Hardened Cement Paste Specimens Research Articles

Performance of Silica Fume on Preventing Strength Retrogression in Hardened Cement Paste and Mortar at Elevated Temperatures

This study examines the effect of silica fume (SF) as a partial replacement for cement to prevent strength retrogression in hardened cement paste (HCP) and mortar at elevated temperatures. An experimental procedure was conducted on 320 specimens, replacing 0, 10, 20, and 30% of the cement by weight with SF. The residual compressive strength of the specimens was evaluated at room temperature (25 °C) and at 100, 200, 300, and 400 °C for 7, 28, and 56 days. The results indicate that the addition of SF to the cement paste and mortar improves the compressive strength both at 25 °C and at temperatures up to 400 °C. That is attributed to the formation of C-S-H phases, such as tobermorite and xonotlite. Additionally, the optimal residual compressive strength was achieved by adding 30% of SF. Therefore, XRD, SEM, and EDS techniques were employed to evaluate the microstructure of HCP specimens with 30% of SF. The results show that adding SF leads to a denser microstructure and lower porosity, resulting in more durable cement paste and mortar at ambient and elevated temperatures. In conclusion, using SF as a partial replacement for cement can be an effective way of developing sustainable fire-resistant construction materials.

The Effect of Elevated Temperature on Ordinary Portland Cements

A study was directed to analyze the influence of the different types of Ordinary Portland Cements (OPC) on the hardened cement paste exposed to high temperatures. Hardened cement paste specimens with three types of cement and nine water-cement ratios were investigated and heated to 11 heat steps at 28 days of age. Residual compressive strength and relative residual compressive strength were compared to the unheated results. It was shown that the expected range for the relative residual strength of the different types of OPC up to 400 °C is between 90 and 100%. From the local minimum at 150 °C to the maximum at 300 °C. The CEM I 52.5 type of cement shows the least favorable behavior to the increased (over 400 °C) temperature, according to the results of the residual compressive strengths curves and the newly introduced Integrated Temperature Endurance values after the first water-cement ratio (0.3). Nine water-cement ratios were tested with the favorable types of OPC (CEM I 42.5 and CEM I 32.5). At all water-cement ratios, the residual strength tendencies for the two types of cement were parallel. It has been observed that changes in the water-cement ratio have little to no impact on the relative strength curve's shape or the relationship between strength and the water cement ratio. As a result of our experiments applying simply Ordinary Portland Cement, it is obvious that the CEM I 32.5 type of OPC cement significantly outperformed the CEM I 42.5 type in regards of post-heating strength tendencies.

The Synergistic Effect of Secondary Raw Materials and Nano Additive on the Properties of Cement Matrix

The purpose of the research presented in this paper was to analyse the synergistic effect of nano SiO2 (NS), fluidized catalytic cracking catalyst waste (FCCCw) and calcinated at 700°C temperature paper sludge waste (PSw) on the properties the cement matrix. Nano materials are known to reduce voids between cement particles, accelerate cement hydration, and increase the strength of cement matrix. A certain amount of FCCCw and PSw in the cement mix also accelerates cement hydration and increase the compressive strength of the system, but it is important to analyse the synergistic effect of these materials on the properties of cement matrix. The results showed that both waste materials are active pozzolanic additives. The optimal quantities in terms of positive effect on the cement matrix are 10% of FCCCw and 2.5% of PSw. The paper presents the results of XRD analysis, density, ultrasonic pulse velocity and compressive strength tests with hardened cement paste. A 22% increase in compressive strength compared to the control specimen was observed in specimens where 10% of cement was replaced by FCCCw and where NS was additionally added. The replacement of cement by 2.5% of PSw increases the compressive strength about 7%. Moreover, the compressive strength of specimens containing 10% of FCCCw, 2.5% of PSw and NS increases even though the content of cement is reduced. Hardened cement paste specimens containing FCCCw, PSw, and NS have better density and ultrasonic pulse velocity values. XRD analysis showed that a combination of FCCCw, PSw, and NS added to the cement mix increases the amount of calcium hydrosilicates formed, reduces the amount of free portlandite and unreacted cement minerals (allite and belite).

Combined experimental and numerical study of uniaxial compression failure of hardened cement paste at micrometre length scale

The aim of this work is to investigate the mechanical performance of hardened cement paste (HCP) under compression at the micrometre length scale. In order to achieve this, both experimental and numerical approaches were applied. In the experimental part, micrometre sized HCP specimens were fabricated and subjected to uniaxial compression by a flat end tip using nanoindenter. During the test, the load-displacement curves can be obtained. In the modelling part, virtual micrometre sized specimens were created from digital material structures obtained by X-ray computed tomography. A computational compression test was then performed on these virtual specimens by a discrete lattice fracture model using the local mechanical properties calibrated in the authors' previous work. A good agreement is found between the experimental and numerical results. The approach proposed in this work forms a general framework for testing and modelling the compression behaviour of cementitious material at the micrometre length scale.

Synergistic effect of dry sludge from waste wash water of concrete plants and zeolitic by-product on the properties of ternary blended ordinary Portland cements

Abstract This paper analyses the use of two types of supplementary cementing materials, – dry sludge from waste wash water of concrete plants and zeolitic by-product – for producing hardened cement paste specimens. X-ray powder diffraction, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, and thermal analysis were used as investigation methods. Test results showed that the addition of dry sludge reduced the compressive strength of hardened cement paste at both 7 and 28 days. In hardened cement paste specimens where Portland cement was substituted with 5 %–30% of dry sludge the compressive strength decreased significantly after 7 and 28 at days. After 28 days blended Portland cement containing up to 10% of supplementary cementing materials ((SCM) - dry sludge and zeolitic by-product) demonstrated higher compressive strength than the reference specimen as a result of synergistic interactions, whereas higher replacement levels led to reduction in compressive strength.

The Influence of Fluidized Bed Combustion Fly Ash on the Phase Composition and Microstructure of Cement Paste

Fly ashes from coal combustion in circulating fluidized bed boilers in three power plants were tested as a potential additive to cement binder in concrete. The phase composition and microstructure of cement pastes containing fluidized bed fly ash was studied. The fractions of cement substitution with fluidized bed fly ash were 20% and 30% by weight. X-ray diffraction (XRD) tests and thermal analyses (derivative thermogravimetry (DTG), differential thermal analysis (DTA) and thermogravimetry (TG)) were performed on ash specimens and on hardened cement paste specimens matured in water for up to 400 days. Quantitative evaluation of the phase composition as a function of fluidized bed fly ash content revealed significant changes in portlandite content and only moderate changes in the content of ettringite.

Influence of Cementitious Capillary Crystalline Waterproofing Material on the Water Impermeability and Microstructure of Concrete

The main objective of this study is to investigate the effect of cementitious capillary crystalline waterproofing (CCCW) material on the water impermeability and microstructure of concrete. The water impermeability of concrete covered with or without CCCW material was tested according to the Chinese standard GB 18445-2012. The results indicate that concretes coated with CCCW material showed much higher water impermeability than blank ones, and the ratio of water impermeability pressure between them reached 275. The samples obtained in various depths of hardened cement paste specimens with or without CCCW coating were analyzed through scanning electron microscopy (SEM) and thermogravimetry-differential scanning calorimetry (TG-DSC), to study the differences in microstructure and hydration products. The results present that after a 28-day standard curing, there were lots of ettringite crystals and CaCO3 formed in the paste in 1 cm from the coating, but the action depth of the CCCW coating could not reach 3 cm. The ettringite and CaCO3 is precipitated in the pore structure of cement matrix and filling the voids, which leads to the significant enhancement in water impermeability.

Effect of AlF3 production waste on the processes of hydration and hardening of the alkali-activated Portland cement with sodium silicate hydrate

Generally, the early strength of hardened Portland cement paste is controlled, by changing the mineral composition of clinker and grinding fineness. By using clinker (calcium silicates) and liquid glass (sodium silicate) together, these materials have high degree of coagulation and rapid hardening. For this reason, it is important to regulate the setting times. One solution of this problem is the use of chemical admixtures. In this research, the admixture made from liquid glass and AlF3 production waste was suggested (the waste from phosphates fertilizers production). The cement system “Portland cement clinker–sodium silicate hydrate–AlF3 production waste” was characterized by X-ray diffraction, differential scanning calorimetry, measurements of hydration temperature and the compressive strength of hardened cement paste specimens. The microstructure of hardened cement pastes was evaluated by scanning electron microscopy. Reference specimens were made from pure Portland cement clinker. A possibility to regulate hydration rates both at the early stage of hardening and at the later ones by addition of this admixture was demonstrated. The effect of AlF3 production waste was described. By using this admixture, the hydration temperature increased, the hydration process delayed, and compressive strength of hardened Portland cement clinker paste samples increased, especially at later ages. This gave the opportunity for composition: clinker–liquid glass–AlF3 production waste to create slowly curing cement; which is suitable for long transportation and for massive constructions.

The effect of multi-walled carbon nanotubes on the rheological properties and hydration process of cement pastes

The properties of cementitious materials can be noticeably improved by the addition of carbon nanotubes (CNTs). It is important to detect the effects of the pellets containing multi-walled CNTs, dispersed in carboxyl-methyl cellulose, on the hydration process of fresh cement pastes, and physical or mechanical properties of hardened cement paste specimens, obtained without the use of commercially available surfactants and plasticisers, in order to use CNTs in cementitious materials more effectively. The influence of dispersed CNTs in different amounts (from 0 to 0.5%) on the pH and electrical conductivity values of water solutions and fresh cement pastes, as well as on the rheological properties, setting time and exothermic reaction of fresh cement pastes or on the physical and mechanical properties of hardened cement paste specimens was analysed in this work. It can be stated, that 0.005% is the marginal amount of CNTs, as amounts above 0.005% deteriorate the hydration parameters of cement pastes. This was confirmed by decreased values in electrical conductivity (approximately 17%), pH (approximately 6.8%) and dynamic viscosity (approximately 3 times) tests. Lower pH values lead to a significant retardation of the dissolution of cement minerals and this process is reflected in the setting time (prolonged by 110–130%) and maximum time (extended by 148%) or temperature (decreased by 18.4%) of exothermic reaction. Consequently, the mechanical properties of hardened cement paste specimens (after 28 days) decreased about 10%, as well as water absorption increased up to 10.6%.

Influence of cement type on resistance to attack from two carboxylic acids

In a number of circumstances, concrete may be required to possess resistance to organic acids. These are frequently carboxylic acids. This paper examines the effect of two such acids – acetic and butyric – on hardened cement paste specimens made from three cement types – Portland cement (PC), a combination of PC and fly ash (PC/FA), and a calcium sulfoaluminate cement (CSA). Specimens were exposed to solutions of the acids and deterioration characterized in terms of mass loss and pH measurements, micro-CT scanning, and chemical and mineralogical analysis. Additionally geochemical modelling was used to further examine the mechanisms involved during acid attack. The CSA cement was most resistant to attack, with the PC paste displaying the least resistance. This resistance has been partly attributed to the higher acid neutralization capacity of CSA cement. However, this paper demonstrates that the enhanced performance is most probably the result of a denser microstructure.

Blended Cement Systems with Zeolitized Silica Fume

In this paper the properties and the hydration of Portland cement pastes containing zeolitized silica fume were studied. Hydrosodalite was obtained by zeolitization of silica fume. XRD, DSC, SEM and energy-dispersive X-ray spectroscopy were used as investigation methods. The compressive strength of hardened cement paste was measured after 28 days. The levels of cement replacement by additives were 5 %, 10 % and 15 % by weight in the specimens. Changes in the strength of hardened cement paste were observed. The influence of additives on hydration rate and hydration temperature was also observed. Both 1Hs (hydrosodalite) and 2Hs (modified hydrosodalite) additives accelerate the hydration of the cement paste. The highest hydration temperature was achieved in a mixture with 5 wt. % 1Hs additive and with 10 wt. % 2Hs additive. The microstructure of hardened cement paste specimens with 1Hs and 2Hs additive is denser compared with reference specimens without the additives. The maximum compressive strength of the specimens with 10 wt. % of 2Hs reached up to 101 MPa under laboratory conditions.DOI: http://dx.doi.org/10.5755/j01.ms.22.2.7018

Synthesis and characterization of a new polymeric microcapsule and feasibility investigation in self-healing cementitious materials

This paper presents work towards development of a new type of polymeric microcapsule with phenol–formaldehyde (PF) resin as shell and dicyclopentadiene (DCPD) as healing agent for self-healing microcracks in cementitious materials. The PF/DCPD microcapsules were synthesized via in-situ polymerization and characterized by means of optical microscope (OM), scanning electron microscope (SEM) and thermal analysis (TGA). The chemical stability of synthesized microcapsules and the trigger performance were studied respectively in simulated concrete pore solution and hardened cement paste specimens. The results revealed that the synthesized microcapsules possessed excellent stability in both simulated pore solution and real cement environment. X-ray computed tomography (XCT) was applied to observe the status and fracture behavior of microcapsules inside cement paste matrix. Further segmentation and 3D rendering of the reconstructed data obtained from XCT showed that the microcapsules had a good dispersibility with desired trigger sensitivity. The OM investigation of a fractured surface of a cement paste incorporated with microcapsules confirmed that the self-healing function of the microcapsules could be triggered by cracking and the healing agent could be released simultaneously to heal the cracks.

Properties and acceleration mechanism of cement mortar added with low alkaline liquid state setting accelerator

Low alkaline liquid state setting accelerator(LSA) for Portland cement was prepared in laboratory from aqueous solution of several inorganic sulfate salts and some organic chemical substances. Properties of cement with addition of LSA relating to its setting time and strength development as well as its resistance to sulfate attack for short and long term exposure were experimentally examined. The experimental results showed that 5%–7% addition of LSA significantly accelerated the initial and final setting of Portland cement in the presence or absence of the blending of mineral admixtures, the initial and final setting time being less than 3 min and 6 min respectively. Meanwhile, the early 1 day curing age compressive strength increased remarkably by 20%, while the late 28th day curing age compressive strength remained almost unchanged as compared with that of the reference accelerator free cement mortar specimen. Furthermore, mortar specimens of cement added with LSA and exposed to 5% Na2SO4 solution showed their excellent resistance to sulfate attack, with their short and long term curing age resistance coefficient to sulfate attack being around 1.04 to 1.17, all larger than 1.0. XRD analysis on hardened cement paste specimens at very early curing ages of several minutes disclosed the existence of more ettringite in specimens added with LSA than that of the reference specimens, meanwhile SEM observation also revealed the existence of well crystallized ettringite at very early hydration stage, suggesting that the accelerated setting of Portland cement can be attributed to the early and rapid formation of ettringite over the whole cement paste matrix due to the introduction of LSA. MIP measurement revealed that hardened cement paste specimens with the addition of LSA presented less medium diameter pores, more proportion of small pores and less proportion of large capillary pores, which is in a very good coincidence with the improvement of strength development of cement mortars added with LSA.

Determination of air-void parameters of hardened cement-based materials using X-ray computed tomography

This paper presents an attempt to tackle limitations in the two-dimensional (2D) stereological characterization of the air-void parameters of hardened cement-based materials by employing three-dimensional (3D) X-ray computed tomography (CT), a technique capable of simultaneously imaging numerous sections within a specimen. Using three hardened cement paste specimens composed of different air-void systems, we performed sensitivity analyses in terms of the number of traverse lines employed for a single section and the number of sampling sections across the height of a specimen. Parameters for a single section converged rapidly as the number of traverse lines increased, although unacceptable variations were in evidence across multiple sections. When the number of sampling sections exceeded about 10, a set of representative air-void parameters was successfully obtained within a standard variation of less than 10% of average values. The spacing factor and air content measures obtained via CT image analysis were in good agreement with previously reported data and with the original spacing factors defined for 3D space. Some advantages found in the use of X-ray CT imaging for determining air-void parameters are discussed.

FUNDAMENTAL STUDY ON WATER TRANSFER IN PORTLAND CEMENT PASTE

Water transfer behavior in hardened cement paste is evaluated. For evaluation of water transfer, mass loss of specimens over time is measured and Boltzmann's method is applied to measured data. At the same time, desorption isotherm of hardened cement paste is evaluated. Using the diffusion coefficient for nominal water content and desorption isotherm, water conductivity for chemical potential of water in hardened cement paste is derived. Nine hardened cement paste specimens, which have different porosity, are tested and using these data, predicting model of water conductivity is proposed. The proposed model needs the properties of hardened cement, such as, maximum water content at 20°C degree, water vapor BET surface area, and water content for predicting the water conductivity. According to the simulated results, the temperature dependency of water conductivity is obvious. Evaluation of temperature dependency of water conductivity is remained to be future issue.

Mercury porosimetry of hardened cement pastes

Mercury porosimetry was performed on 92 hardened cement paste specimens of water/cement (w/c) ratios 0.3, 0.4, 0.5, 0.6, and 0.7 and curing times of 1, 3, 7, 14, 28, and 56 days. This paper presents the experimental techniques, results, and their possible implications with respect to pore connectivity. As expected, longer curing times and lower w/c ratios resulted in smaller indicated total porosities and smaller threshold pore widths. Longer curing times and higher w/c ratios resulted in greater degrees of hydration. In most of the mercury intrusion results, two peaks could be observed in the differential curves that were identified as the “initial” and “rounded” peaks. The initial peak may correspond to the intrusion of mercury through a connected capillary network, while the rounded peak may correspond to the crushing of interposed hydration products.

Autoclaved aerated concrete behavior under explosive action

Autoclaved aerated concrete (AAC) is widely used in construction, mainly in masonry as infill walls. Exterior AAC walls may be subjected to different actions including accidental lateral dynamic loading and local fragments' impact. The present paper studies some of the dynamic characteristics of AAC walls under localized high-intensity impact, such as the number of cracks and their dependence on the boundaries location, the initiation of spalling and the contribution of face treatment and face reinforcement on enhancing the materials response. Some comparisons are made with earlier results on hardened cement paste specimens.

The disc-bending point load test for the early strength of hardened cement paste

A circular disc specimen of finite thickness is freely bottom-supported by a tube that has diameter less than the disc diameter. The specimen is subjected to bending stresses by applying a concentrated load to the centre of its top surface, and the load is increased gradually until the disc fails. A theoretical basis for using this loading configuration as a strength test is derived. This so-called disc-bending point load test is used in this paper for testing the strength of API Class G hardened cement paste specimens of various curing ages. The measured disc-bending point load strength index (Idb) is used for computing the rupture moduli of the test specimens, and for estimating their compressive strengths. The test results are compared with data from conventional strength tests such as the uniaxial compression test, as well as the diametral and axial point load strength tests. Correlations between Idb and uniaxial compressive strength are shown jor disc specimens of various thicknesses, and the effect of indentor penetration and disc thickness is investigated.

Sulfate attack on hardened cement paste

Hardened cement paste specimens made with fifferent cement types and varying water-cement ratios (w/c) were immersed in 5% sodium sulfate solution maintained at constant pH value of 6. The distribution curves for ettringite, gypsum, and portlandite phases were obtained by using layer by layer XRD analysis and interpreted in terms of material damage due to sulfate attack. The mechanism of sulfate attack is evaluated in regard to the leaching of Ca(OH) 2 and formation of gypsum and ettringite.

Controlled dynamic cracking of hardened cement paste specimens

A new method of studying the dynamic development of microcracks in hardened cement paste (HCP) plates subjected to dynamic loads is developed. The dynamic loading is produced by special cylindrical explosive charges containing about 10 mg PETN each. These charges, when initiated, produce a dynamic pressure pulse for about 1–2 μ/s. The charges are inserted inside boreholes perpendicular to the hardened cement paste plate faces in predefined locations. The dynamic loading causes cracking of the plates, enabling one to measure the crack propagation velocity and to investigate the crack interaction phenomenon. Crack propagation velocities were found to be in the range of 60 to 200 m/sec, much lower than the theoretical values. Crack arresting phenomena of adjacent cracks, which were observed during the experiments, could explain delays in crack propagation as well as the increase in hardened cement paste specimen strength under dynamic stresses. The new method enables simple measurements of crack propagation velocities in HCP samples of different shapes and dimensions under dynamic tensile stresses, including small samples, in which crushing of concrete is observed in common methods during similar tests.