The effect of clay fines in aggregate on the properties of mortar
Within Europe, harmonisation of standards for aggregates has resulted in a suite of new and modified aggregate test methods and a reappraisal of specification limits through the work of the European Committee for Standardisation (CEN) Technical Committee (TG) 154. The acceptance in many countries, particularly France and The Netherlands, that fines quality is more significant than fines quantity has led to the drafting of test methods to assess the harmfulness of fines. The methylene blue dye test, a quantitative test method for clay fines, and the Sand Equivalent test for fines content are currently draft European standards and their use is included in the new European Standard for aggregates for mortar being drafted by Sub-committee 1 of Technical Committee 154. Information gathered on fines content and fines quality has led to concerns that our European partners would demand onerous restrictions on harmful fines that would prejudice the use of what may be perfectly satisfactory materials traditionally used by brick-layers in the UK. This thesis describes research work conducted to relate the methylene blue value and fines content of building sand to the performance of mortar and to address criticisms of the methylene blue dye test for harmful fines in aggregates. A review of literature relating to mortar properties and the effects of the aggregate used has been undertaken. Attempts have been made to characterise the fines in natural building sands and relate their properties to the effects they have on mortar. A correlation has been established between clay content and durability performance. A novel test has been developed to measure the shrinkage potential of mortars cast on bricks of varying capillary suction. The clay and fines content of the sand used has been correlated to plastic shrinkage cracking in mortar. The role of air entrainment, substrate action and mortar type has been discussed in relation to durability performance and shrinkage potential. Combinations of materials have been described that give optimum durability performance for a minimum shrinkage potential. A method has been developed that can selectively alter the dye adsorption of clay minerals so that results of dye adsorption tests on clays are in proportion to their damaging effects in mortar. Aggregate has been recovered from hardened mortar and successfully tested for methylene blue dye adsorption. The method has been applied to a sample of mortar recovered from the site of a mortar failure.