Advances in Applied Ceramics: Structural,Functional and Bioceramics

The main product of Portland cement hydration is C-S-H. Despite constituting more than half of the volume of hydrated pastes and having an important role in strength development, very little is known about the factors that determine its morphology. To investigate the relationship between the chemical composition, silicate anion structure and morphology of C-S-H, samples were synthesised via silica-lime reactions and by the hydration of C₃S under controlled lime concentrations and with/without accelerators. The silicate anion structure of the samples was studied by ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and the morphology and chemical composition by TEM and SEM. All samples prepared via silica-lime reactions with bulk Ca/Si up to 1.5 were foil-like. The hydration of C₃S at fixed lime concentration yielded foil-like C-S-H for [CaO]<22 mmol L⁻ and fibrillar C-S-H for [CaO]>22 mmol L⁻¹. A relationship between the silicate anion structure and the morphology of C-S-H was found for the samples fabricated with accelerators.

The extent to which the composition of the reaction mix affects the formation of a biphasic C–(A)–S–H/N–A–S–H geopolymer framework, and how the interaction of these phases affects geopolymer microstructure, can only be studied by strict stoichiometric control. Stoichiometrically controlled geopolymers containing both C–(A)–S–H and N–A–S–H gels are produced here by reaction of a sodium silicate solution with calcium aluminosilicate powders, which were synthesised via a novel solution-polymerisation method utilising polyethylene glycol as a polymer carrier to sterically inhibit movement of precursor cations. Increased Ca content in the reaction mix appears to promote greater formation of a C–(N)–(A)–S–H gel, while reduced Ca content and increased Al and Si content promote greater formation of an N–A–S–H gel in addition to the main C–(A)–S–H reaction product. The stoichiometrically controlled geopolymers constitute a chemically simplified model system through which the nature of the biphasic C–(A)–S–H/N–A–S–H gels present in alkali activated binders can be studied.

Geopolymer binders are generally formed by reacting powdered aluminosilicate precursors with alkali silicate activators. Most research to date has concentrated on using either pulverised fuel ash or high purity dehydroxylated kaolin (metakaolin) in association with ground granulated blast furnace slag as the main precursor material. However, recently, attention has turned to alternative calcined clays that are abundant throughout the globe and have lower kaolinite contents than commercially available metakaolins. Due to the lack of clear and simple screening protocols enabling assessment of such geological resources for use as precursors in geopolymer systems, the present paper presents results from experimental work that was carried out to develop a functional binder using materials containing kaolinite taken from the Interbasaltic Formation of Northern Ireland. The influence of mineralogy has been examined, and a screening process, using three Interbasaltic materials as examples, that will assist in the rapid selection of suitable geopolymeric precursors from such materials is outlined.

The fire performance of magnesium potassium phosphate cement (MKPC) binders blended with fly ash (FA) and ground granulated blast furnace slag (GBFS) was investigated up to 1000°C using X-ray diffraction, thermogravimetric analysis and SEM techniques. The FA/MKPC and GBFS/MKPC binders dehydrate above 200°C to form amorphous KMgPO₄, concurrent with volumetric and mass changes. Above 1000°C, additional crystalline phases were formed and microstructural changes occurred, although no cracking or spalling of the samples was observed. These results indicate that FA/MKPC and GBFS/MKPC binders are expected to have satisfactory fire performance under the fire scenario conditions relevant to the operation of a UK or other geological disposal facility

We present a multimodel simulation approach, targeted at understanding the behaviour of comminution and the effect of grinding aids in industrial cement mills. On the atomistic scale, we use molecular dynamics (MD) simulations with validated force field models to quantify elastic and structural properties, cleavage energies as well as the organic interactions with mineral surfaces. Simulations based on the discrete element method (DEM) are used to integrate the information gained from MD simulations into the clinker particle behaviour at larger scales. Computed impact energy distributions from DEM mill simulations can serve as a link between large scale industrial and laboratory sized mills. They also provide the required input for particle impact fragmentation models. Such a multiscale, multimodel methodology paves the way for a structured approach to the design of chemical additives aimed at improving mill performance.

From rational and micromechanical arguments on energy levels of particle/particle and particle/ fluid interactions, we first provide here a conceptual diagram of predominant interactions within flowing cement pastes under simple shear in steady state as a function of shear rate and solid volume fraction. We focus then on what is universal and common to all polymers used in the cement construction industry and describe at first order the main changes in the underlying physics occurring when such molecules are added to a cement suspension. We finally discuss the upscaling between cement paste rheology and concrete fresh properties.