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Portland cement is one of the principal sources of anthropomorphic CO2 emissions. It is estimated that cement production contributes up to 10% of greenhouse gas emissions and annual cement production over 4 billion tons. This has led to the development of a range of alkali activated materials (AAM), the most common precursor materials being class F fly ash and blast furnace slag. At present Class C Fly Ash is not widely utilized as an AAM due to the chemical composition and activation requirements. However, initial research on high Calcium German Class C Fly Ash suggests that the material may have potential for application as an AAM. This paper reports the development of ambient cured alkali activated mortar optimised by varying the alkali modulus and w/b ratio. The evolution of the mechanical and microstructural properties is reported over the initial 28 day period. Compressive strength in excess of 10 MPa at 7 days and 15 MPa at 28 days was achieved at ambient temperature. Similar strengths were observed for both 10% and 15% dosage but as dosage increases the optimal Alkali Modulus reduces.
Due to environmental concerns regarding the use of Portland cement as the principal binder material in concrete and mortar it is imperative to identify alternative materials that could reduce the carbon footprint of the construction industry. One alternative to address these issues is the use of alkali activated materials, in particular, when based on waste streams that currently have no or only limited industrial application. This paper reports a preliminary study into the synthesis of geopolymer mortar utilizing Brown Coal Fly Ash. The ash had a CaO content of ~39%, indicating that synthesis at ambient or low temperature may be feasible. The paper reports initial trials on the effect of curing temperature, ambient to 120 °C, on the mechanical properties of the mortars produced. The results showed that ambient cured mortar achieved a compressive strength of 6.5 MPa at 3 days. A curing temperature of 60 °C gave the optimum results with a compressive strength of almost 20 MPa and a flexural strength of 3.5 MPa obtained.
Mortars for application on concrete, e.g. repair mortars or protective coatings, need to have a durable bond to the substrate. This bond is determined by the adhesion between the two materials and by the differential deformations of the mortar and the substrate. In the present contribution, the hygric deformations (shrinkage/expansion) of novel one-part alkali-activated mortars and their bond to concrete substrates are studied. Shrinkage of the mortars was studied at 50 % r.H., while expansion was studied on mortars stored over an open water surface (> 99% r.H.). The bond behaviour was studied by pull-off tests according to DIN EN 1542 and by optical microscopy.
The alkali-activated mortars exhibit hygric deformations much lower than the deformations of an established, commercial mortar for sewer maintenance that was tested as reference in parallel with the alkali-activated mortars. The bond behaviour of the alkali-activated mortars depends strongly on their mix-design and curing. Optical microscopy showed that in the
mortars with lower bond strength, cracks developed in the mortar during curing. Mortars with appropriate mix-design and curing did not exhibit cracking, and their pull-off strength (up to >3 MPa) conformed to the requirements of relevant standards.
Mixing and curing of geopolymers and related alkali-activated materials without storage and handling of highly alkaline solutions possesses advantages regarding safety and economic viability. One possible approach is to produce these materials from solid silica feedstocks and solid sodium aluminate, and subsequent mixing with water. We present a comparison between geopolymers and aluminosilicate gel-zeolite composites synthesized by this route from different silica feedstocks (by-product silica from chlorosilane production, microsilica, rice husk ash) and with different SiO2/Al2O3 ratios, using results from XRD, NMR, SEM, thermal analysis, mechanical and acid resistance testing. The use of rice husk ash favors formation of a fully amorphous geopolymer with high strength. Utilization of the other silica feedstocks leads to formation of aluminosilicate gel-zeolite composites, the amount and kind of zeolites depending on the feedstock and the SiO2/Al2O3 ratio. These composites show beneficial dehydration behavior, viz. no distinct dehydration step of thermal strain, with the phase assemblage after heating to 1000°C controlled by the starting composition. Mortars produced from both, the geopolymers as well as the composites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.