Early Age Hydration Research Articles

Stray Current-Induced Development of Cement-Based Microstructure in Water-Submerged, Ca(OH)<sub>2</sub>-Submerged and Sealed Conditions

This work reports on the development of microstructural and mechanical properties of mortar cubes under the synergetic action of stray current and various environmental/curing conditions. The study refers to specimens cured for 24h only, followed by a 112 days period of partial or full submersion in water or alkaline medium. Additionally, equally prepared mortar specimens were tested in sealed conditions. The outcomes for submerged and saturated conditions were compared to sealed conditions. Three current density regimes were employed i.e. 1 A/m2, 100 mA/m2, and 10 mA/m2, simulating different levels of stray (DC) current environment. The highest level of 1A/m2 was also comparable to stray current densities, as measured in field conditions. The tests were designed in a way, so that the effects of diffu-sion-controlled transport (ions leaching due to concentration gradients), were distinguished from migration-controlled ones (ion/water transport in stray current conditions). Mechanical, microstructural and electrical properties were moni-tored throughout the test. For water-conditioned specimens, the stray current was found to accelerate degradation pro-cesses. This was reflected by decreased compressive strength, reduced electrical resistivity and increased porosity of the matrix. The results were attributed to leaching-out of alkali ions due to concentration gradients, where except diffusion, migration took place i.e. the leaching-out effect was accelerated by water and ions migration in conditions of stray cur-rent flow. In contrast, stray current flowing through mortar in sealed conditions (as well as through mortar in alkaline medium) resulted in increased compressive strength and electrical resistivity. These were accompanied by densification of the bulk matrix and reduced porosity. It can be concluded that for a cement-based material at early hydration age, both positive and negative effects of stray current flow can be expected. The level and direction of these effects are dependent on the external environment and the current density levels, where stray currents above 100 mA/m2 and in conditions of concentration gradients with the external medium, would lead to more pronounced negative effects on microstructural and micromechanical performance.

Monitoring early hydration of reinforced concrete structures using structural parameters identified by piezo sensors via electromechanical impedance technique

Abstract Concrete is the most widely used material in civil engineering construction. Its life begins when the hydration process is activated after mixing the cement granulates with water. In this paper, a non-dimensional hydration parameter, obtained from piezoelectric ceramic (PZT) patches bonded to rebars embedded inside concrete, is employed to monitor the early age hydration of concrete. The non-dimensional hydration parameter is derived from the equivalent stiffness determined from the piezo-impedance transducers using the electro-mechanical impedance (EMI) technique. The focus of the study is to monitor the hydration process of cementitious materials commencing from the early hours and continue till 28 days using single non-dimensional parameter. The experimental results show that the proposed piezo-based non-dimensional hydration parameter is very effective in monitoring the early age hydration, as it has been derived from the refined structural impedance parameters, obtained by eliminating the PZT contribution, and using both the real and imaginary components of the admittance signature.

A methodology for assessing the chemical and physical potential of industrially sourced rice husk ash on strength development and early-age hydration of cement paste

Among supplementary cementitious materials (SCMs), rice husk ash (RHA) is a by-product of the agricultural industry that is recognized by its high reactivity and ability to enhance performance of cement-based materials. The dependence of RHA reactivity on burning conditions and grinding procedures makes it sensitive to uncontrolled combustion/production. This study assesses 5 RHAs from industrial sources to understand how uncontrolled combustion impacts the reactivity of RHA. A fundamental model is proposed that combines physical (specific surface area) and chemical (amorphous silica content) characteristics of RHA with hydration and strength gain, considering the relative contribution of each one over time. Isothermal calorimetry was used to continuously assess strength development at early ages. The results show that the model is well correlated with strength development at different ages and allows the computation of an equivalent water-to-cement ratio considering solely the characteristics of a RHA sample, and prior to producing and testing specimens.

Low-Temperature Curing Strength Enhancement in Cement-Based Materials Containing Limestone Powder.

With the ongoing sustainability movement, the incorporation of limestone powder in cementitious binders for concrete in the U.S. has become a subject of renewed interest. In addition to accelerating the early age hydration reactions of cementitious systems by providing additional surfaces for nucleation and growth of products, limestone powder is also intriguing based on its influence on low-temperature curing. For example, previous results have indicated that the utilization of limestone powder to replace one quarter of the fly ash in a high volume fly ash mixture (40 % to 60 % cement replacement) produces a reduction in the apparent activation energy for setting for temperatures below 25 °C. In the present study, the relationship between heat release and compressive strength of mortars at batching/curing temperatures of 10 °C and 23 °C is investigated. For Portland-limestone cements (PLC) with limestone additions on the order of 10 %, a higher strength per unit heat release is obtained after only 7 d of curing in lime water. Surprisingly, in some cases, the absolute strength of these mortar cubes measured at 7 d is higher when cured at 10 °C than at 23 °C. Solubilities vs. temperature, reaction stoichiometries and enthalpies, and projected phase distributions based on thermodynamic modeling for the cementitious phases are examined to provide some theoretical insight into this strength enhancement. For a subset of the investigated cements, thermogravimetric analysis (TGA), quantitative X-ray diffraction (XRD), and scanning electron microscopy (SEM) are conducted on 7-d paste specimens produced at the two temperatures to examine differences in their reaction rates and the phases produced. The strength enhancement observed in the PLC cements is related to the cement hydration products formed in the presence of carbonates as a function of temperature.

Effects of Nanoalumina and Graphene Oxide on Early-Age Hydration and Mechanical Properties of Cement Paste

The effects of nanoalumina (NA) and graphene oxide (GO) on the early-age hydration and mechanical properties of portland cement pastes were investigated in this study. The hydration heat release rate and cumulative heat of cement pastes incorporating different dosages of NA and GO were evaluated using an isothermal calorimeter measurement method. Early-age electrical resistivity development was investigated by a noncontact electrical resistivity technique. The results show that both NA and GO could efficiently accelerate cement hydration. As a physical filler, NA significantly accelerates the hydration of tricalcium aluminate (CA) in cement. On the other hand, GO is able to obviously reduce the dormant period of cement hydration and shift the heat flow peaks to the left by accelerating the hydration of tricalcium silicate (CS) in cement. Compared to plain cement pastes, both the compressive and flexural strengths of cement pastes incorporating NA or GO are significantly increased. However, when NA and GO contents exceed the optimal amounts, improvements in flexural strength tend to decline, which is probably due to particle agglomeration. NA-cement paste exhibited slightly higher electrical resistivity than plain cement paste during hydration acceleration and deceleration stages. But GO-cement paste clearly showed lower electrical resistivity, which might be attributed to iron diffusion caused by GO with large surface areas.

Early-age hydration characteristics of composite binder containing iron tailing powder

The early-age hydration characteristics of composite binder containing iron tailing powder were investigated by determining the hydration heat, non-evaporable water content, pore structure, and morphology of hardened paste as well as the compressive strength of mortar. The results show that the reaction degree of iron tailing powder is extremely low at early age. Addition of coarse iron tailing powder negatively affects the properties of composite binder paste and mortar. Compared to samples containing coarse iron tailing powder, the samples containing fine iron tailing powder show a large amount of hydration heat, high non-evaporable water content, fine pore structure of hardened paste and high compressive strength of mortar. The iron tailing powder significantly promotes the hydration of binder at low water-to-binder ratio. The iron tailing powder particles are well graded with cement particles in the paste, but the large iron tailing powder particles bond poorly with the surrounding hydrates. The decreasing ratio of compressive strength of mortar is much lower than the replacement level of iron tailing powder, especially for the mortar containing fine iron tailing powder at low water-to-binder ratio.

Characterization of the absolute volume change of cement pastes in early-age hydration process based on helium pycnometry

Based on detection technology of helium pycnometry, this study developed a novel approach for characterizing the absolute volume change of cement pastes in early-age hydration process. This method gives an accurate and continuous surveillance on the absolute volume change of cement pastes excluding all the empty pore space even below nm-scale. Traditional methods, such as chemical shrinkage test (based on ASTM C 1608), isothermal calorimetry test and X-ray diffraction (XRD) analysis, are also adopted for comparison and analyzation. It was found that the data obtained by chemical shrinkage test is discontinuous and inaccurate, the data gotten by isothermal calorimetry test is vague and unspecific, and the data gained by XRD analysis is discontinuous and unspecific. Compared with the traditional methods, the results captured by helium pycnometry are continuous, accurate and specific for describing the hydration process, especially the changing details of the absolute volume change. But at the same time, helium pycnometry has a good correspondence with the traditional methods, whose trends are almost the same. Based on the trends of the absolute volume change and heat flow, the absolute volume change of cement pastes in early-age hydration process was then divided into four periods (dissolution period, induction period, fast reaction period and stable period).

Effects of graphene oxide on early-age hydration and electrical resistivity of Portland cement paste

The effects of graphene oxide (GO) on the early-age hydration process and mechanical properties of Portland cement paste were experimentally investigated in this study. Based on an isothermal calorimeter measurement, the hydration rate of cement was observed to increase with the increase of GO content by nucleation effect. On the other hand, the electrical resistivity development of GO-cement paste was monitored using a non-contact electrical resistivity device. The result showed that electrical the resistivity of GO-cement paste was evidently higher than that of plain cement paste. However, cement paste with excessive amounts of GO exhibited a decreased electrical resistivity due to the massive ion diffusion caused by GO. Compared to plain cement paste, the GO-cement paste exhibited obviously higher compressive and flexural strengths, but the enhancements in compressive strength began to decline when the GO amount was greater than 0.04%. The microstructure characterization indicated that GO can apparently densify the cement pastes with less porosity and hydrates networking, which is consistent with the results of hydration acceleration and strength enhancement.

Effects of compound chemical activators on the hydration of low-carbon ferrochrome slag-based composite cement

Low-carbon ferrochrome slag (LCFS), a by-product of the ferrochrome alloy industry, has potential for use as a cementitious material due to its pozzolanic characteristic. The objective of the present study was to determine the optimum compound chemical activators for LCFS-based composite cement using an orthogonal test, in which 7 d and 28 d compressive strengths were used as the evaluating indices. The influences of compound chemical activators on the hydration of a composite cement mix were investigated using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) and thermogravimetry-differential scanning calorimetry (TG-DSC). The optimum activator to activate the composite cement was a compound of NaCl (NC) at a dosage of 0.6%, Na2SO4 (NS) at a dosage of 1.2%, NaF (NF) at a dosage of 0.6% and Al2(SO4)3 (AS) at a dosage of 0.9% or 0.7%. The compressive strengths of the optimum composite cement mix at ages of 3, 28 and 180 d increased by 50.1%, 22.4% and 16.5%, respectively. More hydration products including ettringite and calcium silicate hydrate were formed at an early age of hydration. The compound chemical activators effectively activated the ferrochrome slag (FS), blast-furnace slag (BFS) and fly ash (FA) in the composite cement.

Effect of Additional Surfaces on Ordinary Portland Cement Early-Age Hydration

Early-age hydration of Ordinary Portland Cement (OPC) was studied in the presence of two additional surfaces. Additional surfaces are known to accelerate the early-age hydration of OPC. Autocatalytic reaction modelling was used to determine acceleration mechanism of additional surfaces. Heat development of the hydration was measured with semi-adiabatic calorimetry and the results were modelled with an autocatalytic reaction. Autocatalytic reaction modelling was able to determine number of initially active nucleation sites in early-age hydration. OPC hydration followed autocatalytic reaction principles throughout induction period and accelerating period. Both of the added surfaces, limestone filler and calcium-silicate-hydrate (C-S-H) coated limestone filler accelerated the early-age hydration. According to autocatalytic modelling, the C-S-H coated filler increased the number of initially active nucleation sites. Pristine limestone filler accelerated the early-age hydration by providing the additional nucleation sites throughout the early-age hydration. The difference was explained with common theories of nucleation and crystal growth. Autocatalytic model and measured calorimeter curve started to significantly deviate at the inflection point, where the reaction mode changed. The reaction mode change depended on the average particle distance. Early-age hydration, modelled as autocatalytic reaction was able to improve understanding of OPC early-age hydration and quantify the number of initially active nucleation sites. Understanding and quantifying the acceleration mechanisms in early-age hydration will aid larger utilization of supplementary cementitious materials where understanding the early-age strength development is crucial.

Electrical Current Flow and Cement Hydration: Implications on Cement-Based Microstructure

Stray current is an electrical current “leakage” from metal conductors and electrical installations. When it flows through cement-based materials, electrical energy is converted to thermal energy that causes increasing temperature due to Joule heating phenomena. The aim of this paper is to shed light on the influence of electrical current flow on cement hydration, thermal properties and pore structure changes of cement-based materials. Calorimetry tests show that degree of cement hydration increases as a results of temperature increase due to electrical current flow through cement-based materials. To evaluate the influence of electrical current on the thermal properties of cement paste, the specific heat of cement paste was calculated based on the degree of cement hydration and temperature development during the hydration process. MIP tests were carried out to quantify changes in the pore structure due to electrical current flow. The results shows that if no other factors are present, leaching is avoided and for relatively early cement hydration age, the electrical current flow accelerates cement hydration, leading to an initial decrease in porosity of the cement paste.

From Hydration to Strength Properties of Fly Ash Based Mortar

Fly ash (FA) is important alternative or supplement to cement to reduce the environmental impact of concrete industry. However, early age strength development of FA is a concern due to the slower reaction rate of FA compared to cement. This paper examines the early age hydration properties of cement paste containing typical Australian FA and establishes correlations between the degree of hydration and the early-age strength development properties of mortar mixes. All mixes have the same mixture proportion of water to binder (w:b) ratio. FA sourced from different power plants are used for the tests. Cement replacement levels of 0%, 10%, 30% and 40% by mass are considered. The degree of hydration was established from the heat production of the mixes by using isothermal calorimetry. The hydration properties were characterized by hydration curve parameters obtained from curve fitting. The results show that both hydration rate and strength of the binder materials (FA and cement) were reduced with higher replacement levels of FA contributing to a reduced hydration rate at early ages. Linear relationship could be obtained between degree of hydration and strength at early ages for all the fly ash binders.

Self-restraint thermal stress in early-age concrete samples and its evaluation

The self-restraint thermal stress caused by mismatch in thermal and mechanical behaviors of cementitious material and coarse aggregates could be a factor associated with early-age cracking in massive concrete structures, which might be even complicated when considering the mesostructure of concrete. This paper presents the application of mesoscopic model based numerical approach to trace the progression of thermo-mechanical mismatch and consequent self-restraint thermal stress development during early-age hydration in concrete. The mesostructure of concrete sample which contains aggregates, mortar and ITZ is modeled, with the material properties for each phase rationally incorporated. Thermo-mechanical behaviors of both homogeneous mortar and heterogeneous concrete samples under standard curing condition are simulated, focusing on the production mechanism of self-restraint stress. Numerical results show that the spatial-time distribution of temperature and expansion coefficient fields in concrete samples is the main origin of the self-restraint stress. The magnitude of this stress is non-negligible, which may become an influential factor to the load-bearing capacity of concrete.

In Situ Soft X-ray Spectromicroscopy of Early Tricalcium Silicate Hydration.

The understanding and control of early hydration of tricalcium silicate (C3S) is of great importance to cement science and concrete technology. However, traditional characterization methods are incapable of providing morphological and spectroscopic information about in situ hydration at the nanoscale. Using soft X-ray spectromicroscopy, we report the changes in morphology and molecular structure of C3S at an early stage of hydration. In situ C3S hydration in a wet cell, beginning with induction (~1 h) and acceleration (~4 h) periods of up to ~8 h, was studied and compared with ex situ measurements in the deceleration period after 15 h of curing. Analysis of the near-edge X-ray absorption fine structure showed that the Ca binding energy and energy splitting of C3S changed rapidly in the early age of hydration and exhibited values similar to calcium silicate hydrate (C–S–H). The formation of C–S–H nanoseeds in the C3S solution and the development of a fibrillar C–S–H morphology on the C3S surface were visualized. Following this, silicate polymerization accompanied by C–S–H precipitation produced chemical shifts in the peaks of the main Si K edge and in multiple scattering. However, the silicate polymerization process did not significantly affect the Ca binding energy of C–S–H.

Effect of water and nano-silica solution on the early stages cement hydration

The nanoscopic young’s modulus of cement paste during early ages of hydration in water and nano-silica was investigated by the application of atomic force microscopy (AFM). Peak-Force Quantitative Nano-Mechanical (PFQNM) imaging mode in atomic force microscopy was used to determine elastic modulus of cement paste before and after immersion in water and nano-silica solution. The results clearly show how mechanical and physical properties of cement grains changes progressively to higher values during the hydration process in water and nano-silica.Moreover, an empirical theory on mechanism of hydration development is proposed in the current study based on experimental observations. Average Young’s modulus of condensed cement powder was changed from about 15GPa, for unhydrated state, to 11.3GPa after two hours of hydration in water environment. The average roughness trend was also altered from 8.31nm to 18.2nm during hydration process. Results show that the cement hydration rate in nano-silica solution is much higher than water environment. Moreover transition zone between C-S-H colonies have much higher mechanical properties than the hardest colonies in every ages. In the presence of nano-silica, transition zone has a width of less than 10nm and a surface area of 5% of the overall surface area.

Physicochemical characteristics of cementitious building materials derived from industrial solid wastes

In this study the possibility of using alternative waste materials with cementitious properties was studied. Lime rich sludge (LRS) (solid waste of acetylene industry), ground granulated blastfurnace slag (GGBFS) and fine kaolinite sand (FKS) (by-product during kaolin mineralization) were used in the production of cementitious building materials. Different mixes were prepared using percentage weight ratios 80/20, 75/25, 70/30 of GGBFS and LRS, respectively, at water/cement ratio of 0.20 by weight. Fine kaolinite sand (5wt%) was used as addition in these mixes. The mechanical and hydration characteristics of the prepared pastes were studied at various time intervals up to 90days. The results showed that LRS acts as an alkaline activator for GGBFS and the optimum mix composition for activation was 75% GGBFS and 25% LRS. Addition of fine kaolinite sand (FKS) to these mixes resulted in a marked improvement in the hydraulic properties especially in the early hydration ages; this is related to the higher rate of pazzolanic interaction between the lime in LRS and the aluminosilicate constituents in fine kaolinite sand leading to formation of excessive amounts of calcium silicate hydrates (CSH) which act as binding centers between the remaining unhydrated parts of GGBFS and FKS. XRD patterns and DSC thermograms obtained for all pastes showed that the main hydration products are nearly amorphous and microcrystalline calcium silicate hydrates as well as minor amounts of calcium aluminosilicate hydrates.

Influence of the synergy between mineral additions and Portland cement in the physical-mechanical properties of ternary binders

The paper deals with the synergistic effect of mineral additions on the physical-mechanical performance of ternary blends prepared with different Portland cements (PC). The effect in setting and heat flow release is also analyzed. The mineral additions used are blast furnace slag (BFS), fly ash (FA) and limestone filler (LF). PCs with different C3A and alkali content have been tested to study the synergy in ternary blends. Ternary binders with PC low in C3A and alkali content achieve similar mechanical strength gain as plain PC and refinement of pore size distribution from early hydration ages due to the acceleration of PC hydration induced by the mineral additions. In contrast, ternary binders with PC higher in C3A and alkali content have a delayed in mechanical strength at early hydration ages, but significantly higher at long hydration times.

Parameters controlling early age hydration of cement pastes containing accelerators for sprayed concrete

The objective of this work is to parametrize the early age hydration behavior of accelerated cement pastes based on the chemical properties of cement and accelerators. Eight cements, three alkali-free and one alkaline accelerators were evaluated. Isothermal calorimetry, in situ XRD and SEM imaging were performed to characterize kinetics and mechanisms of hydration and the microstructure development. The reactivity of all accelerators is directly proportional to their aluminum and sulfate concentrations and to the amount and solubility of the setting regulator contained in cement. Alite hydration is enhanced if a proper C3A/SO3 ratio (between 0.67 and 0.90) remains after accelerator addition and if limestone filler is employed, because undersulfated C3A reactions are avoided. Combinations of compatible materials are recommended to enhance the performance of the matrix and to prevent an undesirable hydration behavior and its consequences in mechanical strength development.

Formation and early hydration characteristics of C<sub>2.75</sub>B<sub>1.25</sub>A<sub>3</sub>$ in binary system of C<sub>2.75</sub>B<sub>1.25</sub>A<sub>3</sub>$-C<sub>2</sub>S

C 2.75 B 1.25 A 3 $ (2.75CaO•1.25BaO• 3Al 2 O 3 • SO 3 ) is one of the important minerals and it govern-directly the early-strength of belite-barium calcium sulphoaluminate cement. In this paper a binary system C 2.75 B 1.25 A 3 $-C 2 S is selected to investigate the formation of C 2.75 B 1.25 A 3 $. In the range of 1100 °C–1200 °C, the earlier formed C 2 S hinders the formation of C 2.75 B 1.25 A 3 $. On the contrary, when the temperature is in the range of 1200 °C–1350 °C, the initially formed C 2 S could provide a surface for the nucleation of C 2.75 B 1.25 A 3 $ and cut down the potential barrier (?Gk*) for the heterogeneous nucleation of C 2.75 B 1.25 A 3 $, which contributes to its formation. Moreover, at 1350 °C, the large amount of previously formed C 2 S benefits the extent of formation of C 2.75 B 1.25 A 3 $. The possible reason was that it could prevent sulfur evaporation. In early hydration age, AFm and AFt originating from C 2.75 B 1.25 A 3 $ hydration are found within 2 h and 12 h under 95% RH at 1 °C, respectively, whereas C 2 S is unhydrated at this moment.

Water dynamics in cement paste at early age prepared with pozzolanic volcanic ash and Ordinary Portland Cement using quasielastic neutron scattering

Early age hydration kinetics of Portland cement with pozzolanic volcanic ash was examined using quasielastic neutron scattering. Volcanic ash consisting of two different particle sizes was used to prepare cement pastes with different ratios of Portland cement to volcanic ash. The concentration of the volcanic ash played a major role in the bound water index and self-diffusion coefficients of hydration water confined in the cement paste. An increase in the particle size of the volcanic ash affected the degree of hydration by allowing more free and mobile water in the gel pores, suggesting that volcanic ash may not have completely reacted during the experimental time frame. This study shows that the particle size along with variation in volcanic ash composition governs the early age hydration process in volcanic ash cements.