Today's concrete is no longer a simple combination of cement, aggregates and water. With increased use of various types of waste materials as supplementary cementitious materials and chemical admixtures, material incompatibility problems have been observed in concrete construction. As a result, some of the greatest problems in concrete manufacturing occur when concrete does not stiffen or harden on time. To this end, a new innovative type of cementing technique (based on the principles of magnetorheology) is presented that allows for the real-time control over the stiffening or setting behavior of concrete. In traditional magnetorheological (MR) fluids, magnetic particles are mostly submerged in Newtonian carrier fluids using high volumetric contents (40-50%) of magnetic particles. A key interest in this work was to investigate if using a non-Newtonian carrier fluid like cement paste with low dosages of magnetic particles would yield an MR effect. Rheological tests were conducted on paste mixtures containing small dosages of magnetic particles (less than 2% volume fraction) and when a magnetic field was applied, it was determined that the shear resistance of the paste could be altered significantly. The response of the paste was found to be dependent on the magnitude of the applied field, concentration of the magnetic particles and surface chemistry of magnetic particles. Furthermore the magnetic particles used in this research to create the MR cement paste did not have any effect on cement hydration products or on compressive strength results. It was shown that the rheological behavior of cement paste could even be adapted to simulate "setting" behavior when an MR-based approach is used. Thus, the potential to create a cement-based material whose fresh state behavior can be adapted on-demand by the user to achieve a desired behavior may soon be a reality. Such a material can be useful in applications in which controlling the fresh-state behavior is critical, and could transform the way cement-based materials are cast. In addition, possibilities to create a smart cement-based composite from the fresh to the hardened state may be possible if the magnetic particles could later be used for structural health monitoring. / text

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This dissertation covers concrete and cement paste (CP) containing portland-limetsone cement (PLC) for the purpose of understanding mainly mechanical property behaviors. PLC has been consistently investigated by researchers over the past decade who have found equal and often superior performance with PLC, but few researchers have found reasons why. Throughout this dissertation CP cylinders are used to help understand concrete. By understanding the CP (cementitious materials, water, admix, and no fine or coarse aggregates) portion of concrete, it is believed further understanding can be achieved. The interaction of CP and aggregates, or paste aggregate bond (PAB), is exceptionally important. Literature review suggests PAB can be affected by chemical and physical properties of aggregates as well as cement which dictates the mechanical property performance of concrete. CP measurements used herein are mainly compressive strength, however there were other measurements, such as thermal setting, collected as well. CP properties whether thermal or mechanical, generally agreed with concrete results. Additionally, CP compressive strengths were found to have value beyond what was previously known. Mainly that CP compressive strengths can help diagnose concrete behavior. CP used with concrete measurements can give some indication, not previously available, concerning PAB and bonding efficiency. However, the CP measurements had high variability. As such, a new CP production method was developed alongside different analysis techniques to reduce the variability. CP was further used with concrete to recommend a factor for balancing fineness and limestone percentage in PLC. Next, CP measurements were used with 74 concrete mixtures for investigating the Mississippi Department of Transportation fly ash replacement level limits for ordinary portland cement (OPC) and PLC for implementation into the Mississippi market. Lastly, this dissertation delved into non-production concrete mixtures with few ingredients (washed aggregates, cementitious materials, and water) to discover mechanisms behind PLC and OPC behaviors. From CP and concrete measurement perspectives, it is recommended that PLC in the MS marketplace be implemented without hesitation. From the conclusions herein, there does not appear to be many cases where OPC considerably outperforms PLC. In almost every case, PLC performs equivalent or better than OPC when numerous properties are considered.

Modeling of the time-dependent behavior of cement paste has always been a difficulty. In the past, viscoelastic behavior of cementitious materials has been primarily attributed to the viscoelastic properties of C-S-H components. Recent experimental results show that C-S-H may not exhibit as much creep and relaxation as previously thought. This requires new consideration of different mechanisms leading to the viscoelastic behavior of cement paste. Thus the objective of this thesis is to build a computational model using finite element method to predict the viscoelastic behavior of cement paste, and using this model, virtual tests can be carried out to improve understanding of the mechanisms of viscoelastic behavior. The primary finding from this thesis is that the apparent viscoelastic behavior due to dissolution of load bearing phases is substantial. The dissolution process occurring during the hydration reaction can change the stress distribution inside cementitious materials, resulting in an apparent viscoelastic behavior of the whole cementitious materials. This finding requires new consideration of mechanisms of time-dependent behavior of cementitious materials regarding the dissolution process of cement paste.

The focus of this research was to investigate the use of functionalized/non-functionalized multi walled carbon nanotubes (MWCNTs) as reinforcements for the Portland cement paste. The unique geometrical characteristics of the carbon nanotubes (CNTs), as well as its unique mechanical properties such as high strength, ductility and stiffness, were the vital motivation for this study. In this research, we combined this unique material (CNTs) with concrete which is the most used man-made material. When compared to other composite materials, a limited amount of research has been conducted on the CNTs/cement composites. In order to investigate how the aspect ratio of functionalized/non-functionalized MWCNTs affects the mechanical properties of cementitious composites, ten different mixes of the MWCNTs/cement composites were prepared and tested. The different batches had a fixed water/cement ratio of 0.4, and variations of MWCNTs length, concentration and surface treatment. The cement nanocomposites were cast in small-scale specimens (beams) for the three-point flexural testing. Four major mechanical properties were evaluated at ages of 7, 14, and 28 days from the casting day: the maximum flexural strength, ultimate strain capacity (ductility), modulus of elasticity, and modulus of toughness. The results for the different nanocomposite batches were compared with the plain cement (reference) batch. The mechanical testing results showed that at 28 days almost all of the MWCNTs composites increased the flexural strength of the cement nanocomposites. At 28 days, the long MWCNTs increased the flexural strength more than the short MWCNTs. In general, the ultimate strain (ductility) of the short MWCNTs nanocomposites was higher than the ultimate strain of the long MWCNTs nanocomposites. The flexural strength of short 0.2 percent MWNT and long 0.04 percent MWNT (OH) increased by 269 percent and 83 percent, respectively, compared to the plain cement sample at 28 days. The highest ductility at 28 days for the short 0.1 percent MWNT and the short 0.2 percent MWNT was 86 percent and 81 percent, respectively. Clear evidence was obtained from the SEM images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks. In conclusion, CNTs as nano reinforcements, can effectively improve certain mechanical properties of the cement paste composites.

The present research seeks to study the decrease in diffusivity rate as relative humidity (RH) decreases and modeling drying shrinkage of hardened cement paste as a poroviscoelastic respose. Thin cement paste strips of 0.4 and 0.5 w/c at age 3 and 7 days were measured for mass loss and shrinkage at small RH steps in an environmental chamber at constant temperature. Non-linear drying diffusion rate of hardened cement was modeled with the use of Fick's second law of diffusion by assuming linearity of diffusion rate over short drops of ambient relative humidity. Techniques to determine drying isotherms prior to full equilibration of mass loss, as well as converting mass loss into concentration of water vapor were developed. Using the measured water vapor diffusivity, drying shrinkage strain was modeled by the theory of poroviscoelasticity. This approach was validated by determining viscoelastic properties from uniaxial creep tests considering the effect of aging by the solidification theory. A change in drying diffusion rate at different RH was observed in the 0.4 and 0.5 w/c pastes at different ages. Drying diffusion rate decreases as RH drops. This can be attributed to a change in diffusion mechanisms in the porous media at smaller pore radius. Shrinkage modeling with an average diffusion coefficient and with determined viscoelastic parameters from creep tests agreed well compared to the shrinkage data from experiments, indicating that drying shrinkage of cement paste may be considered as a poroviscoelastic reponse.

The NIST miniature paste prism test was developed to assess sulphate resistance of cements faster than the commonly used ASTM C 1012 test. The goal of this research is to address the current limitations regarding the NIST procedure to determine the optimum testing parameters and to establish appropriate expansion limits. A range of variables including details of specimen design, curing regime, water-to-cementitious materials ratio, and prism length were tested on the materials with a wide range of sulphate performance. The findings of the study demonstrate that even though it can yield results in only three months, the NIST test can provide an erroneous differentiation between certain cementitious materials. Reliability and repeatability of the test results can be improved by using longer specimens and longer curing times. The NIST test can be used as a preliminary screening test, but users need to be aware of its limitations.

The NIST miniature paste prism test was developed to assess sulphate resistance of cements faster than the commonly used ASTM C 1012 test. The goal of this research is to address the current limitations regarding the NIST procedure to determine the optimum testing parameters and to establish appropriate expansion limits. A range of variables including details of specimen design, curing regime, water-to-cementitious materials ratio, and prism length were tested on the materials with a wide range of sulphate performance. The findings of the study demonstrate that even though it can yield results in only three months, the NIST test can provide an erroneous differentiation between certain cementitious materials. Reliability and repeatability of the test results can be improved by using longer specimens and longer curing times. The NIST test can be used as a preliminary screening test, but users need to be aware of its limitations.

<p>Superabsorbent polymer (SAP) hydrogel particles have been used as internal curing agents in concrete mixes as they are capable of absorbing and subsequently releasing large amounts of water. This reduces autogenous shrinkage during early stages of hydration. The size, shape, and composition of the hydrogel particles can be controlled during the synthesis, hence providing the opportunity to custom synthesize these internal curing agents to elicit desired structure-property relationships. Utilization of optimized dosage and formulation of SAP has the potential to improve the microstructure, durability, and strength of internally cured concrete. </p> <p>The first study focuses on the synthesis and application of novel composite hydrogel particles as internal curing agents in cementitious mixes. Composite polyacrylamide hydrogel particles containing two different amorphous silica–either nanosilica or silica fume–were used to investigate whether the internal curing performance of hydrogel particles could be enhanced. The dosage and type of silica, crosslinker amount were varied to identify the composite polyacrylamide hydrogel particle composition that provides optimum benefits to internally cured cementitious systems. The synthesized hydrogels were characterized by means of absorption capacity tests, compositional and size analysis. The beneficial impacts of the addition of composite hydrogels on cement paste microstructure are highlighted, including the preferential formation of cement hydration products (such as portlandite) within the hydrogel-induced voids that appeared to be influenced by the composition of the hydrogel particles. The interrelationship between extent of hydration, size of hydrogel voids, and void-filling with hydration products was found to strongly influence mechanical strength and is thus an important structure-property relationship to consider when selecting hydrogels for internal curing purposes. This study informs the design of composite hydrogel particles to optimize performance in cementitious mixes. Additionally, it provides a novel means of incorporating other commonly used admixtures in concrete without facing common challenges related to dispersion and health hazards.</p> <p>The second study focuses on the utilization of two retarding admixture-citric acid and sucrose-to custom synthesize composite polyacrylamides to investigate whether the composite hydrogels could delay hydration of cement paste. Isothermal calorimetry analysis results showed that composite sucrose-containing polyacrylamide hydrogel particles were successfully able to retard main hydration peak of cement paste, beyond the retardation capabilities of the pure polyacrylamide hydrogels. Thus, this study provides avenues of exploring the utilization of common admixtures to formulate novel composite hydrogels that imparts specific properties to cementitious systems.</p> <p>In another study, SAP formulated by admixture industries were used to investigate the feasibility of internal curing of bridge decks and pavement patches with SAP particles. The microstructure and early age hydration properties of SAP-cured cementitious systems were studied. Mitigation of microcracks in the matrix, along with portlandite growth in SAP voids, were observed in SAP-cured mortars. Presence of SAP also mitigated autogenous shrinkage and improved early age hydration as observed by isothermal calorimetry analysis. This thesis highlights some of the beneficial impacts of SAP-cured cementitious systems, and the potential to harness those benefits in large-scale applications of SAP-cured concrete.</p> <br>