International Concrete Abstracts Portal

Results describing investigations of the kinetics of hydration and struc-ture formation of cement systems (C3 S, C3 A, C3 A + gypsum, and portland ce-ment) in the presence of naphthaleneformaldehyde type superplasticizers (SNF ) of different compositions are discussed Using the individual components of SNF sodium polymethylenenaphthalenesulfonates ( PNS ) with polycondensation degrees ranging from 2 to 17, the basic dependences of the kinetics of the processes on the average molecular mass of the admixture ( Mn were shown In the case of C3 S the retarding effect of SNF on hydration and structure formation processes depends lineally on Mn , as a general rule, whilst for C3 A the retarding effect is more pronounced for mid-condensated components of superplasticizer. This difference is due to the different adsorption mechanism of the superplasticizer on the above-mentioned minerals. The unusual form and the analogy of ther-mokinetic curves for C3 A and C3 A - gypsum hydration in the presence of PNS are the first experimental proof that organic-mineral compounds are formed in these systems.

The importance of molecular weight as a parameter influencing the performance of water-reducing admixture in cement paste was investigated. The sulphomethylolated ALCELL@ lignin sample was divided into four fractions of different molecular weight. The fractionation was performed by membrane ultrafiltration on a small laboratory unit. Advanced hydrophilic membranes with a 1, 10 and 50 kd nominal MW cut-off and effective area of 63 cm2 were used. Molecular weight distribution, average molecular weights (Mn , M,, Mz and Mz+1 and polydispersity of the original sample and its fractions were determined by high-performance aqueous size-exclusion chromatography. The five samples, prepared as 20% aqueous solutions, were used as water-reducing admixtures. Their influence on fresh cement pastes was examined using the torque and mini-slump tests. The torque test was performed to investigate the fluidifying and retarding effects of the samples. Changes in torque resistance were monitored by a computerized system giving an indication of the degree of dispersion and set retardation. The mini-slump test was used to determine the workability and air-entrainment of the fresh cement paste. The dependence of the properties of the fresh cement pastes on the lignosulphonate molecular weight was studied. It was found that the lower the molecular weight of the lignosulphonate sample the higher the set retardation of the cement paste. The highest fluidification and lowest air-entrainment were observed when the intermediate molecular weight samples were used. The optimum molecular weight fraction in each particular experiment was always more effective than the original unfractionated sample. Fractions with extremely low efficiency were also identified. The data obtained can be utilized in optimization of lignosulphonate admixture performance.

A deflocculation test based on the method proposed by A. Verhasselt and J. Pairon has been applied to different mixes of cement-admixtures in order tc distinguish quickly between plasticizer and superplasticizer. The admixtures used were commercial products containing different proportions of lignosulphonates (L), sulphonated naphthalene formaldehydecondensate (SNF) and sulphonated melamine formaldehyde condensate (SMF). Three different cements were employed: a low tricalcium aluminate (C3 A = 2.5 %), a high alkali content (Na2 O equivalent = 0.95%) and a high fineness Portland cement (S.A. Blaine = 5002 cm2/gr). In addition, the minimum active dosage of superplasticizer was calculated. The influence of the type of cement in the minimum active dosage is clearly shown by the results obtained through this method.

Polybasic organic phosphate and lignosulphonate were combined in appropriate proportions to be a kind of high-range retarder-plasticizer(KM) for use of higher temperatures. Various retarders have been used in ordinary concrete and roller-compacted concrete(RCC)for testing under 45 “C and 25% relative humidity. Rheology and time of setting of fresh concrete and mechanical properties of hardened concrete, allowance of exposure time of bond between two construction layers of RCC,etc. were investigated. Simulated testing for Three Gorges Project was also carried out by testing a t the Development General Co. and the Testing Center of Gezhouba group.

This paper describes the effect of a lithium nitrate-based admixture on the hydroxide ion concentration of the pore solution of hydrating pastes made from portland cement and water. No significant increase in the hydroxide ion concentration results from using this admixture in the mix, which is thus different than any published study with any other lithium compound. It has been reported that underdosing with lithium salts can increase the expansion due to ASR. The authors propose that this is mainly due to increases in hydroxide concentration observed with other lithium salts and therefore this admixture will not show such an effect. Mortar bar tests with the new admixture verify the hypothesis that the lithium nitrate-based admixture does not increase expansion at any dose. This is then a much safer admixture to use in the field with respect to risk from damaging ASR expansions. It is also much safer to handle than lithium hydroxide-based admixtures since solutions of lithium nitrate are much closer to neutral pH than lithium hydroxide solutions.