Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
South, Warren, A study of the compressive strength and drying shrinkage of cementitious binders prepared using natural pozzolans, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2009. http://ro.uow.edu.au/theses/3100
The world cement industry, of which New Zealand is a small proportion, faces the prospect of mandated reductions in carbon emissions due to the societal focus on climate change. The use of supplementary cementitious materials, such as fly ash, ground granulated blast furnace slag or other pozzolanic materials can facilitate this reduction through the substitution of portland cement clinker in cement. However, the acceptance of the use of these materials will only be achieved by the thorough evaluation of the performance of the resultant cement blends. This thesis examines two main performance parameters, compressive strength and drying shrinkage, in a range of cement blends based on New Zealand-sourced natural pozzolans with the aim of supporting their use in reducing the aggregate emissions of that country.
This work takes four candidate materials – pumicite, amorphous silica, and two diatomites, all indigenous to New Zealand -and evaluates their efficacy as mineral additions to Portland cement binders. Each material was characterised for their chemical and physical nature. They were then used to prepare binders for subsequent testing, Substitution levels of 5%, 10%, 20% and 40% were used with a Portland cement in a series of tests used to determine compliance to local Standards and indicate field performance.
The blends were prepared by blending and intergrinding the precursor materials.
Testing of compressive strength up to 90 days and drying shrinkage up to 3 years maturity was undertaken using standard mortars of a 0.5 water-to-binder ratio and also with a water addition sufficient to produce equal workability to the standard mortar.
The precursor pozzolans were prepared to two different fineness levels to investigate the effect of fine or coarse processing on performance.
A further series of experiments involved the addition of a small amount of gypsum to determine the optimum level of sulphate in the binder for compressive strength.
The additions of pozzolans to cement blends up to 40% substitution levels was found to affect the properties of mortar compressive strength and drying shrinkage. In general, the early strengths were depressed but later strengths (greater than 28 days) were found to be enhanced by the pozzolan addition. The performance of pumice blends were little affected by the fineness of the material at the two specific surface areas tested. A finer amorphous silica did not enhance compressive strengths.
The addition of water to improve workability was generally detrimental to compressive strengths.
Processing options such as intergrinding and the addition of sulphate were evaluated for their affect on binder properties. Intergrinding the components of pumicite and amorphous silica did not increase mortar compressive strengths for the pumicite blends. Amorphous silica blends, at 5% and 10% substitution showed increased compressive strengths, while only one of the diatomites showed improved strengths.
Some advantage was seen in a raised sulphate content of selected blends utilizing pumicite and amorphous silica. For the diatomite blends, increasing sulphate content was detrimental to compressive strength at blends above 10% substitution. However, some increases in 28 and 90 days compressive strengths were noted for the 5% substitution blends.
Drying shrinkage was evaluated using a mortar method. Increased drying shrinkages were noted for increased substitution rates for all materials. Pumicite blends exhibited lower drying shrinkages, due to the pumicite being coarse relative to the reference cement. The other materials showed equal or elevated mortar drying shrinkage relative to the reference cement.
A series of mortar and concrete tests were subsequently carried out on 20% pumicite blends at three specific surface areas. Later concrete compressive strengths were higher as the fineness of the pumicite increased. Concrete drying shrinkage also increased with increasing fineness but 10% and 20% substitution exhibited drying shrinkage close to that of the reference.
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