Della M. Roy

Alkali-activated cements Opportunities and challenges

Abstract Alkali-activated cements as discussed here are those with compositions falling in the Me2O-MeO-Me2O3-SiO2-H2O system. This paper reviews their history of development and discusses their present status. Currently, there are major opportunities for such cements based upon (a) substantial knowledge of properties and mechanisms; (b) good track record of field performance in various applications and; (c) future orientation as environmentally friendly materials in accord with making use of substantial amounts of by-product and waste materials, thereby consuming less energy and generating less waste. The equivalent performance to Portland cement materials is one target for these cements, but, in many cases, the properties of alkali-activated cements actually are superior. It is important for assuring long-term durability to characterize more fully the complex solid phases, including determining the combined state of alkali in the solid hydration products, and of the residual soluble species in the pore fluids as a function of time.

Ettringite and CSH Portland cement phases for waste ion immobilization: A review

Abstract The formation, structure and chemistry of the ettringite and CSH phases of Portland cement have been reviewed as they relate to waste ion immobilization. The purpose of this review was to investigate the use of Portland cement as a host for priority metallic pollutants (Cr, Pb, Ba, Se, Zn, Ag, Hg, As and Cd) as identified by the Environmental Protection Agency and as a host for radioactive waste ions (various isotopes of Am, C, Cs, I, Np, Pu, Ra, Sr, Tc, Th, Sn, U) as identified in 40 CFR 191. Ettringite acts as host to a number of these ions in both the columnar and channel sections of the crystal structure. Substitutions have been made at the calcium, aluminum, hydroxide and sulfate sites. CSH also hosts a number of the waste species in both ionic and salt form. Immobilization mechanisms for CSH include sorption, phase mixing and substitution. The following ions have not apparently been reported as specifically immobilized by one of these phases: Ag, Am, Np, Pu, Ra, Tc, Th and Sn; however, some of these ions are immobilized by Portland cement.

Diffusion of ions through hardened cement pastes

Diffusion of Na+ and Cl− ions through thin hardened paste section was determined as a function of water/cement ratio, curing time, curing temperature, and diffusion temperature. Diffusivities of Na+ ions were smaller than for Cl− ions in the temperature range measured up to 60°C. Activation energies for diffusion were calculated, and contrasts were made with diffusion through a low porosity quartzite rock having minimal surface interaction with the diffusing species.

New Strong Cement Materials: Chemically Bonded Ceramics

New cements developed in recent years have strengths that are greater by an order of magnitude than those of conventional hydraulic cements. These low-temperature materials, whose strengths approach those of many traditional high-temperature ceramics, are termed chemically bonded ceramics. The different routes to generating strong cementitious materials, including warm pressing, chemical modification, high-shear mixing with polymer additions, and the making of fiber and particulate composites, are reviewed. Strength, toughness, durability, impermeability, and abrasion resistance of these new materials have been greatly improved, as have certain electrical and acoustical properties.

Effect of silica fume, metakaolin, and low-calcium fly ash on chemical resistance of concrete

Effects of aggressive chemical environments were evaluated on the mortars prepared with ordinary portland cement (OPC) and silica fume (SF)/metakaolin (MK)/low-calcium fly ash at various replacement levels. The natural adverse chemical environmental conditions were simulated using sulfuric acid, hydrochloric acid, nitric acid, acetic acid, phosphoric acid, and a mixture of sodium and magnesium sulfates. Chemical resistance information was used in conjunction with compressive strength measurements to propose realistic OPC/mineral admixture proportions.

Sulfoaluminate-belite cement from low-calcium fly ash and sulfur-rich and other industrial by-products

The study describes the preparation and characterization of an environmentally friendly cement with performance characteristics similar to those of Portland cement, from a lime kiln bag house dust, a low-calcium fly ash, and a scrubber sludge. Promising preliminary results show the formation of relatively low-temperature phases calcium sulfoaluminate (4CaO-3Al 2 O 3 -SO 3 ) and dicalcium silicate (2CaO-SiO 2 ) at ∼1250°C if nodulized raw meal is used for clinker preparation and at 1175°C if powdered raw meal is used as compared to the ∼1500°C sintering temperature required for Portland cement. Phases of the developed cements were predicted using modified Bogue calculations. Isothermal calorimetric measurements indicate the hydration properties of the cements are comparable to ordinary Portland cement. Mechanical properties and microstructural evaluations also were carried out.

Investigation of relations between porosity, pore structure, and C1− diffusion of fly ash and blended cement pastes

By-product fly ash from coal combustion influences the properties of cement pastes significantly. A study has been carried out to contribute to understanding the behavior and function of fly ash and other substituents in cement pastes. Comparisons are made with the properties of pastes of as-received portland cement and preliminary comparisons made with granulated blast furnace slag cement and silica fume-cement. Relations between the median pore size, porosity and Cl− diffusion of fly ash-cement pastes, and the implications of observations for diffusion mechanisms and the probable pore structure of the blended cement pastes are discussed. Measurements include non-evaporable water (Wn), porosity and pore structure by mercury porosimetry, Cl− transport by a rapid method under the influence of an applied electric field, and the permeability to water. X-ray diffraction phase characterization has been made before and after Cl− diffusion.

27Al and29Si magic angle spinning nuclear magnetic resonance spectroscopy of Al-substituted tobermorites

Solid-state27Al and29Si NMR spectroscopy with magic angle spinning (MAS) of samples was used to study several 1.13 nm tobermorites, most of which were deliberately substituted with aluminium.27Al MASNMR clearly showed that aluminium is tetrahedrally co-ordinated in tobermorite structures. In addition two different aluminium environments resonating at ∼ 57 and 64 ppm from [Al(H2O)6]3+ were detected.29Si MASNMR of pure, anomalous tobermorites showed resonances at −85.7 and −95.7 ppm from tetramethylsilane representing chain middle groups (Q2) and branching sites (Q3), respectively. Anomalous Al-substituted tobermorites, on the other hand, showed two to four resonances representing different silicon environments. One Al-substituted tobermorite showed two resonances at −84.6 and −91.5 ppm which were assigned to Q2(0 Al) and Q3 (1 Al), respectively. In the above tobermorite aluminium appeared to have substituted into branching sites only. Two other Al-substituted tobermorites, however, showed four distinct resonances at ∼ −82.0, −85.2, −92.0 and −96.0 and these were assigned to Q2 (1 Al), Q2 (0 Al), Q3 (1 Al) and Q3 (0 Al), respectively. Thus these two tobermorites showed substitution of aluminium in the chain middle groups as well as branching sites. Another Al-substituted tobermorite which showed a normal thermal behaviour exhibited, as expected, only Q2(0 Al) and Q2 (1 Al) sites resonating at −84.7 and ∼ −80.2 ppm, respectively. No Q3 sites were detected because few or no branching sites are present in this normal tobermorite. The results reported here clearly demonstrate the usefulness of solid-state27Al and29Si MASNMR spectroscopy for the investigation of short-range order in alumino-silicate materials.

Very high strength cement pastes prepared by hot pressing and other high pressure techniques

Abstract Very high strengths approaching zero-porosity values have been measured on materials prepared by hot pressing pastes of conventional cements, using pressures of 25, 000–50, 000 psi at temperatures near 150°C. Typical strengths for hot pressed samples are: 73, 900 (compressive); 59, 300; 6320; 11, 820 (compressive, indirect tensile, and shear, all expressed in psi). High pressure specimens prepared at 100, 000 psi without elevated temperatures resulted in strengths of 46, 100; 4020; and 8400 (compressive, indirect tensile, and shear, respectively). The microstructures as revealed by scanning electron microscopy are very dense, and the hot pressed samples, particularly, are dimensionally stable.

The effect of w/c ratio and curing temperature on the permeability of hardened cement paste

Abstract The importance of concrete permeability is discussed relative to its effect on durability. Experimental studies were made of the effect of water/cement ratio and curing temperature on the porosity, pore size distribution and permeability of cement pastes. Although total porosities of samples cured at 60°C are smaller than in those cured at 27°C, the pore volume larger than 750A radius is greater in the 60°C samples and is related to higher permeabilities also in the latter.