Study of Cement Mortar Strength at Different Ages

Study of Cement Mortar Strength at Different Ages
1 Introduction
Cement mortar is a fundamental construction material composed of cement, fine aggregate, and water in specified proportions. It serves as the binding agent in masonry construction, plastering, and as a matrix in concrete. Understanding the strength development of cement mortar over time is essential for quality control, structural design, and construction scheduling.
The strength of cement mortar is not a static property but evolves continuously as cement hydrates. The rate and magnitude of strength gain depend on multiple factors including cement composition, water-to-cement ratio, curing conditions, and the age at testing. The compressive strength at 28 days is conventionally considered the standard reference value for cement classification and quality assessment. However, early-age strength (3 and 7 days) provides critical information for construction operations such as formwork removal and loading schedules, while long-term strength (beyond 28 days) is relevant for durability and service life considerations.
This report presents a study of cement mortar strength at different ages, examining the patterns of strength development from early ages through extended curing periods, and analyzing the factors that influence these patterns.
2 Strength Development Mechanisms
2.1 Hydration Process
The strength development of cement mortar is fundamentally governed by the hydration of cement compounds. The principal cement phases—tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), and tetracalcium aluminoferrite (C₄AF)—react with water to form calcium silicate hydrate (C-S-H) gel, calcium hydroxide (portlandite), and other hydration products.
C₃S hydrates relatively rapidly, contributing primarily to early-age strength (up to 28 days). C₂S hydrates more slowly but continues to contribute to strength development over extended periods, providing long-term strength gain. The hydration of C₃A contributes to early strength but is less significant for ultimate strength development.
2.2 Strength-Age Relationship
The relationship between strength and age for cement mortar follows a characteristic pattern. Strength increases rapidly during the first few days as the initial hydration reactions proceed at high rates. The rate of strength gain subsequently decreases, with long-term strength development following a logarithmic or hyperbolic function.
A theoretical model based on first-order reaction kinetics describes strength development as a decelerating process . The model assumes that strength gain follows a relationship in which the rate of hardening is proportional to the remaining strength, leading to an asymptotic approach to a limiting strength value.
3 Experimental Data on Strength Development
3.1 Compressive Strength at Standard Ages
Experimental studies consistently demonstrate progressive strength gain with curing age. In a study of commercial cement-lime mortar, the average axial compressive strength at 3, 7, 14, and 28 days under a confining pressure of 250 kPa was reported as 2.9, 3.20, 4.84, and 6.91 MPa, respectively . This represents a strength increase of approximately 138% from 3 days to 28 days.
For ordinary Portland cement mortar, data from a comprehensive study shows compressive strength increasing from 14.42 MPa at 3 days to 23.86 MPa at 14 days, and reaching 31.61 MPa at 28 days . The strength at 28 days was approximately 119% of the 14-day strength and 219% of the 3-day strength.
3.2 Flexural Strength Development
Flexural strength also increases with curing age, though the pattern differs from compressive strength. In the study of dry-cured and standard-cured mortars, bending strength was found to increase with curing age but at a rate that decreased with age . The increasing rate of bending strength was observed to decrease relative to compressive strength as curing age increased.
Research on nano-clay cement mortar reported flexural strength ratios at 7 and 28 days, with values indicating that flexural strength development follows a similar progressive pattern to compressive strength but with different absolute values .
3.3 Long-Term Strength Gain
Strength development continues beyond 28 days, though at a decreasing rate. A study on cement mortar with various mix proportions (1:2 to 1:7) demonstrated that compressive strength continued to increase up to 90 days for all mix proportions . Mathematical models were developed to describe this behavior, with logarithmic functions providing good fit for the entire 3–90 day period.
The relationship can be expressed as:
f′c=aln⁡(t)+bf′c=aln(t)+b
where f′cf′c is the compressive strength, tt is the age in days, and aa and bb are constants dependent on mix proportions.
For a 1:2 mortar mix, the relationship was determined as f′c=7.805ln⁡(t)+13.28f′c=7.805ln(t)+13.28 . This indicates that strength continues to increase logarithmically with age, with no abrupt plateau at 28 days.
4 Factors Influencing Strength Development
4.1 Water-to-Cement Ratio
The water-to-cement (w/c) ratio is the single most important factor controlling mortar strength. Lower w/c ratios produce higher strength at all ages because reduced water content results in lower porosity and denser microstructure. The strength difference between mortars of different w/c ratios is established at early ages and persists throughout curing.
Research on multi-walled carbon nanotube (MWCNT) mortar demonstrated that the strength enhancement was more significant at lower w/c ratios, with the improvement at 28 days being 34% for w/c = 0.4 compared to only 4% for w/c = 0.6 .
4.2 Curing Conditions
Curing conditions significantly influence strength development. Standard curing (moist curing) produces higher strength than dry curing at all ages . The strength difference between standard-cured and dry-cured mortar increases with curing age, as dry-cured specimens experience reduced hydration due to moisture loss.
Temperature effects on strength development are complex. The maturity concept—which assumes that strength depends on the cumulative effects of time and temperature—provides a framework for understanding these effects. However, research has shown that the strength-maturity relationship is not unique but varies with curing temperature . Mortar cured at higher temperatures initially gains strength more rapidly but may exhibit lower ultimate strength compared to mortar cured at lower temperatures.