Iranian Journal of Materials Science and Engineering

Different mineral admixtures of Indian metakaolin, Iranian silica fume and nanosilica were used to produce high performance mortars. Two different sands types with grain size of 0.015-4mm were mixed with type II Portland cement, polycarboxylate superplasticizer,mineral admixture with 650kg/m³ cement content and water/cement ratio of 0.35. Different amount of cement was replaced by metakaolin or silica fume (5-15wt%) or nanosilica (0.8-5wt%). After mixing, moulding and curing, compressive strength, electrical resistivity and abrasion resistance were studied. The maximum compressive strength of 28 days samples were 76MPa, 79MPa and 75MPa for 15wt% substitution of cement with metakaolin, silica fume and 5wt% with nanosilica. The compressive strength of these samples showed 28%, 33% and 26% increment in comparison with reference sample, respectively. X-ray patterns showed that replacing silica fume leads to reduction of Portlandite (Ca(OH)₂) phase. This can be attributed to the pozzolanic reaction and formation of new hydrated calcium silicate phase (CSH) that caused improvement of strength of admixtures containing samples. The microstructure of silica fume containing sample also showed better bond between sand and matrix. The electrical resistivity of samples with 15wt% metakaolin or silica fume and 5wt% nanosilica reach to 21kΩ.cm, 15 kΩ.cm and 10kΩ.cm, respectively. These samples showed high durability and corrosion resistance relative to reference samples (3.4 kΩ.cm). The abrasion resistance of different admixtures, specially silica fume containing samples were improved.

Microscopic studies has shown that adjacent to the interface between cement paste and aggregate, there exists an area with high porosity and low binding compounds that is referred to as interfacial transition zone (ITZ). ITZ in concrete and mortar imposes a number of negative effects, including flexural and compressive strengths reduction and permeability enhancement. That’s why many research attempts have been devoted to limit ITZ and its negative effects. The present study investigates the possibility of utilizing fine Portland cement (PC) clinker as a reactive aggregate in mortar for the same purpose. For this, natural quartz sand in normal mortar (NM) was totally replaced with PC clinker of the same particle size distribution and the most important engineering properties of the new mortar referred to as Reactive Aggregate Mortar (RAM) were measured and compared with NM as control. The results of compressive strengths measurements represented 65% and 21% increases at curing ages of 7 and 90 days, respectively, for RAM compared to NM. Chloride penetration depth in RAM displayed reductions by about 33% and 26% after 14 and 28 days of exposure, respectively. The effect of PC clinker reactivity on the microstructure and size of ITZ was studied by using scanning electron microscopy.