Thermal behavior of cement matrix with high-volume mineral admixtures at early hydration age

Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinder and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition have been examined at early hydration age. Results show that with addition of fly ash, coal gangue and blast furnace slag the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases.There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.     

Effect of Chloride‐Based Accelerator in the Presence of Water‐Reducing and Retarding Admixture on Autogenous Shrinkage

Rapid repair concrete mixtures commonly used for full‐depth concrete pavement repair sections can use large dosages of accelerating admixtures to increase strength gain rates and decrease the time to traffic opening. Most often, these mixtures also contain water‐reducing and retarding admixtures (WRRAs) to allow for the use of a low water–cementitious material ratio in order to meet strength requirements. The use of large dosages of accelerating admixtures in combination with retarding admixtures could have significant side effects on concrete. Autogenous shrinkage of low water–cementitious concrete can contribute to high tensile stresses and cracking problems. The effect of calcium chloride‐based accelerating admixture dosage, when used with WRRAs, on autogenous shrinkage was measured. It was found that the inclusion of calcium chloride‐containing accelerating admixtures has a nonlinear effect on the pore size distribution and consequently a nonlinear increase on the autogenous shrinkage.     

Combined effect of expansive and shrinkage reducing admixtures to obtain stable and durable mortars

In order to improve the dimensional stability of cement based mortars, the effects produced on cement hydration of a shrinkage reducer (propyleneglycol ether based—SRA) and an expansive admixture (calcium oxide based—EXP) were investigated. Mortar samples (prepared without admixtures or with SRA or EXP or SRA and EXP) were compared through compressive strength measurements, water evaporation, restrained shrinkage and restrained expansion measurements. Setting time and free expansion were also detected on cement paste specimens.

A synergistic effect on the shrinkage reduction was observed when the shrinkage reducing admixture and the expansive agent were used together. In order to clarify this phenomenon, the hydration of cement pastes containing these kinds of admixtures was followed by ESEM-FEG (environmental scanning electron microscopy–field emission gun), TG (thermogravimetry), specific surface area measurements (by BET—Brunauer–Emmet–Teller-method) and XRDS (X-ray diffraction spectroscopy).     

A model to predict the amount of calcium hydroxide in concrete containing mineral admixtures

This study examines in detail the degree of reactivity of admixtures, such as fly ash and blast furnace slag, and their effect on the levels of calcium hydroxide in cement paste. Experimental results indicate that reactivity between calcium hydroxide and mineral admixture is dependent on the amount of calcium hydroxide and the degree of hydration of mineral admixtures.From these results, a model was formulated to predict the reaction between calcium hydroxide and mineral admixtures, and its validity verified by comparing calculated data with the data from the tests with cement mortar specimens. The calculated values of calcium hydroxide agree well with the test results. The parameters of the prediction model are dependent on the physical and chemical characteristics of mineral admixtures.     

预湿水泥浆颗粒内养护材料对混凝土早期自收缩及抗压强度的影响

制备不同粒径和水灰比的水泥浆颗粒作为低水灰比混凝土内养护材料.以最佳内养护水灰比原则,设计了使用三种水灰比分别为0.6、0.7和0.8的同粒径水泥浆颗粒等体积取代砂子的混凝土.研究了不同水灰比水泥浆颗粒对混凝土早期自收缩、抗压强度和内部微结构的影响.结果表明:颗粒吸水率与其水灰比正相关、与其粒径负相关;预湿水泥浆颗粒可显著降低混凝土早期的自收缩,颗粒水灰比越大,自收缩降低效果越明显;但是掺入水泥浆颗粒也会降低混凝土的抗压强度,颗粒水灰比越高抗压强度降低越多,应用中应优化选择预湿颗粒的水灰比;水泥浆颗粒作为高性能混凝土内养护材料,可改善微观界面的孔隙结构,提高界面的密实性,减少混凝土早期的收缩和开裂.     

Influence of clays on the rheology of cement pastes

The fresh state of concrete is becoming increasingly important in furthering the types of applications of today's construction world. Processing techniques have resulted in technologies such as self-consolidating concrete and depend on the microstructural changes that take place during and immediately after mixing and placing. These changes to the microstructure reflect the flocculation behavior between the particles in suspension. The ability to modify this behavior allows control over the balance among flowability and shape-stability of concrete. This study investigates how clay admixtures affect the microstructure of cement pastes from a rheological stand point. Shear and compressive rheology techniques are used to measure how the solids volume fraction of suspensions with different admixtures evolves with stress. Based on these relationships, the effectiveness of clays on the balance between flowability and shape-stability is measured. Results are consistent with green strength tests performed on concrete mixes derived from the cement paste mixes.     

Creep and drying shrinkage of calcium silicate pastes III. A hypothesis of irreversible strains

Irreversible strains, measured on pastes of pure calcium silicates which were loaded without drying, dried to 53% rh without loading, or were loaded and simultaneously dried to 53% rh, are correlated with indications of structural changes occurring within the hydrated pastes. These changes are interpreted in terms of microstructural models of the hydrated pastes. It is assumed that irreversible strains are caused only by changes in the “pore component” and the “CSH component.” In young pastes irreversible shrinkage can be explained by a reduction in pores in the range 40–100 Å diameter. In well-hydrated pastes structural changes in the silicate framework of the “CSH component” dominate irreversibility. Microshearing between CSH particles is thought to occur under load. An increase in degree of silicate polymerization bonding occurs on drying, and this increase is greater when drying takes place under load.     

碱-矿渣水泥性能研究及应用

研究了碱-矿渣水泥水化产物组成、水泥石孔结构和孔液成分、力学性能、水化热、抗化学腐蚀性能和对钢筋的钝化作用等;并对混凝土的力学性能、干缩变形、耐久性和对外加剂的适应性等进行了研究。结果表明,碱-矿渣水泥具有早强、高強、低水化热和高耐久性等优良性能,但是干缩较大;选择适当的外加剂可以改善部分性能。介绍了利用该水泥的特性在多种工程中的应用情况。     

Morphology and microstructure of hydrating portland cement and its constituents I. Changes in hydration of tricalcium aluminate alone and in the presence of triethanolamine or calcium lignosulphonate

Morphological changes which occur during the hydration of C₃A^∗ paste were observed by electron microscopy from the age of 5 minutes to 3 months. X-ray analyses were used to identify phases. The effects of the admixtures, triethanolamine and calcium lignosulphonate, were investigated with and without gypsum. The dosage of admixture was 0.5%; the water to C₃A ratio was 1.0. The results showed clearly the morphological changes that took place as hydration proceeded, and showed the specific influences of each of the two admixtures.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Prediction of diffusivity of concrete based on simple analytic equations

Proposed is a simple analytic model that can predict realistically the diffusivities of concrete and mortar. The basic concept of the model comes from the relation between the diffusivities and the microstructure of concrete. The microstructure that affects the diffusivity includes the interfacial transition zone (ITZ) between aggregates and cement pastes as well as the microstructure of cement paste itself. The general effective media (GEM) equation was introduced to derive the diffusivity of cement paste. The effective diffusivity of concrete is derived on the basis of the composite sphere assemblage model, which considers the diffusivities of both ITZ and cement paste. The main parameters in the proposed model are the microstructural properties of cement paste such as capillary porosity and pore structure parameter, solid phase diffusivity, aggregate volume fraction, and interfacial zone properties. To validate the proposed model, many series of concrete and mortar specimens have been tested to measure the diffusivities. The major test variables include the water-to-binder ratios, the types and amount of mineral admixtures on the diffusivities. The effects of compressive strength, water-to-binder ratio, and mineral admixtures have been investigated comprehensively. The comparison of the proposed theory with the test data exhibits reasonably good correlation. The proposed model allows more accurate prediction of diffusion process and, thus, more realistic durability design of concrete structures.     

Calcium hydroxide distribution and calcium silicate hydrate composition in tricalcium silicate and β-dicalcium silicate pastes

Pastes of tricalcium silicate (C₃S) and β-dicalcium silicate (C₂S) 23 years old were studied by electron probe microanalysis. In both cases, regions consisting entirely or largely of calcium hydroxide and of CSH were distinguished on a scale of 2–50 μm. The regions high in CSH accounted for 75–80 percent of the whole in the C₃S paste and about 96 percent in the C₂S paste; these values are much higher than those initially occupied by anhydrous starting materials. Within the high CSH areas, no compositional variation was detected that could have corresponded to the so-called inner and outer hydrates. The ratio of mean Ca to mean Si in the high CSH areas was found to be 1.72 for the C₃S paste and 1.78 for the C₂S paste with an exciting beam energy of 10 keV.     

An investigation on microstructure of cement paste containing fly ash and silica fume

In this study, high-calcium fly ash (HCFA) and silica fume (SF) were used as mineral admixtures. The effect of these admixtures on the microstructure of cement paste was investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reaction of HCFA and SF with portlandite, which occurs in Portland cement (PC), forms a new calcium-silicate-hydrate (C-S-H) gel.     

EVA对硫铝酸盐水泥净浆微观结构和力学性能的影响

研究了乙烯-醋酸乙烯酯(EVA)对硫铝酸盐水泥(CSA)净浆抗压强度、凝结时间、干燥收缩、质量损失及浆体内部温度的影响规律,并通过XRD、FTIR、SEM及EDS等测试手段对6 h、28 d龄期时的水化产物及微观结构进行分析。结果表明：掺入EVA后CSA净浆的凝结时间显著缩短,6 h的抗压强度升高,而1 d、3 d、28 d的抗压强度降低;CSA净浆的干燥收缩和质量损失率随着EVA掺量的增加逐渐减小;EVA的掺入提高了CSA净浆内部温度曲线的峰值,加快了峰值出现的时间。微观分析表明：EVA对CSA净浆6 h的水化具有促进作用,使其生成了更多的钙矾石,而对其28 d的水化具有抑制作用,水化产物有所减少。     

Bond strength between C3S paste and iron,copper or zinc wire and microstructure of interface

Bond strength between C₃S paste and iron, copper or zinc wire and microstructure of the interfacial region were examined by pull-out test and scanning electron microscope respectively. The bond strength between C₃S paste and iron wire at 28-day curing is larger than that between C₃S paste and copper or zinc wire. In the case of iron and copper wires, CSH and Ca(OH)₂ are formed in the paste at the interface. On the other hand, Ca[Zn(OH)₃·H₂O]₂, CSH and Ca(OH)₂ are formed in between C₃S paste and zinc wire. The relation between the development of bond strength and curing time is characteristic according to the kind of metal wires.     

水泥浆体的微结构及其与强度的关系

在亚微观尺度上描述了水泥石的微结构，简要回顾了其研究进程，详细介绍了主要水化产物以及掺加不同混合材可改变水化产物组分含量。归纳了水泥石的微观结构与强度的关系，并展望了其研究前景。     

Granulometric synthesis and characterisation of dispersed nanosilica powder and its application in cementitious system

Abstract

Dispersed, spherical particles of nanosilica with controllable size have been synthesised using a metal alkoxide, i.e. tetraethoxysilane, as starting material, ammonia as base catalyst and non-ionic surfactant as template by sol–gel method. Size of particles and dispersivity were controlled by varying the surfactant chain length and temperature conditions of the reaction mixture. Silica nanoparticles were synthesised using a series of non-ionic surfactants, namely Polyoxyethylene (20) sorbitan monolaurate (Tween 20) and Polyoxyethylene (80) sorbitan monooleate (Tween 80), at different reaction temperatures of 25, 50, 70 and 90°C. The particle size of silica nanoparticles gradually decreased with increasing carbon chain length of the surfactant and at higher temperature particle size became larger. Furthermore, these silica nanoparticles are incorporated into the cementitious system to improve the mechanical properties and reduce calcium leaching in the hydration process. Addition of silica nanoparticles into cement paste improves the microstructure of the paste, and calcium leaching is significantly reduced as silica nanoparticles react with calcium hydroxide, thereby forming a denser calcium–silicate–hydrate gel structure. Synthesised silica nanoparticles and microstructure of cement paste incorporated with silica nanoparticles were analysed using scanning electron microscopy, powder X-ray diffraction, infrared spectroscopy (IR), ²⁹Si MAS NMR and thermogravimetry analysis for morphological and mineralogical attributes.     

Morphology and microstructure of hydrating portland cement and its constituents III. Changes in the hydration of a mixture of C3S,C3A and gypsum with and without triethanolamine and calcium lignosulphonate present

Systematic sequential observations with the electron microscope were made of the morphological changes which occurred during the hydration of a paste mixture of C₃S, C₃A and gypsum. It was found that this system produced hydration products similar in nature to those produced by the monomineral systems with gypsum present. The two organic admixtures studied had some effect on changes in morphology and microstructure of the hydrating mixture, but they showed a pronounced influence on the rate of the hydration processes.     

Capillary porosity depercolation in cement-based materials: Measurement techniques and factors which influence their interpretation

The connectivity of the capillary porosity in cement-based materials impacts fluid-and-ion transport and thus material durability, the interpretation of experimental measurements such as chemical shrinkage, and the timing and duration of curing operations. While several methods have been used to assess the connectivity of the capillary pores, the interpretation of some experimental procedures can be complicated by the addition of certain chemical admixtures. This paper assesses capillary porosity depercolation in cement pastes using measurements of chemical shrinkage, low temperature calorimetry (LTC), and electrical impedance spectroscopy. The experimental results are analyzed to identify the time of capillary porosity depercolation. In addition, the factors that influence the interpretation of each technique are discussed. Experimental evidence suggests that capillary porosity depercolation, as defined by Powers, occurs after hydration has reduced the capillary porosity to around 20% in cement paste systems. The influence of capillary porosity depercolation on the transport properties is demonstrated in terms of a reduction in the electrical conductivity of the cementitious material. Special attention is paid to understand and interpret the influence of shrinkage-reducing admixtures (SRAs) on the freezing behavior of cementitious systems, particularly in regard to the inapplicability of using LTC to detect porosity depercolation in cement pastes containing such organic admixtures.     

Reaction mechanisms of concrete admixtures

Concrete admixtures influence the kinetic of cement hydration mainly during the dormant period. The dominant influence of admixtures seems to lie in different bound forces between dissociated ions in the pore water solution. Repulsive forces characterize the solvation process while attractive forces dominate during crystallization. These changes of ion bound forces lead twice to volume changes during phase transitions of hydration. Volume changes measured with an immersion weighing setup show clearly the effect of concrete admixtures on cement reaction. Retarder agents produce a volume swelling while accelerators force an immediate shrinkage behaviour. A mechanism as introduced by Le Chatelier involving a solution-crystallization step seems to describe the hydration process most adequately. As long as repulsive forces dominate, a volume swelling occurs and no strength gain can take place. The dormant period is defined by the length of the swelling process. Hardening and strength growth start at the point at which volume shrinkage appears.