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High-strength-performing concretes (HSPC) have been used extensively due to new building requirements and their special properties suitable for modern cities. Superplasticizers (SPs) are essential components in the mix design of these concretes since they control their fresh properties and improve durability. In practice, superplasticizers are used to achieve a desired workability without increasing the water content by dispersing agglomerated cement particles. The aim of this research was to investigate the effectiveness of a blended polycarboxylate (PCE) superplasticizer on the rheological behavior for three different cements. It was found that two SP agents with the same molecular structure but with different weight and side-chain length provided an SP product that had a greater effect on the rheological properties of the cement paste. Yield stress values remained unaffected in the presence of blended SP with an SP fraction above 50% and with long side chains while its adsorption ability increased with the decrease in SP fraction with shorter side chains and lower molecular mass.

A busca por estruturas que apresentam durabilidade e comportamento mecânico cada vez superiores tem sido alvo de pesquisas no meio técnico científico da construção civil. Neste contexto, as aplicações de nanomateriais em materiais cimentícios tradicionais têm atraído muitas atenções nos últimos anos, principalmente porque algumas propriedades destes materiais foram melhoradas pela aplicação de concentrações adequadas de nanotubos de carbono (NTC) e nanofibras de carbono (NFC). O entendimento de como a incorporação destes nanomateriais altera as propriedades das pastas cimentícias tem importantes implicações científicas quanto à resistência mecânica e à durabilidade, sobretudo devido aos impactos nas propriedades reológicas da pasta, tanto após a mistura quanto em decorrência de sua hidratação. No entanto, foi relatado na literatura que as propriedades reológicas dos compósitos cimentícios nanoestruturados tiveram a trabalhabilidade alterada. Assim, neste trabalho foram avaliadas as propriedades reológicas de pastas de cimento no estado fresco, fabricadas com NTC/NFC sintetizados diretamente sobre o clínquer, por meio do mini-slump de Kantro e reometria rotacional de fluxo. Os testes reológicos foram realizados, aos 30, 60 e 90 após a mistura, em pastas produzidas com cimentos Portland CPIII-40 RS, CPV-ARI e Classe G. Os teores de NTC/NFC adotados foram de 0,15% e 0,30% com rendimentos de 10,5% e 25,0%, sem aditivos. Os dados reológicos foram ajustados pelos modelos matemáticos de Bingham e Bingham modificado. Para relações água/cimento superiores a 0,4, os resultados não mostraram alteração no comportamento reológico das pastas contendo NTC, independentemente do tipo de cimento bem como do teor de NTC. Os resultados revelaram também uma boa correlação do comportamento reológico no estado fresco da pasta de cimento nanoestruturada ao longo do tempo The search for concrete structures with high performance in terms of mechanical strength and durability has been the target of extensive research of the construction industry. In this context, the use of nanomaterials in traditional cementitious composites has attracted more attention in recent years, mainly because of some properties of these materials can be enhanced with the appropriate concentration and dispersion of carbon nanotubes (CNT) and carbon nanofibers (CNF). Understanding how the incorporation of these nanomaterials can modify the properties of cementitious pastes has important implications, since their impact on rheological properties, either right after mixing and during the initial time of hydration can affect mechanical strength and durability. However, it has been reported in the literature that the rheological properties of nanostructured cementitious composites have changed their workability. In this scenario, the aim of this investigation was to evaluate the rheological behavior of fresh pastes produced with different types of Portland cement manufactured with CNT/CNF synthesized directly on clinker. The mini-slump of Kantro and rotational flow rheometry were applied in this evaluation. Rheological tests were performed 30, 60 and 90 min after mixing in pastes produced with Portland cements CPIII-40 RS, CPV-ARI and Class G. The CNT/CNF content was 0.15% and 0.30% with respect to cement content. No chemical admixtures were used to disperse the NTC. The water/cement ratio varied between 0.4 and 0.5. The rheological behavior was adjusted to Bingham and modified Bingham mathematical models. For a water/cement ratios greater than 0.4, the results indicated no changes in rheological behavior of pastes containing CNT, regardless of the cement type and CNT content. The results also reveal a good correlation of fresh nanostructured cement paste rheological behavior in over time

The current compressive strength test for cement classification is performed under a fixed water to cement ratio according to the Brazilian and European standards, regardless the consistency obtained. However, under practical conditions the amount of water required is related to the rheological need to obtain and maintain a desired workability. Intrinsic cement characteristics influence the water demand such as the cement particle’s granulometry, specific surface area, density, the presence or not of water reducing admixtures, chemical and mineralogical nature of raw materials, etc., influence particles agglomeration state. Because water demand influences the mechanical properties of cement based products, the compressive strength class under a fixed water to solids ratio specified by the standards may not be representative for the user. The present work investigates the influence of mixing conditions on mini-slump spread results, a test that has been used for many years but never standardized. Cement paste samples were produced with varied mixing conditions (time and rotation speed) using a conventional stirrer and subjected to mini-slump spread test immediately after mixing and at fixed hydration times. Results show that mixing and hydration time do influence on mini-slump spread. At lower rotation speeds, results variability increases. Under fixed time after first contact with water, increasing rotation speed leads in a reduction of results variability and increase the test’s repeatability on cement pastes. Resumo O ensaio de resistência do cimento, segundo às normas brasileiras e europeia atuais, é realizado com uma relação água/cimento fixa em massa, independente da consistência obtida. No entanto em condições normais de uso, a quantidade de água é uma variável definida com a finalidade de manutenção da consistência dos materiais cimentícios para obter trabalhabilidade adequada. Características intrínsecas dos ligantes, como distribuição de tamanho de partículas, área superficial especifica, densidade e natureza química ou mineralógica, ou extrínsecas, como presença de aditivos, temperatura ambiente e da água, tempo e energia de processamento, etc., podem afetar de forma distinta o estado de aglomeração e, consequentemente, a necessidade de líquido para a mistura. Como a demanda de água influencia a resistência mecânica dos produtos, a utilização da relação água/cimento fixa torna, em termos práticos, a classificação do cimento pela resistência pouco significativa para o usuário. Neste trabalho foi investigada a influência das condições de mistura na repetibilidade do ensaio de mini-slump, bastante usado para avaliação da quantidade de água nas composições, mas não normatizado. Pastas de uma mesma amostra de cimento foram misturadas por diferentes tempos e/ou rotações, usando um agitador mecânico convencional, e submetidas ao ensaio de mini-slump imediatamente após o ensaio ou após o mesmo tempo de contato com a água. Observou-se influência significativa nos resultados: quanto maior a rotação ou a manutenção do tempo de mistura, menor a variabilidade do espalhamento mini-slump sendo que a influência é mais sensível para diferenças de rotação.

Growing demand for creating more sustainable and durable concretes lead to the increased usage of various cementitious materials and chemical admixtures in the mixtures. However, the increased usage of these components resulted in more complex mixtures that sometimes cause unexpected incompatibility problems.This report summarizes the results of the investigation of the parameters that may lead to workability problems, early age hydration irregularities and difficulties in achieving quality air void system in both plain and fly ash cementitious mixtures. The present research work was performed in three major phases and the statistical modeling was used to aid in interpretation. Phase I involved evaluation of more than 100 different paste and mortar mixtures with respect to potential slump loss and hydration irregularities. The results showed that cements with high C(3)A and low SO(3)content were more prone to incompatibility problems. It was also observed that mixes with lignin based water reducing agent (WRA) had higher tendency for rapid stiffening than mixes with polycarboxylate type superplasticizer (PCSP). Increased replacement of cement by class C ashes resulted in the development of abnormal secondary peaks in semi-adiabatic calorimetry curves and accelerated the setting behavior.The focus of phase II was on identifying material combinations that can result in problems related to air void generation and stability. The experiments were conducted on 18 different systems and included determination of foam drainage and foam index parameters. The results show that the amount of air entrainers required to obtain target air percentage, increased with the increase in the fly ash content in the mixture. Lignin based WRA had, in general, a higher air entraining effect than the super-plasticizer when used in combination with air entrainers. Also, five out of the six mixtures with most unstable air void system, identified using the foam drainage experiments, contained the PCSP.The third (and final) phase of the study involved production of 10 concrete mixtures to verify the incompatibility findings from the paste and mortar experiments performed in phases I and II. The observations from the concrete testing were in agreement with the findings from the paste and mortar testing.Statistical modeling (performed using the material properties and results from phase I) identified the total C(3)A, SO(3)A and Na(2)0(=) contents of the binder system along with dosage of PCSP (if present in the mixture) as statistically significant in predicting the initial set time and area of spread (measured using the mini-slump test).

The properties of high performance concrete, mainly in the fresh state are governed by the flow behaviour of the paste phase, which is controlled by the dispersion of cement particles by the superplasticizer. In this paper, experimental procedures for evaluating the flow behaviour of cement paste with different types and dosages of superplasticizers are studied. The rheological nature of the paste is represented through the Bingham and Herschel-Bulkley models. It is observed that the Marsh cone flow time, mini-slump spread and the rheological parameters show the same trend with change in dosage of superplasticizer. Also, the rheological parameters increase with time and rate of increase is more significant at dosages less than the saturation dosage, as determined from the Marsh cone test. It is confirmed that the Marsh cone flow time can be predicted using an appropriate model. � 2008 RILEM.