Sorptivity and strength of air-cured and water-cured PC-PFA-MK concrete and the influence of binder composition on carbonation depth

The paper reports the influence of the composition of Portland cement-pulverised fuel ash-metakaolin (PC-PFA-MK) binders on sorptivity and strength development of PC-PFA-MK concrete cured both in air and in water and on carbonation depth, and relates this to measured changes in sorptivity of the concrete. Concrete mixtures covering four different total cement replacement levels (10%, 20%, 30% and 40%) for PC-PFA-MK concrete with various MK/PFA proportions, water and air cured for up to 18 months, were investigated. The change in compressive strength and sorptivity with age at all cement replacement levels under both water and air curing are compared with those of the control PC concrete. The results presented in this paper form part of an investigation into the optimisation of a ternary blended cementitious system based on ordinary PC, PFA and MK for the development of high-performance concrete.     

Heat of hydration of Portland Cement–Metakaolin–Fly ash (PC–MK–PFA) blends

In the present study two pozzolanic materials are used, Metakaolin (MK) and Fly Ash (PFA), as binary and ternary partial replacement binders with Portland cement (PC) to investigate their effect on the rate of heat evolution (dQ/dt in J/gh) during hydration, and the heat of hydration, (Q(t) in J/g). For binary PC–PFA blends PC hydration is enhanced in the very early stages of hydration, but at extended periods (up to 120 h) an increase in PFA replacement level causes a systematic reduction in heat output. For binary PC–MK blends the results suggest that the MK initially diminishes PC hydration but the subsequent pozzolanic reaction of MK increasingly contributes to the heat output causing some blends to exceed the heat output of the PC control. For both systems a principal controlling factor in the PC hydration rate (and the heat evolution rate) is the water requirement of the pozzolan, but for PC–MK blends the pozzolanic reaction of the MK makes a significant contribution to the heat output. However this reaction is controlled both by the availability of water and the supply of Ca²⁺ ions from the hydrating PC which introduces an increasing level of complexity to the heat output versus time profiles. When combining MK and PFA in ternary PC–MK–PFA blends the MK has a dominant influence on the heat output versus time profiles.     

Property improvement of Portland cement by incorporating with metakaolin and slag

The physical and mechanical properties of Portland cement (PC) containing metakaolin (MK) or combination of MK and slag and the compatibility between such materials and superplasticizers were investigated in present study. After MK was incorporated into PC, the compressive strength of the blended cement was enhanced. However, the fluidity of MK blended cement became poorer than that of PC at the same dosage of superplasticizer and the same water/binder ratio. When both MK (10%) and ultra-fine slag (20% or 30%) were incorporated into PC together, not only the compressive strength of the blended cement was increased, but also the fluidity of the blended cement paste was improved comparing to MK blended cement. This indicates that ultra-fine slag can improve the physical and mechanical properties of MK blended cement. The physical and chemical effects of two mineral admixtures were also discussed.     

Binder content influences on chloride ingress in concrete

The reported study looked at the effect of reducing free water contents, and thereby binder contents, on the ingress of chloride in concrete. Concretes with equal water/binder ratio (and design strength), but with water contents reduced by up to 30 litres/m³, were tested for chloride diffusion (D) and penetration. The quality of the microstructure was inferred from initial surface absorption tests (ISAT). The results show no practical difference in chloride durability between the corresponding concretes, and that reducing the binder content, (providing that the water/binder ratio is maintained) is not likely to be detrimental. However, the results reported underline the importance of binder type, in this case PFA. Implications of the results are discussed and, in light of the findings, whether specifications which demand minimum cement Contents are justified.     

Pore structure characteristics of MgO–SiO2 binder

This paper reports on an investigation into the pore structure characteristics of MgO–SiO₂ binders. Paste specimens were prepared using two MgO/SiO₂ ratios (1 and 1.5), two silica sources (Silica fume-SF and Metakaolin-MK), two water to binder ratios (0.5 and 0.6) and compared to a portland cement (PC) control mix. Mercury intrusion porosimetry was used to understand the evolution of pore structural features of the binder with hydration period. A continued reduction in porosity and refinement of pores was observed with the progress in hydration irrespective of the type of the silica source or the initial MgO/SiO₂ ratio. The MgO–MK mixes exhibited a finer and denser microstructure as compared to the equivalent MgO–SF mixes. The formation of hydrotalcite in addition to M-S-H in MgO–MK mixes was postulated to be the primary reason for the difference in microstructural characteristics, compared to MgO–SF mixes. The space-filling capacity of hydration products of MgO–MK binder was found to be better than PC.     

Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures

In this paper, the effects of elevated temperatures on the compressive strength stress–strain relationship (stiffness) and energy absorption capacities (toughness) of concretes are presented. High-performance concretes (HPCs) were prepared in three series, with different cementitious material constitutions using plain ordinary Portland cement (PC), with and without metakaolin (MK) and silica fume (SF) separate replacements. Each series comprised a concrete mix, prepared without any fibers, and concrete mixes reinforced with either or both steel fibers and polypropylene (PP) fibers. The results showed that after exposure to 600 and 800 °C, the concrete mixes retained, respectively, 45% and 23% of their compressive strength, on average. The results also show that after the concrete was exposed to the elevated temperatures, the loss of stiffness was much quicker than the loss in compressive strength, but the loss of energy absorption capacity was relatively slower. A 20% replacement of the cement by MK resulted in a higher compressive strength but a lower specific toughness, as compared with the concrete prepared with 10% replacement of cement by SF. The MK concrete also showed quicker losses in the compressive strength, elastic modulus and energy absorption capacity after exposure to the elevated temperatures. Steel fibers approximately doubled the energy absorption capacity of the unheated concrete. They were effective in minimizing the degradation of compressive strength for the concrete after exposure to the elevated temperatures. The steel-fiber-reinforced concretes also showed the highest energy absorption capacity after the high-temperature exposure, although they suffered a quick loss of this capacity. In comparison, using PP fibers reduced the energy absorption capacity of the concrete after exposure to 800 °C, although it had a minor beneficial effect on the energy absorption capacity of the concrete before heating.     

Durability of metakaolin concrete to sulfate attack

This study investigates the effect of metakaolin (MK) replacement of cement on the durability of concrete to sulfate attack. Three MK replacement levels were considered in the study: 5%, 10%, and 15% by weight of cement. The other experimental parameters investigated in the study were: water to binder ratio (0.5 and 0.6), initial moist curing period (3, 7, and 28 days), curing type (moist and autoclaving), and air content (1.5% and 5%). After the specified initial moist curing period, concrete specimens were immersed in 5% sodium sulfate solution for a total period of 18 month. The degree of sulfate attack was evaluated by measuring expansion of concrete prisms, compressive strength reduction of concrete cubes, and visual inspection of concrete specimens to cracks. The study showed that MK replacement of cement increased the sulfate resistance of concrete. The sulfate resistance of MK concrete increased with increasing the MK replacement level. The sulfate resistance of MK concrete at w/b ratio of 0.5 was found higher than that at w/b ratio of 0.6. Autoclaved MK concrete specimens showed superior sulfate resistance compared to moist cured ones. The pore volume of autoclaved MK concrete was found less than that of moist cured MK concrete. The air entrained MK concrete showed higher improvement in the sulfate resistance than the non-air entrained MK concrete. However, the air entrained plain concrete showed lower improvement in the sulfate resistance than the non-air entrained concrete.     

Effects of mineral and chemical admixtures on high-strength concrete in seawater

The effects of mineral and chemical admixtures namely fly ash, ground granulated blast furnace slag, silica fume and superplasticizers on the porosity, pore size distribution and compressive strength development of high-strength concrete in seawater curing condition exposed to tidal zone were investigated. In this study, three levels of cement replacement (0%, 30% and 70% by weight) were used. The total cementitious content used was 420 kg/m³. A water/binder ratio of 0.4 was used to produce concrete having a target compressive strength ranging between 54 and 63 MPa at the age of 28 days. At the age of 364 days, the compressive strength of the specimens produced ranged between 59 and 74 MPa. The pore size distribution of both high-strength concrete (MSS-0 and MSS-40) was significantly finer and the mean volume pore radii (MVPR) at the age of 6 months were reduced about three times compared to NPC concrete. Results of this study indicate that both concrete mixes (30% and 70%) exhibited better performance than the NPC concrete in seawater exposed to tidal zone. Hence, it is believed that both high-strength concrete produced would withstand severe seawater exposure without serious deterioration.     

A new way to increase the long-term bond strength of new-to-old concrete by the use of fly ash

The short-term and long-term bond strengths of new-to-old concrete were experimentally investigated with an emphasis on the influence of new concretes and binders. These new concretes included ordinary Portland cement concrete, expansive concrete and high-volume fly ash concrete, while the binders included pure cement paste (C-binder), expansive binder (E-binder) and fly ash mortar (F-binder). The results showed that the short-term bond strength of all specimens with fly ash concrete was lower than that with ordinary Portland cement concrete, which in turn was lower than that with expansive concrete. The bond strength of the specimens with F-binder was the lowest at the age of 7 days. However, the long-term bond strength of all specimens with added fly ash was the highest and strength losses were observed in the specimens repaired with expansive concrete or E-binder at the age of 3 years. The microstructure of the transition zone with F-binder was also studied by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) at the ages of 28 days and 1 year, respectively.     

Influence of the composition of cement kiln dust on its interaction with fly ash and slag

Cement kiln dust (CKD), a by-product of the cement industry, contains significant amounts of alkali, free lime, chloride and sulfate. Wide variation reported in the chemical composition of CKDs limits their potential application as a sustainable binder component in concrete. In the current study, the performance of two different CKDs as components in a novel binder is evaluated. Several binders are developed by blending CKDs with fly ash or slag. Binders with 70% CKD were prepared at a water-to-binder ratio of 0.4, and heat-cured at 75 °C to accelerate the strength development. The hydration progress was monitored using X-ray diffraction, and morphological examination was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ettringite and calcium aluminosilicate hydrate (C-A-S-H) were identified as the main hydration products in the hardened binder system. Strength development of CKD-based binder was found to be significantly influenced by its free lime and sulfate contents.     

掺钢渣-矿渣-粉煤灰复合微粉混凝土性能研究

研究了由钢渣-矿渣-粉煤灰制备的复合微粉对混凝土强度、收缩性能和氯离子渗透性能的影响。结果表明：在同水胶比下,复合微粉等量取代水泥后,混凝土7d强度低于普通混凝土的强度,当复合微粉掺量小于45％时,其28d及以后强度高于普通混凝土。在同水胶比下,复合微粉等量取代水泥后,可有效降低混凝土的干燥收缩,且混凝土的抗氯离子渗透性能显著提高。     

Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties

High strength can be obtained at early ages for precast concrete elements by the use of CEMI 52.5R cement (OPC) and thermal treatment (steam curing). To compensate for the announced withdrawal of CEM I cements because of high CO₂ emissions during their production and the ecotax that this will imply, one attractive alternative is the use of composed cements resulting from the combination of clinker with mineral admixtures. In steam curing conditions, previous studies have shown an increase in the compressive strength at one day of age for mortars incorporating an OPC/blast furnace slag (GGBS)/metakaolin (MK) combination, in comparison with mortars incorporating OPC only. The present study investigates the connection between the compressive strength, at one day of age, of steam cured mortars made with various binders and the hydration of these binders. The progress of the hydration was characterised by means of XRD, thermal and microprobe analyses. The results indicate that the increase in compressive strength when MK is incorporated (OPC/MK or OPC/MK/GGBS) can be explained by an increase in the amount of C-S-H, C-A-H, C-A-S-H phases, a decrease in the amount of CH and a change in the chemical nature of the matrix (decrease in C/S ratio). The decrease in compressive strength of OPC/slag-based material can be explained by a reduction in the amount of hydrated phases (particularly C-S-H) and compactness.These are promising results for precast concrete manufacturers who are concerned about preserving the environment.     

Effect of inorganic materials on the solidification of heavy metal sludge

Portland cement, cement-fly ash and lime-fly ash binders were used to solidify a synthetic heavy metal sludge containing nitrates of Cr, Ni, Cd and Hg. The sludge to binder (cement, cement-fly ash and lime-fly ash) ratio was kept at 3.33, 1.43 and 1.25, respectively. In addition inorganic substances like Cu, Zn, Pb, Sodium hydroxide and sodium sulfate were added. The molded samples were cured at room temperature for 28 days. The solidified samples with and without interference were examined for the change in their bulk density and compressive strength at definite time intervals during curing. All the metals and sodium salts added increased the average bulk density of the final product with increase in concentration (2% to 8%) with all the binder systems. The samples containing copper and lead decreased the compressive strength at all the concentrations added with CFA and LFA binders. Zn had the largest effect on all the three binder systems, lowering the strength of all samples at all the days and concentrations except the 2% Zn with CEM binder. However, Pb had only minor effect on the compressive strength with CEM binder and values remained almost constant at all the times and concentrations studied. In contrast, the effect of sodium sulfate was less marked while sodium hydroxide increased the rate of set and 28-day compressive strength of samples containing cement as binder. These observations confirm the need for specific studies of the waste and binder prior to the selection of a solidification process for the treatment of hazardous wastes. The results provide a better understanding of materials that may interfere with the immobilization of waste constituents and provide information on the possible mechanism of the interfering effects.     

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.     

Medium strength self-compacting concrete containing fly ash: Modelling using factorial experimental plans

Fresh self-compacting concrete (SCC) flows into place and around obstructions under its own weight to fill the formwork completely and self-compact, without any segregation and blocking. The elimination of the need for compaction leads to better quality concrete and substantial improvement of working conditions. SCC mixes generally have a much higher content of fine fillers, including cement, and produce excessively high compressive strength concrete, which narrows its field of application to special concrete only. To obtain maximum benefit from SCC, it has to be adopted in general concrete construction practice. Such practice requires inexpensive and medium strength concrete.This investigation aims to develop medium strength SCC (MS-SCC). The cost of materials will be decreased by reducing the cement content and by using pulverised fuel ash (PFA) with a minimum amount of superplasticizer (SP). A factorial design was carried out to mathematically model the influence of five key parameters on filling and passing abilities, segregation and compressive strength, which are important for the successful development of medium strength self-compacting concrete incorporating PFA. The parameters considered in the study were the contents of cement and PFA, water-to-powder (cement+PFA) ratio (W/P) and dosage of SP. The responses of the derived statistical models are slump flow, fluidity loss, Orimet time, V-funnel time, L-box, JRing combined to the Orimet, JRing combined to cone, rheological parameters, segregation and compressive strength at 7, 28 and 90 days. Twenty-one mixes were prepared to derive the statistical models, and five were used for the verification and the accuracy of the developed models. The models are valid for mixes made with 0.38 to 0.72 W/P, 60 to 216 kg/m³ of cement content, 183 to 317 kg/m³ of PFA and 0% to 1% of SP, by mass of powder. The influences of W/P, cement and PFA contents, and the dosage of SP were characterised and analysed using polynomial regression, which can identify the primary factors and their interactions on the measured properties. The results show tha MS-SCC can be achieved with a 28-day compressive strength of 30 to 35 MPa by using up to 210 kg/m³ of PFA.     

Strength, permeability, and carbonation of high-performance concrete

This investigation is aimed at developing high-performance concrete and form part of an investigation into the optimization of a blended cementitious system for the development of high-performance concrete. Binary and ternary blended cementitious systems based on ordinary Portland cement (OPC), pulverised fuel ash (PFA) and silica fume (SF) were investigated. PFA up to 40% was used, and to these blends, 0%, 5%, 10% and 15% SF were incorporated as partial cement replacements. Results of compressive strength, tensile strength, oxygen permeability and carbonation of concrete are reported. A water-binder (w/b) ratio of 0.27 was used for the main group of mixes and w/b ratios of 0.40 and 0.50 were used for some selected mixes. Based on the experimentally obtained results, prediction models were developed which enabled the establishment of isoresponse contours showing the interaction between the various parameters investigated. It was found that the incorporation of 8-12% SF as cement replacement yielded the optimum strength and permeability values.     

机制砂粉煤灰混凝土中钢筋锚固性能试验研究

通过拉拔试验研究了机制砂粉煤灰混凝土中热轧带肋钢筋和高强预应力钢绞线的粘结锚固性能,结果表明:水胶比、粉煤灰替代水泥率和超量系数等因素对钢筋和钢绞线在机制砂粉煤灰混凝土中的粘结锚固强度的影响规律,与机制砂粉煤灰混凝土轴心抗拉强度的影响规律是一致的,其粘结锚固强度取决于机制砂粉煤灰混凝土轴心抗拉强度的高低。通过分析粘结锚固强度与轴心抗拉强度之间的关系,提出了机制砂粉煤灰混凝土中热轧带肋钢筋和高强预应力钢绞线的锚固长度计算建议。     

二水石膏对海工高性能复合胶凝材料性能的影响

为满足海洋工程混凝土结构的耐久性和施工等方面的需求，需采用复合胶凝材料配制海工高性能混凝土。采用硅酸盐水泥＋粒化高炉矿渣粉＋硅灰制成复合胶凝材料，与普通硅酸盐水泥进行性能比较研究，并重点研究了二水石膏（即三氧化硫含量）对复合胶凝材料性能的影响。研究结果表明，三氧化硫含量对复合胶凝材料的工作性和凝结时间的影响不大，但对其强度和反应抗氯离子渗透性的电量产生显著影响，三氧化硫含量高的复合胶凝材料具有较高的早期强度和良好的抗氯离子渗透性能。     

超细矿渣粉和偏高岭土对硫铝酸盐水泥水化和强度的影响

掺入矿物掺合料是改善硫铝酸盐水泥(CSA)混凝土凝结硬化性能和降低生产成本的主要技术途径之一。研究了水胶比为0.4时,单掺超细矿渣粉(UFS)、偏高岭土(MK)与复掺超细矿渣粉、偏高岭土对硫铝酸盐水泥凝结时间、流动度、电阻率、抗压强度的影响,并对其1 d、28 d龄期时的水化产物进行XRD半定量分析。结果表明,单掺和复掺缩短了水泥浆体的凝结时间,但单掺偏高岭土时的缩短效果更明显,且水泥浆体的流动度随着超细矿渣粉和偏高岭土掺量的增加而减小。掺入超细矿渣粉、偏高岭土缩短了水泥浆体电阻率变化速率曲线峰值出现的时间,峰值大小与掺量成递减关系。当掺量从0%(质量分数,下同)增大到20%时,单掺超细矿渣粉试样的28 d抗压强度减小了24.7%,单掺偏高岭土试样的28 d抗压强度减小了17.7%,两者复掺试样的28 d抗压强度减小了17.3%。超细矿渣粉和偏高岭土对水泥水化产物没有明显影响,但促进了硅酸二钙(β-C₂S)的早期水化。     

有替代硅酸盐水泥熟料的生产的选择吗?(英文)

人们一直在寻找CO2排放量低的水硬性胶凝材料,用它替代传统的以硅酸盐水泥熟料(Portland clinker,PC)为主的水泥。介绍了几种处于不同发展时期的新型非PC基的胶凝材料体系。目前大多数水泥生产商都尽可能多地用辅助性胶凝材料替代硅酸盐水泥熟料。火山灰材料具有低的水硬活性,它可使用高浓度碱金属溶液来激发,得到介于"地聚合物"和石灰激发火山灰胶凝材料间的复合胶凝材料。较远期可以期待基于贝利特、硫铝酸钙和铁铝酸钙矿物组成的水泥熟料,如拉法基公司的AetherTM已投入生产。更远的将来,不产生CO2的原材料,如硅酸镁等,可能使得水泥生产中实现CO2零排放,然而,这些胶凝材料的耐久性有待验证,用其配制的混凝土中钢筋锈蚀的防护是实际应用中的关键问题。