Shrinkage Of Alkali-activated Slag Mortars Research Articles

Effect of Modified Cow Dung Fibers on Strength and Autogenous Shrinkage of Alkali-Activated Slag Mortar.

Alkali-activated slag (AAS) presents a promising alternative to ordinary Portland cement due to its cost effectiveness, environmental friendliness, and satisfactory durability characteristics. In this paper, cow dung waste was recycled as a renewable natural cellulose fiber, modified with alkali, and then added to AAS mortar. The physico-chemical characteristics of raw and modified cow dung fibers were determined through Fourier transform infrared (FTIR), X-ray diffraction (XRD), and Scanning electron microscope (SEM). Investigations were conducted on the dispersion of cow dung fibers in the AAS matrix, as well as the flowability, strength, and autogenous shrinkage of AAS mortar with varying cow dung fiber contents. The results indicated that modified fiber has higher crystallinity and surface roughness. The ultrasonic method showed superior effectiveness compared to pre-mixing and after-mixing methods. Compared with raw cow dung fibers, modified fibers led to an increase of 11.3% and 36.3% of the 28 d flexural strength and compressive strength of the AAS mortar, respectively. The modified cow dung fibers had a more significant inhibition on autogenous shrinkage, and the addition of 2 wt% cow dung fibers reduced the 7 d autogenous shrinkage of the AAS paste by 52.8% due to the "internal curing effect." This study provides an alternative value-added recycling option for cow dung fibers as a potential environmentally friendly and sustainable reinforcing raw material for cementitious materials, which can be used to develop low autogenous shrinkage green composites.

Mitigation of autogenous shrinkage of alkali-activated slag mortar by stearate salts

The autogenous shrinkage of the stearates-modified alkali-activated slag (AAS) mortar was investigated in this study. The experimental results suggested that the maximum reduction (25 %) of autogenous shrinkage of AAS can be achieved by adding 0.5 wt% calcium stearate, while potassium stearate and sodium stearate seem less effective in decreasing shrinkage. According to the results obtained, the mechanisms of shrinkage mitigation are mainly associated with the retardation of hydration, the coarsening of pore structure, and the increased contact angle. It is estimated that the capillary pressure of pores was reduced by 30 % ∼ 60 % when stearates were used. However, the elastic modulus of AAS was reduced by 20 % due to the bubble-stabilizing effect of stearates, especially at higher dosages, which would partially offset its decreasing effects on autogenous shrinkage. It is also found that the traditional physical model could be used to predict the autogenous shrinkage of plain and modified AAS samples, provided the effect of both elastic and creep parts is considered.

Long-term (2 years) drying shrinkage evaluation of alkali-activated slag mortar: Experiments and partial factor analysis

Alkali-activated slag with many excellent properties was regarded as a novel low carbon building material, has received more and more attention. This research aims to study the impacts of alkali solution (alkali content and modulus), cement and gypsum contents on compressive strength, weight loss and drying shrinkage for alkali-activated slag mortar. Gypsum used as expanding source could compensate the drying shrinkage caused by silica and alkali components in the first three months, but it has a negative impact on the strength. The mainly results can be concluded that the alkali-activated slag blended with a cement content up to 20 wt% could effectively reduce the shrinkage and weight loss and increase the strength. Furthermore, the alkali content was below 3 wt%, the specimens possess relatively lower drying shrinkage. Based on the results of the test and analysis, the partial factors of combined activation on compressive strength and drying shrinkage of alkali-activated slag mortar were put forward. In the meantime, the relationships between compressive strength and combined activation factor are liner at 28, 90 and 365 days. Compressive strength and drying shrinkage could be estimated according to the combined activation and partial factor analysis. This work could provide a reasonable method for preparing the alkali-activated slag mortar and predict the shrinkage at different periods.

Effect mechanism of slag activity on the basic tensile creep of alkali-activated slag mortar

This study aims to investigate the effect mechanism of slag activity on the basic tensile creep of alkali-activated slag mortar (AM) using a combined activator of quicklime and sodium carbonate (molar ratio 1:1). Based on the results of setting time, compressive strength, dynamic modulus, autogenous shrinkage, basic tensile creep and microstructure, the basic tensile creep mechanism of AM with different activity slag is discussed. Results indicate that as slag activity increases, the degree of hydration increases and more C-A-S-H gel generates; meanwhile, the nominal crystal-binder ratio decreases, resulting in a high autogenous shrinkage of AM. Autogenous shrinkage is the driving force of the basic tensile creep. The increased amount of Q2(1Al) results that more Na+ is absorbed by C-A-S-H gel, which promotes the slippage of C-A-S-H gel. Moreover, the collapsing of C-A-S-H gel is easier with its shorter mean chain length (MCL). Therefore, the basic tensile creep increases as slag activity increases. Because of the higher autogenous shrinkage, smaller nominal crystal-binder ratio and larger amount of Q2(1Al), AM has larger basic tensile creep than ordinary Portland cement mortar (OM).

Impact of heat curing regime on the compressive strength and drying shrinkage of alkali-activated slag mortar

Heat curing has been recognized as one of the strategies to mitigate the shrinkage of alkali-activated slag (AAS) mortar. However, the impacts of heat curing regime on the drying shrinkage and the compressive strength of AAS mortar remain unclear. Therefore, this paper investigated the performance of AAS mortars cured at three different temperatures and four different durations. The results show that the heat curing hinders the late-age strength development of AAS mortars, particularly for those with an insufficient curing duration. This is related to the retarded early alkaline reaction and the increased volume of coarse pores inside the mortars. Furthermore, increasing the curing temperature is more effective in mitigating the drying shrinkage of AAS mortars than extending the curing duration. The decrease of drying shrinkage is primarily attributed to the reduced viscous deformation during the drying process, although the heat curing tends to increase the effective capillary pore pressure.

Effect of hydration assemblage on the autogenous shrinkage of alkali-activated slag mortars

The purpose of this work is to characterise the autogenous shrinkage performance of alkali-activated slag (AAS) mortars prepared with four activator types (i.e. sodium hydroxide, waterglass, calcium hydroxide/sodium carbonate and calcium oxide/sodium carbonate), with ordinary Portland cement (OPC) mortar as the reference sample. The hydration assemblage and nanostructure of calcium aluminosilicate hydrate (C-A-S-H) were investigated using a series of microscopic characterisations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA) and silicon-29/aluminium-27 nuclear magnetic resonance (29Si/27Al MAS NMR) analysis combined with thermodynamic modelling. The results show that the autogenous shrinkage of mortar samples is highly associated with C-A-S-H type and mass content in the cementitious systems. Compared with the OPC system, the larger C-A-S-H mass fractions in AAS systems affect the microstructure rearrangement, contributing to higher autogenous shrinkage. Furthermore, the autogenous shrinkage in AAS systems is negatively correlated with the medium chain length and aluminium (Al) incorporation degree of C-A-S-H.

Mitigating Autogenous Shrinkage of Alkali-Activated Slag Mortar by Using Porous Fine Aggregates as Internal Curing Agents

Alkali-activated slag (AAS) is beneficial for resource conservation in that it consumes little primary industrial energy, and it also performs well in terms of its mechanical properties and durability. However, its higher autogenous shrinkage compared to OPC mortars is a serious issue impeding AAS-based binder development for practical applications. This study investigated the feasibility and performance of active recycled aggregates when applied as man-made internal curing agents (MAs) for AAS mortars. They were applied as aggregate replacements for sand in this study to investigate the effects on the autogenous shrinkage, internal relative humidity (IRH), compressive strength, hydration properties and pore structure of AAS mortars. Three MAs with the sizes of 0.63–1.25 mm (MA 0.63), 1.25–2.5 mm (MA 1.25) and 2.5–4.75 mm (MA 2.5) were used. The results showed that MAs have potential as internal curing agents to mitigate the autogenous shrinkage of AAS mortars. When using saturated MAs, the autogenous shrinkage of AAS mortars was reduced by 87.68%. The addition of MAs also significantly prolonged the critical time taken for the IRH to start decreasing from 100%.

Effect of CaO on the Autogenous Shrinkage of Alkali-Activated Slag Mortar

The autogenous shrinkage of alkali-activated slag (AAS) is significantly higher than that of ordinary Portland cement (OPC). The higher risk of concrete cracking due to autogenous shrinkage is a critical drawback to wider use of this promising alternative binder. The effect of CaO content on the autogenous shrinkage of AAS mortar was investigated. The autogenous shrinkage of AAS mortars was determined by comparator. The pore structure of the pastes was determined by mercury intrusion porosimetry. The hydration products of the pastes were determined by Fourier transform-infrared, thermogravimetric analysis, X-ray diffraction, and 29Si solid-state magic-angle spinning nuclear magnetic resonance. The results show that the amount of portlandite increases as CaO content increases. CaO in the paste causes the partial replacement of C-S-H(I) (low stiffness) by C-S-H(II) (high stiffness). The hydration reaction of AAS is inhibited by the addition of CaO. The increase of polymerization degree of C-(A-)S-H and rearrangement of C-S-H(I) during hydration are inhibited by the addition of CaO, and micropores closure is also inhibited. Therefore, the autogenous shrinkage of AAS mortar decreases with the increase of CaO content.

Role of aggregate and fibre in strength and drying shrinkage of alkali-activated slag mortar

This paper presents an investigation on the influences of aggregate-to-binder (A/B) ratio, aggregate size, polypropylene (PP) and steel fibre dosages on the strength and drying shrinkage of alkali-activated slag mortar (AASM). The results indicate that increasing either A/B ratio up to 2.0 or aggregate size up to 1.18 mm first enhances both compressive and flexural strengths of AASM, followed by a reduction as they further increase. This is mainly attributed to the densified aggregate skeleton formed inside AASM. Furthermore, increasing the A/B ratio decreases the moisture loss in AASM and subsequently decreases their drying shrinkage. The use of coarse sand (e.g. 1.18–2.36 mm) enhances the volumetric stability of AASM prepared with the same binder content. For fibre reinforced AASM, the addition of PP fibres reduces the compressive strength of AASM, while the addition of steel fibres has a positive impact. The use of PP fibres not only causes fibre cluster problem, but also creates the weakened interfaces between fibres and binder. Increasing the PP or steel fibre dosage can enhance the flexural strength of AASM, particularly for steel fibres as they have a better bond with the matrix. Incorporation of PP or steel fibres can reduce the drying shrinkage of AASM. As compared to PP fibres, steel fibres are more effective in restraining the drying shrinkage of AASM.

Effect of Internal Curing by Super Absorbent Polymer on the Autogenous Shrinkage of Alkali-Activated Slag Mortars.

Alkali activated slag (AAS) mortar is becoming an increasingly popular green building material because of its excellent engineering properties and low CO2 emissions, promising to replace ordinary Portland cement (OPC) mortar. However, AAS’s high shrinkage and short setting time are the important reasons to limit its wide application in engineering. This paper was conducted to investigate the effect of internal curing(IC) by super absorbent polymer (SAP) on the autogenous shrinkage of AAS mortars. For this, an experimental study was carried out to evaluate the effect of SAP dosage on the setting time, autogenous shrinkage, compressive strength, microstructure, and pore structure. The SAP were incorporated at different dosage of 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 percent by weight of slag. The workability, physical (porosity), mechanical, and shrinkage properties of the mortars were evaluated, and a complementary study on microstructure was made. The results indicated that the setting time increased with an increase of SAP dosage due to the additional activator released by SAP. Autogenous shrinkage decreased with an increase of SAP dosage, and was mitigated completely when the dosage of SAP ≥ 0.2% wt of slag. Although IC by means of SAP reduced the compressive strength, this reduction (23% at 56 days for 0.2% SAP) was acceptable given the important role that it played on mitigating autogenous shrinkage. In the research, the 0.2% SAP dosage was the optimal content. The results can provide data and basis for practical application of AAS mortar.

Effect of internal curing by superabsorbent polymers – Internal relative humidity and autogenous shrinkage of alkali-activated slag mortars

This study experimentally investigated the effect of internal curing by superabsorbent polymers (SAPs) for mitigating autogenous shrinkage of alkali-activated slag (AAS) mortars. Measured were the compressive strength, the internal relative humidity (IRH), and autogenous shrinkage of AAS mortars incorporating different dosages of SAPs. Test results showed that as the dosage of SAPs increased, the reduction of IRH owing to self-desiccation and autogenous shrinkage both decreased, indicating that the SAPs can be effectively used as internal curing agents. The resulting modeling of the internal curing zone is expected to be useful in determining the appropriate dosage of SAPs in AAS mortars.

Influence of activator on the strength and drying shrinkage of alkali-activated slag mortar

The development of new binders, as an alternative to traditional cement, by the alkaline activation of industrial by-products (i.e. ground granulated slag and fly ash) is an ongoing research topic in the scientific community [Puertas F, Amat T, Jimenez AF, Vazquez T. Mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres. Cem Concr Res 2003;33(12): 2031–6]. The aim of this study was to investigate the feasibility of using and alkaline activated ground Turkish slag to produce a mortar without Portland cement (PC). Following the characterization of the slag, mortar specimens made with alkali-activated slag were prepared. Three different activators were used: liquid sodium silicate (LSS), sodium hydroxide (SH) and sodium carbonate (SC) at different sodium concentrations. Compressive and flexural tensile strength of alkali-activated slag mortar was measured at 7-days, 28-days and 3-months. Drying shrinkage of the mortar was measured up to 6-months. Setting times of the alkali-activated slag paste and PC paste were also measured. Setting times of LSS and SH activated slag pastes were found to be much slower than the setting time of PC paste. However, slag paste activated with SC showed similar setting properties to PC paste. LSS, SH and SC activated slag mortar developed 81, 29, and 36 MPa maximum compressive strengths, and 6.8, 3.8, and 5.3 MPa maximum flexural tensile strengths at 28-days. PC mortar developed 33 MPa compressive strength and 5.2 MPa flexural tensile strength. LSS and SH activated slag mortars were found to be more brittle than SC activated slag and PC mortars. Slag mortar made with LSS had a high drying shrinkage, up to six times that of PC mortar. Similarly, slag mortar made with SH had a shrinkage up to three times that of PC mortar. However, SC activated slag mortar had a lower or comparable shrinkage to PC mortar. Therefore, the use of SC as an activator for slag mortar is recommended, since it results in adequate strength, similar setting times to PC mortar and comparable or lower shrinkage.