Little published data is available to guide specification calcium sulfoaluminate cement (CSA) mixtures with citric acid retarder dosage rates capable of achieving adequate field working times, nor to ...understand the effect of retarder dosage on hydration and property development, representing a significant barrier to widespread CSA concrete use. Thus, this study investigated the use of citric acid with two commercially-available CSA cements, tracking the effects of dosage on phase development, hydration, setting, and compressive strengths. Key findings were that: citric acid successfully retarded initial set past 120min for both cements despite significantly different chemistries; increasing cement anhydrite content reduced retarder effectiveness and altered hardened binder microstructure, reducing compressive strengths; greater retarder dosages did not negatively affect cumulative hydration, nor strengths; and the time at which the maximum rate of heat evolution occurred correlated with final setting, a relationship useful for predicting field mixture behavior based on laboratory testing.
Modification of fineness, including by grinding or classification, is a commonly used beneficiation method for improving fly ash performance. In this study, five fly ashes, including off-spec and ...reclaimed ashes, were size fractionated by sieving into the following fractions: < 20 µm, < 45 µm, and > 45 µm. The original fly ash and the size fractions were characterized, and their reactivity was measured. For Class C fly ashes, the CaO contents decreased and SiO
2
contents increased as the size fractions became coarser, with the > 45 µm size fractions having lower CaO contents that would allow them to be classified as Class F fly ashes. The > 45 µm size fractions had more angular or irregular particles (impurities and unburnt carbon), while the < 20 µm fractions had very limited amounts of such particles. There was a general reduction of reactivity from finer to coarser fractions as measured using a direct reactivity test. As expected, cement pastes made with finer fractions showed higher heat release and calcium hydroxide consumption. The magnitude of changes between the size fractions depended on the original fly ash composition and particle size distribution.
This work investigated the use of acid treatment as a method for increasing the reactivity of natural zeolite used as a supplementary cementitious material. The effects of treating a natural ...clinoptilolite zeolite with nine acid solutions, 0.1M, 0.5M, or 1M hydrochloric or nitric acid or 0.1M, 0.5M, or 0.87M acetic acid, were measured using x-ray diffraction, particle size analysis, pore size distribution and surface area analysis. The zeolite pozzolanic reactivity was determined by measuring the quantity of portlandite in hydrated zeolite-cement paste after 28 and 90days. Results showed that acid treatment increased zeolite surface area, resulting in increased zeolite pozzolanic reactivity, independent of the solution concentration used. Cement hydration was also increased, evidenced by greater rates of heat evolution from cement-zeolite pastes. Additionally, although reductions of portlandite occurred most quickly in pastes with zeolites treated with strong acids, by 90days the zeolites treated with acetic acid solutions showed comparable portlandite reductions.
Wet curing improves ordinary portland cement (OPC) concrete durability and strength by increasing total hydration, densifying microstructure, and decreasing concrete permeability. In general, wet ...curing is recommended for OPC until it gains >70% of the designed compressive strength, typically for at least 7 days. Calcium sulfoaluminate (CSA) cement may allow for decreased curing time requirements due to its different phase composition from that of OPC, rapid hydration, and strength gain. Rapid hydration may also prevent some disadvantages associated with OPC curing requirements, such as long curing times, costs associated with the use of high-water quantities, and supervision needs for curing processes. In addition, it is important to understand the effect of use of various curing regimes that are currently specified for use with OPC when applied in CSA systems. This study investigated a variety of curing durations and curing solution compositions to understand their effects on CSA hydration, strength development, and shrinkage. The results demonstrate that 3-day moist curing promotes adequate strength gain and completion of hydration reactions. Additionally, wet curing CSA samples even for 1 day led to lower shrinkage than 7-day cured OPC samples and may result in reduced cracking in concrete pavements. Curing through ponding of samples in deionized water or calcium sulfate-saturated solution resulted in strength reductions of 18% or greater relative to fog-curing.
•Water-to-cement ratio affected the very early age hydration of the CSAB cements differently depending upon their chemical makeup (phase content, specifically the crystallographic form of ye'elimite ...and the ratio of anhydrite/ye'elimite present in the system).•Setting time was more strongly linked with anhydrite content than any other variable investigated.•Despite the variances in initial hydration kinetics due to chemistry, percent hydration, and compressive strength correlated with w/c independent of cement chemistry.•Greater w/c was found to lead to increased overall hydration levels (percent hydration), but lower strength, and increased setting times.•Similar to OPC systems, use of low w/c (0.3 – 0.4) is recommended in order to maximize compressive strength and minimize binder porosity.
Little published data is available to guide engineers in designing calcium sulfoaluminate belite (CSAB) cement mixtures with adequate workability, strength, and durability. This lack of understanding of design factors, especially the effect of varying w/c, represents a significant barrier to widespread CSAB use. In this study hydration, setting time, and strength development of two CSAB cements with w/c from 0.3 – 0.6 were evaluated. CSAB reaction kinetics varied with increased w/c depending on CSAB composition, specifically calcium sulfate content – with higher w/c increasing retardation in higher anhydrite/ye'elimite content cement, but reduced retardation in lower anhydrite/ye'elimite cement. For both cements, greater w/c led to greater total hydration, increased setting times, and reduced compressive strengths in the pastes and mortar samples. Setting time was linked more closely to anhydrite content than w/c, with greater sulfate volumes shortening setting times.
Using supplementary cementitious materials (SCMs) to decrease concrete diffusivity, which slow the ingress of chlorides into concrete that results in reinforcement corrosion, is one of the most ...economical ways to increase concrete service life. To show the effect of ternary SCM blends on concrete diffusivity, bulk chloride diffusion testing was performed on 12 SCM concrete mixtures cured in air, with a curing compound, or under continuous wet curing for 28 days, 91 days, and 180 days. The ternary mixtures interacted synergistically, reducing diffusivity at greater rates than binary SCM mixtures. The chloride diffusion coefficients of specimens cured outdoors were six times greater than for wet-cured specimens and the application of curing compound did not result in significant diffusivity reductions. An experimental data were compared to service-life-modeling software predictions for the experimental mixtures and showed that current software service-life predictions correspond to worst-case curing conditions.
•Calcination negatively affected zeolite particle size, pore size distribution, and surface area.•Calcination reduced zeolite crystallinity and increased amorphous material quantity.•Zeolites reacted ...pozzolanically in cement paste, but calcination did not increase reactivity.•Calcining above the zeolite’s decomposition temperature reduced pozzolanic reactivity.•Amorphous content did not correlate with zeolite reactivity in cementitious systems.
This study examined several sources of natural clinoptilolite zeolite calcined at temperatures of 300 °C-965 °C, tracking changes in phase content, particle size, surface area, and pore size distribution. Changes in pozzolanic reactivity with calcination were measured through calcium hydroxide content of hydrated zeolite-cement pastes and compressive strengths of cement mortars. Calcination increased zeolite particle size and reduced porosity and surface area, resulting in decreased zeolite pozzolanic reactivity despite increased amorphous content in the samples. Overall, in contrast to what has been shown previously in literature, calcination was ineffective in increasing the reactivity of clinoptilolite zeolites for use as SCMs.
One of the challenges associated with creating bacterial-concrete systems capable of biomineralizing CaCO
3
to fill cracks is the high pH environment of the hydrated cement paste. In this study two ...approaches to address this challenge were investigated: (i) the use of calcium sulfoaluminate (CSA) cement, which develops a naturally lower pH, and (ii) the use of non-axenic bacterial cultures, which may facilitate growth of bacterial strains more resilient to harsh alkaline conditions. Axenic
B. subtilis
and a non-axenic bacterial system from soil were produced and utilized in ordinary portland cement (OPC) and CSA samples. The mechanical properties, water absorption, calcium carbonate precipitation capability, and survivability of bacteria were investigated. The highest
B. subtilis
and soil bacteria viability was obtained through use of CSA cement and may enable greater later age crack healing potential than mixtures using OPC. Incorporation of axenic bacteria resulted in increased bacteria survivability in the mortar samples when compared to non-axenic bacteria mixes. However, in both cementitious systems, use of
B. subtilis
and soil bacteria led to similar improvements, suggesting that non-axenic cultures may be used in concrete effectively.
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