International Concrete Abstracts Portal

The economic use of chemical admixtures depends on supply chains. Therefore, in most regions ins sub-Saharan Africa (SSA), the use of admixtures is not common practice. This amplifies the unfavorable framework for concrete construction such as fragmentary supply chains, high local cement prices, and unfavorable construction site facilities in this region significantly. The use of superplasticizer (SP) and stabilizing agents (STA) can enhance the concrete technology in SSA, since they can disassociate the concrete quality from external boundary influences. After providing a general overview of the peculiarities of the SSA boundary framework, economic concepts are provided, how existing material solutions can be significantly improved by the use of SPs and STAs based on locally available materials such as lignosulphonates and cassava starch. Finally a three step optimization process is described that helps developing flowable concrete based on materials that can be accessed in most locations in SSA.

Macromonomers for MPEG type of PCEs are produced through esterification of methacrylic acid (MAA) with methoxypoly(ethylene glycol) (MPEG) yielding the MPEG-MAA ester. However, PEG impurities present in MPEG may lead to MAA diester (PEG-di-MAA) formation. Such diester can cause crosslinking of the PCE polymer which might reduce its dispersing power. To investigate this effect, MPEG-MAA macromonomers containing 0 – 20 wt. % of PEG-di-MAA diester were used in PCE synthesis. It was found that when the PEG-di-MAA content in the macromonomer exceeds 2 wt. %, then dispersing effectiveness starts to decrease and the solution viscosity of the PCE increases. Surprisingly, incorporation of the diester into the PCE polymer does not occur randomly. Instead, two distinct species of crosslinked PCE molecules (Mw ~ 300.000 and ~ 3 mio g/mol) are formed within the first minutes of copolymerization. Apparently, the crosslinked PCE species counteract the dispersing effect of the main product.

The measure of wettability of cement particles and the evaluation of the type and the strength of active surface sites of clinker particles were carried out to score the efficacy of organic grinding aids. The first analysis, based on the Washburn method, allows measuring the contact angle of cement with different solvents and is directly related to the surface tension. The second analysis, based on the Hammett method, helps to classify the sites that may be responsible of the surface tension. Milled clinker (portland clinker CEM I 52, 5R; 0,025% w/w DEG and TEA) was investigated. Firstly the wetting rates of powder with 4 different solvents (ethanol, n-hexane, toluene, and formamide) were detected using a tensiometer. Then the powder dispersions in a solvent were titrated by acid solutions to get information on the acid/basic character of the surface sites. Techniques and results are shown.

The interaction of methoxy-capped poly(ethylene glycol) polymers (MPEG) and a poly(methacrylic acid) anionic polymer (PMA) from water onto sodium Montmorillonite (Na-Mmt) particles untreated or treated by calcium chloride was studied at 20°C. In the absence of Ca2+, MPEGs are able to intercalate by displacing the water molecules present in the interlayer space, as shown by XRD and TGA analyses. In contrast, the adsorbed amount of PMA remains low. The saturation of Mmt with Ca2+ prevents MPEG intercalation through replacing sodium by a stronger water coordinator in the interlayer space, but slightly increases PMA adsorption possibly through a calcium bonding mechanism. This was confirmed with PCE superplasticizers and Na- and Ca-saturated Mmt clays. Whatever the PCE, a larger amount was consumed on Na-Mmt than on Ca-Mmt. This confirms the occurrence of two consumption mechanisms: (i) a superficial adsorption via cation bonding of the carboxylate groups with anionic sites on clay surfaces, (ii) intercalation of ether units of the grafts in the interlayer space by displacement of water molecules coordinated to the exchangeable cations.

The composition of the aqueous solution in alkali-activated binders, i.e., the high alkalinity and the high ionic strength challenge chemists to design molecules exhibiting the same plasticizing effects as in cementitious materials. The highest difficulty probably lies in alkali-silicate activated systems due to the presence of multivalent silicate oligomers in solution. Reported here are new insights about the adsorption of polymers in presence of various concentrated electrolyte solutions in order to mimic the harsh conditions present in geopolymer pastes. In order to eliminate the problem of the reactivity of such systems, TiO2 nanoparticles were used as a model substrate. The adsorption of polymer molecules as well as the specific adsorption of monovalent and divalent ions is revealed. Those results are compared to the rheological characteristics of alkali-hydroxide or alkali-silicate activated geopolymers. The conclusions which can be drawn from the model system fit qualitatively very well with the classical slump tests done on real systems.