam-cured at 55 °C maximum temperature over 8 h. For the materials and test conditions reported in this study, it was found that steam-cured concretes were more porous as indicated by the much higher sorptivity values compared with standard-cured specimens. Mixes with SF have the best performance and hold promise in precast manufacturing due to their high early strength development and low sorptivity values.     

A new model for the estimation of compressive strength of Portland cement concrete

In this article, on the basis of the existing experimental data, an empirical equation for calculating the compressive strength of Portland cement concrete is developed. The determination of the compressive strengths by the equation described here relies on accurate determination of the water to cement ratio which gives maximum compressive strength and the analysis of its variation with the curing time. The results obtained for the plain (without admixture) and latex modified concretes at the age of 28 days show that this ratio ranges from 0.18 to 0.23. These values are reasonably close to the non-evaporable water content reported for the Portland cement. On the other hand, this range as determined by the above procedure limits the usefulness of the proposed equation for predicting the compressive strength of silica fume blended Portland cement concretes. However, a general method of solving problems of this type allows the determination of upper and lower bounds of this range. This method requires the measurement of at least two compressive strengths corresponding to two different water to cement ratios.     

Properties of plain and latex modified Portland cement pastes and concretes with and without superplasticizer

This paper deals with the effects of latex concentration on the workability and strength characteristics of Portland cement pastes with and without superplasticizer. Durability assessments are made by immersing these pastes in 5% Na₂SO₄ and 2.5% NaCl solutions. From the results obtained, it is found that the superplasticizer and superplasticizer-latex combinations may improve the workability of the Portland cement pastes. The Portland cement pastes with superplasticizer have much higher strengths than the latex modified Portland cement pastes with and without superplasticizer. In general, curing in lime-saturated water adversely affect the strength of the pastes containing latex from about 28 days onwards. In the durability test, the resistance of latex modified Portland cement pastes with and without superplasticizer to NaCl is decreased. Degradation mechanism depends on the characteristics of the corrosive medium as well as the resistance of the material itself to the resulting chemical action. The character of strain-stress data of latex modified concretes becomes more prominent as the latex concentration increases. These data are anomalous when compared with the data normally observed for concretes without admixture. The proposed equations are found adequate to describe the stress-strain behaviour of latex modified concretes in compression. These equations can also be applied in calculating the initial modulus of elasticity and proportional limit in the case of polymer modified concretes, which exhibit non-linear behaviour at high stress.     

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.     

Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete

The effect of mineral admixture and curing condition on the sorptivity of concrete are investigated. In the present work, the maximum particle size and the grading of coarse aggregate, the cement content and water/cement ratio of the concrete are kept constant. Then, in the ordinary Portland cement (OPC) 42.5 concrete, a portion of the sand is replaced by a mineral admixture such as fly ash (FA), limestone filler, sandstone filler or silica fume (SF). This paper presents the results of both the sorptivity coefficient and the compressive strength of OPC 42.5 concretes with these mineral admixtures, and concretes with OPC 32.5, blended cement (BC) or trass cement (TC). The results obtained indicate that the sorptivity coefficient of concrete decreases as the compressive strength of concrete increases. It is also shown that the sorptivity coefficient of concrete is very sensitive to the curing condition. The effect of curing condition on the sorptivity coefficient of concrete seems to be higher in low-strength concretes.     

Pessimum effect of externally applied chlorides on expansion due to delayed ettringite formation: Proposed mechanism

Mortars and concretes were subjected to a heat treatment cycle consisting of a pre-set period of 4 h at 23 °C followed by accelerated curing at 95 °C prior to storage at room temperature in water or limewater, 0.5 M, 2.8 M sodium chloride solutions. It was found that the specimens stored in 0.5 M sodium chloride solution gave a much greater expansion than those stored in limewater or 2.8 M sodium chloride solution. This pessimum influence of chlorides on expansion due to delayed ettringite formation deviates from the commonly held view that chlorides mitigate sulphate attack in concretes. The mechanism of the pessimum effect of chlorides on expansion due to delayed ettringite formation, and the final products of the associated phase transformations have been proposed. X-ray diffraction and differential thermal analysis techniques were used to follow phase transformations.     

The effect of excessive steam curing on Portland composite cement concrete

Steam curing at atmospheric pressure is an important technique for obtaining high early strength values in precast concrete production. Cement type, as well as curing period and temperature, is an important parameter in the steam-curing process. PC42.5 is the type of cement that is most commonly used in Turkish precast concrete plants. Its behavior is well known. Nowadays, the production of composite cements is becoming more popular every other day due to its advantages. The object of this study was to determine the properties of this relatively new binder comparatively with conventional PC42.5 under steam curing. For this purpose, 15-cm concrete cubes were prepared with a water/cement ratio (W/C) of 0.44 and were subjected to steam curing for five different curing periods of 4, 8, 16, 24 and 36 h under curing temperatures of 65 and 85 °C. Cement dosage was kept constant (400 kg/m³) for all specimens. The variation of compressive strength values and maturity for each condition has been presented comparatively within this study. Test results indicated that Portland composite cement (PKC/A42.5) can be used in place of PC42.5 for steam curing at atmospheric pressure in precast concrete production. However, in case of early high strength demand for early demolding purposes, curing temperature should be increased to 85 °C for PKC/A42.5 cement concretes.     

Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes

In this investigation, two mixes were used: ordinary Portland cement (OPC) and a blended cement prepared with the partial substitution of OPC by 10 mass% silica fume (SF). The setting and hardening characteristics were monitored by the aid of electrical conductivity as a function of curing time. The shear stress and electrical conductivity were studied at different temperatures, namely, 20, 35, 45 and 55 °C. As the temperature increases, the shear stresses decrease with the increase of shear rate. The height of electrical conductivity peaks of superplasticized cement pastes increases due to the increase of the paste fluidity. In the presence of 1.0% polycarboxylate (PC), the electrical conductivity of cement pastes decreases from 1 to 28 days. PC retards the hydration of cement pastes. The presence of PC extended the setting times of cement pastes at 35 °C than at 20 °C due to the increase in the adsorption capacity at this temperature. PC extends the dormant stage of the hydration process and delays the onset of the accelerating stage, without affecting its rate.     

碱–磷渣–粉煤灰混凝土力学性能和耐久性(英文)

研究了用碱激发磷渣_粉煤灰胶凝材料(atkali activated phosphor slag fly ash cement,AAPFC)制各的混凝土的力学性能和耐久性,并用扫描电子显微镜观察了形成的水泥石与骨料的界面结构.结果表明:相对于硅酸盐水泥混凝土,AAPFC混凝土具有强度高,弹性模量较低的特点;其抗冻性和抗氯离子渗透性显著优于硅酸盐水泥混凝土,但抗碳化性不及后者.硅酸盐水泥混凝土中水泥石与骨料界面上存在大量定向排列的Ca(OH)2,造成弱结合,而AAPFC混凝土中水泥石与骨料问结合紧密.     

Effect of temperature and aging on the mechanical properties of concrete: Part I. Experimental results

This paper reports the results of curing temperature and aging on the strength and elastic modulus and the Part II paper suggests a prediction model based on these experimental results. Tests of 480 cylinders made of Types I, V, and V cement+fly ash concretes, cured in isothermal conditions of 10, 23, 35, and 50 °C and tested at the ages of 1, 3, 7, and 28 days are reported. According to the experimental results, concretes subjected to high temperatures at early ages attain higher early-age compressive and splitting tensile strengths but lower later-age compressive and splitting tensile strengths than concretes subjected to normal temperature. Even though the elastic modulus has the same tendency, the variation of elastic modulus with curing temperature is not so obvious as compressive strength. Based on the experimental result, the relationships among compressive strength, elastic modulus, and splitting tensile strength are analyzed, considering the effects of curing temperature, aging, and cement type.     

养护措施和湿养护时间对掺与未掺矿渣混凝土性能的影响

研究了采用湿棉絮覆盖、喷养护剂、塑料薄膜密封3种养护措施对混凝土强度、收缩、中心温升、氯离子渗透性的影响,并通过不同龄期强度、氯离子渗透深度(5%NaCl溶液介质,质量分数)、钢筋腐蚀电位、加速腐蚀保护层开裂时间研究湿养护时间(1,7,14d和28d)对未掺和掺矿渣(等量取代水泥40%,质量分数)混凝土强度、抗渗性、护筋性的影响.结果表明:合适的养护措施能有效提高混凝土强度和抗渗性,降低水化温升和早期收缩.对未掺与掺矿渣的混凝土,更长的湿养护有助发展更高强度、耐久性和护筋性;而不充分养护导致混凝土较差的抗渗性和护筋性,该影响对掺矿渣混凝土尤为明显.7d湿养护对掺矿渣混凝土发展更高的潜在抗渗性是不够的.     

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.     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Effect of internal curing on durability-related properties of high performance concrete

Internal curing of high performance concrete (HPC) by pre-saturated lightweight aggregates is a well-established method of counteracting self-desiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of high-performance concretes as a function of water to cement ratio is reported.     

Development of high volume fly ash cements for use in concrete construction

The paper describes a study undertaken to examine the use of high levels of low-lime fly ash (high volume FA) as a cement component in concrete, beyond the 30% level commonly adopted. The results indicate that FA levels up to 45% by mass can be combined with Portland cement (PC, C1) to produce the range of practical concrete design strengths, although early strength, which may be critical in construction, can be reduced compared to PC, and lower level FA concretes. The study progressed to consider the use of a rapid hardening Portland cement (C2) and low energy clinker (C3) combined with FA at 45%, as a means of overcoming these early strength shortfalls. Both were found to be effective in matching early strength behaviour of PC concrete. Tests covering fresh (workability loss, bleeding and moisture loss), engineering (strength development, modulus of elasticity, drying shrinkage and creep) and durability (absorption, permeability, carbonation rates and chloride diffusion) properties of these concretes were then carried out. The results indicate that in almost all cases, either similar or enhanced performance was achieved with the high volume FA concrete, compared to that of PC and these findings offer a route to extending FA use. The practical implications of the study are also examined.     

Ceramic cements and ceramic concretes of quartz-chamotte composition

Conclusions On the basis of quartz sand and scrap chamotte refractories (28.0% Al₂O₃, 67.0% SiO₂) we obtained highly concentrated bonding suspensions (cement slips) of mixed composition enabling us to form castings with a porosity of 12.2–18.3% characterized by bending strengths of up to 4.5 MPa and compressive strengths of up to 32.4 MPa.During the use of quartz-chamotte suspensions as bond and of chamotte scrap (grog) as the filler we obtained ceramic concretes with an original compressive strength of 18.4–25.6 MPa, and a porosity of 14.6–19.2%.We studied the change in the physicomechanical properties of the bonds (cements) and the ceramic concretes during heat treatment in the range 100–1450°C.The chamotte ceramic concretes based on quartz-chamotte bonds of the optimal compositions possess an increased spalling resistance (16 water-heat cycles from 1300°C during testing of specimen cubes with edges of 50 mm).Translated from Ogneupory, No. 5, pp. 5–9, May, 1986     

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.     

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.