Investigation of Cement–Bentonite Slurry Samples Containing PFA in the UCS and Triaxial Apparatus

Abstract

Three mixtures of cement-bentonite slurry containing 28, 36 and 44 % PFA (as a proportion of cementitious materials) were tested using the unconfined compressive strength and triaxial apparatus to determine the stress-strain and shear strength relationships for samples cured for various periods. The samples were batched using 4 % bentonite and 20 % cementitious materials (by mass of water) and allowed to cure underwater once extruded from sealed moulds. Curing periods of 14, 28 and 90 days were selected to investigate the changes in behaviour at durations commonly specified (28 and 90 days) as well as providing insight into changing behaviour with curing (additional curing periods of 7 and 60 days were investigated on a smaller number of samples to increase understanding). Two rates of displacement were used (1.0 and 1.3 mm/min) and four confining pressures (0, 50, 100 and 200 kPa). Shear strength and strain at peak deviator stress of the samples do not appear to vary considerably with confining pressure. For samples containing 28 % PFA, the majority of physical properties exhibited by the cement-bentonite samples change with curing period up to 60 days, where after the properties become similar to those cured for 90 days.

Deformation and Compression Behaviour of a Cement–Bentonite Slurry for Groundwater Control Applications

“…2, 3). The majority of samples exhibited a notable peak deviator stress (even after only 7 days of curing, which was not the case with samples containing PFA as the cement replacement material, where an obvious peak was not apparent until at least 28 days of curing, Royal et al 2013), with strain softening after this threshold (Fig. 3).…”

“…The cementitious materials were a Rugby cement (CEM II/B-V, 32.5 N) and GGBS, supplied by Hansen Aggregates); 80% of the cement was replaced with GGBS (based on the recommendations of Garvin andHayles 1999, andOpdyke andEvans 2005). The slurry was prepared in commercially available food mixers before being decanted in plastic cylindrical moulds (50 mm internal diameter by 150 mm in height for UCS-TX-UU samples and 75 mm internal diameter moulds for testing in the oedometer) to form bulk samples (following the procedure used by Royal et al 2013). The bentonite powder was mixed into the RO water (for at least 20 min) and allowed to hydrate for 24 h; the cement and GGBS were subsequently added to the hydrated bentonite slurry and mixed for a period of 5-10 min; the slurry was decanted into the moulds (which were agitated on a vibrating table to remove bubbles of air); and the moulds were sealed with flexible plastic covers and stored in water.…”

“…Richardson and Groves (1992) observed changes in the structure of the CSH formed in hardened cements pastes that contained high levels of GGBS (70% or greater); the products were finer and more ''foil like'' when compared to the ''fibrillar'' structure more commonly associated with CSH formation with OPC. It is suggested that this change in physical structure of cementitious products, as well the chemistry of the products, associated with the inclusion of significant proportions of GGBS that results in considerable variation in physical properties when compared to other CBs containing PFA (Royal et al 2013); as illustrated in the range of physical response presented below.…”

“…Samples were prepared for testing (following the methodology by Royal et al 2013) on the UCS and TX-UU, by placing them in a split mould (100 mm in length), which was secured in a simple jig, and shaving away the protruding length of CB using various saws and pallet knives. This produced samples with perpendicular faces and a length to diameter ratio of 2:1.…”

“…CB samples batched from slurry are likely to exhibit a range of physical properties due to differences in material composition within the slurry (ICE 1999;Jefferis 2012;Royal et al 2013). Therefore, the procedure described in Royal et al (2013) was used: at least three samples were prepared for each test (the numbers of samples investigated in each test are presented in Table 1) and the mean behaviour of these samples was used to consider changes in deformation behaviour. Similar approaches were previously used by Opdyke and Evans (2005) and Williams and Ghataora (2011).…”