International Concrete Abstracts Portal

Chemical admixtures play a central role in modern concrete materials and technologies. In conjunction with mineral additives such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability. Chemical admixtures have also assisted in developing new concrete technologies, for example, concrete pumping and self-leveling, underwater concreting and shotcreting. Chemical admixtures have further promoted the use of secondary industrial materials (blast furnace slag and fly ash) in cementitious systems, contributing to resource conservation and environmental sustainability. In the continuing quest for more cost-efficient and environmentally acceptable materials and technologies, it may thus be expected that chemical admixtures will continue to play an important role in future generations of concrete. Probing into the future, how will concrete chemical admixtures evolve in the coming decades? What trends can be anticipated in future developments and use of these admixtures ? What will be the driving influences for these developments? This paper addresses some of the issues that are considered relevant driving forces to promote changes in the use of currently available chemical admixtures, or in the development of new admixtures. The trends already apparent in cementitious materials and concrete applications provide a reasonable basis for proposing probable trends in the evolution of concrete admixtures into the 2 lSt Century.

Superplasticized high-volume fly ash concretes with 50% of portland cement replacement were made by incorporating two different chemical admixtures based on sulfonated naphthalene (SN) or acrylic polymer (AP). Portland cement with a Blaine fineness of about 400 or 500 m2/Kg was replaced by 50% of ground or un-ground fly ash. The content of the cementitious material (Portland cement + fly ash) was about 470 kg/m3. The concretes with SN were manufactured with a slump in the range of 190-200 mm, whereas the slump of the concretes with AP was in the range of 220-230 mm. Due to the different effect of the superplasticizers, the water-cementitious material ratio (w/cm) was 0.32 or 0.29 for the SN or AP admixture, respectively, although the dosage was slightly lower for the latter. Cube specimens, 150 x 150 mm in size, were cured at 5°C or 20°C and compressive strength was measured at 1 to 90 days. Due to the lower w/cm, the strength of the concretes with the acrylic polymer was significantly higher with respect to those with SN. The better performance of the AP superplasticizer, in terms of compressive strength, was obtained at early and later ages independently of the curing temperature (5 and 2OC), and the fineness of the portland cement and fly ash. Due to the lower w/cm of concrete with the AP admixture with respect to those with the SN superplasticizer, the durability of high-volume fly ash concrete can be improved in terms of lower penetration rate of CO2 or chloride ions.

Dispersion mechanisms of superplasticizers have received much attention over the past years. Recent developments have brought very efficient superplasticizers where the dominant stabilizing mechanism is thought to be via steric repulsion. These new superplasticizers contain an adsorbing backbone onto which non adsorbing side chains are grafted with the objective of getting them to stretch out into the solution from the cement particle surface and induce the steric repulsion upon approach of other particles. Another feature of these polymers is that they induce only very small zeta potentials. Calculations of interaction energies indicate that these polymers act predominantly through steric repulsion. However, the same calculations could lead to the conclusion that all polymers can only act through steric repulsion. The calculation of the steric and electrostatic contributions are greatly dependent on the polymer adsorption conformation and the distribution of charge at the particle surface associated with these adsorbed polyelectrolytes. Many of the assumptions made in calculating interparticle forces are not necessarily good approximations for polyelectrolytes. This paper discusses the limits of the approximations currently used in such calculations and presents a more accurate model for the calculation of these forces. The main result, applicable for a wide range of superplasticizers, is that both electrostatic and steric repulsions should be taken into account, provided the electrostatic charge can be assumed to lie at the outer-bound of the adsorbed layer of superplasticizers. Such information is of primary importance for understanding and solving cement and superplasticizer incompatibilities, as well as for developing novel products.

This research was aimed at the use of agroresources as admixtures for mortar and concrete. Thirteen modified starches from potato, wheat, corn and waxy corn were investigated. They were compared to methylhydroxyethylcellulose (MHEC) and Welan gum for the following properties: thicknening effect, water retention, setting time and 28-day compressive strength. The study was carried out on standard mortars and the starch dosage was either 0.75 % or 0.25 % of the total dry matter (sand + cement). The results obtained show that the recommended dosage is 0.25 %, which is still economic compared to MHEC and Welan gum. The best performances were obtained with waxy corn starches and, to a less extent, with potato starches. The latter performed well except for water retention.

Certain di-phosphonate terminated monofunctional polyoxyethylene are widely used as concrete superplasticizers. In order to understand its action mode, its effect on hydration of pure tricalcium silicate suspensions has been investigated by conductimetry, isothermal calorimetry and ionic and total organic carbon (TOC) analyse of the liquid phase. The polyoxyethylene di-phosphonate modifies the nucleation and growth process of C-S-H by reducing the number of initial nuclei, decreasing the growth rate in the accelerated period and increasing the rate during the difision limited period comparatively to control samples. The evolution of ionic concentrations in the solution during hydration reveals an apparent increase of the critical supersaturation required to nucleate both C-S-H and portlandite. This is due to the formation of a calcium-diphosphonate polyox complex. The calcium complexation constant and pKa of the phosphonate polyox have been determined from pH and conductivity studies of calcium-hydroxide and sodium-hydroxide polymer solutions and well account for portlandite solubility in presence of phosphonate. The polyox phosphonate does not seem to adsorb on C3S surface but is rather adsorbed on C-S-H. This is probably the origin of the decrease of the growth rate leading to the modification of the texture of C-S-H and subsequently the modification of the rate during the diffusion limited period.