International Concrete Abstracts Portal

The effects of Na2S04 addition to cement pastes containing PNS superplasticizer with different molecular weight were investigated in different parameters of cement pastes, such as the rheological properties, the adsorption of PNS superplaticizer, and the development of the heat of cement hydration. Na2S04 addition significantly improves the fluidity of cement pastes made with a low-alkali-cement and a high-molecular-weight PNS. On the contrary, Na2S04 addition to a high-alkali-cement has a negative effect on the fluidity of cement pastes in the presence of PNS superplasticizer. The effect of Na2S04 addition on the fluidity of cement pastes made with a low-molecular-weight PNS is relatively small. When sodium sulfate is added to the low-alkali cements in order to adjust the soluble alkali content, it is confirmed that the cement pastes containing the high-molecular-weight PNS have better fluidity than those containing the low- molecular-weight PNS. The Na2S04 addition reduces the amount of PNS adsorbed on cement particles and the effectiveness of Na2S04 in reducing the amount of PNS adsorbed is independent to the molecular weight of the PNS. The Na2S04 addition to low-alkali-cements retards cement hydration during induction period in the presence of a high-molecular-weight PNS and then accelerates it during the acceleration period. However, the Na2S04 addition to high-alkali-cements just accelerates cement hydration and this acceleration effect is not dependent to the molecular weight of PNS super-plasticizer.

Carboxylic acid ester superplasticizers (CAE) consist of polymers in which hydrophilic polyoxyethylene ester chains (CE) are grafted onto a main chain bearing carboxylic groups (CA). In the present work, CAE copolymers characterized by different molar carboxylic acid - carboxylic ester ratios (CAKE) were synthesized and evaluated as super-plasticizers by using two different cements. The efficiency of CAE copolymers as superplasticizers was found to be dependent on the carboxylic acid - carboxylic ester ratio (CAKE) and the optimum CA/CE value in order to attain the best flowability was different for the two cements. Adsorption measurements indicated an increase of adsorption onto both the cements by increasing CAKE. On the other hand, zeta potential of cement pastes was not substantially influenced by the addition of the different superplasticizers. The results of the present work seem to indicate that both adsorption and steric stabilization are the main factors which determine the performances of CAE as superplasticizers and that CA/CE is an important parameter influencing the cement/CAE superplasticizer compatibility.

The results presented and discussed are those of a study aimed at understanding how and why tint heterogeneities occur at the surface of concrete. In particular, the study involved laboratory tests on mortars which were aimed at examining the effect of the following parameters on the phenomenon of tint heterogeneity: W/C, super-plasticizer content, cement alkali content, mould type, and curing conditions. The results appear to show that two of the parameters aforementioned have a major influence on tint homogeneity: the homogeneity of the binder volume fraction distribution resulting from the use of super-plasticizers, and the absorption properties of the mould.

A new conductivity method is proposed to monitor the behavior of fresh cement-based materials during the consolidation, setting, and early hardening periods. The method relies on differences in electrical conductivity measured at different depths, and as function of time, to evaluate variations in the local composition (solids, fluids) of the material. To perform these measurements in a practical and cost-effective way, simple disposable conductivity probes were designed with multiple electrodes, after initial work with similar laboratory-type probes. The approach was tested with several cement-based systems ncluding grouts, mortars, and concrete. During the dormant period, the conductivity readings reflect changes in the homogeneity of the samples as a function of time, which are qualitatively well related to the bleeding-segregation behavior of the cement-based system. From the conductivity data obtained as a function of sample depth, a stability index could be defined using the least-squared deviation from the mean conductivity at a given time. The time-dependence of such a stability index reflects the evolution of the bleeding and segregation phenomena in the material. The multi-electrode conductivity approach also yields other valuable information, namely a determination of the initial setting, and a reasonable estimate of the rate of strength development during the early stages of the hardening period.

The time dependence of the interaction between hydrating cement particles and a poly-naphthalene sulfonate (PNS) superplasticizer has been investigated using rapid response calorimetry and other physico-chemical approaches. The study focuses on the processes which occur during the first instants following the immersion of the cement particles into the solution, in the presence, or absence, of the PNS superplasticizer. Specifically, the investigation aims to elucidate the dominant phenomena in the coupled processes taking place in the water/cement/sulfate/PNS system, and the consequences of these phenomena on 1- the reaction rates and products and 2- the cement-superplasticizer compatibility The systems discussed here are cement pastes at W/C=3 containing cements having highly different alkali sulfate contents and a normal PNS super-plasticizer. The kinetics of the initial reactions (O-30 min.) are monitored by fast response adiabatic calorimetry; the superplasticizer adsorption, and variations in the ionic composition of the interstitial solution, are also determined at short time intervals during the same period. The evolution of the hydrate phases as a function of time is monitored through XRD analyis.