Lucia Alarcon-Ruiz

Bio: Lucia Alarcon-Ruiz is an academic researcher. The author has contributed to research in topics: Thermal decomposition & Portlandite. The author has an hindex of 1, co-authored 1 publications receiving 559 citations.

The use of thermal analysis in assessing the effect of temperature on a cement paste

Abstract: Upon heating, the cement paste undergoes a continuous sequence of more or less irreversible decomposition reactions. This paper reports studies on a cement paste fired to various temperature regimes up to 800 °C in steps of 100 °C for a constant period of 24 h. This work has been carried out using thermal analysis technique to study the effect of temperature in the mineralogical composition of cement hydration products. The thermal decomposition of the cement paste is analysed with the thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) curves. Such techniques can be used to determine fire conditions and the consequent deterioration expected in the cement paste. Therefore, the aim of this work is to have a better knowledge of the reactions that take place in a cement paste during a fire and thus to be able to determine the temperature history of concrete after a fire exposure. The results show that even if the dehydroxylation reaction is reversible, the portlandite formed during the cooling has an onset temperature of decomposition lower than the original portlandite and can thus be considered as a tracer for determining the temperature history of concrete after a fire exposure.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

Abstract: The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Mix design and properties assessment of Ultra-High-Performance Fibre Reinforced Concrete (UHPFRC)

Abstract: This paper addresses the mix design and properties assessment of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC). The design of the concrete mixtures is based on the aim to achieve a densely compacted cementitious matrix, employing the modified Andreasen & Andersen particle packing model. One simple and efficient method for producing the UHPFRC is utilised in this study. The workability, air content, porosity, flexural and compressive strengths of the designed UHPFRC are measured and analyzed. The results show that by utilizing the improved packing model, it is possible to design UHPFRC with a relatively low binder amount. Additionally, the cement hydration degree of UHPFRC is calculated. The results show that, after 28 day of curing, there is still a large amount of unhydrated cement in the UHPFRC matrix, which could be further replaced by fillers to improve the workability and cost efficiency of UHPFRC.

Development of an eco-friendly Ultra-High Performance Concrete (UHPC) with efficient cement and mineral admixtures uses

Abstract: This paper addresses the development of an eco-friendly Ultra-High Performance Concrete (UHPC) with efficient cement and mineral admixtures uses are investigated. The modified Andreasen & Andersen particle packing model is utilized to achieve a densely compacted cementitious matrix. Fly ash (FA), ground granulated blast-furnace slag (GGBS) and limestone powder (LP) are used to replace cement, and their effects on the properties of the designed UHPC are analyzed. The results show that the influence of FA, GGBS or LP on the early hydration kinetics of the UHPC is very similar during the initial five days, while the hydration rate of the blends with GGBS is mostly accelerated afterwards. Moreover, the mechanical properties of the mixture with GGBS are superior, compared to that with FA or LP at both 28 and 91 days. Due to the very low water amount and relatively large superplasticizer dosage in UHPC, the pozzolanic reaction of FA is significantly retarded. Additionally, the calculations of the embedded CO2 emission demonstrate that the cement and mineral admixtures are efficiently used in the developed UHPC, which reduce its environmental impact compared to other UHPCs found in the literature.

Effect of nano-silica on the hydration and microstructure development of Ultra-High Performance Concrete (UHPC) with a low binder amount

Abstract: This paper presents the effect of nano-silica on the hydration and microstructure development of UltraHigh Performance Concrete (UHPC) with a low binder amount. The design of UHPC is based on the modified Andreasen and Andersen particle packing model. The results reveal that by utilizing this packing model, a dense and homogeneous skeleton of UHPC can be obtained with a relatively low binder amount (about 440 kg/m 3 ). Moreover, due to the high amount of superplasticizer utilized to produce UHPC in this study, the dormant period of the cement hydration is extended. However, due to the nucleation effect of nano-silica, the retardation effect from superplasticizer can be significantly compensated. Additionally, with the addition of nano-silica, the viscosity of UHPC significantly increases, which causes that more air is entrapped in the fresh mixtures and the porosity of the hardened concrete correspondingly increases. In contrary, due to the nucleation effect of nano-silica, the hydration of cement can be promoted and more C–S–H gel can be generated. Hence, it can be concluded that there is an optimal nano-silica amount for the production of UHPC with the lowest porosity, at which the positive effect of the nucleation and the negative influence of the entrapped air can be well balanced.

Hydration and properties of nano-TiO2 blended cement composites

Abstract: Two types of nano-TiO2 particles were blended into cement pastes and mortars. Their effects on the hydration and properties of the hydrated cement pastes were investigated. The addition of nano-TiO2 powders significantly accelerated the hydration rate and promoted the hydration degree of the cementitious materials at early ages. It was demonstrated that TiO2 was inert and stable during the cement hydration process. The total porosity of the cement pastes decreased and the pore size distribution were also altered. The acceleration of hydration rate and the change of microstructure also affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of the nano-TiO2. The compressive strength of the mortar was enhanced, practically at early ages. It is concluded that the nano-TiO2 acted as a catalyst in the cement hydration reactions.