Cost-efficient one-part alkali-activated mortars with low global warming potential for floor heating systems applications

Increasing building energy efficiency is one the most cost-effective ways to reduce emissions. The use of thermal insulation materials mitigates heat loss in buildings, therefore minimising heat energy needs. In recent years, several papers were published on the subject of foam alkali-activated cements with enhanced thermal conductivity. However, on those papers cost analysis was strangely avoided. This paper presents experimental results on one-part alkali-activated cements. It also includes global warming potential assessment and cost analysis. Foam one-part alkali-activated cements cost simulations considering two carbon dioxide social costs scenarios are also included. The results show that one-part alkali-activated cements mixtures based on 26%OPC + 58.3%FA + 8%CS + 7.7%CH and 3.5% hydrogen peroxide constitute a promising cost-efficient (67 euro/m3), thermal insulation solution for floor heating systems. This mixture presents a low global warming potential of 443 KgCO2eq/m3. The results confirm that in both carbon dioxide social cost scenarios the mixture 26 OPC + 58.3 FA + 8 CS + 7.7 CH with 3.5% hydrogen peroxide foaming agent is still the most cost efficient

Role of soluble aluminum species in the activating solution for synthesis of silico-aluminophosphate geopolymers

An aluminosilicate precursor, such as metakaolin, can be transformed into a cement-like geopolymer binder via a phosphate activation approach. This paper identifies the effect of the addition of aluminum species into the phosphate activating solution on the formation of such geopolymers, from the fresh to the hardened state. Activating solutions with Al/P molar ratios of 0, 0.1, and 0.3 were prepared by blending monoaluminum phosphate (MAP) and orthophosphoric acid (OPA). The rheological properties, fluidity, and setting times of the fresh geopolymer pastes and the compressive strength of the hardened geopolymer matrices were studied. Liquid-state 27Al and 31P nuclear magnetic resonance (NMR) measurements for the chemical environments of Al and P, and spectroscopic, thermal, and microscopic analyses revealed that the soluble aluminum in the phosphate activating solution played an important role during the geopolymerization process. Seeding of aluminum species through inclusion in the activating solution allowed a rapid sol/gel transition that improved the rheological properties and setting time of the fresh geopolymer pastes at ambient temperature. However, although the increased concentration of aluminum phosphate oligomers promoted by the soluble aluminum addition contributed to the formation of a compact matrix with high early strength, it hinders the ongoing reaction of metakaolin in the later period, which has a detrimental influence on ongoing strength development beyond 7 days of curing

Nanostructural evolution of alkali-activated mineral wools

Mineral wools are the most widely used building insulation material worldwide. Annually, 2.5 million tonnes of mineral wool waste are generated in the EU alone, and this is a largely unutilised material that is landfilled or incinerated. However, mineral wool wastes are promising precursors for production of alkali-activated cementitious binders due to their favourable chemical and mineralogical composition and high surface area. Alkali-activation is therefore a valuable route for valorisation of large quantities of mineral wool waste. This study resolves the phase assemblage and nanostructure of reaction products formed upon alkali activation of stone wool and glass wool by sodium hydroxide and sodium silicate solutions with X-ray diffraction, electron microscopy and solid state nuclear magnetic resonance spectroscopy experiments probing ^27Al and ^29Si. The stone wool-based alkali-activated binder comprises an amorphous sodium- and aluminium-substituted calcium silicate hydrate (C-(N-)A-S-H) gel, an amorphous sodium aluminosilicate hydrate (N-A-S-H) gel and small amounts of the layered double hydroxide phase quintinite and zeolite F. The glass wool-based alkali-activated binder comprises an amorphous Ca- and Al-substituted sodium silicate (N-(C-)(A-)S–H) gel. Gel chemical composition and reaction kinetics of alkali-activated mineral wools are shown to be dependent on the activating solution chemistry, with reaction rate and extent promoted by inclusion of a source of soluble Si in the reaction mixture. This work provides the most advanced description of the chemistry and structure of alkali-activated mineral wools to date, yielding new insight that is essential in developing valorisation pathways for mineral wools by alkali activation and providing significant impetus for development of sustainable construction materials

Influence of activator type on reaction kinetics, setting time, and compressive strength of alkali-activated mineral wools

Alkali activation is a promising utilisation route for mineral wool wastes, due to suitable chemical composition, high reactivity, and surface area. One key factor in the development of alkali-activated binders is the selection of the suitable alkali activator. Here, the effect of sodium hydroxide, sodium silicate, sodium aluminate, and sodium carbonate solution on the alkali-activation kinetics of two main types of mineral wools, stone wool and glass wool, is investigated. Setting time and compressive strength development results are presented, which are explained and discussed in the context of isothermal calorimeter data obtained at temperature of 40 °C. Sodium hydroxide and sodium silicate solutions provided fast reaction with both mineral wools, evidenced by high heat release, high early strength, and fast setting. The reaction with sodium aluminate solution took several days to initiate, but it produced high compressive strength after 28 days of curing with both mineral wools. Glass wool reacted and hardened rapidly with sodium carbonate solution, but stone wool reacted slowly with sodium carbonate and exhibited a low extent of reaction, likely due to lower extent of reaction of stone wool under less alkaline conditions. These results show that mineral wool alkali activation kinetics and binder gel formation are controlled by the activator type and highlight the importance of choosing the most appropriate activator for each desired application

High strength/density ratio in a syntactic foam made from one-part mix geopolymer and cenospheres

By designing a composite of one-part mix geopolymer and hollow cenospheres, a commercially viable and environmentally-friendly foam was synthesised with a high strength/density ratio. The composite is made of a dry mix powder of geopolymer source materials, sodium silicate alkali activator and cenospheres, which starts to react when mixed with water. As the geopolymer reacts and gains strength over time, the surface of the cenospheres takes part in the reaction and forms a strong bond with the binding matrix. Synchrotron-based Fourier transform infrared microspectroscopy revealed, for the first time, the chemical bonding interaction of the amorphous interfacial layer between the geopolymer and cenospheres. The resulting foam composite gained a strength of 17.5 MPa at a density of 978 kg/m3, which is noticeably higher than that of existing environmentally-friendly lightweight foams made under ambient conditions. The thermal conductivity of the foam was measured to be around 0.28 kW/mK, which is similar to that of foam concrete. This foam produced in this study is found to be lightweight, strong and possess a desirable insulating capacity, while the preparation process of the one-part mix composite is maintained simply by adding water and curing the mixture at an ambient temperature

On the sustainable development of cement

Cement is the most manufactured product on earth. Unfortunately, the manufacture of cement is accompanied by the emission of carbon dioxide gas. Among all manufacturing industry sectors in the UK, the cement industry is the largest CO2 emitter and these emissions are damaging our planet. The sustainable development of cement will allow future generations to develop without being compromised by the cement industry. This work identifies some of the routes to reducing the environmental burden of the cement industry

Microstructure as a key parameter for understanding chloride ingress in alkali-activated mortars

This study aims to evaluate the influence of microstructural properties on the chloride diffusion resistance of alkali-activated materials (AAMs) based on blast furnace slag and/or fly ash, with variable activator doses (represented as Na2O%). Resistance to chloride penetration was tested using accelerated chloride penetration (NT BUILD 443) and chloride migration (NT BUILD 492) tests. Addition of slag to alkali-activated mortars mainly based on fly ash reduced porosity and chloride permeability. Chloride penetration decreased with increasing Na2O%, but porosity and pore structure did not follow the same trend. The pore threshold (dth) and critical pore radius (rcrit) determined by mercury intrusion porosimetry had a good correlation with the chloride diffusion coefficient. Both the quantification of reaction products and the correlation between chloride penetration and pore surface area indicated that physical chloride adsorption on the C-A-S-H/N-A-S-H gel surfaces predominated over chemical chloride binding

Steel corrosion in reinforced alkali-activated materials

The development of alkali-activated materials (AAMs) as an alternative to Portland cement (PC) has seen significant progress in the past decades. However, there still remains significant uncertainty regarding their long term performance when used in steel-reinforced structures. The durability of AAMs in such applications depends strongly on the corrosion behaviour of the embedded steel reinforcement, and the experimental data in the literature are limited and in some cases inconsistent. This letter elucidates the role of the chemistry of AAMs on the mechanisms governing passivation and chloride-induced corrosion of the steel reinforcement, to bring a better understanding of the durability of AAM structures exposed to chloride. The corrosion of the steel reinforcement in AAMs differs significantly from observations in PC; the onset of pitting (or the chloride ‘threshold’ value) depends strongly on the alkalinity, and the redox environment, of these binders. Classifications or standards used to assess the severity of steel corrosion in PC appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage