Reducing setting time of blended cement paste containing high-SO3 fly ash (HSFA) using chemical/physical accelerators and by fly ash pre-washing.

Reducing the carbon footprint of the cement industry has become one of the main concerns of researchers in the field. This study explores different strategies to reduce the setting retardation effect of high-SO3 fly ash (HSFA) on cement paste. The SO3 phase was found to correspond to hannebachite (CaSO3·0.5H2O). Chemical (calcium chloride), physical (fine limestone powder), and pre-washing strategies were investigated as means to reduce or eliminate the retardation. Each of these strategies showed some potential to decrease the retardation effect. A combination of fine limestone powder and HSFA pre-washing showed almost the same accelerating power as the calcium chloride, offering a good alternative when chloride incorporation is restricted. The retardation effect can be associated with a combined extension of the induction period and a depression of the initial silicate reactions of the clinker phases. A methodology to assess the hannebachite content based on a thermogravimetric analysis (TGA) technique is proposed, allowing a good alternative control approach for field conditions or for where X-ray (XRD or XRF) equipment is not readily available.

Influence of Substrate Moisture State and Roughness on Interface Microstructure and Bond Strength: Slant Shear vs. Pull-Off Testing.

There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.

Measurement and Modeling of the Ability of Crack Fillers to Prevent Chloride Ingress into Mortar.

A common repair procedures applied to damaged concrete is to fill cracks with an organic polymer. This operation is performed to increase the service life of the concrete by removing a preferential pathway for the ingress of water, chlorides, and other deleterious species. To effectively fulfill its mission of preventing chloride ingress, the polymer must not only fully fill the macro-crack, but must also intrude the damage zone surrounding the crack perimeter. Here, the performance of two commonly employed crack fillers, one epoxy, and one methacrylate, are investigated using a combined experimental and computer modeling approach. Neutron tomography and microbeam X-ray fluorescence spectrometry (µXRF) measurements are employed on pre-cracked and chloride-exposed specimens to quantify the crack filling and chloride ingress limiting abilities, respectively, of the two polymers. A two-dimensional model of chloride transport is derived from a mass balance and solved by the finite element method. Crack images provided by µXRF are used to generate the input microstructure for the simulations. When chloride binding and a time-dependent mortar diffusivity are both included in the computer model, good agreement with the experimental results is obtained. Both crack fillers significantly reduce chloride ingress during the 21 d period of the present experiments; however, the epoxy itself contains approximately 4 % by mass chlorine. Leaching studies were performed assess the epoxy as a source of deleterious ions for initiating corrosion of the steel reinforcement in concrete structures.

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.

NEUTRON RADIOGRAPHY MEASUREMENT OF SALT SOLUTION ABSORPTION IN MORTAR.

Some concrete pavements in the US have recently exhibited premature joint deterioration. It is hypothesized that one component of this damage can be attributed to a reaction that occurs when salt-laden water is absorbed in the concrete and reacts with the matrix. This study examines the absorption of CaCl2 solution in mortar via neutron imaging. Mortar specimens were prepared with water to cement ratios, (w/c), of 0.36, 0.42 and 0.50 by mass and exposed to chloride solutions with concentrations ranging from 0 % to 29.8 % by mass. Depth of fluid penetration and moisture content along the specimen length were determined for 96 h after exposure. At high salt concentration (29.8 %), the sorption rate decreased by over 80 % in all samples. Along with changes in surface tension and viscosity, CaCl2 reacts with the cement paste to produce products (Friedel's salt, Kuzel's salt, or calcium oxychloride) that block pores and reduce absorption.

Limestone and Silica Powder Replacements for Cement: Early-Age Performance.

Developing functional concrete mixtures with less ordinary portland cement (OPC) has been one of the key objectives of the 21st century sustainability movement. While the supplies of many alternatives to OPC (such as fly ash or slag) may be limited, those of limestone and silica powders produced by crushing rocks seem virtually endless. The present study examines the chemical and physical influences of these powders on the rheology, hydration, and setting of cement-based materials via experiments and three-dimensional microstructural modeling. It is shown that both limestone and silica particle surfaces are active templates (sites) for the nucleation and growth of cement hydration products, while the limestone itself is also somewhat soluble, leading to the formation of carboaluminate hydration products. Because the filler particles are incorporated as active members of the percolated backbone that constitutes initial setting of a cement-based system, replacements of up to 50 % of the OPC by either of these powders on a volumetric basis have minimal impact on the initial setting time, and even a paste with only 5 % OPC and 95 % limestone powder by volume achieves initial set within 24 h. While their influence on setting is similar, the limestone and silica powders produce pastes with quite different rheological properties, when substituted at the same volume level. When proceeding from setting to later age strength development, one must also consider the dilution of the system due to cement removal, along with the solubility/reactivity of the filler. However, for applications where controlled (prompt) setting is more critical than developing high strengths, such as mortar tile adhesives, grouts, and renderings, significant levels of these powder replacements for cement can serve as sustainable, functional alternatives to the oft-employed 100 % OPC products.

Numerical Simulation of the Freeze-Thaw Behavior of Mortar Containing Deicing Salt Solution.

This paper presents a one-dimensional finite difference model that is developed to describe the freeze-thaw behavior of an air-entrained mortar containing deicing salt solution. A phenomenological model is used to predict the temperature and the heat flow for mortar specimens during cooling and heating. Phase transformations associated with the freezing/melting of water/ice or transition of the eutectic solution from liquid to solid are included in this phenomenological model. The lever rule is used to calculate the quantity of solution that undergoes the phase transformation, thereby simulating the energy released/absorbed during phase transformation. Undercooling and pore size effects are considered in the numerical model. To investigate the effect of pore size distribution, this distribution is considered using the Gibbs-Thomson equation in a saturated mortar specimen. For an air-entrained mortar, the impact of considering pore size (and curvature) on freezing was relatively insignificant; however the impact of pore size is much more significant during melting. The fluid inside pores smaller than 5 nm (i.e., gel pores) has a relatively small contribution in the macroscopic freeze-thaw behavior of mortar specimens within the temperature range used in this study (i.e., +24 °C to -35 °C), and can therefore be neglected for the macroscopic freeze-thaw simulations. A heat sink term is utilized to simulate the heat dissipation during phase transformations. Data from experiments performed using a low-temperature longitudinal guarded comparative calorimeter (LGCC) on mortar specimens fully saturated with various concentration NaCl solutions or partially saturated with water is compared to the numerical results and a promising agreement is generally obtained.

On the Relation of Setting and Early-Age Strength Development to Porosity and Hydration in Cement-Based Materials.

Previous research has demonstrated a linear relationship between compressive strength (mortar cubes and concrete cylinders) and cumulative heat release normalized per unit volume of (mixing) water for a wide variety of cement-based mixtures at ages of 1 d and beyond. This paper utilizes concurrent ultrasonic reflection and calorimetry measurements to further explore this relationship from the time of specimen casting to 3 d. The ultrasonic measurements permit a continuous evaluation of thickening, setting, and strength development during this time period for comparison with the ongoing chemical reactions, as characterized by isothermal calorimetry measurements. Initially, the ultrasonic strength-heat release relation depends strongly on water-to-cement ratio, as well as admixture additions, with no universal behavior. Still, each individual strength-heat release curve is consistent with a percolation-based view of the cement setting process. However, beyond about 8 h for the systems investigated in the present study, the various strength-heat release curves merge towards a single relationship that broadly characterizes the development of strength as a function of heat released (fractional space filled), demonstrating that mortar and/or concrete strength at early ages can be effectively monitored using either ultrasonic or calorimetry measurements on small paste or mortar specimens.

Chemical vs. Physical Acceleration of Cement Hydration.

Cold weather concreting often requires the use of chemical accelerators to speed up the hydration reactions of the cement, so that setting and early-age strength development will occur in a timely manner. While calcium chloride (dihydrate - CaCl2·2H2O) is the most commonly used chemical accelerator, recent research using fine limestone powders has indicated their high proficiency for physically accelerating early-age hydration and reducing setting times. This paper presents a comparative study of the efficiency of these two approaches in accelerating hydration (as assessed via isothermal calorimetry), reducing setting times (Vicat needle), and increasing early-age mortar cube strength (1 d and 7 d). Both the CaCl2 and the fine limestone powder are used to replace a portion of the finest sand in the mortar mixtures, while keeping both the water-to-cement ratio and volume fractions of water and cement constant. Studies are conducted at 73.4 °F (23°C) and 50 °F (10 °C), so that activation energies can be estimated for the hydration and setting processes. Because the mechanisms of acceleration of the CaCl2 and limestone powder are different, a hybrid mixture with 1 % CaCl2 and 20 % limestone powder (by mass of cement) is also investigated. Both technologies are found to be viable options for reducing setting times and increasing early-age strengths, and it is hoped that concrete producers and contractors will consider the addition of fine limestone powder to their toolbox of techniques for assuring performance in cold weather and other concreting conditions where acceleration may be needed.

D90: The Strongest Contributor to Setting Time in Mineral Trioxide Aggregate and Portland Cement.

INTRODUCTION: The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It was hypothesized that much of this variation was caused by differences in particle size distribution. METHODS: Two gram samples from 11 MTA-type cements were analyzed by laser diffraction to determine their particle size distributions characterized by their percentile equivalent diameters (the 10th percentile, the median, and the 90th percentile [d90], respectively). Setting time data were received from manufacturers who performed indentation setting time tests as specified by the standards relevant to dentistry, ISO 6786 (9 respondents) or ISO 9917.1 (1 respondent), or not divulged to the authors (1 respondent). In a parallel experiment, 6 samples of different size graded Portland cements were produced using the same cement clinker. The measurement of setting time for Portland cement pastes was performed using American Society for Testing and Materials C 191. Cumulative heat release was measured using isothermal calorimetry to assess the reactions occurring during the setting of these pastes. In all experiments, linear correlations were assessed between setting times, heat release, and the 3 particle size parameters. RESULTS: Particle size varied considerably among MTA cements. For MTA cements, d90 was the particle size characteristic showing the highest positive linear correlation with setting time (r = 0.538). For Portland cement, d90 gave an even higher linear correlation for the initial setting time (r = 0.804) and the final setting time (r = 0.873) and exhibited a strong negative linear correlation for cumulative heat release (r = 0.901). CONCLUSIONS: Smaller particle sizes result in faster setting times, with d90 (the largest particles) being most closely correlated with the setting times of the samples.

The Visible Cement Data Set.

With advances in x-ray microtomography, it is now possible to obtain three-dimensional representations of a material's microstructure with a voxel size of less than one micrometer. The Visible Cement Data Set represents a collection of 3-D data sets obtained using the European Synchrotron Radiation Facility in Grenoble, France in September 2000. Most of the images obtained are for hydrating portland cement pastes, with a few data sets representing hydrating Plaster of Paris and a common building brick. All of these data sets are being made available on the Visible Cement Data Set website at http://visiblecement.nist.gov. The website includes the raw 3-D datafiles, a description of the material imaged for each data set, example two-dimensional images and visualizations for each data set, and a collection of C language computer programs that will be of use in processing and analyzing the 3-D microstructural images. This paper provides the details of the experiments performed at the ESRF, the analysis procedures utilized in obtaining the data set files, and a few representative example images for each of the three materials investigated.